Figure 1. Overview of the gesture-recognition wristband (GRW): a) Configuration of the proposed GRW. b) Schematic illustration of a single nanofiber-based pressure-sensor unit. c) Cross section of the wrist with GRW. d) Photograph of the GRW.
One key to the success of the GRW in this work is its highly sensitive NFPSU, as shown in Figure 1b . It consists of a filmy optical-nanofiber sensor, a soft liquid sac, and a rigid three-dimensional (3D)-printed resin shell. The optical nanofiber, serving as a light waveguide to detect skin deformation with high sensitivity, is U-shaped for increased compactness and is encapsulated by polydimethylsiloxane (PDMS) films (length: 21 mm; width: 14 mm; thickness: 200 μm) to form a filmy optical nanofiber sensor. A soft liquid sac (length: 27 mm; width: 23 mm; thickness: 2.5 mm) filled with glycerol is sealed by the filmy optical nanofiber sensor; it is used to transfer the applied pressure due to skin deformation to the optical nanofiber. The rigid 3D-printed resin shell is designed to isolate the sensor from mechanical stimuli other than those generated by finger movements. The sac base (length: 6 mm; width: 6 mm; thickness: 200 μm) penetrates the hole in the center of the resin shell to contact the skin surface and collect pressure signals. To analyze the relationship between hand gestures and the related tendon movements, two NFPSUs are positioned on the dorsal wrist (back side) and one on the volar wrist (palm side), respectively (Figure 1c ). Using only these three NFPSUs, twelve typical hand gestures can be recognized. To make the GRW suitable for testers with different physiques, we used elastic straps to connect the three NFPSUs as shown in Figure 1d . Each NFPSU has dimensions 3×2.5×0.3 cm3 (comparable to a coin) and can be comfortably worn on the wrist.