Coatings have been applied to surfaces of medical devices because it is believed that the coatings provide the medical devices with certain advantages. Coatings containing antimicrobial agents have been applied to medical device surfaces to prevent infection. For example, U.S. Pat. No. 6,468,649 to Zhong et al. teaches an implantable medical device having a substrate with a hydrophilic coating composition to limit in vivo colonization of bacteria and fungi. Also, coatings containing therapeutic agents have been applied to stent surfaces because it is believed that such coatings help treat or prevent restenosis. For example, U.S. Pat. No. 6,258,121 to Yang et al. discloses a stent having a polymeric coating for controllably releasing an included active agent such as taxol, to inhibit restenosis following angioplasty.
Various methods are known in the art for coating medical devices. These methods include spraying a coating solution onto a device and then evaporating the solvent, to leave a coating of desired components on the surface of the device. For example, U.S. Pat. No. 6,258,121 to Yang et al. discloses stents that are coated by spraying the solution and then evaporating certain components, thereby leaving a coat of the agent on the surface of the stents. Other methods include dipping the medical device into the solution to be coated., as in U.S. Pat. No. 6,156,373 to Zhong et al. U.S. Pat. No. 6,569,195 to Yang et al. discloses dipping a stent into a mixture comprising a solvent, a polymer, and a therapeutic agent, with subsequent evaporation of the solvent to leave a polymeric coating. Another method that can be used is an electrohydrodynamic coating process described in U.S. Pat. No. 6,669,980 to Hansen where a coating solution is electrically charged, atomized and deposited on the device by an attractive electrostatic force.
However, it has been found in the present invention that the use of cryogenic means to coat medical devices has many advantages that were not known in the art before now.