Organic materials chemist Shawn Dirk focuses a projector during work on neural interfaces, which are aimed at improving amputees’ control over prosthetics with direct help from their own nervous systems. Focusing prior to exposing polymers ensures that researchers pattern the desired feature sizes for the interfaces. Photo: Randy Montoya |
Sandia
National Laboratories researchers, using off-the-shelf equipment in a chemistry
laboratory, have been working on ways to improve amputees’ control over
prosthetics with direct help from their own nervous systems.
Organic
materials chemist Shawn Dirk, robotics engineer Steve Buerger, and others are
creating biocompatible interface scaffolds. The goal is improved prosthetics
with 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 connected to separate, implanted electronics.
Neural
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.
Sandia’s
research focuses on biomaterials and peripheral nerves at the interface site.
The idea is to match material properties to nerve fibers with flexible,
conductive materials that are biocompatible so they can integrate with nerve
bundles.
“There
are a lot of knobs we can turn to get the material properties to match those of
the nerves,” Dirk said.
Buerger
added, “If we can get the right material properties, we could create a healthy,
long-lasting interface that will allow an amputee to control a robotic limb
using their own nervous system for years, or even decades, without repeat
surgeries.”
Researchers
are looking at flexible conducting electrode materials using thin evaporated
metal or patterned multiwalled carbon nanotubes.
The
work is in its early stages and it might be years before such materials reach
the market. Studies must confirm they function as needed, then they would face
a lengthy Food and Drug Administration approval process.
But
the need is there. 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.
Before
joining Sandia, Buerger worked with a research group at MIT developing
biomedical robots, including prosthetics. Sandia’s robotics group was
developing prosthetics before his arrival as part of U.S. Department of
Energy-sponsored humanitarian programs to reduce proliferation risks.
Robotics
approached the problem from a technical point of view, looking at improving
implantable and wearable neural interface electronics. However, Buerger said
that didn’t address the central issue of interfacing with nerves, so
researchers turned to Dirk’s team.
“This
goes after the crux of the problem,” he said.
The
challenges are numerous. 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.
Dirk
presented a paper on potential neural interface materials at the winter meeting
of the Materials Research Society, describing Sandia’s work in collaboration
with the University of New Mexico and MD Anderson Cancer Center in Houston. Co-authors are
Buerger, UNM assistant professor Elizabeth Hedberg-Dirk, UNM graduate student
and Sandia contractor Kirsten Cicotte, and MD Anderson’s Patrick Lin and
Gregory Reece.
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.
Robotics engineer Steve Buerger displays implantable and wearable neural interface electronics developed by Sandia as he sits in the prosthetics laboratory with a display of prosthetic components. He is part of a research team that is working on ways to improve amputees’ control over prosthetics with direct help from their own nervous system. Photo: Randy Montoya |
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 electrospun scaffolds with two types of material—PBF, or poly(butylene
fumarate), a polymer developed at UNM and Sandia for tissue engineering, and PDMS,
or poly(dimethylsiloxane).
PBF
is a biocompatible material that’s biodegradable so the porous scaffold would
disintegrate, leaving the contacts behind. PDMS is a biocompatible caulk-like
material that is not biodegradable, meaning the scaffold would remain.
Electrodes on one side of the materials made them conductive.
Sandia’s
work was funded through a late-start Laboratory Directed Research &
Development (LDRD) project in 2010; afterward the researchers partnered with MD
Anderson for implant tests. Sandia and MD Anderson are seeking funding to
continue the project, Dirk said.
Buerger
said they’re using their proof-of-concept work to obtain third-party funding “so we can bring this technology closer to something that will help our wounded
warriors, amputees and victims of peripheral nerve injury.”
Sandia
and UNM have applied for a patent on the scaffold technique. Sandia also filed
two separate provisional patent applications, one in partnership with MD
Anderson and the other with UNM, and the partners expect to submit full
applications this year.
The
MD Anderson collaboration came about because then-Sandia employee Dick Fate, an
MD Anderson patient who’d lost his left leg to cancer, thought the hospital and
the Labs were a natural match. He brokered an invitation from Sandia to the
hospital, which led to the eventual partnership.
Fate,
who retired in 2010, views the debilitating effect of rising health care costs
on the nation’s economy as a national security issue.
“To
me it seems like such a logical match, the best engineering lab in the country
working with the best medical research institution in the country to solve some
of these big problems that are nearly driving this country bankrupt,” he said.
After
Sandia researchers came up with interface materials, MD Anderson surgeons
sutured the scaffolds into legs of rats between a transected peroneal nerve.
After three to four weeks, the interfaces were evaluated.
Samples
fabricated from PBF turned out to be too thick and not porous enough for good
nerve penetration through the scaffold, Dirk said. PDMS was more promising,
with histology showing the nerve cells beginning to penetrate the scaffold. The
thickness of the electrospun mats, about 100 microns, were appropriate, Dirk
said, but weren’t porous enough and the pore pattern wasn’t controlled.
The
team’s search for a different technique to create the 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.
Using
a laptop and a projector, Dirk said the researchers initially tried using a
mirror and a 3X magnifying glass, but abandoned that because it produced too
much distortion. They now use the magnifying glass to focus UV light onto the
PDMS-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.
“It’s
exciting because we’re getting the feature size down close to what is needed,”
Buerger said.
