Humidity-Dependent Viscoelasticity of a Single Human Corneocyte at Nanoscale Contact Using AFM Force Spectroscopy for Haptic Design Considerations

Electronic device users interact with applications through fingertip contact on haptic touchscreens, where the human finger skin directly interfaces with a capacitive surface under varying environmental conditions. The skin plays a crucial role as the first point of contact, absorbing and applying force while being exposed to different levels of humidity. As a soft biomaterial and living tissue, human skin is inherently flexible due to its viscoelastic properties; however, the extent of this flexibility, especially in the nanoscale contact regime, has not been fully characterized. Recent advances have enabled the quantification of viscoelastic properties in human tissues and other living cells using atomic force microscopy (AFM), revealing distinct stress relaxation behaviors and relaxation times depending on the cell type. Despite this progress, the viscoelastic properties of corneocytes, the primary cells found in the outermost layer of the finger skin, remain largely unexplored in the fields of biomechanics and nanotribology. This study employs AFM force mapping with a dwell-time technique to quantify the linear viscoelastic properties of corneocytes, simulating a single-touch interaction between human skin and a haptic surface under a constant contact load of 5 nN across five different humidity levels. The results demonstrate a significant effect of humidity on the viscoelastic behavior of corneocytes. Specifically, stress relaxation and characteristic relaxation times decrease with increasing humidity, showing a reduction of roughly 2 times compared to those under low humidity conditions. Additionally, corneocytes exhibit rapid stress relaxation within 3 s, suggesting that the mechanical robustness of the skin-surface interfacial contact could be considerably moderated by moisture at high humidity levels.