Morphological aspects of stereocilia hair bundle leading to the its exceptionally ultra-high sensitivity

This numerical work investigates effects of morphological aspects of stereocilia bundle in bullfrogs on its sensitivity. Stereocilia are ubiquitously present nano-sized structures that use mechanoelectrical transduction for many sensing applications in nature, such as flow sensing in fish and frogs and sound sensing in humans. Although biologists have made progress in understanding its geometry and functioning, there are still challenges in comprehending its complexities due to the limitations of its nanoscale structure and in vivo experiments. Our work used an engineering approach to comprehend the contribution of the stereocilia bundle's morphology to its ultrahigh sensitivity. Fully coupled fluid-structure interaction simulations (at Re<<1) with piezoelectricity were performed using COMSOL Multiphysics with finite element method as a numerical tool. Our results show that the top connector is the primary connecting member in the bundle and leads to its cohesive motion, validating the experimental hypothesis. We observed that the tapered base of stereocilia pillars makes the bundle more sensitive than the prismatic one. Another insight showed that the sensitivity of the stereocilia bundle increases as the cupula Young's modulus decreases. This may explain why different species have varying Young's modulus of their cupula. Along with understanding biology, these findings could help design an optimized ultrasensitive miniaturized flow sensor for drug infusion.