Wearable Piezoresistive Sensors with Ultrawide Pressure Range and

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Functional Nanostructured Materials (including low-D carbon)

Wearable Piezoresistive Sensors with Ultrawide Pressure Range and Circuit Compatibility Based on Conductive-Island-Bridging Nanonetworks Hochan Chang, Sungwoong Kim, Tae-Hyung Kang, SeungWoo Lee, Gil-Tae Yang, Ki-Young Lee, and Hyunjung Yi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10194 • Publication Date (Web): 09 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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ACS Applied Materials & Interfaces

Wearable Piezoresistive Sensors with Ultrawide Pressure Range and Circuit Compatibility Based on Conductive-Island-Bridging Nanonetworks Hochan Chang†, Sungwoong Kim†, Tae-Hyung Kang†, Seung-Woo Lee†‡, Gil-Tae Yang§, KiYoung Lee†, and Hyunjung Yi†#* † Post-Silicon Semiconductor Institute, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea ‡ Department of Fine Chemistry, Seoul National University of Science and Technology, Seoul 01811, Republic of Korea § SEED Tech. Co., 261 Doyak-ro, Wonmi-gu, Bucheon, Gyeonggi-do 14523, Republic of Korea # Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Republic of Korea

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KEYWORDS. Piezoresistive sensors; conductive islands; nanonetworks; printed circuit board; selective surface coating.

ABSTRACT

Wearable pressure sensors with wide operating pressure ranges and enhanced wearability via seamless integration with circuits can greatly improve the fields of digital healthcare, prosthetic limbs, and human-machine interfaces. Herein, we report a conductive island-bridging nanonetwork-based approach for realizing wearable resistive pressure sensors that are operative over ultrawide pressure ranges >400 kPa and circuit-compatible. The sensor has a simple two-layered structure where nanonetworks of single-walled carbon nanotubes selectively patterned on a surface-modified elastomeric film interface and bridge conductive Au island patterns on printed circuit boards (PCBs). We show that varying the design of the Au islands and the conductivity 2 ACS Paragon Plus Environment

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of nanonetworks systematically tunes the sensitivity, linearity, and the operation range of the pressure sensor. In addition, introducing microstructured lead contacts into the Au-island-bridging nanonetwork-based sensor produces a record-high sensitivity of 0.06 kPa-1 at 400 kPa. Furthermore, the PCB that serves as the bottom layer of the pressure sensor and contains embedded interconnects enables facile integration of the sensor with measurement circuits and wireless communication modules. The developed sensor enables the monitoring of wrist pulse waves. Moreover, an insole-shaped PCB-based pressure sensing system wirelessly monitors pressure distributions and gait kinetics during walking. Our scheme can be extended to other nanomaterials and flexible PCBs and thus provides a simple yet powerful platform for emerging wearable applications.

INTRODUCTION



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Wearable devices capable of monitoring minute changes in exerted pressure levels and converting this change into an electrical signal have attracted tremendous interest1-3 due to their potential for in situ monitoring of health condition-related information over a long period of time and for providing smart interfaces between humans and robotics/machines or electronics.4-7 The wide pressure ranges exerted by human that vary over several orders of magnitude up to several hundred kPa have demanded the development of wearable pressure sensors capable of demonstrating both high sensitivity and wide operating pressure ranges.4,

6

Moreover, the mobility of wearable

sensing systems requires compatible integration of the pressure sensor with various circuit systems and wireless communication modules.2, 8

Several types of pressure sensors such as field-effect transistors, piezoelectric devices, capacitors, and piezoresistive devices have been suggested for the development of wearable pressure sensors.9-17 Among these, piezoresistive sensors have attracted significant interest over the past few years owing to their simple measurement scheme, low operating voltage and low power consumption.9,

18-27

Interesting template or


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substrate materials such as silk, sandpaper, and tissue paper have been also employed for the development of high-performance pressure sensors.16, 20-21, 25 Moreover, various electronic materials, nanostructures and device schemes have been reported to show high sensitivity and wide pressure ranges.26-28 However, most previous piezoresistive sensors were based on hybrid integration approach in which pressure sensors were fabricated separately and electrically connected to measurement circuits using external wires. Wearable pressure sensors based on the hybrid approach can potentially suffer from limited mobility/wearability and parasitic capacitances/noise due to the externally wired electrical connections. These limits could become serious when fabricating arrays of pressure sensors.

A strategy to utilize metallic layers or electrodes as a component of the piezoresistive pressure sensor is attractive in terms of integration perspective because the metallic pattern can be fabricated with commercially available printed circuit board (PCB) techniques and thus enables seamless integration of the pressure sensor with measurement circuits.21 However, resistive pressure sensors based on metallic patterns



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have shown ready saturation and limited operating pressure ranges and so far, an integrative approach to develop high-performance pressure sensors with wide pressure ranges and circuit compatibility has been elusive.

Herein, we developed wearable resistive pressure sensors that were highly operative over ultrawide pressure ranges > 400 kPa and circuit compatible based on a concept of conductive island-bridging nanonetworks. We employed a PCB substrate that contained isolated Au patterns on its surface and embedded interconnects to serve as a bottom layer of the piezoresistive sensor. Nanonetworks of single-walled carbon nanotubes (SWNTs) patterned on a surface-treated elastomeric film was interfaced with the PCB layer to electrically bridge the Au islands. We showed that the narrower conductive Au islands extended the operating pressure ranges toward higher than 400 kPa and the wider islands enhanced the sensitivity in the low pressure range whereas the more Au islands increased the overall sensitivity. The sensitivity of the developed pressure sensor with flat lead contacts enabled the monitoring of wrist pulse waves and this sensitivity was retained even at a high pressure level of 400 kPa. Introducing


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microstructured lead contacts further enhanced the sensitivity of the Au-island-bridging nanonetwork-based sensors over wide pressure range, enabling a record-high sensitivity of 0.06 kPa-1 at 400 kPa. We further demonstrated that an array of highperformance pressure sensors was facilely fabricated using PCBs containing multiple Au island patterns and embedded interconnects. An insole-shaped PCB-based pressure sensing system successfully demonstrated wireless monitoring of pressure distributions and gait kinetics during walking. These results altogether highlight the potential of our approach and suggest a simple yet powerful platform for digital healthcare, prosthetic limbs and interactive intelligent robotics.



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RESULTS AND DISCUSSIONS

Figure

1.

Fabrication

of

the

conductive-island-bridging

nanonetwork-based

piezoresistive sensors. (a) Schematic structure of the developed pressure sensor, with key components labeled. The PCB with Au islands and electrical leads on its surface serves as the bottom layer of the piezoresistive sensor and also body material for


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embedded interconnects. (b) Photographs of PCBs with various arrangements of rectangular Au islands. Upper panel: various island widths at a fixed areal coverage (50%). Lower panel: various number of islands at a fixed island width (600 μm). (c) Absorption spectra of SWNTs dispersed at various concentrations in DCB. (d) Schematics of procedures for forming SWNT nanonetworks on selected locations of microstructured PDMS films. (e) AFM images of PDMS surfaces that were dip-coated in SWNT dispersions in DCB after O2 plasma-treatment (upper panel) and without O2 plasma-treatment (lower panel). SWNT networks formed only on the O2 plasma-treated surface. (f, g) Schematics of the cross-section of the nanonetwork-deposited PDMS films and images of the produced corresponding PDMS films molded using (f) a Si/SiO2 wafer having regular pyramidal microstructures and flat lead contacts and (g) sandpaper showing an irregular surface morphology and microstructured lead contacts. (h) Sheet resistance values of nanonetworks dip-coated using different SWNT concentrations.



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The stucture of the developed pressure sensor was illustrated in Figure 1a. It consisted of two layers, that is, PCB and a microstructured elastomeric film. The PCB contained conductive islands (Au) patterns and electrical leads on the surface and embedded interconnects so that the PCB served as both the bottom layer of the pressure sensor and the body material for electrical connections. The elastomeric film contained resistive nanonetworks of SWNTs on the surface facing the Au islands to electrically bridge the Au islands. The deformation of the elstomeric film with microstructures upon loading changed the contact resistance between the nanonetworks and the Au islands and thus was required for the operation of the piezoresistive sensor developed in this study. Photographs of PCBs containing various arrangements of the rectangular Au islands were shown in Figure 1b. Here, the PCB was denoted as Lx-Py, where x and y indicated the number of Au islands and the width of each island divided by 300 μm, respectively. For example, L8-P2 contained eight Au rectangles with a width of 600 μm and L1-P16 contained one Au rectangle with a width of 4800 μm. In the upper panel, the width of the Au islands was varied at a fixed total area (50% of the basal area between the electrical leads), whereas the number of Au islands and accordingly the total area 10 ACS Paragon Plus Environment

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was varied at a fixed island width (600 μm) in the lower panel. In this study, a thin PCB with a thickness of 200 μm was used for flexibility rather than thick rigid PCB. All PCBs containing the Au islands and electrical leads were fabricated using the commercially available PCB technology (See methods).

To conformably form the electronic nanonetworks pattern of SWNTs at a designated location on an elastomeric film, the surface of the polydimethylsiloxane (PDMS) was first treated with oxygen plasma since the oxygen plasma treatment of the PDMS surface enhanced the wettability due to the increased surface energy and thus allowed selective coating of SWNT solutions.29 In this study, SWNTs were chosen as the network material due to their high mechanical and chemical stability and the ability to systematically tune the electrical conductivity of their resulting networks.27,

30

SWNTs

were dispersed in dichlorobenzene (DCB) since DCB dispersed SWNTs without surfactants or stabilizers, obviating washing steps after deposition of SWNTs. The absorption spectra of SWNTs dispersed in DCB showed the characteristic peaks of SWNTs (Figure 1c) and suggested that SWNTs were well dispersed. The SWNTs



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dispersed in DCB then dip-coated on the oxygen plasma-treated PDMS surface, as illustrated in Figure 1d. Acquired atomic force micrographs of dip-coated PDMS surfaces with and without plasma treatment shown in Figure 1e confirmed that SWNT networks were well formed on the oxygen plasma-treated surface whereas no noticeable SWNT network was formed on any of the non-treated surfaces. Moreover, nanonetworks were conformably formed along the surface of various types of microstructured PDMS surfaces (supplementary Figure S1), as shown in scanning electron micrographs of the SWNTs on the PDMS films (Figure 1f and 1g). Our approach also produced SWNT networks with tunable sheet resistance by varying the concentration of SWNTs in the DCB solution (Figure 1h). Fabrication of the pressure sensor was completed by layering the SWNT nanonetwork-deposited elastomeric film onto the PCB containing the Au islands.

The two island arrangement sets shown in Figure 1b were then compared to systematically

investigate

the

characteristics

of

the

conductive-island-bridging

nanonetwork-based pressure sensors. PDMS films studded with regular pyramidal


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microstructures of fixed dimensions (the base dimension of the pyramid was 150 μm and the centers of adjacent pyramids were 300 μm apart) and flat lead contacts were used as the top layer. Thus, for the L8-P2 sensor, each of the eight narrow lines of the rectangular Au islands were in contact with two columns of pyramids; for the L1-P16 sensor, a single wide Au island made contact with 16 columns of pyramids. Photographs of two representative pressure sensors, L8-P2 and L1-P16, along with parts of optical micrographs and their equivalent circuits are shown in Figure 2a, 2b, and supplementary Figure S2. The overall device resistance consisted of series of leads (Rlead), a nanonetwork (Rsingle_nn) not in contact with the Au islands, and the complex of nanonetwork and Au islands in contact each other, Rcomplex. 31-32

The performance of each piezoresistive sensor was characterized by measuring the current of the device at a fixed voltage level (1 V) under various pressure loadings from 0 to 400 kPa. The pressure was increased stepwise by 1 kPa from 0 to 10 kPa, then by 10 kPa from 10 to 100 kPa, and finally by 50 kPa from 100 to 400 kPa. The measured relative current change, (II0)/I0, for each set of the various island arrangements is



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shown in Figure 2c and 2d. In Figure 2c, the measured relative current change compared with the overall current change of the device, i.e., (II0)/(I400I0), is also shown in the inset for clear comparison of the different linearity among various island arrangements. The sensitivity increased with increasing Au island width in the lowpressure range whereas the overall linearity increased with decreasing Au island width. That is, the pressure sensor employing wider Au islands showed most of its response in the low-pressure range whereas the pressure sensor employing narrower Au islands showed almost an equal sensitivity over wide pressure ranges up to 400 kPa. The Rsquare value of the linear-fitted curve of the L8-P2 was 0.999 whereas that for L1-P16 was 0.726 (supplementary Figure S3). In contrast, the shape of the piezoresistive response remained constant but the degree of current change linearly increased as the number of the Au islands was increased from zero to eight at a fixed island width, as shown in Figure 2d and Figure S4. These results suggested that the width of the Au islands affected the shape of the piezoresistive response rather than the total areal coverage. This large degree of control of the shape of the piezoresistive response has not been previously reported. 14 ACS Paragon Plus Environment

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In order to explain the large dependence of the piezoresistivity’s degree of linearity on the Au width, numerical calculation was performed. Details are described in the Supporting Information. The contact resistance, Rct, between the nanonetwork on the microstructure and the conductive islands Rct was extracted from the experimental data,

Rdevice (P) for L8-P2 sensor using the equivalent circuit and schematics shown in Figure 2a and 2e. The extracted Rct (P) is shown in Figure 2f. The result shows that Rct drastically decreases upon loading in the low pressure range and gradually decrease with increasing applied pressure. Since the same nanonetwork materials and pyramidal structure were used for all sensors, it was reasonable to assume that each Rct would be the same for all devices. Comparison of the Rct (P) with the complex nanonetwork resistance, Rcomplex_nn, of various island arrangements (Supporting Information) indicated that for the widest Au island, L1-P16, the Rct value was lower than Rcomplex_nn (150 000 Ω) for all pressure loadings. Therefore, the contribution of the Rct was significant for wider Au even in the low pressure range and thus the response curve showed a similar shape with the Rct curve. In contrast, for narrow Au islands such as L8-P2, the Rct at low pressure levels was much higher than the nanonetwork resistance, Rcomplex_nn (18 750 15 ACS Paragon Plus Environment


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Ω), and thus the Rcomplex_nn dominated the Rcomplex. The portion of the Rcomplex_nn would be gradually lowered with increasing loadings due to the decreasing Rct. In conclusion, the competition between the Rct and Rcomplex_nn of each Au island-nanonetwork complex determined the overall behavior of the sensor performance. In order to further confirm whether the extracted Rct simulated the measured piezoresistive responses of various Au arrangements, Rdevice (P) for all the sensor designs were calculated using the extracted Rct (P) (Supporting Information). The calculated results are shown in Figure 2g and 2h and supplementary Figure S5. The calculated curves well correlate with the experimental results shown in Figure 2c and 2d in terms of the increased linearity with deceasing Au island width and the increased response with increasing number of the Au islands. The slight discrepancy between the calculated values and experimental values could be ascribed to the variances in Rlead and Rnn values for real samples since we used a fixed Rlead of 100 kΩ and Rnn of 300 kΩ for all samples for the calculations. The corresponding sensing mechanism is schematically illustrated in Figure 2i. The red arrow lines represent the current flow. Note that the overall behavior of L8-P2 and L16P1 were similar, although the sensitivity of L8-P2 was higher than that of L16-P1 (Figure 16 ACS Paragon Plus Environment

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2c). This difference was due to the inability of the narrow Au islands in the L16-P1 device to connect neighboring columns of pyramids along the directions of the current flow, hence still yielding a high device resistance. The sensitivity of the pressure sensor was decreased when more conductive nanonetworks (SWNT/DCB 0.05 mg/mL) were employed (supplementary Figure S6), possibly by inducing less partitioning of the current into the conductive Au patterns. The extreme tunability of the linearity of the piezoresistive response over large pressure ranges enabled by introducing conductive islands has not been reported before and provides different opportunities from previously reported works.21, 24, 33-40



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Figure 2. Working principles and characteristics of the conductive-island-bridging nanonetwork-based resistive pressure sensors. (a,b) Photographs of representative pressure sensors, (a) L8-P2 and (b) L1-P16, shown with the parts of the optical micrographs along with the equivalent circuits. (c) Piezoresistive responses of the developed pressure sensors with various Au island widths but a fixed areal coverage of


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50%. (d) Piezoresistive responses of the developed pressure sensors with various numbers of Au islands but at a fixed island width of 600 μm. (e) Schematic of the Au island-nanonetwork complex of L8-P2 with key components being indicated. (f) The extracted contact resistance Rct function from Rdevice (P) of L8-P2. (g-h) Calculated (II0)/I0 for (g) sensors with various Au island widths but a fixed areal coverage of 50% and (h) sensors with varying numbers of Au islands but a fixed island width of 600 μm. (i) Working principle of the L8-P2 and L1-P16 pressure sensors. The red arrow lines represent the current flow.

The piezoresistive response of the developed L8-P2 sensor was then examined in more detail, due to its highly linear and large response over an ultrawide pressure range. The detailed real-time response of this pressure sensor, in which the pressure level was increased by 1 kPa from 0 to 10 kPa, then by 10 kPa from 10 to 100 kPa, and finally by 50 kPa from 100 to 400 kPa and each loading was repeated five times, is shown in Figure 3a. The sensor response was responsive even at a high pressure loading at 400



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kPa. The sensitivity at 400 kPa was 0.0012 kPa-1 (supplementary Table S1). The detection limit of the sensor was ~ 120 Pa (supplementary Figure S7). The response time was estimated to be ~48 ms (Figure 3b). The stability of the L8-P2 pressure sensor was then tested by cycling it from 0 to 100 kPa 10,000 times; the resultant data are shown in Figure 3c. Detailed responses in the vicinities of the 500th and 10,000th cycles are shown in the inset of Figure 3c. After 10,000 cycles, the device current changed only by approximately 2%. This robust response of the sensor suggests an excellent stability of the developed device. The sensor was also sensitive enough to detect arterial pulse waves from the wrist (Figure 3d); the real-time response of the L8P2 pressure sensor attached to a wrist is shown in Figure 3e. In Figure 3f, three distinct wave peaks in one periodic pulse are shown: P1, P2, and P3. P1 is the sum of the incident (ejected) and reflected (from the hand) waves and P2 is the peak of the reflected end-diastolic pressure of the wrist subtracted from that of the lower body. The radial artery augmentation index (AIr) was calculated as P2/P1 and Δ TDVP is the difference between the first and second peaks of the pressure wave and is used as a


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measure of arterial stiffness. The value of ΔTDVP using the L8-P2 pressure sensor was estimated to be 0.2 s, indicating that the person wearing the sensor was in a healthy condition.41 Here, it is noted that although the sensor was sensitive enough to detect wrist pulse wave, the magnitude of the current change by wrist pulse was only ~ 50 nA, which might require complicated amplifying circuits for wearable applications. The sensitivity was significantly improved when microstructured contact leads were introduced as demonstrated below.

An array of individual sensors can provide information on locations and magnitudes of loaded pressure simultaneously. In our approach, a sensor array can be easily fabricated by employing PCBs containing multiple arrays of conductive Au patterns and electrical leads. A schematic illustration of a 3 ⅹ 3 sensor array is shown in Figure 3g. The area for each sensor was 1 cm ⅹ 1 cm. The capability of the sensor array to discriminate loading levels and locations is demonstrated in Figure 3h. Weighing weights of known weight of 50 g, 100 g, 500 g, and 1000 g were placed on various sensor locations, as illustrated in Figure 3g, and the pressure levels detected by each



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pressure sensor were compared. The result suggested that the sensor array well identified the magnitudes and locations of different weighing weights. The 5 x 5 sensor array also identified an alphabet M of 330 g and placed on the sensor array, as demonstrated in Figure 3i. These results indicated that the piezoresistive sensors based on conductive-island-bridging nanonetworks were highly sensitive in the low pressure ranges and yet operative over ultrawide pressure ranges, and also enabled arrays of high-performance pressure sensors in a scalable manner.


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Figure 3. Characteristics of the L8-P2 pressure sensor. (a) Detailed response of the pressure sensor over a ultrawide pressure range of 0 to 400 kPa. The pressure level



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was increased stepwise by 1 kPa from 0 to 10 kPa, then by 10 kPa from 10 to 100 kPa, and finally by 50 kPa from 100 to 400 kPa. Each loading was repeated five times. (b) Response time of the piezoresistive sensor. (c) Stability of the pressure sensor upon 10,000 cycling of loading at 100 kPa and unloading. (d) Photograph of the pressure sensor placed on the wrist and (e) detection of the wrist pulse waves. (f) Detailed curve of the wrist pulse wave indicated three distinct peaks: P1, P2 and P3. (g) Schematic illustration of a 3 ⅹ 3 sensor array. (h) Comparison of the weight of the weighing weights of known values of 50 g, 100 g, 500 g, and 1000 g placed on various sensor locations, as illustrated in (g), and the pressure levels detected by each sensor. (i) Photograph of a 5 ⅹ 5 sensor array loaded with an alphabet M of 330 g and the pressure distribution mapped by the sensor array.

The conductive-island-bridging nanonetwork-based pressure sensor was further explored for different types of elastomeric films in order to additionally utilize the contact resistance between the electrical leads and nanonetworks for significantly enhanced


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sensitivity. Elastomeric films fabricated using sandpaper as a mold was used to fabricate these sensors (Figure 1g). Unlike the pyramid-studded film that formed a flat contact with the electrical leads (Figure 1f), the entire film of these studied piezoresistive sensors was rough. Thus, the contact between the nanonetwork and the electrical leads also affected the overall piezoresistive response. In fact, the pressure sensor that solely depended on this contact resistance with the electrical leads, i.e., did not employ Au islands, was analogous to the interdigitated electrode (IDE)-based pressure sensor. Therefore, this type of sensor was also compared with the responses of the sensors that included Au islands. The pressure sensor without Au islands was highly sensitive in the low-pressure range but became readily saturated above 100 kPa; however, the use of Au islands greatly enhanced the piezoresistive response and the operating pressure range, as shown in Figure 4a. The pressure sensor employing eight lines of rectangular Au islands did not become saturated, even at 400 kPa. The sensitivity was 3.86 kPa-1 at 10 kPa, 0.3 kPa-1 at 100 kPa, and 0.06 kPa-1 at 400 kPa. The sensitivity in the low pressure range was comparable with or better than other reported values and the sensitivity in the high pressure range (400 kPa) was the highest among reported values 25 ACS Paragon Plus Environment


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(supplementary Table S1). This high operating range and sensitivity enabled precise monitoring of pressure distribution during walking and a detailed analysis of walking kinetics as demonstrated below. The detection limit and response time were ~120 Pa and 23 ms, respectively (supplementary Figure S8, S9) and the wrist pulse wave was successfully detected (supplementary S10) with a much higher magnitude in current change. Overall, these results clearly suggest the advantages and versatility of the proposed approach.

The circuit-compatible pressure sensors with high sensitivity and ultrawide operating pressure ranges enabled insole-shaped PCB-based pressure sensing system that included a data processing unit, wireless data transmission, and charging module. An insole-shaped PCB that included Au islands (L8-P2) for six pressure sensors (marked as 1–6) was fabricated (Figure 4b and supplementary Figure S11, S12). Au islands for each pressure sensor were fabricated at specific positions important for gait analysis.4244

The insole-shaped PCB that included the Au islands, electrical leads, and

interconnects was fabricated as a unit and the SWNT/elastomeric films were then


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layered onto the specific locations, completing the fabrication of the pressure-sensing system for in situ monitoring of walking patterns. The voltage response of the pressure sensor was converted to the corresponding pressure level using a pre-measured calibration curve for each sensor (supplementary Figure S13). The detailed curves of the pressure levels from the pressure sensors placed at various positions during walking and the detailed curves are shown in Figure 4c. An analysis of the curves according to the general gait cycle shown in Figure 4d indicated a stance and swing time of approximately 1.04 and 0.59 s, respectively, which are consistent with prior report.45 Moreover, mapping the distribution of the piezoresistive response according to the gait cycle across the entire insole during walking indicated that the pressure propagated from the heel (1) to the toe (6) through the middle positions (2, 3, 4 and 5), as shown in Figure 4e. These capabilities for the analysis of walking kinetics could be used to help a therapist correct an ineffective walking style, to accelerate the recovery of a patient undergoing rehabilitation, or to analyze daily activities for digital healthcare.

CONCLISIONS



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In summary, we have demonstrated high-performance wearable resistive pressure sensors that were highly tunable, sensitive, stable and operative over ultrawide pressure range and circuit-compatible using a conductive-island-bridging nanonetworkbased approach. Rational design of the arrangement of the conductive islands and of the resistivity of the nanonetwork was used to systematically tune the sensitivity, linearity, and operating pressure range of the sensor. The extreme tunability of the linearity of the piezoresistive response over large pressure ranges demonstrated here has not been reported before. Furthermore, the interconnect-embedded PCB layer of the pressure sensor also allowed for a facile integration of an array of high-performance pressure sensors with measurement circuits and wireless communication modules. An insole-shaped PCB-based pressure sensor system containing the developed pressure sensors enabled sensitive and stable wireless monitoring of pressure distributions and gait kinetics during walking. Our approach is cost-effective, simple, and versatile. Moreover, flexible sensors can be fabricated using flexible PCBs (supplementary Figure S14) for conformable devices. Therefore, this approach could provide a simple yet


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powerful sensing platform for emerging applications such as digital healthcare, intelligent human-machine interfaces, and internet of things.



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Figure 4. An insole-shaped PCB-based pressure sensing system for wireless monitoring of foot pressure distribution during walking and gait dynamics. (a) Comparison of piezoresistive responses of pressure sensors with microstructured lead contacts fabricated using SWNTs deposited on sandpaper-molded PDMS and with different arrangements of Au islands. The addition of Au islands also greatly enhances the response and the operating pressure range (>100 kPa) of sensors with microstructured lead contacts. (b) Photographs of the insole-shaped PCB containing six pressure sensors at various locations (1–6). The insole-shaped PCB containing the Au islands and interconnects for all six pressure sensors was fabricated as a single unit. The circuits for data processing, a module for wireless data communications, and a battery are also shown in the back side image. (c) Pressure levels obtained from various pressure sensors during walking. The detailed responses of each pressure sensor are also shown. (d) Illustration of gait cycles and the analyzed times for the stance and swing phases. (e) Mapping of piezoresistive response across the entire insole during the stance phase of a single gait.


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Methods

Selective deposition of SWNT nanonetworks on microstructured elastomers

The mold for regular pyramidal patterns was prepared via anisotropic wet-etching of a Si wafer using a KOH solution, as previously reported.26 The mold for irregular rough morphologies was prepared using an abrasive paper (No. 220 sandpaper). A PDMS solution having a base: curing agent ratio of 10:1 (w/w) and degassed for 1 h was spin-coated on the mold surface at 500 rpm for 30 s and cured at 60°C for 6 h. The area of the elastomer to be deposited with SWNTs was O2 plasma-treated using a mask. Single-walled carbon nanotubes (SWNTs, Nanointegris Inc.) were dispersed in a dichlorobenzene (DCB) solution to concentrations of 0.005 mg/mL, 0.025 mg/mL, and 0.050 mg/mL. The dispersions of SWNTs in DCB were dip-coated onto the O2 plasmatreated elastomer films, dried in ambient condition, and then heat-treated on a hot plate set at 200°C for 30 min.



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Fabrication of resistive pressure sensors based on conductive-island-bridging nanonetworks

Conductive Au islands and interconnects were prepared on a thin PCB substrate based on FR-4 materials using commercially available PCB technology (HANSEM DIGITEC). The width of the rectangular Au islands was varied from 300 μm to 4 800 μm whereas the length was set at 1 mm. The production of pressure sensor was completed by stacking the nanonetwork-deposited elastomeric film onto the PCB containing the Au islands and electrical leads.

Characterizations of materials

The microstructures of the SWNTs on the PDMS film were characterized using a fieldemission scanning electron microscope (FE-SEM, Jeol) operating at an acceleration voltage of 20 kV and also using an atomic force microscope (XE-100, Park systems) operating in non-contact mode. The sheet resistance of the nanonetwork was measured by using the van der Pauw method (HMS-3000, Ecopia). The absorbance levels of the


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SWNT nanonetworks prepared using the various concentrations of the SWNTs in DCB were measured by performing UV-Vis spectroscopy (SynergyMx, BioTek).

Characterizations of the piezoresistive response of the developed sensor

A motorized vertical stage (z-stage) integrated with a force gauge was used for applying pressure to the device and the pressed area was 1 cm2. A voltage of 1 V was applied to the device using a LabVIEW-controlled source meter (Keithley 236), and the resultant current data under various pressure levels were collected using the same source meter. For the full characterization of the piezoresistive response, the pressure was increased stepwise by 1 kPa from 0 to 10 kPa, then by 10 kPa to 100 kPa and finally by 50 kPa to 400 kPa. Loading at each pressure level was repeated five times. The stability of each tested device was performed by applying 10,000 cycles of loading at 100 kPa and unloading and recording the current level of the device.

Detection of arterial pulse waves on the wrist

A thin L8-P2 pressure sensor employing SWNT nanonetworks (0.025 mg/mL in DCB) on an elastomeric film studded with pyramidal microstructures was fabricated and 33 ACS Paragon Plus Environment


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placed onto the wrist of a subject in a manner so that the elastomeric surface was in contact with the skin. Additional pressure was applied to improve the stability of the measurement. The current level was monitored using a potentiostat (PARSTAT MC, Princeton Applied Research) at an applied voltage of 1 V and a sampling interval of 10 ms.

Wireless monitoring of the distribution of foot pressure and walking kinetics

An insole-shaped PCB containing six L8-P2 patterns at specific locations including heel and toe locations and electrical leads was designed and fabricated using a commercial PCB technology (HANSEM DIGITEC). SWNT nanonetworks (0.025 mg/mL in DCB) on sandpaper-molded elastomer films were layered onto each of the L8-P2 patterns where electrical leads for each sensor were connected to the wireless measurement module. The insole was covered using a tape (supplementary Figure S11). The real-time response of each of the six pressure sensors under normal gait was recorded using a wireless measurement system integrated with the insole-shaped PCB (supplementary Figure S12). The voltage level across a resistor of a known resistance value that was


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connected to the pressure sensor in series was measured and the corresponding pressure level was calculated using a calibration curve (supplementary Figure S13). Calibration curves were obtained for all the sensors.

ASSOCIATED CONTENT

Supporting Information.

Schematic illustration of the process used to fabricate various types of elastomeric PDMS films, Optical micrographs of pressure sensors with various Au islands widths at a fixed areal coverage of 50% and PDMS films studded with pyramidal micropatterns, Comparison of the piezoresistive responses of pressure sensors having various Au arrangements, Comparison of the piezoresistive responses of pressure sensors having different sheet resistances of the SWNT nanonetwork, Photograph of the packaged insole-shaped pressure sensing system, A schematic of the insole-shaped pressure sensing system for wireless monitoring of foot pressure distribution and gait dynamics,



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A representative calibration curve of the pressure sensor used for the wireless monitoring of foot pressure distribution during walking and gait dynamics.

AUTHOR INFORMATION

Corresponding Author *Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes Korea Institute of Science and Technology has filed a patent related to this work (application no. 2016-0127141 (KR) and application no. 15/722,266 (US)).

ACKNOWLEDGMENT This work was supported by Korea Institute of Science and Technology (KIST) through Young Fellow Program and Institutional Program. This work was also supported in part


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by Small and Medium Business Administration of Korea through Convergence Program (S2272818) and by the National Research Foundation of Korea (NRF) grant (No. NRF2017R1C1B2004765) by the Korea government (MSIT).

ABBREVIATIONS PCBs, printed circuit boards; SWNTs, single-walled carbon nanotubes; PDMS, polydimethylsiloxane; DCB, dichlorobenzene; AFM, atomic force microscopy; IDE, interdigitated electrode.

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Table of Contents (TOC) Graphic



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Wearable Piezoresistive Sensors with Ultrawide Pressure Range and

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