Fig. 1

Simulations for guiding the layout of the flexible device and characterization of the fabricated device. a Stress simulation of and strain sensor in the flexible device. The stress gradually decreased from the center of the device along the radial direction, and stress concentration occurred at the fixed edges of the device. b The electrical field simulation of the electrode in the device with and without insulating layer. The up-top inset revealed that the electrical field could transport across the whole device. The bottom inset showed that electrical field only exists along the strain sensor. The electric field strength of other locations, especially around the microelectrodes were nearly 0 V/m after adding an insulating layer. b The fabrication process of flexible device. First a thin PDMS film was spin-coated onto a 2 cm × 2 cm acrylic slide. Then lithography was employed to form a spiral channel and carbon fiber was assembled into the channel. The isolation layer was spin-coated onto the carbon fiber. Finally, carbon fiber adhered to the top layer of PDMS. d Determination of flexible device dimension during its fabrication. e The schematic for the developed device. f An image of the fabricated flexible device. g XRD for determining the existing of carbon fiber and nano-copper. h SEM of bare carbon fiber. i SEM of enlarged bare carbon fiber for showing its smooth surface. j SEM of and the carbon fiber modified with nano-copper (electroplating time = 80 s). k SEM of enlarged carbon fiber modified with nano-copper, which showing nano-copper with feather-shape. The inset showed that the feather-shaped nano-copper was constructed by spherical nanoparticles