The one constant when it comes to live organ transplants is that it varies. The variables are endless, from the type of transplant and the medical staff, to the donor and the recipient. There are big differences between minor transplants such as blood transfusions, transplants of renewables like bone marrow, common transplants like kidneys, and really risky transplants like hearts. Needless to say, the country, hospital and medical team make a significant difference in the outcome of the transplant as well. Potential donors also vary in mental, economical and social state, and patients differ in chance of rejection and recovery.

Thus the laws regarding transplants vary as transplants themselves. The underlying issue is that life is sacred, and as much as that drives us to do everything to save a life, it simultaneously forbids us from endangering (another) life in the process. Therefore, generally speaking, for a transplant to be permissible it needs to be with low risk to the donor, and high chance of success for the recipient.

1. Surgery to remove the organ is not (unreasonably) dangerous²
2. The donor must be able to continue his life normally after the donation
3. The donor must not require prolonged and chronic medical care, and
4. The success rate in the recipient must be high

He adds that "informed consent must be obtained from the donor".

The bottom line: in each case a qualified Halachic expert must be consulted. 

The use of artificial organs for transplants poses no halachic problems as long as the prospects for success are greater than the risks.³