The utilization of prosthetics dates back to ancient Egyptian era. Owing to the increase in the number of amputations during the American Civil War, the utilization of artificial limbs became popular. The discovery of anesthetics also aided use of prosthetics.
In the current scenario, prosthetics have become more light, compliant and easily adaptable to different shapes and style with the invention of more sophisticated materials and electronics. The state-of-the-art C-Leg is one of the best prosthetic available today with a hydraulic cylinder, a built-in computer and a carbon fiber frame, which ensures high performance grades. Upper-limb prosthetics, unlike lower-limb prosthetics, are not versatile enough to function as a healthy limb.
Light and compact exoskeletons have been developed recently. Robotic devices used nowadays have an edge over prosthetics and exoskeletons as the user need not wear them.
The following video by Ekso Bionics is a fine example of wearable robotics.
Example of an Exoskeleton Design
Exoskeletons are articulated at about seven single-axis revolute joints, which are as follows:
- Wrist radial–ulnar (rad-uln) deviation
- Wrist flx-ext, fore-arm pronation–supination (pron-sup)
- Elbow flx-ext
- Shoulder internal–external (int-ext) rotation
- Shoulder flexion–extension (flx-ext)
- Shoulder abduction–adduction (abd-add)
Some of the joints configurations are discussed in the following section:
- Anthropomorphic joints - This configuration can be classified into three types namely, 90°, 180° and axial.
- Human–machine interfaces - The design of this configuration is challenging as the human arm occupies the joint axis of rotation.
- Joint cable routing - In cable-driven devices, the mechanical joint ROMs are achieved by routing the cables either through or around the joint axes at constant cable length.
- Singularity placement – Device configuration known as a singularity involve the loss of DOF while aligning two rotational axes. However, in an exoskeleton arm, the singularities occur while aligning joints 1 and 3 or joints 3 and 5.
- Power transmission - Power can be transmitted from one location to another through gear trains, fluid lines, cables and shafts. However, the bandwidth and rigidity of the wearable robots can be achieved by selecting suitable transmission type and adjusting the placement of actuators.
The following are the system requirements for the exoskeleton joint design:
- Kinematic and dynamic requirements - These requirements can be met by conducting a pilot study of activities of daily living (ADL).
- Mechanical human–machine interfaces - These interfaces are the components that mechanically link the exoskeleton structure and the human arm and also transmit force between them.
- Safety requirements - Safety precautions are required to be implemented on three levels, namely, software, electrical and mechanical.
- Modeling the human arm - Modeling of anthropomorphic joint approximations can be performed at varying degrees of complexity and accuracy.
- Performance - The performance of the system is measured with the help of its bandwidth.
The design and analysis of a multi-fingered hand prosthesis has been presented in a paper by Jingzhou Yang et al. and is described below.
The IOWA hand prosthesis consisting of multi-segmental joints was developed in order to actuate each finger segment with the help of a cable-conduit system that is connected using two or three mechanical springs that help capture the functional capacity of the hand. As seen in the human hand, flexor tendons help bend and rotate this structure and this is can also be seen with the prosthetic hand by using a single cable-conduit mechanism that transfers the linear force into axial and lateral deflection to help mobilise flexible element.
The five active fingers of the IOWA hand are capable of bending at the distal interphalangeal, proximal interphalangeal and metacarpo-phalangeal joints. However, each finger consists of conduits, cables, compression links for cable, and springs acting as joints. Compression links functions as a restrainer for the conduit and as a connecting holder for the cable.
The key merits and demerits of the IOWA hand are listed below:
- The actuators can be fixed anywhere on the body as the cable conduit system realizes remote actuation of the flexible element.
- Each joint in each finger can be bent or rotated at different angles, which allows the hand to grasp complex geometry.
- The IOWA hand has adjustable stiffness/compliance characteristics to meet the user’s needs.
- Though pinch force is high at fingertips, fine motion control between two fingers is hard to achieve.
- With adjustable hand compliance and consistent design parameters, the IOWA hand has realistic finger movement.
Nearly 34,500 knee amputations are being carried out every year to improve the living standard of individuals. However, a completely functional prosthetic knee has not been developed yet. During amputation, a femur is cut 10 cm above the knee. The femur is joined to muscles which are located above the amputation point and this does not affect other muscles. Some of the problems faced with currently available knee prosthetics are as follows:
- Inability to turn and use stair case
- Abnormal gait
- Navigating obstacles
The C-leg is currently the most widely used prosthetic knee. It has hydraulic cylinders to enable leg movement and a microprocessor to determine the movement by acquiring muscle impulses. It also uses strain gauges to identify the point at which the force is applied to the knee. However, the user will face difficulties while walking downhill and running while using the C-leg.
A customized prosthetic knee for different individual needs has been proposed through the following steps:
- Data mining should be carried out to imitate normal gait by measuring length and diameter of the leg, knee flexion angles and structure of the muscles in a group of amputees and non-amputees.
- Muscle impulses can be acquired to improve the maneuverability and neuro signals are recorded by using electrodes.
- The impulses are transferred to a microprocessor and power source mounted on the prosthetic. The power source can be charged using 120V outlet.
- Muscles are simulated using actuators, which generate walking movements by controlling the microprocessor.