This invention relates generally to a method for manufacturing an implantable medical device. More specifically, the invention relates to a method for manufacturing an implantable medical device having a coated surface. More particularly, the invention is directed to a method for manufacturing an implantable medical device, having a surface covered with a coating that can include a desired amount of a biologically active material, using an ultraviolet (UV) laser. The invention also pertains to a method for manufacturing an implantable medical device having a surface covered with a coating having more than one layer wherein a desired portion of the top or outer layer is ablated with an ultraviolet (UV) laser. Also, the invention relates to a method for measuring a thickness of a coating applied to an implantable medical device. Furthermore, the invention is directed to a method for manufacturing an implantable medical device having a surface covered with a coating free of webbing or cracking.
There are various kinds of medical devices that can be implanted in a human body. For example, medical devices, such as stents, are implanted into a body lumen, such as a blood vessel, where it stays permanently, to keep the vessel open and to improve blood flow to the heart muscle and relieve symptoms and used to reduce restenosis after balloon angioplasty or other procedures involving catheters. Usually, the suitable stents include a stent having a cylindrical shape. The walls of the cylindrical structure can be formed of metal or polymer with openings therein, e.g., a mesh. The medical devices can also be positioned in other parts of the body, such as the kidneys or the brain. The procedure for implanting the medical device is fairly common, and various types of medical devices or stents have been developed and actually used.
To make the medical device surface more biocompatible, they have been coated with polymers. Further, there are various types of polymer coatings for medical devices that may contain a biologically active material, such as a drug, that are delivered to an afflicted area of a body. Drugs may be either bonded chemically, physically or absorbed in the polymer matrix of the coating. Also, for the purpose of obtaining drug delivery medical devices or stents, the drugs may be directly coated or immobilized onto the devices, e.g. using a binding molecule between the drug molecule and the device surface. For example, U.S. Pat. No. 6,099,562 to Ding et al. discloses a stent having an undercoat containing a biologically active material covered by a topcoat substantially free of pores, and U.S. Pat. No. 5,879,697 to Ding et al. discloses a coated stent wherein the coating contains a reservoir layer containing a biologically active material. Pinchuk, in U.S. Pat. No. 5,092,877, discloses a stent of a polymeric material that may have a coating associated with the delivery of drugs. A patent to Sahatjian, U.S. Pat. No. 5,304,121, discloses a coating applied to a stent consisting of a hydrogel polymer and a pre-selected drug such as cell growth inhibitors or heparin. Thus, a number of various coatings for medical devices have been used. Such coatings have been applied to the surface of a medical device mostly by either spray-coating or dip-coating the device with a coating solution.
When a drug whose dosage must be strictly controlled is contained in the coating of the medical device, the amount of coating present on the medical device must be accurately adjusted. Previously, the only way to adjust the amount of coating on a medical device is to control the process parameters used to spray-coat the coating composition on the surface of the medical device to form the coating, such as controlling the spraying time and the flow rate of the coating solution. However, such control does not permit sufficiently accurate placement of the desired amount of coating material or drug contained in the coating material to be placed on the medical device. Also, when a dip coating method is used to form the coating, the amount of coating placed on the surface of the medical device cannot be controlled precisely. In addition, no matter what method is used for forming the coating, there has been no way to efficiently remove or trim excess or undesired coating from the coated medical device. Therefore, a method to manufacture a medical device having a desired amount of coating is needed.
Also, due to complex geometry of certain medical devices such as a stent, a webbing of coating material can form in the openings of these medical devices. More specifically, for instance, when a stent having openings in its sidewall is coated with a coating material, webbings, bindings or bridges of the coating material can form in the openings, at small gaps or corners between stent struts. This is especially true, when the stent has struts that are very close to each other or has struts that have bends in them. However, there has been no efficient way to remove or trim such webbings, bindings or bridges of coating material. Hence, an object of the invention is to provide a method to remove or trim this webbing, binding or bridging from a coated medical device.
In addition, it is not always desirable to have an even or uniform coating on an entire coated surface of a medical device. For example, depending on its geometry, a stent may have a portion where a thick coating may easily crack and cause problems. More specifically, when a self-expandable stent is placed into its restrained state, its struts lie in close proximity to each other. The coating on some struts may adhere to coating on other struts. When the stent is expanded, the adhered coating may be torn off. Likewise when a balloon-expandable stent is collapsed for implantation, the coating on certain struts may adhere to the coating on other struts because the struts are placed in close proximity to each other. Such adhered coating may be cracked or removed from the struts when the stent is expanded. If a portion of the coating can be removed from the struts so that the coating on the struts are made thinner and less likely to adhere to each other, the cracking of the coating may be reduced. However, previously, there has been no way to efficiently make a portion of a coating on a stent thinner. Thus, a further object of the invention is to provide a method to thin a portion of the coating on a medical device.
These and other objectives are accomplished by the present invention. To achieve the aforementioned objectives, a method has been invented for manufacturing an implantable medical device having a surface adapted for exposure to body tissue of a patient, wherein at least a portion of the surface is covered with a coating having a desired amount of a biologically active material. Specifically, in the method, a coating composition containing the biologically active material is applied to a portion of the surface of the medical device in a manner such that a coating containing an amount of the biologically active material in excess of the desired amount of biologically active material is formed. Then the amount of biologically active material in the coating that is in excess of the desired amount of biologically active material is determined. A portion of the coating is ablated using an ultraviolet (UV) laser in order to remove the coating containing the excess biologically active material.
Another embodiment of the present invention is a method for manufacturing an implantable medical device having a surface adapted for exposure to body tissue of a patient, wherein at least a portion of the surface is covered with a coating having at least two layers and containing a biologically active material. In the method, a first coating composition and a second composition are applied, in turn, on at least a portion of the surface of the medical device. A portion of the second coating layer is then ablated using an ultraviolet (UV) laser.
Yet another embodiment of the invention is a method for measuring a thickness of a coating applied to at least a portion of a surface of an implantable medical device. In the method, a portion of the coating is ablated with an ultraviolet (UV) laser having pulse length shorter than about 100 nanoseconds and a repetition rate less than about 100 Hertz to expose a portion of the surface of the medical device and to create a step having a height in the coating. The thickness of the coating is determined by measuring the height of the step by using a white light interferometer.
Furthermore, another embodiment of the present invention is a medical device having a surface adapted for exposure to body tissue of a patient, wherein the surface has a plurality of openings therein and wherein at least a portion of the surface is covered with a coating in a manner such that the openings are substantially free of coating and a method for manufacturing the medical device. In the method, after applying a coating composition to the surface of the medical device to form a coating thereon, coating present in the openings of the surface is ablated using an ultraviolet (UV) laser having pulse length shorter than about 100 nanoseconds and a repetition rate less than about 100 Hertz.
Another embodiment of the present invention is a method for manufacturing an expandable stent having a surface adapted for exposure to body tissue of a patient. At least a portion of the surface of the stent is comprised of a plurality of struts, and the struts are covered with a coating substantially free of cracks. In the method, after applying a coating composition to at least one of the struts to form a coating thereon, a portion of the coating on the strut is removed using an ultraviolet (UV) laser, having pulse length shorter than about 100 nanoseconds and a repetition rate less than about 100 Hertz, to prevent the coating from cracking.