Semi-Active Variable-Impedance Materials: Biomechanical Design and Control

In collaboration with the team at the Institute for Soldier Nanotechnologies, and under the combined guidance of Drs. Neville Hogan and Gareth McKinley, this project is devoted to the study of semi-active variable-impedance materials in the context of enhancing human biomechanical performance. The challenge of this project is to develop biomechanically sound devices with semi-active variable-impedance materials to cushion the effects of falls, limit or avoid fractures, support continued mobility despite injury, etc. Variable impedance materials change their resistance to deformation, for example their damping and stiffness, in response to control, making them interesting for tunable protection for military applications, first response teams, etc. The variable impedance materials in question are categorized as field responsive fluids and include electrorheological (ER), magnetorheological (MR), and shear-thickening (ST) fluids. These fluids change their resistance to deformation in response to electric, magnetic, or shear fields, respectively. ER and ST are the primary fluid types studied in this project.

Electrorheological fluids -- ER fluids quickly vary their rheological properties in response to an imposed electric field. There are two types of ER fluids, heterogeneous and homogeneous, shown below along with a sketch of the corresponding shear stress response of each type of fluid to an increasing shear rate and electrical field.

A prototype was developed using a layered architecture with ER fluid between sheets of aluminized mylar to study, among other things, the effect of unconstrained boundary conditions and of the inter-electrode spacer material. Research showed that homogeneous ER fluid exhibits a magnitude higher energy absorption for the same power input as heterogeneous fluid. The research also showed that ER fluid was most effective when implemented without a spacer in the tested geometry. This result could be a reflection of the increased ER effect with decreasing electrode spacing or the interference of the ER effect by an intermittent dielectric layer. Ongoing research aims to address these possibilities and to explore new geometries that have the potential for higher force transmission and more efficient operation. A mathematical model was also developed that could reproduce the experimental performance of this system and support continuing device design.