Lotus leaf inspired superhydrophobic rubber composites for temperature stable piezoresistive sensors with ultrahigh compressibility and linear working range

Highlights

• Conductive sponge composite with lotus leaf inspired microstructure is prepared.

• The sponge composite shows excellent anticorrosion and temperature insensitive properties.

• The sponge composite possesses ultrahigh compressibility and linear working strain range.

• The sponge composite shows potential usage in body motion detection.

Abstract

Piezoresistive sensors have promising applications in wearable electronics; however, developing multi-functional piezoresistive sensors that possess ultrahigh compressibility and linear working range and could be used in tough environment (e.g., high humidity, corrosive media and low temperatures) remains a challenge. Herein, a flexible electrically conductive polymer foam composite (CPFC) with the lotus leaf inspired microstructure is prepared by anchoring carbon nanotubes (CNTs) onto the skeleton of the polymer foam with the assistance of ultrasonication and simultaneous non-solvent induced phase separation (NIPS). Hemisphere arrays are produced on the skeleton of the polymer foam, while CNTs are decorated on the surface of these arrays, forming conductive network. The obtained superhydrophobic CPFC exhibits excellent anti-corrosive and photothermal conversion performance, making it possible to be used in some harsh environment. When used as the piezoresistive sensor, the CPFC exhibits stable electrical conductivity, extremely high compressibility and linear working range (up to 90%), superb sensing stability and durability (over 2300 cycles). Furthermore, the piezoresistive sensing performance is impervious to the ambient temperature, and the CPFC sensor can work in the temperature from −20 ℃~80 ℃ with stable sensing signals. Also, the CPFC can detect various human body movements even in the corrosive condition.

Introduction

Flexible and wearable piezoresistive sensors have become an emerging research area and displayed potential applications in flexible electronics [1], [2], health monitoring [3], [4], artificial skin [5], [6], etc. Many pressure sensitive materials [7], [8], [9], [10] are explored as the piezoresistive sensors, among which the conductive polymer foam composites (CPFCs) have received increasing attention, due to their light weight, high elasticity, low cost and controllable electrical conductivity [11], [12], [13]. Various conductive nanofillers such as carbon nanotubes (CNTs) [14], graphene [15] and carbon black (CB) [16] have been introduced to the polymer foam, and two methods are usually used for preparation of the CPFCs. One is based on the uniform mixing of the conductive nanofillers with the polymer, followed by the foaming process (e.g., freeze drying). Zhai et al. fabricated CB/thermoplastic polyurethane (TPU) foam with a pinnate-veined aligned structure by unidirectional freeze drying the mixture of CB and TPU solution [17]. The CB/TPU foam has a low density of 0.13 g cm⁻³, high porosity of 83.7%, and the maximum compression strain could reach up to 80%.

However, in this case, the nanofillers are easily wrapped by one layer of insulating polymer, and a high nanofiller concentration is hence required to achieve a relatively high conductivity for the CPFCs, which could not only increase the cost but also deteriorate the processability. Furthermore, the flexibility and compressibility may, to some extent, be sacrificed, due to the incorporation of a large number of rigid fillers [18], [19]. To settle this problem, the conductive nanofillers are controlled to be decorated on the skeleton of the polymer foam via hydrogen bonding or other interfacial interactions [19], [20], [21], [22]. For example, graphene oxide (GO) can be successfully decorated on the skeleton surface of polyurethane (PU) foam by multiple dipping coating, and the conductive PU foam composite was finally obtained after the GO was chemically reduced [23]. When used as the piezoresistive sensors, the cyclic sensing tests could be conducted under a large pressure strain of 60% because its great conductivity and compressibility. Although the multiple or layer by layer coating can bring the conductive nanofillers onto the foam surface, it is indeed a complicated and time-consuming process, and delicate manipulation is usually required. In addition, the working strain of obtained foam composites is often relatively low and the strain sensing displays a non-linear characteristic, which is undesirable in practical applications.

Another issue of the CPFCs is the environmental applicability, and it is often ignored in the previous research [24], [25], [26] that is mainly focused on the structure design for the CPFCs. For example, the flexible electronics are often exposed or used in some harsh environment (e.g., high moisture, corrosive media and low temperatures). In such cases, the polymer foam may be gradually decomposed if the corrosive solution is diffused into the interior of the CPFCs, deteriorating material performance and shortening their working life. To solve this problem, conductive elastic rubber composite with superhydrophobicity were fabricated by decorating CB on the surface of rubber band and subsequently PDMS modification in our previous work [27]. Although PDMS could increase the interfacial adhesion between the CB nanoparticles, the preparation became multi-step and thus complicated. Also, the PDMS introduction decreases the electrical conductivity of the composite. Then, we developed a self-derived superhydrophobic rubber/Ag nanoparticles sponge composite [28], which can avoid the modification by using low surface energy agents. The obtained sponge composite could be used as both strain and piezoresistive pressure sensors. But the material resistance displayed nonlinear response to the compressive strain and the sensitivity is relatively low (GF = −0.82, pressure strain: 17%-80%). In many cases, the composite is usually used over a wide temperature range such as from winter to summer, which may cause the instability of the material resistance and hence the sensing behavior.

Though some work has been done for preparation of multifunctional CPFCs piezoresistive sensors [29], [30], it is, to date, still challenging to develop a simple but versatile technique for fabricating anti-corrosive, durable and temperature stable CPFC piezoresistive sensors with a high compressibility and linear sensitivity. Herein, inspired from the lotus leaf with many individual small microscale papillae, a superhydrophobic CPFC with polymer hemispheres arrays on their skeleton surface is prepared based on the ultrasonication induced CNTs decoration and simultaneous nonsolvent induced phase separation (NIPS). The obtained superhydrophobic CPFC possessing the microdome like structure displays outstanding water proof, corrosion resistance and photo-thermal effect with good durability. Moreover, the entangled CNT network is insensitive to the temperature, leading to a stable strain sensing performance regardless of the variation of the ambient temperature. The CPFCs based piezoresistive sensors exhibit extremely high compressibility and linear working range (as high as 90%), relatively high sensitivity with a GF of 1.14 and excellent recyclability. The multifunctional CPFCs are promising as piezoresistive sensors especially used in many harsh environments.