Microvascular-Based, Tunable Stiffness Elastomers

Compared to traditional robotic systems, soft robots will conform to obstacles in their environment, rather than inducing a potentially catastrophic collision. The compliant nature of the material and structural design may allow the robot to move continuously, without distinct links or joints. To locomote, a multi-chambered design is employed in soft robots and the robot is actuated by pressurizing different chambers. Motion is induced by differential stretching of the individual chambers. Soft robots can undertake complex motion, but the control and motion planning systems are also often complicated. Many designs must remain tethered to a pressurized air source, restricting movement and limiting the range of motion. Similar complex motion is exhibited in nature where continuous motion occurs by varying wall stiffness. We adopt this approach by incorporating a network of magnetorheological fluid (MRF)-filled channels in the robot wall to control stiffness.<br/>Methods to control the stiffness in a soft robot body generally fall into two categories: active approaches introduce fluid or electrical currents into the material, while semi-active approaches alter the operating conditions of the material, inducing phase changes via heat or electricity. Semi-active approaches typically have slow responses to stiffening [1]. Phase changing fluids, like melted wax, which solidifies when cooled, impart a change to the bulk material properties in which it is embedded. Magnetic particles in MRFs form chains when in the presence of a magnetic field and increase fluid viscosity by 2 orders of magnitude on the order of 1-2 seconds with a relaxation time slightly faster on the order of 1 second [1]. In order to implement MRF into the silicone elastomer, specimens were manufactured through the vaporization of sacrificial components (VaSC). The VaSC method is mostly used in hard materials like epoxy resin with PLA that has been printed and then catalyzed prior to curing [2] whereas in this approach the VaSC method is applied to an elastomer. Hollow channels in silicone rubber were constructed by extruding PLA with a catalyst, 3D printing the channel geometry and then curing it into a specimen mold. After depolymerization, the now hollow channels are infilled with MRF.<br/>Fluid channels filled with MRF are critically affected by the orientation of the magnetic field. Fields applied parallel to a channel of MRF allow particles to form longer chains than those formed in a transversely applied field. This greatly increases the stiffness of the specimen in compression. Finite element analysis of a simple planar truss comprised of MRF in an elastomeric channel was investigated using MATLAB. Results indicate a factor of two increase in stiffness for a compressive force applied to a support node of this type of material [3]. Initial tensile tests were conducted on specimen with varying channel geometry and digital image correlation (DIC) was used to establish a stress strain field. Under the presence of a magnetic field oriented perpendicular to the uniaxial tension, specimens exhibited a decrease in stiffness, with changes from 63% to 50% depending on network architecture. Here we explore the further mechanical property changes induced in this material with the optimization of channel geometry.<br/>[1] Manti, M., Cacucciolo, V., & Cianchetti, M. (2016). Stiffening in soft robotics: A review of the state of the art. IEEE Robotics and Automation Magazine, 23(3), 93–106.<br/>[2] Esser-Kahn, A.P. et al., (2011), Three-Dimensional Microvascular Fiber-Reinforced Composites. Adv. Mater., 23: 3654-3658.<br/>[3] J. G. Williamson, C. Schell, M. Keller and J. Schultz, "Extending the reach of single-chamber inflatable soft robots using Magnetorheological Fluids," 2021 IEEE 4th International Conference on Soft Robotics (RoboSoft), 2021, pp. 119-125.