Understanding the effect of skin mechanical properties on the friction of human finger-pads

The aim of this work is to achieve an understanding of the effect of skin mechanical properties on the friction of human finger-pads. This project primarily concentrates on gaining a more fundamental understanding of the frictional properties of skin. To achieve this, various parameters (epidermis thickness, sweat-gland counts, etc.) affecting skin friction were evaluated using an in-vivo technique, Optical Coherence Tomography (OCT) and a friction testing device. This project is also interested in investigating how those parameters alter the friction for different ages, genders, ethnicities and different contact conditions, such as moisture, temperature, loads, etc. Experimental studies were conducted to investigate the skin frictional behaviour. The findings showed that the skin friction obeys a two-term relationship. The skin friction was found to be strongly associated with its Young’s modulus. Tests on the skin structural properties showed the moisture level of the skin, skin thickness and skin morphological properties play important roles in determining the skin friction. The findings gained can be applied to explain how the skin friction varies among different participants. Further tests showed that physico-chemical properties of the skin can have a significant effect on the skin friction. The OCT system was combined with a multi-axis force plate to measure the contact area between fingers and smooth surfaces. Static measurement showed both apparent and real contact area increase with normal load following a power-law relationship. This is associated with the skin mechanical properties. The dynamic contact area was investigated using a Digital Image Correlation (DIC) method. As a finger was sliding along a flat surface, the dynamic apparent contact area was found to decrease with time.