Flexible pressure sensors show great promise for applications in such fields as electronic skin, healthcare, and intelligent robotics. Traditional capacitive pressure sensors, however, face the problem of low sensitivity, which limits their wider application. In this paper, a flexible capacitive pressure sensor with microstructured ionization layer is fabricated by a sandwich-type process, with a low-cost and simple process of inverted molding with sandpapers being used to form a thermoplastic polyurethane elastomer ionic film with double-sided microstructure as the dielectric layer of the sensor, with silver nanowires as electrodes. The operating mechanism of this iontronic pressure sensor is analyzed using a graphical method, and the sensor is tested on a pressure platform. The test results show that the sensor has ultrahigh pressure sensitivities of 3.744 and 1.689 kPa−1 at low (0–20 kPa) and high (20–800 kPa) pressures, respectively, as well as a rapid response time (100 ms), and it exhibits good stability and repeatability. The sensor can be used for sensitive monitoring of activities such as finger bending, and for facial expression (smile, frown) recognition, as well as speech recognition.

  • •A capacitive flexible pressure sensor with microstructured ionization layer is fabricated by a sandwich-type process, with a thermoplastic polyurethane elastomer ionic film with microstructures as the dielectric layer of the pressure sensor and silver nanowires as electrodes.

  • •Test results show that the sensor has ultrahigh sensitivity, with a pressure sensitivity of 3.744 kPa−1 at low pressures (0–20 kPa) and 1.689 kPa−1 at high pressures (20–800 kPa) and a rapid response time (100 ms), and it exhibits good stability and repeatability.

  • •The sensor is capable of sensitively monitoring activities such as finger bending, can be used for facial expression (smile, frown) recognition, and also enables speech recognition.

In recent years, flexible wearable sensors have become a focus of research in electronics by virtue of their flexibility, variable size, good stability, and bio-adaptability, and they are widely applied to e-skin,1–3 physiological signal detection from the human body,4 and human–computer interaction.5,6 According to their working mechanism, flexible pressure sensors can be classified into four categories: piezoresistive,7 capacitive,8,9 piezoelectric,10 and friction-type.11 With capacitive pressure sensors, changes in pressure cause changes in the spacing of capacitor plates and the dielectric constant of a dielectric layer, resulting in changes in the equivalent capacitance of the sensor. However, traditional capacitive pressure sensors suffer from low sensitivity. Therefore, attempts have been made to increase the pressure-induced deformations and thus improve sensitivity through the use of a microstructured dielectric layer and electrodes or by employing a porous microstructure.12,13 For example, in our previous study,14,15 a microstructured dielectric layer was formed with frosted glass by polydimethylsiloxane (PDMS) inverted molding, and a sensor based on this construction had a sensitivity of 0.24 kPa−1 at low pressures. The use of a porously microstructured dielectric layer was also investigated, but this did not lead to any substantial increase in sensitivity. Unfortunately, the reduced compression modulus of the dielectric layer imposes limits on further improvements in the sensitivity of capacitive sensors, and thus the manufacture of a capacitive pressure sensor with high sensitivity and high signal strength remains a challenge.

Pan and co-workers16,17 have proposed a novel supercapacitive pressure sensor, the iontronic pressure sensor, which is characterized by an ionic dielectric layer containing a large quantity of movable cations and anions.18,19 The ionic dielectric layer in contact with the electrodes forms a nanometer-distance bilayer at the dielectric/electrode interface, with the capacitance density much higher than that of conventional parallel-plate capacitance sensors by a factor of five to six.20,21 The interfacial capacitance varies significantly with the contact area formed by the bilayer, which offers the possibility of manufacturing capacitive pressure sensors with high sensitivity and signal strength.22 

Key to the fabrication of ionic pressure sensors is the use of ionic solutions, which are rich in anions and cations and have the advantages of good stability, strong solubility, and high conductivity.23 Such solutions have found widespread application in, for example, solar cells,24 supercapacitors,25 lithium-ion batteries.26 In addition, the good compatibility of ionic solutions with polymers21 has enabled the preparation of flexible ionic gels. Such gels have excellent compressibility and high ionic conductivity and are widely used to fabricate flexible pressure sensors.27 For instance, researchers from Hangzhou Normal University, taking natural rose petals as a template, used a two-step inverted molding process to prepare an ionic gel media layer with a rose bionic pattern and thereby fabricate a high-performance ionic pressure sensor.28 

In this paper, to manufacture an ionization sensor through a sandwich-type process. The microstructure on the surface of sandpaper is replicated on top of a thermoplastic polyurethane elastomer (TPU) ionic film by means of an inverted molding process, with sandpaper as the mold, TPU ionic film as a dielectric layer, and silver nanowires (AgNWs) as electrodes. The capacitive signal of the sensor is greatly enhanced by the double capacitance created between the ions on the TPU ionic film and the electrons on the AgNW electrodes. In addition, owing to the compressibility of the raised structures on the top and bottom of the TPU ionic film, the contact area between the raised areas of the film and the electrodes of the AgNWs is greatly increased under pressure, and thus the capacitance is significantly increased even when the pressure is weak. The results show that this ionic pressure sensor exhibits a rapid response speed and high sensitivity and could be used for monitoring of finger bending, facial expression recognition, and pulse measurement.

The operating mechanism of the iontronic pressure sensor is shown in Fig. 1(a). The uneven microstructure (with small indentations) on the top and bottom surfaces of the ionic film is the most important component providing the pressure sensitivity of this type of pressure sensor. In contrast with a conventional capacitive pressure sensor, the capacitance of the iontronic pressure sensor has three components: the capacitance between the upper electrode and the upper layer of the ionic film, that between the lower electrode and the lower layer of the ionic film, and that between the upper and lower electrodes and the ionic film. Change in the capacitance between the ionic film and the electrodes depend primarily on the contact area between the two. For example, in the initial state, there is less contact between the electrodes and the small indentations on the ionic film, therefore only a relatively small capacitance is generated at the contacting interface of the two layers of electrodes and the ionic film. As the pressure increases, the raised microstructures are slightly flattened and embedded into the AgNW electrodes, resulting in a significant increase in the contact area of the ionic film with the electrodes and thus in the equivalent capacitance of the sensor. As the pressure continues to increase, these protrusions tend to be almost completely flattened, thereby increasing the contact area between the electrodes and the dielectric layer, and thus the equivalent capacitance of the sensor. Therefore, variations in external pressure can be monitored by the variations in output capacitance.

FIG. 1.

(a) Operating mechanism of iontronic pressure sensor. (b) Finite element simulations showing internal stresses resulting from different external pressures.

FIG. 1.

(a) Operating mechanism of iontronic pressure sensor. (b) Finite element simulations showing internal stresses resulting from different external pressures.

Close modal

Figure 1(b) shows a finite element simulations of the deformation of the microstructure ionic layer under low and high pressure. As can be seen, under low pressure, the stress concentration is distributed in the upper parts of the microstructure surface. As the pressure gradually increases, the lower parts of the microstructure also undergo contact deformation.

A flow chart of the preparation of the iontronic pressure sensor is shown in Fig. 2. Specifically, 20 ml of dimethylformamide (DMF) and 20 ml of tetrahydrofuran (THF) were poured into a beaker, and 3.5 g of TPU particles were poured into the mixed solution, and full dissolution was achieved by magnetic stirring for 3 h. Then, 5 ml of 1-ethyl-3-methylimidazole bis(trifluoromethanesulfonyl)imide salt ionic solution ([EMIM]+[TFSI]) with a concentration of 99% was poured into the stirred solution, which was thus rich in anions and cations. The solution thereby prepared was in the form of a slightly yellow transparent gel, and was subjected to magnetic stirring for 3 h to uniformly disperse the ionic solution in the TPU. The stirred liquid in gel was poured onto sandpaper with a mesh number of 2000, and was homogenized by a spin coater at a speed of 250 rad/s. The homogenized ionic gel was then covered with another piece of sandpaper and placed on a heating plate at 110 °C for 2 h. The sandpaper on both sides of the ionic film was then peeled off to leave an ionic film with a microstructure on both sides, and this was then combined with two layers of TPU film with AgNW electrodes, thus completing the fabrication of the sensor through a sandwich-type process. A photograph of the sensor is shown in Fig. 2(b).

FIG .2.

(a) Preparation process of the pressure sensor. (b) Photograph of sensor. (c) Surface structure of ionogel dielectric layer.

FIG .2.

(a) Preparation process of the pressure sensor. (b) Photograph of sensor. (c) Surface structure of ionogel dielectric layer.

Close modal

The surface structure of the ionic film dielectric layer is shown in Fig. 2(c). It can be seen that there are multiple high and low protruding structures on the surface of the ionic film, because of the inverted molding with the sandpaper. The introduction of these protruding microstructures effectively improves the sensitivity of the sensor.

Figure 3(a) shows testing platform for the iontronic sensor, comprising the sensor, a press machine, an impedance analyzer, and LabVIEW software. The sensor was placed on the placement platform of the press, which was used to apply pressure to it via a pressure bar. The leads of the sensor were connected to the interface of the impedance analyzer, such that the changes in the capacitance of the sensor could be monitored in real time by LabVIEW.

FIG. 3.

(a) Sensor testing platform. (b) Sensitivity test of sensor. (c) Repeatability test of sensor. (d) Response time test of sensor. (e) Capacitance–time curve with loading and unloading of 0.01 g sponge. (f) Variation of output capacitance of sensor during 1000 loading cycles.

FIG. 3.

(a) Sensor testing platform. (b) Sensitivity test of sensor. (c) Repeatability test of sensor. (d) Response time test of sensor. (e) Capacitance–time curve with loading and unloading of 0.01 g sponge. (f) Variation of output capacitance of sensor during 1000 loading cycles.

Close modal
The sensitivity of the iontronic pressure sensor can be obtained as
(1)
where S denotes the pressure sensitivity of the pressure sensor, ∆C is the absolute value of the difference between the testing capacitance and the initial capacitance, C0 is the initial capacitance, and P is the pressure applied to the sensor. The sensitivity curve of the sensor shown in Fig. 3(b) was plotted by gradually applying a pressure of 0–800 kPa to the sensor and recording the output capacitance values. It can be seen from the results that the sensor is able to withstand a wide range of pressure (0–800 kPa). To better analyze the pressure sensitivity of the sensor, the applied pressure was divided into two ranges based on the test curves: low pressure (0–20 kPa) and high pressure (20–800 kPa). The pressure sensitivities of the sensor were 3.744 and 1.689 kPa−1 in the low- and high-pressure ranges, respectively. The sensitivity at low pressure was greater than that at high pressure because the initial capacitance of the sensor was smaller at low pressure, and the AgNW electrodes and the ionic film had just started to come into contact with each other. There were microstructured protrusions on the surface of the ionic film, and so deformations occurred owing to extrusion under low pressure, and simultaneously the double-capacitance effect of the electrode interface was enhanced, and thus the sensor’s pressure sensitivity was larger. With increasing pressure, the microstructure on the surface of the ionic film tended to flatten, and it was mainly the action of the anions and cations in the ionic film that enhanced the equivalent capacitance of the sensor under high pressure.

Different pressures (3, 60, 280, 500, and 750 kPa) were applied to the sensor, each for three times, and the test results are shown in Fig. 3(c). The results showed that the sensor could sensitively identify pressure values in ranges from low pressure to high pressure, and the changes of output capacitance under the same pressure were basically the same, demonstrating the good repeatability of the sensor. Figure 3(d) presents the test results for the response time of the sensor, and it can be seen that the response and recovery times of the sensor are both 100 ms.

A 0.01 g sponge was placed on the pressure sensor, and the test result is shown in Fig. 3(e). The iontronic pressure sensor could sensitively detect the loading of the sponge with a small detection limit (∼1 Pa). The mechanical stability of a pressure sensor is an important indicator characterizing its performance. To verify the mechanical stability of the sensor, we cyclically loaded and released pressure for 1000 cycles to obtain its capacitance output curve, as shown in Fig. 3(f). After 1000 cycles, the output capacitance of the sensor remained roughly the same as during the first loading–unloading, verifying that the sensor is suitable for long-term operation.

With their excellent pressure sensing performance, safety, and wearing comfort, iontronic flexible pressure sensors have potential applications in wearable electronic devices for monitoring human physiological signals. To verify this, we designed several demonstration experiments using the proposed sensor to monitor human activities.

The sensor was attached to the finger joints, and the degrees of bending of the fingers was monitored by changes in sensor capacitance. The test results are shown in Fig. 4(a). It was found that the sensor could sensitively detect changes in the various angles of finger bending. The output capacitance of the sensor reached about 1300 nF when the finger was bent to 90°, which indicated that the sensor had a wide range of detectable pressure and large output capacitance values.

FIG. 4.

Use of sensor for testing (a) finger bending, (b) smiling, (c) frowning, and (d) swallowing.

FIG. 4.

Use of sensor for testing (a) finger bending, (b) smiling, (c) frowning, and (d) swallowing.

Close modal

The sensor was attached to the cheeks for monitoring the changes that occur in smiles. During smiling, the cheeks arched such that a squeezing force was exerted on the sensor, increasing the pressure and thus its capacitance. The sensor was restored to its original state following the smile, and its capacitance became smaller. The test results are shown in Fig. 4(b).

The sensor was fixed between the eyebrows to detect frowning activity. During a frown, the force on the sensor was reduced, and so the pressure was less and its capacitance became smaller. The test results are shown in Fig. 4(c).

The sensor was fixed at the throat, and the test results presented in Fig. 4(d) show that the sensor could sensitively monitor swallowing activities.

Also with the sensor fixed at the throat [Fig. 5(a)], different capacitance waveforms could be observed on the response curves when volunteers uttered words with different syllables (monosyllable, A; two syllables, HELLO; multisyllables, INTERESTING), and the test results are shown in Figs. 5(b)5(d). It can be seen that the pressure sensor baselines for the three voice tests were different, because the three voice tests were performed separately. The initial capacitance of the pressure sensor is ∼20 pF. The sensor needed to be fixed with tape at the protrusion of the Adam’s apple. Owing to the different fixing forces each time, the baseline values of the sensor changed, but this did not affect speech recognition. Each word was repeated multiple times, and similar capacitive waveforms occurred multiple times in the test results, indicating good repeatability and rapid response speed of the sensor. This indicates that the sensor has a wide range of potential applications in the field of speech recognition.

FIG. 5.

(a) Schematic of setup for sensor to monitor speech. (b) Sensor speech test word: A. (c) Sensor speech test word: HELLO. (d) Sensor speech test word: INTERESTING.

FIG. 5.

(a) Schematic of setup for sensor to monitor speech. (b) Sensor speech test word: A. (c) Sensor speech test word: HELLO. (d) Sensor speech test word: INTERESTING.

Close modal

When the stressed area is relatively large, a single sensor is insufficient. Therefore, a 2 × 2 array sensor was designed. The changes in output capacitance of the pressure-sensitive units were tested by placing different weights (5, 10, and 20 g) on each unit as shown in Fig. 6(a). The results showed that the heavier the applied weight, the higher was the output capacitance of the corresponding sensitive unit. The array sensor was attached by adhesive to the back of a hand, and the output capacitance of each pressure-sensitive unit was utilized to monitor the changes in the pressure caused by the bending of the palm. The capacitance of the pressure sensor was relatively large in the initial state. The test results presented in Fig. 6(b) showed that each sensitive unit of the array sensor could sensitively monitor the force at different locations on the back of the hand, demonstrating the potential for wide application of the array sensor in the field of wearable skin.

FIG. 6.

(a) Pressure distribution from 2 × 2 pixel array of sensors loaded by weights. (b) Pressure distribution on back of hand covered by 2 × 2 pixel array.

FIG. 6.

(a) Pressure distribution from 2 × 2 pixel array of sensors loaded by weights. (b) Pressure distribution on back of hand covered by 2 × 2 pixel array.

Close modal

High sensitivity and a wide linear range are important indicators of sensor performance. Sensors with high sensitivity and wide linear range can more accurately collect information on human tactile perception and physiological signals. Figure 7 compares the proposed sensor with other capacitative sensors in terms of sensitivity and maximum linear range, and it can be seen show that the proposed sensor has a wider linear range (750 kPa) and higher sensitivity (3.744 kPa−1).

FIG. 7.

Comparison of sensitivity and linear sensing range of proposed pressure sensor with those of other capacitive sensors from Refs. 46, 18, 19, 23, 25, and 26.

FIG. 7.

Comparison of sensitivity and linear sensing range of proposed pressure sensor with those of other capacitive sensors from Refs. 46, 18, 19, 23, 25, and 26.

Close modal

A capacitive flexible pressure sensor with microstructured ionization layer has been fabricated by a sandwich-type process, with a TPU ionic film incorporating microstructures as the dielectric layer and AgNWs as the electrodes. The ionic film produces a double capacitance effect under pressure, and the interlayer spacing and the contact area of the ionic film are increased by the presence of the microstructure on the ionic surface. This greatly improves the sensitivity of the sensor, with pressure sensitivities of 3.744 and 1.689 kPa−1 in the low-pressure and high-pressure ranges, respectively. The sensor also has a rapid response time (100 ms) and good stability and repeatability. It can be used for sensitive monitoring of activities such as finger bending, and for facial expression (smile, frown) recognition, as well as speech recognition. Thus, the proposed sensor has a wide range of potential applications in the fields of human–computer interaction, smart skin and healthcare.

This work was supported by the Youth Project of the National Natural Science Foundation of China (Grant No. 52105594), the Youth Project of the Applied Basic Research Program of Shanxi Province (Grant No. 20210302124274), the Key Research and Development Program of Shanxi Province (Grant No. 202102030201005), the Natural Youth Science Foundation of Shanxi Province (Grant Nos. 202103021223005 and 202203021212015), the Fund for Shanxi 1331 Project, the Science and Technology Innovation Plan for Colleges and Universities in Shanxi Province (Grant No. 2022L575). the Science and Technology Innovation Project in Higher Schools in Shanxi (Grant No. J2020383), and Teaching Reform and Innovation Project of the Education Department of Shanxi Province (Grant No. J20221195).

The authors have no conflicts to disclose.

The data that support the findings of this study are available within the article.

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Hairong Kou received her Ph.D. degree from the North University of China, Shanxi, China, in 2020. She is currently an Associate Professor at Taiyuan University. Her research interests focus on the design and production of wireless passive sensors.

Yuhang Pang received the B.S. degree from the North University of China, Taiyuan, China, in 2014, where he is currently pursuing the M.S. degree through the Graduate Program. His research interest is wireless flexible wearable sensors.

Libo Yang received the B.S. degree from the North University of China in 2002 and the Master’s degree in Software Engineering from Beijing University of Technology in 2010. He is currently a Professor in the Department of Intelligence and Automation, Taiyuan University.

Xiaoyong Zhang received the Ph.D. degree in Instrument Science and Technology from the North University of China, Taiyuan, China, in 2021. He is currently an Associate Professor at Taiyuan University. His research interests focus on the design of computer systems and signal processing.

Zhenzhen Shang was born in Shanxi, China, in 1992. She received the B.S. degree from the North University of China, Taiyuan, China, in 2015. She received the Ph.D. degree in Instrument Science and Technology from the North University of China, Taiyuan. She engages in teaching in micro/nano-measurement technology and computational science technology.

Liang Zhang received the B.S. degree from the Lvliang University, Lvliang, China, in 2018 and he received the Master’s degree from the North University of China in 2021. His research interests are data mining and image processing.

Lei Zhang received the Ph.D. degree in Instrumentation Science and Technology from the North University of China, Shanxi, China, in 2020. He is currently a Lecturer at the North University of China. His research interests include high-temperature, wireless, and flexible sensors.