Bioinspired microneedle insertion for deep and precise skin penetration with low force: Why the application of mechanophysical stimuli should be considered

A mosquito is known to precisely and easily insert its proboscis to the human skin by pressing down a labium and vibrating a fascicle bundle. Its advanced skin-piercing mechanisms indicate that skin resistance to the insertion of needle-like objects can be changed by the application of mechanophysical stimuli. Here, we characterize the effect of the application of mechanophysical stimuli on skin resistance to microneedle insertion to find clues for inserting a microneedle in a deep and precise fashion with low force. Microneedles with a diameter of 60-140 gm are inserted at a velocity of 0.1-2.0 mm/s to full-thickness porcine skins while either uniaxial/ equibiaxial stretch of 0-20% or mechanical vibration at a frequency of 1 to 1000 Hz and an amplitude of 1-10 pm is applied to the skins as static or dynamic mechanophysical stimulus, respectively. The values of force and depth at two events of skin puncture and maximum penetration are measured to explore changes in skin resistance induced by the application of external stimuli. The static mechanophysical stimulus applied to the skin mainly affects the precision of microneedle insertion; the application of dynamic mechanophysical stimulus controls the value and deviation of skin resistance to microneedle insertion. The application of mechanophysical stimuli, inspired from a mosquito, therefore allows a microneedle to be deeply and easily inserted to the skin in a controlled way. The findings will have broad impacts on microneedle-mediated applications and lead to an indepth understanding of skin biomechanics.