Numerically modelling skin-product interaction: applications to tactile perception and skin injury

Abstract

As we interact with objects and surfaces, mechanical stimuli propagate from the skin surface towards our receptors, after which neurological impulses are sent to the central nervous system, thereby initiating our psychological and behavioral response. Due to the skin’s complexity, cutaneous biotribological phenomena are not fully understood. The objective of this study is to use computational skin models to help deliver insight into skin-product interaction. Through collaboration with L’Oréal Research & Innovation, numerical models were utilized to better understand tactile perception. Parameterized finite element finger skin models were developed to simulate the transmission of stresses, strains and energy from the skin-surface boundary to the tactile mechanoreceptors. Three parametric studies were conducted to understand the effects of skin ageing, counter surface topography and cosmetic film properties on receptor excitation. Key findings include: • Age-related biomechanical skin changes reduce the magnitude of mechanical stimuli being transmitted to the receptor sites, offering an additional explanation for tactile perceptive degradation amongst the elderly. • When in contact with rigid sinusoidal counter surfaces, the similarity of the fingerprint and counter surface wavelength plays a dominant role in influencing mechanoreceptor excitation. • Contact between the finger and facial skin generates mechanical signals at the receptor sites which exhibit three characteristic behaviours. Through including and modifying the properties of interfacial cosmetic polymer films, it is possible to influence each of these behaviours and thus modulate tactile perception. During the COVID-19 pandemic, healthcare workers wearing high-grade personal protective equipment for extended periods experienced facial skin irritation and injury. Therefore, research efforts were briefly focused on a final parametric study, where the effect of altering the properties of respirator masks on the skin’s damage propensity was investigated. It was found that soft, low friction materials should be utilized, contact area should be maximized and the use of soft, incompressible materials should be avoided. Overall, this thesis describes the use of computational models to analyse the propagation of subsurface mechanical stimuli within soft tissue during skin-product interaction. Product designers could use the outputs of this thesis in conjunction with existing experimental data to enhance sensorial experiences amongst consumers.