A shape-memory composite material developed for satellite antennas could prove useful in prosthetics and next-generation ski boots. The material, identified by the trade name TEMBO®, was developed by Composite Technology Development (CTD) of Lafayette, CO. The TEMBO concept grew out of government-funded research to develop elastic memory composite (EMC) hinges to deploy structures in space-based applications. This EMC hinge has been used in place of motor-driven or spring-actuated hinges to deploy devices such as a solar array. Such an approach relies on electrical heaters to activate the shape memory material in the hinges, which then deploys the packaged structure to the shape needed for operation on orbit.
CTD initially was funded by the Federal government to conceptualize elastic memory composite-based deployable satellite structures. These EMC materials were adapted into hinges, which were developed and tested under a Small Business Innovation Research (SBIR) Phase II contract from Missile Defense Agency (MDA) predecessor BMDO in 2000. Since that time, the use of EMC material in space has been validated through repeated deployment, reliability, and integrity tests aboard the International Space Station.
How it Works
To manufacture EMC components, CTD develops polymers using a proprietary combination of thermoset materials such as epoxies and cyanate esters — materials commonly used to make composites for aerospace applications. The polymers are heated to the glass-transition temperature, generally around 80-90 °C for satellite structures. (The glass-transition temperature is the point at which properties of polymers change quite significantly, with the stiffness or modulus dropping by as much as a hundredfold.) At this temperature, the material generally becomes soft and weak. CTD engineers and chemists are able to physically modify these materials above the glass transition temperature to enable the carbon-fiber-reinforced materials to withstand very high strain levels without damage or degradation.
The EMC material combines the structural properties of fiber-reinforced composites with shape-memory characteristics of shape-memory polymers. Components and structures fabricated with the material can be folded, rolled, or otherwise packaged to a different shape for storage and later returned to the original as-manufactured shape, without loss of performance. This shape change is affected by use of a thermomechanical process where heat and force are required to package the component; heat alone is needed to return the component back to its “as-fabricated” shape. EMC material is manufactured using most composite manufacturing processes including thermally cured fabrics, filament winding, and vacuum-assisted resin transfer molding.
CTD’s most visible success with EMC material is through the manufacture of hinges. The hinges are made using standard processing and manufacturing techniques in which reinforcing fibers are thermally combined with EMC polymers into a desired shape, cured, and then assembled into hardened parts as necessary. Hinges manufactured for space-based satellite structures consist of three parts: a composite laminate, a heater, and end settings. CTD manufactures both composites and heaters. The company designs the pliable resistive wire heater to bend and form to the shape of the composite part, while held together by the shape-memory polymer material. After the heater is in place, end fittings made of metal or various other composites are applied to both ends of the composite, allowing it to be attached to any two objects desired.
One of the most demonstrated space-based applications is the use of EMC structures for use on deployable solar arrays. This device uses passive heating from the Sun to deploy a large solar array for powering a satellite. By using the Sun’s energy to initiate this deployment, the extra weight of heaters and power supplies is eliminated. Moreover, the lightweight, carbon-fiber-reinforced structure provides strength and stiffness to the deployed structure to the solar panels.
On Earth, the hinges soon could provide aid and support to the physically disabled and skiing enthusiasts. Specifically related to prosthetics, the company is looking at the needs of amputees for whom composites are regularly used to interface in the socket fitting between the residual and mechanical limbs. The fit and interface between the residual and mechanical limbs are critical, so CTD aims to solve the problem of precise fitting using shape-memory materials. As a body changes shape — whether because of swelling due to activity or too many morsels of chocolate — so does the residual limb. CTD officials believe that the shape-memory polymer prosthetic sockets should emerge as marketable products within the next few years, pending the company’s ability to obtain any necessary approval by Federal regulatory agencies.
Where it Stands
Beyond the technology’s space-based applications, CTD researchers are working to move the material into a variety of markets, including biomedicine. The company is developing its material into a prosthetic “socket,” the part that connects a person’s residual limb with the prosthetic appendage, such as a foot. And for the athletic footwear market, the technology can provide a custom fit for ski boots; for example, providing additional comfort, support, and performance.
CTD would make the shape-memory material for prosthetics in the same way it makes an EMC hinge — with polymers, resins, and fibers heated to form a brace-like socket fitting, then cooled to keep the desired shape. The company also is considering an offshoot of the prosthetic concept, using the basis of the composite hinge technology to develop “body-interfacing” inserts for ski boots and other protective sporting gear. Such boot inserts would be made using the same process as the hinges and prosthetic devices. A heat-reactive, shape-memory polymer insert would be put into boot linings to provide a proper fit, while allowing some expansion when the user’s feet start to swell, such as with prolonged activity.
Beyond the realm of space, sports, and prosthetics, CTD is looking at other applications for its shape-memory technology, including components for aircraft, automotive equipment, composite tooling, down-hole drilling, marine and maritime structures, and composite tooling. The company also is looking at the technology’s potential in the development of small, unmanned aerial vehicles that can be folded up and carried on a warfighter’s back for surveillance purposes.
For more information on CTD’s hinge technology, visit http://info.hotims.com/28050-516. (Source: Joe Singleton/NTTC; MDA TechUpdate, Missile Defense Agency, National Technology Transfer Center Washington Operations)