A small US biotech company has claimed that it has removed a major safety obstacle to using "reprogrammed" human stem cells, which pose a risk of turning cancerous.

PrimeGen, based in Irvine, California, claims that its scientists have converted specialised adult human cells back to a seemingly embryonic state, employing methods that are much less likely to trigger cancer than those deployed previously.

The company also says that it has been able to produce reprogrammed cells faster and much more efficiently than other scientists.

Presently, the newest area in stem cell biology is that of induced pluripotent stem cells, or iPS cells, which have the ability to develop into several different tissue types, first created by Shinya Yamanaka of Kyoto University in Japan.

Following the discovery, researchers in Japan and Wisconsin had reprogrammed skin cells to make primordial stem cells without destroying embryos.

However, little-noted in the news reports at the time was that to make these cells, the scientists needed to introduce cancer-causing genes into the cells using gene-altered viruses, thus making the resultant cells unsuitable for human therapy.

But now, PrimeGen has claimed that it had circumvented this problem.

Rather than using retroviruses to ferry the genes into the cells, PrimeGen used tiny carbon-based particles coated with DNA that codes for the same four reprogramming genes used by Yamanaka, including Oct3/4 and a fifth gene called Nanog.

The team then mixed the particles with human skin cells, kidney cells, or cells from the retina. The particles were quickly taken up by the adult cells, which were reprogrammed into an iPS cell-like state, says PrimeGen president John Sundsmo.