Highly sensitive pressure sensor with broad linearity via constructing a hollow structure in polyaniline/polydimethylsiloxane composite

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Abstract

Flexible pressure sensors are increasingly required for their widely practical applications in health monitoring, humanoid robotics and wearable electronics. Although the geometric microstructure design or active material selection for sensor has been manipulated by many researchers, achieving the desired pressure sensor possessing high sensitivity over a broad linear detection range is still challenging. Herein, a flexible and wearable pressure sensor is demonstrated based on the polyaniline/polydimethylsiloxane (PANI/PDMS) composite with hollow structure and micro-protrude surface structure. The sensitivity and linearity of the as-fabricated sensor could be greatly enhanced through the construction of hollow structure and micro-protrude surface structure, reaching a sensitivity of 0.641 kPa−1 over a broad linear range (0.05–60 kPa), and the sensor also exhibits stable cycling performances (6000 cycles), fast response time (200 ms) and recovery time (150 ms). Importantly, the sensing mechanism of this sensor is theoretically studied, a sensing model based on tunneling effect and contact mechanic is proposed and its validity is experimentally verified. Finally, the pressure sensor demonstrates practical applications in object operations, answering mouse click and feet movements, thereby providing a significant guidance for constructing advanced electronic devices.

Introduction

Flexible pressure sensors have attracted tremendous attention because of their great potential for applications in human-machine interfaces, human motion monitoring, and artificial electronic skin [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. Currently, various sensing mechanisms have been proposed, such as transistor sensing [14,15], capacitive sensing [16,17], piezoelectric sensing [18,19], triboelectric sensing [20,21] and piezoresistive sensing [4,5,[22], [23], [24], [25], [26]]. Among them, the flexible piezoresistive pressure sensors, which transduce the mechanical stimuli into an electrical signal, have been excessively explored for their advantages of low cost, feasible fabrication and easy signal collection. To date, enormous efforts have been devoted to enhancing the sensitivity, response time, durability and working pressure range [[27], [28], [29]].

Generally, incorporation of electroactive materials into elastomers is conventionally suggested for the fabrication of flexible pressure sensors. Polydimethylsiloxane (PDMS) is popularly used as an elastic material in the flexible sensors due to its excellent flexibility, good adhesion to the skin, and convenient fabrication process. Numerous electroactive materials, including carbon nanotubes (CNTs) [7,30,31], graphene [22,[32], [33], [34]], metal nanowires [27,35], and conductive polymers [1,5,23,[36], [37], [38], [39], [40]], have been widely utilized as active components of piezoresistive pressure sensors. Polyaniline, one of most promising conductive polymers, has been intensively researched owing to its good environmental stability, acceptable electrical conductivity, particularly, ease of deposition onto a wide variety of substrates. Ge et al. [23] achieved a flexible pressure sensor based on reduced graphene oxide/polyaniline (rGO/PANI) wrapped melamine sponge, which exhibited a sensitivity of 0.152 kPa−1 in the pressure range of 0–3.24 kPa. Furthermore, diverse microstructures, such as micropillars [5,41], hemispheres [42,43], wrinkles [36,44], and triangular pyramid [45,46], have been introduced in the fabrication of pressure sensors with higher sensitivity. For instance, Park et al. [5] developed a pressure sensor based on polyaniline nanofibers and Au-deposited PDMS micropillars to obtain a sensitivity of 2.0 kPa−1 in the pressure range below 0.22 kPa. Yang and co-workers [36] demonstrated a wearable pressure sensor based on three-scale nested wrinkling microstructures of polypyrrole (PPy) film with a high sensitivity of 19.32 kPa−1 in the low-pressure region (<0.5 kPa). Unfortunately, the high sensitivity was expressed only in a low-pressure region for the vast majority of reported sensors, inhibiting their applications in wearable flexible devices. Too much attention has been paid to enhancing the sensitivity of pressure sensor, rather than their operational range and linear pressure-sensing capacity. Therefore, fabricating pressure sensors simultaneously possessing high sensitivity and linearity over a broad detection regime to extend their practical applications remains a huge challenge currently. Additionally, the theoretical investigation of the relations between structural evolution and linearity was not set forth very clearly. This lack of theoretical guidance greatly impedes the development and design of the highly desirable sensor. Thus, it is highly necessary to explore the influence of structure evolution of pressure sensor on its sensing performance.

Herein, we demonstrate a material strategy to realize both the high sensitivity and linearity over a wide pressure regime for the composite-based pressure sensor, which was prepared by assembling the PANI/PDMS composite with hollowed structure and rough surface and a pair of coplanar interdigitated Au electrodes. The pressure sensor exhibits a high sensitivity (0.641 kPa−1) throughout a broad linear pressure-sensing range (0.05–60 kPa), excellent durability (6000 cycles), fast response time (200 ms) and recovery time (150 ms). Furthermore, by combination of the tunneling effect and contact mechanic theory, a sensing model is proposed and the sensing mechanism is theoretically studied. In addition, the flexible pressure sensor could be mounted onto human skin to monitor various human motions, including object operations, answering mouse click and feet movement, revealing a great potential application in the field of wearable electronic devices.

Section snippets

Experimental section

Materials and chemicals: Aniline (99%) and PDMS (Sylgard 184, Dow Corning) were purchased from Aladdin Industrial Inc. Sulfuric acid (H2SO4), acetone, absolute ethyl alcohol and ferric chloride (FeCl3) were purchased from ChengDu Chron Chemicals Co. Ltd and were used without further purification. Nickel foam (120 ppi, 0.2 mm and 0.5 mm) was supplied by RuiQue Electronics Technology Co. Ltd.

Fabrication of Hollow-structured PANI/PDMS (Hs-PP) composite film: Firstly, the commercial Ni foam was

Results and discussions

The fabrication process of the Hs-PP composite is illustrated in Fig. 1a. Firstly, during the electro-polymerization process, the PANI network was deposited on a commercial Ni foam, which was confirmed by the SEM images in Fig. 1b. The PANI nanoparticles were observed on the surface of Ni scaffold from the zoom-in SEM image (Fig. 1b inset). Secondly, the PDMS matrix was introduced to infiltrate the PANI-coated Ni foam. Finally, the cured PDMS-infiltrated PANI-coated Ni foam was immersed into

Conclusions

In summary, we have demonstrated a conductive PANI/PDMS composite with a hollow structure and micro-protrude surface, which could be utilized for a flexible pressure sensor with high sensitivity (0.641 kPa−1) and excellent linearity over a wide working pressure regime (0.05–60 kPa). The pressure sensor also exhibits stable cycling performances (6000 cycles), a fast response time (200 ms) and recovery time (150 ms). More importantly, a theory sensing model based on the tunneling resistance and

CRediT authorship contribution statement

Shaodi Zheng: Methodology, Investigation, Writing - original draft, Data curation, Writing - review & editing. Yuanping Jiang: Investigation, Writing - review & editing. Xiaotian Wu: Formal analysis, Writing - review & editing. Zewang Xu: Methodology, Validation. Zhengying Liu: Resources, Writing - review & editing, Supervision, Project administration, Writing - review & editing. Wei Yang: Validation, Funding acquisition. Mingbo Yang: Validation, Funding acquisition.

Declaration of competing interest

We wish to confirm that there are no known conflicts of interest associated with this publication.

Acknowledgment

This research was supported by the National Natural Science Foundation of China (Grant No. 51873126, 51721091), the Opening Project of Jiangxi Province Key Laboratory of Polymer Micro/Nano Manufacturing and Devices (No. 1410601133) and the Research Foundation for Advanced Talents of East China University of Technology (No. 2400100073).

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