Figure 4.

Capacitive mechanical sensor based on SHARK. (a and b) Comparison of the structure and sensing mechanism of sandwich-shaped artificial skin and SHARK. In sandwich-shaped artificial skin (a), the capacitance increased upon stretching because of deformation of the dielectric layer. In contrast, in SHARK (b), the capacitance decreased because of separation of graphene layers. (c) Reciprocal of capacitance (1/capacitance) vs strain and stress vs strain for a band-shaped SHARK sensor in the tensile strain range of 0–26 mm mm⁻¹. (d and e) Change in the capacitance of a SHARK sensor in multiple bending (d) and stretching (e) cycles at a strain of 0.5 mm mm⁻¹. Insets correspond to zoomed-in views of typical cycles. (f) Stress and capacitance relaxation of a band-shaped SHARK sensor at strains from 0 to 30 mm mm⁻¹. (g–i) Zoomed-in views of stress and capacitance relaxation at strains of 2 (g), 10 (h) and 20 (i) mm mm⁻¹. (j) Capacitance vs strain and stress vs strain curves for a disc-shaped SHARK sensor in the compressive strain range of 0–0.65 mm mm⁻¹. (k and l) Change in the capacitance of a SHARK sensor in multiple compression cycles in the strain range of 0–0.15 mm mm⁻¹ (k) and upon stepwise ramping at strains from 0 to 0.4 mm mm⁻¹ (l). The strain refers to the normalized distance change of the two ends of the hydrogels. The inset in (k) corresponds to a zoomed-in view of a typical cycle.