WO2023018380A2

WO2023018380A2 - A flexible pressure sensor

Info

Publication number: WO2023018380A2
Application number: PCT/SG2022/050576
Authority: WO
Inventors: Kang DU; Jee Chin TEOH; Chwee Teck Lim
Original assignee: National University Of Singapore
Priority date: 2021-08-13
Filing date: 2022-08-12
Publication date: 2023-02-16
Also published as: WO2023018380A3

Abstract

A flexible pressure sensor for sensing pressure, and method of generating such are provided. The flexible pressure sensor comprising a film and a microtube sensor integrated into the film, the microtube sensor comprises a first microtube filled with a liquid metal.

Description

A FLEXIBLE PRESSURE SENSOR

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] International application PCT/SG2018/050076 (published as WO 2018/160135 Al on September 7, 2018), and United States Application Serial No. 16/487,983 (published as US 2020/0025699 Al on January 23, 2020), by the current applicant are incorporated herein by reference. The present application claims the benefit of a Singapore Application No. 10202108844S filed on August 13, 2021, the content of which is incorporated herein by reference.

TECHNICAL FIELD

[0002] The present disclosure relates to a device for pressure sensing. In particular, the present disclosure relates to a flexible and conformable silicone sensor for pressure sensing.

BACKGROUND

[0003] Existing pressure sensors are generally rigid thin film sensors with screen printed electrical ink. These sensors are made of non-stretchable constituent materials which limits the use of such sensors in pressure mapping on irregular surfaces as the stiff sensor usually does not conform well to a surface and instead creates its own shape. Therefore, existing pressure sensors are unable to measure pressure on irregular body surfaces. Even if the existing sensors are able to measure pressure on such surfaces, the resulting measurement is usually not precise in light of the inflexibility of the sensors.

[0004] Existing pressure sensors are not suitable for long-term continuous monitoring of compressible environments. For example, existing pressure sensors when placed on the skin of a human or animal would cause irritation due to their rigid nature. Existing pressure mapping devices usually exists in the form of mats, with flexible thin film sensors sandwiched between two pieces of fabric. The overall device thickness is close to 1 mm and the fabric breathability is generally low. This less conformable design is likely to cause discomfort and create stress points over time, this poses a serious threat to the already vulnerable skin region.

[0005] In view of the above limitations, therefore there is a need to provide a device for measuring pressure that overcomes or at least ameliorates the above limitations. SUMMARY

[0006] The disclosure relates to a flexible and conformable 3D printing-supported sensor for pressure sensing, which is not limited by surface regularity. The sensor may be a mesh sensor. The flexible pressure sensor may comprise a stretchable silicone mesh integrated with one or more elastomeric microtube sensors. The flexible pressure sensor may be stretchable and conformable and capable of measuring pressure on irregular body surfaces over long periods of time. Thus, the flexible pressure sensor may advantageously be placed directly onto or integrated into cushion and irregular regions such as seat edges, head rests and hand grips for various applications.

[0007] The entire device, including the microtube sensor, 3D printed film and even the connector and sealant, are all flexible and stretchable. The only solid constituent in the sensing device is the wire that links the sensor to a readout module. It can therefore be appreciated that this almost 100% flexible and stretchable design enables the sensing device to be stretched and deformed extensively to fit various measurement surfaces. The flexible connection also further enhances the device durability and stability.

[0008] The presently disclosed flexible pressure sensor may comprise nontoxic liquid state metal and elastomers, with superior flexibility and stretchability (elongation up to 100%). It is highly conformable and adapts excellently to any free form surfaces (not limited to single curved surfaces). Thus, in some embodiments, the sensor may be easily placed at high risk and irregularly shaped body surfaces to monitor the interfacial pressures, hence efficiently prevent the event of ulceration.

[0009] Furthermore, the presently disclosed flexible pressure sensor may be made of biocompatible skin-like material, which is excellent for skin contact and wearable applications. The flexible pressure sensor is ultrathin (less than 0.5 mm thickness) and porous. The material breathability enables this device to be applied close to the skin with little irritation and compensation on the user comfort. This unobstructive intervention with minimal presence enhances comfort and improves user experiences. These are critical factors that ensure user adherence, which is the key to achieve the goal of long-term monitoring.

[0010] Finally, the presently disclosed flexible pressure sensor is fully customizable. Specifications such as sensor size, quantity and location of sensing points, measurement resolution and pressure detection range, can be modified to suit various measurement and application requirements. The sensors can also be placed directly on top or integrated into the bed mattress, wheelchair seat cushion, back rest and any irregular regions such as seat edge, head rest and hand grip. Higher degree of customization onto prosthesis and residual limbs is also possible.

[0011] There is provided, according to an embodiment of the present disclosure, a flexible pressure sensor comprising: a film; and a microtube sensor integrated into the film, the microtube sensor comprises a first microtube filled with liquid metal. In some embodiments, the film may be made of silicone. In some embodiments, the film may comprise one or more openings. In some embodiments, the film may be a silicone mesh. In some embodiments, the microtube sensor may be woven through the one or more openings of the film. In some embodiments, the microtube sensor may be connected to a conductive wire at a connection point. In some embodiments, the connection point may be substantially surrounded by a second microtube. In some embodiments, the liquid metal may comprise a metal element or a metallic alloy.

[0012] Further, there is provided, according to an embodiment of the present disclosure, a method of manufacturing a flexible pressure sensor, the method comprising: providing a microtube sensor comprising a first microtube filled with liquid metal; and integrating the microtube sensor with a film. In some embodiments, integrating the microtube sensor with the film may further comprise forming the film around the microtube sensor. In some embodiments, integrating the microtube sensor with the film may comprise weaving the microtube sensor through openings in the film. In some embodiments, forming the film around the microtube sensor may comprise of 3D printing the film around the microtube sensor, such that openings are formed around the microtube sensor. In some embodiments, forming the film around the microtube sensor may comprise printing the film around the microtube sensor, such that a mesh is formed. In some embodiments, the film may be a silicone mesh. In some embodiments, the liquid metal may comprise a metal element or a metallic alloy. In some embodiments, providing the microtube sensor may further comprise connecting said microtube sensor to a conductive wire at a connection point. In some embodiments, connecting may further comprise sealing the connection point with a silicone glue. In some embodiments, after sealing the connection point, the method may further comprise encapsulating the sealed connection point with a second microtube. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order for the present disclosure to be better understood and for its practical applications to be appreciated, the following Figures are provided and referenced hereafter. It should be noted that the Figures are given as examples only and in no way limit the scope of the invention.

[0014] Figs. 1A and IB are photographs depicting a process of preparing a microtube sensor for incorporation into a mesh sensor, in accordance with embodiments of the present disclosure;

[0015] Fig. 1A is a photograph of a microtube sensor, in accordance with embodiments of the present disclosure;

[0016] Fig. IB is a photograph of a microtube sensor strengthened at a connection point between a liquid metal filled microtube and a conductive wire, in accordance with embodiments of the present disclosure;

[0017] Figs. 2A and 2B are photographs illustrating 3D-printed silicone meshes, in accordance with embodiments of the present disclosure;

[0018] Fig. 2A is a photograph showing various silicone meshes with different infill densities, in accordance with embodiments of the present disclosure;

[0019] Fig. 2B is a photograph showing a silicone mesh with a microtube sensor integrated, in accordance with embodiments of the present disclosure;

[0020] Figs. 3 A to 3C are photographs depicting a process of integrating a microtube sensor into a film to form a pressure sensor, in accordance with embodiments of the present disclosure;

[0021] Fig. 3 A is a photograph showing a microtube sensor attached to an eye of a needle, in accordance with embodiments of the present disclosure;

[0022] Fig. 3B is a photograph showing the weaving of microtube sensor through a film, in accordance with embodiments of the present disclosure;

[0023] Fig. 3C is a photograph showing a pressure sensor, in accordance with embodiments of the present disclosure; [0024] Figs. 4A to 4D are photographs illustrating the flexibility of a pressure sensor, in accordance with embodiments of the present disclosure;

[0025] Fig. 4A is a photograph showing a pressure sensor in an original state, in accordance with embodiments of the present disclosure;

[0026] Fig. 4B is a photograph showing a pressure sensor wrapped around a polygonal pyramid, in accordance with embodiments of the present disclosure;

[0027] Fig. 4C is a photograph showing a pressure sensor wrapped around a curved surface, in accordance with embodiments of the present disclosure;

[0028] Fig. 4D is a photograph showing a pressure sensor wrapped around an angular surface, in accordance with embodiments of the present disclosure;

[0029] Figs. 5 A to 5 C are photographs illustrating the conformability of a pressure sensor as compared to a prior art thin film sensor, in accordance with embodiments of the present disclosure;

[0030] Fig. 5A is a photograph showing a pressure sensor on a highly deformable foam material, in accordance with embodiments of the present disclosure;

[0031] Fig. 5B is a photograph showing the result of interfacial pressure measurement with a pressure sensor, in accordance with embodiments of the present disclosure;

[0032] Fig. 5C is a photograph showing the result of interfacial pressure measurement with a prior art thin film sensor, in accordance with embodiments of the present disclosure;

[0033] Figs. 6A and 6B illustrate the results of a cyclic tensile test carried out on pressure sensor 300, in accordance with embodiments of the present disclosure;

[0034] Fig. 6A illustrates the resistive response of a single cycle, in accordance with embodiments of the present disclosure;

[0035] Fig. 6B illustrates the resistive response of one hundred cycles, in accordance with embodiments of the present disclosure;

[0036] Figs. 7A and 7B illustrate the impact of compressive pressure on the resistance of a pressure sensor, in accordance with embodiments of the present disclosure; [0037] Fig. 7A illustrates the resistive response of a pressure sensor with a single strand of microtube sensor with the corresponding compressive pressure applied, in accordance with embodiments of the present disclosure;

[0038] Fig. 7B illustrates a close-up view of the small resistance change at low pressure range, in accordance with embodiments of the present disclosure;

[0039] Figs. 8A and 8B illustrate the results obtained from a stepwise compressive test, in accordance with embodiments of the present disclosure;

[0040] Fig. 8A illustrates the pressure, step increase in pressure, and corresponding resistance change of a cycle, with an inset showing a close-up of the small resistance change at low pressure ranges, in accordance with embodiments of the present disclosure;

[0041] Fig. 8B illustrates the resistive response throughout one hundred cycles of the stepwise compressive test, in accordance with embodiments of the present disclosure;

[0042] Figs. 9A and 9B illustrate the change of resistance when high and low pressure is exerted over an extended period of time, in accordance with embodiments of the present disclosure;

[0043] Fig. 9A illustrates the resistive response of a pressure sensor subjected to constant high pressure at 180 kPa over twelve hours, in accordance with embodiments of the present disclosure;

[0044] Fig. 9B illustrates the resistive response of a pressure sensor subjected to constant low pressure at 30 kPa over 7 hours, in accordance with embodiments of the present disclosure;

[0045] Figs. 10A and 10B illustrate the results of a cyclic compressive test with load up to 30 kPa, in accordance with embodiments of the present disclosure;

[0046] Fig. 10A illustrates the resistive response of a pressure sensor over a single cycle, in accordance with embodiments of the present disclosure; and

[0047] Fig. 10B illustrates the resisting response of a pressure sensor over 5000 cycles, in accordance with embodiments of the present disclosure. [0048] Identical or duplicate or equivalent or similar structures, elements, or parts that appear in one or more drawings are generally labelled with the same reference numeral, optionally with an additional letter or letters to distinguish between similar entities or variants of entities and may not be repeatedly labelled and/or described. References to previously presented elements are implied without necessarily further citing the drawing or description in which they appear.

DETAILED DESCRIPTION

[0049] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, modules, units and/or circuits have not been described in detail so as not to obscure the invention.

[0050] Dimensions of components and features shown in the figures are chosen for convenience or clarity of presentation and are not necessarily shown to scale or true perspective. For convenience or clarity, some elements or structures are not shown or shown only partially and/or with different perspective or from different point of views.

[0051] Although embodiments of the invention are not limited in this regard, the terms “plurality” and “a plurality” as used herein may include, for example, “multiple” or “two or more”. The terms “plurality” or “a plurality” may be used throughout the specification to describe two or more components, devices, elements, units, parameters, or the like. Unless explicitly stated, the method embodiments described herein are not constrained to a particular order or sequence. Additionally, some of the described method embodiments or elements thereof can occur or be performed simultaneously, at the same point in time, or concurrently. Unless otherwise indicated, use of the conjunction “or” as used herein is to be understood as inclusive (any or all of the stated options).

[0052] The term “microtube” as used herein means a tube having an outer diameter in the micrometer range, e.g., between about 1 (pm) micrometer and about 999 micrometers (pm), for example about 1 pm, about 5 pm, about 10 pm, about 50 pm, about 75 pm, about 80 pm, about 100 pm, about 150 pm, about 200 pm, about 500 pm, about 600 pm, about 800 pm or about 900 pm. [0053] The term “flexible” as used herein means capable of bending easily without breaking.

[0054] The term “polymer” as used herein means a substance that has a molecular structure consisting predominantly or entirely of a large number of similar units bonded together, e.g., many synthetic organic materials used as plastics and resins. The polymer will have at least one of the following properties: flexibility, stretchability, softness, and biocompatibility. Examples of polymers include but are not limited to silicone elastomer, ultraviolet sensitive polymer, polyurethane, polyimide, conductive polymer, conductive rubber, thermoplastic, and thermoset polymer.

[0055] The term “elastomer” as used herein means an elastomer, e.g., a rubber-like material, composed of silicone containing silicon together with carbon, hydrogen, and oxygen. Examples of elastomers include but are not limited to polydimethylsiloxane (PDMS), phenyl-vinyl silicone, methyl-siloxane, fluoro-siloxane, or platinum cured silicone rubber.

[0056] The term “liquid metal” as used herein means a metal element or a metal alloy that is a liquid at or near room temperature and that is conductive to electrical current. Examples include but are not limited to metals such as gallium (Ga) and liquid metallic alloys, such as Gallistan and eutectic gallium -indium (eGain). Other examples include conductive elements, such as carbon nanotubes, silver nanowires, metallic ink, and graphene.

[0057] The term “resistance” as used herein means an electrical quantity that measures how a device or material reduces the electric current flow through it. The electrical resistance of an electrical conductor is a measure of the difficulty of passing an electric current through that conductor. The resistance explains the relationship between voltage (amount of electrical pressure) and the current (flow of electricity) and is measured in units of ohms (E ).

[0058] The term "force-induced deformation" as used herein refers to a deformation of a material in response to or in reaction to application of a force to the material.

[0059] All numeric values herein can be modified by the term “about,” whether or not explicitly indicated. The term “about” generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value (i.e., having the same function or result). In some versions the term “about” refers to ± 10% of the stated value, in other versions the term “about” refers to ± 2% of the stated value. [0060] With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the disclosure. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the disclosure may be practiced.

[0061] Figs. 1A and IB are photographs depicting a process of preparing a microtube sensor for incorporation into a mesh sensor, in accordance with embodiments of the present disclosure. Fig. 1A is a photograph of a microtube sensor, and Fig. IB is a photograph of a microtube sensor strengthened at a connection point between a liquid metal filled microtube and a conductive wire, in accordance with embodiments of the present disclosure. According to some embodiments of the disclosure, the microtube sensor may be produced through the method previously disclosed by the applicant in International Application No. PCT/SG2018/050076 (Publication No. WO 2018/160135 Al published September 7, 2018), and United States Application Serial No. 16/487,983 (U.S. Patent Publication No. US 2020/0025699 Al published January 23, 2020). A microtube may be fabricated using a customized fabrication platform which comprises a cylindrical container to house premixed elastomer, a metal tube heater, and a step motor to draw wires through the cylindrical container and the metal tube heater. The elastomer is prepared and then poured into the cylindrical container. A nylon wire is then placed into the cylindrical container. The nylon wire is then coated with the elastomer as it is drawn up by the stepper motor. The elastomer coating may then be cured when the coated nylon wire passes through the metal tube heater chamber, which may be heated at 100°C. The wire may then be removed from the cured elastomer coating to form a microtube. This method can produce microtubes of various sizes as the inner diameter of the microtube may be adjusted by using nylon wires of varying thickness or diameter. The size of the microtube may be adjusted depending on the application of the microtube.

[0062] In some embodiments, the microtube sensor disclosed herein may have the property that a change in electrical resistance of the liquid-state conductive element is indicative of a force-induced deformation of the flexible microtube. Hence, in some embodiments, the sensing mechanism of the microtubular sensor is based on the deformation when exposed to external mechanical forces. When the sensor is compressed, the flexible microtube flattens and constricts, and the cross-sectional area decreases. Subsequently, the reduced volume and displacement of the liquid metal (for example eGain metallic alloy) at compressed region will result in an increase in its electrical resistance. It can be appreciated that the flexible microtube as disclosed herein may return to its original state when the pressure (for example compressive pressure) is removed.

[0063] In some embodiments, the microtube may be filled with liquid metal to form a liquid metal filled microtube 104. In some embodiments, the microtube may be vacuum filled with liquid metal to form a liquid metal filled microtube 104. To introduce the liquid metal into the microtube, one end of the microtube may be dipped into liquid metal at atmospheric pressure, while the other end may be connected to a valve linked to a vacuum pump. When the valve is opened, a pressure difference may be created between the two terminals. Atmospheric pressure may then push the liquid metal into the microtube, towards the other end of the microtube which was kept in vacuum state. The liquid metal filling process is almost instantaneous, and it may be easily scaled up to fill in longer and more individual microtubes in one go.

[0064] In some embodiments, each end of the liquid metal filled microtube 104 may be connected to a conductive wire 108 to form a microtube sensor 100 (see Fig. 1A). Connection point 112 is the part where liquid metal filled microtube 104 connects to conductive wire 108. The connection point 112 may be the weakest point of microtube sensor 100 and may be prone to rupture. To address this issue, the connection point 112 may be substantially sealed with silicone glue and protected or encapsulated by a small segment of microtube 116 with a larger internal diameter in some embodiments. In some embodiments, such small segment of microtube 116 is referred as a second microtube. In some embodiments, the second microtube substantially encapsulates or surrounds the connection point 112 or the sealed connection point 112. In some embodiments, the second microtube may strengthen the connection between the liquid metal fdled microtube 104 and the conductive wire 108. Hence, in such an embodiment, the microtube sensor may be termed as strengthened microtube sensor.

[0065] Figs. 2A and 2B are photographs illustrating 3D-printed silicone meshes, in accordance with embodiments of the present disclosure. Fig. 2A is a photograph showing various silicone meshes with different infill densities (for example 10%, 20%, 30%, 40% or 50%, this range is not limited by these definite values and it may be varied accordingly), and Fig. 2B is a photograph showing a silicone mesh with a microtube sensor integrated, in accordance with embodiments of the present disclosure. Fig. 2A describes the various infill densities (20%, 30%, or 40%) of the silicone meshes according to embodiments of the present disclosure. As used herein, the term “infill density” refers to the volume of the silicone materials occupying the silicone meshes, where the silicone meshes comprising the space occupied by the silicone materials and voids (or openings). Hence, it is to be understood that a higher value of infdl density means that more silicone materials are present in the silicone meshes, which may be represented by smaller size of openings of the meshes. According to some embodiments of the disclosure, microtube sensor 100 may be integrated into a fdm 200 to form a pressure sensor. As disclosed herein, the pressure sensor is a pressure sensorthat is substantially flexible. Film 200 may be a silicone mesh 200 with a grid-like pattern with openings. In some embodiments of the disclosure, film 200 may be made of other suitable materials for e.g. flexible and stretchable materials like polydimethylsiloxane (PDMS), nylon, polyurethane, thermoplastic polyurethane (TPU), rubber, or other similar materials. Additionally, the film 200 may have no openings, or with openings of various densities, dimensions, and arrangements. In some embodiments of the disclosure, when the film 200 is provided with openings, said openings may be of uniform or variable sizes, shapes and/ or patterns. The various aspects of film 200 may be changed and customised depending on the usage of the pressure sensor described in the present disclosure.

[0066] According to some embodiments, silicone mesh 200 may be produced using 3D printing. A mesh design may be generated on a commercial computer aided design (CAD) software, or any other design software, to generate a geometry model. This geometry model may be sliced using a slicing software compatible with a 3D printer. The 3D printer used may be a liquid additive manufacturing (LAM) 3D printer, or any other suitable 3D printer. For example, a LAM 3D printer may be used to displace liquid silicone rubber precisely onto a printing platform via a G-code program. Preferably, the printout from the 3D printer is heat cured upon completion of each print layer. Various infill densities may be used to suit various application requirements by adjusting the printing parameter. In some embodiments of the disclosure, silicone mesh 200a may have an infill density of 20%, silicone mesh 200b may have an infill density of 30%, and silicone mesh 200c may have an infill density of 40%. It can be appreciated that the infill density will increase with the decrease in total size of the openings. According to some embodiments of the disclosure, a microtube sensor 100 with 500 pm diameter may be integrated into a 40% infilled silicone mesh 200c (see Fig. 2B).

[0067] Figs. 3 A to 3C are photographs depicting a process of integrating a microtube sensor 100 into a film 200 to form a pressure sensor 300, in accordance with embodiments of the present disclosure. Fig. 3A is a photograph showing a microtube sensor 100 attached to an eye of a needle 304, Fig. 3B is a photograph showing the weaving of microtube sensor 100 through the film 200, and Fig. 3C is a photograph showing a pressure sensor 300, in accordance with embodiments of the present disclosure. According to some embodiments, a pressure sensor 300 comprises a microtube sensor 100 integrated into a film 200 which comprises a silicone mesh. The physical dimension of film 200 which is made of a silicone mesh, as well as the quantity, location, size, and design of the integrated microtube sensor 100 are customizable based on user preferences. According to some embodiments ofthe disclosure, microtube sensor 100 may be integrated into film 200 by manual weaving. Prior to weaving microtube sensor 100 into film 200, microtube sensor 100 is secured to a needle 304. In accordance with some embodiments of the disclosure, securing microtube sensor 100 to needle 304 is done by attaching the microtube sensor 100 to an eye of needle 304 (see Fig. 3A). In accordance with some embodiments, securing microtube sensor 100 to needle 304 is done by passing the microtube sensor 100 through the eye of needle 304 (not shown). Microtube sensor 100 may then be integrated into film 200 by weaving needle 304 with the secured or attached microtube sensor 100 through the mesh holes or openings of film 200 (see Fig. 3B). In some embodiments, needle 304 may be used to create openings on film 200 to weave microtube sensor 100 through film 200. Once the microtube sensor 100 is integrated into film 200, a pressure sensor 300 is formed (see Fig. 3C).

[0068] In accordance with some embodiments ofthe disclosure, microtube sensor 100 may be integrated into film 200 through direct printing (see Fig. 2B). With direct printing, microtube sensor 100 is first placed on a thin plastic substrate and secured with double-sided tape. The microtube sensor 100 secured on the substrate is then positioned on the 3D printing platform. Based on the desired mesh design, liquid silicone rubber is displaced directly from the nozzle onto the thin substrate which is lined up with microtube sensor 100. Curing will cause the film 200 and microtube sensors 100 bonded firmly through the film 200.

[0069] In accordance with some embodiments of the disclosure, pressure sensor 300 may advantageously be adapted for pressure-sensing wearable or measurement devices to detect localized contact pressure on high-risk body surfaces, such as irregularly shaped bony prominences and residual limbs of amputated extremities, which may improve the management and prevention of pressure ulceration in the vulnerable community that are prone to pressure sores. In accordance with some embodiments of the disclosure, pressure sensor 300 may be adapted for pressure -sensing devices embedded in mattresses, seat cushions, garments and etc. to monitor user’s physical state (i.e., restlessness) and vital sign (i.e., breathing pattern), which may assist bedridden users and infants who are not able to speak for themselves. In accordance with some embodiments of the disclosure, pressure sensor 300 may be adapted and included in remote controllers for gaming and robotics, wearables, and accessories, which may enhance the gaming and user experience. In accordance with some embodiments of the disclosure, pressure sensor 300 maybe adapted for motion capture for biomechanics and sports purposes, allowing kinematics (strain and range of motion) and kinetics (pressure) data to be extracted from the user’s actions with minimal intervention. In accordance with some embodiments of the disclosure, pressure sensor 300 may be adapted to be used in relation to any compressible environment or surface, or where in a compressible environment a correct measurement of the surface compressed or which force is applied is desired.

EXAMPLES

[0070] Several experiments were carried out to quantify the characteristics of the pressure sensor 300. For the experiments carried out, pressure sensor 300 was made of 40% infilled a silicone mesh type film 200, manually weaved with a single strand of 500 pm diameter microtube sensor 100 as shown in Fig. 2B, in accordance with some embodiments of the disclosure.

[0071] Figs. 4A to 4D are photographs illustrating the flexibility of a pressure sensor 300, in accordance with embodiments of the present disclosure. Fig. 4A is a photograph showing a pressure sensor in an original state, Fig. 4B is a photograph showing a pressure sensor 300 wrapped around a polygonal pyramid, Fig. 4C is a photograph showing a pressure sensor 300 wrapped around a curved surface, and Fig. 4D is a photograph showing a pressure sensor 300 wrapped around an angular surface, in accordance with embodiments of the present disclosure. During pressure measurement, pressure sensor 300 may be placed directly onto a surface or embedded into a region of interest. As illustrated in Figs. 4B to 4D, pressure sensor 300 advantageously conforms well to different surfaces, including an inclined or flat planar surface (see Fig. 4B), a curved contour (see Fig. 4C), and a sharp edge (see Fig. 4D).

[0072] Figs. 5A to 5C are photographs illustrating the conformability of a pressure sensor 300 as compared to a rigid thin film sensor, in accordance with embodiments of the present disclosure. Fig. 5A is a photograph showing pressure sensor 300 on a highly deformable foam material, Fig. 5B is a photograph showing the result of interfacial pressure measurement with pressure sensor 300, and Fig. 5C is a photograph showing the result of interfacial pressure measurement with a rigid thin film sensor 504, in accordance with embodiments of the present disclosure. To illustrate the conformability of pressure sensor 300, interfacial pressure measurement was carried out. Pressure sensor 300 and a rigid thin film sensor 504 were placed onto a piece of highly deformable foam material 508. A sharp-edged indenter 512 was then used to apply vertical force onto pressure sensor 300 (see Fig. 5B) or rigid thin film sensor 504 (see Fig. 5C). As illustrated in Fig. 5B, pressure sensor 300 advantageously conforms well to the contour of the deformed surface. On the other hand, as illustrated in Fig. 5C, rigid thin film sensor 504 created its own shape and altered the deformation profile.

[0073] The stretchability and consistency of pressure sensor 300 to stretching was analysed by carrying out a cyclic tensile test. Figs. 6A and 6B illustrate the results of a cyclic tensile test carried out on pressure sensor 300, in accordance with embodiments of the present disclosure. Fig. 6A illustrates the resistive response of a single cycle, and Fig. 6B illustrates the resistive response of one hundred cycles, in accordance with embodiments of the present disclosure. As illustrated in Figs. 6A and 6B, pressure sensor 300 may easily stretch up to 100% of its original length. It is to be understood that the pressure sensor 300 may be stretch or extended by 20%, 30%, 50%, 80% or 100% of its original length. The results obtained from the cyclic tensile test show that pressure sensor 300 is stretchable and consistent. The strong friction between the two components (microtube and silicone mesh) was sufficient to hold the microtube sensor 100 firmly in place during the measurement. No significant slipping of microtube sensor 100 within the silicone mesh 200 was observed in all tensile and compressive tests conducted.

[0074] Sensor resistance changes when a compressive pressure applied. Therefore, pressure acting on a sensor may be evaluated based on its resistive response. Figs. 7A and 7B illustrate the impact of compressive pressure on the resistance of pressure sensor 300, in accordance with embodiments of the present disclosure. Fig. 7A illustrates the resistive response of pressure sensor 300 with a single strand of microtube sensor 100 with the corresponding compressive pressure applied, and Fig. 7B illustrates a close-up view of the small resistance change at low pressure range, in accordance with embodiments of the present disclosure. As illustrated in Fig. 7A, resistance and pressure are positively associated. However, the results show that the relationship is not a perfect linear line, and that the resistive behaviour may be subdivided into two states. At the initial stage, as the pressure applied was less than the threshold pressure (about 50 kPa in this case), sensor resistance changed slowly in very small quantity (see Fig. 7B). It was only after the threshold value was exceeded that resistance changed linearly and significantly as the pressure exerted. It can be appreciated that the threshold pressure of the pressure sensor disclosed herein may be about 30 kPa, 40 kPa, 50 kPa, 60 kPa, 80 kPa or 100 kPa. In some embodiments, the pressure sensor may detect and measure the pressure accurately in the linear region of the curve described in Fig. 7A above.

[0075] The minimum detection limit was investigated by subjecting pressure sensor 300 to a stepwise compressive test with step pressures ranging from 20 kPa to 100 kPa. Figs. 8A and 8B illustrate the results obtained from a stepwise compressive test, in accordance with embodiments of the present disclosure. Fig. 8A illustrates the pressure, step increase in pressure, and corresponding resistance change of a cycle, with an inset showing a close-up of the small resistance change at low pressure ranges, and Fig. 8B illustrates the resistive response throughout one hundred cycles of the stepwise compressive test, in accordance with embodiments of the present disclosure. As illustrated in Fig. 8A, the change of resistance was only significant when the applied pressure exceeded 50 kPa. In other words, for pressure sensor 300, the minimum pressure detection limit was 50kPa. The sensor response remained consistent throughout the 100 cycles as depicted in Fig. 8B. It is to be understood that the pressure detection limit of the pressure sensor disclosed herein may be about 30 kPa, 40 kPa, 50 kPa, 60 kPa, 80 kPa or 100 kPa.

[0076] The ability of pressure sensor 300 to withstand pressure over extended periods of time was investigated using a holding test. Figs. 9A and 9B illustrate the change of resistance when high and low pressure is exerted over an extended period of time, in accordance with embodiments of the present disclosure. Fig. 9A illustrates the resistive response of pressure sensor 300 subjected to constant high pressure at 180 kPa over 12 hours, and Fig. 9B illustrates the resistive response of a pressure sensor subjected to constant low pressure at 30 kPa over 7 hours, in accordance with embodiments of the present disclosure. As illustrated in Figs. 9A and 9B, pressure sensor 300 advantageously responded well to both high and low pressures sustained over long periods of time.

[0077] Long-term performance of pressure sensor 300 was investigated by indenting pressure sensor 300 up to 30 kPa for 5000 cycles. A low pressure of 30 kPa was selected to examine the ability of pressure sensor 300 to detect small pressures well below the detection limit of the pressure sensor 300 (50 kPa in this case). In another exemplary embodiment, if the detection limit of the pressure sensor 300 is 30 kPa, a lower pressure such as 10 kPa may be used to examine the ability of pressure sensor 300 to detect small pressures. Figs. 10A and 10B illustrate the results of a cyclic compressive test with load up to 30 kPa, in accordance with embodiments of the present disclosure. Fig. 10A illustrates the resistive response of pressure sensor 300 over a single cycle, and Fig. 10B illustrates the resisting response of pressure sensor 300 over 5000 cycles, in accordance with embodiments of the present disclosure. As can be seen from Figs. 10A and 10B, pressure sensor 300 has a consistent resistive response over small pressure cycling.

[0078] It should be appreciated that the above-described methods may be varied in many ways, including omitting or adding steps, changing the order of steps and the type of devices used. It should be appreciated that different features may be combined in different ways. In particular, not all the features shown above in a particular embodiment are necessary in every embodiment of the disclosure. Further combinations of the above features are also considered to be within the scope of some embodiments of the disclosure.

[0079] Those skilled in the art will appreciate that the invention described herein in susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within the spirit and scope. The invention also includes all of the steps, features, compositions, and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of said steps or features.

[0080] Throughout this specification and the statements which follow, 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.

[0081] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

[0082] It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims, which follow.

Claims

1. A flexible pressure sensor comprising: a film; and a microtube sensor integrated into the film, the microtube sensor comprises a first microtube filled with a liquid metal.

2. The flexible pressure sensor of claim 1, wherein the film is made of silicone.

3. The flexible pressure sensor of claim 1 or 2, wherein the film comprises one or more openings.

4. The flexible pressure sensor of claim 3, wherein the microtube sensor is woven through the one or more openings in the film.

5. The flexible pressure sensor of any of claims 1 to 4, wherein the film is a silicone mesh.

6. The flexible pressure sensor of any of claims 1 to 5, wherein the microtube sensor is connected to a conductive wire at a connection point.

7. The flexible pressure sensor of claim 6, wherein the connection point is substantially surrounded by a second microtube.

8. The flexible pressure sensor of claim 1, wherein the liquid metal comprising a metal element or a metallic alloy.

9. A method of manufacturing a flexible pressure sensor, the method comprising:

(i) providing a microtube sensor comprising a first microtube filled with a liquid metal; and

(ii) integrating the microtube sensor with a film to form said flexible pressure sensor.

10. The method of claim 9, wherein integrating the microtube sensor with the film further comprises forming the film around the microtube sensor.

11. The method of claim 9, wherein integrating the microtube sensor with the film comprises weaving the microtube sensor through one or more openings in the film.

12. The method of claim 10, wherein forming the film around the microtube sensor comprises 3D printing the film around the microtube sensor, such that openings are formed around the microtube sensor.

13. The method of claim 10, wherein forming the film around the microtube sensor comprises 3D printing the film around the microtube sensor, such that a mesh is formed.

14. The method of claim 9, wherein the film is a silicone mesh.

15. The method of claim 9, wherein the liquid metal comprising a metal element or a metallic alloy.

16. The method of claim 9, wherein providing the microtube sensor further comprises connecting said microtube sensor to a conductive wire at a connection point.

17. The method of claim 16, wherein connecting further comprises sealing the connection point with a silicone glue.

18. The method of claim 17, wherein after sealing the connection point, encapsulating the sealed connection point with a second microtube.