A Capacitive Pressure Sensor with Linearity and High Sensitivity over a Wide Pressure Range using Thermoplastic Microspheres

A flexible pressure sensor with a linear response over a wide pressure range with high sensitivity is extremely desirable for high‐pressure applications, such as monitoring body weight and plantar pressure distributions. As a promising candidate to achieve the high sensitivity, a porous structure in capacitive pressure sensors is essential. However, this structure invokes the early saturation of sensitivity owing to a low compression modulus. In this study, thermoplastic microspheres (TPM) as porous materials are utilized, and demonstrate a solution‐processed simple structure that is highly sensitive (3.1 × 10−3 kPa−1) over a wide pressure range (1 MPa) with linearity (R2 = 0.996). The TPM‐containing pressure sensor responds within 100 ms and can withstand a 10 000 times cyclic test under 150 kPa. The linearity may be attributed to the rigid characteristics of the TPM‐containing membrane. These results show that the TPM‐containing pressure sensor has sufficient sensitivity and linearity to reduce the need for complex system calibrations and has potential applications in pressure distribution and weight monitoring.


A Capacitive Pressure Sensor with Linearity and High Sensitivity over a Wide Pressure Range using Thermoplastic Microspheres
Yusaku Tagawa, Sunghoon Lee, Takao Someya,* and Tomoyuki Yokota* DOI: 10.1002/aelm.202201304 foot of a diabetic patient because a local plantar pressure of 200 kPa or higher may be a risk factor for ulceration. [7][8][9] Studies have also demonstrated that certain diabetic patients have plantar pressures of 500 kPa or more. [10,11] As another example, a rapid change in body weight for patients with heart failure is a critical biomarker. [12] The body weight monitoring is typically required from several hundred kPa to several MPa. [13] In summary, a wearable flexible pressure sensor with a high-pressure sensitivity range (1 MPa) can provide precise critical biomedical information by measuring the weight and plantar pressure distribution.
Among the various flexible pressure sensors, capacitive pressure sensors are one of the most promising candidates owing to their simple fabrication process, rapid response, small hysteresis, and low power consumption. [14] In spite of these advantages, typical silicone rubber have a relatively low dielectric constant (2)(3), which contributes to a low sensitivity. Therefore, porous structures at the dielectric layer and dielectric/conductor interface are essential for achieving a higher sensitivity because they reduce the initial dielectric constant and compression modulus.
Porous structures in a capacitive pressure sensor are fabricated by applying porous structure processes or utilizing porous materials. Microstructured templates, such as pyramids, [15,16] bumps, [17,18] hemispheres, [19] and pillars [20,21] can be used to fabricate the interfacial airgap. Other porous structure processes include nanofibers fabricating fluffy lamination with air vacancy. [22,23] Using porous materials is favorable because it offers various advantages, such as the adaptability to solution processes, simplicity of fabrication methods due to the large area, and low cost. Foaming materials such as ammonium bicarbonate (NH 4 HCO 3 ) and sodium hydrogen bicarbonate (NaHCO 3 ), [24,25] or water-dissolvable materials such as sugar, have been utilized to introduce pores in the bulk dielectric layer with forming processes. Thermally expandable polymers with an elastic solvent have been heated and coated to create homogeneous and simple porous structures without using dissolving processes. [26,27] Other methods include introducing flowershaped porous structures by laser cutting and implementing them as insole pressure sensors, [3] combining piezoresistive and capacitive types, [28] and using multiple composite materials capable of fabricating large and small porous structures. [29,1] A flexible pressure sensor with a linear response over a wide pressure range with high sensitivity is extremely desirable for high-pressure applications, such as monitoring body weight and plantar pressure distributions. As a promising candidate to achieve the high sensitivity, a porous structure in capacitive pressure sensors is essential. However, this structure invokes the early saturation of sensitivity owing to a low compression modulus. In this study, thermoplastic microspheres (TPM) as porous materials are utilized, and demonstrate a solution-processed simple structure that is highly sensitive (3.1 × 10 −3 kPa −1 ) over a wide pressure range (1 MPa) with linearity (R 2 = 0.996). The TPM-containing pressure sensor responds within 100 ms and can withstand a 10 000 times cyclic test under 150 kPa. The linearity may be attributed to the rigid characteristics of the TPM-containing membrane. These results show that the TPM-containing pressure sensor has sufficient sensitivity and linearity to reduce the need for complex system calibrations and has potential applications in pressure distribution and weight monitoring.

Introduction
Flexible electronics provide various benefits toward wearable devices for healthcare, and medical applications owing to the bio-adaptability. [1] Flexible pressure sensors enable to measure various biological signals for long-term monitoring. In particular, obtaining plantar pressure distributions and body weight measurements through wearable devices is significant not only for healthcare applications, [2,3] but also for monitoring various diseases such as diabetes, obesity, and heart failure. [4][5][6][7][8] For example, measuring the distribution of the plantar pressure is an essential guideline for reducing the burden on the www.advelectronicmat.de On the other hand, essential characteristics of stable operation such as linearity over a wide range have yet to be dealt with enough. [30] Nonlinear dynamic response by the sensitivity saturation of porous materials increases the complexity of the measurement system and limits the measurement range. As a result, balancing the linear response and high sensitivity in porous structures remains challenging. [31,32] The maximum linear response range of the flexible capacitive bulk pressure sensor was ≈800 kPa but the sensitivity was under 10 −3 kPa −1 . [33,34] On the other hand, the porous structure sensors achieve high sensitivity more than 10 −2 kPa −1 but the linear response range is under 200 kPa. [3,15,19,[24][25][26]28,[35][36][37][38] To the best of the author's knowledge, no capacitive pressure sensor with porous structures that maintains linearity up to 1 MPa with a high sensitivity (>10 −3 kPa −1 ) exists.
In this study, A capacitive flexible pressure sensor containing thermoplastic microspheres (TPM) that has linearity and a high sensitivity over a wide pressure range is proposed. The TPM expanded at relatively low temperatures (95-125 °C) and increased 50 times in volume. The TPM-containing Polydimethylsiloxane (PDMS) was spin-coated to fabricate an elastomeric membrane with a homogeneous porous structure. Notably, porous PDMS with TPM has a higher Young's modulus than that of bulk PDMS. This rigid stiffness of TPM-containing PDMS indicates that the relatively high modulus and porous structure prevent a loss of sensitivity and enhancing the linear response. Owing to this characteristic of TPM, we achieved a pressure sensor with a high sensitivity (3.1 × 10 −3 kPa −1 ) while maintaining linearity (R 2 = 0.996) under a high compression pressure (1 MPa). Furthermore, the sensor responded within 100 ms under 140 kPa and indicated the durability by 10 000 cyclic loadings. As a pressure distribution application, we implemented 3 × 3 multichannel pressure sensors with a large area (3.4 cm × 3.4 cm) to demonstrate the uniformity of the materials. In addition, we fabricated geta wearables that can detect body weight. As a result, a 4.9 kPa load fluctuation can be detected equally well in a high and low weight range without sensitivity compensation systems, such as nonlinear fitting, which implies the significant potential to achieve reliable and simple wearable applications. Figure 1 presents the characteristics of the TPM polymer. TPM is constructed by a polymer shell containing liquid gas. Figure 1a,b juxtaposes the TPM schematic images and actual pictures before and after the heat treatment. The heat treatment softens the TPM polymer shell and expands the liquid gas inside the TPM. The volume of TPM increases by ≈50 times after heat treatment. We captured the SEM images in Figure 1c,d to observe the TPM before and after expansion. The results of the particle size distribution after heat treatment at different temperatures are shown in Figure 1e to investigate its effect on the expansion rate of TPM ( Figure S1, Supporting information). The average area of TPM under each thermal condition (pristine microsphere, 95 °C, 105 °C, 115 °C, 125 °C) is 108 µm 2 , 135 µm 2 , 1164 µm 2 , 1329 µm 2 , and 1533 µm 2 , respectively. The TPM polymer dramatically expanded at temperatures greater than 105 °C. With more than 125 °C annealing, the TPM polymer was no longer countable because of the adhesion to each other. This TPM has a smaller diameter after the expansion than that of the polymer material in the thermally expandable pressure sensor previously reported. [26,27]

Fabrication of TPM Containing Capacitive Pressure Sensor
The fabrication process is demonstrated in Figure 2a through five steps: i) preparing the thin film polyimide substrate with  www.advelectronicmat.de the patterned gold electrodes and placing the substrate on a glass slide, ii) preparing and pouring a mixture of PDMS and TPM onto the substrate shown in (i), iii) coating the mixture over the entire polyimide substrate by spin-coating, iv) a two-step heat treatment making the TPM expand (95-125 °C for 5 min), and PDMS cross-link (70 °C for 2 h), and finally (v) coating the pure PDMS again as an adhesive layer on the TPM-containing the PDMS and attaching the top electrode by thermo-compression bonding. The fabrication process is described in detail in the Materials & Methods section. The overall thickness varies with the TPM content, expansion temperature, and spin-coating speed; however, the total thickness of the sensor is <350 µm (Figure 2b). Figure 2c demonstrates the fabricated pressure sensor and a laser microscope image of the TPM-containing PDMS film. The TPM-containing PDMS film is a flexible material that can be bent, twisted, and stretched, as shown in Figure 2d. Table 1 to compare the pressure sensitivity S = (ΔC/C 0 )/ΔP and linearity (correlation coefficient R 2 ). Here, ΔC, C0, ΔP were capacitance change from initial capacitance, initial capacitance, and pressure differences. The membrane at the 125 °C expansion condition demonstrated the most significant response in both sensitivity and linearity. At a temperature greater than 125 °C, the volume of TPM decreases. We also compared the process condition of TPM concentrations: 0, 3, 5, 8 wt.% (Figures 3c, S4, Supporting Information). The total thickness of 0, 3, 5, and 8 wt.% were 71 ± 4.2, 151 ± 2.6, 178 ± 8.6, and 226 ± 19 µm, respectively. The results indicated that the 8 wt.% TPM achieved the best sensitivity. Measurements of the Young's moduli of the three samples indicated that the high concentration of TPM caused it to become rigid and hard to deform in the low-pressure range ( Figure S2, Supporting Information). For TPM contents above 8 wt.%, the films expanded nonuniformly and the dielectric layers became bumpy. Based on the aforementioned, the 8 wt.% and 125 °C membrane-forming conditions are preferable for the material used in this study. The optimized pressure sensor demonstrates a rapid response (within 100 ms, as shown in Figure 3d), a high durability under the 10 000 times cycling pressure of 150 kPa (Figure 3e), and relatively low hysteresis ( Figure S3, Supporting Information). This rapid response and durability demonstrate the potential of the proposed pressure sensor to stably operate in a high-pressure range. The optimized sensor presents significant superiority compared to the previous porous structured capacitive pressure sensor (Figure 3f).

Multichannel Pressure Sensor Applications
To demonstrate that the proposed pressure sensors can be fabricated over a large area with uniformity, Figure 4a displays an experiment of a sensor capable of 3 × 3 multipoint measurements. The average initial capacitance of each sensor is 5.02 ± 0.23 pF (Figure 4b). We placed a weight with a constant load of 100 g on the large-area multipoint sensors, as shown in Figure 4c, and measured the capacitance change using a multiplexer (Figure 4d). The capacitance change was smaller at the peripherical region of the sensor. One to three fingers were used to press the sensors to demonstrate that each sensor works independently, as shown in Figure 4e. We confirmed that the pressure sensor of each pixel can be measured independently despite multiple fingers pressing several pixels.

Wearable Body Weight Monitoring Using Geta
Body weight measurements using wearable devices are important for monitoring healthcare problems and detecting symptoms of heavy diseases. [3,39] Using the linear response of the proposed pressure sensor over a wide range of pressures, we implemented Geta wearables ( Figure 5) to accurately measure the body weight. Geta is a traditional Japanese footwear that usually supports the entire body weight on one or two surfaces (Figure 5a). Therefore, by attaching flexible pressure sensors with large areas to the entire sole of shoes, it is possible to cover the entire body weight with the sensors (Figure 5b). As a result, the rates of change in the capacitance for the 4.9, 9.8, and 25 kPa weights at low loads are equivalent to the rates of change for the 4.9, 9.8, and 25 kPa weights at high loads (under 300 kPa) as shown in Figures 5c, S5, 5d, Supporting Information. Furthermore, this demonstrates that a flexible pressure sensor can accurately measure weight fluctuations without a Wheatstone bridge circuit or nonlinear fitting. Figure 5e indicated that the proposed pressure sensor also could detect the dynamic change of ground reaction force (GRF) owing to the fast response.

Discussion
Unlike previous capacitive pressure sensors with a soft porous structure, [26,27] we simultaneously achieved the linear response and high sensitive pressure sensor by the hard porous materials (Figure 3f). The previous porous materials reduce the compression modulus and initial permittivity, that invoke high sensitivity although the linear response was sacrificed. The relatively small TPM polymer (Figure 1e) can endure significant pressures and become rigid compared to the PDMS materials, whereas the porous structure reduces the initial capacitance ( Figure S4, Supporting Information). In the low-pressure region (0-50 kPa), the initial capacitance increase may be canceled owing to the high moduli. In typical porous materials, the sudden decrease in dielectric thickness in the low-pressure region often induces a dramatic capacitance change and invokes a nonlinear response. [24] The TPM polymer reduced the air gap

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in the middle-pressure region (50-200 kPa), contributing to a similar sensitivity toward the conventional porous structure.
In the high-pressure region (200-1000 kPa), the stress-strain curve shown in Figure S2 plateaued, indicating that the modulus became lower than the lower-pressure region. [38] The low modulus in the high-pressure region may be one of the most significant points of the linearity of the TPM materials.
To improve further sensitivity and linearity of the porous capacitive pressure sensor using TPM materials, two strategies may help. One is to improve the dielectric constant. [30] By introducing nanofiller structures, the dielectric constant can be increased without significantly affecting the mechanical properties of the dielectric film. Increasing the porosity can achieve a sensor with lower initial capacitance, high sensitivity, and linear response. [40,41] Sensor area increase during loading is another candidate for a strategy that contributes to sensitivity. Since the proposed TPM is a solution-processable material, the microstructures could be utilized. Square structures [19] or biomimetic structures such as Calathea Zebrine Leaf [42] are promising because they are relatively possible to combine linearity and high sensitivity. This research has several limitations. First, the sensor has nonnegligible hysteresis ( Figure S4, Supporting Information). The combination of the microstructured template could solve the problem due to the fast redundancy of the interfacial layer. In addition, the intrinsic creep phenomenon in the highpressure region could be another problem in reading out the absolute value for the weight measurements. The capacitance gradually increased when we used the constant weight because of the continuous deformation. In Section 2.5., We eliminated this effect by taking the difference before and after putting on the constant weight. The thinner sensor could erase these problems using some additional time-dependent calibration. Finally, we annealed the TPM-containing PDMS on the polyimide substrate. The process sometimes affects the substrate flatness, and the more TPM concentration, the more bump structures are constructed. Such instability may become critical for printing or other flexible and stretchable substrates. The preheating process before mixing PDMS curing agent could solve the problem like in previous work. [26] On the other hand, with increasing of TPM concentration, the solution becomes high viscosity and hard to treat by solution process. This phenomenon leads to a decrease in the sensitivity owing to the limitation of the TPM concentration.

Conclusion
In this study, we demonstrated the flexible capacitive sensor with linearity and a high sensitivity over a wide range of pressures using the TPM polymer. The TPM materials have the unique characteristics of a high compression modulus, that might invoke the linearity and high sensitivity of the proposed sensor. As a result, the capacitive pressure sensor with a TPMcontaining PDMS membrane as a dielectric layer demonstrates a high sensitivity (3.1 ×10 −3 kPa) and linearity (R 2 = 0.996) over a wide pressure range (0 to 1 MPa). We fabricated two types of prototypes for the application of the linear response pressure sensor, including the 3×3 multichannel matrix and Geta wearables, to clarify the processability of a large area and emphasize the importance of linearity with the flexibility for monitoring human weight. Accurate and accessible wearable body weight monitoring devices using TPM-containing PDMS sensors can contribute to the expansion of applications in healthcare and medical device fields.

Experimental Section
Evaluation of TPM Diameter Distribution: For sample preparation, ≈0.05 g of TPM powder was placed on the slide glass and annealed at 95 °C, 105 °C, and 115 °C, for 5 min and at 125 °C for 2 min. The expanded TPMs were transferred to the black double layer adhesive tape on the slide glass and blown with a nitrogen gun. A digital microscope (VHX-7000, Keyence) was used for imaging and analyzing (Supplemental Figure S1). It was manually divided the connected particles and reduced the tiny particles; the analyzer automatically calculated the particle size and area.
Fabrication of Capacitive Pressure Sensor: A polyimide (PI) film (12.5 µm thickness) was prepared having top and bottom electrode substrates; 50 nm of gold was deposited on the PI film by thermal evaporation. The thermoplastic microspheres (Expancel 031 DU 40, Nouryon) were mixed with a polydimethylsiloxane (PDMS, silpot184, Dow Corning Toray Corp., Japan) base and a curing agent having a weight ratio of 10:1. The ratios of the microspheres and PDMS were 0, 3, 5, 8 wt.% toward the base solvent. The total weight of the solution was between 3 and 5 g. Mixing was conducted at 2000 rpm for 5 min with a vacuum state using a mixer (Awatori Rentaro, ARV-310, Thinky Corp., Japan). To eliminate the bubbles in the mixed solution, a vacuum pomp and chamber were used for an hour. The mixed solution was spin-coated on a PI substrate attached to the glass substrate at 300 rpm for 5 s and 1000 rpm for 60 s. The spin-coated substrate was then inserted into an oven at www.advelectronicmat.de 95, 105, 115, and 125 °C for 10 min to expand the thermoplastic microspheres. The substrate was additionally annealed at 70 °C for 2 h for curing. After the substrate returned to room temperature, a pure PDMS solution (weight ratio of 10:1) was spin-coated at 500 rpm for 5 s and 3000 rpm for 60 s. The PDMS-coated substrate and top electrode were attached by a thermal pressing machine (heater press, NPa Systems) using a temperature and force of 70 °C and 30 N, respectively, for 2 min. After 2 h of curing at 70 °C, the capacitive sensor was detached from the glass substrate.
Evaluation of Capacitive Pressure Sensor: A force gauge (ZTA-500N, IMADA Corp.) and vertical motorized test stand (MX2-2500N, IMADA Corp.) were used. The sensor was placed on the center of the test stand via an acrylic plate. The contact area of the force gauge was covered by the insulating tape. The contact area between the sensor and force gage was 1 cm 2 and the following forces were applied at a sampling frequency of 100 Hz: 0.2, 0.5, 1, 2, 5, 10,15,20,30,40,50,60,70,80,90, 100 N (corresponding to 2, 5, 10, 20, 50, 100, 150, 200, 300, 400, 500, 700, 800, 900, 1000 kPa, respectively). The sensor was connected to an LCR meter (E4980AL, Keysight) and measured at 10 kHz and an amplitude of 2 V. The capacitance was derived using BenchVue (Keysight) at the middle accumulation time mode; the sampling frequency was ≈5 Hz. The rapid accumulation time mode was used in the software for the response time measurement and the sampling frequency was ≈10 Hz. The response time was calculated by 0 to 90% differences in compression phase and 100% to 10% differences in release phase. Cyclic tests were conducted by constant displacement measurement for 10 000 times. The sudden increase of capacitance in several cyclic periods was owing to the sudden increase in the pressure.
Measurement of Young's Modulus: A die-cutting machine was used to prepare the testing samples, and the shape complied with the JIS standard (K 7161-2). Following the cutting process, the thickness of the testing samples was measured by 3D laser microscopy (VK-9700, Keyence). A high-precision mechanical system (AG-X, Shimadzu) was used for the strain and stress measurements. Young's modulus was derived from the slope of the strain and stress fitting of the least squares methods from the 4 to 5% strain region.
Evaluation of Surface Morphology: The surface morphology of PDMS with TPM was measured by scanning electron microscopy (SEM, TM3030Plus, Hitachi High-Tech Corp.). The PDMS membrane was cut off by a medical mess and attached to the black double-sided adhesive layer. The SEM AC voltage was 5 kV. The other microscopic images of the top view were obtained by 3D laser microscopy (VK-9700, Keyence), and the output images combined color and laser images.
Evaluation of 3 × 3 Sensors: The fabrication process of the 3 × 3 pressure sensor was entirely the same having a single channel capacitive pressure sensor with an 8 wt.% TPM-containing PDMS and heated at 125 °C, except the electrode patterns. Each of the six electrodes were connected to the multiplexer via wires (Mogami-wire, AWG36(2706)). The impedance between the two electrodes that were selected by the multiplexer and LCR meter using a Python control module was measured. The 100 g constant weight was covered by a nonconductive film (Bemis, Parafilm PM-992) for shielding. Both hands were covered with gloves during the finger press measurements to reduce the effects of touching on the capacitance.
Fabrication and Evaluation of Geta Wearables: The fabrication of the pressure sensor part was the same as the condition of PDMS with 8 wt.% and 125 °C TPM. The pressure sensors were embedded in two regions of Geta to measure GRF. Capacitance was derived from the two pressure sensors. 100 g, 200 g, and 500 g weights were chosen to evaluate the linearity measurement under almost no weight and large weight (corresponding to 300 kPa). The area of pressure sensors was 2 cm 2 in the linearity test to increase the pressure range. To confirm the sensor ability precisely, the capacitance difference before and after putting on the constant weight were derived under 300 kPa. After confirming the linearity, the author used the geta wearables for dynamic weight change monitoring by alternative standing on one side leg.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.