Understanding the compatibility between spider silk and conducting materials is essential to advance the use of spider silk in electronic applications. Spider silk is tough, but becomes soft when exposed to water. Here we report a strong affinity of amine-functionalised multi-walled carbon nanotubes for spider silk, with coating assisted by a water and mechanical shear method. The nanotubes adhere uniformly and bond to the silk fibre surface to produce tough, custom-shaped, flexible and electrically conducting fibres after drying and contraction. The conductivity of coated silk fibres is reversibly sensitive to strain and humidity, leading to proof-of-concept sensor and actuator demonstrations.
The immense demand for electronics, and thus the electronic waste and environmental pollution it generates, poses a growing problem that will require innovative solutions1. Many toxic elements and non-biodegradable plastics are commonly found in conventional electronics, and efforts to develop new eco-friendly electronic designs are therefore desirable. Incorporation of natural materials into these designs is advantageous to reduce the quantity of toxic components of the electronic devices. Moreover, natural materials often possess complex and robust physical properties that can be harnessed for electrical and sensor applications. Spider silk (SS) is one such material and the combination of its toughness and bio-compatibility makes the material strategically important for implant, electrical, sensor and actuating applications.
Single f-CNT-SS fibre surface profiles.
The essential aspects leading to the realization of the f-CNT-SS material are as follows. The f-CNTs are polar, with positive charge at the amine sites. The SS is a protein-based polymer where the amino acid groups vary along the backbone, some are neutral and some polar. By mechanical mixing, the dry f-CNT powder is partially dispersed and adheres to the SS fibres due to polar interaction. When water is applied to the mixture, the f-CNTs disperse further and the SS fibres experience hydrogen bond breaking, resulting in fibre swelling and softening. As a result, the surface area of the fibre is increased, allowing more f-CNTs to adhere to the fibre. Applications of shear and pressure bring the f-CNTs in closer proximity to the surface of the fibre, promoting both physical and chemical interactions between them. Upon drying, the SS fibre matrix shrinks further as hydrogen bonding is re-established concentrating the CNT array and making it electrically conducting.