The invention relates generally to a method for coating a stent or a medical device having a tubular wall. More particularly, the invention is directed to a method for electrostatic spray-coating a stent or a medical device having a tubular wall.
Medical devices, such as implantable stents, have been coated with a coating comprising a biocompatible polymer to reduce adverse physiological reactions, such as restenosis, caused by uncoated surfaces of medical devices inserted or implanted in patient""s body. Also, the coating can incorporate a biologically active material. For example, implanted stents have been used to carry medicinal agents, such as thrombolytic agents. See, U.S. Pat. No. 6,099,562 to Ding et al., U.S. Pat. No. 5,879,697 to Ding et al., Pinchuk to U.S. Pat. Nos. 5,092,877, 5,304,121 to Sahatjian.
Such coatings have been applied to the surface of a medical device by various methods, e.g., spray coating and dip coating. When a tubular wall, such as a stent, having openings therein is coated by conventional methods, it has been extremely difficult to coat only the inner surface of a tubular wall without coating the outer surface and vice versa. Also, the ratio of coating thickness placed on the inner surface of the tubular wall and placed on the outer surface of the tubular wall created by a conventional method is fixed and cannot be varied. For example, when a spray coating method is employed to coat such a tubular wall, the ratio of coating thickness depends on the configuration of the tubular wall, specifically, the size and shape of the openings therein. Accordingly, this ratio cannot be controlled. When a dip coating method is employed, the thickness of the coating on the inner surface and the outer surface is the same and cannot be varied. Also, conventional coating methods lack the ability to coat a tubular wall so that the coating thickness along the longitudinal axis of the tubular wall is varied.
Furthermore, in some medical devices having a tubular wall, all of the surfaces of the medical device or portions thereof may not need to be coated, or may not need to be coated with a coating comprising a biologically active material. For instance, the inner surface of a stent does not have to be coated with a coating containing a biologically active material when the biologically active material is intended to be delivered to a body lumen wall, which only directly contacts the outer surface of the stent. The inner surface of the stent does not come in direct contact with the body lumen wall and does not apply the biologically active material to the body lumen wall. On the other hand, if the biologically active material is intended to be delivered to a body fluid rather than a body lumen wall, then the coating containing the biologically active material should be placed on the inner surface of the stent wall but is not needed on the outer surface.
Also, in some instances, a release profile of a biologically active material can be optimized by varying coating thickness along longitudinal axis of the tubular wall. Specifically, in some stents, the amount of a coating containing a biologically active material may be preferably increased at the end sections of the tubular wall or stents as compared to the middle portion to reduce a risk of restenosis caused at the end sections.
In addition, coatings on different portions of the tubular wall may require different physical properties. For example, an expandable stent must be put in its unexpanded state or xe2x80x9ccrimpedxe2x80x9d before it is delivered to a body lumen. Thus, the coating on portions of the stent which contact each other in the stent""s crimping state must not stick to each other and cause damage. In the case of a balloon expandable stent, the inner surface of the stent that contacts the balloon must not stick to the balloon during expansion. On the other hand, it is desirable to provide a relatively soft or xe2x80x9cstickyxe2x80x9d coating on the outer surface because it comes in direct contact with a body lumen wall.
Accordingly, there is a need for a method of coating a medical device comprising a tubular wall, such as a stent, that can control the thickness of coating on inner surface and outer surface. Furthermore, there is also a need for a method of coating a tubular wall, such as a stent, that can vary the thickness of coating along the longitudinal axis of the structure.
This and other objectives are accomplished by the present invention. To achieve these objectives, we have developed a method which is efficient to realize a controlled thickness of a coating on at least a portion of a medical device comprising a tubular wall, such as a stent, having an inner surface, an outer surface and openings therein. Specifically, in the method of the present invention, the tubular wall is grounded or electrically charged, and a conductive core wire is located axially through the tubular wall. A potential is applied to the conductive core wire to impart an electrical charge to the conductive core wire. The tubular wall is exposed to an electrically charged coating formulation, and the charged coating formulation is deposited onto a portion of the tubular wall to form a coating on the tubular wall. In one embodiment, the tubular wall is grounded, and the conductive core wire and the coating formulation has the same electrical charge. In another embodiment, the tubular wall is grounded, and the conductive core wire and the coating formulation has opposite electrical charges. In yet another embodiment, the tubular wall and the coating formulation has the same electrical charge, and the conductive core wire has an electrical charge opposite that of the tubular wall and the coating formulation. Alternatively, the potential applied to the conductive core wire may be pulsated to cyclically impart a positive electrical charge to the conductive core wire followed by a negative electrical charge.
In an embodiment, a core wire comprising a resistor material is located axially through the tubular wall instead of the conductive core wire, and a current is directed in the core wire. Two resistor wires may be located axially through the tubular wall.
Furthermore, in the method of the present invention, the core wire can be kept free of the coating formulation by, for example, using two bobbins, wherein one is feeding the core wire through the tubular wall and the other is winding the core wire. Also, in the method of the present invention, a pair of deflector plates can be used to direct the charged coating formulation.