The Massachusetts team believes it has overcome one of the major stumbling blocks encountered in the past by chemists trying to develop similar drug delivery systems - drugs leaching out before they are meant to.

The approach used by Dr Paula Hammond and her team, along with David Lynn from the University of Wisconsin-Madison, builds on work by Gero Decher, of Louis Pasteur University, in Strasbourg, France, in 1997.

Decher devised a method for depositing thin polymer films on objects of any shape in the hope that drugs embedded in these films would be slowly released as the layers broke down in the body.

The problem was that water could penetrate these layers and dislodge the drug early.

Dr Hammond's team say they have prevented this by making the polymer form chains or a mesh that means water cannot enter as readily.