LEIPZIG, Germany — Bionics are the science and engineering of extreme interfaces between the human body and synthetics, according to a keynote speaker at the OT World Congress, here.
Hugh Herr, PhD, the director of Massachusetts Institute of Technology Center for Extreme Bionics and BIOM Inc. founder, described three interfaces between the body and synthetics. The first interface is mechanical by attaching a synthetic machine in a comfortable, healthy manner. A good mechanical interface should relieve the discomfort caused by a prosthetic socket. This is achieved by “spatially varying mechanical impedance based on underlying tissue biomechanics, as well as temporally varying socket impedance by using microprocessor and smart materials.”
“If we know how the skin moves, we can design synthetic interfaces that move the same way, and mitigate shear between the skin and synthetic interface,” Herr said.
Imaging tools and cameras provide data that show tissue geometry, while robotic tools measure tissue stiffness and damping. With these tools, a biomechanical model is created that shows the underlying tissue structure impedances.
“The human body is stiff in some places and soft in other places. So our sockets need to be of variable impedance to reflect that biomechanical fact,” Herr said.
A prosthetic socket can be designed to provide support and give where needed. He demonstrated a smart material that can be used in socket fabrication that is pliable, yet stiffens when a small voltage is applied. An added microprocessor can make the socket stiffen or relax as needed.
Dynamic interfaces are required by synthetic limbs to make them behave like their biological counterpart, he said. Conventional prostheses are lacking, even for level ground walking.
“During your day, most of what you walk on is ordinary level surfaces. As a field we need to build bionic limbs that emulate biologic limb capability for normal flat level surfaces and walking. Once we mastered that, we can extend to going down hills and steps. We need to start there,” he said.
Herr said the most powerful human joint is the ankle, and there is a need to build a foot-ankle device that emulates normal biological dynamics. A passive prosthetic foot cannot emulate what the biologic foot does, and secondary conditions are costly.
“We should emphasize less the cost of the prosthesis, and more the cost of all the secondary conditions of leg amputations. If we can build technology that fully emulates biological function, we could mitigate or perhaps eliminate these secondary conditions.”
Electrical interfaces with the nervous system via neural implants provide the means to communicate with the nervous system bi-directionally. Using surface electrodes with electromyography provides simple, conventional control of bionic devices but “we want to do more. What we ultimately want to do is communicate to nerves,” Herr said.
He described current research in modeling spinal reflex to help amputees modulate the sensitivity of the reflex to perform a certain action with a bionic limb. His research in animal models uses skin and muscle cells placed adjacent to the nerve. The transected nerve grows and innervates the skin and muscle cells, while the animal vascularizes the tissues.
“You end up with a stable, viable implant. The muscle is an amplifier of the nerve signal, so you can take motor signal from the muscle cells and stimulate for cutaneous feedback from the prosthesis,” he said. — by Carey Cowles
For more information:
Herr H. On the design of bionic leg devices: The science of extreme interface. Presented at: OT World Congress; May 12-16, 2014; Leipzig, Germany.
Disclosure: Herr is the founder of BIOM Inc.