Various approaches are being explored to provide alternatives to organ transplants.

Artificial organs: Kidney dialysis machines illustrate that machines can take over the function of biological organs. To be an alternative to a transplant, though, an artificial organ must be portable. Despite much research, even a relatively simple organ such as the heart has proven an engineering challenge. Commonly, they are used as a 'bridge' to keep patients alive until a heart becomes available.

Several mechanical hearts are in experimental use, including the JARVIK-7 and the Heart Mate II. Early 'pulsatile' devices mimicking the heart's action proved unreliable, and continuous pumping systems are now more common, offering support to a struggling heart.

For other organs, the complexity of the physiological processes and biochemical reactions carried out by organs - and their coordination with the rest of the body - is very hard to mimic. One exciting line of 'bioengineering' research involves combining nanotechnology and biological materials so very sophisticated processes can be carried out in relatively small spaces.

Cell therapy: Instead of mechanical solutions, the body's own cells could come to the rescue. Stem cells, for example, could be used to provide new cells and tissues to repair or replace damaged cells. There have been some trials of adult stem cells to repair failing hearts, with mixed results. Some researchers have argued that not enough is yet known about stem cells for clinical work to be done.

Other possibilities include using cloning techniques to generate embryonic stem cells that exactly match a recipient, or 'reprogramming' adult cells so that they are turned into stem cells. These could then be used to generate new tissues. Research in this area is highly promising, but it remains some way from routine use.

Hybrid devices: Some approaches try to marry the benefits of precision engineering with biological responsiveness. For example, progress is being made towards an artificial pancreas, combining insulin pumps with encapsulated pancreatic beta-cells that sense glucose levels.

Xenotransplantation: Much research has gone into turning animals such as pigs into a source of organs for transplant. The main problem is that pig organs are rejected extremely violently by the human body. Attempts are being made to genetically engineer pigs so that they do not stimulate such responses. But many technical challenges remain, and there are also concerns that pig viruses might be transmitted to people.

Transplantation raises a number of ethical issues, the importance attached to each varying from country to country:

Presumed versus informed consent: Should donors opt in or should everyone be considered a potential donor unless they ask not to be? [link to more detail]

Access to organs: How should we decide who gets an organ? Is a doctor more deserving than a road sweeper? Or a convicted criminal? A young person more deserving than an old one? What about heavy drinkers or smokers?

Organ trade: In most countries, donated organs are considered a 'gift'. Should people be rewarded for donating a kidney, say, if they are in short supply?

Altruistic donation: Should people be able to donate organs for use by strangers, as with blood donation?

Restrictions on use: Should donors have any say how donated organs are used? In the USA, some attempts have been made to specify use, but this has generally been resisted.

Point of death: How do we define death? Is 'brain-stem death' really death of the body? Is there a temptation to 'speed up' a death in order to provide someone else with an organ? (Donor and recipient teams are kept separate to avoid such conflicts of interest.)

Saviour siblings: Is it OK to use techniques to 'select' particular embryos so that they can be used as a source of tissue for a seriously ill sibling?

Face transplants: The links between the face and personal identity have added ethical issues to the technical challenges of face transplantation.

Executed prisoners: China obtains many organs from executed prisoners, without consent. Several groups have queried the ethics of this practice (as well as of the executions themselves).