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  Method for coating implantable devicesUSPTO Application #: 20080102193Title: Method for coating implantable devices Abstract: A method of forming a coating for an implantable medical device, such as a stent, is provided which includes applying a composition to the device in an environment having a selected pressure. (end of abstract) Agent: Squire, Sanders & Dempsey LLP - San Francisco, CA, US Inventor: Stephen D. Pacetti USPTO Applicaton #: 20080102193 - Class: 427002100 (USPTO) Related Patent Categories: Coating Processes, Medical Or Dental Purpose Product; Parts; Subcombinations; Intermediates (e.g., Balloon Catheter, Splint) The Patent Description & Claims data below is from USPTO Patent Application 20080102193. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This application is a Continuation from U.S. patent application Ser. No. 10/792,551, filed Dec. 5, 2003, which is a Divisional Application from U.S. patent application Ser. No. 09/872,816, filed on 31 May 2001, now U.S. Pat. No. 6,743,462, the entire disclosure of which is incorporated by reference. FIELD OF THE INVENTION [0002] This invention relates to an apparatus and method for coating implantable devices such as stents. DESCRIPTION OF THE BACKGROUND [0003] Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed, so that they can be inserted through small cavities via catheters, and then expanded to a larger diameter once they are at the desired location. Mechanical intervention via stents has reduced the rate of restenosis; restenosis, however, is still a significant clinical problem. Accordingly, stents have been modified to perform not only as a mechanical scaffolding, but also to provide biological therapy. [0004] Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. In order to provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more favorable results. [0005] A common method of medicating a stent is by depositing a polymeric coating, impregnated with the therapeutic substance, on the surface of the stent. A polymer dissolved in a solvent is applied to the stent. A therapeutic substance can be dissolved or dispersed in the composition. The solvent is allowed to evaporate to form the coating. The application of the composition can be performed by spraying the composition on the stent or immersing the stent in the composition. [0006] The solvents employed with the composition can be categorized as having a high vapor pressure or low vapor pressure. Non-volatile solvents evaporate very slowly from the composition causing coating defects such as inconsistency in the coating thickness and formation of "cob webs" or "pool webs" between the stent struts. A solution to this problem is to coat the stent at elevated temperatures to increase the evaporation rate of the solvent. However, not all drugs are stable at elevated temperatures. Volatile solvents have the tendency to evaporate very quickly from the composition resulting in a coating which has a powdered consistency and adheres poorly to the surface of the stent. Accordingly, what is needed is an apparatus and process for coating stents that does not suffer from the aforementioned drawbacks. SUMMARY OF THE INVENTION [0007] In accordance with one aspect of the invention, a method of forming a coating for an implantable medical device, such as a stent, is provided. The method comprises applying a composition to the stent in an environment having a pressure other than ambient pressure. For compositions including a non-volatile solvent, the pressure can be less that 760 torr; for compositions including a volatile solvent, the pressure can be greater than 760 torr. The composition can include a polymer, such as an ethylene vinyl alcohol copolymer dissolved in a solvent, such as dimethylacetamide. Optionally, a therapeutic substance can be added to the composition, such as actinomycin D, paclitaxel, docetaxel, or rapamycin. In accordance to one embodiment, the composition can be applied by spraying the composition on the stent. During the act of applying, the stent can be rotated and/or moved in a linear direction along the longitudinal axis of the stent. The stent can be a radially expandable stent, such as a balloon expandable or self-expandable type. [0008] In accordance with another aspect of the invention, a method of forming a coating for a stent is provided, comprising positioning a stent in a chamber; applying a fluid to the stent; and adjusting the pressure of the chamber to increase or decrease the evaporation rate of the fluid. [0009] In accordance with another aspect of the invention, an apparatus for coating implantable medical devices such as stents is provided. The apparatus includes a chamber for housing a stent and a pressure controller for adjusting the pressure of the chamber during the coating process to a pressure below or above 760 torr. In one embodiment, an applicator can be provided for spraying a composition at the stent. A support assembly holds the stents in the chamber and can be connected to a motor for providing rotational and/or translational motion to the stent. A temperature controller can also be provided for adjusting the temperature of the chamber. BRIEF DESCRIPTION OF THE FIGURE [0010] FIG. 1 illustrate a pressure chamber for forming a coating on a stent. DETAILED DESCRIPTION OF THE EMBODIMENTS Embodiments of the Pressure Chamber [0011] Referring to FIG. 1, there is illustrated a pressure chamber 10 defining a workspace 12 for depositing a composition on a stent 14 for forming a coating. A chamber opening (number omitted) can be provided for allowing a user to gain access into workspace 12. A hatch 16 can be placed over the chamber opening for tightly sealing the opening during the deposition process. The size of workspace 12 needs to be large enough so as to enclose a support assembly 18, such as a mandrel, for adequately supporting stent 14 during the coating process. Workspace 12 can be large enough so as to support any suitable number of support assemblies 18 and stents 14. [0012] In one embodiment, support assembly 18 can be connected to a first motor assembly 20A for rotation of support assembly 18 along the central, longitudinal axis x of stent 14. A second motor assembly 20B can be additionally provided for translational movement of support assembly 18 in a linear direction, back and forth, along a railing 22. The rotational and translational motion of stent 14 during the application of the composition can result in a more uniform deposition of the coating. [0013] An applicator 24, such as a spray valve, penetrates through the wall of pressure chamber 10 and is positioned in the vicinity of stent 14. Commercial applicators are available from Spray Systems Co., EFD International Inc., and Badger Air-Brush Co., one specific model of which is the EFD 780S spray device with VALVEMATE 7040 control system. To avoid spray rate alterations due to the pressure difference, applicator 24 can be placed entirely within pressure chamber 10. The nose of applicator 24 can be positioned at any suitable distance away from stent 14, for example at about 1 cm to about 10 cm. An operator should be capable of adjusting the distance depending on the particular circumstances of the deposition process. Applicator 24 is capable of applying the composition at a pressure of, for example, about 10 torr to about 1000 torr. In accordance with an alternative embodiment, support element 18 can be in a vertical position and applicator 24 spraying in a horizontal direction. [0014] A pressure controller such as a pump 26 is in fluid communication with workspace 12 so as to create pressures below or above 760 torr (1 atm) in pressure chamber 10. In one embodiment, a cold trap 28 can be provided for preventing the solvent or condensation from penetrating into pump 26 should pump 26 be used to create a vacuum in pressure chamber 10. A filter 30, such as a mist filter, can also be provided to prevent droplets of coating composition from possibly reaching and damaging pump 26. Other components of pressure chamber 10 can include a throttle valve 32 for opening and closing the communication line to pump 26, a baratron vacuum gauge 34 for measuring the pressure in workspace 12 independent of the type and composition of the solvent vapor, and an absorbent 36 for capturing the bulk of the composition over-spray. Gas, such as air, can be pumped or bled into pressure chamber 10 for creating a convection flow inside pressure chamber 10, to actively scavenge the solvent vapor from workspace 12 and out through pump 26 so as to prevent solvent vapor build-up. A diffuser 38 can be used to diffuse or "spread out" the flow of gas so as to minimize disturbance of the spraying process. A bleed valve 40 can be used for adjusting the flow rate of gas through diffuser 38. In addition to rapidly removing the solvent vapor from pressure chamber 10, bleed valve 40 can also be used to control the chamber pressure by working in concert with throttle valve 32. [0015] Pressure chamber 10 can also be connected to a heating and/or cooling source 44 so as to control the temperature of workspace 12. A cooler deposition environment, such as temperatures of less than 50.degree. C. may be preferred depending on the chemical stability of the therapeutic substance and the type solvent used. In lieu of providing and external heating source, an internal component, such as heating and/or cooling coils, can be provided. Method of Applying the Composition [0016] To form a coating on a surface of stent 14, the surface of stent 14 should be clean and free from contaminants that may be introduced during manufacturing. However, the surface of stent 14 requires no particular surface treatment to retain the applied coating. Stent 14 is mounted on mandrel 18 and the composition is sprayed via applicator 24 at a pressure of, for example between 10 to 1000 torr. During the spraying of the composition, stent can be rotated at about 1 to about 120 rotations per minute. Stent 14 can also be moved in a linear direction at speed of about 1 to about 20 cm/sec. The temperature of chamber 10 should be maintained at a temperature that does not adversely affect the therapeutic substance or the coating process--for example at about 20.degree. C. to about 50.degree. C. [0017] For a solvent having a low vapor pressure (e.g., below 30 torr at the temperature of application), or in other words non-volatile substances, the solvent evaporates very slowly from the composition, leading to irregularities in the coating thickness and "cob webs" or "pool webs" between the stent struts. Accordingly, compositions have been applied in short bursts, interrupted by the drying of the composition between each application step to minimize coating defects. Reducing the pressure of chamber 10 below ambient pressure during the coating process allows the solvent to evaporate more rapidly. Rapid evaporation of the solvent allows the composition to be applied continuously for depositing a coating of a suitable thickness or weight while minimizing coating defects such as "pool webs." The pressure employed in pressure camber 10 depends on the type of solvent employed. Table 1 is an exemplary list of non-volatile solvents and the suitable range of pressure which can be used in the process of the present invention: TABLE-US-00001 TABLE 1 Exemplary Pressure Ranges Solvent torr @ 20.degree. C. Dimethylsulfoxide 0.8-<760 Dimethlacetamide 0.9-<760 Dimethylformamide 5.4-<760 Continue reading... Full patent description for Method for coating implantable devices Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Method for coating implantable devices patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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