WO2020149793A1

WO2020149793A1 - Pressure sensor

Info

Publication number: WO2020149793A1
Application number: PCT/SG2020/050020
Authority: WO
Inventors: Longteng YU; Joo Chuan Yeo; Chwee Teck Lim; Fazila ALOWENI; Shin Yuh ANG; Sivagame MANIYA
Original assignee: National University Of Singapore; Singapore Health Services Pte Ltd
Priority date: 2019-01-15
Filing date: 2020-01-15
Publication date: 2020-07-23
Also published as: SG10201900359TA

Abstract

A pressure sensor for a pressure dressing comprises at least one sensor element. The at least one sensor element comprises a wire formed from a compressible insulating sheath that encapsulates an electrically conductive liquid core. The pressure sensor also comprises a readout component connected to respective ends of the wire for measuring changes in resistance of the wire when the sensor element is under compression. The wire is arranged in a non-linear shape having an extended area over which pressure can be measured, such as a spiral or a helix.

Description

PRESSURE SENSOR

TECHNICAL FIELD

[001] The present disclosure relates to a pressure sensor, a method of manufacturing a pressure sensor, and a method of monitoring pressure in a pressure dressing.

BACKGROUND

[002] The concept of compression therapy is based on a simple and efficient mechanical principle consisting of applying an elastic garment around the affected body parts. The purpose of compression therapy is to reduce swelling and aid in recovery.

[003] Depending on the pathology, medical compression therapy can be applied in different forms, such as socks, stockings (calf or thigh-length), pantyhose, and sleeves. It is commonly prescribed for patients with chronic venous ulcer, burns patients and patients with lymphedema after breast surgery and lymph node dissection.

[004] In burns patients, compression therapy is an important component during the rehabilitation program. Elastic bandages or compression garments are used to provide pressure over healing burns and grafts when they are durable enough to tolerate the shearing that occurs from the fabric against the skin. This compression minimizes the development of scars by interfering with the production of collagen and helping to realign the collagen fibers. Pressure garment therapy (PGT) is well accepted and commonly used by clinicians in the treatment of burns scars and grafts. The medium to high pressures (24-40 mmHg) in these garments can support scar minimisation, and evidence is well documented for such an application.

[005] For breast cancer patients, compression therapy is an effective treatment for lymphedema and can also be used for anti-edematous prevention in patients who underwent removal of axillary lymph nodes as well as radiotherapy.

[006] Compression bandaging is the gold standard for the treatment of venous ulcers, which are caused by micro-circulatory damage triggered by chronic venous hypertension. The compressive pressure exerted by the bandages is key to the healing of venous ulcers.

[007] The physician prescribes the compression level corresponding to the pathology of the patient. Therefore, it is important for clinicians and patients to be informed of the amount of pressure that is being delivered so that effective treatment is provided. This is important because inadequate pressure will lead to an ineffective healing while excessive pressure can result in tissue damage, pressure injury and necrosis.

[008] However, quantifying the intra-bandage pressure throughout the treatment is an issue as the application of compression bandages is largely based on the expertise and experience of the nurse. Currently available devices do not allow for continuous monitoring.

[009] For example, one known type of monitoring device is sold under the trade marks Kikuhime™ and PicoPress™. This type of device operates according to pneumatic principles. To measure the sub-bandage pressure, an air bladder is sandwiched in the bandages and connected to a bulky manometer which measures the air pressure. Due to the nature of these devices, it is only possible to measure the pressure when the bandage is first applied.

[010] It is important to have continuous monitoring of sub-bandage pressure, as a fall in pressure during a week-long compression treatment will reduce the efficacy of the treatment.

[Oil] It is desirable therefore to provide a means of monitoring intra bandage pressure that overcomes or alleviates one or more of the above difficulties, or which at least provides a useful alternative.

SUMMARY OF THE PRESENT DISCLOSURE

[012] Disclosed herein is a pressure sensor for a pressure dressing formed from one or more bandages each having a bandage width, the pressure sensor comprising :

at least one sensor element comprising a wire formed from a compressible insulating sheath that encapsulates an electrically conductive liquid core; and a readout component connected to respective ends of the wire for measuring changes in resistance of the wire resulting from changes in compression of the sensor element;

wherein the wire is arranged in a non-linear shape having an area over which pressure can be measured.

[013] By arranging the wire in a non-linear shape (which may be two- dimensional or three-dimensional) such that it covers an area (i.e., a two- dimensional region), it is possible to not only increase the path length between the terminals of the sensor to improve sensitivity, but also to reduce or substantially avoid the deleterious effects of tension on the pressure measurement. Additionally, by covering an area, a more reliable pressure measurement may be obtained since the compression is distributed over the area rather than being concentrated at a line.

[014] In some embodiments, the non-linear shape has at least one dimension that is less than half of the bandage width.

[015] The non-linear shape may be a spiral or a helix, for example. In the case of a spiral, the spiral may be substantially circular or substantially elliptical. In some embodiments, the spiral is a Fermat spiral.

[016] In some embodiments, the non-linear shape is a serpentine shape.

[017] The wire may be arranged such that its respective ends are substantially parallel.

[018] The sheath may be formed from a material having a Young's modulus in the range between about 55 kPa and 3 MPa, and/or a Shore hardness between about Shore-A 10 and Shore-A 55.

[019] In some embodiments, an inner diameter of the sheath is in the range between about 120 microns and 1000 microns.

[020] In some embodiments, an outer diameter of the sheath is in the range between about 400 microns and 2000 microns.

[021] The core may comprise a eutectic alloy, such as eutectic gallium- indium or galinstan. Alternatively, the core may comprise an ionic liquid or ionic hydrogel.

[022] Also disclosed is a method of manufacturing a pressure sensor for a pressure dressing that has one or more bandages each having a bandage width, the method comprising : providing a mould having formed therein a template defining a non linear shape;

forming a sensor element by:

arranging a compressible insulating sheath in the mould such that it conforms to the non-linear shape;

filling the compressible insulating sheath with an electrically conductive liquid to form a wire; and

transferring the compressible insulating sheath to a substrate; and

connecting a readout component to respective ends of the wire of the sensor element for measuring changes in resistance of the wire resulting from changes in compression of the sensor element.

[023] In some embodiments, the non-linear shape has at least one dimension that is less than half of the bandage width.

[024] The compressible insulating sheath may be filled with the electrically conductive liquid before arranging it in the mould. In some embodiments, the compressible insulating sheath may be transferred to the substrate after the arranging step but before the filling step. That is, the sheath may be patterned and then affixed to the substrate before it is filled with the conductive liquid to form the wire.

[025] The non-linear shape may be a spiral or a helix, such as a substantially circular or substantially elliptical spiral.

[026] In some embodiments, the non-linear shape is a serpentine shape.

[027] Further disclosed herein is a method of monitoring pressure in a pressure dressing applied to a subject, comprising:

providing a pressure sensor according to any one of the preceding paragraphs, for embedding between layers of the pressure dressing, or between the pressure dressing and the subject;

receiving, from the readout component, a first resistance measurement at a first time;

receiving, from the readout component, one or more subsequent resistance measurements at one or more subsequent times; and

determining, from the first resistance measurement and the one or more subsequent resistance measurements, whether the pressure dressing requires adjustment. BRIEF DESCRIPTION OF THE DRAWINGS

[028] Some embodiments of pressure sensors, methods of forming pressure sensors, and methods of monitoring pressure, in accordance with present teachings will now be described, by way of non-limiting example only, with reference to the accompanying drawings in which :

FIG. 1A shows a schematic depiction of a pressure sensor in accordance with certain embodiments;

FIG. IB is a close-up of the part of the pressure sensor indicated at B in FIG. 1A;

FIG. 2 depicts some steps for forming a pressure sensor in accordance with certain embodiments;

FIG. 3 is a graph showing the response of an exemplary pressure sensor under cyclic loading over time;

FIG. 4 is a series of curves of normalised resistance as a function of pressure, for varying internal diameters of the tube of an exemplary pressure sensor;

FIG. 5 is a series of curves of normalised resistance as a function of pressure, for varying external diameters of the tube of an exemplary pressure sensor;

FIG. 6 shows resistance response of an exemplary pressure sensor under cyclic loading;

FIG. 7 shows force-extension curves for an exemplary pressure sensor during cyclic loading, where the area bounded within a force- extension curve is the energy dissipated in one loading and unloading cycle;

FIG. 8 shows overall responsiveness of an exemplary pressure sensor from the start to the end of a test process;

FIG. 9 is a zoomed-in section of the curve centred around the region labelled 9 in FIG. 8;

FIG. 10 shows sensor characteristics as a function of time;

FIG. 11 shows comparative pressure data for a prior art pressure sensor and a pressure sensor according to certain embodiments; and

FIG. 12 shows an example configuration of a wire of a pressure sensor according to certain embodiments. DETAILED DESCRIPTION

[029] Embodiments of the present disclosure relate to a pressure sensor having at least one sensor element comprising a wire that is arranged in a non-linear shape, which may be a two-dimensional or three-dimensional shape. At least some embodiments provide a flexible, stretchable, wearable and washable piezoresistive microtubular sensor capable of continuously monitoring intra-layer bandage pressure in real time.

[030] FIG. 1A, which is not to scale, shows a pressure sensor 100 comprising a sensor element 112. The sensor element 112 comprises a wire 110 that is arranged in a non-linear shape, in this case a two- dimensional shape, having a width W and length L. The dimensions are such that the width W of the two-dimensional shape is less than half of the width of a compression bandage 150 with which the sensor 100 is to be used. In particular, the wire 110 is preferably dimensioned such that a bandage 150 which is to applied on top of the sensor 100 for compression therapy can be wound in such a manner that its edges do not overlap the sensor element 112, as this may cause erroneous readings. For example, in at least some compression therapies, when the pressure dressing is applied, consecutive loops of the bandage may overlap each other by only half the bandage width. Accordingly, it is advantageous in such circumstances for the sensor element not to be too large (and in particular, for the width W to be less than half the bandage width).

[031] In this embodiment, the wire 110 is arranged into a circular spiral, for example a shape that is substantially a Fermat spiral. The wire 110 may be affixed to at least one substrate, such as a polymer film (such as film 220 shown in FIG. 2), such that it retains its shape. The wire 110 may be sandwiched by, and sealed between, two films such that it is more resistant to losing its shape, and further so that the connection (e.g. an epoxy seal) between the wire ends 114, 116 and their connections to an external circuit are better protected. One of the films may have adhesive on both sides, such that a first side can adhere to both the wire and the other film, and a second, opposed side can adhere to a bandage (e.g., bandage 150 or another bandage over which bandage 150 is applied in a compression bandaging arrangement), for ease of application of the sensor 100 during compression bandaging. For example, one of the films may be a Tegaderm™ film (3M, Maplewood, MN) or the like. It will be appreciated that many other types of film having adhesive on both sides may be used.

[032] It will be appreciated that many other two-dimensional shapes are possible for the wire 110. For example, as shown in FIG. 12, an alternative wire 310 may have a serpentine configuration. The serpentine configuration may have a plurality of turns 320. For example, the wire 310 shown in FIG. 12 has five turns 320. Preferably, a serpentine wire 310 has an odd number of turns such that its ends 314, 316 are aligned in the same direction, for ease of connecting the wire 310 to a readout component 120.

[033] It will also be appreciated that the wire 110 need not be arranged in a two-dimensional shape. For example, the wire 110 may be arranged in a three-dimensional shape such as a helix. When the sensor is applied during compression therapy, the three-dimensional shape covers an extended area over which pressure can be measured. For example, the three-dimensional shape may have a two-dimensional projection that covers the extended area. In some embodiments, the wire 110 may have a multi-layer arrangement. For example, the wire may be woven into textiles or the like.

[034] The respective ends 114 and 116 of the wire are connected to a readout component 120 via connecting wires 124 and 126 respectively. The readout component 120 is connected to a power source 130 (such as a battery) and comprises positive and negative terminals of a resistance measurement circuit that are connected to the ends 114, 116 of the wire, and at least one processor that is in communication with the resistance measurement circuit.

[035] The readout component 120 may also include computer-readable storage for storing program code that is executable by the at least one processor to, for example, receive resistance measurements, and compare the resistance measurements to one or more calibration curves that are also stored on the computer-readable storage, and that relate resistance to pressure.

[036] The readout component 120 may further include at least one interface for receiving data from and/or sending data to an external device. For example, the readout component 120 may include a wireless communications interface (such as a Bluetooth™ interface) for transmitting resistance and/or pressure measurements to an external device, such as a mobile computing device or desktop computing device.

[037] The readout component 120 may comprise a voltage comparator or a Wheatstone bridge circuit to measure electrical inputs from the sensor. The readout component 120 may also comprise a power management unit to power the readout component and to transmit measurements.

[038] The respective ends 114 and 116 of the wire emerge from the spiral in substantially parallel fashion. Advantageously, this facilitates connection of the ends 114, 116 of the wire to the readout component 120.

[039] The wire 110 has a sheath-core structure in which a liquid conductor 208, such as a metallic alloy, is enclosed in an elastomeric microtube 202, as shown in the zoomed view of FIG. IB (corresponding to B in FIG. 1A). In certain embodiments, the liquid conductor is a eutectic alloy, such as eutectic gallium-indium (eGaln) which is a conductive liquid at room temperature.

[040] When exposed to external pressure, the wire 110 of the sensor element 112 deforms and thereby exhibits an increase in the electrical resistance. In particular, the resistance dR of an infinitesimal length element dl located at point l along the wire can be determined based on the relationship dR = ( p - dl)/A(l ), where p is the electrical resistivity of the core material and A(l is the cross-sectional area at point 1. When the sensor wire 110 is compressed, the sheath 202 flattens and constricts, and the cross-sectional area decreases. The consequent reduced volume and displacement of the conductive eGaln metallic alloy core in the region of the compression causes an increase in its electrical resistance. The increase in resistance is closely correlated with the pressure, as shown in FIG. 4 and FIG. 5, for example. [041] As will be demonstrated in the discussion below, a pressure sensor according to certain embodiments is responsive (< 100ms response time), sensitive (sensitivity of 0.145 mmHg ^_1), and able to withstand common laundry procedures. To validate its accuracy for compression therapy, clinical tests on healthy volunteers were conducted, in which an exemplary pressure sensor was embedded in a bandage applied to the study subjects. The results in FIG. 11 show that the response of the sensor agrees well with that of a prior art pneumatic sensor (PicoPress™), implying that the sensor when embedded in a bandaging system can provide high precision pressure measurements whilst also being usable for continuous monitoring.

[042] Advantageously, by measuring electrical resistance (or changes therein relative to that of the sensor prior to use in a compression bandage), sensors according to certain embodiments avoid the need for a bulky meter, and can work wirelessly and continuously using a tiny IC chip that communicates with a smartphone.

[043] The piezo-resistive behaviour of embodiments of the sensor is suitable for prolonged monitoring for the duration of compression bandaging in compression therapy. This provides an advantage over conventional digital pneumatic pressure monitors which are unable to monitor the pressure continuously in real time.

[044] Referring now to FIG. 2, an exemplary method of forming a pressure sensor will now be described.

[045] In certain embodiments, the sheath 202 of the wire 110 may be prepared using an extrusion technique. In one embodiment, uncured polydimethylsiloxane (PDMS) base and curing agent were mixed in a container with a weight ratio of 10: 1. The mixture was degassed for 30 minutes to remove air bubbles. A metal filament was drawn vertically from within the container. Due to the viscosity and surface tension of the PDMS, a uniform thin layer of PDMS was formed around the wire as it was drawn above the mixture. To further improve its wettability, hot water («98 °C) was added to the water bath to allow partial curing of PDMS and ice-cold water was used to maintain the PDMS viscosity. Next, a cylindrical electrical heating unit («150 °C) was positioned above the container to cure the PDMS on the filament entirely. Once it was fully cured, the PDMS coated wire was soaked in acetone and sonicated for 2 hours, making the wire easier to remove and form the hollow tubular structure. The PDMS microtubular pressure sensor measured with an internal diameter of 100 pm with a wall thickness of 10 pm, which is five times smaller than the smallest sensor reported in the literature at present. The inner diameter of the microtube could also be varied with different diameters of wire.

[046] Owing to the uncomplicated fabrication method, the entire tube is consistently uniform in diameter and flexible. Furthermore, the microtube is very thin and soft and could be bent over a sharp tip with a bend radius of «200 pm, indicating the conformability of the microtubular sensor over tight curvatures. Overall, the microtubular sensor is highly bendable, flexible, twistable, and stretchable. Importantly, it is believed that the combination of an ultrathin (100 pm or less) wall thickness and low modulus (between about 55 kPa and 3 MPa) allows an efficient mechanotransduction of the forces to the liquid metal core, hence extending this capability to measure physiological signals.

[047] In other embodiments, substantially the same microtube 202 fabrication technique may be used, but with different elastomeric materials. For example, a platinum catalysed silicone rubber such as Ecoflex™ (Smooth-On, Macungie, PA) may be used in place of PDMS. Specifically, Ecoflex Part A and Part B are mixed with a weight ratio of 1 : 1 to obtain Ecoflex rubber. Before that, SLIDE^® STD Liquid Surface Tension Diffuser (Smooth-On, Macungie, PA) may be added into Ecoflex Part B at 1.5% w/w, to facilitate the removal of the wire from the Ecoflex coating.

[048] In any event, it is advantageous for the elastomeric material to have a hardness that is approximately equal to, but not less than, that of human skin. Ecoflex has a hardness close to skin, which is much softer than PDMS. Some suitable elastomeric materials may have a Young's modulus in the range between about 55 kPa and 3 MPa, for example. Equivalently, suitable materials may have a hardness between about Shore-A 10 and Shore-A 55. To maximise the sensitivity of the sensor element 112, the elastic modulus of the sheath 202 is preferably made as low as possible, while still approximating the softness of human skin. Accordingly, an elastic modulus of about 55 kPa has been found to be particularly advantageous.

[049] Once the sheath 202 has been formed as described above, it can be formed into the desired non-linear shape. In certain embodiments, this can be done with the aid of a plastic mould carrying the desired pattern. For example, as depicted in FIG. 2A, a plastic mould 200 having a spiral channel 210 can be prepared by 3D printing. The sheath 202 may then be fitted into the channel 210 of mould 200 as shown in FIG. 2B. Following this, the patterned sheath 202 may be extracted using an adhesive plastic film 220, as shown in FIG. 2C. This maintains the spiral pattern of the sheath 202. In some embodiments, the adhesive plastic film 220 may have an adhesive layer on both sides, with one side having a backing layer applied to the adhesive layer.

[050] Eutectic gallium-indium (eGaln), a conductive liquid at room temperature, may then be injected into the sheath 202. The liquid forms a conductive channel 208 within the sheath 202 (FIG. IB) that exhibits similar resistivity characteristics to that of a conductive metal wire whilst having the advantage of being elastic and flexible. It will be appreciated that essentially any other conductive liquid may also be used to form the conductive channel (core) 208 of the wire 110. For example, a galinstan alloy, an ionic liquid, or an ionic hydrogel may form the core of the wire 110.

[051] It will be appreciated that the order of the patterning, filling and substrate transfer steps may be changed. For example, the conductive liquid may be injected into the sheath 202 prior to arranging the sheath 202 in the mould 200, after which the filled and patterned sheath is transferred to the substrate (film) 220. Alternatively, the sheath 202 may be arranged in channel 210 to pattern it, the conductive liquid then injected into the patterned sheath while it is still in the mould 200, and the filled and patterned sheath 202 then being transferred to the film 220.

[052] After the microtube 202 is filled with the conductive liquid 208 to form the wire 110, metal wires may be inserted into both ends 114, 116 of the eGaln injected microtube 110 and sealed with epoxy, for example. Alternatively, the microtube 110 may be sealed with the same material from which the sheath is constituted (for example, PDMS or platinum catalysed silicone rubber). An adhesive may also be used. The microtube may also be sealed via a mechanical seal, such as a clamp. Typically, the microtubular wire 110 may be ready to use 15 minutes after the application of the epoxy.

[053] In certain embodiments, a second layer of plastic film (not shown) may be attached to the patterned wire 110 on the opposite side to the first film 220. The two films form an envelope to protect the patterned wire 110. Where the first film 220 has adhesive layers on both sides, as discussed above, the backing layer may be removed from the first film 220 to expose the adhesive layer to facilitate attachment to a bandage.

[054] It has been found, using the fabrication technique described above, that sheaths 202 having an ID (inner diameter) as small as 120pm and an OD (outer diameter) as small as 400pm can be fabricated with substantially constant wall thickness. A sheath of constant thickness may provide a more accurate mechanical characterisation as the applied pressure is distributed evenly.

[055] Compressive testing using an Instron tester was carried out to characterise the response of the spiral microtubular sensor 100 to the external pressure loading. A loading unit was repeatedly actuated to apply compression to the sensor element 110 and the nominal pressure on the spiral microtubular sensor 100 was calculated from the force exerted by the loading unit over a fixed area of 5.0 x 5.0 cm².

[056] By repeating each compressive test at a given pressure thrice, the peak resistance and the corresponding pressure were recorded, as shown in FIG. 3. The relationship between the sensor's resistance and the average external pressure loading can then be obtained. The resistance vs. pressure curve is used to convert the resistance readout obtained from the sensor to a pressure value.

[057] Various additional experiments were performed to characterise the sensor 100. The studies were conducted to optimise the size of the sheath 202 for maximum sensitivity, and to test for hysteresis and response time of the sensor 100.

[058] To obtain the optimal sensor dimension configuration, the response of each sensor 100 was compared over the pressure range of interest. The first test compared the resistance response of spiral microtubular sensors 100 of varying ID. The time for polymer curing before drawing the fibre was kept constant and hence, the thickness of the polymer coating was constant. The length of the wire 110 (prior to forming into the spiral shape) of sensor 100 was also kept constant.

[059] Sensors were tested at the 25-55 mmHg pressure range and their response curves were obtained. Results showed that a spiral microtubular sensor of smaller ID, with all other parameters constant, was relatively more sensitive than those with larger ID, as indicated by a steeper gradient in the resistance-pressure curve in FIG. 4. A higher sensitivity is advantageous for pressure monitoring in compression therapy. Hence, the subsequent tests utilised spiral microtubular sensors with an ID of 200pm.

[060] Following the comparison of sensors with ID of 200pm, a series of sensors with ID 200pm and varying OD were tested to see the effect of OD on the sensitivity. It was hypothesized that a larger thickness would increase the microtube's stiffness, resulting in a smaller extent of deformation relative to a smaller thickness microtube when exposed to the same pressure. Microtubes of OD 800, 1000, 1200 and 1500 pm (all with ID 200 pm) were tested to obtain their resistance-pressure relations to compare their sensitivities at the 25-55 mmHg pressure range.

[061] Test results show a trend of increasing sensitivity with decreasing sheath thickness, as shown in FIG. 5. Experimentally, at smaller OD of 800 and 1000 pm, the sensor sensitivities were relatively similar. For the practical testing of the sensor in clinical tests, a spiral microtubular sensor of ID 200 pm and OD 800 pm was used.

[062] As the spiral microtubular sensor may be subjected to repeated use for extended periods of time, a hysteresis test was performed to determine whether the sensor 100 is able to provide consistent results with repeated use. To study if hysteresis was present in the spiral microtubular sensor, a sensor of ID 200 pm and OD 800 pm was subjected to cyclic loading (n=500). The resistance response over the cyclic loading was obtained, as shown in FIG. 6. The resistance-pressure relation of the sensor over repeated compression was relatively stable. The maximum resistance at the same applied pressure obtained experimentally dropped by < 5% after 500 compression cycles. [063] The force-extension curves of the 125th, 250th, 325th and 500th compressions are shown in FIG. 7. The area contained within the force- extension curve signifies the energy dissipation during the compression and release cycles. The resistance response of the prototype sensor obtained showed an insignificant drop in sensor sensitivity over a 500- compression cycle. The energy dissipation in the spiral microtubular sensor were relatively similar at the 25th, 50th, 75th and 100th percentile compression during the 500-compression cycle test. The energy lost as heat in one compression and decompression cycle in the highest-pressure scenario was approximately 3.5 mJ.

[064] Hysteresis is more pronounced when the loading cycle is repeated numerous times. In the case of the pressure monitoring in compression therapy, continual pressure is applied onto the sensor and maintained over a few days. From these tests, it was determined that hysteresis does not significantly affect the functionality and accuracy of the spiral microtubular sensor 100.

[065] A test was conducted to verify the spiral microtubular sensor's responsiveness to changes in applied pressure. In the test, a fixed pressure (30 mmHg) was applied to the spiral microtubular sensor. The pressure was then maintained for 30 seconds before it was removed. FIG. 8 shows the resistance response of the spiral microtubular sensor 100 (ID 200 pm, OD 800 pm) in the test. As shown in the close-up view of FIG. 9, the spiral microtubular sensor 100 had a response time of 100 ms upon application of pressure.

[066] The variation of the resistance response over time was also assessed. As shown in FIG. 10, the relationship between normalized resistance and applied pressure did not vary significantly over time, demonstrating stability of the sensor 100.

[067] To assess the performance of the spiral microtubular sensor for compression therapy, pilot tests were conducted with healthy volunteers. The testing included the use of the spiral microtubular sensor 100, and a prior art PicoPress™ device as a validation of the pressure measurements obtained during application of a Four Layer Bandage (4LB) by trained nurses. 4LB is a multi-layer compression therapy system, consisting of the following layers with their purposes stated in Table Al. [068] Table Al. Purpose of bandage layers in 4LB

Purpose

1 Soft padding layer that absorbs exudate and redistribute pressure evenly around the leg

2 Conformable bandage that increases the absorbency of exudate and smoothens layer 1 to preserve elasticity of layer 3 and 4.

3 Highly comfortable bandage that is the main source of compressive pressure.

4 Cohesive bandage that applies slight compression and maintains the 4LB system for the duration of treatment.

[069] Application of the 4LB system starts with layer 1 up to 4. The main purposes of the first 2 layers are wound management and preparation of the system for compression. Layer 3 will provide the bulk of the compression for the treatment while layer 4 seals the compression bandaging and maintains its integrity for the duration of treatment.

[070] A spiral microtubular sensor 100 having a sheath 202 made of Ecoflex™ with ID 200 pm and OD 800 pm was chosen to be used in the tests. 4LB bandaging was applied by trained nurses. Both the spiral microtubular sensor 100 and a PicoPress™ transducer were placed onto the 2nd layer of the 4LB. PicoPress™ was used as a benchmark to measure the pressure upon bandage application.

[071] Resistance readings from the spiral microtubular sensor 100 were converted to a pressure value using the sensor's resistance-pressure curve, as seen in FIG. 4 and FIG. 5. As described above, the sensor element 112 of the spiral microtubular sensor 100 is connected to an IC chip which can measure the electrical resistance of the sensor element 112 and may also send signals to a smartphone (not shown) via Bluetooth. An app executing on the smartphone may display the intensity of the pressure applied on the spiral microtubular sensor 100 in real time.

[072] After the application of the bandage, the sub-bandage pressure was recorded with the subject lying down supine, sitting up-right with leg bent at 90° and standing on the floor.

[073] As seen in FIG. 11, in the supine position, the spiral microtubular sensor 100 gave pressure readings that were best comparable to that of the PicoPress™, showcasing its ability to give accurate pressure readings (FIG. 11A-F, FIG. 111). However, in some tests, there may be a discrepancy between the pressure readings from the spiral microtubular sensor and the PicoPress™ (Figure FIG. 11C, FIG. HE, FIG. 11F, FIG. 11G-I).

[074] Advantageously, production of sheaths 202 fabricated from Ecoflex™ can be scaled-up at low cost. Moreover, the high level of control of the specification of the sheath 202 allows it to be applied in varying environments, from detecting and measuring small pressure in a small area to larger pressure over a larger area. The mechanical properties of the sheath 202 can be adjusted by varying the dimensions and material to suit a wide variety of needs.

[075] As described above, the sensor 100 according to certain embodiments was embedded into a bandage and was able to achieve similar pressure readings as the current state of the art. Advantageously, however, due to the elegant design and working principle of the sensor 100, it can achieve continuous pressure monitoring of the bandage pressure levels, thereby giving it a crucial advantage over the prior art.

[076] Many modifications will be apparent to those skilled in the art without departing from the scope of the present disclosure.

[077] Throughout this specification, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Claims

1. A pressure sensor for a pressure dressing formed from one or more bandages each having a bandage width, the pressure sensor comprising:

at least one sensor element comprising a wire formed from a compressible insulating sheath that encapsulates an electrically conductive liquid core; and

a readout component connected to respective ends of the wire for measuring changes in resistance of the wire resulting from changes in compression of the sensor element;

2. A pressure sensor according to claim 1, wherein the non-linear shape has at least one dimension that is less than half of the bandage width.

3. A pressure sensor according to claim 1 or claim 2, wherein the non-linear shape is a spiral or a helix.

4. A pressure sensor according to claim 3, wherein the spiral is substantially circular or substantially elliptical.

5. A pressure sensor according to any one of the preceding claims, wherein the non-linear shape is a serpentine shape.

6. A pressure sensor according to any one of the preceding claims, wherein the wire is arranged such that its respective ends are substantially parallel.

7. A pressure sensor according to claim 3, wherein the non-linear shape is a Fermat spiral.

8. A pressure sensor according to any one of the preceding claims, wherein the sheath is formed from a material having a Young's modulus in the range between about 55 kPa and 3 MPa, and/or a Shore hardness between about Shore-A 10 and Shore-A 55.

9. A pressure sensor according to any one of the preceding claims, wherein an inner diameter of the sheath is in the range between about 120 microns and 1000 microns.

10. A pressure sensor according to any one of the preceding claims, wherein an outer diameter of the sheath is in the range between about 400 microns and 2000 microns.

11. A pressure sensor according to any one of the preceding claims, wherein the core comprises a eutectic alloy.

12. A pressure sensor according to claim 11, wherein the eutectic alloy is eutectic gallium-indium or galinstan.

13. A pressure sensor according to any one of claims 1 to 10, wherein the core comprises an ionic liquid or an ionic hydrogel.

14. A method of manufacturing a pressure sensor for a pressure dressing that has one or more bandages each having a bandage width, the method comprising:

providing a mould having formed therein a template defining a non linear shape;

forming a sensor element by:

transferring the compressible insulating sheath to a substrate; and

15. A method according to claim 14, wherein the non-linear shape has at least one dimension that is less than half of the bandage width.

16. A method according to claim 14 or claim 15, wherein the compressible insulating sheath is filled with the electrically conductive liquid before arranging it in the mould.

17. A method according to claim 14 or claim 15, wherein the compressible insulating sheath is filled with the electrically conductive liquid after arranging it in the mould and after transferring it to the substrate.

18. A method according to any one of claims 14 to 17, wherein the non-linear shape is a spiral or a helix.

19. A method according to claim 18, wherein the spiral is substantially circular or substantially elliptical.

20. A method according to any one of claims 14 to 17, wherein the non-linear shape is a serpentine shape.

21. A method of monitoring pressure in a pressure dressing applied to a subject, comprising:

providing a pressure sensor according to any one of claims 1 to 13, for embedding between layers of the pressure dressing, or between the pressure dressing and the subject;

determining, from the first resistance measurement and the one or more subsequent resistance measurements, whether the pressure dressing requires adjustment.