Ultra-sensitive and durable strain sensor with sandwich structure and excellent anti-interference ability for wearable electronic skins

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abstract

Smart and wearable strain sensors have sparked enormous research interests in various applications of flexible electronic devices. For this topic, it remains a huge challenge to acquire wide sensing range, high sensitivity, superior durability and fast response synergistically. Herein, we present an ultra-sensitive and durable strain sensor with sandwich structure to address the issues, which is mainly composed of the composite of carbon black (CB)/aligned thermoplastic polyurethane (TPU) fibrous mat and the Ecoflex. The CB/TPU/Ecoflex strain sensor (CTESS) is prepared via decorating CB nanoparticles onto the aligned electrospun TPU fibrous mats by ultrasonication, then encapsulated with Ecoflex to develop a sandwich structure. This structure provides effective protection for the conductive CB/TPU fibrous network, endowing the strain sensor with excellent sensing performances, including low detection limit (0.5% strain), wide response range (up to 225% strain), ultrahigh sensitivity (maximum gauge factor of 3186.4 at strain of 225%), fast response time (70 ms) and favorable repeatability even after 5000 stretching/releasing cycles. CTESS also shows an excellent anti-interference capability to external humidity and temperature. The CTESS is then assembled as artificial electronic skins to monitor various human motions, exhibiting great application prospects in next-generation wearable electronics.

Introduction

With the development of multifarious smart terminals, flexible sensors have aroused enormous research interests in the wide application of wearable electronic skins [1,2], soft robotics [3], human motion monitoring [4], human-machine interfaces [5,6] and artificial intelligence [7]. Among them, stretchable strain sensor with favorable flexibility and wearability has been investigated extensively [8]. For a satisfactory strain sensor, comprehensive sensing performances of nice flexibility, good response stability, wide response range and high sensitivity are urgently desired. However, there are still many challenges to develop a favorable strain sensor, for instance, the balance between high sensitivity and broad sensing range, complicated preparation technology, expensive raw materials, and so on.

To realize these sensing performances simultaneously, conductive polymer composites (CPCs) as the candidate materials of strain sensors have aroused wide attention because of good flexibility, cost-efficiency and excellent workability [[9], [10], [11], [12], [13]]. Up to now, in order to develop flexible strain sensors with CPCs, a lot of research has been carried out through employing conductive carbonaceous materials (carbon nanotubes (CNTs), carbon black (CB) and graphene) or metallic materials (Ag and gold nanowires/nanoparticles) as conductive fillers, and polymers with nice flexibility (silicon rubber, thermoplastic polyurethane (TPU), Ecoflex and polydimethylsiloxane (PDMS)) as matrix materials [[14], [15], [16], [17], [18], [19], [20]]. For example, Li et al. achieved a strain sensor with high sensitivity by transferring assembled graphene films onto PDMS matrix [21], which obtained a high gauge factor (GF) of 1037 in a range of 0–2% strain. Kang et al. sputtered Pt on the flexible polyurethane acrylate matrix and then constructed crack structures [22], obtaining a strain sensor with micro-crack structure and high sensitivity (GF of 2000 in strain range of 0–2%). Moreover, Wang et al. prepared an ultra-sensitive strain sensor based on the composite of gold/titanium thin film and PDMS, achieving an ultra-high GF of 5000 at 1% strain [23]. These strain sensors possessed high sensitivity (GF > 1000) and favorable stability except narrow sensing range (always about 1%–2%) and complex preparation process. In order to acquire a wider sensing range, TPU was chosen as the matrix material on the basis of the nice flexibility in our previous work. Ren et al. fabricated an ultra-stretchable strain sensor through decorating CNTs on electrospun TPU fibrous mat (CNTs/TPU) by a simple ultrasonic process [24]. The strain sensor based on CNTs/TPU composite mat showed extremely broad sensing range (900% strain), while its GF needs to be improved (only 19.96). Wang et al. developed a CNT/TPU based strain sensor with high performance by using a simple, low-cost and large scale wet spinning technology [15], and achieved a broad sensing range (320% strain) and a high GF of 97.1 at strain of 160–320% simultaneously. While the detection limit (5% strain) and response time (200 ms), which play an important role in sensing performance, still need to be promoted. Therefore, synchronously achieving low detection limit (<1% strain), high sensitivity (GF > 500), large stretchability (>200% strain), fast response time (<100 ms) and excellent stability (>5000 cycles) to meet the requirements of human motion detection remains a challenge for CPCs based strain sensors.

In this work, to address the issues as mentioned above, we present a wearable CB/TPU/Ecoflex strain sensor (CTESS). Zero-dimensional CB is applied as the conductive material due to its low-cost and good electrical conductivity, TPU is selected as the matrix material owing to the nice flexibility and cost efficiency. The CTESS is fabricated by decorating CB particles on aligned electrospun TPU fibrous mat through ultrasonication and encapsulation using Ecoflex to develop a sandwich structure. CB nanoparticles are decorated on TPU mat through the ultrasonication treatment. The electrospun TPU mesh coated with CB particles forms an intact 3D conductive network, guaranteeing the CTESS to suffer large deformations. The point-to-point contact mode of zero-dimensional CB nanoparticles is easy to respond to the deformation of the net structure during stretching, which endows the CTESS with good sensing ability. Sensing behaviors of CTESS, including strain range, sensitivity, response time, response stability and anti-interference performance towards temperature and humidity are investigated systematically. To evaluate the potential in practical applications, CTESS is assembled as artificial electronic skins to detect various human movements. The present study provides an effective strategy to achieve next-generation wearable electronic devices.

Section snippets

Materials

TPU (Elastollan 1185A) was purchased from BASF Co., Ltd. Tetrahydrofuran (THF) and N, N-dimethylformamide (DMF) were both supplied by Fuyu Fine Chemical Co., Ltd, Tianjin, China. CB (Vulcan XC72) was supplied by Cabot Corp., USA. Sodium dodecyl sulfonate (SDS) was supplied by Kangpuhuiwei Technology Co., Ltd, Beijing, China. All the chemicals are analytical grade and used directly without any treatment.

The preparation of CTESS

The preparation process of CTESS mainly includes electrospinning, ultrasonication and

Results and discussion

Fig. 1a illustrates the preparation process of CTESS, which mainly includes electrospinning, ultrasonication and encapsulation. Firstly, the pure TPU fibrous mat is fabricated by electrospinning. As shown in Fig. S2, the surface of the pure electrospun TPU fiber is smooth, and the electrospun TPU fibrous mat displays an interesting aligned wave-like structure. The thickness (only 54 μm) of TFM is displayed in Fig. S3a. Meanwhile, as shown in Fig. S3b, the joint points between adjacent fibers

Conclusions

In summary, we have successfully developed a smart and wearable CTESS with a sandwich structure by electrospinning, ultrasonication and encapsulation. The CTESS exhibits broad sensing range (225% strain) and ultrahigh GF of 3186.4 at the strain of 210–225%. The CTESS displays ultralow detecting limit (0.5% strain) and short response time (70 ms). The electrical response for CTESS is very steady even after 5000 stretching/releasing cycles under 40% strain, exhibiting good repeatability and

CRediT authorship contribution statement

Yi Zhao: Investigation, Writing - original draft. Miaoning Ren: Investigation, Data curation. Ying Shang: Investigation, Methodology. Jiannan Li: Investigation, Data curation. Shuo Wang: Investigation, Software. Wei Zhai: Writing - review & editing. Guoqiang Zheng: Formal analysis, Methodology. Kun Dai: Conceptualization, Supervision. Chuntai Liu: Resources, Supervision. Changyu Shen: Supervision, Validation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge the financial support of this research by National Natural Science Foundation of China (51773183, U1804133), National Natural Science Foundation of China-Henan Province Joint Funds (U1604253), Henan Province University Innovation Talents Support Program (20HASTIT001), Innovation Team of Colleges and Universities in Henan Province (20IRTSTHN002).

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