EP4654886A1

EP4654886A1 - Apparatus and method for eit

Info

Publication number: EP4654886A1
Application number: EP24717749.6A
Authority: EP
Inventors: Richard Bayford; Andy BARDILL; Andreas Demosthenous; Neil CHILTON
Original assignee: Middlesex University Higher Education Corp; Printed Electronics Ltd; UCL Business Ltd
Current assignee: Middlesex University Higher Education Corp; Printed Electronics Ltd; UCL Business Ltd
Priority date: 2023-01-23
Filing date: 2024-01-23
Publication date: 2025-12-03
Also published as: GB202300955D0; GB2633283A; WO2024156999A1

Abstract

A wearable device (100) for use in electrical impedance tomography on an animal (102) having a body part, which wearable device comprises: a carrier layer substantially conformable to the general shape of the body part; a plurality of electrically conductive ink electrodes spaced apart across the carrier layer; a plurality of electrically conductive ink tracks extending along the carrier layer, one electrically conductive ink track for each electrically conductive ink electrode, each electrically conductive ink track having a first end electrically connected to a respective one conductive ink electrode and a second end for providing an electrical connection to an external electrical circuit; at least one region of an interface layer for adhering the wearable device to the body part and for providing electrical conductivity between the plurality of electrically conductive ink electrodes and the body part; wherein, in use during movement of said body part, said wearable device permits at least some flexing, elongation and/or contraction of at least a portion of the wearable device and at the same time remains in contact with the body part substantially in conformance with its general shape, without buckling or twisting up and away from the body part during said flexing, elongation and/or contraction.

Description

APPARATUS AND METHOD FOR EIT FIELD OF THE INVENTION Embodiments of the present invention relate to a wearable device for use in electrical impedance tomography (EIT), to a kit comprising the device, to an EIT hub, to an apparatus for performing EIT, to various methods of manufacturing such a wearable device, and to use on a laboratory animal of a wearable device. BACKGROUND TO THE INVENTION Electrical impedance tomography (EIT) is a non-invasive, radiation-free technique that provides an image of the electric conductivity of an object by injecting small currents (typically ^5 mA rms) through electrodes placed on the object’s boundary and measuring the resulting potentials from the same or other electrodes. EIT images are generated based on solving an ill-posed inverse problem. Publicly available tutorials and software are available at EIDORS for enabling reconstruction of EIT images. EIT has been successfully used to image regional changes in pulmonary ventilation and perfusion in real time. The images do not provide static structural equivalent to magnetic resonance imaging (MRI) or computed tomography (CT) of the lung but yield the air volume change in the lung with high temporal resolution (up to about 120 frames per second). EIT offers the potential for continuous bedside respiratory monitoring of patients. In order to perform EIT on patients to monitor lung function, the patient must wear a plurality of electrodes around the torso. Often the electrodes are placed in a plane at approximately the nipple line, but other locations up and down the torso are possible. Various devices have been proposed as a mount for the electrodes to ease placement on the patient. One such device is the SensorBelt by Sentec that is complicated and expensive to manufacture and involves a lengthy fitting procedure with straps passing over the patient’s shoulders. This is not straightforward when the patient is lying down. Another device is provided by Draeger for use with the PulmoVista 500 system that has an elastomeric belt having electrodes, each electrode MUH,016-UK with a projecting press-stud. The belt must be fitted to the patient first followed by an interface cable to connect the electrodes to an EIT system. The interface cable itself has large connectors for connecting to the press studs on the belt. Again, the fitting procedure is complicated and neither system facilitates patient comfort whilst being worn. US 2004/0236202 discloses an assembly that incorporates an expandable strap for use in EIT. Each strap is made from an inelastic (but bendable and flexible) material such as a polymer film (e.g. Mylar). The nature of the strap in US 2004/0236202 is such that it buckles under tension to permit it to expand during the breathing cycle of the patient, which is shown in Fig.4 of that document. One problem with the assembly of US 2004/0236202 is that the buckling will tend to lift one or more electrodes away from the patient’s skin. In turn this will increase noise in measured voltages, compromising the EIT image reconstruction process. SUMMARY OF THE INVENTION According to some embodiments there is a provided a wearable device for use in electrical impedance tomography on an animal having a body part. The wearable device may comprise a carrier layer. The carrier layer may be substantially conformable to the general shape of the body part. The wearable device may comprise a plurality of electrically conductive ink electrodes spaced apart across (e.g. along the length) the carrier layer. The plurality of electrically conductive ink electrodes may be arranged as a 1D array or a 2D array for example. The wearable device may comprise a plurality of electrically conductive ink tracks extending (e.g. lengthwise) along the carrier layer. One electrically conductive ink track may be for each electrically conductive ink electrode. Each electrically conductive ink track may have a first end electrically connected to a respective one conductive ink electrode and a second end for providing an electrical connection to an external electrical circuit. The wearable device may comprise at least one region of an interface layer. The interface layer may be for adhering the wearable device to the body part and for providing electrical conductivity between the plurality of electrically conductive ink electrodes and the body part. The wearable device may be adapted so that, in use during movement of said body part, said wearable device permits at least some flexing, elongation and/or contraction of at least a portion of the wearable device (e.g. along its length). The MUH,016-UK wearable device may, at the same time, remain in contact with the body part substantially in conformance with its general shape, without buckling or twisting up and away from the body part during said flexing, elongation and/or contraction. In any embodiment described herein, the wearable device may be suitable for use and/or adapted for use on a human or a non-human animal. In some embodiments the body part may be the thorax and the wearable device may be adapted to permit the flexing and/or elongation and/or contraction under action of the breathing cycle of the animal (whereby the wearable device is usable for monitoring lung function, e.g. air recruitment). In some embodiments the body part may be a limb on which a prosthetic is worn, and the wearable device may be adapted to permit the flexing and/or elongation and/or contraction under movement the limb. In some embodiments the carrier layer may have an elasticity in at least one direction (e.g. along at least a part of its length). The carrier layer may have a flexibility in at least one plane. The plurality of electrically conductive ink tracks may be arranged to permit said flexing and/or elongation and/or contraction. In some embodiments the carrier layer may comprise an elongate strip. The elongate strip may have a length sufficient to span a portion of said body part. Each electrically conductive ink track may comprise a meandering, winding or indirect path along at least a portion of the length of the elongate strip. In some embodiments the elongate strip may have a length sufficient to span anteriorly or posteriorly, a portion of a perimeter of the thorax of the animal. In some embodiments the portion may extend between a sinistral side and a dextral side of the animal thorax within a chest region. In some embodiments the carrier layer may comprise a patch which may have a square, rectangular, round or other shape for example. The array of electrodes may be arranged on the patch in a 2-dimensional array or other 2-dimensional pattern. In some embodiments the meandering, winding or indirect path may comprise a deviation from and return to a direction extending substantially along the carrier layer. The direction of extension may be substantially lengthwise along the carrier MUH,016-UK layer. In some embodiments the meandering, winding or indirect path may comprise a sinusoid, serpentine shape, zig-zag shape or a kink. In some embodiments the meandering, winding or indirect path may comprise a portion that oscillates between two imaginary parallel lines running in a direction (e.g. lengthwise) along the elongate strip and/or the carrier layer. In some embodiments each electrically conductive ink track may comprise an electrically conductive ink. The electrically conductive ink may have a flexibility allowing it to remain substantially conductive and within workable levels of impedance after repeated bending and stretching of the wearable device during normal use on the animal. In some embodiments the wearable device may comprise a feature for facilitating substantially uniform flexing and/or elongation and/or contraction of the wearable device (e.g. along its length). In some embodiments the feature may comprise a printed feature. In some embodiments the feature may comprise a redundant conductive ink track. In some embodiments the redundant conductive ink track may comprise a shape, pattern or path the same or similar to said plurality of electrically conductive ink tracks. In some embodiments the redundant conductive ink track may comprise a shape, pattern or path that is dissimilar to said plurality of electrically conductive ink tracks. In some embodiments there may a mixture of similar and dissimilar shapes. In some embodiments the redundant electrically conductive ink tracks may be arranged so that, at points along the length of the elongate strip, the total number of conductive ink tracks across a width of the elongate strip is constant. In some embodiments the plurality of electrically conductive ink electrodes may comprise a conductive ink layer formed by a printing process. In some embodiments the carrier layer may comprise a plurality of openings, MUH,016-UK a respective electrode of the plurality of electrically conductive ink electrodes accessible through a corresponding opening. In some embodiments the plurality of electrically conductive ink tracks may comprise a conductive ink layer formed by a printing process. In some embodiments the wearable device may further comprise a dielectric encapsulant that substantially encapsulates each of said plurality of electrically conductive ink tracks except at said first end at least sufficient to provide said electrical connection with a respective one conductive ink electrode and at said second end at least sufficient to provide said electrical connection to said external circuit. In some embodiments the dielectric encapsulant may comprise a plurality of dielectric encapsulant layers formed by a printing process. In some embodiments the plurality of electrically conductive ink electrodes, the plurality of electrically conductive ink tracks and the dielectric encapsulant form a unitary structure. In some embodiments, when measured at an electrode of said plurality of electrically conductive ink electrodes, the unitary structure may have a thickness of less than 0.20 mm, preferably less than 0.15 mm, and preferably is about 0.14 mm, not including said carrier layer and said interface layer. In some embodiments, when measured at an electrically conductive track of said plurality of electrically conductive ink tracks, said unitary structure may have a thickness of less than 0.25 mm, preferably less than 0.20 mm, and preferably is about 0.16 mm, not including said carrier layer and said interface layer. In some embodiments the wearable device may further comprise an adhesive between said unitary structure and said carrier layer. In some embodiments the adhesive may be a powder adhesive of the kind used in a heat transfer printing process. In some embodiments the carrier layer may comprise a textile carrier layer. The textile carrier layer may comprise a breathable and/or moisture-wicking fabric. MUH,016-UK The textile carrier layer may comprise woven or non-woven textile or material that exhibits properties of flexibility and elasticity. In some embodiments the carrier layer or the textile carrier layer may have a thickness less than about 1.0 mm. In some embodiments the carrier layer or the textile carrier layer may have a thickness within one of the following ranges: between about 0.1 mm and about 0.7 mm; between about 0.2 mm and about 0.6 mm; between about 0.3 mm and about 0.5 mm; and between about 0.35 mm and about 0.45 mm. In some embodiments the textile carrier layer may comprises a plant fibre, such as bamboo. The bamboo may be in the form of a bamboo viscose. In some embodiments the region of an interface layer may comprise multiple separate and distinct regions. Each separate and distinct region may cover at least one of said plurality of electrically conductive ink electrodes. In some embodiments the region of an interface layer may comprise a single region covering multiple electrodes. In some embodiments the region of an interface layer may have a thickness between about 0.5 mm and about 1.0 mm. In some embodiments the at least one region of an interface layer may comprise a cross-linked hydrophilic polymer, such as a hydrogel. In some embodiments the carrier layer comprises a jacket or harness for a laboratory animal. In some embodiments, the wearable device may comprise a location indicator for assisting a medical professional to locate the wearable device on the body part. In some embodiments the wearable device may be universal (e.g. useable on the anterior and posterior of the thorax). MUH,016-UK In some embodiments, the wearable device may comprise a plurality of orientation indicators that assist the medical professional to orient the wearable device correctly on the body part. In one embodiment, the plurality of indicators comprises a plurality of numbers, a first group of which are oriented in a first direction and a second group of which are oriented in a second direction opposite to the first direction. When viewed in one sense the first group of numbers is the correct way up to the medical professional and the second group of numbers is upside down, and when view in the opposite sense by turning the wearable device around, the second group of numbers is the correct way up to the medical professional and the first group of numbers is upside down. In some embodiments the indicator comprises a functional component (such as one of the aforementioned plurality of numbers) and an aesthetic component. According to some embodiments there is provided a kit for use in performing electrical impedance tomography on an animal, which kit comprises at least two wearable devices as set out above, as described anywhere herein or as set out in the claims. According to some embodiments there is provided the use on a laboratory animal of a wearable device as set out above, as described anywhere herein or as set out in the claims. According to some embodiments there is provided an apparatus comprising: a wearable device as set out above, as described anywhere herein or as set out in the claims; and a processor and a memory (which may be a microcontroller, ASIC, etc.), the memory storing computer-executable instructions that when executed by the processor cause the processor to: take EIT measurements of a body part to which the wearable device is adhered; process the EIT measurements to determine an impedance distribution in the body part; and provide an output signal representative of said impedance distribution. In an embodiment, the apparatus may comprise a controllable device, and the MUH,016-UK output signal may be used to control the controllable device. In an embodiment, the controllable device may be a prosthetic limb and the output signal may be used to control motion of the prosthetic limb using one or more motor. In another embodiment the controllable device may be a material having controllable properties (e.g. stretchability, stiffness, etc.), and the output signal may be used to control the properties of the material (e.g. to make it more or less stretchy, or more or less stiff). According to some embodiments there is provided an EIT hub for use with a wearable device as set out above, as described anywhere herein or as set out in the claims. The EIT hub may comprises a port for connecting the EIT hub to the wearable device. The EIT hub may comprise circuitry adapted for applying an alternating current through pairs of electrodes of the plurality of electrodes of the wearable device. The circuitry may be adapted for measuring voltages across other pairs of electrodes of the plurality of electrodes of the wearable device, whilst said alternating current is applied. The circuitry may be adapted for digitising measured voltages for use in an EIT reconstruction algorithm. In some embodiments the EIT hub may further comprise a rechargeable battery for powering the circuitry. There may be a memory for storing data. There may be a network interface from which said data may be transmitted to a remote computing device. In some embodiments the EIT hub may further comprise a base station for holding the EIT hub during use, wherein the EIT hub is separable from the base station substantially without resistance, such as by movement of the patient wearing the wearable device connected to the EIT hub. In some embodiments the EIT hub and the base station may be electrically and/or physically connectable when the base station is holding the EIT hub. The base station may comprises a cable permitting data and/or power transfer. In some embodiments the EIT hub may be adapted for charging the rechargeable battery by wireless power transfer, such as a near-field wireless power transfer technique. MUH,016-UK In some embodiments the EIT hub may further comprising a coil for wireless power transfer by inductive coupling. In some embodiments the EIT hub may further comprise a housing comprising said port and containing said circuitry. According to some embodiments there is provided an apparatus for performing electrical impedance tomography (EIT) on an animal, which apparatus comprises: a wearable device as set out above, as described anywhere herein or as set out in the claims; and an EIT hub as set out above, as described anywhere herein or as set out in the claims. In some embodiments the apparatus may further comprise a transmitter for transmitting the digitised measured voltages to a remote computer for use in said EIT reconstruction algorithm. In some embodiments the apparatus may further comprise a first port and a second port to which a respective connector of each wearable device is connectable. The apparatus may further comprise at least one indicator for indicating to a user which of the first and second ports each wearable device should be connected according to the location of the first and second wearable devices on the patient. In some embodiments the apparatus may further comprise a computing device for communicating with the EIT hub. The computing device may have a Graphical User Interface (GUI) permitting a user to control said EIT hub. According to some embodiments there is provided a method of manufacturing a wearable device for use in electrical impedance tomography. The method may comprise providing a temporary release substrate. The method may comprise forming on said temporary release substrate a plurality of electrically conductive ink electrodes spaced apart across (e.g. along the length) of the temporary release substrate. The method may comprise forming on said temporary release substrate a plurality of electrically conductive ink tracks extending along (e.g. lengthwise) the temporary release substrate, one electrically conductive ink track for each electrically conductive MUH,016-UK ink electrode. Each electrically conductive ink track may have a first end electrically connected to a respective one conductive ink electrode and a second end for providing an electrical connection to an external electrical circuit. The method may comprise applying an adhesive to at least a part of said plurality of electrically conductive ink electrodes and to at least a part of said plurality of electrically conductive ink tracks, and/or to a carrier layer. The method may comprise bringing the temporary release substrate and said carrier layer together so as to adhere the carrier layer to said plurality of electrically conductive ink electrodes and to said plurality of electrically conductive ink tracks. The method may comprise removing the temporary release substrate to leave said plurality of electrically conductive ink electrodes and said plurality of electrically conductive ink tracks adhered to said carrier layer. In some embodiments the step of forming a plurality of electrically conductive ink electrodes may comprise a printing process. The step of forming a plurality of electrically conductive ink tracks may comprise a printing process. In some embodiments the method may further comprise the step of forming a dielectric encapsulant on said temporary release substrate such that each of said plurality of electrically conductive ink tracks is substantially encapsulated except at said first end at least sufficient to provide said electrical connection with a respective one conductive ink electrode and at said second end at least sufficient to provide said electrical connection to said external circuit. In some embodiments the step of forming a dielectric encapsulant on said temporary release substrate may comprise a printing process. In some embodiments the printing process may comprises one of: screen printing, inkjet printing, and LED ultra violet light printing. In some embodiments the step of forming a plurality of electrically conductive ink electrodes and the step of forming a plurality of electrically conductive ink tracks may comprise forming a unitary structure on said temporary release substrate. In some embodiments the step of forming said unitary structure may be performed layer by layer. MUH,016-UK In some embodiments the step of forming a plurality of electrically conductive ink tracks may comprise applying to said temporary release substrate a first layer of a dielectric encapsulant approximately following a desired layout of the electrically conductive ink tracks. The method may comprise curing said first layer of dielectric encapsulant. The method may comprise applying to said first layer of dielectric encapsulant a layer of a conductive ink. The conductive ink may then be cured. The method may comprise applying to said layer of conductive ink a layer of a second layer of a dielectric encapsulant. The second layer of dielectric encapsulant may then be cured. In some embodiments the step of forming a plurality of electrically conductive ink electrodes may comprise applying to said temporary release substrate a layer of conductive ink approximately following a desired layout of the plurality of electrically conductive ink electrodes. In some embodiments the method may further comprise the step of providing a region of an interface layer over each electrode of said plurality of electrically conductive electrodes. In some embodiments the region of interface layer may comprise separate and distinct regions of the interface layer, each separate and distinct region covering a respective electrode. Each separate and distinct region of interface layer may cover an area of the wearable device greater than the respective electrode. In some embodiments the region of interface layer may covers at least two electrodes of said plurality of electrically conductive electrodes. In some embodiments the region of interface layer may comprise a cross-linked hydrophilic polymer such as a hydrogel. In some embodiments the method may further comprise the step of manufacturing two substantially identical wearable devices. Each wearable device may be useable on a body part of an animal, such as the anterior or a posterior portion of the thorax of a patient (human or non-human) at the choice of a medical or MUH,016-UK veterinary professional. According to some embodiments there is provided a method of manufacturing a wearable device for use in electrical impedance tomography. The method may comprise forming on a carrier layer a plurality of electrically conductive ink electrodes spaced apart along the length of the carrier layer. The method may comprise the step of forming on said carrier layer a plurality of electrically conductive ink tracks extending lengthwise along the carrier layer. There may be one electrically conductive ink track for each electrically conductive ink electrode. Each electrically conductive ink track may have a first end electrically connected to a respective one conductive ink electrode and a second end for providing an electrical connection to an external electrical circuit. According to some embodiments there is provided a computer-implemented method of manufacturing a wearable device for use in electrical impedance tomography. The method may comprise receiving at a computing device a measurement of at body part of an animal. The computing device may use said measurement to create or adapt a data structure representing a wearable device as set out above, as described anywhere herein or as claimed, so that a layout of the plurality of electrically conductive ink tracks and the plurality of electrically conductive ink electrodes is suitable for at least a portion of the body part. The method may comprise outputting the data structure for use in manufacturing a wearable device based on said data structure. According to some embodiments there is provided a wearable device for use in electrical impedance tomography on a human patient. The wearable device may comprise a textile carrier layer. The textile carrier layer may be in the form of an elongate strip. The elongate strip may have a length sufficient to span, anteriorly or posteriorly, a portion of a perimeter of a human torso. The portion may extend between a sinistral side and a dextral side of the human torso within a chest region. The textile carrier layer may be substantially conformable to the general shape of the human torso between the sinistral and dextral sides. The wearable device may comprise a plurality of electrically conductive ink electrodes spaced apart along the length of the textile carrier layer. The wearable device may comprise a plurality of electrically conductive ink tracks extending lengthwise along the textile carrier layer. There may be one MUH,016-UK electrically conductive ink track for each electrically conductive ink electrode. Each electrically conductive ink track may have a first end electrically connected to a respective one conductive ink electrode and a second end for providing an electrical connection to an external electrical circuit. The wearable device may comprise at least one region of an interface layer for adhering the wearable device to the skin of the human torso and for providing electrical conductivity between the plurality of electrically conductive ink electrodes and the skin. In use, the wearable device may permit at least some flexing and/or elongation and/or contraction of at least a portion of the wearable device along its length under action of the human breathing cycle. At the same time the wearable device may remain in contact with the human torso and substantially in conformance with its general shape, without buckling or twisting up and away from the torso during said flexing and/or elongation and/or contraction. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is schematic illustration of a wearable EIT apparatus according to the present invention in use on a patient; Fig. 2 is a plan view of the wearable EIT apparatus of Fig. 1; Fig.3A is a plan view of a first embodiment of a wearable device according to the present invention, showing an inward-facing side; Fig. 3B is a side view of the wearable device of Fig. 3A; Fig. 3C is a plan view of the wearable device of Fig. 3A showing an outward- facing side; Fig. 3D is a cross-section of the wearable device along line D-D of Fig. 3A; Fig. 3E is a cross-section of the wearable device along line E-E of Fig. 3A; Fig.4A is a plan view of a second embodiment of a wearable device according to the present invention, showing an inward-facing side; Fig. 4B is a cross-section of the wearable device along line B-B of Fig. 4A; Fig. 4C is a cross-section of the wearable device along line C-C of Fig. 4A; Fig. 5 is a plan view of a third embodiment of a wearable device according to the present invention; Figs.6, 7 and 8 are plan views of various hydrogel regions on a wearable device according to the present invention; Figs. 9A and 9B are steps in a method of manufacturing a wearable device according to the present invention; MUH,016-UK Fig. 10A is a schematic perspective view of three states of a dock usable with a wearable EIT apparatus according to the present invention; Fig. 10B shows the dock of Fig. 10A in use on a patient; Fig.11 is a schematic perspective view of an EIT hub useable with a wearable EIT apparatus according to the present invention; Fig. 12 is a schematic side view of a computing device and a DC control box useable with a wearable EIT apparatus according to the present invention; Fig.13 is a schematic block diagram of the components of the EIT hub of Fig. 11; Fig. 14A and 14B are schematic diagrams illustrating current injection and voltage scan sequences when the wearable EIT apparatus is in use; Fig. 15 is a schematic diagram illustrating data processing method performed by the EIT hub of Fig. 11; Fig. 16 is a table illustrating the organisation of data in a byte stream output from the EIT hub of Fig. 11; Fig. 17 is a schematic diagram illustrating the wearable EIT apparatus in use on a patient lying in a hospital bed; Figs. 18A – 18J are schematic illustrations of different states of a Graphical User Interface as displayed on the computing device of Fig. 12 during use of the wearable EIT apparatus; Fig. 19A is a schematic perspective view of another embodiment of an EIT hub; Fig. 19B is schematic side view of the EIT hub of Fig. 19A; Fig.19C is a schematic perspective view of the EIT hub of Figs. 19A and 19B in use on a patient; Fig.20A is a schematic side view of an embodiment of a monitor in accordance with an embodiment, shown in an off state; Fig. 20B is a schematic side view of an embodiment of the monitor of Fig. 20A, shown in an on state; Fig. 20C is a schematic perspective view of an embodiment of the monitor of Fig. 20B; Figs. 21A and 21B are a schematic plan views of an outward-facing side of different embodiments of a wearable device for use on different patient groups; Fig. 22 is a schematic plan view of an inner side of an embodiment of a wearable device; and MUH,016-UK Fig. 23 is a schematic diagram of a system and method for controlling prosthetic limb motion using a wearable device in accordance with an embodiment. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to Fig. 1 a wearable EIT apparatus generally identified by reference numeral 100 is worn by a human patient 102 on a body part. In an embodiment the body part is the upper torso. Whilst embodiments of the wearable EIT apparatus 100 described herein are shown in use on human patients for monitoring lung function, the wearable EIT apparatus may be used on other animals, and is not limited to monitoring lung function, as described in greater detail below. Furthermore, the wearable EIT apparatus may be used on other body parts, including head, arm, leg, lower torso and breast. The human patient has a sinistral, or left, side 104 and a dextral, or right, side 106. A line 108 schematically indicates the delineation between the sinistral side 104 and dextral side 106. The wearable EIT apparatus 100 is positioned at the 4^th/5^th intercostal. It is noted that this is not essential, and the wearable EIT apparatus 100 may be worn in a different position up or down the torso, and there may be two or more wearable EIT apparatus used simultaneously to construct a 3D image for example. How the wearable EIT apparatus may be applied and then worn by the patient 102 will be described in greater detail below. As shown in Fig. 2, the wearable EIT apparatus 102 comprises two separate wearable devices, a first wearable device 108 and a second wearable device 110. In other embodiments described elsewhere herein the wearable EIT apparatus may comprise a single wearable device. In Fig.2 the first and second wearable devices 108, 110 are shown approximately in the shape that is adopted when in use as shown in Fig. 1. However, it is important to note that the first and second wearable devices 108, 110 are not rigid and that the shape shown in Fig. 2 is not a permanent shape. Upon application to the human patient 102, a first portion 112 of the first wearable device 108 substantially conforms to the shape of the anterior, or front, of the patient’s torso. A first portion 112 of the second wearable device 110 substantially conforms to the shape of the posterior, or back, of the patient’s torso. Second portions 114 of both the first and second wearable devices 108, 110 are not in contact with the patient 102 but MUH,016-UK are free to extend out from the sinistral side 104 as a ‘tail’ at the end of which is a respective connector 116, which permits the first and second wearable devices to be connected to an EIT hub (not shown in Fig. 1 or 2). The details of the connector 116 and EIT hub will be described in greater detail hereinafter. It should be noted that the second portion 114 does not have be oriented on the sinistral side 104 of the patient but may be positioned on the dextral side 106 instead. As will be explained below, this orientation may be chosen at the convenience of the user (e.g. healthcare professional), and does not require manufacture of ‘sinistral’ and ‘dextral’ versions of the first and second wearable devices 108, 110. The first and second wearable devices 108, 110 are universal and therefore identical with one another. Thus, the first wearable device 108 and second wearable device 110 may be applied either to the anterior or posterior part of the patient’s torso and will substantially conform to the shape under light pressure by the hand of the medical professional. In the following embodiments, reference to a ‘wearable device’ should understood as referring to features of either the first wearable device 108 or the second wearable device 110 respectively. Referring to Figs. 3A, 3B and 3C an embodiment of a wearable device is generally indicated by reference numeral 150. Fig. 3A shows the inner side of the wearable device 150 that, in use, is removably adhered to the patient’s skin as will be described later. Fig. 3B is a side elevation view illustrating the very thin profile of the wearable device 150, which is a significant factor in increasing patient comfort whilst wearing the device. For example, in an embodiment the maximum thickness (thickness being shown in Fig. 3B) of the wearable device 150 may be less than approximately 1.5 mm. Other areas may have smaller thicknesses. It is envisaged that other wearable devices could be manufactured with different thicknesses to those mentioned herein, whilst retaining the same functions of patient comfort, performance, etc. Fig. 3C is an obverse plan view of Fig. 3A, and shows the outer side of the wearable device 150 visible during use on the patient 102. The wearable device 150 may have a shape in the form of an elongate strip or belt 152. For example, when viewed in plan as in Fig. 3A, a length of the strip 152 may be much greater than its width, the length being sufficient extend over either the anterior or posterior part of the perimeter of the thorax of the patient 102, from the MUH,016-UK sinistral side 104 to the dextral side 106 (or vice-versa) (as shown by first portion 112 in Figs. 1 and 2), and to leave a part of the length free as the aforementioned ‘tail’ comprising the connector 116 (as shown by the second portion 114 in Fig.2). It is not essential for the elongate strip 152 to have a uniform width along its length. For example, in the embodiment of Fig.3A the elongate strip 152 comprises shoulders 154 which divide the length into the aforementioned first portion 112 (in this embodiment a wider region) to be worn by the patient 102, and the aforementioned second portion 114 (in this embodiment a narrower portion) generally comprising the tail and connector 116. However, embodiments of the wearable device 150 (or any wearable device described herein) are contemplated in which the width of the elongate strip 152 is substantially uniform along the length. In fact, embodiments comprising a substantially uniform width may improve handling and application by a healthcare professional of the wearable device 150. The wearable device 150 may comprise an array of electrodes 156 (indicated generally by a dashed box in Fig. 3A). In an embodiment the array of electrodes may be arranged substantially linearly along the wearable device 150. In other embodiments there may be different arrangements of electrodes as described elsewhere herein. In an embodiment the array of electrodes 156 may comprise eight individual electrodes 158, although other numbers of electrodes are possible, depending on the EIT system in use. Such EIT systems may use 4, 8 or 16 electrodes for example. The array of electrodes 156 may be arranged on the elongate strip 152 so that, in use, each electrode 158 will be approximately evenly spaced around either the anterior or posterior part of the torso of the patient 102. Fig. 3C is an obverse view of Fig. 3A, showing an external side 157 of the wearable device 150 when worn by the patient 102. The external side 157 may comprise a centre line indicator 157a whose function is to assist the medical professional to position the wearable device 150 correctly on the patient. For example, if the wearable device 150 is to be applied to the anterior of the patient 102, the centre line indicator 157a may be positioned at the 4^th/5^th intercostal and the centre line indicator 157a in line with the sternum of the patient. If the wearable device 150 is to be applied to the posterior of the patient 102, the centre line indicator 157a may be positioned at the 4^th/5^th intercostal and the centre line indicator 157a in line with the spine of the patient. In this way the medical professional is assisted in placing the two wearable devices 150 approximately in the same plane and ensuring that the electrodes are evenly spaced around the patient’s torso. The MUH,016-UK centre line indicator 157a may also comprise a size indicator, which in this embodiment is an ‘L’ indicating ‘large’ size. Each electrode 158 may have a dedicated electrically conductive track 160 which provides an electrical connection between a respective electrode 158 and the connector 116. Each electrically conductive track 160 may comprise an electrically conductive ink encapsulated in, or sandwiched between, a dielectric encapsulant as will be described in greater detail below. The electrically conductive tracks 160 may have a thickness of between about 8 and 15 µm, in one embodiment may have a thickness of about 10 µm, and a width of between about 0.5 mm and about 1.5 mm. Within the first portion 112 of the elongate strip 152 (to be attached to the patient’s torso), each electrically conductive track 160 may comprise a meandering, winding or indirect path over the surface of the elongate strip 152 toward its respective electrode 158. The meandering, winding or indirect path may oscillate between two imaginary parallel lines running in a lengthwise direction along the elongate strip 152, and could be in the form of a sinusoid, serpentine shape, zig-zag shape, etc. It may be advantageous if the shape is substantially smooth (such as sinusoid, serpentine) rather than with sharp changes in angle (such as sawtooth or zig-zag). The meandering, winding or indirect path may form a repeating or non-repeating pattern along the elongate strip 152. The meandering, winding or indirect paths of the various electrically conductive tracks 160 may be ‘nested’, or in phase, with one another to allow the width of the elongate strip to be kept smaller than a non-nested, or out phase, arrangement. Within the second portion 114 of the elongate strip 152, each electrically conductive track 160 follows a substantially linear path to/from the connector 116. The electrically conductive tracks 160 may be spaced apart from one another between about 0.5 mm and 1.5 mm in the second portion 114. At the shoulder 154 of the elongate strip 152, the paths of the electrically conductive tracks 160 may diverge so that a first group 162 travels down one side of the elongate strip 152 and a second group travels down the opposite side of the elongate strip 152 with the array of electrodes 156 in between the first and second groups 162, 164. To that end the innermost electrically conductive track 160 of the first and second groups may be spaced about 20 mm at their closest points on either side of the electrodes 158. The spacing of the electrically conductive tracks 160 may be as mentioned above within MUH,016-UK each of the first and second groups 162, 164. As the path of one of the innermost electrically conductive tracks 160 nears an electrode 158, it may branch off to connect to that electrode. Fig. 3D is a cross-section of the wearable device 150 along line D-D of Fig. 3A showing the construction of the electrically conductive tracks 160 and how they are bonded to a biocompatible textile carrier layer. It is noted that the size of the electrically conductive tracks is greatly exaggerated in Fig. 3 for the purposes of clarity. The wearable device 150 comprises a textile carrier layer 166, which may be cut or stamped from a sheet of textile to form the elongate strip 152. The textile carrier layer 166 may be a bamboo viscose of approximately 0.4 mm thickness, although other textiles are envisaged. The textile carrier layer 166 is breathable, flexible and elastic, and feels generally similar to ordinary clothing worn directly next to the skin, e.g. a close-fitting cotton T-shirt (in fact many clothes are made from bamboo viscose nowadays). When not being worn by a patient, the textile carrier layer 166 is not stiff, and ‘flops’ under its own weight when picked up by hand, much like ordinary clothing. These clothing-like properties make textile carrier layer 166 well-suited for close contact with the patient’s skin, for comfort and for accommodating patient movements, e.g. breathing, as described in greater detail below. In other embodiments the wearable device 150 (and other wearable devices and EIT apparatus described herein) may comprise a carrier layer having properties of flexibility and elasticity. In an embodiment the carrier layer may comprise a non- textile material. An example of a non-textile carrier layer is a stretchable circuit material such as BEYOLEX (RTM) available from Panasonic. Adjacent the textile carrier layer 166 are areas of adhesive 168. In an embodiment the adhesive may be heat-activated, such as a thermoplastic polyurethane (TPU). The thickness of the adhesive layer may be approximately 30 µm. The width of the adhesive area 168 may be fractionally wider than the areas of dielectric encapsulant (described below). The areas of adhesive 168 follow closely the patterns of the electrically conductive tracks 160 and electrodes 158 as shown in Fig. 3. The purpose of the regions of adhesive 168 is to bond the electrically conductive tracks 160 and electrodes 158 to the textile carrier layer 166, as described in greater detail below. MUH,016-UK The electrically conductive tracks 160 comprise first areas of dielectric encapsulant 170, second areas of dielectric encapsulant 171 and regions of conductive ink 172. The first and second regions of dielectric encapsulant 170, 171 encapsulate the regions of conductive ink 172. In this way the regions of conductive ink 172 are electrically insulated from one another and from the surroundings. In an embodiment the dielectric encapsulant may comprise a non-conductive ink, such as Magnatrans White (available from An additive such as Magnatrans Fixer (also available from may be added to the Magnatrans White to reduce curing time of the ink. . The conductive ink 172 may be a printable conductive ink, such as a printable silver ink. An example of such a printable conductive ink is CI-1036 available from Engineered Conductive Materials, LLC. The conductive ink 172 may have properties of flexibility, allowing it to remain conductive and with acceptable levels of impedance after repeated bending and stretching in use on the wearable device 150. Fig.3E is a cross-section of the wearable device 150 along line E-E of Fig. 3A showing an electrode 158 and a portion of an electrically conductive track 160 as it branches off to make an electrical connection with the electrode 158. The size of the electrically conductive tracks 160 and the electrode 158 is greatly exaggerated in Fig. 3E to assist understanding of the structure. The areas of adhesive 168 follow the pattern of the electrically conductive tracks 160 and the electrode 158 (see Fig.3A) and adhere these structures to the textile carrier layer 166. The region of conductive ink 172 connects the region of the outermost conductive track 160 (see Fig.3A, leftmost track 160) and the centre of the textile carrier layer 166 where it spreads out to cover an electrode interconnect area that is similar in size and shape to the electrode 158 (see Fig. 3). The region of conductive ink 172 is encapsulated by the first and second regions of dielectric encapsulant, except for the electrode interconnect area. An electrode layer 174 is positioned above and in contact with the electrode interconnect area of the region of conductive ink 172 and is similar in shape to the electrode 158 (in Fig.3). In this way the conductive ink 172 provides electrical conductivity between the connector 116 and the electrode layer 174. The electrode layer 174 may comprise a printable conductive ink, such as a silver/silver chloride ink (e.g with a Ag:AgCl ratio of 80:20). An example of such a conductive ink is CI-4040 available from Engineered Materials Systems, Inc. MUH,016-UK Adjacent the electrode layer 174 is a region of hydrogel 176 or other suitable interface layer for the body part of the patient that will allow current flow during the EIT process. The hydrogel may comprise any suitable cross-linked hydrophilic polymer that does not dissolve in water. The hydrogel may comprise a physical hydrogel having non-covalent bonds. The hydrogel may comprise a sensing hydrogel typically used for ECG electrodes. An example of a hydrogel 176 is ACG635 available from Axelgaard Manufacturing Co., Ltd. The hydrogel 176 has a shape corresponding to the electrode 158 (see Fig. 3) and a thickness of approximately 0.5 mm – 1.0 mm. The hydrogel 176 may permit the wearable device 150 to be removably adhered to the patient’s skin, to provide electrical conductivity between the patient’s skin and the electrode layer 174, and to hold the electrodes 158 in place on the patient during use. In an embodiment the hydrogel may exhibit good “quick-grab” and re-stick properties on the patient’s skin, facilitating placement and adjustment of the wearable device 150 (if necessary). Figs. 4A, 4B and 4C illustrate another embodiment of a wearable device is generally indicated by reference numeral 250. The wearable device 250 is generally similar to the wearable device 150, with like numerals indicating like parts. The main differences are that the electrically conductive traces 260 are on the outer side of the wearable device 250 (and do not come into contact with the patient’s skin during use), and the layers of the electrodes 258 are reversed so that the electrode layer is nearest to the skin during use. In this way the electrodes 258 are accessible through openings 259 in the textile carrier layer 266. This arrangement ensures that only the textile carrier layer 266 and hydrogel 276 to come into contact with the patient’s skin. The openings 259 may be cut (e.g. by laser) or stamped out of the textile carrier layer 266. Fig. 4C schematically illustrates this the reversed arrangement of one the electrodes 258 (reversed at least compared to the arrangement in Fig. 3 and 3A) and how the region of hydrogel and, indirectly, electrode layer 274 are accessible through the opening 259 in the textile carrier layer 266. Although the region of hydrogel 276 appears thicker in Fig.4B than in Fig.3A (to accommodate the thickness of the region of adhesive 268) the reality is that since each layer is relatively thin in the first place, the same thickness of hydrogel 276 as shown in Fig. 3A (e.g. approximately 1.0 mm) will function through opening 259. MUH,016-UK Fig.5 illustrates another embodiment of a wearable device generally identified by reference numeral 350. The wearable device 350 differs from wearable devices 150 and 250 in that it may comprise redundant electrically conductive tracks 351. By ‘redundant’ it is meant that the tracks 351 are not connected to any of the electrodes 358 (or a power source during use) but do function in at least another way as described below. The redundant electrically conductive tracks 351 may follow generally the same direction and pattern as the electrically conductive tracks 360. The redundant electrically conductive tracks 351 may be provided in shorter lengths between adjacent pairs of electrodes 358 than shown in Fig. 5. It is noted that a number of redundant electrically conductive tracks 351 may be provided between each adjacent pair of electrodes 358 so that there is the same total number of tracks 351, 360 (used and redundant) between any pair. Another way of stating this is that the total number of tracks 351, 360 between each adjacent pair of electrodes is equal to the total number of electrodes 358 on the wearable device 350. It is just the number of each type of track that changes between adjacent electrodes. For example, it will be observed that there are eight electrodes 358 on the wearable device 350. There are eight tracks 351, 360 between any adjacent pair of electrodes, but there are seven ‘used’ tracks 360 and one redundant track 351 between the rightmost pair of electrodes in Fig. 5, and one ‘used’ track 360 and seven redundant tracks 351 between the leftmost pair of electrodes 358. The redundant electrically conductive tracks 351 may be used in any embodiment described or contemplated herein. With this arrangement it may be possible to improve the uniformity of expansion and contraction of the wearable device 350 along its longitudinal axis due to chest excursion of the patient 102 during the breathing cycle, and other patient movements. For example, the wearable device 350 may facilitate more even expansion between each pair of adjacent electrodes as the patient breathes in. This may help to keep each electrode 358 in position on and in contact with the patient’s torso. Figs.6, 7 and 8 illustrate the wearable device 150 with different options for the region of hydrogel 176. In Fig. 6 there may be individual regions of hydrogel 176 associated with each electrode 158. Each region of hydrogel may correspond approximately to the shape of each electrode 158. In Fig.7 the regions of hydrogel 176 are larger and this may improve retention of the wearable device 176 on the patient. In MUH,016-UK Fig. 8, there is a single region of hydrogel 176 which covers all eight electrodes 158 and the majority of the wearable device 150 that is brought into contact with the patient’s skin. Other arrangements are envisaged, including for example regions of hydrogel 176 that each cover any number of the electrodes 158, as well as other shapes of the hydrogel including, but not limited to, square, rectangular, round, oval. Steps in a manufacturing process of the wearable device 150 are shown in Figs. 9A and 9B. The manufacturing process may comprise a screen printing process, although other printing processes are envisaged, such as inkjet printing, 3D printing and LED UV printing. The manufacturing process comprises two main parts: forming a transfer by screen printing of layers onto a temporary release substrate, applying the transfer to a fabric substrate, and then removing the temporary release substrate to leave the printed layers on the fabric substrate. Following the printing process the various layers form a unitary layered structure. By ‘unitary’ it is meant that following the manufacturing process the layers are joined together as a single structure, the layers not being separable during ordinary use. To form the transfer, the screen printing process may comprise at step S1 providing a temporary release substrate. The temporary release substrate may comprise a transfer paper or film, or other substrate suitable for receiving printed plastisol or related ink (such as polyurethane-based inks) layers and enabling transfer of the printed layers onto the fabric substrate. At step S2 a first layer may be printed onto the temporary release substrate. The step of printing may comprise applying a stencil or mesh over the temporary release substrate. The stencil or mesh may comprise a pattern or design formed as an opening(s) through which the relevant material (e.g. printable conductive ink, dielectric encapsulant) is urged onto the temporary release substrate by a squeegee head. Upon removal of the stencil or mesh, the pattern or design in the relevant material is left on the temporary release substrate. The design or pattern of the stencil or mesh will produce any desired pattern of the electrically conductive tracks and electrodes of the wearable device 150, e.g. as shown in Figs. 3, 4 and 5. It is also envisaged that the stencil or mesh may comprise more than one pattern (e.g. more than one of the same pattern, or different patterns) to be printed simultaneously on the temporary release substrate in different areas, and then later separated. Following the printing of the first layer it is then cured with heat (e.g. blown hot air) in a dryer such as a tunnel oven. The dryer may provide a hot air flow onto to MUH,016-UK the temporary release substrate to enable the first layer (and other layer described below) to be cured effectively. The dryer may comprise a conveyor for transporting the temporary release substrate therethrough. In an embodiment, the blown air temperature inside the dryer is typically set at one hundred and twenty degrees Celsius (120°C) for three minutes for drying the or each non-conductive ink layer. For the or each electrically conductive ink layer the temperature is raised to typically one hundred and thirty degrees Celsius (130°C) for three minutes. In an embodiment, the dryer used comprises 3 m drying section. It is appreciated that the temperatures indicated here are dependent on the curing system used and the temperatures indicated can be lower or higher depending on the system. Once the first layer has cured, at step S3 a second layer (generally comprising a different material to the first layer) is then printed on top of the first layer using the same technique of stencil and squeegee, and then the second layer is cured with heat in the dryer. It is noted that the design or pattern of the stencil may vary with each layer. For example, it may be desirable to print the layers of dielectric encapsulant with a slightly wider track width than the conductive ink track. In this way, the dielectric encapsulant will encapsulate the conductive ink so that it is insulated in the correct areas. This encapsulation of the conductive ink can be seen in Fig. 3D (layers 170 and 171 encapsulating track 172) and in Fig. 4C (layers 270 and 271 encapsulating track 272) for example. At step S4 it is checked whether all layers have been printed and dried. If not, step S3 is repeated until all layers have been printed and dried. Using this process the various layers of the embodiments of the wearable device 150 may be built up by printing onto the temporary release substrate. The final layer to be applied at step S4 is the adhesive that will be subsequently bonded to the fabric substrate. Depending on the nature of the adhesive it may or may not be necessary to cure, or at least partly cure, the previous layer before applying the adhesive. For example, if the adhesive is a printable adhesive, the previous layer may be dried before applying the adhesive. However, if the adhesive is a solid powdered hot-melt adhesive suitable for use in a heat transfer printing process (such as a plastisol transfer adhesion powder) it may be preferable to apply the powdered adhesive to the previous layer before the drying step. It is noted that the layers are printed in reverse order so that the layer that will MUH,016-UK be closest to the fabric substrate (e.g. region of adhesive) is uppermost on the temporary release substrate with the other layers beneath it. For example, referring to Fig. 3D the first layer printed at step S1 would be the dielectric encapsulant 171, then at step S2 the conductive ink 172, and so on until the region of adhesive 168 is applied. At step S5 the transfer is positioned on the textile carrier layer 166 or 266. The positioning on the textile carrier layer is so that the printed and cured layers are ‘sandwiched’ between the textile and the temporary release substrate of the transfer. At step S6 the cured layers are adhered to the textile (for example by application of heat and/or pressure) by the region of adhesive. At step S7, once adhered, the temporary release substrate of the transfer may be removed (e.g. by peeling) leaving the cured layers attached to the textile carrier layer. After the temporary release substrate has been removed, the region or regions of hydrogel 176, 276 are applied at step S8 to the various electrode layers 174. The hydrogel may overlap onto the dielectric encapsulant 171 as described above (and also Fig. 3E for example), or may extend over a wider area of the textile carrier layer 166, 266. It may be that the region of hydrogel is in the form of a bulk roll (e.g.0.23 m wide by 91 m long) as supplied by the manufacturer. As supplied, the hydrogel in the bulk roll is sandwiched between two removeable liners: a disposable top liner (e.g. LDPE) and a disposable bottom liner (e.g. PET). The bottom liner protects a tie layer of the hydrogel that will adhere to various substrates (such as the electrode layer 174) and the bottom liner protects a skin-layer gel formula for adhesion with the patient’s skin. In step S8 a length of hydrogel may be dispensed from the bulk roll and, with the top and bottom liners in place, the desired shape of hydrogel (see Figs. 6, 7 and 8) may be cut or stamped from the bulk roll. The disposable top liner is removed from the cut or stamped shape to reveal the tie layer of the hydrogel, which is then applied to the relevant area of the textile carrier and/or cured layers (see Figs.6, 7 and 8 for examples of the cut or stamped shapes) so that it is adhered in place. The disposable bottom layer of the hydrogel is left in place and would only be removed just before use on the patient 102. It is envisaged that the formation of the region or regions of hydrogel may be incorporated into the printing process and would be the first layer to be printed onto (or formed on) the temporary release substrate. For example, with use of appropriate MUH,016-UK machinery and/or hydrogel formulations, step S8 may be carried out between step S1 and step S2. Optional step S9 may comprise a cleaning process to sterilise the wearable device 150. The cleaning process may involve the use of ultraviolet light, application of alcohol and/or a gas. A separate step of cleaning may not be necessary if the preceding steps are performed in a clean environment. If the layers have been adhered to a larger sheet of the textile carrier there may be a further optional step S10 of cutting or stamping the elongate strip 150 from the sheet of textile, thereby forming the wearable device 150. Optional step S11 may comprise application of a disposable protective flexible or rigid packaging (which hermetically seals the wearable device 150). The packing may comprise materials such as: a ‘zip-lock’, heat sealed or adhesive sealed flexible polymer such as polyethelene; a treated paper, edge bonded envelope such as used for a medical dressing; a rigid polymer box with a polymerised seal such as used in food packaging. The purpose of the disposable protective flexible or rigid packaging is to protect the wearable device 150 during shipment, packaging and transportation, and for hygiene. The disposable protective flexible or rigid packaging is removed by the medical professional just prior to use on the patient 102. It may be possible to automate or semi-automate the printing process using appropriate machinery on a production line such as those used for large scale textile printing, those used to produce multi-layered medical dressings or those used to produce sanitary products such as nappies or sanitary towels. In some embodiments, individual layers are batch printed (e.g. the same layer for multiple wearable devices) and cured before moving to the next layer. It is envisaged that the manufacturing process may be computer-controlled and performed substantially or entirely by machine. An advantage of the manufacturing process of the wearable EIT apparatus 100 is that it is possible to mass produce relatively quickly and at low cost. At the same time, the manufacturing process is easily adapted permitting the wearable EIT apparatus 100 may be manufactured to order with a bespoke size for individual patients. In such embodiments a patient may MUH,016-UK arrive at a hospital and a medical professional may decide that lung-function (or any other clinical observation using EIT) of the patient should be monitored using EIT. Measurements of the patient’s torso (e.g. chest circumference) are taken by medical professionals at the hospital, and input into a computer at the hospital. The computer then transmits the measurements to a computer at the manufacturing site, which may be converted (with or without human intervention) into an electronic data structure representing dimensions for a wearable EIT apparatus 100 bespoke for that patient (e.g. overall length, electrode spacing, conductive track layout, etc.). The electronic data structure is sent to a computer-controlled manufacturing line which manufactures the wearable EIT apparatus according to the electronic data. The wearable EIT apparatus is then delivered to the hospital for use on the patient. It is envisaged that such delivery may take place quickly (e.g. same day or within one or two days) from the point at which the patient’s measurements were taken. After the wearable EIT apparatus 100 has been used on the patient it may be disposed/recycled. In other embodiments wearable EIT apparatus 100 may be manufactured quickly (at least more quickly than existing devices for mounting electrodes, such as the Sentec SensorBelt and the Pulmovista 500 from Draeger which are usually made by hand) in large volumes in fully-automated or semi-automated manufacturing lines. By large volumes it may be meant on a mass production scale similar to medical dressings, medical plasters and sanitary wear. Such capability may be useful for hospitals dealing with large numbers of patients requiring lung-function monitoring for example, or any other clinical monitoring using EIT. In some embodiments, wearable EIT apparatus may be manufactured in several standard sizes such as a small, medium and large, each size capable of covering a range of patient chest sizes. One example of a range of different sizes is shown in the following table: Other size ranges are possible of course. It will be appreciated that although MUH,016-UK each wearable device 150 of the wearable EIT apparatus 100 is marked as suitable for a certain size, each individual wearable device 150 is approximately half the nominal size plus the length of the tail 114. The length of tail 114 may be approximately 200 – 300 mm for adults, but may be varied as desired (e.g. scaled for infants and children), although the length of the tail may be dependent on the size of the production machinery available (e.g. larger belts may have shorter tails and vice-versa). In general, a shorter tail has a better electrical performance compared to a longer tail. One advantage of using printable materials and a printing process is that the wearable device may be manufactured comparatively quickly, at lower cost and in large numbers compared to EIT wearable devices that the applicant has seen to date. Another advantage is that the size of the belt can be adjusted quickly and easily in software to suit groups of patients of a certain size range (e.g. chest size), or even made bespoke for each patient. Turning to the EIT system that is useable with the wearable EIT apparatus 100, Fig. 10A shows three states of a dock 300. The dock 300 enables the first and second wearable devices 108, 110 to be connected to equipment required to drive the electrodes and record data, as described in greater detail below. A first state of the dock 300 is shown at the top left of Fig. 9A in which a first port 301 and a second port 302 are available to receive corresponding connectors 116 of the first wearable device 108 and the second wearable device 110. The first and second ports 301, 302 may comprise 8 channels, each channel corresponding to a single electrode 158, 258 on the respective wearable device 108, 110. The first and second ports 301, 302 may each have a width greater than a height, and the two ports stacked width-wise on top of one another as shown. The dock 300 comprises a housing 304 which may be constructed using a resilient plastics material to provide a hard shell for protecting electrical wires and connections (not shown) inside the connector 116, the electrical wires connected to the housing 304 by an electrical cable 306. A first indicator 308 is provided on a first face 310 of the connector 116. A second indicator (not shown) is provided on a second face 312, opposite to the first face 310. Recalling that the first and second wearable devices 108, 110 are universal and can be worn on either the anterior or posterior part of the torso of the patient 102, the first and second indicators may assist the medical MUH,016-UK professional to connect the wearable device 108, 110 to the correct port 300, 302 as shown in Fig. 10B. In this embodiment, the first indicator 308 illustrates the anterior part of the head and torso, and the second indicator (not shown) indicates the posterior part of the head and torso. Alternative indicators intended to achieve a similar function may be used. A second state of the dock 300 is shown in the central portion of Fig. 10A. In the second state, the first wearable device 108 has been connected to the second port 302 by means of its connector 116. The second wearable device 110 is being connected to the first port 301 by its connector 116. Note that the shading differences of the two wearable devices 108, 110 in Fig. 10A are purely for illustration and are not intended to indicate any difference in function between the two devices, and that only a part of each device is shown in the drawing. A third state of the dock 300 is shown in the lower right portion of Fig 10A. In the third state both the first and second wearable devices 108, 110 have been connected to the dock 300. Fig.11 shows an EIT hub 400 that is electrically connected to the dock 300 by the electrical cable 306, and which is connected to a DC power control box 401 (see Fig. 11) by a power cable 402 that delivers two different voltages to the EIT hub 400 (as described below) with a total power output of 42 W. A data cable 404 (in this example a USB cable capable of carrying 100 Mbps, but wireless transmission of data is also envisaged) facilitates data communication between the EIT hub 400 and an external computing device (see Fig 11). The EIT hub 400 comprises a housing 406 of a resilient plastics material or metal material that may be Ingress Protection rated at IP65 or better, measuring approximately 160 mm by 100 mm by 30 mm (L x W x D). A printed circuit board (not shown) is held within the housing 406. The main function of the EIT hub 400 is to deliver currents (^ 5 mA rms) to the patient via a pair of electrodes, measure voltages across other pairs of electrodes, digitise the measured voltages and output the data via the data cable 404 to the external computing device. This functionality is described in greater detail below with reference to Figs. 12, 13 and 14. Fig. 12 shows the aforementioned DC control box 401 that comprises a first MUH,016-UK power isolation switch 408 and a second power isolation switch 410. The DC control box 401 is connected to a medical grade AC/DC power supply 412 by a power cable 414. The AC/DC power supply 410 may be model PCM50UT04 available from The AC/DC power supply 410 receives AC power from a mains supply 413 (and may receive input mains voltages in the range 90 – 264 VAC) and outputs two DC voltages, + 5 V and ± 12 V DC. The first isolation switch 408 isolates the + 5 V output voltage from the AC/DC power supply 412, and the second isolation switch 410 isolates the ± 12 V output voltage. It is envisaged that the two isolation switches could be combined into a single isolation switch to isolate/connect both output voltages simultaneously. Figs.11 and 12 also show the EIT hub 400 in communication with a computing device 416 (which may be a ‘rugged’ tablet PC such as a Getac T800 G2). The computing device 416 comprises a processor, memory (volatile, e.g. RAM, and non- volatile, e.g. solid-state drive), an operating system (such as Windows (RTM)), and a touchscreen 418 (e.g. 8.1” in size) upon which, in use, a Graphical User Interface (GUI) (not shown) is displayed. The touchscreen 418 and GUI enable the medical professional to control the EIT hub 400, and to start and stop an EIT scan on the patient amongst other things, as described below in conjunction with Fig. 18A – 18J. The computing device 416 receives (and may store) data from the EIT hub 400 for immediate or later processing, either locally on the computing device 400 or remotely on another computing device 416. The printed circuit board of the EIT hub 400 comprises the following subsystems: 1. A DC power interface ± 12 V and + 5 V to power the various subsystems. 2. A power module providing ± 9 V, 3.3 V, 1.8 V to power the digital and analogue parts of the device. 3. An electrode interface to connect to the first and second wearable devices 108, 110 on the patient 102. 4. A programmable logic controller which may comprise a field programmable gate array (FPGA, e.g. CMOD A7 which may be in the form of the Artix-7 FPGA Evaluation Board) and a complex programmable logic device (CPLD) to control the device and to communicate with the computing device 416. MUH,016-UK 5. An Ethernet chip for communication, control and data transfer with the computing device 416, with a speed of up to 100 Mbps (although higher or lower speeds may be used). 6. Current driver circuitry to source/sink current. The amplitude and frequency are adjusted digitally. 7. Two stages of amplification readout circuitry to amplify the voltage signal on the electrodes. Automatic gain calibration is used to enhance resolution. 8. A switch matrix subsystem with programmable injection/readout pattern. 9. An electronic discharge (ESD) and protection subsystem that ensures patient safety. 10. A simple light emitting diode interface. 11. Ingress Protection (IP) device enclosure (not shown). Fig 13 shows the system architecture 500 of the printed circuit board of the EIT hub 400. The printed circuit board comprises two main parts: an analogue frontend 502 and a finite state machine 504 (shown both by the larger shaded block and by a smaller block therein) implemented by the programmable logic controller FPGA and CPLD chips. The CPLD chip may be a Xilinx XC2C256-7VQ100I for example. Functions of the analogue fronted 502 include: generating input current with a digital- to-analogue converter (‘DAC’) 505 and a differential current driver 506 for pairs of electrodes on the first and second wearable devices 108, 110; applying the input current to the correct pair of input electrodes via an analogue switch matrix 508; and using the analogue switch matrix 508 to connect the correct pair of output electrodes to readout circuitry 510 for amplifying the measured voltage across the pair of electrodes. An analogue-to-digital converter (‘ADC’) 512 returns digital voltage values to the finite state machine 504. The ADC 512 may be implemented using a 12- bit 40 MSps ADC (e.g. AD9237) and the DAC 505 implemented using a 12-bit DAC (e.g. AD5445), both available from Analog Devices, Inc. Functions of the finite state machine 504 include: direct digital synthesis (‘DDS’) logic 512 for synthesizing the signals for the current driver 506; electrode multiplex control logic 516 for controlling the analogue switch matrix 508; dynamic gain control logic 518 for controlling the gain of the readout circuitry 510; and multiply-add logic 520 for processing digital voltage values from the ADC 512. An Ethernet interface 522 (not part of the finite state machine) enables data MUH,016-UK communication with the computing device 416 via the data cable. The Ethernet interface 522 may be the AC320004-3 daughter board (available from Premier Farnell Ltd) for example. The Ethernet interface 522 could be supplemented with, or replaced by, a wireless network interface for wireless data communications. A lookup table 522 is used with direct digital synthesis logic 510 to derive sinusoidal signals to drive the current sources/sinks and in-phase and quadrature (I/Q) demodulation of the readout values. The input to the differential current driver 506 is taken from the DAC 505 and low pass filtered using passive components (not shown). The current driver supply rail +/- 9 V is chosen to drive the electrodes at a wide range of frequency and controlled current levels. The dynamic gain control logic 518 is implemented on the readout circuitry 510 so that readout values cover the range of the ADC 512. Current injection and voltage readout patterns are fully programmable using the finite state machine 504. The readout circuitry 510 comprises two stages. The first is a low-noise instrumentation amplifier (IA1) with programmable gain and the second is a programmable gain amplifier (IA2). The wearable device EIT apparatus 100 and a horizontal section through the thorax of the patient 102 are shown schematically at the top right of Fig.12 to indicate the interaction between the analogue frontend 502 of EIT hub 400 and the electrodes of the wearable EIT apparatus 100. The electrodes are numbered 1 to 16 in Fig. 12, and the pair of electrodes numbered 1 and 2 are shown injecting current to the patient 102 at the particular moment in time. Lines of current 524 inside patient 102 are shown schematically. The injection of current gives rises to a potential gradient inside the patient 102, and corresponding lines of isopotential 526 are also schematically shown. Whilst current is injected by the pair of electrodes 1 and 2, one or more voltage reading is measured between adjacent pairs or electrodes, e.g. 3 and 4, 4 and 5, 5 and 6, … 15 and 16. The voltage readings are processed by the EIT hub 400 and stored for subsequent transmission to the computer device 416 as described below. In use one complete scan, or frame, comprises injecting current across a first pair of adjacent electrodes, taking a voltage measurement between all pairs of adjacent electrodes (including the injecting pair), then injecting current across a second pair of MUH,016-UK adjacent electrodes (which may be adjacent the first pair and which may comprise one electrode of the first pair), and taking voltage measurements between all adjacent pairs of electrodes, and so on. This process is repeated until all adjacent pairs of electrodes have been used to inject current. This represents a frame of data. Once a frame of data has been gathered and stored by the EIT hub 400, it is sent to the computing device 416. The EIT hub 400 has a programmable frame rate of 20 – 200 fps, although other rates outside this range are envisaged. In an embodiment there may be 89 fps. In this way each frame comprises a plurality of voltage measurements, which for convenience may be a multiple of the number of electrode pairs. In an embodiment there may be 256 voltage measurements in each frame so that, for each pair of electrodes injecting current, there are then 256/16 = 16 voltage measurements, i.e. one for each pair of electrodes taking voltage measurements. Figs. 14A and 14B illustrate this current injection and voltage measurement pattern at two current injection positions. In Fig. 13A, the EIT hub 400 provides at a first time a first current injection is across electrodes E1 and E2 and is labelled ‘Cycle 1’. One voltage measurement is taken between E1 and E2 (‘Position 1’), then between E2 and E3, and so on in sequence until E16 and E1 (‘Position 16’), thereby totalling 16 voltage measurements for the Cycle 1 current injection. Fig. 13B shows that, for ‘Cycle 2’ at a second time later than the first time, the EIT hub 400 provides a second current injection across electrodes E2 and E3. Voltage measurements are taken between E2 and E3 (‘Position 1’), then between E3 and E4, and so on in sequence until E1 and E2 (‘Position 16’), thereby totalling 16 voltage measurements for the Cycle 2 current injection. This pattern of sequential current injection, and sequential voltage measurements, is repeated around the wearable EIT device 100 until current is injected across the last pair of electrodes, E16 and E1 in this example. In the embodiment shown there are thus 256 electrode pair measurements per frame. Turning now to each voltage measurement, the readout circuitry 510 measures the voltage across the particular pair of electrodes for a window of time t. The EIT hub 400 performs I/Q demodulation to calculate the real and imaginary parts of each such electrode voltage measurement. This process is illustrated in Fig. 15 in which the relevant parts of the EIT hub 400 are shown on the left side with the same reference numerals as in Fig. 12. The right-hand side of Fig.14 is a schematic illustration of the MUH,016-UK digitisation and processing of the voltage measurements. It is recalled that the injection current between each pair of electrodes is a sinusoid. Accordingly, the voltage measurement 526 from the readout circuitry 510 is also a sinusoid over the window of time t. In an embodiment the window of time t comprises a settling time 527a and a voltage acquisition time 527b. During the settling time 527a, there may be a user- selectable forced reset for ~ 10 µs to set the amplifier and filter to the reference dc voltage, so that the settling can be faster. The window of time t may be 32 µs long, with the setting time 527a being the first 16 µs and the voltage acquisition time 527b being the time 16 – 32 µs. After the settling time 527a within the window each voltage measurement 526 is sampled 528 at 32 MSps and is then digitised by the ADC 512, producing individual voltage samples. The input dynamic range of the ADC is 4 V and the step size of digitisation is 4/2¹² = 4096 (recalling that the ADC is 12-bit in this embodiment). Each individual voltage sample is converted into an integer multiple of this step size, which is between -2048 and 2047, and is hereafter referred to as a digitised voltage sample. The real and imaginary components in the measurement voltage are calculated using digital I/Q demodulation, where 512 of the digitised samples each multiply a sample from pre-stored sine and cosine sequences in the lookup table 522, and then the products are accumulated. Since the pre-stored sine sequence is also used to generate the excitation signal for the injection current, the pre-stored sine sequence is in phase with the excitation current, neglecting the phase delay in the analogue frontend 502. The pre-stored sine and cosine sequence comprise 12-bit samples of single frequency sine and cosine waves, respectively, so an ideal sine wave between - 1 and 1 is represented by a digital sequence between -2048 and 2047. The calculated real and imaginary components of each digitised sample (voltage measurement) ^_^^^, V_I and V_Q’ can be calculated as: MUH,016-UK where N=512, floor( ) is a function of rounding the value inside the parentheses to the closest smaller integer, and m is the total number of cycles of the voltage signal sampled in the window of time t. Since the sampling window per measurement is fixed in this embodiment at 16 ^s and the sampling frequency is fixed at 32 MSps, the 512 samples cover different number of cycles of the excitation signal depending on the excitation frequency. The value m is 2 for 125 kHz, 4 for 250 kHz, 8 for 500 kHz and 16 for 1 MHz. ^_^^^ ^{^}^^{^} is the nth digitised voltage sample: where ^_()(*+,-.(^^^ is the sampled voltage on the electrodes before being amplified. The amplifiers in the readout circuitry 510 have a fixed gain of 20 and a dynamic gain, Set_gain, set by the dynamic gain control logic 518. Once a frame of data (comprising one scan as described above) has been gathered, the EIT hub 400 transmits the frame to the computing device 416. In this embodiment the output bitstream for one frame comprises 2048 bytes. The sequence from Byte 1 to Byte 2048 is shown in Fig.16. As can been seen, the real and imaginary parts of each voltage measurement comprise 4 bytes of data respectitvely: Bytes 1 to 4 are the real part, VI, and Bytes 5 to 8 are the imaginary part, VQ. The values for both the real and imaginary parts are in a 32-bit 2’s complement format. In principle the EIT hub 400 may run for a time that is controllable by the medical professional, including times of seconds, minutes and hours. Each voltage measurement taken during this time comprises 8 bytes of data, where the highest 4 bytes are the calculated real part of the measured voltage, and the lowest 4 bytes are the imaginary part. In various embodiments, this data may be all be stored on the EIT hub 400 for subsequent processing, may be sent frame-by-frame from the EIT hub 400 to the computing device 416, or sent as individual measurements as they are taken. The computing device 416 then extracts the real and imaginary data for the received frame. Before using the values in EIT image reconstruction, the actual MUH,016-UK electrode voltages are determined by dividing the real and imaginary voltages by the amplifier gain. In particular, for calculating the actual measured voltage on the electrodes from V_I and V_Q the process is as follows. Ignoring the floor( ) function, equation (3) can be substituted into equation (1) giving: —^_()(*+,-.( is in phase with the excitation signal When the non-amplified electrode voltage ^_()(*+,-.( is in-phase with the excitation signal, equation (4) can be written as: where ^_{()(*+,-.(38(9:3;^8<9=(} is the peak voltage of ^_()(*+,-.(. Equation (5) can be further simplified to: Equation (6) can be simplified to: —^_()(*+,-.( has a phase difference from the excitation signal MUH,016-UK In more general situations, the measured voltage on the electrodes has a phase difference from the excitation signal. The peak voltage ^_{()(*+,-.(38(9:}, of the measured electrode voltage, ^_()(*+,-.(, can be written as: Once the computing device 416 has received at least one frame of data, it may begin EIT image reconstruction using known software and techniques. Image reconstruction may be performed using any suitable known or future linear or non- linear computer-implemented reconstruction algorithm. Known reconstruction algorithms can be found at the EIDORS website (http://eidors3d.sourceforge.net/), including GREIT (described at http://eidors3d.sourceforge.net/GREIT/index.shtml) and Truncated Singular Value Decomposition (tSVD). Use of the wearable EIT apparatus The wearable EIT apparatus 100 may be used as part of a procedure to monitor lung function (e.g. air recruitment by the lungs) of a patient using EIT. The patient may be a human of any age, including a neonate, baby, child and adult (with the wearable device sized accordingly). To facilitate correct placement, the wearable EIT apparatus 100 is placed onto the patient 102 by a medical professional. The first wearable device 108 and the second wearable device 110 are removed from their packaging (including removing any protective layer applied in step S9 in Fig. 10) and laid flat with the electrodes uppermost. Next, the or each disposable liner of the hydrogel is peeled from a group of electrodes, or from individual electrodes, on each wearable device 108, 110. Recalling that the wearable devices are universal, the medical professional chooses either the first or second wearable device and applies the uppermost side with the electrodes to the anterior of the patient 102, aligning the indicator 157a with the sternum at the 4^th/5^th intercostal with the second portion 114 and connector 116 exiting MUH,016-UK sinistral (or dextral depending on convenience). A light press by the hand should be sufficient to adhere the hydrogel to the patient’s skin, in a similar fashion to an EXG electrode. Then the remaining wearable device is applied to the posterior side of the patient’s torso aligning the indicator 157a with the spine, with the second portion 114 and connector 116 exiting same side (sinistral or dextral) as the wearable applied to the anterior. At this point the first and second wearable devices should be substantially co-planar forming a cross-section at the 4^th/5^th intercostal of the patient 102. Through the process the medical professional should take care to apply the belts smoothly around the patient 102 avoiding either stretching the wearable device or bunching the electrodes, so that each wearable device substantially follows the contours of the patient’s torso. As mentioned previously, the wearable devices 108, 110 may be placed at other positions up or down the torso, and there may be more than one pair used on a patient simultaneously. An advantage of at least some embodiments of the wearable EIT apparatus 100 is that it may be applied to the patient 102 when the patient is standing, sitting or lying down. The latter may be helpful for patient’s that are unconscious for example, or otherwise unable to sit or stand: the patient can be rolled on one side to apply one wearable device and then rolled to the other side to apply the other wearable device. An advantage of at least some embodiments of the wearable EIT apparatus 100 is that patient comfort is improved when compared to EIT belts of at least some previous designs. In particular, the wearable EIT apparatus 100 is relatively light in weight and is comparatively thin (2 mm or less for example). It may provide a feeling similar to a textile on the patient’s skin. It may permit a degree of breathability, helping to keep the electrodes in position and in proper contact with the skin during extended periods of use (e.g. hours or days). Following application to the patient, the patient can return to a lying position either on the front or back as needed and without significant discomfort caused by the wearable EIT apparatus. An advantage of at least some embodiments of the wearable EIT apparatus 100 is that, although following closely to the contours of the patient’s torso, the materials and/or shape and/or construction of the wearable EIT device 100 permit a degree of flexing/expansion/contraction of at least a portion of the wearable EIT apparatus 100 along its length during chest excursion of the patient under the breathing cycle. In this MUH,016-UK way the chance of slippage of the electrodes relative to the patient’s torso (either around the torso, or up/down the torso, or some combination of both) is reduced during the patient’s normal breathing cycle. Furthermore, electrodes are kept in contact with the patient’s skin over a period of time, which is important for injecting currents and measuring voltages. Maintaining the position of the electrodes is helpful to ensure good quality EIT image reconstruction results. Fig.17 illustrates the wearable EIT apparatus 100, EIT hub 400 and computing device 416 in use on the patient 102. As shown the patient can adopt a normal lying position (either front or back), substantially unencumbered by the wearable EIT apparatus 100. Performing an EIT data collection Figs. 18A – 18J are screen shots of a graphical user interface (GUI) on the computing device 416 during collection of frames of data from the patient 102. The purpose of the GUI is to guide the medical professional through the collection of data from the patient 102. The software is accessible via an icon (not shown) on the touchscreen 418. Once loaded, the software checks whether any the wearable EIT device 100 is connected and the EIT hub 400 powered on. If not, a message box 602 is displayed asking the use to “check hardware turned on” and then click the retry button, as shown in Fig. 18A. In Fig. 18B, once the hardware is connected, the medical professional enters a patient identifier in box 604. The patient identifier could be a pseudo- anonymised alpha-numeric code for example, indicated by ‘example1’ in Fig. 18C. A ‘confirm’ button 606 may be clicked after the patient identifier has been entered. Once button 606 is clicked the GUI changes to that shown in Fig. 18D, at which point the software will perform checks on the electrodes. The patient 102 and electrodes are shown schematically to the user in the GUI by a diagram 608. In the example shown in Fig. 18D, the checks by the software have revealed that three electrodes 610 are detached or otherwise not in proper contact with the patient’s skin. The affected electrodes are indicated to the user in a red colour and a message box 612 invites the medical professional to re-position these electrodes. The remaining electrodes 614 pass the check and are shown to the user in a green colour. MUH,016-UK Once all of the electrodes 610 have passed the checks, the system enters a calibration phase shown in Fig. 18E, comprising a number of different rounds (e.g. 1 – 5 rounds). During calibration, the gain for each electrode pair is determined and stored (e.g. in an array) with a number of values equal to the number of electrodes. It is important to note that the gain setting for each electrode pair is relative to the drive (current injecting) electrodes at each point in time and is not set for physical pairs of electrodes: for the purposes of setting gain, the drive electrodes are always considered to be electrodes 1 and 2 with the remaining electrodes numbered sequentially (to avoid confusion these ‘rotating’ electrode numbers will be shown in italics). In this way the gain settings ‘rotate’ around the electrodes as the drive electrodes change (see e.g. Fig. 13, 14A and 14B) with time during collection of each frame of data. The reason is that the measured voltage decreases rapidly with distance from drive electrodes, and this is far more significant than variations between the physical electrodes. Accordingly, the gain setting for electrode pairs is set dependent on the position of the drive electrodes. The gain settings stored for electrodes 1, 2 and 16 are fixed at 1 as these always used as either one or two of the drive electrodes. To determine the gain settings for the other electrodes the calibration process begins by recording several frames of data sufficient to cover at least one whole breathing cycle of the patient. An Upper_Threshold parameter is initialised to a value representing the maximum acceptable DSP output. The Upper_Threshold is used at the end of the calibration process as described below. The first value for Upper_Threshold may be chosen manually based on visual inspection of initial data sets, in which gains may also be chosen manually. The gain setting for electrode pair 3 and 4 is determined first (although it could be done in any order). In each frame of data there are n voltage measurements between electrode pair 3 and 4, where n is the number of electrodes. In some embodiments n = 16, and there are 16 voltage measurements between electrodes 3 and 4 in each frame. The median value of the n voltage measurements is determined for the real part, and then the median value of the n voltage measurements is determined for the imaginary part. Ignoring the +/- select and store the maximum of the two median values. Repeat this process for each frame of data to generate a series of voltage values, and then take the average, V, of this series. Finally, determine a ‘new_gain’ for electrode pair 3 and MUH,016-UK 4 such that V/old_gain*new_gain < Upper_Threshold. The new_gain is stored in the array for use during EIT data collection. This process is then repeated for each electrode pair 4 and 5, 5 and 6 … and 14 and 15 whereby the array stores gain values for each electrode pair. The values in the array are compared to those stored in the hardware. If any have changed, the values stored in the hardware are updated with the relevant new_gain value. The calibration process is then repeated several times, e.g. up to a maximum of five. A progress bar 616 indicates how far the calibration has progressed. In Fig. 18F, the GUI 600 invites the medical professional to select the patient orientation using a number of radio buttons 618 after which the feet 620 on the GUI will rotate to match the selection and the “start recording” button 622 will become available (see Fig.18G). Whenever the patient’s position changes, the correct radio button should be selected. Fig. 18H shows recording underway, as indicated by a pulsating green dot 624. A “pause recording” function is available on button 622 to pause data recording and application of current to the patient 102. To prevent accidental re-positioning of the patient in the GUI 600 and accidental pausing of the recording, a screen lock/unlock function is provided by a drop-down box 626. During recording, the electrodes are checked periodically for correct contact with the patient’s skin. Fig. 18J shows that one of the electrodes 610 has a high but acceptable impedance (indicated to the user by an amber colour). If the impedance is too high, this may be indicated to the user by a red colour. If three or more electrodes have an impedance that is too high, the user may be advised by the GUI 600 to pause recording and make a visual check of the electrodes on the patient and to adjust/re-apply as necessary. Once that has been done, recording may be resumed using button 622. To end the data recording the button 622 is pressed and the software closed. Fig.19A is a schematic perspective view of an embodiment of an EIT hub 700 shown during use. The features and functionality of the EIT hub 700 are generally the same as the dock 300 and EIT hub 400. As such, the description of the corresponding features and functionality will not be repeated; reference is made to the features and functionality described above in relation to the dock 300 and the EIT hub 400. These features and functionality are expressly incorporated into this description of the EIT hub 700, except for necessary changes to accommodate the differences described below. MUH,016-UK The functionality of the dock 300 has been incorporated into the EIT hub 700 so that a separate dock is not required. In Fig.19A the EIT hub 700 is shown connected to the first wearable device 108 and the second wearable device 110 (only part of each shown). Each device 108, 110 has been directly connected to a port (not shown) on the EIT hub 700. The form factor of the EIT hub 700 is smaller and lighter than the EIT hub 400. In this embodiment, the EIT is approximately 30 mm by 60 mm by 10 mm and weighs approximately 100 g. Other sizes and weights are envisaged, but generally the EIT hub 700 is light enough for its weight to be supported when dangling from the wearable devices 108, 110. The EIT hub comprises a housing 702 comprising a resilient plastics material that is suitable for injection moulding, e.g. ABS, which may be formed and shaped in a way that facilitates cleaning (e.g. sanitising or sterilising) and that can achieve an Ingress Protection rate of IP65 or better. Where there were comparatively sharper edges and corners on the housing 406 of EIT hub 400, the housing 702 has comparatively ‘softer’ edges and corners (comprising generally larger radii of curvature) for improved handling and patient comfort for example. The housing 702 contains a PCB, similar in functionality to the PCB of EIT hub 400 although it is envisaged that the PCB of EIT hub 400 and components may be subject to a degree of miniaturisation to reduce weight and save space, through the integration of components into a single Application Specific Integrated Circuit (ASIC). A further difference is that the EIT hub 700 comprises an on-board power source, such as a rechargeable battery, within the housing 702. The rechargeable battery may be chargeable wirelessly using a base station 704 (see Fig. 19B). As such the base station 704 may comprise a primary coil and the EIT hub 700 may comprise a secondary coil to facilitate wireless charging (e.g. by induction). There are a number of standards for wireless charging (e.g. Qi, PMA) and the EIT hub 700 may utilise any such standard, either existing or future, which may be based on the IEEE 802.15.4 standard or similar. The base station 704 comprises a cable 706 for delivery of electrical power for recharging the rechargeable battery. Optionally, the cable 706 may be capable of data transfer, and the EIT hub 700 may be capable of bidirectional or unidirectional data transfer with the base station 704 using a short-range wireless data communication protocol such as NFC, Bluetooth, etc. In other embodiments, the EIT MUH,016-UK hub 700 may be provided with a longer-range wireless communication interface using WLAN, Zigbee, or any similar standard which may be based on IEEE 802.11 for example. In such embodiments it may not be necessary for the base station to have a data communication capability as the EIT hub 700 may transmit and receive data with other wireless network interfaces in other devices. In some embodiments, the base station 704 may comprise its own on-board power source (such as a rechargeable battery) for powering wireless charging of the EIT hub 700. The power source of the base station 704 may be rechargeable via port (not shown) to which a suitable charging cable may be connected. In some embodiments the cable 706 may be omitted entirely. In other embodiments the cable 706 may be capable of power transfer and/or data transmission. Referring to Fig.19B arrow 708 illustrates generally how the EIT hub 700 may be lifted from and placed on to the base station 704 by a user. In an embodiment the EIT hub 700 simply rests on the base station 704 during which wireless charging and/or data transfer may take place, and there is no retaining mechanism between the EIT hub 700 and the base station 704. In some embodiments a retaining mechanism (e.g. magnetic, Velcro (RTM)) may be provided to releasably hold the EIT hub 700 and the base station 74 together until such time as they are to be separated. Generally, it is preferred that there is no retaining mechanism, or a retaining mechanism that permits very easy separation. In either case, the EIT hub 700 may be separated from the base station 704 if the patient moves or is lifted, causing a corresponding ‘tug’ on the EIT hub 700 via the wearable devices 108, 110. Fig. 19C shows the EIT hub 700, wearable devices 108, 110, and base station 704 in use on a patient 710. Although the patient 710 shown in Fig.19C is an infant, the EIT hub 700 and base station 704 are useable on all patient groups including children and adults. Different sized wearable devices 108, 110 will be required for different patient groups to accommodate different chest sizes as explained elsewhere herein. As shown, the small form factor and light weight of the EIT hub 700 enables the infant to be picked up by an adult, when the EIT hub 700 will simply be lifted off the base station 704 and will ‘dangle’ from the wearable devices on the infant. This is advantageous because a parent or healthcare professional can hold and cuddle the infant whilst monitoring of the infant’s lung function continues uninterrupted, and/or without the need to remove and replace the wearable devices 108, 110. When used MUH,016-UK with older patients the small form factor and light weight enables the patient to move relatively freely, for example to sit up, turn over and get up from bed, whilst lung function monitoring continues and/or without the need to remove the wearable devices 108, 110. In some embodiments, data collection may pause when the EIT hub 700 is separated from the base station 704 and resume once placed back on the base station 704. In some embodiments, data collection may continue whilst the EIT hub 700 is separated from the base station 704, and data may be transmitted wirelessly back to the base station 704 (or other wireless network interface) and/or may be stored temporarily on the EIT hub 700 until the EIT hub 700 is back on the base station 704 and/or within range of a wireless network interface to which data may be transmitted. Referring to Figs. 20A to 20C an embodiment of a monitor is generally identified by reference numeral 800. Fig. 20A illustrates the monitor and when switched off, and Figs. 20B and 20C illustrate the monitor when on. The monitor 800 comprises the features and functionality of the DC control box 401 and the computing device 416 described above. As such, the description of the corresponding features functionality will not be repeated; reference is made to the features and functionality described above in relation to the DC control box 410 and the computing device 416. These features and functionality are expressly incorporated into this description of the monitor 800, except for necessary changes to accommodate the differences described below. The monitor 800 comprises a housing 802 formed from a resilient plastics material suitable for injection moulding such as ABS which may be formed and shaped in a way that facilitates cleaning (e.g. sanitising or sterilising). The housing may have dimensions 200 mm by 150 mm by 30 Mm, with the total weight of the monitor being approximately 0.7 kg and capable of achieving an Ingress Protection rating of IP65 or higher. A handle 804 may be provided to facilitate transportation of the monitor 800. The monitor comprises a display 806 which may or may not comprises a touchscreen. Around the periphery of the display 806, on the housing 802, there are a number of buttons and switches including: a power switch 808, a rotary dial 810 function, menu access and a series of buttons 812a – 812e for rapid access to various functions. Referring to Fig. 20B a side 814 of the housing 802 comprises various ports including: an SD card slot 816, a USB C port 818, an Ethernet port 820 (although the MUH,016-UK monitor 800 may be provided with wireless communication capability in addition to or instead of the Ethernet port 820), DC power control I/P and O/P, ports 822 for connecting the cable 706 of the EIT hub 700. Referring also to Fig.20C, the display 806 may display a GUI interface during use. The GUI may be similar to the screen shots shown in Figs.18A – 18J. During EIT recording (i.e. data collection) the GUI may also appear as shown in Fig.20C and may comprise a variety of regions providing information for the healthcare professional. A first region may comprise a schematic illustration of the wearable EIT apparatus 100 inside which a reconstructed image 826 of the patient’s lungs may be displayed. The remainder of the GUI is an indicative clinical interface displaying parameters that are customisable to the nature of the condition under investigation. Referring to Fig. 21A an embodiment of a wearable device is generally indicated by reference numeral 900. The wearable device 900 may be similar in all respects to the wearable devices 150 and 250, and details will not be repeated here but are incorporated by reference to the earlier parts of this disclosure. In Fig. 21A the outer side of the wearable device 900 is shown, being that side visible to the medical professional during application to the patient. Recalling that the wearable device 900 is reversible and can be used on either the anterior or posterior side of the patient, the wearable device 900 comprises various indications to assist the medical professional to place the wearable device 900 correctly on the patient. There may an anterior/posterior indicator 902. The anterior/posterior indicator 902 may comprise a printed indicator that is printed onto the textile layer of the wearable device 900. The printed indicator may be in the form of text, one or more symbols, or a combination of these. In an embodiment, the text may read ‘FRONT’ and ‘BACK’. In an embodiment, the anterior/posterior indicator 902 may be adapted so that in a first orientation of the wearable device 900 the medical professional knows that it is to be used on the anterior of the thorax, and in a second orientation of the wearable device 900 the medical professional knows that it is to be used on the posterior of the thorax. For example, the text ‘FRONT’ and ‘BACK may be printed so that only one is the correct way up in the first orientation and only the other is the correct way up in the second orientation of the wearable device 900. The wearable device 900 also comprises n electrode indicators 904 (only one MUH,016-UK indicated in Fig. 21A). The number n of electrode indicators 904 is the same as the number of electrodes on the wearable device 900. However, each electrode indicator 904 comprises two numbers. Recalling that the wearable device 900 is universal and that two wearable devices 900 will be worn by each patient around the thorax, there are 2n electrodes on the patient when both wearable devices 900 are in place. For EIT image reconstruction purposes it is important that the computer-implemented reconstruction algorithm knows where each electrode is on the perimeter of the patient’s thorax. To facilitate this, each electrode indicator 904 comprises two numbers, a first number being in the range 1 to n and a second number being in the range n +1 to 2n, where n is the number of electrodes on each wearable device 900. Furthermore each the first and second number are printed with opposite orientations, like the text ‘FRONT’ and ‘BACK’. In this way, when the medical professional orients the wearable device 900 so that the word ‘FRONT’ is the correct way round, the numbers 1 to n are also the correct way round. The medical professional proceeds to place the wearable device onto the anterior of the patient’s thorax. Having done so, electrode number 1 will be to the dextral-most side of the patient and electrode number n will be to the sinistral-most side. The medical professional then takes a second wearable device 900 and orients it so that the text ‘BACK’ is the correct way round. Then the numbers n + 1 to 2n are the correct way round. The medical professional proceeds to place the second wearable device 900 on to the posterior of the patient’s thorax. Having done so, electrode number n + 1 will be to the sinistral-most side of the patient and electrode number 2n will be to the dextral-most side. In this way the anterior/posterior indicator 902 and the electrode indicators 904 ensure that the electrodes are positioned around the patient’s thorax in numerical order, as expected by the computer-implemented EIT reconstruction algorithm. Fig.21B is another embodiment of a wearable device 950 comprising a similar anterior/posterior indicator 952 to the anterior/posterior indicator 902 of Fig. 21A. However, the electrode indicator 954 is different to the electrode indicator 904. The electrode indicator 954 comprises an aesthetic component 954a and a functional component 954b (only one of each indicated in Fig. 21B), whereas the electrode indicator 904 comprises principally a functional component. In an embodiment the aesthetic component 954a may comprise an image or shape that has appeal to infants and children, and their parents. In the embodiment of Fig. 21B, the aesthetic component 954a comprises an image of a panda holding two leaves. Other images and MUH,016-UK shapes are possible of course at the choice of the designer, although a panda is a useful image as good likeness can by achieved when printing in black and white only. The functional component 954b comprises an electrode number in each leaf, a first number being in the range 1 to n and a second number being in the range n +1 to 2n, where n is the number of electrodes on each wearable device 950. A location indicator 956 is provided to assist the healthcare professional to ensure that the correct part of the wearable device 950 is centred on the sternum or spine of the patient. In use, the various indicators on the wearable device 950 function in the same way as the indicators on the wearable device 900 and reference is made to the description above. Fig.22 is a plan view of the inner side of an embodiment of a wearable device generally indicated by reference numeral 1000. The design of the wearable device 1000 may be suitable for all patient groups (by making the device longer or shorter, and by adjusting the inter-electrode spacing), although there may be certain advantages of this design for particular patient groups. For example, the wearable device 1000 may be useful for infants, including premature babies. In this patient group, chest size is small and space on the wearable device is at a premium. The wearable device 1000 comprises a array of electrodes 1002 (comprising individual electrodes 1004) and electrically conductive ink tracks 1006. The array of electrodes may be arranged substantially linearly along the wearable device 1000. The layered structure, arrangement and manufacturing process of the wearable device 1000 is the same as the other wearable devices as described herein, such as wearable devices 108, 110, 150, 250, 900 and 950. Differences include that the shape of the wearable device 1000 is rectangular, each electrode 1004 has a generally stadium shape (two parallel sides with a semi-circle at either end), and the electrically conductive tracks 1006 are substantially linear between a connection end 1008 to a zone containing the array of electrodes 1002. Once within the array of electrodes 1002, the electrically conductive ink tracks 1006 may comprise one or more region 1010 in which the electrically conductive ink tracks 1006 leave the substantially linear path for a distance and then return to it. In this way, the path of the electrically conductive ink tracks 1006 may comprise at least one non-linearity, such as a ‘kink’. In some embodiments the kink may be such that each track does not cross back over a point where the linear path of that track would have pass through (but for the kink). This is contrast to the meandering, zig-zag or wandering paths described in conjunction with the wearable MUH,016-UK device 150 shown in Fig. 3A for example: there the meandering path of each track does cross the linear line, much like the graph of a sinusoid crosses back and forth over the x-axis. Using a kink in the track pattern (as shown in the embodiment of Fig. 22 for example), rather than an oscillatory track pattern, may facilitate a reduction width of the wearable device 1000 compared to the wearable device 150 for example. As mentioned, the shape of each electrode 1004 has been changed from a square shape (see e.g. Fig. 3A) to a stadium shape. In an embodiment the size of each electrode may be approximately 10 mm in length by 2 mm in width. The long axis of the stadium shape of each electrode is oriented along the long axis of the wearable device 1000. By using electrode shapes of this kind, space is saved across the width of the wearable device 1000 leaving more space for the electrically conductive ink tracks 1006 to pass by each electrode 1004. Other shapes of electrode 1004 could be used, including rectangular, oval, ellipse, or any other shape that may have a longer axis of symmetry and a shorter axis of symmetry. The wearable device 1000 may be manufactured in a variety of sizes to suit different patient groups, or different patients within a patient groups. For example, for an infant patient group the wearable device may be made in three different sizes based on the weight of the baby, e.g.2 kg, 3.5 kg and 5 kg. The length of the wearable device 1000 and the centre spacing of the electrodes may increase with the weight indicated by each wearable device. For example, the wearable device 1000 for a 2 kg weight may have a length (including tail) of 240 mm, to be suitable for a nominal chest circumference of 255 mm, minimum chest circumference of 240 mm, and a maximum chest circumference of 270 mm. The electrode spacing may be 5mm. For example, the wearable device 1000 for a 3 kg weight may have a length (including tail) of 268 mm, to be suitable for a nominal chest circumference 295 mm, a minimum chest circumference of 270 mm, and a maximum chest circumference of 320 mm. The electrode spacing may be 7 mm. For example, the wearable device 1000 for a 5 kg weight may have a length (including tail) of 310 mm, to be suitable for a nominal chest circumference of 350 mm, a minimum chest circumference of 310 mm, a maximum chest circumference of 390 mm. The electrode spacing may be 10 mm. Use of the wearable EIT apparatus and wearable device on animals MUH,016-UK The various embodiments of the wearable devices and wearable EIT apparatus described herein may be used or adapted for use (e.g. by sizing appropriately) on animals. The animals may any of the following kinds: vertebrates; warm-blooded vertebrates; mammals. Particular examples of animals that may use the wearable EIT apparatus include domesticated animals (farm, laboratory and pets) including mice, rabbits, pigs, dogs, cats, cows, horses. In an embodiment, the wearable device may be used in the same or similar way as described above for a human patient, i.e. adhered to the animal body, possibly with the use of additional electrolytic agent under each electrode to facilitate electrical conductivity with the skin (especially if the animal has a fur on its body). Alternatively the hair may be removed from the relevant region before use. In some embodiments (for example smaller animals such as rabbits, pigs, mice, etc.) the wearable EIT apparatus may comprise a single wearable device rather that two wearable devices as described above for a human patient. The single wearable device may be sized to extend around at least a portion of the thorax of the animal, including entirely around the thorax. In some embodiments, the carrier layer part of the wearable EIT apparatus (comprising one or two wearable devices for example) may be a jacket or harness sized for use on an animal such as a mouse or rabbit. Examples of jackets and harness which could be adapted in this way are available from Lomir Biomedical Inc. Use of the wearable EIT apparatus and wearable device to control a controllable object The various embodiments of the wearable devices and wearable EIT apparatus described herein may be used in computer-implemented apparatus and methods for controlling a controllable object based on impedance patterns or changes identified in a body part. The general concept is that one or more wearable device a described herein may be used to determine an impedance pattern or a change in impedance inside a body part as the body part moves (e.g. the bones and muscle inside a limb). The impedance pattern or change may be processed by a processor to recognise or classify (e.g. following training by a neural network) the impedance pattern or change and then produce a corresponding output. The output may be used to control a controllable MUH,016-UK object, e.g. to change its physical properties and/or control movement. For example the wearable devices and wearable EIT apparatus described herein may be used as part of a prosthesis motion control system. An example of a prothesis motion control system is described in “A Human-Machine Interface Using Electrical Impedance Tomography for Hand Prosthesis Control”, Wu, Y. et al., IEEE Transactions on Biomedical Circuits and Systems, Volume: 12, Issue: 6, Dec. 2018, the contents of which is incorporated herein for all purposes. A schematic representation of a prosthesis motion control system in use on a patient is shown in Fig. 23. The prosthesis motion control system is generally identified by reference numeral 1100 which comprises a prosthetic limb 1102 (in this embodiment comprising a forearm and hand) and a wearable EIT apparatus 1104. The prosthesis motion control system 1100 is worn on a patient’s limb 1106 (in this example, the patient’s forearm). It is noted that in Fig. 22 the shape and size of the wearable EIT apparatus 1104 is bulkier than it is in reality. As will be appreciated, an advantage of the wearable EIT apparatus 1104 as described herein is that it is comparatively thin, thereby improving patient comfort and permitting it to be worn for extended periods of time (e.g. all day). The wearable EIT apparatus 1104 may be thin enough to permit ordinary clothing to be pulled over or rolled down easily by the patient over the limb 1106 and prosthetic limb 1102. Inside the prosthetic limb 1102 (but not shown in Fig. 22) there is a power source (e.g. a rechargeable battery), a microcontroller or other small form factor computing device, and one or more motor for moving parts of the prosthetic limb 1102 such as the fingers and wrist. The microcontroller may comprise the functionality of the EIT hub 400 or 700 as described above, the details of which are expressly incorporated into the Fig. 23 embodiment. Alternatively, the electrical components of EIT hub 400 or 700 may be provided separately inside the prosthetic limb 1102, for example in the form of and ASIC. There may be no need to provide the housing and other non-electrical components of the EIT hub 400 or 700 since the electrical components are housed inside the prosthetic limb 1102. In an embodiment the wearable EIT apparatus 1104 need not comprise a long tail portion as described in other embodiments elsewhere herein. The wearable EIT apparatus 1104 may comprise a shorter tail portion with the connector (like the MUH,016-UK connector 116) positioned close to the first electrode, for example within a few centimetres, to facilitate patient comfort. In some embodiments the wearable EIT apparatus 1104 may comprise a single wearable device rather that two wearable devices. The single wearable device may be sized to extend around at least a portion of the patient’s limb 1106 (e.g. arm, leg), including entirely around the limb. There may also be an overlapping portion of the wearable EIT apparatus 1104 which could be provided with some releasable retaining mechanism (e.g. VELCRO (RTM), buckle, etc.) to facilitate placement and retention on the limb 1106. The upper part of Fig. 23 identified by reference numeral 1108 illustrates the general method of prosthetic motion control using system 1100. EIT measurements are made at step 1110 (which may use the same method described above in conjunction with Figs. 13 to 16, details of which are expressly incorporated into the Fig. 22 embodiment). At step 1112 the EIT hub or microcontroller inside the prosthetic limb 1102 processes the EIT measurements (e.g. using time difference EIT) to determine bone and/or muscle movement of the patient’s limb 1106 in the region of the wearable EIT apparatus 1104 and corresponding to a position of the hand and/or wrist. According to the pattern of the bone and/or muscle motion of the patient’s limb recognised by the microcontroller, the microcontroller may output control signals at step 1114 to the one or more motor to articulate one or more joint of the prosthetic limb 1102 in a particular way (e.g. pinching thumb and forefinger, grasping an object, etc.). It is noted that in the embodiment of Fig. 23, it is not necessary for an image to be generated and displayed and the microcontroller may calculate just impedances from the EIT measurements and then analyse the calculated impedances. As described in the aforementioned paper by Wu et al., impedance patterns related to bone and/or muscle motion may be recognised by the microcontroller following training using a neural network with the particular patient. As described above in conjunction with other embodiments the wearable EIT apparatus 1104 permits a degree of stretching along its length and is flexible to substantially conform to the shape of the patient’s limb. This feature may help the electrodes of the wearable EIT apparatus 1104 to remain in contact with the patient’s limb during use. For example as the muscles and bones of the patient in the vicinity of MUH,016-UK the wearable EIT apparatus 1104 change position with movement, this stretchability and/or flexibility may assist the electrodes to stay in position on the patient’s limb and to remain in contact with the skin. In this way the reliability of EIT measurements is improved, and consequently the motion control of the prosthetic limb 1102 may also be improved. In another example, the controllable object may comprise a material having properties that can be controlled (e.g. by application of a current and/or voltage to the material). Such properties may include stretchability and stiffness. In a similar way to the prosthesis motion control described above, the wearable device may be used on a limb to monitor muscle and/or bone movements. A patient wears the material on the same limb. When a certain movement is identified the output signal may be used to control the material to make it more or less stretchy or more or less stiff for example. In this way it may be possible to limit the range of movement of the limb to inhibit damage to muscles, ligaments and joints which may otherwise be caused by movements outside the limited range. —Electrode array arrangements and wearable device shape Various embodiments described herein include a substantially linear array of electrodes. Whilst such arrangements may be useful in certain applications (e.g. monitoring of lung function), other arrangements of electrodes are possible for various applications. For example, the wearable device may be provided with a non-linear array of electrodes. In an embodiment the wearable device may be provided with a two-dimensional pattern or array of electrodes. In an embodiment the wearable device may be in the form of a patch that is used on the body part. The patch may have a shape such as a square, rectangle, circle, etc. and need not cover all of or encircle the body part. MUH,016-UK

Claims

CLAIMS 1. A wearable device for use in electrical impedance tomography on an animal having a body part, which wearable device comprises: a carrier layer substantially conformable to the general shape of the body part; a plurality of electrically conductive ink electrodes spaced apart across the carrier layer; a plurality of electrically conductive ink tracks extending along the carrier layer, one electrically conductive ink track for each electrically conductive ink electrode, each electrically conductive ink track having a first end electrically connected to a respective one conductive ink electrode and a second end for providing an electrical connection to an external electrical circuit; at least one region of an interface layer for adhering the wearable device to the body part and for providing electrical conductivity between the plurality of electrically conductive ink electrodes and the body part; wherein, in use during movement of said body part, said wearable device permits at least some flexing, elongation and/or contraction of at least a portion of the wearable device and at the same time remains in contact with the body part substantially in conformance with its general shape, without buckling or twisting up and away from the body part during said flexing, elongation and/or contraction. 2. A wearable device as claimed in claim 1, wherein said carrier layer has an elasticity in at least one direction and/or a flexibility in at least one plane, and said plurality of electrically conductive ink tracks are arranged to allow said flexing and/or elongation and/or contraction. 3. A wearable device as claimed in claim 1 or 2, wherein said carrier layer comprises an elongate strip having a length sufficient to span a portion of said body part; and/or/ wherein each electrically conductive ink track comprises a meandering, winding or indirect path along at least a portion of the length of the elongate strip. 4. A wearable device as claimed in in claim 3, wherein said meandering, winding MUH,016-UK or indirect path comprises a deviation from and return to a direction extending substantially along said carrier layer. 5. A wearable device as claimed in claim 3 or 4, wherein said meandering, winding or indirect path comprises a sinusoid, serpentine shape, zig-zag shape or a kink. 6. A wearable device as claimed in any preceding claim, wherein each electrically conductive ink track comprises an electrically conductive ink that has a flexibility allowing it to remain substantially conductive and within workable levels of impedance after repeated bending and stretching of the wearable device during normal use on the animal. 7. A wearable device as claimed in any preceding claim, further comprising a feature for facilitating substantially uniform flexing and/or elongation and/or contraction of the wearable device, and optionally wherein said feature comprises a printed feature. 8. A wearable device as claimed in claim 7, wherein said feature comprises a redundant conductive ink track. 9. A wearable device as claimed in claim 8, wherein said redundant conductive ink track comprises a shape, pattern or path the same or similar to said plurality of electrically conductive ink tracks. 10. A wearable device as claimed in claim 8, wherein said redundant conductive ink track comprises a shape, pattern or path that is dissimilar to said plurality of electrically conductive ink tracks. 11. A wearable device as claimed in any preceding claim, wherein said plurality of electrically conductive ink electrodes comprise a conductive ink layer formed by a printing process. 12. A wearable device as claimed in any preceding claim, wherein said plurality of electrically conductive ink tracks comprise a conductive ink layer formed by a printing MUH,016-UK process. 13. A wearable device as claimed in any preceding claim, further comprising a dielectric encapsulant that substantially encapsulates each of said plurality of electrically conductive ink tracks except at said first end at least sufficient to provide said electrical connection with a respective one conductive ink electrode and at said second end at least sufficient to provide said electrical connection to said external circuit. 14. A wearable device as claimed in claim 15, wherein said dielectric encapsulant comprises a plurality of dielectric encapsulant layers formed by a printing process. 15. A wearable device as claimed in claim 15 or 16, wherein said plurality of electrically conductive ink electrodes, said plurality of electrically conductive ink tracks and said dielectric encapsulant form a unitary structure. 16. A wearable device as claimed in claim 15, wherein when measured at an electrode of said plurality of electrically conductive ink electrodes said unitary structure has a thickness of less than 0.20 mm, preferably less than 0.15 mm, and preferably is about 0.14 mm, not including said carrier layer and said interface layer. 17. A wearable device as claimed in claim 15 or 16, wherein when measured at an electrically conductive track of said plurality of electrically conductive ink tracks said unitary structure has a thickness of less than 0.25 mm, preferably less than 0.20 mm, and preferably is about 0.16 mm, not including said carrier layer and said interface layer. 18. A wearable device as claimed in claim 15, 16 or 17, further comprising an adhesive between said unitary structure and said carrier layer. 19. A wearable device as claimed in any preceding claim, wherein said carrier layer comprises a textile carrier layer, and optionally wherein said textile carrier layer comprises a breathable and/or moisture-wicking fabric, and further optionally wherein said textile carrier layer comprises woven or non-woven textile or material that exhibits properties of flexibility and elasticity. MUH,016-UK 20. A wearable device as claimed in any preceding claim, wherein said textile carrier layer has a thickness less than 1.0 mm, and optionally wherein said textile carrier layer has a thickness within one of the following ranges: between 0.1 mm and 0.7 mm; between 0.2 mm and 0.6 mm; between 0.3 mm and 0.5 mm; and between 0.35 mm and 0.45 mm. 21. A wearable device as claimed in any preceding claim, wherein said region of an interface layer has a thickness between 0.5 mm and 1.0 mm. 22. A wearable device as claimed in any of claims 1 to 21, wherein said at least one region of an interface layer comprises a cross-linked hydrophilic polymer, such as a hydrogel. 23. A wearable device as claimed in any preceding claim, wherein said carrier layer comprises a jacket or harness for a laboratory animal. 24. A method of manufacturing a wearable device for use in electrical impedance tomography, which method comprises the steps of: providing a temporary release substrate; forming on said temporary release substrate a plurality of electrically conductive ink electrodes spaced apart across the temporary release substrate; forming on said temporary release substrate a plurality of electrically conductive ink tracks extending lengthwise along the temporary release substrate, one electrically conductive ink track for each electrically conductive ink electrode, each electrically conductive ink track having a first end electrically connected to a respective one conductive ink electrode and a second end for providing an electrical connection to an external electrical circuit; applying an adhesive to at least a part of said plurality of electrically conductive ink electrodes and to at least a part of said plurality of electrically conductive ink tracks, and/or to a carrier layer; bringing the temporary release substrate and said carrier layer together so as to adhere the carrier layer to said plurality of electrically conductive ink MUH,016-UK electrodes and to said plurality of electrically conductive ink tracks; removing the temporary release substrate to leave said plurality of electrically conductive ink electrodes and said plurality of electrically conductive ink tracks adhered to said carrier layer. 25. A method as claimed in claim 24, wherein: said step of forming a plurality of electrically conductive ink electrodes comprises a printing process; and/or said step of forming a plurality of electrically conductive ink tracks comprises a printing process. MUH,016-UK