Better neural control of prosthetics for amputees
March 6, 2012
The idea is to match material properties of prosthetics to nerve fibers, using conductive materials that are biocompatible so they can integrate with nerve bundles.
The prosthetics will have flexible nerve-to-nerve or nerve-to-muscle interfaces through which transected nerves can grow, putting small groups of nerve fibers in close contact to electrode sites,which are connected to separate, implanted electronics.
These interfaces operate where the nervous system and an artificial device intersect. Interfaces can monitor nerve signals or provide inputs that let amputees control prosthetic devices by direct neural signals, the same way they would control parts of their own bodies.
The researchers are looking at flexible conducting electrode materials using thin evaporated metal or patterned multiwalled carbon nanotubes.
The Amputee Coalition estimates 2 million people in the United States are living with limb loss. The Congressional Research Service reports more than 1,600 amputations involving U.S. troops between 2001 and 2010, more than 1,400 of those associated with the fighting in Iraq and Afghanistan. Most were major limb amputations.
Interfaces must be structured so nerve fibers can grow through. They must be mechanically compatible so they don’t harm the nervous system or surrounding tissues, and biocompatible to integrate with tissue and promote nerve fiber growth. They also must incorporate conductivity to allow electrode sites to connect with external circuitry, and electrical properties must be tuned to transmit neural signals.
The researchers began with a technique first patented in 1902 called electrospinning, which produces nonwoven fiber mats by applying a high-voltage field between the tip of a syringe filled with a polymer solution and a collection mat. Tip diameter and solution viscosity control fiber size.
Collaborating with UNM’s Center for Biomedical Engineering and department of chemical engineering, Sandia researchers worked with polymers that are liquid at room temperature. Electrospinning these liquid polymers does not result in fiber formation, and the results are sort of like water pooling on a flat surface. To remedy the lack of fiber formation, they electrospun the material onto a heated plate, initiating a chemical reaction to crosslink the polymer fibers as they were formed, Dirk said.
Researchers were able to tune the conductivity of the final composite with the addition of multiwalled carbon nanotubes.
The team’s search for a technique to create porous substrates led to projection microstereolithography, developed at the University of Illinois Urbana-Champaign as an inexpensive classroom outreach tool. It couples a computer with a PowerPoint image to a projector whose lens is focused on a mirror that reflects into a beaker containing a solution. They focus UV light onto the coated silicon wafer to form thin porous membranes.
While the lithography technique is not new, “we developed new materials that can be used as biocompatible photo-crosslinkable polymers,” Dirk said.
The technique allowed the team to create a regular array of holes and to pattern holes as small as 79 microns. Now researchers are using other equipment to create more controlled features.