WO2020162836A1

WO2020162836A1 - Low hysteresis and flexible pressure sensitive composite

Info

Publication number: WO2020162836A1
Application number: PCT/SG2020/050061
Authority: WO
Inventors: Chee Keong Tee; Haicheng YAO; Weidong Yang; Yu Jun TAN
Original assignee: National University Of Singapore
Priority date: 2019-02-08
Filing date: 2020-02-07
Publication date: 2020-08-13
Also published as: EP3921616A1; EP3921616B1; EP3921616A4; US20220128420A1; SG11202108467YA; EP3921616C0

Abstract

The invention relates to a sensing structure and a method of fabricating a sensing structure for a compressive-type pressure sensor. The method comprises the steps of providing an elastic micropatterned substrate defining a plurality of 3-dimensional microstructures, each microstructure comprising a tip portion pointing away from the substrate in a first direction; forming a conductive film on the elastic micropatterned substrate such that the 3-dimensional microstructures are substantially covered by the conductive film; and forming cracks in the conductive film in areas on 3-dimensional microstructures by applying load on the conductive film coated substrate while cushioning with a thin layer of soft material between a rigid substrate and the conductive film coated substrate, wherein the cracks comprises substantially annular cracks in areas on the 3-dimensional microstructures and the conductive film is free from cracks at the tip portions of the 3-dimensional microstructures.

Description

LOW HYSTERESIS AND FLEXIBLE PRESSURE SENSITIVE COMPOSITE

FIELD OF INVENTION

The present invention relates broadly to compressive-type sensors, sensing structures for a compressive-type pressure sensor, methods of fabricating a sensing structure for a compressive-type pressure sensor, and methods of fabricating a compressive-type pressure sensor.

BACKGROUND

Any mention and/or discussion of prior art throughout the specification should not be considered, in any way, as an admission that this prior art is well known or forms part of common general knowledge in the field.

Kang et al. [7] developed a crack-based strain sensor by sputtering Pt on a plain polymeric film. When stretching is applied, parallel cracks are formed on the surface of the deposited Pt layer, which increases the resistance of the strain sensor. The resistance also decreases when the strain is released. However, the sensors described in Kang et al. can only be used to detect tensile strains.

Embodiments of the present invention seek to address at least one of the above problems.

SUMMARY

In accordance with a first aspect of the present invention there is provided a sensing structure for a compressive-type pressure sensor, the sensing structure comprising:

an elastic micropattemed substrate defining a plurality of 3-dimensional microstructures, each micro structure comprising a tip portion pointing away from the substrate in a first direction; and

a conductive film on the elastic micropattemed substrate such that the 3-dimensional microstmctures are substantially covered by the conductive film, wherein the conductive film comprises cracks in areas on 3-dimensional microstructures.

In accordance with a second aspect of the present invention, there is provided a compressive pressure sensor exhibiting a ratio of peak sensitivity to electrical hysteresis of more than about 1E9 WRh ¹·% ¹ In accordance with a third aspect of the present invention there is provided a method of fabricating a sensing structure for a compressive-type pressure sensor, the method comprising the steps of:

providing an elastic micropatterned substrate defining a plurality of 3-dimensional microstructures, each micro structure comprising a tip portion pointing away from the substrate in a first direction;

forming a conductive film on the elastic micropatterned substrate such that the 3-dimensional microstructures are substantially covered by the conductive film; and

forming cracks in the conductive film in areas on 3 -dimensional microstructures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

Figure 1 shows schematic drawings illustrating a method of fabricating a sensing structure, according to an example embodiment.

Figure 2A shows a microscope image of a single 3-dimensional micro structure covered with a conductive film with annular cracks, according to an example embodiment.

Figure 2B shows ab SEM image of a single 3-dimensional micro structure covered with a conductive film with annular cracks, according to an example embodiment.

Figure 3A shows a schematic drawing illustrating an analytical model of the conductive path during loading of a sensor according to an example embodiment.

Figure 3B shows a graph illustrating pressure-induced electrical performance of a sensor according to an example embodiment, compared with a traditional PEDOT:PSS coated micropyramids sensor.

Figure 4 shows a graph illustrating the pressure-induced sensitivity of a sensor according to an example embodiment.

Figure 5 A shows a graph illustrating the result of the detection of a small pressure (15Pa), using a sensor according to an example embodiments, together with a schematic of the circuit design.

Figure 5B shows a detail of the groan of Figure 5A illustrating fast response time (1.4ms) is exhibited when the pressure is applied on the sensor according to an example embodiment. Figure 6 shows illustrating the detection of small pressure (5Pa, lOPa and 15Pa) under a load of 7kPa using a sensor according to an example embodiment, together with photographs showing the experimental set-up.

Figure 7A shows a graph illustrating cyclic testing a sensor according to an example embodiments, specifically repeat of 10 cycles.

Figure 7B shows a graph illustrating cyclic testing a sensor according to an example embodiments, specifically maximum conductivity change over 10,000 cycles.

Figure 8A shows a graph illustrating cyclic testing of a sensor according to an example embodiment with a rigid electrode, specifically pressure response of the sensor after 1, 10, 100, 1000 and 10000 cycles of compression.

Figure 8B shows a graph illustrating cyclic testing of a sensor according to an example embodiment with a flexible electrode, specifically pressure response of the sensor after 1, 10, 100, 1000 and 10000 cycles of compression.

Figure 9 shows a graph illustrating comparable pressure responses of a sensor according to an example embodiment under different compressive strain rates.

Figure 10A shows schematic drawings illustrating localized pulse tracing and PWV measurement using a sensor according to an example embodiment, specifically a schematic of localized pulse tracing through multi-channel electrodes.

Figure 10B shows a graph illustrating results of pulse tracing at radial artery using a sensor according to an example embodiment.

Figure 11 shows a schematic drawing illustrating simultaneous pulse measurement at carotid artery and radial artery using sensors according to example embodiments.

Figure 12 shows graphs illustrating pulse measurement and tracing from carotid artery to radial artery using sensors according to example embodiments together with a zoom-in pulse wave at radial artery.

Figure 13 shows a graph illustrating the numerical method of computing sensitivity in terms of resistance-pressure curve.

Figure 14 shows a graph illustrating a comparison of sensitivity-pressure curve under loading/unloading of a sensor according to an example embodiment and existing sensors.

Figure 15 shows a graph illustrating a comparison of sensitivity-pressure curve under loading path of a sensor according to an example embodiment and existing sensors.

Figure 16 shows a graph illustrating the numerical method of calculating electromechanical hysteresis. Figure 17 shows a graph illustrating a comparison of electromechanical hysteresis under loading path of a sensor according to an example embodiment and existing sensors.

Figure 18 shows a graph illustrating a comparison of peak Sensitivity and Hysteresis values of a sensor according to an example embodiment and existing sensors.

Figure 19 shows a graph illustrating a comparison of SpHe value of a sensor according to an example embodiment and existing sensors.

Figure 20 shows a flow-chart 2000 illustrating a method of fabricating a sensing structure for a compressive-type pressure sensor.

DETAILED DESCRIPTION

Embodiments of the present invention can provide a piezo-resistive tactile sensor, which is also referred to herein as TRACE (Tactile Resistive Annularly Cracked E-skin) according to example embodiments, by designing nano-scaled metallic regular annular cracks on a polymeric micro-structured array. Tactile sensors according to example embodiment can advantageously achieve a high sensitivity (more than 10⁷ ohm/kPa) in pressure sensing. Tactile sensors according to example embodiments are advantageously also highly sensitive over a wide pressure range, which is a property not demonstrated by current tactile sensors. In addition, due to the ultra-thin metallic coating used in example embodiments, the sensor shows low hysteresis (2.37%) where the changes of the electrical signals are advantageously independent of the direction of pressure change. Various organic or inorganic thin conductive materials can be used on the polymeric micro- structured arrays according to various embodiments of the present invention, and good testing results were obtained. The facile fabrication method according to example embodiments advantageously enables a low-cost and large-area fabrication of the sensor. TRACE sensor according to example embodiments can be used to trace the arterial pulse. A pulse wave velocity (PWV) of 11 m/s was attained from the carotid artery to radial artery from measurements using an example embodiment, which is comparable to the standard measurement result. By designing the flexible electrodes with multiple channels on a TRACE sensor according to an example embodiment, the localized PWV was traceable only at the radial artery for the first time (~ 0.6 m/s), to the best of the inventors’ knowledge. These excellent performance capabilities make TRACE sensors according to example embodiments of the present invention promising in numerous e-skin applications, e.g. smart home devices, health monitoring system and robotics.

According to example embodiments of the present invention, a technique to create regular cracks on a 3D surface is provided, which advantageously results in the ability to sense both in plane and out of plane forces.

Sensors with a patterned 3D microstructure according to example embodiments have superior sensitivity compared to the planar sensor structures. Designable cracks on 3D microstructure for the detection of compressive tactile information in a wide range according to example embodiments is crucial for considerable sensory applications.

Fabrication Method according to an example embodiment:

The fabrication flow of TRACE sensor according to an example embodiment comprises, generally, of three major steps (see Figure 1). First, micropattemed (here micropyramidal in this embodiment by way of example, not limitation) elastomer 100 is cast on a substrate 102 (such as, but not limited to, Si or PET) by using, by way of example, not limitation, a Si mould 104 with the inverse micropyramidal pattern. After peeling off the micropattemed elastomer 100 from the mould 104, the micropattemed elastomer 100 is surface treated and, in this embodiment by way of example, not limitation) platinum (Pt) 106 is sputtered onto the micropattemed surface of the elastomer 100 to form a thin conductive layer (~ 30 nm). Then, load 107 is applied on the Pt-coated elastomer 106/100 while cushioning with a thin layer 108 of soft material between a rigid substrate 110 and the Pt-coated elastomer 106/100 supported, for example, on a rigid base (not shown) for regular cracks generation. The soft material layer 108 was found to advantageously prevent the cracks from initiating from the tip of the pyramids. Cracks on the tip may limit the sensor response to sudden drop in resistance when pressure increases, instead of the preferable gradual resistance change with pressure. Also, the soft material layer 108 cushioning was found to advantageously distribute the applied load uniformly over the whole surface of the micropyramids for the generation of annular cracks. Images 200, 202 in Figures 2A and 2B show laser microscope and SEM images, respectively, of an individual micropyramid of a TRACE sensor according to an example embodiment, with the regular peripheral cracks e.g. 203 clearly visible, as well as the absence of cracks at the tip. The elastomer layer with the microstmctures 106/100 remains adhered to the Si/PET/any other substrate 102 in preferred example embodiments.

Results and Discussion of example embodiments:

With reference to Figure 3A, to obtain the resistive pressure response, patterned electrodes e.g. 300 are assembled on the Pt side of micropattemed Pt/elastomer structure 302 of a TRACE sensor 304 according to an example embodiment. When pressure is applied on the sensor 304, conductive paths e.g. 306 can be generated from one electrode e.g. 300 to the other e.g. 308 through the reconnected cracks e.g. 310 of Pt layer 312. According to example embodiments, the finite element method is used to analyse the stress field of the 3D cracked microstmcture, to optimize the sensing performance by tuning the nanoscale crack morphology. Finally, we propose the optimal design of sensor structure with controllable regular annular cracks on polymeric substrate according to a preferred embodiment which has regular cracks arranged in a staggered fashion along an incline plane.

To characterize the electrical performance of the TRACE sensor 304 according to an example embodiment, Au/Ti layers were deposited on a piece of ultra-flat glass substrate 314 as the electrodes e.g. 300, 308. When applying 0-20 kPa on the TRACE sensor 304 (strain rate: 5 pm/s), the registered resistance continuously and reversibly decreases from 10⁸ ohms to 10³ ohms as shown in curves 316, 317 in Figure 3B. The sensor 304 exhibited high sensitivity and wide sensitive range when comparing with other piezo-resistive sensors [8]. A control of such a piezo-resistive sensor, i.e. PEDOT:PSS-coated micropyramidal sensor, was fabricated and compared, curves 318, 319 with the TRACE sensor, curves 316, 317. As can be seen from Figure 3B, the TRACE sensor (curves 316, 317) demonstrated a much higher sensitivity and wider sensitive range as compared to the control sensor (curves 318, 319). Specifically, the control (curves 318, 319) shows a good sensitivity only under low loadings, with a rapid decline of sensitivity when the pressure applied exceeds 2 kPa. The TRACE sensor (curves 316, 317) also shows a low hysteresis (2.37%) in pressure response between loading and unloading. In contrast, the PEDOT:PSS-coated micropyramidal control sensor (curves 318, 319) shows a high hysteresis (79%). The low hysteresis of the TRACE sensor (curves 316, 317) advantageously enables a high accuracy of the sensor according to example embodiments.

Sensitivity of the pressure sensor 304 according to an example embodiment is computed from the differential of the pressure -resistance curves 316, 317, and the resulting forward and backward sensitivity curves 400, 401 are shown in Figure 4. The sensor 304 has a high sensitivity of 10⁹ ohm/kPa. With 20 kPa of load applied, sensitivity remains reasonably high at 10² ohm/kPa. The result indicates both high sensitivity and wide- sensitive range are advantageously achievable according to example embodiments. Loading (curve 400) and unloading (curve 401) showed comparable pressure induced sensitivity change. In addition, sensitivity at different loads remained consistent over several cycles.

Figures 5 A and 5B, the latter being a zoomed portion of the former, show that a small pressure of 15 Pa can be detected by the TRACE sensor 500 according to an example embodiment. Upon applying the load, a fast response time of 1.4 ms is attained. It was found that the detection of smaller pressure was dependent on the split of cracks. Generally, it was found that a threshold force was applicable to make the split cracks sufficiently reconnected and form the conductive path. It was found that the resistance change is also contributed by the contact area increase of the metal film with the conductive electrodes compressing on the cracked metal film 3D structures.

It was also found that the TRACE sensor according to an example embodiment can advantageously maintain its sensitivity even under a high load, i.e. it can detect small pressure even when a high load has already been applied. For example, with reference to Figure 6, the small pressures of 5 Pa, 10 Pa and 15 Pa are detected by the TRACE sensor 600 according to an example embodiment under a load of 7 kPa by way of pre-loading, as shown in curves 601-603 in Figure 6. This advantage is evidently superior when comparing with other pressure sensors that are sensitive only in low-load range.

The TRACE sensor according to an example embodiment was tested to about 10,000 compression and release cycles when pressure of 20 kPa was applied. With reference to Figures 7 A and 7B, the sensor according to an example embodiment was tested with cyclic loads. Here, the conductance of the sensor is defined as s (o=l/R), which will increase with loads. When the pressure of 20 kPa was applied on the sensor at the specific strain rate (10 mih/s), s was acquired to quantify the cyclic pressure response. o_max refers to the conductance when the load is 20 kPa. The TRACE sensor according to an example embodiment exhibited reversible behavior and comparable o_max over multiple cycles, see Figure 7A. The o_max was further compared before and after 10,000 times of cyclic loads. Before 10,000 cycles of compression, the value remained stable and continuous over 100 cycles of measurement (compare Figure 7A). After 10,000 cycles, o_max showed a slight decline but remained steady over another 100 cycles of measurement 700, see Figure 7B. The slight decline is believed to be because of the crack propagation during cyclic loads, which has been discussed in previous research work [9]. It is believed that over sequential compression cycles, there will be some change in the crack configuration in example embodiments, and hence a preferred embodiment may have a less rigid conductor applied on the cracked metal film, e.g. conductive textiles/cloth.

To study the crack propagation during cyclic loads, pressure responses of a TRACE sensor according to an example embodiment were measured up to 10,000 times of cyclic compression (see Figure 8A). Under the same pressure, resistance increased during cyclic loads and the pressure response got to shift, compare e.g. curve 801, 1^st forward cycle, and curve 802, 10000^th forward cycle. This is believed to be induced by the crack propagation, during which the increasing number of cracks can be regarded as the increasing number of resistors in series connection. Besides the increase of resistance, propagated cracks are also believed to escalate the threshold force needed to form the conductive path. This could be observed from the pressure response of the TRACE sensor according to an example embodiment after 10,000 times of compression. To ameliorate the issue caused by crack propagation, flexible electrodes were present according to another example embodiment. Conductive cloth was selected as electrodes in this example embodiment. After cyclic loads on the sensor, the increase in resistance and the shift of pressure response got attenuated (compare e.g. curve 811, 1^st forward cycle, and curve 812, 10000^th forward cycle in Figure 8B). This provides a promising solution for the development of a more reliable sensor based on metal cracks according to example embodiments flexible electrodes can, for example, be fabricated by patterning conductive cloth, or depositing conductive polymer (e.g. CNTs, silver nanowires) or depositing thin metal film on polymeric substrates (e.g. PDMS, Ecoflex) according to example embodiments, noting that the flexible electrodes can also have a low flexural modulus similar to or less than the modulus of the sensor layer and metal layers. The TRACE sensor according to example embodiments advantageously displays a strain rate independent performance, which is an important characteristic of a pressure sensor. Specifically, when pressure was applied on the sensor at different strain rates (2 pm/s, 5 pm/s and 10 pm/s), consistent resistance versus pressure curves were attained, see curves 901-903 in Figure 9.

Accordingly, the TRACE sensor according to example embodiments exhibits high sensitivity, wide- sensitive range and low hysteresis, indicating the good potential in the application of wearable devices, by way of example, not limitation. To demonstrate its feasibility in health monitoring, pulse measurement was carried out by designing the TRACE sensor 1000 according to an example embodiment on flexible electrodes. With reference to Figure 10A, gold was deposited on PET 1001 as flexible electrodes e.g. 1002 and they were assembled with a micropatterned Pt layer/elastomer structure (compare 302 in Figure 3A) to form a TRACE sensor according to an example embodiment. Pulse waves e.g. 1004 could be detected at radial artery, indicated at numeral 1100 in Figure 11, and carotid artery, indicated at numeral 1102 in Figure 11, simultaneously using two TRACE sensors 1104a, b.

By tracing the pulse transit time (PTT) through pulses at these two sites 1100, 1102, the pulse wave velocity (PWV) can be acquired by dividing the distance by PTT (PWV=Distance/PTT). The distance refers to the length difference between heart- to-radial artery and heart-to-carotid artery (-75 cm). Pulse waves detected at radial artery and carotid artery are displayed in curves 1200 and 1202, respectively, in Figure 12. Various waves e.g. 1204, 1206 and dicrotic notch 1208 of radial pulse could be clearly distinguished, which was attributed to the fast response time of the TRACE sensor according to an example embodiment. A PTT of 67 ms was traced through a foot-to-foot recognition of pulses and a PWV of -11 m/s was acquired from carotid artery and radial artery.

Returning to Figure 10A, for the first time measuring the localized PWV only at the radial artery was demonstrated, to the best of the inventors’ knowledge, through the design of multi-channel electrodes in the TRACE sensor 1000 according to an example embodiment. Specifically, a first measurements channel extends between electrodes 1002 and 1006, and a second measurement channel extends between electrodes 1002 and 1008. When the pulse passes through the respective electrode pairs, distinct waves from the two different channels were attained and their time difference was recognized as AT between the PTT measurement pulses 1010 and 1012 from the respective channels, as shown in Figure 10A. The overlaid detected pulse curves 1014, 1016 at the two sites of the radial artery are shown in Figure 10B. The disparity of the configurations of these two pulses is attributed to the different amplitude and timing of reflected waves at the different sites. However, both pulse curves 1014 and 1016 exhibited a recognizable pulse peak, e.g. 1018, 1020. The PTT was obtained through a peak-to-peak as -7 ms, and the localized PWV was acquired as -0.6 m/s.

Benchmarking sensor hysteresis versus sensitivity for TRACE sensors according to example embodiments:

Current sensors technology can be evaluated by having an index that incorporates both sensitivity and hysteresis.

Here, we introduce an index value, termed SpHe, which can be used to determine ratio of peak sensitivity (Sp) to electrical hysteresis (He) of a compressive type pressure sensor.

It is shown that a TRACE sensor according to an example embodiment can achieve the highest SpHe value compared to current published sensors. It is noted that many literature papers do not include hysteresis data. Here, a TRACE sensor according to an example embodiment is compared to publications with published hysteresis data available to the present inventors. Benchmark methodology for TRACE sensors according to example embodiments:

1. Sensitivity

In order to compare the sensitive property, one unifying computing methodology is presented here to evaluate piezoresistive pressure sensors in references. The original data of resistance/current and correspondent pressure was collected from scientific papers. Then, all data was transferred into the presented unified indicators and units, namely, resistance- pressure curve, where W for resistance and kPa for pressure. The definition of sensitivity is generally considered to be the change rate of resistance with the increasing pressure applied on sensors. In the benchmark methodology presented here, the numerical method such as centered difference formula was employed to obtain the sensitivity curve from the relation of resistance and pressure. The derivative at any one resistance data point was calculated by selecting the average of the slopes between that data point and its two neighbor data points. Figure 13 illustrates the schematic of numerically calculating sensitivity from resistance- pressure curve 1300. The derivative function applied to discrete data points e.g. S_k can be expressed as:

where SF, SB and RF, RB denote forward and backward sensitivity and resistance, respectively, P is applied pressure, and R_k is any one point of resistance. Figure 14 and Figure 15 show the sensitivity -pressure curves for loading/unloading and only forward loading conditions, respectively, of a TRACE sensor (curves 1401, 1402, 1501) according to an example embodiment as compared to published sensors.

2. Electromechanical hysteresis

The electromechanical hysteresis is one critical factor of pressure sensors. First, the area difference between resistance-pressure curves of loading/unloading paths are calculated. Then the hysteresis is considered as the ratio of the area difference against loading curve. The hysteresis data is numerically calculated by trapezoid approximation integration method. Figure 16 depicts the schematic of the algorithm for getting the hysteresis. The electromechanical [hysteresis indicating the ability to maintain electric performance after loading/unloading, is expressed as:

and

where H is the indicator of electric hysteresis, AF, AB denote integration area of resistance- pressure curve for forward and backward path, and P is pressure. Figure 17 gives hysteresis data for various piezoresistive pressure sensors, including a TRACE sensor according to an example embodiment.

Benchmarking results using index value SpHe for TRACE sensor according to an example embodiment:

Figure 18 shows a plot of peak sensitivity and hysteresis values for current piezoresistive sensors compared to a TRACE sensor according to an example embodiment (data point 1801).

Figure 19 shows a plot of index value SpHe for current piezoresistive sensors compared to a TRACE sensor according to an example embodiment.

In one embodiment, there is provided a sensing structure for a compressive-type pressure sensor, the sensing structure comprising:

The cracks may comprise substantially annular cracks in areas on the 3-dimensional microstmctures.

The conductive film may be free from cracks at the tip portions of the 3 -dimensional microstmctures.

The sensing structure may further comprise:

electrodes configured to be disposed on the conductive film covered tip portions of the 3- dimensional microstmctures for measuring a resistance of a conductive path comprising portions of the conductive film in the areas on the 3-dimensional microstmctures when a load is applied to the sensor for deforming the 3-dimensional microstmctures.

The sensing structure may exhibit a ratio of peak sensitivity to electrical hysteresis of more than about 1E9 WRh ¹·% ¹. The electrodes may be formed on an electrode substrate.

The electrodes may comprise flexible electrodes.

The flexible electrodes may be formed on a flexible electrode substrate.

In one embodiment, there is provided a compressive pressure sensor exhibiting a ratio of peak sensitivity to electrical hysteresis of more than about 1E9 WRh ¹·% ¹

At step 2002, an elastic micropatterned substrate defining a plurality of 3-dimensional microstmctures is provided, each microstmcture comprising a tip portion pointing away from the substrate in a first direction. At step 2004, a conductive film is formed on the elastic micropatterned substrate such that the 3-dimensional microstmctures are substantially covered by the conductive film. At step 2006, cracks are formed in the conductive film in areas on 3-dimensional microstmctures.

Forming the cracks may comprise providing a rigid substrate on the conductive film covered 3-dimensional microstmctures and applying a load to the 3-dimensional microstmctures via the rigid substrate.

The method may comprise providing a soft cushioning material between the rigid substrate and the conductive film covered 3-dimensional microstmctures during the applying of the load.

Forming the cracks may comprise forming substantially annular cracks in areas on the 3- dimensional microstmctures.

Forming the cracks may comprise leaving the conductive film free from cracks at the tip portions of the 3-dimensional microstmctures.

The method of may further comprise:

disposing electrodes on the conductive film covered tip portions of the 3-dimensional microstmctures for measuring a resistance of a conductive path comprising portions of the conductive film in the areas on the 3-dimensional microstmctures when a load is applied to the sensor for deforming the 3-dimensional microstmctures.

The method may comprise forming the electrodes on an electrode substrate.

The electrodes may comprise flexible electrodes.

The method may comprise forming the flexible electrodes on a flexible electrode substrate. Embodiments of the present invention can have one or more of the following features and associated benefits/advantages:

Applications of embodiments of the present invention include, but are not limited to:

- A highly sensitive pressure sensor to detect small pressure under different loading conditions.

- A multipurpose pulse analyser for pulse measurement and pulse tracing.

Aspects of the systems and methods described herein, such as, but not limited to, the electrical measurements and/or analysis, may be implemented as functionality programmed into any of a variety of circuitry, including programmable logic devices (PLDs), such as field programmable gate arrays (FPGAs), programmable array logic (PAL) devices, electrically programmable logic and memory devices and standard cell-based devices, as well as application specific integrated circuits (ASICs). Some other possibilities for implementing aspects of the system include: microcontrollers with memory (such as electronically erasable programmable read only memory (EEPROM)), embedded microprocessors, firmware, software, etc. Furthermore, aspects of the system may be embodied in microprocessors having software-based circuit emulation, discrete logic (sequential and combinatorial), custom devices, fuzzy (neural) logic, quantum devices, and hybrids of any of the above device types. Of course the underlying device technologies may be provided in a variety of component types, e.g., metal-oxide semiconductor field-effect transistor (MOSFET) technologies like complementary metal-oxide semiconductor (CMOS), bipolar technologies like emitter-coupled logic (ECL), polymer technologies (e.g., silicon-conjugated polymer and metal-conjugated polymer-metal structures), mixed analog and digital, etc.

The above description of illustrated embodiments of the systems and methods is not intended to be exhaustive or to limit the systems and methods to the precise forms disclosed. While specific embodiments of, and examples for, the systems components and methods are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the systems, components and methods, as those skilled in the relevant art will recognize. The teachings of the systems and methods provided herein can be applied to other processing systems and methods, not only for the systems and methods described above.

The elements and acts of the various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the systems and methods in light of the above detailed description.

In general, in the following claims, the terms used should not be construed to limit the systems and methods to the specific embodiments disclosed in the specification and the claims, but should be construed to include all processing systems that operate under the claims. Accordingly, the systems and methods are not limited by the disclosure, but instead the scope of the systems and methods is to be determined entirely by the claims.

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise," "comprising," and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in a sense of "including, but not limited to." Words using the singular or plural number also include the plural or singular number respectively. Additionally, the words "herein," "hereunder," "above," "below," and words of similar import refer to this application as a whole and not to any particular portions of this application. When the word "or" is used in reference to a list of two or more items, that word covers all of the following interpretations of the word: any of the items in the list, all of the items in the list and any combination of the items in the list.

References:

[1] Chwee-Lin Choong, ..., Jong-Jin Park. Highly Stretchable Resistive Pressure Sensors Using a Conductive Elastomeric Composite on a Micropyramid Array. Advanced Materials. 2014.

[2] Lijia Pan, Alex Chortos, Guihua Yu, Yaqun Wang, Scott Isaacson, Ranulfo Allen, Yi Shi, Zhenan Bao. An ultra-sensitive resistive pressure sensor based on hollow-sphere

microstructure induced elasticity in conducting polymer film. Nature Comms. 2014. [3] Shu Gong, Willem Schwalb, Yongwei Wang, Yi Chen, Yue Tang Jye Si, Bijan

Shirinzadeh, Wenlong Cheng. A wearable and highly sensitive pressure sensor with ultrathin gold nanowires. Nature Comms. 2014.

[4] Changhyun Pang, Gil-Yong Lee, Tae-il Kim, Sang Moon Kim, Hong Nam Kim, Sung- Hoon Ahn and Kahp-Yang Suh. A flexible and highly sensitive strain-gauge sensor using reversible interlocking of nanofibers. Nature Materials. 2012.

[5] Benjamin C.-K. Tee, Alex Chortos, Andre Berndt, Amanda Kim Nguyen, Ariane Tom, Allister McGuire, Ziliang Carter Lin, Kevin Tien, Won-Gyu Bae, Huiliang Wang, Ping Mei, Ho-Hsiu Chou, Bianxiao Cui, Karl Deisseroth, Tse Nga Ng, Zhenan Bao. A skin-inspired organic digital mechanoreceptor. Science. 2015.

[6] Ling Qiu, M. Bulut Coskun, Yue Tang, Jefferson Z. Liu, Tuncay Alan, Jie Ding, Van-Tan Truong, and Dan Li* Ultrafast Dynamic Piezoresistive Response of Graphene Based Cellular Elastomers. Advanced Materials. 2016.

[7] Kang, D. et al. Ultrasensitive mechanical crack-based sensor inspired by the spider sensory system. Nature 516, 222-226 (2014).

[8] Choong, C. L. et al. Highly stretchable resistive pressure sensors using a conductive elastomeric composite on a micropyramid array. Adv. Mater. (2014). doi: 10.1002/adma.201305182

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Claims

1. A sensing structure for a compressive-type pressure sensor, the sensing structure comprising:

2. The sensing structure of claim 1, wherein the cracks comprise substantially annular cracks in areas on the 3-dimensional microstructures.

3. The sensing structure of claims 1 or 2, wherein the conductive film is free from cracks at the tip portions of the 3-dimensional microstmctures.

4. The sensing structure of any one of claims 1 to 3, further comprising:

electrodes configured to be disposed on the conductive film covered tip portions of the 3- dimensional microstmctures for measuring a resistance of a conductive path comprising portions of the conductive film in the areas on the 3-dimensional microstmctures when a load is applied to the sensor for deforming the 3-dimensional microstmctures in a second direction substantially opposite to the first direction.

5. The sensing structure of claim 4, exhibiting a ratio of peak sensitivity to electrical hysteresis of more than about 1E9 WRh ¹·% ¹.

6. The sensing structure of claims 4 or 5, wherein the electrodes are formed on an electrode substrate.

7. The sensing structure of any one of claims 4 to 6, wherein the electrodes comprise flexible electrodes.

8. The sensing structure of claim 7, wherein the flexible electrodes are formed on a flexible electrode substrate.

9. A compressive pressure sensor exhibiting a ratio of peak sensitivity to electrical hysteresis of more than about 1E9 WRh ¹·% ¹

10. A method of fabricating a sensing structure for a compressive-type pressure sensor, the method comprising the steps of:

providing an elastic micropattemed substrate defining a plurality of 3-dimensional microstmctures, each microstmcture comprising a tip portion pointing away from the substrate in a first direction; forming a conductive film on the elastic micropattemed substrate such that the 3-dimensional microstructures are substantially covered by the conductive film; and

11. The method of claim 10, wherein forming the cracks comprises providing a rigid substrate on the conductive film covered 3-dimensional microstructures and applying a load to the 3-dimensional microstructures via the rigid substrate.

12. The method of claims 10 or 11, comprising providing a soft cushioning material between the rigid substrate and the conductive film covered 3-dimensional microstructures during the applying of the load.

13. The method of any one of claims 10 to 12, wherein forming the cracks comprises forming substantially annular cracks in areas on the 3-dimensional microstructures.

14. The method of any one of claims 10 to 13, wherein forming the cracks comprises leaving the conductive film free from cracks at the tip portions of the 3 -dimensional microstructures.

15. The method of any one of claims 10 to 14, further comprising:

disposing electrodes on the conductive film covered tip portions of the 3-dimensional microstructures for measuring a resistance of a conductive path comprising portions of the conductive film in the areas on the 3-dimensional microstructures when a load is applied to the sensor for deforming the 3-dimensional microstructures in a second direction substantially opposite to the first direction.

16. The method of claim 15, comprising forming the electrodes on an electrode substrate.

17. The method of claims 15 or 16, wherein the electrodes comprise flexible electrodes.

18. The method of claim 17, comprising forming the flexible electrodes on a flexible electrode substrate.