A Monolithic Fabrication Strategy for Millimeter-Scale, Self-Sensing Actuators

Advances in soft robotic technology helped to overcome many challenges in minimally invasive surgery by improving dexterity, flexibility, and safety. However, there are tradeoffs as well in using soft robots for surgical applications including complicated fabrication and assembly processes, low output force, poor controllability, and limited sensing functionality. The confined, tortuous workspace of minimally invasive surgery adds additional challenges, such as restricting the size of a robot to a millimeter scale and requiring a wide range of motion for navigation. Thus, the fabrication of these millimeter-scale soft robots becomes more delicate and convoluted, resulting in inconsistent fabrication outcomes. The manufacturing and assembly process becomes more onerous as the number of sensing and actuation elements increases. In addition, the control of these soft robots can be complicated due to the nature of their non-linear behavior, requiring computationally expensive control algorithms. Likewise, some aspects of mechanical performances (i.e., output force and torque) are limited due to the use of low elastic modulus materials. Bridging these gaps, we present the design, fabrication, and characterization of programmable composite actuators (PCAs). PCAs implement the concept of using an ionic solution as a working fluid for actuation and as a sensing medium for self-sensing. Unlike other soft robots, which require a manual assembly process to combine two discrete components, an actuator and a sensor, PCAs seamlessly integrate actuators and sensors into a single unit by embedding both self-sensing and actuation capabilities in a monolithic laminate structure. Leveraging a layer-by-layer fabrication method, PCAs are manufactured by stacking and bonding various types of films (i.e., rigid, flexible, soft, adhesive, and conductive films). PCAs include three components: 1) soft, inflatable Teflon balloons, 2) a rigid-flexible origami structure, and 3) conductive electrodes for self-sensing. The soft, inflatable balloons consist of adhesive films and Teflon films whose surface is chemically modified by using hydrogen plasma to promote adhesion. The Teflon films are selectively bonded to create multiple bellow-shaped actuation chambers. By fluidically inflating these chambers, the flat Teflon balloons can be transformed from 2D to 3D bellow shapes, producing motion. Similarly, the rigid-flexible origami structure is fabricated by selectively bonding rigid (i.e., fiber-reinforced epoxy laminate) and flexible (i.e., polyimide) films, and it surrounds the soft, inflatable Teflon balloons. Encompassing the soft, inflatable Teflon balloons, the origami structure helps to increase the output force of the actuator, and it mechanically programs the actuator to produce various types of motion and degrees of freedom in an actuator (i.e., extension, bending, and rotation). The conductive electrodes are composed of copper and graphene films embedded within the actuator to perform as a proprioceptive ionic sensor when the ionic solution is used as a working fluid. Thus, the position of the actuators can be sensed by measuring the change in impedance across the electrodes, which depends on the volume of the balloons. To further understand the performance of these PCAs, they are characterized in terms of the range of motion and output force. In addition, multiple PCAs are combined as a continuum robot to demonstrate their potential usage as a surgical robot for minimally invasive surgery.