Bionic prosthetic arms are in direct association with the body’s nerves as well as muscles. They pick up electrical signals from the user’s remaining limb and transmit them to the prosthetic hand for use. These prosthetic arms use a combination of sensors, motors, batteries, electrodes, and prosthetic components for effective functioning. The electrodes situated within the prosthetic socket are responsible for picking up the signals from the residual limb, relaying this information to a controller that will then translate this information into commands suitable for the electric motors located within the prosthetic components for their operation. In the situation where the muscle signals are not able to control the prosthesis, switches or touch pads are used to operate the components.
BASIC COMPONENTS OF A BIONIC ARM PROSTHESIS
- Controller or control systems
- Prosthetic components
Microprocessors are electronic components that are used by a computer or other electrical systems for doing their work. They are the central processing component of the entire unit, responsible for controlling and coordinating the activities of all the other components. In a bionic prosthetic arm, the powered prosthetic components are controlled by motors and gears that are supplied energy by the batteries. In these prosthetic arms, the system’s microprocessor acts as the brain of the entire system as it designates, controls and coordinates all the activities.
These systems can either have a single, central microprocessor that can operate all the components, mounted inside the socket, or there can be separate microprocessors for controlling the function of each component, mounted on or inside the specific component. These microprocessors run through algorithms that are programmed through handheld, wireless personal devices or through interfaced cable system controlled by a computer. Algorithms are programmed to adjust different parameters of the bionic prosthetic arms, for instance, mode selection parameters, input signal amplification, and thresholds, output voltages, and currents, among a multitude of other parameters. When multiple microprocessors are involved, the programming and adjustments become more complex.
For pattern recognition, whereby the microprocessor recognized and learns the muscle function pattern, a separate microprocessor is used to establish control of different prosthetic components simultaneously. The major advantage of this pattern recognition function is that it provides direct prosthetic control for a more natural and easy usage pattern.
Therefore, in any bionic prosthetic arm, microprocessors are responsible for the regulation of input signals, to manage any power and energy requirements of the system as well as the regulation and control of the output of different components.
The electrodes in the bionic arms are the connection between the source of the signal in the residual limb and the control systems. These are generally surface electrodes (some also use implantable electrodes) that are usually placed in pairs over the opposite muscle groups for obtaining the electromyographic signals produced by the contraction of the muscles. The electrodes are placed inside the socket over the limb and have a control unit to multiply and adjust the gain (increase or decrease the signal). When the electrodes detect signals, they inject them into the control system to imbalance it, causing the prosthetic components to perform their desired function.
The battery is the energy storehouse of the bionic prosthetic arm and functions to supply the required electric power to the motors for the operation of the prosthetic components, as needed. The batteries are rechargeable and are of 2 types – (i) the battery is built into the arm where the arm has to be plugged in directly for recharging, (ii) a removable battery that can be ejected for charging.
For a good cosmetic appearance, the batteries are built into the arms that have a low profile design. This provides a natural-looking, streamlined prosthetic shell.
Generally, the controller is made up of an amplifier, a signal processor, and a logic unit that makes all the control decisions. The amplifier functions to amplify and increase the electrical signal in magnitude. Common practice is to utilize differential amplifiers as they can easily discriminate and separate any external disturbances. The signal processor is responsible for processing the signal and works on the mean average of its absolute value. The muscles give out alternating current that has to be processed and converted to direct current in order to be picked up by the electrodes. This signal is then used to control the motors in of the prosthetic components.
The simplest type of control system consists of a switch to regulate the flowing current to the motor. Some systems use electronic switches that are used for prosthetic functions. Switches are consistent, simple, and easy to use and work on a push or a pull mechanism. A button like micro switch can be pushed or a transducer can be pulled.
Touch pads are force-sensitive resistors that function as temporary switches and often substitute for electrode functioning when the user cannot effectively contract the residual limb muscles.
Sockets – Depending on the level of amputation, sockets are fabricated by capturing the anatomy of the residual limb and the electrodes of the bionic prosthetic arm are placed inside the socket, to maintain total contact. It is preferable to make the socket self-suspending or held onto the limb through suction, with the goal to avoid any additional harnessing as it hampers movement. The arm/forearm setups are attached with the sockets, with space to hold the batteries.
Terminal Devices – The terminal devices of bionic prosthetic arms are the main, voluntary prehensors that perform grasp and gripping activities, enabling the patients to perform their daily tasks. Terminal devices are referred to as the ‘muscle’ of the system as they perform different movements and motions. Most of the technologically advanced terminal devices consist of opposable thumbs and fingers equipped with sensors that allow them to use different grip patterns. The electronic terminal devices have an electronic unit, with a switch and motor to allow the opening and closing of the terminal device. The terminal device is connected to the controller and the threshold and gain is adjusted, as needed by the user.
Powered wrist, elbow, and shoulder units – Wrist units help in positioning the terminal device and also allow for interchangeability of terminal devices, for example – replacing a hand with a hook to perform a specific activity. Powered elbows provide the bionic prosthetic arm, the ability to lift objects, and this ability is limited by the power of the unit’s motor and drive systems. For any user to achieve the maximum lifting capacity of a system, the elbow must be fixed or locked.