A passive artificial hand that has no power source or control over its movement can be made from a range of materials whose physical properties are important to provide a lifelike visual impact, feel and durability. In contrast, a hand that needs to be designed to hold and manipulate objects requires as at least some form of mechanical structure, c.f., a skeleton but also other components so that it is functional. It has a palm or central structure for the attachment of finger joints and links that is similar to the anatomy of a natural hand. Also there is provision for actuators, i.e., electric motors or if the device is body powered, cables to react against the mechanism. For a powered anthropomorphic hand, choices are made about which of the fingers and thumb should be powered. A single actuator could move the first finger or groups of fingers and the thumb to form pinch grips where the fingertips oppose the thumb tip. With two actuators a finger or group of fingers can move independently of the powered thumb. Alternatively, one actuator could move both the first finger and thumb together and the second actuator moves the other fingers. Three actuators allow for the thumb to have two degrees of freedom with the third actuator powering all the fingers. Alternatively, the first finger could be powered with one actuator and the three other fingers with a second actuator. The powered thumb then has only one movement of extension/flexion and no rotation. The thumb can also have no power and be moved into position using a natural hand, in which case all three actuators are available for moving the fingers. With four actuators, there are more possible combinations for grip patterns but each additional actuator adds mass not only to the hand but also is in need of more coordination and algorithmic control of the fingers and thumb.

A person wearing an artificial hand will want to control the movement of the fingers and thumb or generate grip force. They will need some device that could be an electronic, electromechanical or mechanical interface. The person may simply desire to open and close the hand. An electronic signal generated from some action by the person can be sent to the actuator(s) to cause the desired effect. A simple open-loop controller can be made to achieve such an action (Figure 4.1). For the position of the fingers to be controlled precisely requires a detailed mathematical model of all the parts of the system. The model parameters will need to be determined through measurements made on the components. Only then can precise control be achieved. However, most systems will have variable parameters such as friction and backlash in the finger mechanisms. A much better idea is to use negative feedback shown in Figure 4.2. The advantages of using a feedback control system have been extensively researched. For the artificial hand, there are the advantages of improved performance in terms of making precise finger movements and automatic grips. The person controlling an artificial hand can control posture and force generated in an open-loop way, but this would require continuous monitoring of the hand and have a cognitive burden. Adopting a hierarchical control structure frees the person of having to think all the time about state of the hand (Figure 4.3). The lower levels of control are automatic electronic systems, so that the person simply instructs the controller to instigate grip of an object and monitor any slipping that may occur. People who use an artificial hand would like to have some sensory feedback, i.e., be able to feel some property of an object such as surface texture. Here, sensors can estimate these physical properties and through the skin surface stimulate neurons to elicit a sense of feeling, i.e., haptic feedback.

An outline of the engineering requirements for a prosthetic hand has been described in this book where the techniques and ideas presented are also applicable to robotic hands. Of all the components that form the system of an artificial hand, it is perhaps the algorithms, sensors and the processing of signals that will play an increasing role to improve the functionality of advanced hand designs. The new developments in 3-D printing (additive manufacturing) allow for custom-made designs, modular components and low cost in the development of designs. However, the polymer-based materials have non-isometric properties and can be relatively weak compared to blocks of materials that are machined into the desired shape. Also if a high tolerance is required, then a 3-D printed component may lack the precision required. This technology has seen considerable development in the recent years and opens up the possibility of low cost and modular component design. Further developments, especially using low-density metals, will greatly enhance the durability of future prosthetics.