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Triple-state liquid-based microfluidic tactile sensor with high flexibility, durability, and sensitivity Joo Chuan Yeo, * Kenry, Jiahao Yu, Kian Ping Loh, Zhiping Wang, and Chwee Teck Lim ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00115 • Publication Date (Web): 07 Mar 2016 Downloaded from http://pubs.acs.org on March 10, 2016

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Triple-state liquid-based microfluidic tactile sensor with high flexibility, durability, and sensitivity Joo Chuan Yeo1,2,3,#, Kenry1,2,4,#, Jiahao Yu2, Kian Ping Loh4,5, Zhiping Wang3, Chwee Teck Lim2,4,6,* 1

NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore 117456 2

Department of Biomedical Engineering, National University of Singapore, Singapore 117575 3

4

Singapore Institute of Manufacturing Technology (SIMTech), A*Star, Singapore 638075

Center for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore 117546 5 6

Department of Chemistry, National University of Singapore, Singapore 117543

Mechanobiology Institute, National University of Singapore, Singapore 117411 #

These authors contributed equally to the work.

*Correspondence Chwee Teck Lim ([email protected]) Department of Biomedical Engineering National University of Singapore 9 Engineering Drive 1 Singapore 117575, Singapore

Keywords: Flexible microfluidics; Liquid-state wearable device; Tactile sensor; Resistive sensing; Liquid metallic alloy; Eutectic gallium indium (eGaIn).

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Abstract Here, we develop a novel triple-state liquid-based resistive microfluidic tactile sensor with high flexibility, durability, and sensitivity. It comprises a platinum-cured silicone microfluidic assembly filled with 2 µL liquid metallic alloy interfacing two screen-printed conductive electrodes on a polyethylene terephthalate (PET) film. This flexible tactile sensor is highly sensitive (i.e., 2 – 20 × 10-3 kPa-1) and capable of distinguishing compressive loads with an extremely large range of pressure (i.e., 2 to 400 kPa) as well as bending loads. Owing to its unique and durable structure, the sensor is capable of withstanding numerous strenuous mechanical load applications, such as foot stomping and car wheel rolling over it, without compromising its electrical signal stability and overall integrity. Additionally, our sensing device is highly deformable, wearable, and capable of differentiating and quantifying pressures exerted by distinct body movements, including finger touch and foot pressure. As a proof-of-concept of the applicability of our tactile sensor, we demonstrate the measurements of localized dynamic foot pressure by embedding the device in shoes and high heels. This work highlights the tremendous potential of the liquid-based microfluidic tactile sensing platform in a wide range of applications and further facilitates the realization of functional liquid-state device technology with superior mechanical flexibility, durability and sensitivity.

Recent years have seen the rapid rise and development of flexible electronic sensors with a high degree of deformability and conformability 1, 2. Among the various types of sensing devices, tactile sensors are one of the most researched elements

3, 4

as they serve as one of the fundamental components of a wide range

of emerging applications, such as wearable consumer electronics 10

5, 6

, soft robotics

7, 8

, electronics skins

9,

, and healthcare monitoring 11-13. Generally, tactile sensors operate based on force-induced variations in

piezoelectricity, resistance, or capacitance 14-18. Of the many tactile sensors that have been developed, the resistance-based sensors are arguably one of the most popular due to the simplicity in device design and operation as well as the relatively low operational energy consumption. In principle, resistive tactile sensors detect and quantify pressure based on the changes in the electrical resistance under applied mechanical loads 5.

To date, sensors utilizing active components in liquid state embedded within flexible materials, such as soft elastomeric substrates, have demonstrated great potential for various applications 19. This is because liquids represent the ultimate threshold in mechanical deformability. Consequently, with its intrinsic mechanical deformability, the liquid-state device technology is capable of undergoing a high degree of deformation and is poised to overcome the limitations of conventional solid-state materials, such as plastic deformation and fracture. In general, conductive fluids require high physicochemical stability to 2 ACS Paragon Plus Environment

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function optimally in tactile sensors. Numerous studies have reported that the eutectic fluid metal GaIn (eGaIn), an electrically conductive liquid metallic alloy with low viscosity 20, can be easily patterned as electrodes 21-23, configured as pressure sensors 22, and is capable of withstanding extremely high degree of strain without failure. In fact, the eGaIn alloy has high conductivity similar to that of copper and possesses a liquid phase under room temperature. Furthermore, it is non-toxic. Apart from eGaIn, the eutectic alloy of gallium, indium, and tin (i.e., Galinstan) is another common metallic liquid typically explored as the active sensing component of the liquid-state sensing devices

24

. Similar to eGaIn,

Galinstan exists in liquid state at room temperature and exhibits high electrical conductivity.

Here, by making use of the liquid metallic alloy eGaIn for its excellent electrical conductivity and mechanical deformability as well as the soft elastomeric silicone rubber and polyethylene terephthalate (PET) polymer for their superior flexibility and conformability, we devised a highly adaptable and wearable liquid-state device technology. Specifically, by constructing a silicone rubber-PET film microfluidic assembly filled with eGaIn liquid metallic alloy interfacing two screen-printed silver electrodes, we realized a triple-state liquid-based microfluidic tactile sensor with high flexibility, durability, and sensitivity. We demonstrate the versatility of our sensing device in differentiating various compressive and bending mechanical loads through the distinct variations of the electrical resistance of the device. Furthermore, we report the excellent reliability and durability of the tactile sensor through its electrical signal stability, conductive liquid confinement, and overall device integrity after subjecting it to strenuous mechanical load applications, such as foot stomping and car wheel rolling over it. As a proofof-concept of the applicability of our tactile sensor, we show that it is able to distinguish and quantify the localized pressure exerted through distinct body movements like bare foot stepping and walking in different footwear (i.e., shoes and high heels). Overall, this work illustrates the excellent promise of liquid-based microfluidic tactile sensing platform and functional liquid-state device technology in a wide variety of applications.


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Methods Finite element modeling Finite element modeling of the ‘S’-shaped and straight microfluidic-based tactile sensors was performed in SolidWorks® Simulation. Flexible silicone rubber substrate (Ecoflex 0050, Smooth-On, Easton, Pennsylvania) with a density of 1070 kg/m3 and a Young’s modulus of 250 kPa was modeled as a linear elastic material due to the small strains encountered.

Device design and fabrication The liquid-state microfluidic tactile sensor comprises a top layer of an ‘S’-shaped microstructure in silicone rubber (Ecoflex 0050, Smooth-On, Easton, Pennsylvania) and a bottom layer of a PET film screen-printed with two strips of conductive silver electrodes (Zephyr Silkscreen Pte. Ltd., Singapore). The design of the ‘S’-shaped microstructure consists of a central circular region with a diameter of 5 mm, two side circular regions with diameter of 2.5 mm, and two arcs with curvature radius of 6 mm and channel width of 100 µm connecting the side circular regions to the central circular regions. The microstructure has a height of 80 µm. The master mold for the Ecoflex silicone rubber substrate was fabricated from the SU-8 photoresist on a silicon wafer based on the standard soft lithography technique. The soft silicone rubber was mixed in 1:1 base-to-hardener (w/w) ratio and poured directly onto the silanized wafer. It was then baked at 70 °C for 1 h before it was carefully peeled off from the master mold to form the top layer of the sensor. Fluidic inlet and outlet with diameter of 1 mm were formed through hole-punching. Subsequently, the top layer of silicone rubber and bottom layer of PET film were brought together immediately after subjected them to a 3 min UV ozone treatment. Next, the liquid metallic alloy, i.e., eutectic GaIn (eGaIn), was introduced into the microstructure with a 1 mL needle syringe. Finally, the fluidic ports of the microstructure were sealed using adhesive to produce the final working tactile sensor.

Pressure sensing, durability, and mechanical forces differentiation The liquid-based microfluidic tactile sensor was subjected to compressive ramp-hold-release loads starting from 0.05 N to 8 N over a contact diameter of 5 mm (i.e., corresponding to pressure of approximately 2.55 kPa up to 407.44 kPa, respectively) using a universal load machine (5848 MicroTester, Instron, Norwood, MA). The ramp and release rates were set at 5 mm/min and hold duration was set at 30 s for static load evaluations. For dynamic load assessment, the ramp rate, release rate, and hold duration were randomly set. The electrical response of the tactile sensor upon different load


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applications was constantly monitored and recorded using a digital multimeter with data logging function (EX542, Extech Instruments, Nashua, NH).

Results and discussion The liquid-based microfluidic tactile sensor consists of a top layer of Ecoflex silicone rubber patterned with an ‘S’-shaped microfluidic structure, a bottom layer of PET film decorated with two strips of screenprinted silver electrodes, and a 2 µL minute volume of liquid metallic alloy which serves as the working fluid of the sensor (Fig. 1a). The microfluidic element comprises a large central circular region with a diameter of 5 mm and two smaller side circular regions with diameters of 2.5 mm. The two smaller circular regions are connected to the central region by two thin arcs with arc radii of 6 mm, forming a complete ‘S’-shaped microfluidic structure. The silicone rubber top layer was prepared based on the standard soft lithography technique and the assembly of the two layers of silicone rubber and PET film was achieved using the UV ozone bonding. The ‘S’-shaped microfluidic structure was then filled with a liquid metallic alloy of eGaIn through fluidic ports, completing the overall tactile sensing device.

In principle, our eGaIn-based microfluidic tactile sensor operates based on the resistive sensing mechanism (Fig. 1b). Mechanical forces or pressures applied on the device are differentiated and quantified through variations in the electrical resistance of the sensor. The resistance of our sensing device varies following the mechanical deformation experienced by the microfluidic structure due to an applied pressure. Here, we categorized our sensing device into three different states, i.e., State 1 under no pressure, State 2 under low pressure, and State 3 under high pressure applications. In general, when an external load of low pressure is applied on the central circular region of the device (i.e., State 2), there will be a slight reduction in its cross-sectional area. Subsequently, this causes a decreased volume of the conductive eGaIn metallic alloy confined under the central circular microstructure and a slight increased eGaIn volume confined under the side circular regions on which the two electrodes rest. This corresponds to an increase in the electrical resistances of the central circular region (i.e., R2) and a decrease in the side circular regions (i.e., R1). Eventually, an overall reduction in the effective electrical resistance (i.e., RT) of the tactile sensor is achieved. Assuming a uniform pressure P acting on the circular surface, the relative change in the electrical resistance, ∆R/R0 of the tactile sensor at the low pressure regime can be expressed by Eq. 1,

Ew + 4d ∆R E − P = −1 R0 w + 4d 5 ACS Paragon Plus Environment

(1)

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where E is the Young’s modulus of the microfluidic substrate, w is the width of the microfluidic channel, and d is the curvature diameter of the microchannel. On the other hand, when an external load of high pressure is applied on the device (i.e., State 3), a significant reduction in the cross-sectional area of the central circular region will follow. This leads to a much reduced volume of the liquid metallic alloy confined under this region and a significant increase in the electrical resistance of this circular region (i.e., R2). At the same time, due to a huge displacement of the conductive liquid from the central circular region to the side circular regions, a corresponding increase in the electrical resistance of the side circular regions (i.e., R1) will follow, resulting in an increase in the overall effective resistance (i.e., RT) of the tactile sensor. Mathematically, the relative electrical resistance change of the tactile sensor, ∆R/R0, at the high pressure regime can be described by Eq. 2,

Ew 4dE E − P + ln( ) ∆R E − P P E = −1 R0 w + 4d

(2)

Finally, when the external load is released from the device, due to the elasticity of the silicone rubber, the ‘S’-shaped microfluidic structure recovers to its original state and the displaced conductive liquid metallic alloy refills the microstructure (i.e., State 1). Structurally, the conduit is designed such that the working fluid approaches the central or side regions at an angle tangential to the conduit radius in order to reduce the occurrence of turbulent flow along the conduit as well as to minimize the generation of air bubbles. In addition, for a stable and robust function over a substantial period of time, the smooth and continuous flow of conductive fluid within the microfluidic sensor is necessary. For this reason, the total volume of the conductive fluid in the side regions has to be approximately half of that in the central region. The specific details on the derivation of the theoretical models defining the variations in the electrical resistance of the device in relation to the load applications can be found in the SI Text, Deformation Mechanics. The actual fabricated liquid-based microfluidic tactile sensor is illustrated in Fig. 1c. In order to execute its designated functions robustly and effectively, our tactile sensing device is incorporated with several distinctive features, such as small physical size, superior thinness, high degree of flexibility, and excellent large area conformability (Fig. 1c).


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Figure 1 | Design, principle of operation, and unique features of the ‘S’-shaped liquid-based microfluidic tactile sensor. (a) Perspective and exploded views of the ‘S’-shaped liquid-based microfluidic tactile sensor. Liquidbased microfluidic tactile sensor consists of a top layer of Ecoflex rubber patterned with an ‘S’-shaped microfluidic


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structure and a bottom layer of PET film screen-printed with silver electrodes. The microchannel is filled with liquid metallic alloy of eGaIn which serves as the working fluid of the resistive tactile sensor. (b) Principle of operation of the ‘S’-shaped microfluidic tactile sensor. (c) Actual fabricated liquid-based microfluidic tactile sensor with its unique features, i.e., small, thin, flexible, and conformable.

For the liquid-based microfluidic tactile sensor to be highly sensitive, precise localized pressure sensing is of utmost significance. This is normally influenced by the choice of materials to be utilized as the building blocks of the sensing device, in particular, the top layer of the device in contact with the loads, as well as the microfluidic structure embedded within this layer. In our previous work, we had characterized the

localized

pressure

sensing

capability

of

two

soft

silicone

elastomeric

substrates

of

25

polydimethylsiloxane (PDMS) and silicone rubber embedded with straight microfluidic structure . We demonstrated that the straight microfluidic channel embedded in a silicone rubber possessed higher sensitivity towards localized load application. Nevertheless, we aimed to enhance the localized sensitivity of our tactile sensor towards mechanical loads through the unique design of microfluidic structure.

Based on the computational finite element modeling, we characterized the mechanical deformation profile and sensitivity of the two different microfluidic structures, i.e., ‘S’-shaped and straight microfluidic structures, embedded in silicone rubber upon the application of localized loads (Fig. 2). First, we simulated a random localized load of 4 N over a contact area of 19.63 mm2 (i.e., contact diameter of 5 mm) on the two microfluidic devices (Fig 2a and Fig. 2b). We observed that the ‘S’-shaped microfluidic structure in silicone rubber displayed a higher and sharper deformation profile as compared to the straight microfluidic structure, indicating a higher localized load sensitivity of the ‘S’-shaped microfluidic structure. To verify the higher localized sensitivity of the ‘S’-shaped microstructure, both the microfluidic structures were subjected to force loadings with a range of magnitudes spanning from 0.05 to 8 N which corresponded to pressures of 2.55 to 407.5 kPa (Fig. 2c to Fig. 2f). We noted that the two microfluidic structures displayed distinct deformation profiles upon contact with the mechanical loads (Fig. 2c and Fig. 2d). In fact, for every load application with the same magnitudes, we observed that the ‘S’-shaped microfluidic structure constantly exhibited higher mechanical deformations, suggesting a superior strain response of the ‘S’-shaped microchannel (Fig. 2e and Fig. 2f). Importantly, due to the unique design of the ‘S’-shaped microfluidic structure, there is a slight variation in the Young’s modulus between the two thin arcs. As such, this would result in a minute asymmetry in the deformation profile of the ‘S’-shaped microstructure against the middle position of 0 mm upon load applications. Eventually, based on all the simulation data, we inferred that the top layer of the flexible tactile sensor comprising the ‘S’-shaped


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microfluidic structure possessed a better localized sensitivity towards mechanical loads as compared to the straight microchannel.

Figure 2 | Finite element modeling of the mechanical deformation experienced by the microfluidic tactile sensor upon the application of different loads. (a-b) Mechanical deformation of the top layer of the sensor patterned with: (a) ‘S’-shaped and (b) straight microchannels, upon a 4 N load application over a contact area of 19.63 mm2 (contact diameter of 5 mm). (c-d) Mechanical deformation profiles of: (c) ‘S’-shaped and (d) straight microchannels, upon the application of different loads ranging from 0.05 to 8 N over a contact area of 19.63 mm2. (e-f) Comparison of the mechanical deformation experienced by the ‘S’-shaped and straight microchannels upon the application of two distinct loads over a contact area of 19.63 mm2: (e) 0.5 N (i.e., 25.5 kPa) and (f) 8 N (i.e., 407.5 kPa).


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Next, we characterized the pressure sensing and reliability performance of the liquid-state ‘S’-shaped microfluidic tactile sensor in response to both static and dynamic mechanical forces under various loading conditions (Fig. 3). To start with, we assessed the localized precision and sensitivity of our sensor by evaluating its relative electrical resistance change profile over time upon a 1 N load application on the different parts of the device (Fig. 3a). We observed distinct electrical responses when the loads were applied on the central (i.e., P2) and side (i.e., P5) circular regions of the device. At regions 5 mm away from the circular regions (i.e., P1, P3, and P4), load applications did not generate any electrical response. This indicated the localized precision and sensitivity of the tactile sensor.

We then carried out static load evaluation by performing compressive ramp-hold-release load cycles on the device, starting from a load of 0.05 N up to 8 N (i.e., corresponding to pressure of approximately 2.55 kPa up to 407.44 kPa, respectively), using a universal load machine (Fig. 3b to Fig. 3d). Evidently, the static load evaluation revealed the capability of our triple-state device in distinguishing the range of magnitude of the load applied (Fig. 3b). We observed that for mechanical loads possessing low pressure (e.g., 7, 12, and 25 kPa), the device exhibited a negative pressure coefficient (NPC) (i.e., ∆R/R0 < 0) with a sensitivity of 2×10-3 kPa-1. On the other hand, for high pressure loads (e.g., 28 kPa), a positive pressure coefficient (PPC) (i.e., ∆R/R0 > 0) was displayed by the device. It is worth mentioning that the ‘S’-shaped microfluidic structure displayed an NPC or PPC correlation in response to positive or negative bending because of the specific mechanical deformations (i.e., squeezing or stretching) experienced by the central circular region of the microfluidic structure. In fact, differences in stiffness between the substrate (i.e., PET) and microfluidic structure (i.e., Ecoflex) also contributed to the NPC or PPC correlation.

We also compared the static loadings on the ‘S’-shaped microstructure against those on a straight microchannel (Fig. 3c and Fig. 3d). At the low pressure regime up to 25 kPa, we noted that the straight microstructure displayed PPC correlation while that of the ‘S’-shaped microchannel was NPC (Fig. 3c). Interestingly, for pressure above 25 kPa, the pressure correlation state of the ‘S’-shaped microchannel switched to positive while that of straight microchannel remained positive. In fact, we observed constant PPC positive pressure coefficient irrespective of the magnitude of the loads applied on the straight microstructure design. Here, it is interesting to highlight that, instead of a gradual transition, the ‘S’shaped microstructure exhibited an almost instantaneous and discrete jump from NPC to PPC. This unique NPC-PPC transition further emphasized the low pressure differentiating capability of our ‘S’shaped microfluidic-based sensor. At higher pressure range above 70 kPa, we noted that the normalized resistance of the ‘S’-shaped microstructure was higher than that of straight microchannel (Fig. 3d), achieving a high device sensitivity of 20×10-3 kPa-1. Together, all these demonstrated the versatility of the 10 ACS Paragon Plus Environment

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‘S’-shaped microstructure in distinguishing low and high pressure loads as well as its higher sensitivity at the high pressure regime. Additionally, the ‘S’-shaped microstructure exhibited negligible variations in the relative electrical resistance at the high pressure regime while that of straight microchannel significantly varied.

Subsequent to static load measurement, we performed dynamic load assessment on the ‘S’-shaped tactile sensor by subjecting it to continuous cyclical load switching. We then examined its electrical responses to validate the sensing capability and stability of our device over a range of dynamic forces (Fig. 3e). More clearly, we probed the pressure differentiation capability and the cyclical stability of the electrical output of the device over time in response to the applications of random low and high pressure loads. As shown, our tactile sensor was capable of load differentiation based on its distinct pressure correlation states. As such, with the unique pressure correlation states, our device was capable of distinguishing the gentle stroking and finger pressing. Here, it is worth noting that, due to the triple-state characteristic of the sensor, the electrical sensor output ∆R/R0 would yield the same value in the rare case of the static application of two different loads of 0 and approximately 40 kPa. Nevertheless, as continuous real-time measurements of forces were performed, these two loads could be easily distinguished since the sensor output will always cross the zero baseline as the applied load increases (Fig. 3e). The device also exhibited stable electrical responses after undergoing multiple loading-unloading cycles. Moreover, we noticed the high signal-to-noise ratios throughout the measurements, further confirming the force sensing and differentiating capability of the device.

In addition to compressive loads, our tactile sensor could be utilized to detect and distinguish other types of mechanical force like bending load, upon the application of an individual single load, due to the high degree of flexibility of the device. We subjected our ‘S’-shaped microfluidic tactile sensor to four loads with bending angles of -90 o, -45o, +45o, and +90o (Fig. 3f and Fig. 3g) and compared its relative electrical resistance change as a function of bending angles against that of a straight microchannel design (Fig. 3h). Remarkably, once again, the ‘S’-shaped microfluidic tactile sensor exhibited its capability as a triple-state device in differentiating the range of bending angles of the applied loads. Specifically, the ‘S’-shaped microfluidic sensor exhibited an NPC or PPC correlation in response to positive or negative bending, respectively. In contrast, the straight microchannel-based device constantly displayed an NPC correlation regardless of the bending angles.


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Figure 3 | Pressure sensing and reliability performance of the ‘S’-shaped liquid-based microfluidic tactile sensor. (a) Relative electrical resistance change (∆R/R0) profile of the tactile sensor over time upon load application


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on the different parts of the device, illustrating the localized precision of the sensor. (b) ∆R/R0 profile of the device over time, showing two distinct ∆R/R0 ranges of less than one for low pressure (i.e., 7 to 25 kPa) and more than one for high pressure (i.e., 38 kPa) load applications. (c-d) ∆R/R0 profiles of the ‘S’-shaped and straight microchannelsbased tactile sensors as a function of pressure for: (c) low pressure and (d) high pressure loads. (e) ∆R/R0 profile of the tactile sensor upon the applications of random loads of low and high pressures, demonstrating the capability of the triple-state sensor in differentiating and quantifying random load applications. (f) Condition of the bending tests: -90o, -45o, +45o, and +90o. (g) ∆R/R0 profile of the tactile sensor being subjected to bending deformations from -90o to 90o, illustrating the capability of the triple-state sensor in differentiating and quantifying random bending deformations. (h) ∆R/R0 profiles of the ‘S’-shaped and straight microchannels-based tactile sensors as a function of bending angle. (i) ∆R/R0 profile of the tactile sensor over seven days under an applied load of 2 N. (j) ∆R/R0 profile of the tactile sensor as a function of days shows the negligible variation in the relative electrical resistance change of the device, demonstrating its excellent stability and reliability over time. (k) ∆R/R0 profile of the tactile sensor being subjected to 500 loading-unloading cycles over a contact area of 0.95 mm2 (i.e., contact diameter of 1.1 mm). (l) Enlarged view of (k) showing the last five loading-unloading cycles. (m) ∆R/R0 profile of the tactile sensor with respect to temperature variation from 19 to 45 oC as a function of time.

Following a series of static and dynamic compressive and bending load sensing performance evaluations, we probed the stability and reliability of our tactile sensor over a significant period of time (Fig. 3i and Fig. 3j). Static compressive loads were applied on the tactile sensor over a period of seven days and its electrical resistance variations were recorded. Based on the obtained results, we observed that the tactile sensor showed consistent and steady responses with respect to all applied loads. In fact, the relative change in the electrical resistance of the tactile sensor under each load condition was relatively constant. Nonetheless, it is important to highlight that the signal patterns might vary across different days but similar in a single day (Fig. 3i). This was due to the fact that the localized position upon load application on the sensor might vary slightly across different days but would be similar in a single day. Subsequent to this, the tactile sensor was subjected to 500 loading-unloading cycles over a contact area of 0.95 mm2 (i.e., contact diameter of 1.1 mm) (Fig. 3k and Fig. 3l). Again, we noted that the device displayed a consistent resistance change profile over time with negligible drift. Lastly, we examined the device output reliability against variations in temperature ranging from 19 to 45 oC (Fig. 3m). Based on the obtained results, it was evident that the device normalized resistance was highly stable with respect to temperature variations over time. Importantly, this signified the independence of the electrical resistance of our tactile sensor in response to changes in temperature. Overall, in light of all the experimental data, we


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demonstrate the high reliability and robustness of our resistive sensing device as well as its exceptional electrical response stability with negligible time- and temperature-dependent variations.

Further to the characterization of the pressure sensing and reliability performance of the liquid-state microfluidic tactile sensor, we evaluated the durability of our device by subjecting it to numerous strenuous mechanical treatments, such as foot stomping on the device, chair rolling over the device, and even with a car wheel rolling over it (Fig. 4a). Here, the overall integrity and electrical signal stability of a particular device before and after the applications of the mechanical loads were observed and compared. First, the device was subjected to intensive foot stomping followed by office chair rolling. We observed that the device maintained its integrity as the liquid metal eGaIn was well confined within the microfluidic structure (i.e., there was no leakage of conductive liquid) and the test signals maintained its overall shape and relative variations in its electrical resistance, R/R0. Furthermore, we subjected the same tactile sensor with a car wheel rolling over it to further gauge its overall robustness. Once again, the conductive liquid confinement, device integrity, and electrical output signal were similar to those before the device was mechanically acted on. Importantly, we noted that the tactile sensor exhibited excellent mechanical deformability with superior working fluid confinement and maintained its excellent working state even after experiencing mechanically harsh treatments. Notably, our device was capable of quantifying the magnitude of the pressure of the different user-applied deformations it received based on the distinct changes in its electrical resistance (Fig. 4b and Fig. 4c).

To demonstrate the applicability of our liquid-based flexible microfluidic tactile sensor, we embedded our device to the insole parts of the footwear in contact with heel and probed its capability in detecting and distinguishing numerous gait movements. Specifically, the dynamic responses of the device were monitored while the gait movements were executed under different conditions (i.e., with bare foot and footwear) (Fig. 4d to Fig. 4f). We first investigated the electrical output of the device subjected to bare foot gait study over time (Fig. 4d). The gait was divided into three segments of “heel strike”, “foot flat”, and “heel off”. As the heel landed on the sensor, we observed a gradual increase in the electrical output signal reaching a peak as the “heel strike” stance shifted to “foot flat”. A sharp decrease in the electrical output signal back to baseline was noted as the heel was released from the sensor with the “heel off” stance. Next, the tactile sensor was attached to the insole of a shoe and the electrical signal from the same walking gait was recorded over time (Fig. 4e). At “heel strike”, we observed a sudden increase in the electrical output of the device and it slightly increased further as the gesture shifted to “foot flat”. The electrical resistance of the sensor gradually returned to its baseline with “heel off” stance. Notably, the electrical signals produced from the two foot stepping motions were characteristically different from each 14 ACS Paragon Plus Environment

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other. Interestingly, distinct signal patterns were noted from the bare foot and shoe-encapsulated conditions. In particular, we observed that the heel sustained a higher localized pressure with bare foot as compared to in cushioned shoes.

Figure 4 | Durable and wearable liquid-based microfluidic tactile sensor for pressure differentiation and measurement. (a) Durability and stability of the microfluidic tactile sensor as assessed from the device integrity and test signals before and after experiencing the strenuous movements of foot stomping, chair rolling, and car wheel


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rolling over it. (b-c) Electrical resistance profiles of the device when it was subjected to different actions of: (b) foot stomping and chair rolling and (c) car wheel rolling over it. (d-f) Relative electrical resistance change (∆R/R0) profiles of the tactile sensor upon subjected to dynamic loading and unloading cycles of: (d) bare foot stepping, (e) walking in shoes, and (f) walking in high heel. Insets show the characteristic electrical responses of the tactile sensor corresponding to the distinct movements of heel strike, foot flat, and heel off for different actions of: (d) bare foot stepping and walking in (e) shoes and (f) high heels.

To further investigate the force recognition and differentiation capability of our liquid-state tactile sensor, we attached it to the insole of a high heel and assessed the dynamic electrical response produced from the walking gait (Fig. 4f). Remarkably, we observed that the electrical signal generated from foot stepping with high heel was uniquely different and distinguishable from that generated under the other two conditions. The individual characteristic electrical responses from foot stepping under bare foot and other conditions as detected by our tactile sensor are illustrated in the Insets of Fig. 4d to Fig. 4f. In fact, a localized high pressure was sustained for a longer period on the heel of a user walking in high heels. By repeating the same foot stepping cycles for several times, we further showed the signal stability of our device over time. Overall, these results demonstrated that subtle differences in foot stepping as well as the condition and manner in which the motion was executed could be identified with our liquid-based microfluidic tactile sensor. Importantly, this highlights the potential of our device as a wearable sensor for a wide range of applications and we believe that the development of such devices is of significant importance.

Conclusions In summary, we developed a simple and robust triple-state liquid-based resistive microfluidic tactile sensor with high flexibility, durability, and sensitivity. The device consists of a top layer of elastomeric silicone rubber patterned with an ‘S’-shaped microfluidic structure and a bottom layer of PET film deposited with two silver electrode strips. The top and bottom layers were UV ozone-bonded and the conductive eGaIn liquid metal serving as the working fluid of the device is introduced into the microfluidic assembly, completing the tactile sensor set-up. This flexible tactile sensor is highly sensitive and able to distinguish different compressive and bending loads based on the changes in the electrical resistance of the device upon subjected to mechanical loads. Simultaneously, our sensor is very durable, capable of withstanding numerous strenuous mechanical load applications without compromising its electrical output stability, conductive liquid confinement, and overall integrity. Furthermore, this sensing device is highly deformable, wearable, and capable of differentiating forces exerted by distinct body


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movements. We anticipate that this work will significantly facilitate the realization of functional liquidstate device technology with outstanding mechanical flexibility, durability, and sensitivity.

Supporting information Supporting Information Available: The following files are available free of charge. Supplementary Information.docx. Deformation mechanics – detailed derivation of the theoretical models defining the variations in the electrical resistance of the tactile sensor in relation to the applications of low and high pressure loads.

Acknowledgements This research was supported by the National Research Foundation, Prime Minister’s Office, Singapore under its medium-sized centre programme, Centre for Advanced 2D Materials and its Research Centre of Excellence, Mechanobiology Institute, as well as the MechanoBioEngineering Laboratory at the Department of Biomedical Engineering of the National University of Singapore. The authors would also like to thank Ms Trifanny Yeo for her assistance in the car and high heel signal measurements and acquisitions experiments.

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Triple-State Liquid-Based Microfluidic Tactile ... - ACS Publications

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