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Supercritical fluids (SCF) have variable liquid-like densities, gas-like viscosities, and the zero surface tension of gases. These three basic properties provide unique advantages in modifying the surfaces of medical devices to provide better therapies.
Using a non-reactive gas in its supercritical state as the fluid eliminates exposure of the coating or substrate to solvents or extreme temperatures that could alter the structure, chemistry, morphology, and most importantly the therapeutic effectiveness of the drug. In addition, because the fluid is a gas it does not affect materials already coated on the device. This allows for multiple layers containing one or more therapeutic agents to be coated onto the device, while maintaining specific control over drug elution profiles.
The variable density of supercritical fluid allows precise control over the solvation, diffusion and flow of the drugs and polymers during a surface modification process, and facilitates the generation of drug-eluting coatings with precise control over drug morphology and polymer composition, as well as multi-drug and multi-layer coatings. The ability to create multi-drug and multi-layer combination coatings has been a significant challenge using current solvent-based methods since the solvents dissolve or otherwise debilitate the previously coated layer.
Figure 1: Supercritical Fluid Process
The gas-like viscosity and surface tension of supercritical fluids enhances the ability of the coating to access the most intricate dimensions of medical devices, to be unaffected or indifferent to the surface characteristics of the pre-treated medical device surface, and to allow processes up to 10 times faster than those that require transport of materials in traditional solvents. In fact, these properties provide a capability to deliver therapeutic chemistry into micro and nanoporous openings in medical devices.
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