Hybrid‐Filler Stretchable Conductive Composites: From Fabrication to Application

A conductive composite is a conductive material composed of conductive fillers in a nonconductive matrix. Among them, stretchable conductive composites (SCCs) generally consist of conductive fillers dispersed in elastomer and thus have a much higher mechanical stretchability than rigid metal or carbon conductive materials. Due to the ability to preserve electrical conductivity under mechanical deformation, SCCs have shown attractive applications in stretchable devices such as soft robots and wearable electronics. Most SCCs are filled with only one type of conductive filler and are thus called single-filler composites. Single-filler SCCs inevitably suffer from limitations such as low mechanical strength, poor stretchability, uneven filler dispersion, insufficient conductivity, degradation caused by oxidation, and poor repeatability. To solve these problems, many recent studies have introduced secondary fillers to enhance the physical or chemical properties of composites. These composites filled with multiple fillers are called hybrid-filler composites. There are a variety of reasons as to why secondary fillers are attractive. Their main function in hybrid-filler SCCs is to enhance the electrical conductivity by dispersing or bridging the primary fillers or by producing a synergistic effect with them. For instance, filler aggregation is a major problem for conductive composites filled with carbon fillers such as carbon nanotubes (CNTs), graphene, and carbon black (CB), which not only ineffectively uses the filler but also reduces the mechanical strength and electrical conductivity of the composites. There are two main ways to solve this problem by introducing secondary fillers. One method is to use conductive polymers (CPs) (e.g., polyaniline) or dispersants (e.g., polyvinylcarbazole-based compatibilizer, β-cyclodextrin, and silane) as secondary fillers to improve the interface interaction between the carbon fillers and the matrix, thereby more uniformly dispersing them in the matrix. Another method is to add secondary conductive fillers like metal nanoparticles (NPs) or other carbon fillers with different geometric shapes to achieve dispersion by breaking up filler aggregates during mixing. The homogeneous dispersion of the main fillers can generally reduce its percolation threshold and enhance the electrical conductivity and mechanical strength of the composite. In addition, conductive secondary fillers can also fill Dr. G. Yun, H. Lu, Prof. W. Li School of Mechanical, Materials, Mechatronic and Biomedical Engineering University of Wollongong Wollongong, NSW 2522, Australia E-mail: weihuali@uow.edu.au Dr. S.-Y. Tang Department of Electronic, Electrical and Systems Engineering University of Birmingham Edgbaston, Birmingham B15 2TT, UK E-mail: S.Tang@bham.ac.UK Prof. S. Zhang Key Laboratory of Precision Scientific Instrumentation of Anhui Higher Education Institutes Department of Precision Machinery and Instrumentation University of Science and Technology of China Hefei, Anhui 230027, China Prof. M. D. Dickey Department of Chemical and Biomolecular Engineering North Carolina State University Raleigh, NC 27695, USA E-mail: mddickey@ncsu.edu