Implants for soft tissue should ideally mimic the mechanical properties of the surrounding tissue, be resorbable and biocompatible. However, most soft-tissue implants available today do not have these properties.

A team of researchers led by Robert Langer of the Massachusetts Institute of Technology (MIT), George Whitesides of Harvard University and Maria Palasis of 480 Biomedical Inc. in Watertown have developed a platform technology using variants of poly(glycolic) acid (PGA) that are braided and subsequently coated with a poly(glycolide-co-caprolactone crosslinked elastomer, or PGCL. The coating makes the implant stronger mechanically and allows it to expand and contract to the same extent as conventional metallic stents.

Further improving device mechanical properties

The mechanical properties of the devices can be further improved by tuning the branching structure of the elastomer, the elastomer crosslink density and molecular weight of the prepolymer. The researchers reduce the amount by which the elastomers deform, for example, by incorporating a four-arm branched initiator called pentaerythritol into the polymer processing step to create a branched prepolymer of PGCL. The four-arm structure provides crosslink points on the prepolymer and helps to overcome plastic deformation, so improving the elastomers' overall mechanical strength.

Palasis and colleagues tested out their devices by implanting them in the femoral arteries of pigs and sheep, and found that they were naturally resorbed by 18 months. They were also biocompatible in that they did not produce an inflammatory response in the animals in experiments that lasted a year.

The researchers say that the polymer materials used degrade via hydrolysis into metabolites that can be safely eliminated from the body. Thanks to this property, these copolymers are already routinely used as sutures, for orthopaedics devices and drug-delivery systems.

Alternatives to self-expanding metallic stents?

Such strong bioresorbable materials show much promise in the treatment of arterial disease, in which they would hold a vessel open and not recoil as the vessel naturally moves. They could be alternatives to self-expanding metallic stents, which although strong can fracture easily, with the added advantage of being eliminated from the body once their work is done. They are thus less likely to cause irritation or other related problems. They are also highly flexible and therefore conform to the shape of irregular cavities, say Palasis and colleagues, so providing an ideal platform for efficiently delivering drugs directly to tissue.

Given that preclinical safety of the implant has been established, the next step is human clinical studies, they add.

The research is detailed in Nature Materials doi:10.1038/nmat5016