Researchers at the RIKEN Thin-Film Device Laboratory have succeeded in producing a stretchable and conductive electronic material by combining polyurethane nanofibers with silver nanowires, making on-skin electronics a possibility.
To be suitable for use as wearable electronics, a material needs to be both stretchable and conductive. It should also be breathable and durable. Finding a material that satisfies all four criteria is not easy. One promising strategy is to combine elastic polymer nanofibers with a network of conductive metal nanowires, but it is challenging to get the two components to bond tightly to each other.
Importantly, the two components bond strongly to each other. There’s very good adhesion between the silver nanowires and the polyurethane nanofibers. This is why the team simultaneously achieved very good stretchability and conductivity. The team suspects that hydrogen bonds, the same bonds that hold pulp fibers together in a sheet of paper, are responsible for this good adhesion.
The film could find use as electrical wiring that can be used on the skin. Its flexibility allows it to stretch when a person bends their elbow or knee, and it can be stretched to three times its initial length. In the same way, it could also be used for wiring on robots. Another potential use is as a sensor. As its conductivity changes when it is stretched, it can be used to detect movement. The team demonstrated this by using their film to monitor the angle of a bent wrist and detect a person’s pulse.
The film has other advantages besides good stretchability and conductivity. When the researchers tested their film by stretching and releasing it 1,000 times, they observed that its conductivity dropped off only moderately, indicating that it has good durability. Furthermore, it is easy to fabricate. This film can be obtained by solution processes. That is very important for the mass production of the industrial product.
Kenjiro Fukuda (one of the lead researchers on the project) notes that his team consists of researchers from different fields of expertise. Zhi Jiang brought his knowledge of stretchable nanowires to the project, while Fukuda and other team members supported the fabrication of the polymer nanomesh structures.
The team intends to improve the stretchability of the material by using longer silver nanowires. They also plan to experiment with other combinations of materials and pursue applications of these breathable conductors in real-time health monitoring systems and soft robotics.
