Cosmesis
Cosmesis is a term used when making a prosthetic look like a real limb or body part. The skin is commonly made of silicone and urethanes, to make it flexible and similar to the texture of real skin. These prosthetics are made by modeling after the other limb of a person, using its shape to form the prosthetic. It can be customized with the exact same skin color, veins, hair, or freckles as the other limb. This is done by airbrushing the variations onto the prosthetic or adding hairs in the skin.
Targeted Muscle Reinnervation (TMR)
The most advanced types of prosthetics use TMR. In a healthy body, the brain controls the muscles in limbs by sending electrical commands down the spinal cord, then through peripheral nerves to the muscles. It sends these signals through motor neurons, which reach the muscle at neuromuscular junctions. After an amputation, these nerves would still carry the motor commands, but would end at the site of amputation. In the surgical process for TMR, the amputated nerves are redirected to control a substitute healthy muscle elsewhere in the body. For instance, a surgeon will attach nerves that controlled a patient’s arm to a portion of the patient’s chest muscles. Afterward, when the patient tries to move her arm, the signals traveling through the original arm nerve will now contract a portion of the chest muscles. The electrical activity in these muscles can be sensed with electrodes and used to provide control signals to a prosthetic limb.
Myoelectric Prosthesis
Myoelectric describes the electrical properties of muscles. Myoelectric prostheses receive electronic signals from muscles to the skin, which are then sent to microprocessors that relay the signal to the joints and hands. This technology is used in the SensorHand Speed by Otto Bock, along with the Utah Arm 3, released by Motion Control.
3D Printing
A commonly used and cost-effective technique to create parts for prosthetics is 3D printing. Designers Richard Van As and Ivan Owens worked together on Robohand, a project that allows for less expensive finger prosthetics to be available for people in need around the world.
The high cost of a prosthetic can prevent those who need it from receiving one. The Japanese robotics company Exiii has created a 3D printed bionic arm that addresses this problem. The Hackberry is revolutionary in that its design files were released by the company to make the prosthetic available to those who need it. Exiii claims the Hackberry can by 3D printed at home in parts, and costs less than $300.
Agonist-Antagonist Myoneural Interface
Scientists at the Massachusetts Institute of Technology are perfecting a surgical technique that could give more control to prosthetic limbs. The technique is called agonist-antagonist myoneural interface, or AMI, which uses muscles and attached nerve cells to control the prosthetic. A pair of muscles would be taken from a healthy part of the body, attached to create a line, and then implanted near the site of amputation. This would create a pair of agonistic and antagonistic muscles, meaning, when one contracts the other stretches. The ends of the muscle pair would then be attached to severed nerve endings in the limb that would have continued into the removed limb. This would allow the brain to control the actions of the muscle. The ends of the muscle would also be connected to electrodes controlling the prosthesis. In a leg amputee, for example, the brain would send signals to the grafted muscle to bend the ankle. The muscle would contract, sending electrical signals to the prosthesis to bend the ankle. The muscle graft can also receive signals from the prosthesis, to adjust.