A mechanical mimicry of the negative stiffness and adaptation mechanism of stereocilia bundle.

Abstract

Stereocilia is a mechanotransducer that change external mechanical stimuli into electrical signal and has high sensitivity over a broad dynamic range, which is design conflict of the conventional inertia sensor and accelerometer. The mechanism of the retained high sensitivity was hypothesized as an interplay between the negative stiffness and adaptation. The adaptation shifts the nonlinear high sensitivity region toward the operation region of the stereocilia bundle. In this study, we developed a biomimetic mechanical model of stereocilia to demonstrate the interplay between the negative stiffness and the adaptation mechanism. The model consists of an inverted pendulum supported by pivot spring and a fixed bar which represent a pair of adjacent stereocilia. The magnet pairs are attached to pendulum and fixed bar each other to emulate ion channel’s gating force. The magnet on the fixed bar connected to a stepping motor to move the magnet side-to-side which demonstrate readjustment of tip-link tension by slipping down and climbing up of adaptation molecular motors of stereocilia. A displacement clamping equipment which consists of a uniaxial force sensor and actuator was used to measure the mechanical stiffness of the model. Experimental data from mechanical model showed the negative stiffness region near the equilibrium position and shifted the high sensitivity region with the progress of adaptation. Spontaneous oscillation which produced by the interplay between negative stiffness and adaptation mechanism also observed. The results demonstrate that the negative stiffness and adaptation mechanism was mechanically produced by the combination of repulsive force and its continuous readjustment. The change of model parameters of biomimetic mechanical system such as spring stiffness, magnetic force, and adaptation motor speed provided us better understanding of nature’s inertia sensing mechanism.