Progress in the field of prosthetics has led to the development of more functional limbs as well as a new set of scientific and engineering challenges.

In Peter Pan, Captain Hook uses a hook as an artificial hand; in Star Wars, Luke Skywalker uses a remarkably lifelike prosthetic arm that gives him complete control and sensation, equipped with an outer layer of synthetic flesh that has the appearance and texture of organic tissue.  Today, prosthetic technology lies somewhere between these two extremes: amputees are already using bionic limbs controlled by their own rerouted nerves and mind-controlled artificial limbs have just recently entered Phase III testing on human subjects.  Nearly 1.7 million Americans and countless other people living with limb loss now have reasons to believe that new and better prosthetic limbs will soon be available.

One of these people is Jesse Sullivan, an electrical technician who lost both his arms after he was electrocuted by a live wire.  In 2003, he became the first person to receive a bionic limb controlled by a nerve-muscle graft, through a technology called targeted muscle reinnervation (TMR).  Due to the amputation, the nerve cells that were once used to drive Sullivan’s arm motions now ended in the stump of his arm.  Physicians reconnected these neurons to nerves that stimulate some of Sullivan’s chest muscles.  As a result, whenever he would think about moving his amputated arm, his brain sent signals that would cause his chest muscles to contract.  Electrodes that could detect these chest muscle contractions enabled Sullivan to move his prosthetic arm in particular directions using thought alone.

Although other physicians have performed similar procedures on about 30 different people in the United States, Canada, and Europe, there are still major limitations to the procedure.  Patients’ range of motion with these bionic arms is still restricted by the technical challenge of rewiring nerves through surgery.  Moreover, the rewired neurons take months to grow after the operation, and often the electrodes must be further adjusted as the nerves shift position.  This method of developing a bionic arm also does not apply to people with extensive damage to their nerves, such as patients with cervical spine injuries.

In an effort to make bionic arms more versatile and broadly applicable, scientists would like to design robotic arms controlled directly by the neurons in the brain through a brain-machine interface (BMI).  In the 1960s, a noninvasive device called the electroencephalogram (EEG) was developed to measure electrical activity in different regions of the brain. In the 1970s, scientists found ways to measure electrical activity at the resolution of a single neuron using a device that must be inserted into the brain.  Since then, researchers have been developing ways of using primates’ neuronal activity to control external devices.

Today, about a hundred electrodes can be used to measure the firing of neurons in the motor cortex of the monkey’s brain, and the signals detected by the electrodes can then be used to control a robotic arm.  Using this method, scientists have successfully implemented robotic arms that monkeys can use to feed themselves.  This past summer, researchers prepared to test a similar technology on humans, as the Modular Prosthetic Limb (MPL), an artificial arm designed to be controlled by thought, entered Phase III trials.  Researchers are developing implantable micro-arrays that would both record neural signals and stimulate the brain.  Such a micro-array would “read” the mind and use that information to control the prosthesis, and the MPL in turn would send signals back to make the micro-array stimulate the brain as a form of sensory feedback from the prosthesis.

Perhaps due to the fine control necessary for natural arm movement, the greatest developments in mind-controlled artificial limbs have been made with arm prostheses.  The development of leg prostheses faces a whole different set of challenges.  For instance, leg prostheses need to be strong enough to support the amputee, but also sufficiently light and flexible to allow supple movements.  One very popular leg prostheses is the C-Leg, which uses hydraulic controls to move the knee.  In the past years, the C-Leg has been developed into a microchip-controlled prostheses that automatically adapts to changes in speed and direction, and that can be programmed to different modes such as walking, biking, and driving.

Another major challenge for lower extremity prostheses is the site of attachment between the stump and the prosthesis.  The traditional method, developed in the 1980’s, is the Sabolich Socket, which is designed to wrap snugly around the stump of the amputee’s limb.  And alternative option that has been put forth is to screw the artificial leg into the patient’s bone.  In this method, a titanium-coated bolt is inserted into the cavity of the bone, and the bone is then allowed to grow around the bolt.  Next, the bolt is extended using another titanium component that connects to the artificial limb.  The direct attachment of the prostheses to the skeleton gives the amputee greater control over the artificial limb and eliminates some of the problems associated with the socket method, such as skin irritation or the need to readjust the socket due to changes in stump size.

The development of a good prosthetic limb involves many different issues, involving both the mechanical aspect and the electronics involved in the communication between the artificial limb and the patient’s brain.  To create a truly mind-reading artificial limb, scientists will likely need to use more electrodes to allow for a wider range of movement, and to devise electrodes that last longer, read from multiple neurons, and communicate with the robotic device wirelessly, all of which would make it more feasible to use a bionic limb controlled directly by the brain.  Scientists are also working to refine the spatial and temporal resolution of the electrode readings.  Once these challenges have been solved, a bionic limb controlled directly by the brain could become available, allowing for people with amputated or paralyzed limbs to achieve near-normal mechanical function. The future of medicine and its integration with mechanical advancements shows great promise for the many who use artificial limbs.

Katherine Zhou is a junior Molecular Biophysics and Biochemistry major in Saybrook College.