Rethinking Prosthetics

When people lose an arm or a leg, the nerves that control the limbs continue sending signals to the muscles left behind. Decoding those signals to articulate natural movement in a sophisticated prosthetic is the goal of research being conducted by Richard Weir, associate research professor of bioengineering.

Weir’s work is focused on developing a prosthetic hand and fingers that provide a full range of movement as well as a sense of touch for persons with hand amputations.

Life-altering research

According to Weir, the key is the development of implantable myoelectric sensors (IMES), rice-sized capsules that will be implanted into muscles in the forearm. The sensors wirelessly transform muscle signals into signals that can be used to control hand and finger movement. The goal is to give the prosthesis the full 22 degrees of movement articulated in a human hand and wrist.

Current technology provides a control interface that allows only two commands to be delivered to the prosthetic hand: to open and close, Weir says.

After a decade of research on this formidable challenge, Weir and his collaborators—the Alfred Mann Foundation, Illinois Institute of Technology and Sigenics Inc.—are on the cusp of seeing IMES technology reinnervate muscle in amputees, and potentially transform their lives long-term.

“It’s pretty exciting,” Weir says. “If we can go into each of the 18 muscles in the forearm with sensors that give 18 control signals rather than the two we have at the moment, that would advance the science. If they work well, this will completely change the way the devices are controlled. We’ll be able to do much more than just the open-close type of approach.”

The goal is to create a prosthesis that an amputee can control for the remainder of his or her life. So far in lab settings, Weir says, nerve interfaces are functional for two to three years before tissue necrosis sets in. He pointed out that operating such a prosthesis in day-to-day life for an extended period of time “is a much different kettle of fish.”

Weir previously worked with a team at the Rehabilitation Institute of Chicago on a neurally controlled hand, which was part of a project to develop a physiological replacement for the human arm. The Defense Advanced Research Projects Agency (DARPA) initiative assigned different teams to various parts of the arm, such as the elbow, shoulder and hand. Weir was the architect of the hand, and the team’s prototype was featured in a 2010 National Geographic cover story on advances in prosthetics.

Ultimately, DARPA reoriented the project to focus on developing a brain-machine interface to help patients with high-level spinal cord injuries.

Weir opted to continue with the neural and muscle interfaces in the arm because a brain-machine interface has a risk-benefit ratio that’s not necessarily justified for people with amputations. Also, he says, amputees already have been through trauma and are resistant to the more invasive surgery that is required for brain-machine interfaces.

The IMES development is being supported by a grant from the National Institutes of Health from the National Institute on Biomedical Imaging and Bioengineering.

Assisting in Weir’s research are students from the Departments of Bioengineering and Mechanical Engineering, including Matthew Davidson, bioengineering, and Nili Krausz, mechanical engineering, as well as students from CU Boulder and the Colorado School of Mines.

“Hopefully, we’ll get to the point where we’re doing an implantation here in Denver,” says Weir.

Cutting-edge technology

Beyond his IMES research, Weir’s lab is working with a piece of machinery that few have access to. Thanks to a $600,000 capital equipment grant from the Veterans Administration, the BioMechatronics Development Laboratory is home to a cutting-edge 3-D printer: a laser metal sintering machine.

Weir says the fabricator will allow his research team to develop better components—created faster and at a lower cost—for prosthetic fingers, hands and arms.

“It’s a whole new way of thinking about how to make things,” Weir says. “The revolutionary aspect is to be able to do stuff that you can’t using conventional technology. There is the possibility to fabricate impossible-to-machine components and to explore whether that confers advantage to the designs we’re working on.”

While 3-D plastic printers have been available for many years, metal printing is still “a very nascent technology,” Weir says. He estimates that only a couple dozen of the devices, built by German-based EOS e-Manufacturing Solutions, are being used in the United States, mostly for biomedical and aeronautical applications.

Weir first saw a 3-D metal rapid prototype machine being used to create cranial implants—custom titanium plates in the shape of the human skull—at a laboratory at North Carolina State University. “When I saw that I said, ‘I want one of those,’” says Weir.

He got his wish in 2011 when the VA, well aware of how Weir’s pioneering research could benefit veteran amputees, funded the purchase of one of these machines through a capital equipment grant. His lab had already been using a 3-D plastic printer, but a metal prototyping machine dramatically expands the horizons for their prosthetic designs.

“That’s what we have a need for when we’re building our small hands,” says Weir. “We have all of these tiny parts that need to be very strong, and a lot of times steel turns out to be the best material to work in. If we want, we can change the machine’s setup, for a fee of course, that will allow us to print in a different metal. We can print in titanium, nickel, magnesium, cobalt.”

The machine uses a three-dimensional digital image to methodically laser-sinter beads of metal powder into solid metal. Most components will be built overnight in the machine, which has a door—much like a microwave oven—that allows manufacturers, or in this case researchers, to view the progress of each iterative design.

Jacob Segil, a CU Boulder mechanical engineering student who works with Weir, says the machine creates a “whole new modality” to turn ideas into reality, especially in the tricky area of anthropomorphic design. “For things that don’t have hard edges, like our bodies, it makes a world of difference,” he says. “To [create] something like our finger, which has curvature and intricacies, out of metal is a horribly difficult and expensive thing to do using conventional machining processes. Now we have a machine to do it.”

Weir comes from a family of medical and engineering professionals. His father was a professor of medicine at Trinity College Dublin, and an uncle ran an engineering company in London. Weir’s twin sister lost a hand in a lawn mower accident when she was five.

“It’s probably all of that” that contributed to his interest in arm prosthetics, he says.