RasLabs, LLC: Polymer-based Liner for Prosthetic Sockets

Developing an adjustable prosthetic liner to revolutionize the fit and function of prosthetics.

RasLabs believes in developing, fabricating, and distributing customized products that have the power to heal and save lives. They produce Synthetic Muscle, electroactive polymer (EAP) based materials and actuators that contract and expand at low voltages. This EAP material could improve the interface between a child and his/her prosthetic limb. Without using gears or motors, the material contracts or expands like muscle, in response to low-voltage electricity. Using this biomimetic material to line the socket of a pediatri-sized artificial leg or other limb could provide a more snug fit of the prosthetic device during normal daily use, and improve a child's experience using it. With funding in part from the PPDC, as well as support of the Synthetic Muscle Project from the US Department of EnergyMassChallengeCASIS, and the US Department of Defense, the prosthetic liner device is in early development. 


Figure 1. Simple demonstration of sensing using an analog multimeter.


Most polymers, once cooled, do not move or change shape. Ras Labs Synthetic Muscle™ are electroactive polymers (EAPs) that undergo controlled motion and shape-morphing with electric input. Synthetic Muscle™ incorporated into self-adjusting EAP based pads for prosthetic liners and sockets will allow amputees and children born without fully formed limbs to go about their active lives without needing to adjust the fitting of their prosthetic device(s) throughout the day. The purpose of this development is to resolve major issues facing amputees, such as prosthetic slippage and the inconvenience of removing or adding prosthetic sockets to maintain fit. For pediatric patients, this is of paramount importance because of the brain mapping that occurs with full function during childhood, and for all children to comfortably and easily enjoy the full freedom of motion.


Ras Labs Synthetic Muscle™, comprised of electroactive polymers (EAPs), expand and contract at low voltages, and can create a dynamic prosthetic liner or socket to maintain proper fit for the amputee throughout the day (prototype stage). In addition, these polymers offer impact resistance and pressure sensing, and have the potential to combine these properties in one integrated solution. The pressure sensing property of these polymers potentially allows Ras Labs’ prosthetic pads to adjust to the shape of the patient’s residuum without manual adjustment.


When mechanical pressure is applied to Ras Labs’ electroactive polymers, the change in resistance can be recorded. This allows Ras Labs to use these EAPs as resistive sensors in addition to their contractile properties for multiple applications. As resistive sensors, these electroactive polymers have the potential to advance prosthetic technologies by creating self-adjustable dynamic prosthetic liners and sockets. Multiple pads can be placed in the prosthetic liner or socket as different sensor zones to detect changes in pressure. This detection can lead to an automatic adjustment of the prosthetic pads by contracting or expanding to maintain proper fit and can also give feedback to patient on the use of the device in static and dynamic states. Feedback from the device can help the patient and prosthetist adjust rehabilitation practices to improve the patient’s locomotion and balance, and automatic adjustment of dynamic prosthetic pads would enhance proper fit and comfort of prosthetic devices for the amputee patient. The sensing the sensing properties of Ras Labs’ Synthetic Muscle™ through compression and impact. Based on preliminary results, Ras Labs Synthetic Muscle™ can register pressure at different magnitudes by a change in resistance. In the compression sensing tests, the polymer senses that a large amount of force is being applied over a set period of time. For the impact sensing tests, the polymer registers the blunt impact by showing a spiked decrease in resistance followed by an exponential climb to a stable state.


Ras Labs is collaborating with United Prosthetics, Inc., which is taking care of the majority of the Boston Marathon victims, including pediatric patients, is assisting with prototyping, and has offered to help with human clinical trials. The addition of the sensing properties of Ras Labs Synthetic Muscle™ to its shape-morphing actuation properties has the potential to greatly advance prosthetic devices. Due to a combination of activity and dehydration, the amputee’s residuum can undergo volume changes throughout the day causing improper fit and discomfort. These changes can eventually lead to skin and tissue degradation if the patient does not manually adjust his or her prosthetic device to maintain a proper fit.  Dynamic fit is crucial to maintaining a proper fit of the prosthetic device, and the goal of Ras Labs is to increase the patient’s level of comfort and proper fit by incorporating Ras Labs EAPs polymers into prosthetic liners and sockets (Figure 2). A combination of contractile actuation, impact attenuation, and pressure sensing from Ras Labs Synthetic Muscle™ to create dynamic self-adjusting prosthetic pads and liners can enable amputees and children born without fully formed limbs to go about their active days without constant manual adjustment of their devices.


Figure 2. Current prototype incorporating Ras Labs Synthetic Muscle™ based EAP system into an adjustable prosthetic liner.