Artificial Muscles May Help Patients to Blink

artificial-muscle-blinking

Illustration of left eyelid attached to an electroactive polymer artificial muscle (EPAM) device after passing through an interpolation unit that is implanted in the lateral orbital wall. The power supply and artificial muscle are implanted in the temporal fossa. When the normal right eyelid blinks, the electrical sensor (green) sends a signal to the battery to activate the EPAM.

Surgeons from the University of California Davis medical center have demonstrated that artificial muscles can restore the ability of patients with facial paralysis to blink. Funded by a grant from the American Academy of Facial Plastic and Reconstructive Surgery (Alexandria, VA), the new procedure is described in the January/February issue of the Archives of Facial Plastic Surgery.Blinking is an essential part of maintaining a healthy eye. The lid wipes the surface of the eye clean and spreads tears across the cornea. Without this lubrication, the eye is  at risk of developing corneal ulcers, which can eventually cause blindness.Involuntary eye blinking is controlled by a cranial nerve. In most patients with permanent eyelid paralysis, this nerve has been injured because of an accident, stroke, or surgery to remove a facial tumor. Many patients have no other functioning nerves nearby that can be rerouted to close the eyelid. Others are born with Mobius syndrome, which is characterized by underdeveloped facial nerves. Such patients are expressionless and can neither blink nor smile.Developed by engineers at SRI International (Palo Alto, CA) in the 1990s, the three-layered artificial muscle used by the surgeons has an inside layer consisting of a piece of soft acrylic or silicone layered with carbon grease. When a current is applied to it, electrostatic attractions cause the outer layers to pull together and squash the soft center, expanding the artificial muscle. The muscle contracts when the charge is removed and flattens the shape of the sling, blinking the eye. When the charge is reactivated, the muscle relaxes, and the soft center reverts back to its original shape."This is the first-wave use of artificial muscle in any biological system," remarks 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. Based on variable voltage input levels, electroactive polymers act like human muscles by expanding and contracting. EPAM is an emerging technology that has the potential for use in rehabilitating facial movement in patients with paralysis. "The amount of force and movement the artificial muscle generates is very similar to natural muscle," Tollefson says. An implanted battery source similar to that used in cochlear implants would power the artificial muscle. For patients who have one functioning eyelid, a sensor wire threaded over the normal eyelid could detect the natural blink impulse and fire the artificial muscle at the same time. Among patients lacking control of either eyelid, an electronic pacemaker similar to that used to regulate heartbeats could blink the eye at a steady rate and be deactivated by a magnetic switch.There are currently two approaches for treating eyelid paralysis. One method involves transfering a muscle from the leg to face. However, this option requires six to 10 hours of surgery, creates a second wound, and is not always suitable for elderly or medically fragile patients.The other treatment involves suturing a small gold weight inside the eyelid, which closes the eye with the help of gravity. Although successful in more than 90% of patients, this procedure results in a blink that is slower than normal and cannot be synchronized with the other eye. Some patients also have difficulty keeping the weighted lid closed when lying down to sleep.For their study, Tollefson and his team used a novel alternative method for eyelid rehabilitation--an eyelid sling mechanism. Using cadavers, the surgeons inserted a sling made of muscle fascia or implantable fabric around the eye, securing the sling to the small bones of the eye using small titanium screws. The sling was attached to a battery-operated artificial muscle. The artificial muscle device and battery were inserted into a natural hollow or fossa at the temple to disguise their presence.Tollefson and his colleagues found that the force and stroke required to close the eyelid with the sling were well within the attainable range of the artificial muscle. This capability may allow the creation of a realistic and functional eyelid blink that is symmetrical and synchronous with the normal blink. A similar system could also enable children born with facial paralysis to smile.The technique, which uses a combination of electrode leads and silicone polymers, could be used to develop synthetic muscles to control other parts of the body as well. For example, the use of artificial muscles could one day enable people with other types of paralysis to regain the ability to smile or control the bladder. Reanimating faces is a natural first step in developing synthetic muscles to control other parts of the body, comments UC Davis otolaryngologist Craig Senders, a coauthor of the study.The researchers are now refining their technique on cadavers and animal models, estimating that the technology will be available for patients within the next five years.