Quantification of the Real Contact Area of a Finger-Pad During Sliding Using a Novel Optical Coherence Tomography System and the Influence of Skin Thickness

Understanding the interaction of skin with surfaces is important as there are many applications where this is important, such as medicine (skin grafts), bioelectronics (sensors, robotic hands, and wearable technology), and consumer products, depending on the grip, dexterity, and feel of the objects. These phenomena are all strongly governed by frictional forces, which are intrinsically linked to the real area of contact. The real contact area, in turn, is modulated by factors such as skin hydration and the topographical roughness of the interacting surfaces. In this work, a new high-resolution optical coherence tomography (OCT) tool, developed in-house, was used to analyze the real contact area and the stratum corneum thickness in finger-pad interfaces due to its ability to penetrate and image the skin layers at high resolution. Data were compared with a lower-resolution VivoSight® OCT device used in previous work. Forty-seven finger-pad sliding tests against smooth glass were performed using three different volunteers with forces ranging from 0.5 to 3 N. Post-test analyses of the OCT images captured revealed that the real measured contact area was 54 ± 7% of the apparent contact area using the high-resolution OCT device in comparison to 63 ± 10% measured with the VivoSight OCT. This underscores the rationale for employing a high-resolution OCT system, as it enables precise visualization of the contact perimeter at the tribological interface, thereby facilitating a more accurate quantification of interfacial geometry and contact mechanics. The thickness of the stratum corneum was also seen to increase during sliding under the high-resolution OCT device. This resulted from the high skin deformation, which in turn influenced the contact area.