Superhydrophobic, biocompatible and durable nanofiber composite with an asymmetric structure for anisotropic strain sensing and body motion detection

• Rigid CNFs are combined with soft TPU nanofibers to form an asymmetric structure. • CNFs construct the conductive network while aligned TPU nanofibers can provide anisotropic deformation. • The nanofiber composite possesses excellent durability, biocompatibility and biosafety. • The nanofiber composite exhibits anisotropic strain sensing performance. Anisotropic strain sensors show multi-directional sensing that can be used to monitor complex human motions. Aligned electrically conductive nanofiber membranes have been candidates for anisotropic strain sensors. Challenges remain for developing nanofiber composite strain sensors with a large workable strain range, corrosion resistance, excellent durability and biocompatibility. Here, we propose a facile vacuum filtration method to prepare an asymmetric nanofiber composite for the anisotropic strain sensing. The carbon nanofibers (CNF)/polydimethylsiloxane (PDMS) are deposited onto an aligned thermoplastic polyurethane (TPU) nanofiber membrane surface, forming a two layered structure. The PDMS ensures strong interfacial adhesion between the TPU nanofibers and the CNF layer, which endows the nanofiber composite with excellent surface stability and durability. The superhydrophobic CNF/PDMS constructs the conductive network, while the aligned nanofiber layer provides the anisotropic mechanical deformation. The nanofiber composite is cytotoxicity-free and exhibits excellent biocompatibility and biosafety. The asymmetric nanofiber composite shows anisotropy in the mechanical property and sensing behavior. The nanofiber composite demonstrates a large working strain range and outstanding sensing durability, and can be used for monitoring the multidirectional body joint motions even under corrosive conditions. Furthermore, the conductive CNF/PDMS exhibits excellent photothermal conversion performance and a negative temperature coefficient (NTC) behavior, and is hence suitable for isotropic temperature sensing. The asymmetric structure opens a new avenue for multifunctional and anisotropic strain sensing.