Experimental Determination of the Viscoelastic Properties of the Human Fingerpad

Understanding the human tactile system is the motivation for this research. The linear viscoelastic mechanical properties of the human fingerpad describe the stress-strain relationship within the fingerpad for any stimulus geometry and any time history of applied forces or displacements. The stress-strain relationships within the fingerpad tissue govern the stimuli to the mechanoreceptors which are the source of all tactile information to the brain. A device referred to as the tactile stimulator was designed and is presented in this work. The tactile stimulator is a versatile device that can provide a variety of dynamic stimuli to any material and measure the displacement and force responses. The stimuli can vary with time and with indentor shape. The device is currently configured to allow normal or tangential displacement-controlled stimuli. The tactile stimulator has a force resolution of less than 1 mN and a displacement resolution of less than 1 p.m. The solution to the linear elastic problem of a cylindrical punch indenting a semi-infinite half-space can be transformed to a linear viscoelastic solution by means of the correspondence principle. Two of a material functions can completely describe any force-displacement or stress-strain relationship within the material. To determine the mechanical properties of the human fingerpad in vivo, two linearly independent experiments in the linear range of the tissue were performed to solve for the dynamic, viscoelastic mechanical properties of the human fingerpad. The impulse response bulk modulus and the impulse response shear modulus are defined in this thesis as a function of frequency. The impulse response bulk modulus varies from 55 kPa at 2 Hz to 8 MPa at 200 Hz. The impulse response shear modulus varies from 90 kPa at 2 Hz to 8 MPa at 100 Hz. Thesis Supervisor: Mandayam A. Srinivasan Title: Principal Research Scientist