An Ultra-wide Sensing Range and Highly Sensitive Flexible Pressure

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Surfaces, Interfaces, and Applications

An Ultra-wide Sensing Range and Highly Sensitive Flexible Pressure Sensor Based on Percolative Thin Film with Knoll-like Micro-structured Surface Shuwen Jiang, Jiangtao Yu, Yao Xiao, Yangyi Zhu, and Wanli Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019

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ACS Applied Materials & Interfaces

An Ultra-wide Sensing Range and Highly Sensitive Flexible Pressure Sensor Based on Percolative Thin Film with Knoll-like Micro-structured Surface Shuwen Jiang *, Jiangtao Yu, Yao Xiao, Yangyi Zhu, and Wanli Zhang State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 611731, China

Email: [email protected] (Dr. Shuwen Jiang).

Abstract Flexible pressure sensors have attracted considerable research interest and effort owing to their broad application prospects in wearable devices, health monitoring, and human-machine interfacing. High-sensitivity, wide workable range, and low-cost pressure sensors are the primary requirement in practical application. In this work, flexible pressure sensors with high sensitivity in a wide pressure range are constructed by introducing knoll-like micro-structured surface into percolative TPU/CB sensitive film, using a facile, efficient, and cost-effective screen printing route. The prepared pressure sensors exhibit an ultra-wide sensing pressure range of 0-1500 kPa, high sensitivity (5.205 kPa-1 in the range of 0 - 100 kPa and 0.63 kPa-1 over 1200 kPa), fast response, and excellent durability over 30,000 cycles. We demonstrated the applications of our pressure sensors in health monitoring, such as detection of wrist radial artery pulse waves, phonation, and vibrations. In addition, the proposed sensors showed the potential in object manipulation and human-machine interfacing, capable of detecting spatial pressure distribution, measuring grip forces and monitoring gas pressures. 1

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KEYWORDS: flexible pressure sensor, percolation effect, micro-structured surface, screen printing, wearable electronics

1. Introduction Flexible pressure sensors nowadays are receiving significant attention and extensive investigation from both academic and industrial societies due to their potential applications in wearable electronics and industrial fields. The development of wearable electronics, such as electric skin1–5, health monitoring6–9, requires devices with high sensitivity that are capable of detecting subtle pressures; while the applications of the sensors in intelligent robots10 and human-machine interfacing11,12 face new demands for a wide workable range to monitor high pressures. Recently, several types of pressure sensors to sense mechanical quantity are investigated based on resistivity13 , capacitance14, piezoelectricity15, triboelectricity16,17, and transistors4,14,18,19, of which the resistive-type pressure sensors have shown broad real applications due to their simple device structure and facile fabrication process20. For the resistive-type pressure sensors, conductive materials as active portions are crucial to achieve high performance in sensing. Nowadays, many conductive materials with excellent electrical properties have been introduced, including metal nanoparticles21,22, metal nanowires23, carbon nanotubes24,25, carbon nanofibers26,27, graphene/graphite28–32, and conductive polymer composites33–35. Among them, conductive polymer composites(CPCs) are designed by dispersing conductive fillers into flexible polymer matrix to construct conductive network. The resistance of CPCs undergo a drastic change due to percolation effect, which usually is the working principle for piezoresistive sensors36. The pressure sensors based on CPC materials are liable to be prepared by screen printing, a large-scale fabrication route of low-cost and high-efficiency37,38 . Additionally, the CPCs-based pressure sensors usually demonstrate good softness, flexibility and high resilience due to polymer matrix; therefore can undergo large reversible deformations enabling them to function under wide pressure loadings that facilitate their applications from electrical 2


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skin to industrial manipulating36. However, the variations of conductivity are relatively insufficient at detecting slight pressure due to the subtle deformation of CPC materials. As a result, this type of sensors exhibits some drawbacks such as poor sensitivity at subtle pressure, making it be hardly used as wearable sensors or physiological sensors where good sensitivity is needed. Generally, there are many strategies to improve sensitivity of CPCs-based pressure sensors. Advanced materials and designs including porous structure28, carbon sponges6,30 , carbon aerogels39,40, fabrics1,8,23, and introducing microcracks41–43 or adding different conductive fillers15,20,44 have been put forward as active sensing portions to improve sensor’s performances. Meanwhile, various microstructures have also been introduced on the surfaces of the conductive materials aiming to increase the electromechanical sensitivity under loadings. For example, pyramids1,14,18,19, domes45, prisms17, wavy46, and bionic41,47–50 micro-structured surfaces,

which are easily

deformable thereby improve the sensitive performances, have been prepared. These sensors with novel materials and/or microstructures demonstrate good sensitivity, fast response, and good stability, especially under slight pressure loadings. Nevertheless, the complicated and costly fabrication processes are usually required, limiting the largescale fabrication of these pressure sensors. In this work, we prepared knoll-like microstructures on the surfaces of percolative polymer materials to develop pressure sensors by screen printing process. The introduction of the knoll-like microstructures would increase the sensitive performance of the sensors while maintaining wide workable range. Herein, spherical conductive carbon black (CB) was dispersed into environment-friendly water borne polyurethane(TPU) to prepare TPU/CB slurry. Spherical carbon black is used as the active material, which not only improves the mechanical strength enduring large pressure loadings51 but also improves the stability in percolation conductive network upon deformation52, and its good dispersibility in polymer matrix superior to other nanofibers or nanosheets, such as carbon nanotubes and graphene

33,52

has also been

taken into consideration. By deliberately tuning the rheological characteristics of the prepared slurry and the operation factors of screen printing, the knoll-like micro3



structure formed on the surface of the printed TPU/CB film. The pressure sensors based on the knoll-like micro-structured TPU/CB film exhibited high sensitivity in ultra-wide workable pressure range. The sensors have been used to detect wrist pulse, throat phonation and acoustic vibration signal. A pressure sensor array was also fabricated to monitor the spatial pressure distribution. Moreover, the applications of the pressure sensors in measuring grip forces and gas pressures in real time were also demonstrated.

2.Experimental Section 2.1 Fabrication of printable TPU/CB slurry The typical synthesis process of screen-printable TPU/CB slurry is schematically shown in Figure 1a. The conductive CB spherical powder (Black Pearls 2000, diameter ~15nm, Cabot Co.), which has a high surface area of ~1500 m2/g, was added to dimethylacetamide (DMAc) solvent(Chengdu Kelong Co. Ltd.) at a weight ratio of 1:5. The mixture was ultra-sonicated to form a homogeneous DMAc-CB suspension. Then the DMAc-CB suspension was blended with the commercially available transparent TPU slurry (Sanhua Chemicals, viscosity: 5000 ± 1000 mPa · s) to prepare printable TPU/CB slurry. After 160 hours of mechanically stirring in a double-blade planetary vacuum mixer (MSK-SFM-16, Shenzhen Kejing Star Technology Co. Ltd.), carbon black suspension was well dispersed into the slurry, which can be observed from the SEM cross-sectional photographs of the prepared TPU/CB thin film (Figure S1). The resulting TPU/CB slurry was screen-printable with viscosity to be 11000±3000 mPa·s examined using a Bookfield rotary viscometer (Shanghai Sunny Hengping Scientific Instrument Co. Ltd.). It is noted that our TPU/CB slurry possessed a little higher viscosity than the transparent TPU slurry as one of raw materials. To obtain percolative TPU/CB thin film with excellent pressure sensitivity, several types of TPU/CB slurries were prepared in which the weight proportion of carbon black was 2%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%,7%, and 8%, respectively. 4


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Figure 1. (a) Schematics of the synthesis procedure of the screen-printable TPU/CB slurry. (b) Schematics of screen printing process and encapsulation of the TPU/CB pressure sensors. (c) Surface morphologies of TPU/CB films dried at 80℃, 100℃, and 120℃, showing the flat, a little uneven, and knoll-like surface, respectively. (d) SEM images of the knoll-like micro-structured surface of TPU/CB thin films. The inset shows the periodicity of the knolls consistent with the mesh holes of the screen stencil plate. (e) A photograph of the prepared TPU/CB pressure sensor. (f) Schematic of experimental configuration for pressure sensor testing.

2.2 Fabrication of the pressure sensors Figure 1b illustrates the fabrication process of the pressure sensors. The silver electrode and lead wire were firstly prepared on polyimide substrate (PI, 75 μm, 3M) by screen printing (300 mesh) silver paste and followed by levelling at room temperature, and drying at 105 ℃ in air oven. Then the TPU/CB slurry was screenprinted (250 mesh) over the silver electrode to prepare TPU/CB film. Due to a little high viscosity of our TPU/CB slurry, the as-printed raw film possessed a mesh structured surface which formed from the screen stencil plate as template, and the mesh would maintain for a time until it was levelled into a flat surface. The levelling process is usually conducted at room temperature during which the solvents volatize gradually. In this study, in order to obtain the micro-structured surface, instead we dried the asprinted TPU/CB raw film in air oven without prior levelling process. By drying the TPU/CB raw films at different temperatures, we prepared the fully solidified films with micro-structured surfaces. Figure 1c presents the surface morphologies of TPU/CB 6


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films dried at 80℃, 100℃, and 120℃, respectively, from which we can see that the surface morphology of the TPU/CB film exhibits flat, a little uneven, and knoll-like microstructure as increasing the drying temperature. The height of the surface knolls on TPU/CB film solidifying at 120℃ was ~2 μm, and the size and periodicity of the knolls were consistent with the mesh holes of the screen stencil plate, as shown in Figure 1d, indicating the role of the screen stencil plate acting as the template. After the TPU/CB thin film with knoll-like surface was prepared, the pressure sensor was fabricated by combining two pieces of the TPU/CB film together face-to-face where knoll-like surfaces contacted with each other, and encapsulated by polyimide tape. The photograph of the as-prepared pressure sensor is shown in Figure 1e. The total thickness of the prepared sensor is ~170 μm including two layers of 75 μm PI substrates, two layers of ~5 μm silver electrodes, and two layers of ~5 μm TPU/CB films. The diameter of the active sensing region in TPU/CB sensor is 10 mm.

2.3 characterization of the pressure sensors The surface topography and cross-sectional morphology of the pressure sensor were observed using the scanning electron microscopy (SEM, FEI Inspect F). To evaluate the prepared pressure sensor, the continuous or periodical force (F) was loaded by the programmable materials testing machine (Z050, Zwick-Roell). The resistance and current variations in the pressure sensors were measured by a universal data acquisition system (QuantumX, HBM). A constant voltage of 5 V was applied across the sensors throughout the test. All the experimental data are simultaneously recorded with a connected computer. Note that a circular soft gasket made of polydimethysiloxane (PDMS) was used to ensure uniform pressure distribution on the tested pressure sensor. The pressure exerted on the sensor was calculated by P=F/A, where A was the pressed area of active sensing region in the sensors. The experimental configuration of pressure sensor testing was schematically illustrated in Figure 1f.

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3. Results and Discussion Basically, our pressure sensors work in the principle of electrical response of the percolative TPU/CB film. For establishment of electrical percolation threshold, several types of TPU/CB films containing various concentration of carbon black were prepared. Figure 2a presents the conductivity of TPU/CB films varied with the fraction of carbon black. From the curve, it can be seen that the critical amount of CB in the film was 3.5 - 5.5 wt%, during which the drastic insulator-conductor transition appeared, demonstrating obvious percolation characteristics21. The percolative TPU/CB films showed significantly sensitive behavior when exposed to external pressure, as shown in Figure 2b. Clearly, the conductivity sensitivity of the TPU/CB films with 4.5 wt% carbon black content was superior to all the others, so the following tests were performed using the TPU/CB pressure sensors containing 4.5 wt% carbon black. Benefiting from the knoll-like sensing surface, The TPU/CB sensor exhibited excellent sensitive performance in an ultra-wide pressure range. Figure 2c shows the current variations across the pressure sensor with gradually increasing pressure from 0 to 1500 kPa. It can be seen that the current variation curve shows different slopes for different pressure regimes, revealing quite different sensitive response in low-pressure range and high-pressure range. In the range of 0 - 100 kPa and over 1200 kPa, current changes linearly increased as the increasing pressure. To evaluate the sensitive performance of the pressure sensor, we define the sensitivity ( S ) of the pressure sensor as (ΔI/I0)/P, where (ΔI/I0) is the current variation across the sensor and P is the applied pressure. By segmentally fitting the curve, we deduced that the sensitivity of the sensor was 5.205 kPa-1 in the low pressure range of 0 - 100 kPa, and in the high pressure range over 1200 kPa the sensitivity was 0.63 kPa-1. The enlarged views A, and B in Figure 2c showed the good linearity of the sensor in both low and high pressure regions. Between low pressure and high pressure regimes, it was observed that sensitivity decreased with increasing pressure loadings.

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Figure 2. The pressure sensitive performance and corresponding working mechanism of the TPU/CB pressure sensor. (a) Sheet conductivity of TPU/CB films varied with the fraction of carbon black, showing obvious percolation characteristics at 3.5 - 5.5 9



wt%. (b) The percolative TPU/CB film containing 4.5 wt% carbon black showed high sensitivity to external pressure. (c) Current variations across the pressure sensor as a function of pressure, showing the sensitivity in a wide pressure range from 0 to 1500 kPa. The enlarged views A, and B show the good linearity of the sensor in both low and high pressure regions. (d) The equivalent serial circuit diagram of the TPU/CB pressure sensor. (e) The schematic structure of the TPU/CB pressure sensor, with CB dispersed in TPU matrix to form pressure sensitive percolation network. (f) Illustration of microstructural changes of the sensor at low pressure. The deformation of knoll-like surfaces causes the surface percolation effect and increase of contact area, both lead to the significant decrease in resistance. (g) Illustration of microstructural changes of the sensor under high pressure loading. The micro-structured surfaces deformed to be nearly flat due to the high pressure and the contact area between two surfaces is almost unchanged, therefore the decrease in resistance is dominated by percolation effect in bulk TPU/CB film.

The pressure dependent sensitive response of the TPU/CB sensor might be understood by the serial circuit model of the resistive sensor. Figure 2d is the equivalent circuit diagram corresponding to the layered structure of our proposed sensor. The total resistance can be defined as Rtotal=Rc+Rk+Rb where Rc is the contact resistance between two layers of TPU/CB film, Rk is the percolation resistance of the knoll-like microstructured surface, and Rb is the percolation resistance of the bulk TPU/CB film, as illustrated in Figure 2e. The sensitivity of the TPU/CB sensor was therefore due to the change in Rc, Rk, and/or Rb under pressure. In the low pressure regime, the knoll-like surfaces are easier deformable and compressed, as showed in Figure 2f, resulting in a significant resistance decrease in resistance Rk due to the percolation effect in knoll-like surface. Besides, the compression and deformation of the knolls and valleys lead to an increase in contact area between two layers of knoll-like surfaces thereby further decrease the contact resistance Rc due to the increase in the conductive pathways. Therefore, we suggest that the simultaneous decline in Rk and Rc lead to the high sensitivity in low pressure regime. To further clarify the effect of the knoll-like micro10


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structured surface of TPU/CB film on sensitivity at low pressure, the pressure sensitive performance of the proposed micro-structured sensor was compared to the sensors based on other flat and uneven TPU/CB thin films. As shown in Figure S2, the microstructured sensor showed the highest current change with ~10-fold improvement compared to the other ones at the same pressure, i.e. an ~10-fold improvement in sensitivity. This result indicates that the high sensitivity in low pressure region should be ascribed to introducing knoll-like surfaces of the TPU/CB films. Furthermore, we note that our pressure sensors have a superior sensitivity at low pressure compared to other microstructural pressure sensors with no percolation mechanism utilized, such as these with micro-pyramid 53 and wave 46 microstructure on PDMS surface, indicating that to some extent the high sensitivity at low pressure should be attributed to percolation effect induced by the deformation of knoll-like surfaces. With the increase of pressure, the deformation of knoll-like surface becomes more difficult and its effect on sensitivity has begun to be weaker, thus the sensitivity decreases gradually as increasing pressure. In the high pressure regime illustrated in Figure 2g, the microstructured surfaces become nearly flat due to the large deformation and the contact area between two surfaces is almost unchanged, therefore there is no significant change in Rk and Rc resulting from the deformation of micro-structured surfaces. As a result, the resistance of pressure sensor in high pressure region is dominated by the decrease in percolation resistance Rb of bulk TPU/CB thin film, thus the pressure sensor exhibits a much lower sensitivity than at low pressure.

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Figure 3. Characteristics of the TPU/CB pressure sensor. (a) Relative current changes of the TPU/CB sensor under cyclic loadings with low pressure from 10 kPa to 60 kPa. (b) Relative current changes of the TPU/CB sensor under cyclic loadings with high pressure from 500 kPa to 1500 kPa. (c, d) Cyclic durability of the TPU/CB pressure sensor worked as many as 30 000 cycles under low and high pressure, which maintained well-functioned in both cases. (e) The response of the TPU/CB sensor at low pressure of 30 kPa, Insets show the instant and recovery time constant are 77.9 ms and 10 ms, respectively. (f) The response of the TPU/CB sensor at high pressure of 1100 kPa, Insets show the instant and recovery time constant are 22.6 ms and 13.7 ms, 12


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respectively.

The prepared TPU/CB pressure sensor was also evaluated to show high stability and durability under different pressure loadings. Figure 3a and 3b show the relative current changes of the TPU/CB sensor under low and high cyclic loadings, respectively. In the wide pressure range, the current signals excited by cycling pressures remained stable without any attenuation, indicating the TPU/CB pressure sensor has a repeatable response and is capable of detecting accurately the different pressure loadings. To demonstrate the long-term durability of the TPU/CB sensors, we performed the reproducible tests under cycling loading. Figure 3c and 3d exhibits the excellent cyclic durability of the TPU/CB pressure sensor at both low and high pressure regimes, where a pressure of 10 kPa and 1500 kPa was periodically imposed on the sensor, respectively. In both cases the sensor worked as many as 30 000 cycles and still maintained wellfunctioned. The insets show the sensor had almost uniform response to the long-lasting cycling pressure, indicating the TPU/CB pressure sensor is rather robust. The dynamic response characteristics of the pressure sensor is also derived from the test results under cyclic loadings. Figure 3e and 3f present the enlarged current change curves of the TPU/CB sensor at pressure of 30kPa and 1100kPa, respectively. We noted the obvious relaxation of the current changes, especially on loading a low pressure. The similar relaxation was also observed in vertically aligned carbon nanotube embedded PDMS sensors

54.

In this case, we use the time constant of the pressure sensor to

evaluate its dynamic performance. The time constant is a particular case of response time, measured by considering a 36.8% (1/e) deviation from the final value. Accordingly, our sensor exhibited an instant time constant (rise) of 77.9 ms and a recovery time constant (down) of 10 ms at low pressure.

The slow instant response

might be related to stress relaxation effect in polymer matrix, which would result in a retarded change in percolation resistance of surface knolls with low pressure loadings. The relatively slow response due to relaxation was also observed in other sensors based on similar conductive materials 54,55, although the micro-structured surfaces improved their response time compared with the planar structures 46,55. The much faster recovery 13



response of the sensor may be owing to the quick resilience of the TPU/CB film on releasing loadings. At high pressure, the sensor displayed a rise time constant of 22.6 ms and a recovery time of 13.7 ms. The fast response in high pressure regime should be ascribed to the rapid compression and quick resilience of the robust TPU/CB thin film under large loadings. The good response of the TPU/CB sensors are favorable for detecting various frequency signals, even real-time and fast-changing pressure loadings such as pulse and respiration. The flexible TPU/CB pressure sensors with knoll-like microstructural surfaces offers the advantages of ultra-wide sensing range, high sensitivity, good response, and excellent durability and stability. These features will bring wide potential applications for the pressure sensor from slight signal detection to object manipulation. The high sensitivity at low pressure endows the sensor with the ability to detect minute pressure induced by light items and to monitor pulse of human body. Figure 4a showed a piece of chewing gum (1.42g, ~116Pa) was loaded on a TPU/CB pressure sensor and generated obvious current variations, with an instant time constant of 80.8 ms and a quicker recovery time constant of 31.2 ms, where a resilience peak was observed in Figure 4b when the chewing gum was picked up from the sensor. Figure 4c demonstrates the real-time monitoring of radial artery pressure pulses. The TPU/CB pressure sensor was attached onto a 24-year-old volunteer’s wrist skin surface using a transparent medical tape (Tegaderm Film, 3M). From the recorded signal waveform (Figure 4d), the wrist pulse rate of the test person was approximately 73 beats per minute. Furthermore, the pressure sensor can distinguish quantitatively characteristic peaks of the pulse waveform. Figure 4e presents a captured segment of single artery pulse wave, which comprises three distinct peaks (P1, P2, and P3). P1 is pulse pressure which is due to the blood flow ejected by heart contraction, P2 and P3 are reflected wave pressures from the peripheral sites (hand for P2 and lower body for P3) 49,56. The radial augmentation index(AIr), related to vascular aging, can be calculated by P2/P1. The derived AIr from the graph was 0.55, which was matched with the reference data56 of a normal 25-year-old man. The results indicated that our pressure sensors can be used as a wearable diagnostic device for real-time monitoring. 14


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Figure 4. Demonstration of the TPU/CB pressure sensor to detect a slight item and wrist pulse. (a) A piece of chewing gum (1.42g, ~116Pa) was loaded on the sensor. (b) The current variations caused by the gum, showing its ability to detect subtle pressure. (c) The sensor was attached onto a 24-year-old volunteer’s wrist skin surface using a transparent medical tape. (d) Recorded signal waveform of wrist pulse monitored by the pressure sensor. (e) A single waveform of the pulse from which the radial augmentation index(AIr) was derived to infer the age of the volunteer.

Due to the fast response and high sensitivity, the TPU/CB sensors can also be used to detect tiny and rapid pressure variations acting as vibration sensors. Figure 5a illustrates the detection and recognition of phonation using the TPU/CB pressure sensor to monitor the throat muscle movement. A TPU/CB pressure sensor was attached on throat skin of a volunteer with the aid of a transparent medical tape. The vibration of vocal cords induced by pronunciation would change the signal of the sensor. Figure 5b shows the raw vibration signal image of pronouncing the word “hello” repeated for three times. We can clearly recognize three similar peaks representing the word “hello” 15



in the waveform and even distinguish two single "he" and "llo" syllables in each pronunciation. The results show that the TPU/CB sensor might be potential in speech recognition and phonation rehabilitation training. Our TPU/CB sensor can also detect other mechanical vibration signals. As shown in Figure 5c, the TPU/CB pressure sensor, pressed by a heavy bolt, was used to record the call-in signals of the nearby cell phone in vibration mode. The last ten-second segment of 60-second ring-in vibration was presented in Figure 5d. It can be clearly seen in the graph that the cell phone produced a series of vibration signals lasting one second at intervals of one other second, and another three brief vibration signals to remind the user whenever no answer to the callin. This test indicated that our pressure sensor had excellent response to vibration in a compressed state.

Figure 5. The pressure sensor was used to detect phonation and vibration. (a) The sensor attached onto the throat skin of a volunteer. (b) Raw vibration signal image of pronouncing the word “hello” repeated for three times, where two single "he" and "llo" syllables can be distinguished. (c) TPU/CB pressure sensor was applied to record the call-in signals of a nearby cell phone. (d) Last ten-second segment of vibration signal 16


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recorded by the sensor, indicating the excellent response to vibration of our pressure sensor.

The ability to detect spatial pressure distribution might be critical to the applications in wearable electronics and artificial electronic skins. By convenience of large-area production of screen printing process, we designed and fabricated a 16×16 pixelated TPU/CB pressure sensor array on PI substrate for detection of pressure distribution. The total size of the sensor array is 8×8 cm2 with each sensitive pixel of 3 mm in square. Figure 6a showed a volunteer placed three of his fingers onto the pressure sensor array, and the fingers’ pressure on the sensor array was reflected by measuring the resistance variations of each pixel. The color mapping in Figure 6b indicated that the touching position, and force level of the fingers can be well resolved by the sensor array.

Figure 6. Demonstration of the TPU/CB sensor measuring spatial pressure distribution. (a) Three fingers touched on a 16×16 pixelated TPU/CB pressure sensor array. (b) The touching position, and force level of the fingers were well resolved by the sensor array, The color contrast indicates the magnitude of the touching force.

Furthermore, the wide sensing pressure range enables the TPU/CB sensors to monitor strenuous forces and pressures. The TPU/CB sensors can be used to measure the grip forces of fingers, as shown in Movie S1. A TPU/CB sensor was calibrated as a force sensor and fixed onto the index finger of a volunteer to detect the forces generated 17



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by grasping different weights. Figure S3 showed the different magnitudes of grip forces when grasping weights of 200 g, 500g, and 1000g, respectively. We can see the sensor’s ability to differentiate quantitatively the forces, demonstrating the potential applications in object control, industrial measurements, and human-machine interfaces. Additionally, owing to the encapsulation of TPU/CB sensors in a hermetically sealed package by PI substrates, the sensors are capable of monitoring the variations of gas pressure. A sensor was put into a vessel to detect the compressed air pressure. Movie S2 showed the realtime resistance decrease of the sensor when pumping air into the vessel. While the resistance of the sensor immediately increased as the air was released, as shown in Movie S3. The capability of monitoring gas pressure is one of unique and intriguing characteristics of the TPU/CB sensors proposed in this study.

4. conclusion In summary, we have fabricated a flexible pressure sensor based on piezoresitive TPU/CB thin film with knoll-like surface, using facile, efficient, and cost-effective screen printing technology. The sensor exhibited an ultra-wide sensitive pressure range of 0-1500 kPa, high sensitivity (5.205 kPa-1 in the range of

0 - 100 kPa and 0.63 kPa-1

over 1200 kPa), and fast response (the instant and recovery time constant were 77.9 ms and 10 ms in low pressure range, and 22.6 ms and 13.7 ms in high pressure range, respectively). This sensor also shown good durability more than 30,000 cycles. The high sensitivity of the sensor in wide pressure range should be mainly attributed to the knoll-like micro-structured surface of the percolative TPU/CB thin film. The pressure sensor has showed great application prospect in wearable electronics such as radial artery pressure pulse waves monitoring, phonation recognition, and vibration detection. The sensor was also capable of detecting spatial pressure distribution, measuring grip forces and monitoring gas pressures, indicating its potential applications in object control, industrial measurements, and human-machine interfaces.

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Supporting Information Figure S1. High-resolution SEM cross-sectional photographs of the prepared TPU/CB thin film showed carbon black well-dispersed in TPU/CB film. Figure S2. The micro-structured sensor showed the highest current change with ~10fold improvement compared to the other ones at the low pressure. Figure S3. The different magnitudes of grip forces when grasping weights of 200 g, 500g, and 1000g, respectively. Movie S1. The TPU/CB sensors was used to measure the grip forces of fingers when grasping weights. Movie S2. The real-time resistance change of the sensor when pumping air into the vessel. Movie S3. The resistance change of the sensor as the air was released from the vessel.

AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected]. ORCID Shuwen Jiang: 0000-0002-9161-9647

Conflicts of interest There are no conflicts to declare

Acknowledgements We gratefully acknowledge the support of the National Natural Science Foundation of China (Grant No. 50972024, 61223002) and the National Key Research and 19



Development Program of China (2017YFB0406400).

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An Ultra-wide Sensing Range and Highly Sensitive Flexible Pressure

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