What the terms mean
A prosthesis is an artificial replacement for a missing body part. Many are purely mechanical: a hook, a passive hand shaped for appearance, a leg with a spring-like foot that stores and returns energy. “Bionic” is a popular word, rather than a strict technical category, for prostheses that use electronics, sensors and motors to behave more like the limb they replace. This article explains the main ideas in general terms. It is not a guide to choosing or fitting a device, which is the work of a prosthetist and a clinical team.
The parts of a modern prosthesis
Most powered prostheses share a similar structure.
- The socket or interface. This is the part that connects the device to the person’s residual limb. It is often custom made, and it is frequently where problems arise, since the device must be held securely without causing pain or skin damage. Fit can change through the day and over months as the limb changes shape.
- The structure. Lightweight frames, joints and covers, typically made from metals and composites.
- Actuators. Small electric motors with gears move a hand, wrist, elbow, knee or ankle.
- Sensors. These detect the user’s intent and the state of the device.
- A controller and battery. A small computer interprets the sensor data and drives the motors, powered by a rechargeable battery. Weight and battery life are constant design limits.
How the device knows what you want
The central challenge is intent: how does the prosthesis know what the user is trying to do? Several approaches exist.
Body-powered control. Cables and harnesses connect the prosthesis to movements of the shoulder or upper body. This approach uses no electronics, is durable and gives users a direct sense of force, which is one reason some people continue to prefer it.
Myoelectric control. When a muscle contracts, it produces a small electrical signal that can be picked up by electrodes on the skin inside the socket. In a simple arrangement, one muscle site might open the hand and another close it. The signal strength can set the speed or grip force. This works because many people with an amputation still have muscles in the residual limb that they can voluntarily contract, even when the hand they once controlled is gone.
Pattern recognition. More advanced systems use several electrodes and software to recognize distinct patterns of muscle activity for different movements. The software is typically trained by asking the user to attempt various gestures, and uses machine learning to link the signals to actions. Results in the laboratory can look impressive, but everyday use brings sweat, shifting electrodes, tired muscles and different arm positions, all of which change the signals. Recalibration may be needed.
Surgical approaches. Some procedures reorganize remaining nerves and muscles so they provide clearer control signals, or implant electrodes into muscle tissue. These are specialized, they involve surgery, and the evidence is still developing. Research on connecting prostheses more directly with the nervous system is related to the wider field of brain-computer interfaces, although most prostheses today do not read the brain at all.
Lower limb control. Legs mostly need to be stable and well timed rather than dexterous. A powered knee or ankle may use motion sensors and force sensors to detect where the foot is in the walking cycle and adjust resistance or push-off. Microprocessor-controlled knees, for example, adjust how easily the joint bends depending on the phase of the step.
Feedback: the missing sense
A natural limb continuously reports touch, pressure, position and effort. A typical prosthesis offers little of this, so users often rely on watching the device. Researchers are exploring ways to deliver feedback, such as small vibrations on the skin, patterns of pressure, or electrical stimulation of nerves. Some experimental systems have allowed participants to report a sense of touch from a prosthetic hand, but these are mostly confined to research studies. Widespread, durable, everyday sensory feedback is not yet standard.
What is still hard
- Dexterity. Human hands have many joints and fine control. Most commercial hands offer a limited set of grips, and controlling each one takes practice.
- Reliability. Electronics must survive rain, dust, sweat, knocks and daily wear.
- Weight and comfort. A device that is heavy or poorly fitted may be set aside, and abandonment is a recognized problem.
- Cost and access. Advanced devices can be expensive, and insurance coverage and repair options vary.
- Cognitive load. Controlling a device by concentrating on it can be tiring.
- Durability of research results. Many studies involve few participants over a short time.
Related technology
Prostheses overlap with several other areas. Humanoid robots use similar actuators and control ideas, and robot sensing offers techniques that may carry across, though a prosthesis has the added need to be comfortable and safe on a living body. Exoskeletons support a limb rather than replace it. Connected devices that share usage data raise the same data privacy concerns as other body-worn technology.
Anyone considering a prosthesis, or an upgrade, should work with a qualified prosthetist, physician and therapist. They can explain what is realistic for a particular person, including training time, upkeep and the trade-offs between simple and advanced options.