The Newman Lab

In collaboration with team 4.3 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.

Electro-rheological 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.

Applications

The energy absorbing nature of field responsive fluids has proven useful in commercial products ranging from shock absorbers to protective garments. Each of these products relies on a combination of the energy absorption mechanism of a specific field responsive fluid and a matched “transmission geometry” to translate the change in material properties at the molecular scale into a usable macro-scale change in device properties. This project aims to understand the underlying mechanisms that affect human-scale performance and to choose the combination of fluid and transmission geometry that maximize this performance.

Electro-rheological fluids

Applications of ER fluids have historically relied on transmitting torque by shearing fluid between two or more counter-rotating disks/shells or on pushing activated ER fluid through an orifice. Mechanical clutches and dampers are classic examples of applications for ER fluids. Current ISN research puts ER fluid into flexible layers that could act like a variable stiffness fabric, absorbing vibration or impact, and supporting load.

A key consideration in this research is the particular geometry that will work best with ER fluids to provide protection for soldiers. A rigid geometry with constrained boundary conditions is impractical in a field-deployable device. ISN researchers have investigated the effect of unconstrained boundary conditions in creating the ER effect using flexible electrodes, and the effect of different inter-electrode spacers. These spacers were used to prevent contact of the electrodes that would lead to a short circuit.

A recent development in project 4.3 is the concept of using electrodes without a separate spacer but with a set of raised features on one of the electrodes designed to prevent electrode contact. These features may be patterned to enhance the ER effect by both decreasing the minimum allowable distance between the electrodes and by adding additional shear surface area. This approach in intended to be compatible with both rigid and flexible substrates. Experiments to study the effects of nanoscale electrode spacing and surface geometry are in progress.

Shear-thickening fluids

An application of shear-thickening fluid that is currently under consideration is blast lung protection. It has been shown that blast injuries result from small-amplitude, high-rate loading and acceleration of the chest cavity. A possible protective mechanism could be created using STF filled foam layered with an air-filled foam. The impedance mismatch between these foam layers and the energy absorption/dissipation properties of ST fluids may help reduce blast lung injuries by lengthening the duration of the blast pressure wave.