Tying Skin Deformation Patterns with Discriminability of Compliance to Design Soft Haptic Actuators

Of the various dimensions that underlie our sense of touch – e.g., roughness, texture, and stickiness – compliance is particularly important in our daily lives. For example, we routinely inspect the ripeness of fruit and affectively touch others. To discriminate the compliance of soft and compliant objects, we rely upon spatiotemporal cues in the mechanical deformation of the skin, which is embedded with hundreds of neural afferents. However, since direct observations of skin dynamics are challenging, we do not yet understand, in touching objects of various compliance, how skin deformation patterns evolve over time and thereby inform perceptual judgements, how do such relations vary among individuals, and how can we create a close-loop system that simulates tactile sensations with simultaneous observation of skin dynamics. First, to obtain visual access of skin dynamics and quantify skin deformation, we developed a 3-D stereo imaging technique for use in passive touch to observe contact of the skin’s surface with transparent, compliant substrates. In doing so, we derive skin deformation cues to quantify and characterize the skin’s movements with varying stimulus compliance, indentation depth, velocity, and time duration. Our results indicate that compliant stimuli at higher velocities are more difficult to discriminate because they induce smaller differences in deformation, beyond a minimum contact duration of 0.4 seconds. Moreover, we find that several independent cues aid our perception of compliance. In particular, the change rate of contact area best correlates with tactile discriminability regardless of indentation velocities and stimulus compliance, while cues associated with skin curvature and bulk force are predictive for stimuli more and less compliant than skin, respectively. Second, we study the differences in skin properties and tactile acuity among individuals where factors of skin stiffness and fingerprint breadth have been underexplored. Therefore, we recruited a cohort of young participants who present a diverse range of finger size, stiffness, and fingerprint breadth, and investigated relationships between their fingertip properties and perceptual discriminability. We found that the ability of participants to discriminate compliance could be differentiated by their finger stiffness. In support of this finding, in softening the participants’ skin with hyaluronic acid, we observed an improvement in their perceptual discriminability, which further validates the high correlation between finger stiffness and perception. Finally, to develop a close-loop haptic system, we designed a transparent, reconfigurable, multi-channel hydraulic haptic actuator. Through actuation, we can control the movement of skin surface and optically observe skin deformation with distinct elasticities that produce distinct percepts. Profiles of the contacting surface are directly visualized through actuated channels to match observations with solid substrates, and seek to enable personalized calibration in the future.