Palpation of the skin with a robot finger: an attempt to measure skin stiffness with a probe loaded with a newly developed tactile vibration sensor and displacement sensor

Background/aims: Various attempts have been made to measure mechanical properties of the skin quantitatively and noninvasively. However, no attempt has been made to measure a tactile sense of our finger that palpates the skin; when we palpate the skin to search for any change, we at first rub the skin surface softly with a finger tip to detect a surface change and then press the finger against the skin to perceive any alteration in consistency. The problem here is how to record such tactile sense quantitatively, because the palpating examination for smoothness or softness of the skin totally depends on a subjective perception. A new tactile sensor from robot technology is equipped with a fingertip‐like sensing probe oscillating at a resonant frequency of 60 kHz; it is capable of clearly sensing small differences in hardness of solid substances from a change in the oscillating frequency and from a displacement sensor that detects depressability of the skin. We have used this new measuring system to evaluate the hardness and elasticity of skin. Methods: We constructed a probe with two independent sensors, i.e., a newly developed tactile vibration sensor and a displacement sensor. To determine its usefulness we first used an in vitro skin model and subsequently used normal and lesional skin, such as neutrophilic erythema, keloid, hypertrophic scar, and scleroderma. Results: Two parameters were obtained with our probe: one that reflected superficial firmness of the skin, which mainly correlated with the hydration state of the stratum comeum, and the other that reflected the firmness of deeper tissue. Conclusion: Our results showed that this probe is useful for evaluating the physical properties of skin lesions that accompany changes in skin firmness.