When you flex your muscles, the muscle cells contract or expand under neurological stimulus (nerve impulses). Something similar happens with a relatively new material called a dialectric electroactive polymer when it is used to create an electroactive polymer artificial muscle (EPAM). Don’t let the lengthy jargon be confusing. The principle is fairly simple, much like the human muscle, and this principle is now being applied as a substitute for human eyelid muscles.
Think of two coins with a rubbery gum-like mass in between, a sandwich of sorts. Squeeze both coins and the rubbery stuff spreads out. Stop squeezing and the rubbery stuff contracts back to its original shape. With EPAM something similar happens, only the top and bottom part are electrodes that expand (and squeeze) when an electric charge is sent through them. The rubbery stuff in the middle is a layer of polymer (plastic…rubbery stuff), which expands with the electrodes. The whole thing contracts or expands on command from the electrical current and does so in a speed and motion very similar to natural muscle.
EPAM has been used for several years in robotic applications. In fact, NASA had a standing offer to reward anyone who can make an EPAM arm as strong as a human arm…yes, arm wrestling with NASA. The film industry is also beginning to use EPAM, or its variants, in animatronic applications (the more realist the T-Rex, the better). It’s not only the ‘naturalness’ of the EPAM’s actions, it the fine control that’s possible – electric current being not only the actuator but also the driving force.
It was the fine control that attracted a research team at the University of California Davis (Sacramento, California, USA) to consider EPAM as a substitute for the muscles in the human eyelid. An inability to blink has many sources: Injury, stroke, illness, and genetic conditions. It affects many thousands of people every year. Since the eye requires the eyelid for moisturizing (not to mention sleeping), losing the ability to blink or keep eyelids closed can lead to blindness. Sometimes eye muscles can be replaced with muscles from the leg, but this requires a very long operation that is not suitable for some people. Another technique is to use a gold weight that ‘drops’ the lid to blink, but this has problems with eye synchronization.
The behavior of an EPAM muscle – a small piece of EPAM not requiring much energy to operate – seemed to be a promising new approach. The problem was supplying and controlling the energy source. This was solved by creating a kind of ‘sling’ like a canvas sling between two trees, but in this case a cover over the eye, attached to posts embedded into bone on either side. The cover, or sling, is attached to the EPAM material. The batteries and controlling circuitry to drive the EPAM are embedded in bones below the temple to make them less noticeable.
“This is the first-wave use of artificial muscle in any biological system,” said Travis Tollefson, a facial plastic surgeon in the UC Davis Department of Otolaryngology – Head and Neck Surgery. “But there are many ideas and concepts where this technology may play a role.”
In their study, Tollefson and his colleagues were seeking to develop the protocol and device design for human implantation of electroactive polymer artificial muscle (EPAM) to reproducibly create a long-lasting eyelid blink that will protect the eye and improve facial appearance.
It should be stressed that the EPAM approach to eyelids is still in development and is at least five years away from use in humans. Nevertheless, this application, and probably many more like it promise ways to replace critical muscle function. In a decade or two, this example of a synthetic eyelid will seem impossibly crude.