PhD Defence Michel Klaassen

the static friction behaviour of skin with relevance to pressure ulcer prevalence

Michel Klaassen is a PhD student in the department of Mechanics of Solids, Surfaces & Systems (MS3). His supervisor is E. van der Heide from the faculty of Engineering Technology (ET).

Pressure ulcers develop as a result of sustained mechanical loading and are commonly observed in patients with reduced mobility and reduced tissue viability as well as in patients that have undergone an amputation. Shear loading is known to be a risk factor that accelerates the onset of tissue damage. In this thesis it is hypothesized that pressure ulcer prevalence can be reduced by decreasing the shear load acting on the skin. 

The susceptibility of developing pressure ulcers whilst applying a range of shear loads for a prolonged period is demonstrated by means of cytokine measurements performed on volunteers. The expression of the IL-1α cytokine, which triggers the inflammatory response, was found to have a shear stress threshold value; at low levels of applied shear loading no increase was observed, whereas an increased expression was observed when the shear load was raised. The exact level of the threshold varies from one person to another. Considering the critical threshold for cytokine release, people who exhibit a high coefficient of friction might be more prone to developing pressure ulcers as it is more likely that a high shear load might be acting on the skin.

Although previous researchers have established that skin hydration might be responsible for causing interpersonal differences in friction behaviour, correlation was only obtained within individuals, whereas this was not obtained for groups. In this thesis, Fourier transfer infrared spectroscopy (FTIR) has been identified as a useful tool for identifying skin characteristics that may be linked to interpersonal differences in friction behaviour. A strong correlation was obtained between the coefficient of friction and FTIR peaks relating to the hydration of the skin and a measure for the viscosity of the sebum layer, meaning that a direct relationship between FTIR spectrum and an individual's friction behaviour has been obtained.

To reduce the friction coefficient between skin and various counter surfaces, a selection of parameters can be altered to change the frictional behaviour. Results from friction experiments using smooth counter surfaces showed that the interfacial shear strength depends primarily on the environmental conditions while the material of the counter surface is of secondary importance. It was shown that the microenvironment has a major influence on the coefficient of friction. Both temperature and relative humidity had strong effects on the frictional behaviour. This work suggested that, from a tribological point of view, reducing the humidity should be the primary focus. The effect of surface roughness and hardness was studied using silicone counter surfaces which are commonly used for prosthetic liners. Employing surface roughness and varying the compliance of the silicone compound allowed the coefficient of friction to be altered. 

The results obtained are combined into two design maps: one for optimising the micro-environment in the skin-object interface, and one relating to surfaces in contact with skin exhibiting controlled levels of friction, with the objective of optimising conditions for preventing pressure injury.