These are best made to meet the individual's requirements but can be obtained ‘off the shelf’. Artificial joints normally comprise complex mechanisms to stimulate flexion and rotation movements. Leg prostheses are generally more useful than those for arms, because leg movements are easier to duplicate than those of the arm. Modern electronic circuitry that enables nerve impulses to be picked up and converted into appropriate movements is greatly improving the effectiveness of limb prostheses.

Artificial eyes are worn both for appearance and for psychological reasons. They are made of glass or plastic, and are thin shells of a boat-shape, representing the front half of the eye which has been removed. The stump which is left has still the eye-muscles in it, and so the artificial eye still has the power of moving with the other. A glass eye has to be replaced by a new one every year. Plastic eyes have the advantage of being more comfortable to wear, being more durable, and being unbreakable.

are any artificial replacement of teeth. There are three main types: a crown, a bridge and a denture. A crown is the replacement of the part of a tooth which sticks through the gum. It is fixed to the remaining part of the tooth and may be made of metal, porcelain, plastic or a combination of these. A bridge is the replacement of two or three missing teeth and is usually fixed in place. The replacement teeth are held in position by being joined to one or more crowns on the adjacent teeth. A denture is a removable prosthesis used to replace some or all the teeth. The teeth are made of plastic or porcelain and the base may be of plastic or metal. Removable teeth may be held more firmly by means of implants.

The surgical replacement of stenosed or malfunctioning heart-valves with metal or plastic, human or pig valves has been routinely carried out for many years. So too has been the insertion into patients with abnormal heart rhythms of battery-driven artificial pacemakers (see CARDIAC PACEMAKER) to restore normal function. The replacement of a faulty heart with an artificial one is altogether more challenging. The first working attempt to create an artificial heart took place in the early 1980s. Called the Jarvik-7, it had serious drawbacks: patients had to be permanently connected to apparatus the size of an anaesthetic trolley; and it caused deaths from infection and clotting of the blood. As a result, artificial hearts have been used primarily as bridging devices to keep patients alive until a suitable donor heart for transplantation can be found. Recent work in North America, however, is developing artificial hearts made of titanium and dacron. One type is planted into the chest cavity next to the patient's own heart to assist it in its vital function of pumping blood around the body.

Artificial livers work in a similar way to kidney dialysis machines (see DIALYSIS). Blood is removed from the body and passed through a machine where it is cleaned and treated and then returned to the patient. The core of the device comprises several thousand flexible membrane tubules on which live liver cells (from pigs or people) have been cultured. There is an exchange of biological molecules and water with the ‘circulating’ blood, and the membrane also screens the ‘foreign’ cells from the patient's immune system, thus preventing any antagonistic immune reaction in the recipient.