Biohybrid Toggle Switches

INTRODUCTION<br/>Modern automation technologies continue to push the limits of engineering design by packing higher degrees of freedom—actuation freedom—into smaller machine enclosures. Our prospects of producing many life-changing technologies including multi-line braille displays, high-resolution haptic feedback suits, microfluidic single-cell analyzers, and high-dexterity robotic end effectors critically depend on the number of compact actuators which can fit in the given device.<br/> <br/>With muscle, Nature provides an elegant solution problem for all animals, even using its actomyosin parts to provide high-DOF motion to cells. Human beings have over 640 skeletal muscles, enabling the wealth of gracefully dexterous and precise motions we can do. Muscle’s hierarchical structure frees limitations of size and arrangement; caterpillars have over 4000 muscles. Given muscle’s clear actuation advantages, the burgeoning field of biohybrid robotics aims to fabricate in-vitro, 3D muscle tissue to expand its applications beyond animal locomotion.<br/> <br/>Currently, biohybrid robotic actuators are cell-gel composites formed in a ring shape with bounding box dimensions of 5 mm x 10 mm and a radius of ~0.5 mm. Muscle constructions are limited to forces of up to 0.5 mN for C2C12-derived tissues and ~5 mN for primary skeletal muscle tissues. Researchers have used these nascent constructs to make simple walkers and microgrippers which deform an elastic flexural structure. Walking and gripping demonstrations show biohybrid robotic proof of concept, yet we seek to design a mechanism which we can more generally apply to machine components for future high-DOF actuation systems.<br/> <br/>A common actuator design requirement is the control of two or more discreet position states. Switches and valves flip between on/off and open/closed states. Clutches flip between engaged/disengaged states in power transmissions. Though not mechanical, transistors flip between on/off voltage states to form the heart of the computer. Aside from functional necessity, switching mechanisms often have the advantage of only consuming power during state change. Muscle-powered systems would benefit from holding a position without the need for constant exertion. To our knowledge, no biohybrid systems in the literature have this capability.<br/> <br/>RESULTS<br/>With our previously developed C2C12-derived 3D muscle constructs, we have integrated our actuators with a bistable flexure to form a biohybrid toggle switch. The flexure is a molded soft polymer shuttle connected to a base with two beams on either side of the shuttle. Pushing one side inwards slightly preloads the flexure, moving the shuttle upwards. Pulling the shuttle downwards causes compressive stresses on the beams until the shuttle is in line with beams, where an instability causes the beams to push the shuttle downward with any further motion. Operating near the instability, electrical or optical stimulation of a muscle actuator triggers the toggle motion. We are currently tuning system stiffness and setpoints to obtain consistent actuations with muscle actuators. Rational design of the flexure including first order analysis, finite element simulations, and rapid iterative prototyping with 3D printed molds help to match the muscle contractile performance with bistable flexure behavior.<br/> <br/>OUTLOOK<br/>The first instantiation of our system features micron positioners to account for muscle actuator manufacturing variability and to tune the flexure preload. Once geometric specifications are computationally and experimentally verified, we can design a compact device which matches the actuator’s own length scale. Ideally, this device would have an antagonistic pair of muscle actuators to enable cyclic toggle actions, and we will demonstrate this in future experiments.