Veterans who use either lower-limb or upper-limb prosthetic devices, such as the myoelectric arm shown here, stand to benefit from work being done to develop new materials for socket liners.
Photograph courtesy of Providence VAMC.
The Advanced Platform Technology (APT) Center at the Louis Stokes Cleveland VA Medical Center is developing a new socket liner material to better absorb and channel the sweat that can build up inside the prosthesis to provide a more stable and comfortable separation between the limb and the socket.
The team, which also includes members from the University of Chicago and Case Western Reserve University, is developing a three-layer liner system made of advanced hydrated materials that eases the ability of the moisture to pass through, for use in place of the traditional water-impermeable silicone liners. The goal is to develop a system that can absorb and manage sweat, possibly at a rate faster than moderate sweat production, without leaking, changing size, or changing stiffness. The system is also being designed to withstand forces similar to those that would be applied in heavy use by people with lower-limb amputations, while still maintaining a tight seal with the skin to achieve a good fit.
“Suction socket liners are made of silicone, or vinyl polymers, which are long chains of molecules that join to make a rubbery material,” said Paul Marasco, PhD, a neuroscientist and principal investigator at the APT Center who is leading the developmental study. “We just changed how these long chains connect with each other so they make tiny channels in the silicone for the sweat to pass through. The sweat is captured in a layer of aerogel, which is another advanced material that works like a fancy sponge. The material holds sweat, but it doesn’t expand, and the water doesn’t get pushed out when the amputee puts weight on the liner.
“The trick is, to pass sweat, the silicone needs microscopic channels. However, simply having holes prevents the silicone from maintaining a proper suction seal, which is why normal silicone works so well. That’s where the water comes in. The materials we developed are hydrated, which means that water is attracted to the channels and stays inside them. The sweat can move through the liner. But it never really dries out, which explains how we have a silicone material with holes that can also maintain that critical suction seal. There’s a lot of sophisticated chemistry and microscopic structure in these materials that makes the silicone pass and store the sweat to keep the liner dry inside.”
The team has tested different designs of the inner wicking and middle absorption layer, measuring how much and how fast the sweat can be wicked and stored, as well as the stiffness of the layers. They also connected a sample of the inner layer to the middle layer and tested how fast water can be wicked away and stored, how water can be removed from the two-layer system, and the size changes when the two-layer system goes from dry to wet and back to dry.
In a different phase of the project, Marasco and his colleagues are examining the bacterial content of daily-use socket liners in the Cleveland VA amputee clinic. “We’re trying to get our head around the range of normal levels of bacterial load in the socket, from highest to lowest, and then to see if those measured levels correlate to the clinician’s perception of liner cleanliness,” Marasco said. “We don’t necessarily think a socket liner that’s completely devoid of bacteria is what we’re after. We want to try to build a system where the normal bacteria of your skin can live and be happy and be comfortable. But we need to know what that looks like before we can start assessing the performance of our advanced materials.”
The researchers hope the liner system can be commercialized in two to five years. “It will go a long way toward improving comfort on a daily basis for amputees,” Marasco said.
Editor’s note: This story was adapted from materials provided by the US Department of Veterans Affairs.
