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A Revolution in Prosthetics

Researchers in USC’s Biomimetic MicroElectronic Systems Center are poised to build the most lifelike limbs ever imagined

September 22, 2006 — Loss of a limb can be devastating to individuals and their families, and learning to use a prosthetic device may take years.  Today’s prosthetic arms and legs while impressive, provide a limited range of movement and only a primitive ability to grasp objects.  But at Gerald Loeb’s Medical Device Development Facility in the basement of USC’s Denney Research Building, a hotbed of novel technologies promises to make new artificial limbs more lifelike than you ever imagined.    
Gerald Loeb holds a BION chip, a tiny injectable neuro- stimulator that can re-innervate damaged nerves.

Loeb is the inventor of BIONs™, tiny injectable neurostimulators shaped like a grain of rice.  BIONs™ activate weak and paralyzed muscles, bringing them back to life, but they aren’t the first bionic technology that he has developed.  Loeb was also one of the inventors of the cochlear implant, used to restore hearing to the deaf.  He started working on a visual prosthesis while still in medical school in the 1960s, an application now being pursued by Mark Humayun and colleagues in USC’s Engineering Research Center for Biomimetic MicroElectronic Systems (BMES), where Loeb is deputy director.  BMES is an NSF-funded national Engineering Research Center uniquely focused on neural rehabilitation. Launched in 2003 as a collaboration between the Viterbi School and the Keck School of Medicine at USC, as well as UC Santa Cruz, the center now has partnerships with 14 companies and 10 universities, including Caltech.

“Bionic is the word Hollywood invented to explain the ‘Six Million Dollar Man’ in the 1970s,” says Loeb with a smile.  “But today, biomimetic systems are able to restore lost function to complex neural systems.  We use them to restore the electrical signals that are normally sent out from the motor neurons to different parts of the body. Cochlear implants are the most successful biomimetic systems to date, but we hope to use similar biomimetic technology in retinal implants to restore partial vision and in patients who are paralyzed from a stroke or suffering from memory loss.”

Industry Interest
The Department of Defense is keen to develop this rehabilitative technology for soldiers who have lost their arms or legs in combat.  Consequently, Loeb’s lab in the Alfred Mann Institute for Biomedical Engineering at USC has been named one of several major subcontractors in a $30.4-million contract for the Defense Advanced Research Projects Agency (DARPA) to start phase one of a program called “Revolutionizing Prosthetics 2009.” The four-year program aims to develop a next-generation mechanical arm that will look, feel and behave just like one in the flesh.

The contract grows out of an increasing number of U.S. soldiers who are losing their limbs in the Iraq war.  Despite the many advances in body armor and helmets, more than 450 U.S. soldiers have lost an arm or leg in Iraq or Afghanistan.
An artist's rendering of a next generation prosthetic arm with a cutaway of the inner workings at the elbow joint. APL/JHU image.

DARPA’s new prosthetics program is the first step in a long-term effort to give injured military personnel the most advanced medical and rehabilitative care possible.  USC is part of the effort, which will be led by Stuart D. Harshbarger at Johns Hopkins University’s Applied Physics Laboratory (APL).  An impressive list of subcontractors, including other top-notch universities, government agencies and private firms in the U.S. and Europe are also part of the multi-phase, multi-year project. 

“Understanding the biological principles of limb control for coordinated, complex movement has the potential to not only help prosthetic limbs, but also, in the future, to reanimate paralyzed limbs,” says Mark Humayun, director of the Center for Biomimetic MicroElectronic Systems and a professor of ophthalmology, biomedical engineering, and cell and neurobiology at USC.
“The DARPA award will fuel a whole new generation of novel neurotechnologies and innovative engineering applications that are ripe for implementation,” Loeb adds.  “And DARPA’s overall objective is equally exciting: to design a prosthetic device that can be connected directly to the peripheral and central nervous system so that amputees can regain nearly natural use of their artificial arms.”    

A Tall Order To Fill
That’s a tall order to fill, but today’s technologies are a good starting place. For example, myoelectric arms currently give users a limited range of motion – about three degrees of freedom – and the ability to perform one arm or hand motion at a time. The control systems are operated with deliberate flexing of a muscle or through mechanical movement. 

DARPA wants to increase that range of movement to 22 joints, just like a normal arm and hand. As APL’s Harshbarger described it, the team will design an arm that can move at "strengths, speeds and angles with 22 degrees of freedom, including the shoulder, to match the performance of the human arm while maintaining the person's ability to control the arm." The lucky recipients will be injured soldiers who are recuperating at two Department of Defense centers dedicated to amputee care. One is located at Walter Reed Army Medical Center in Washington, D.C.; the other is located at Brooke Army Medical Center in San Antonio, Texas.
A BION chip, about the size of a grain of rice.

Loeb and his biomedical engineering team received a $1.5-million slice of the contract for the first year of the project.  His role will be to build a sophisticated control system that will replace parts of the nervous system and allow the user to operate the artificial limb neurally, just by thinking about it. 

While Loeb is busy developing the control system and modeling arm performance in a virtual reality environment, other biomedical engineers in BMES will be developing state-of-the-art neural implants for the brain and the central nervous system. Collaborative investigations such as Ted Berger’s experimental work with silicon chip brain implants will contribute to the Biomimetic Center’s overall effort to replace parts of the nervous system that must be bypassed to restore useful function.  Berger is a professor of biomedical engineering at the Viterbi School.
To create a model of the prosthetic arm, seen on the screen in the background, Rahman Davoodi watches the movements of his own arm using 3-D goggles.

The Modeling Center: Where It All Begins
Loeb’s work begins in a modeling center adjacent to his BION™ Fabrication Laboratory, where a team of biomedical engineers and graduate students develop computerized models of the musculoskeletal system to mimic movement in the human body.

Before prosthetic systems are built, Loeb’s team wants to know if patients can actually control them. Using unique modeling software and virtual reality simulations, the team, which is led by Viterbi School biomedical research assistant professor Rahman Davoodi, can spend weeks crunching data and modifying algorithms to render a simulation of the way an arm swings, or how much force is needed to reach for an object on a table. The modeling software and virtual reality environment were developed by Davoodi, Mehdi Khachani, a biomedical engineer, Markus Hauschild, a biomedical engineering graduate student, and USC computer programmers.  

“Once we have an accurate model of the human or prosthetic limb, we are able to study its movement under various prosthetic control strategies and external forces, such as those from gravity or interaction with the environment,” Davoodi explains.

“In our simulation environment, a patient produces command signals by voluntarily contracting his or her intact muscles, or moving intact joints, to control the movement of the virtual prosthetic limb,” he says. “As the patient does that, he/she watches the arm’s motion in 3D stereoscopic goggles. If the simulated motion is not satisfactory, the patient learns, by practice, to change his/her command signals until the prosthetic limb can be controlled effectively.”
Simulation of human and prosthetic limbs, and the human-machine interactions between them, are used to develop more effective prosthetic systems for amputees and paralyzed patients.

Prosthetic Simulators: A New Tool
Using such patient-in-the-loop simulations, engineers can test and refine the design of prosthetic limbs before they are built and patients can learn to operate them before receiving them.  “Really, what we’re doing is developing an affordable tool for engineers to design and test new prosthetic systems and a safe environment to train the patients to operate them,” Davoodi says.   “This is similar to the use of flight simulators, where the pilot can safely try different strategies, including those that are novel or even dangerous, until they are ready to fly the real plane.”

In addition to greater arm movement, DARPA wants to add sensory perception to the prosthetic hands, so that users will be able to feel and manipulate objects, lift up to 60 pounds, and conduct normal, everyday tasks, even in the dark.  Loeb says that will truly revolutionize prostheses.  

“A person probably takes in more information about physical objects through his or her fingertips than any of the senses,” Loeb says.  “The fingertips are highly evolved and contain many features that are designed to enhance their sensitivity and the quantity of information they can provide to the central nervous system.”

Loeb and his team have a plan to imitate the structure and mechanical properties of the human finger, combining a rubbery skin, a spongy pulp, a rigid bone at the core and an overlying stiff fingernail. 

“These features appear to be important in the way that contact with objects affects touch sensors,” says Loeb, whose team has already come up with a simple and robust way to build a similar set of distributed sensors in an artificial fingertip.  “Now it’s time to take that to the next level.”   

BION™2 Implants: The Next Generation Chip
Loeb’s BION™ technology has an important role in prosthetics development. Along with his colleague Todd Kuiken of the Rehabilitation Institute of Chicago, the researchers are already testing neurally controlled prosthetics on injured patients. BION™2, Loeb’s next generation implant, will be able to stimulate the muscles near a wound site enough to make the prosthetic arm move just as a biological arm would have moved.

Mehdi Khachani, a biomedical engineer, is part of the team developing the modeling software and virtual reality environment.

“Kuiken took advantage of the fact that peripheral nerves actually re-grow,” Loeb says.  “They want to grow out and innervate muscles but, of course, the muscles they used to innervate don’t exist anymore. So the idea is to remove the nerve supply to some big muscles that aren’t doing anything anymore, such as the pectoralis muscle, and stitch in the nerve ends that used to go to the amputated arm.  When the patient thinks of moving his fingers or his forearm, some part of that re-innervated muscle is activated, producing a relatively large and easily recorded EMG (electromyographic) signal.”

Kuiken spent years working out the surgical techniques in animals.  Now he is mapping out all of the activation spots on the chest walls of patients who have undergone re-innervation.  He correlates specific patches to specific movements the patient is trying to imitate.  The EMG signals are then used to operate a custom-built motorized prosthesis, a still-primitive prototype of the arms and hands that will be built through the DARPA program. 

Before, such a prosthesis had to be driven by a very cumbersome switching system that prevented patients from making more than one rather jerky movement at a time.  Now the control systems can control all of the patient’s movements at the same time when the individual thinks about the task.  

One of the limitations right now is that EMG signals have to be recorded by many small electrodes that have to be stuck on the skin in just the right place every day.

“Our BION™2 implants can sense muscle electrical activity as well as stimulate it,” Loeb says. “ This should make the command signals much more reliable and solve the problem of interference between the electrodes and the rest of the prosthetic system.”  

Radio Frequency Chips
A simple form of this input-output communication already exists in radio frequency identification (RFID) chips, which Loeb also helped to invent, but the existing technology is not capable of transmitting more than a small amount of data. 

“Bionic chips will have to transmit large amounts of continuous EMG data, so we are working on developing a much higher speed data link that will send this information back out,” Loeb says. “And we will also have to make some very specialized microelectronic circuits that can record all of these electrical signals, amplify and digitize them, and telemeter them out to the prosthetic arm.”   
Loeb examines a BION chip under a microscope in the BION Fabrication Lab.

The task seems daunting, but Loeb and his team are busy developing, testing and modifying the technologies. In the long run, Loeb says the devices will transmit data to an external controller, which will then adjust the level of stimulation in a muscle, acting just as the spinal cord and brain act to adjust muscle activity in healthy people.

“It’s really just a bunch of integrated circuit functions, none of which is terribly demanding,” he quips, “it just takes a long time to get it all done.”

But the work will mean much more to the men and women who have lost their limbs in the line of duty.  To them, Loeb’s masterful prosthetics promise to give them a lifelike limb and a new lease on life.  

--Diane Ainsworth/Photos by Irene Fertik