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. In this case, a liquid sac can cover 2-3 adjacent tendons, thus, the cross-talk of acquired signals and the position sensitive response are effectively mitigated. Benefitting from the flexible sac base (200 μm in thickness), the liquid sac can transduce the movements of wrist tendons with high-fidelity and excellent reversibility.
The locations of the three sensors were arranged by considering the distribution of wrist tendons, subcutaneous fat, and the blood vessels. In order to avoid the impact of wrist pules on the hand gesture recognition, two NFPSUs on the dorsal wrist (back side) were used to detect the movements of thumb and middle fingers. One NFPSU on the volar wrist (palm side) was used to detect flexion of index and ring finger. As shown in Figure 1c, the three NFPSUs can recognize 12 hand gestures and the interference of wrist pulse can be neglected.
 To make the GRW suitable for testers with different physiques and avoid random sliding between skins and NFPSUs, we used elastic straps to connect the three NFPSUs as shown in Figure 1d. Note that we don’t recommend wear the wrist band too much tightly. Each NFPSU has dimensions of 3×2.5×0.3 cm3 (comparable to a coin) and can be comfortably worn on the wrist.