Reservoir blocks
19 Apr 2004
Supercritical carbon dioxide is becoming an increasingly popular solvent in industrial processes, often being used to extract organic compounds such as caffeine from natural products. But researchers from Ohio State University have found a way to use it for the opposite process - forcing chemicals into a polymer. The process may be the key to making implantable reservoirs for controlled release of drugs which double up as bone replacement, the team says.
David Tomasako's team is working on medical implants made from polymethyl-methacrylate (PMMA), which is often used in bone replacement implants. Embedding drugs into these implants would give them a double effect - they'd both replace missing bone and deliver drugs to prevent inflammation or infection following surgery. If bone had been removed as a result of cancer, an impregnated implant could also delivery anti-cancer drugs to the site.
The technique is fairly simple: Tomasako's team swabbed a protein solution onto a coin-sized disc of PMMA, then placed the disc into a glass tank which they flooded with supercritical CO2 under pressure.
The protein was driven 30µm into the surface of the PMMA, with the CO2 apparently acting as a lubricant to push the proteins between the tightly-packed polymer chains of the plastic, but the structure of the protein was not affected. Unexpectedly, the inside of the disc foamed up into a series of tiny voids.
The foaming effect turned out to be controllable: the faster the engineers turned off the CO2, the more voids were created. Tomasako believes that these voids could be used as an extra reservoir for drugs.
The CO2 has yet another property - it sterilises the polymer surface. Normally, surgical implants are sterilised with heat, radiation or disinfectant chemicals, all of which can change the structure of drug active ingredients.
There's still some way to go before these materials are usable in surgery. Tomasako is now trying to determine whether drug molecules actually remain effective after being embedded in the plastic.
Further research will look at combining biodegradable materials with PMMA to form a reservoir that would release the drugs once they had been implanted.
