Development of measuring method for softness of epidermis using rotational step response

We have proposed a new measuring method for the softness of epidermis using rotational step response. It can be proceeded that horizontal torsional step is given periodically to the human skin surface by rotational step motor. Then dynamically epidermal characteristic is abstracted selectively from the human skin minimizing influence of the subcutaneous tissue. The cylindrical rotor is attached to the shaft of step motor and its external area is surrounded by the cylindrical guard ring; therefore, only human skin surface of inside the guard ring is screwed periodically by its rotor. At this moment, viscoelasticity of the epidermis is evaluated from analyzing the inducting coil of the step motor. The waveform of voltage of inducing coil can be characterized by overshoot P1, damping ratio D and undamped natural frequency ω_n. The softness Ks that indicates viscoelasticity of the epidermis can be calculated from these parameters. Many experimental results showed that the softness Ks corresponds to the human sense and it is rational as a parameter for the human skin characteristics. This system is very useful as the measurement can be done easily, in vivo, and non-invasively. It also can be constructed simply without special mechanical sensor because step motor acts as not only driving but also sensing