The immunological barriers to this kind of transplantation have been much greater than envisaged and much harder to suppress without rendering the individual completely susceptible to infection from every opportunistic bug around him/her. An alternative to an animal sourced heart is stem cell technology that can be used to either heal the native heart or create one specifically for the patient de novo in the laboratory. Realistically this is NOT going to happen any time soon. This therefore leaves us with the mechanical alternative - i.e. an artificial heart. Whereas a biological solution seems intuitively to be preferable, it is worth remembering that in human development, for every 1 cm of biological know-how gained, there has been a kilometre of technological gains. On example I like to quote is that when the current immunosuppressive drug regimen currently used in many heart transplant centres was first used, home computers were very large, very expensive and still using MS-DOS. Artificial hearts have been around for a number of years. They have until very recently been very large and unwieldy. It is only when it was realised that the flow generated by the new pump did not have to be a pulsatile copy of what happens in the human body that much smaller continuous flow pumps were built. Apart from size, there are a number of further hurdles designers need to jump over before artificial hearts become mainstream - the first relates to the fact that blood will clot when it comes into contact with artificial surfaces. Clots can then fly off into the general circulation (or in medical parlance, embolize) and cause serious illness such as strokes. Developments in chemical engineering will lead to the construction of bio-compatible clotting resistant surfaces that can be used to line the inside of artificial hearts. Advances in pharmacology have led in recent years to the development of many new smart drugs that can be used to control the clotting cascade (process). These new drugs are more subtle in their actions and more controllable than the hitherto available option i.e warfarin or rat poison.
Other hurdles designers of artificial hearts must overcome relate to energy and energy sources. Having to pump 5-8 litres per minute of viscous liquid forever requires shed loads of energy. Currently this means that the device inside the patient must be hooked up to a sufficiently powerful source of energy through a cable that must breach the skin. The large size of the battery and the fact that the cable breach is a source of life threatening infections make them significant weaknesses for current devices.
This story on tetherless power transmission and this one on structural batteries may mean that solutions are on the horizon. Either solution could be a gamechanger for artificial hearts and that is when the real headache for health planners, commissioners and ethicists will start.