Frequency response of human skin in vivo to mechanical stimulation

Abstract

The frequency-dependent response of human skin was measured, in vivo, to mechanical stimulation at a scale useful in the development of tactile displays. Sinusoidal vibration stimuli of varying frequencies and amplitudes were applied with a flat-ended 0.5 mm diameter cylindrical probe under two conditions: it was either glued to the skin or not. Both normal and tangential stimlui were applied to the skin surface in the glued case and only normal stimuli were applied in the nonglued case. The stimuli were applied to live human subjects at four body sites: the finger pad, wrist, forearm, and forehead. The force displacement response was measured and used to calculate mechanical impedance, power absorption and duty factor (an estimate of the fraction of time that the stimulator is in contact with the skin). Results showed the mechanical impedance generally increasing in magnitude with frequency and higher in magnitude for tangential stimulation than for normal stimulation. Power absorption linearly increased with frequency, and duty factor decreased with increasing frequency and amplitude. The measured properties varied widely between body sites and subjects. A mathematical model previously developed to calculate bulk and shear moduli from normal and tangential impedance data was tested against data at the four body sites. However, because the model assumed isotropy and semi-infinite thickness of the stimulated tissue, data taken did not fit the model well, especially at the finger tip.