LONDON - Scientific advances including techniques allowing patients to grow new joints inside their own bodies will allow the elderly to remain active well beyond their 100th birthdays, researchers claim. British scientists are working on a system which should allow the elderly to buy body parts "off the shelf" and even regenerate their own damaged joints and hearts. Their ultimate aim is to fix up the body with customised replacement parts grown to order. They have already carried out human trials on heart valves which are still working four years after they were transplanted. At the University of Leeds, Britain's biggest bioengineering unit and the world leader in artificial joint replacement research is coordinating a project that aims to give people 50 active years after the age of 50."It is the rise of the bionic pensioner," said Professor Christina Doyle, whose company is working with the university to develop the new technologies. "The idea is when something wears out, your surgeon can buy a replacement off the shelf or, more accurately, in a bag."The university is spending £50 million ($114 million) over the next five years on the new project. The main thrust of the research centres on a method of tissue and medical engineering which the university is at the forefront of developing. Led by the immunologist Professor Eileen Ingham, they are pioneering a technique of stripping the living cells from donor human and animal parts, leaving just the collagen or elastin "scaffold" of the tissue. These "biological shells", which could be for knee, ankle or hip ligaments, as well as blood vessels and heart valves, are then transplanted into the patient whose own body then invades them replacing the removed cells with their own. The technique, which could be available within five years, effectively removes the need for anti-rejection drugs. It is similar to the recently developed system of using stem cells to regrow organs outside the body, but costs about a tenth of the price.