Novel Coating for Implants Could Prevent Premature Failure

Researchers at the Massachusetts Institute of Technology (MIT; Cambridge, USA) developed modular nanostructured multilayers--ranging from 100 nm to one µm--which contain hydroxyapatite (HA) particles in complex with the natural polymer chitosan, creating an osteoconductive surface for mesenchymal stem cells (MSCs). When coupled with the sustained release of physiological amounts of osteoinductive bone morphogenetic protein over several days from the degradable poly(β-amino ester) based multilayers, the single coating results in a synergistic accelerated and upregulated differentiation of MSCs into osteoblasts that lay down new bone tissue on orthopedic implants.

The thickness of the film and the amount of growth factor released are controlled by using a method called layer-by-layer assembly, in which the desired components are laid down one layer at a time until the desired thickness and drug composition are achieved. According to the researchers, the coating could be used not only for joint replacements, but also for fixation plates and screws or dental implants, thus reducing significantly the time needed to achieve osseointegration. The study was published in the March 15, 2012, issue of Advanced Materials.
“When bone cement is used, dead space is created between the existing bone and implant stem, where there are no blood vessels. If bacteria colonize this space they would keep proliferating, as the immune system is unable to reach and destroy them,” said lead author graduate student Nisarg Shah, BSc, and colleagues of the department of chemical engineering. “Our idea is to prevent failure by coating these implants with materials that can induce native bone that is generated within the body; that bone grows into the implant and helps fix it in place.”

Chitosan is derived from chitin, a polysaccharide found in the exoskeleton of shellfish such as shrimp, lobster, and or crabs. Chitosan's properties allow it to be used in transdermal drug delivery, since it is mucoadhesive in nature, reactive, and most importantly, its positive charge under acidic conditions leads to an increase in solubility. This means chitosan can be used to transport a drug to an acidic environment, where the chitosan packaging will then degrade, releasing the drug to the desired environment. One example of this drug delivery has been the transport of insulin.

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