A transparent hydraulic actuator to visualize finger pad deformation at programmable material compliances

The virtual rendering of compliance, or ‘softness,’ at the finger pad may enable interactions such as touching the hand of another or of tissue in surgery. Though prior works have demonstrated programmable compliance, most are evaluated only perceptually. Herein, we develop a capability to optically observe the deformation of the skin surface when indented at programmable states of compliance. By imaging the skin through transparent and programmable channels, we introduce a scientific instrument to directly observe and control patterns of skin deformation to decipher cutaneous cues that may drive perception. The transparent, programmable, multi-channel, hydraulic actuator is fabricated of silicone rubber in a pattern of concentric rings, where combinations of channels can be pressurized with fluids of similar refractive index. Profiles of the contacting skin surface are directly visualized to match observations with solid substrates, to enable personalized calibration. A series of biomechanical and psychophysical experiments evaluate the actuator across compliances spanning a modulus range of 33 to 75 kPa and indicate discriminable patterns of skin deformation and perceptual responses.