Scientists have developed a tiny 'scaffold' of stem cells to fill holes in the brain caused by a stroke, which could help reverse cerbral damage. British researchers have used tiny biodegradable 'ping pong' balls, just one-tenth of a millimetre in diameter, to ferry stem cells into stroke-damaged brains.

So far tested in rats, the biodegradable balls loaded with stem cells were able to replace brain tissue in damaged areas. The balls anchored the stem cells in the right place and within a week, healthy new tissue grew.

Although scientists have had some success using stem cells to treat stroke, the cells have tended to ooze out into healthy parts of the brain, cutting their benefit. The new technique ensures they stay where they are needed.

The team, who report the results in Biomaterials, are now planning to add in a protein and other nutrients to encourage new blood vessel growth in the treated areas. But there is still a long way to go in stem cell therapy for stroke survivors.

Previous research in injecting stem cells into rats with stroke damage has had some success but, because they lack structural support, the cells end up migrating into surrounding healthy tissue, the researchers said.

Researcher Mike Modo of the Institute of Psychiatry, King's College London, said, ''We would expect to see a much better improvement in the outcome after a stroke if we can fully replace the lost brain tissue, and that is what we have been able to do with our technique.''

The stem cells were attached to the 'ping pong balls' and injected into the brain through a very fine needle. The balls, which were made of a biodegradable polymer, gradually broke down as the stem cells made new tissue.

Modo said over a few days the researchers were able to see cells migrating along the scaffold particles and forming a primitive brain tissue that interacts with the host brain. Over time, the particles bio-degrade, leaving more gaps and conduits for tissue, fibres and blood vessels to move into.

With researchers at Nottingham, they now plan to use a protein called VEGF that encourages new blood vessel growth in order to link up the new tissue with the rest of the brain. They also need to compare the scaffold technique with results from just injecting stem cells alone.

Still early days: Professor Douglas Kell, chief executive of the Biotechnology and Biological Sciences Research Council who funded the research, said, "It is reassuring to know that the technology for treating stroke by repairing brain damage is getting ever closer to translation into the clinic."

Joe Korner, director of communications at The Stroke Association, said the research had showed the ability to overcome some of the barriers in translating stem cells into a realistic treatment. ''Every five minutes someone in the UK has a stroke and it is vital that we do all we can to help those affected by stroke.

"However the development of stem cell therapy for stroke survivors is still in the early stages and much more research will be needed before it can be tested in humans or used in practice," Korner added.

Professor Anthony Hollander, a stem cell expert at the University of Bristol, said, 'We are only just beginning to understand how to use tissue engineering to cure diseases.

''This study shows the exciting possibility of using a biomaterial to deliver stem cells to a very specific location in the brain.

''It is too early to say if it will be clinically effective in patients, but the more we explore these possibilities the more likely it is that we will develop successful therapies.''

Stroke is the third most common cause of death in Britain, after heart disease and cancer. It affects 150,000 a year, kills more than 67,000 and causes more disability than any other chronic disease.