Patent Application
20080299337
This invention broadly relates to means for modifying surfaces by deposition and etching, and more specifically, to means for creating structures and materials selectively on the inside surfaces of medical devices to render the devices biocompatible, to provide drug elution capability and/or to promote cell growth on and cell attachment to the modified surface.
Many medical devices, such as stents and stent grafts, are designed and manufactured to be inserted into the wall or lumen of a blood vessel. When this is done, complications may arise from the body's natural reaction to a foreign object. For example, inserting a stent into a blood vessel may cause the growth of an undesirable thick layer of smooth muscle tissue, and this new growth may cause restenosis, or re-narrowing of the vessel. The effects of restenosis are often minimized through the use of drug eluting stents, in which a medicated coating on the stent prevents tissue growth for a period of time. Thrombus formation is another serious condition that may occur after insertion of a stent, and recent studies have shown that current drug eluting stents can not prevent, and may even promote, thrombosis formation. See, for example, Windecker, S. et al. Randomized Comparison of a Titanium-Nitride-Oxide-Coated Stent With a Stainless Steel Stent for Coronary Revascularization, Circulation, 111:2617-2622 (2005).
The inner surface of a healthy blood vessel is lined with endothelial cells, which play an important role in controlling thrombosis, inflammation and other factors. It has generally been found that endothelial cells do not readily attach to the smooth inner surfaces of electropolished metal stents or to the polymers typically used for drug eluting stents. U.S. Pat. No. 6,140,127 discusses the desirability of having endothelial cells attach to the inner walls of stents, and overcomes the previously described attachment issue by using an adhesion specific peptide. Similarly, U.S. Pat. No. 6,478,815 discusses means for overcoming the attachment issue, however in this instance a stent is made primarily of niobium which can be coated with iridium oxide or other materials to promote the growth of endothelial cells. Additionally, a roughened surface on a stent has been proposed as a further means for promoting cell growth on a stent. For example, U.S. Pat. No. 6,820,676 B2 and United States Patent Application Publication No. 2005/0232968 discuss the role of surface inhomogeneities and surface structures in promoting endothelial cell growth.
While the growth of endothelial cells on the inner surface of a stent is highly desirable, the growth of smooth muscle tissue at the inner wall of the blood vessel, i.e., the portion in contact with the outer surface of the stent, is undesirable. It has been found that stents coated entirely with a drug imbibed polymer layer designed to prevent growth of smooth muscle tissue have been highly successful in reducing in-stent restenosis. Unfortunately, the smooth polymer surface also inhibits endothelial cell growth on the inside of the stent. For example, the use of a drug eluting coating on the outer surface of stents is taught in United States Patent Application Publication No. 2006/0200231, however tailoring the properties of the inner surface for endothelial cell growth is not addressed. Stents having outer and inner surfaces which function differently would overcome the defects described supra.
Many references that discuss surfaces to control cell growth, i.e., to enhance cell growth in the case of endothelial cells or suppress cell growth in the case of smooth muscle cells, are based on plasma processing and physical vapor deposition. As stents have a generally open structure, when they are coated or treated in a plasma environment both inner and outer surfaces typically receive the same or very similar coatings or treatments. United States Patent Application Publication No. 2006/0200231 describes a well-know means of coating only the outside surface of an object like a stent. The stent is placed on a mandrel which prevents the inner surfaces from receiving a coating while the outer surface is coated. Heretofore, nothing in the prior art suggests a means for plasma treating or coating only the inner surface of a medical device such as a stent, while leaving the outer surface largely unaltered, or allowing the outer surface to receive a different coating or treatment.
As can be derived from the variety of devices and methods directed at coating and treating implantable medical devices, many means have been contemplated to accomplish the desired end, i.e., surface specific coatings wherein a first surface promotes cell growth thereon and a second surfaces prevents cell growth thereon. Heretofore, tradeoffs between preventing cell growth on one surface and promoting cell growth on another surface were required. Thus, there is a long-felt need for a method to treat or coat only the inner surfaces of medical devices such as shunts, stent-grafts and stents, as a means of preparing the inner and outer surfaces of such devices so that they function differently.
The present invention broadly comprises a method of modifying a surface to produce surface structures, coatings and inhomogeneities in order to promote cell growth on and/or attachment to the surface for a variety of applications. Generally, the subject invention includes plasma deposition and removal processes to produce nanometer scale surface structures and coatings primarily on the inner surfaces of devices having both inner and outer wall surfaces, e.g., stents, stent-grafts and shunts. Specifically, the invention includes methods for producing plasma glow discharges on the inside of medical devices.
The present invention also broadly comprises a method of manufacturing a medical device having interior and exterior surfaces, the method includes the steps of: a) shielding the exterior surface; and, b) exposing the interior surface to a plasma, wherein the shielding of the exterior surface substantially prevents exposure of the exterior surface to the plasma. In some embodiments, the medical device further includes a first cross-sectional shape; while the step of shielding the exterior surface further includes the step of: contacting the exterior surface of the medical device with an inner surface of a hollow electrically conducting tube, the inner surface having a second cross-sectional shape substantially similar to the first cross-sectional shape; and, the step of exposing the interior surface to the plasma further includes the step of: igniting a hollow cathode discharge within the hollow electrically conducting tube. In other embodiments, the step of exposing the interior surface to the plasma further includes the step of: simultaneously sputtering the tube and the medical device. In some of these embodiments, the step of simultaneously sputtering the tube and the medical device modifies the interior surface of the medical device to include an inhomogeneous surface having at least two materials, while in some of these embodiments, the inhomogeneous surface includes a plurality of individual regions and each of the individual regions includes at least two materials and is separated from others of the individual regions by a material boundary. In still yet other embodiments, the step of exposing the interior surface to the plasma further includes the step of: cooling the hollow electrically conducting tube.
In further embodiments of the present invention, the medical device further includes a first cross-sectional shape; while the step of shielding the exterior surface further includes the step of: contacting the exterior surface of the medical device with an inner surface of a hollow electrically insulating tube, the inner surface having a second cross-sectional shape substantially similar to the first cross-sectional shape; and, the step of exposing the interior surface to the plasma further includes the step of: igniting a discharge within the hollow electrically insulating tube using a radio frequency power. In some of these embodiments, the radio frequency power includes a capacitively coupled radio frequency field, while in others of these embodiments, the radio frequency power includes an inductively coupled radio frequency field. In some embodiments, the step of exposing the interior surface to the plasma further includes the step of: cooling the hollow electrically insulating tube.
In yet further embodiments of the present invention, the medical device further includes a first cross-sectional shape; while the step of shielding the exterior surface further includes the step of: contacting the exterior surface of the medical device with an inner surface of a hollow electrically insulating tube, the inner surface having a second cross-sectional shape substantially similar to the first cross-sectional shape; and, the step of exposing the interior surface to the plasma further includes the step of: igniting a discharge within the hollow electrically insulating tube using a microwave power. In some embodiments, the step of exposing the interior surface to the plasma further includes the step of: cooling the hollow electrically insulating tube.
In still yet further embodiments, the step of exposing the interior surface to the plasma is performed in an inert gas, while in other embodiments, the step of exposing the interior surface to the plasma is performed in a reactive gas selected from the group consisting of: oxygen, nitrogen, methane and mixtures thereof. In still other embodiments, the step of exposing the interior surface to the plasma is performed in a precursor gas, and the precursor gas is selected to deposit a coating on the interior surface, and in some of these embodiments, the precursor gas is selected from the group consisting of: a hydrocarbon, a metal containing compound, oxygen, nitrogen and mixtures thereof. In some embodiments, the coating includes a plurality of clusters and each of the clusters includes a lateral dimension from about ten nanometers to about one thousand nanometers. In other embodiments, each of the clusters have a size and a distance from others of the clusters, and in some of these embodiments, the size of each of the clusters and the distance from others of the clusters are chosen to preferentially bind at least one biological structure having a specific size.
In yet further embodiments, the step of exposing the interior surface to the plasma removes material from the interior surface of the medical device, while in other embodiments, the present invention method further includes the step of: c) coating at least the interior surface of the medical device with a biodegradable polymer after the step of exposing the interior surface to the plasma. In some embodiments, a medical device is constructed according to the present invention method.
The present invention further broadly comprises a medical device having an interior surface, an exterior surface and means for exposing the interior surface to at least one plasma. In some embodiments, the at least one plasma includes a first plasma and a second plasma, the first plasma deposits a plurality of clusters on the interior surface and the second plasma etches the interior surface. In other embodiments, the first and second plasmas produce a plurality of surface structures on the medical device. In some of these embodiments, each of the surface structures includes a lateral dimension from about ten nanometers to about one thousand nanometers, while in others of these embodiments, each of the surface structures includes a height from about one hundred nanometers to about ten thousand nanometers. In some embodiments, each of said clusters includes a size and a distance from others of the clusters, and in other embodiments, the size of each of the clusters and the distance from others of the clusters are chosen to preferentially bind at least one biological structure having a specific size.
It is a general object of the present invention to provide a medical device including an interior surface having different characteristics than the device's exterior surface.
It is another general object of the present invention to provide a medical device having an interior surface which includes surface structures, coatings and/or inhomogeneities.
It is yet another object of the present invention to provide a method of producing a plasma glow discharge on the inside of a medical device while substantially shielding the outside of the device from such discharge.
These and other objects and advantages of the present invention will be readily appreciable from the following description of preferred embodiments of the invention and from the accompanying drawings and claims.
The nature and mode of operation of the present invention will now be more fully described in the following detailed description of the invention taken with the accompanying drawing figures, in which:
a is a cross sectional view of an arrangement for capacitively coupling RF power into a tube to produce a plasma;
b is a cross sectional view of an arrangement for inductively coupling RF power into a tube to produce a plasma;
At the outset, it should be appreciated that like drawing numbers on different drawing views identify identical, or functionally similar, structural elements of the invention. While the present invention is described with respect to what is presently considered to be the preferred aspects, it is to be understood that the invention as claimed is not limited to the disclosed aspects.
Furthermore, it is understood that this invention is not limited to the particular methodology, materials and modifications described and as such may, of course, vary. It is also understood that the terminology used herein is for the purpose of describing particular aspects only, and is not intended to limit the scope of the present invention, which is limited only by the appended claims.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs. Although any methods, devices or materials similar or equivalent to those described herein can be used in the practice or testing of the invention, the preferred methods, devices, and materials are now described.
Adverting now to the figures,
It is well known in the art of plasmas and plasma deposition that it is possible to produce a glow discharge inside of a tube, even a tube with a diameter of 1 millimeter (mm) or less, for example, using hollow cathode discharges. As one of ordinary skill in the art appreciates, hollow cathode discharges are primarily used as sources of electrons for a variety of applications such as ion beam neutralization, plasma enhancement and electron beam evaporation.
The general principal of operation of hollow cathode discharge system 22 is that electrons 32 emitted from inner surface 34 of tube 24 are confined by reflections at the opposite wall and effectively produce ions 36 in the gas flowing in tube 24 until electrons 32 exit end 38 of tube 24 and are collected by anode 28. Systems similar to hollow cathode discharge system 22 have been used to deposit material and plasma treat surfaces. See, e.g., U.S. Pat. No. 5,716,500 which describes the use of a hollow cathode discharge system as a source of coating material. Systems similar to hollow cathode discharge system 22 are usually operated at sub-atmospheric pressures, but it is also possible to operate some hollow cathode discharge systems at atmospheric pressures. See, e.g., “Characterization of Hybrid Atmospheric Plasma in Air and Nitrogen,” 49th Annual Technical Conference Proceedings of the Society of Vacuum Coaters, 2006. Known methods of using hollow cathode discharge systems include placing a substrate to be coated or modified outside of the hollow cathode tube, e.g., tube 24. Contrarily, in the present invention, a substrate to be treated or coated lines the inside wall of the hollow cathode discharge system, i.e., inner surface 34 of tube 24, making the substrate an electrode in the plasma discharge system. Although the extremely small discharge volume in typical hollow cathode discharge systems limits their usefulness for etching or depositing on most substrates, their very size and shape make them ideal for etching or depositing on the inner surface of small objects having generally cylindrical shapes, such as stents, grafts and shunts.
Various methods exist for using the present invention to treat or coat inner surface 40 of stent 42 or other medical devices having inner and outer surfaces. For example, a precursor gas such as methane or acetylene could be used alone or in combination with other gases such as argon to produce a carbon containing coating on inner surface 40. The formation of a coating by a plasma discharge in a precursor gas, or plasma enhanced chemical vapor deposition (PECVD) is well know in the art and many precursor gases, such as hexamethyldisiloxane, tetrafluoroethylene, and those containing metals such as titanium isopropoxide can be used.
Alternatively, the hollow discharge tube, e.g., tube 44 shown in
If it is desired to simply expose the inner surface of a device such as a stent to the energetic ion bombardment, for example to roughen the device or plasma activate the device for further processing, the hollow cathode discharge system tube can be made of a biocompatible, low sputter yield material, e.g., carbon. Because the device is biased at a negative voltage with respect to the anode, it will be impacted by ions that have been accelerated to high energy. Therefore, the surface of the device can be aggressively plasma etched, a coating can be put down with PECVD, or both can be done simultaneously.
In addition to a hollow cathode discharge, it is possible to create a plasma on the inside surface of a medical device by other means. For example, an inductively or capacitively coupled radio frequency (RF) field can produce a glow discharge on the inside surface of an electrically insulating tube. The tube must have a low enough conductivity that the RF fields are not shielded from the interior portion. A gas, which can be inert or can contain a precursor for depositing a coating, can flow through the tube. In this case, because the stent or device may itself shield the interior of the tube from the RF fields, the treatment or deposition can take place remotely from where the power is coupled.
Alternatively, microwave power can be used to produce a discharge. In this instance, the tube that holds the medical device can be inserted into a microwave cavity, also known as a waveguide, in a manner well known to those of ordinary skill in the art.
It should be appreciated that the present invention method may be used to produce large numbers of devices simultaneously. For example, a number of stents can line the inside of a long tube and be coated or treated at one time. Alternatively, an array of shorter tubes, as shown in the cross sectional view in
The inventive method of the present invention can be used in a variety of ways to alter the interior surfaces of medical devices. For example, it is possible to create an inhomogeneous surface by depositing a discontinuous coating of atoms of a first substance on a substrate comprising a second substance. In some embodiments, the substrate can then be etched via physical sputtering, while in other embodiments, the steps of depositing and etching are performed simultaneously. This deposition and etching sequence is described in U.S. Patent Application Nos. 60/771,834 and 11/704,650, which applications have been incorporated herein by reference and form the basis of priority for this application. In further embodiments, the discontinuous coating of atoms forms a plurality of clusters, each of the plurality of clusters having lateral dimensions from about ten nanometers to about one thousand nanometers. In yet further embodiments, the inhomogeneous surface includes a plurality of structures, each of the structures having heights from about ten nanometers to about ten thousand nanometers. The above described embodiments of the present invention are shown in
Ultra thin coatings deposited using physical vapor deposition, or in other words those layers having average thicknesses from less than a monolayer, i.e., a single atomic layer, to tens of monolayers, do not ordinarily condense as a uniform coating. Rather, the atoms nucleate as clusters whose size and spacing are determined by such factors as substrate temperature, chemical binding energy between the coating and substrate, energy of the arriving atoms, etc. Therefore, the average height of these clusters may be significantly greater than the average thickness of the overall coating, while the regions between the clusters are merely bare substrate material. The instant invention makes use of differences in etch rates that can exist between such clusters and the underlying substrate material, in order to produce structures that have dimensions of tens to hundreds of nanometers in breadth and height in and on the substrate.
In the embodiment shown in
Although coating a substrate with Al2O3 is described in the foregoing embodiment, one of ordinary skill in the art will recognize that a wide variety of coating materials may be used, e.g., metals, oxides, nitrides and alloys, and such variations are within the spirit and scope of the claimed invention. However, it has been found that metal oxides such as Al2O3 as well as oxides of Titanium (Ti), Molybdenum (Mo), Niobium (Nb), Chromium (Cr) and others have very low sputter yields and are, therefore, particularly advantageous when used for coating a substrate. Such materials are good candidates for producing randomly spaced clusters of atoms on a nanometer scale, such as Al2O3 clusters 112. Hereinafter, such nanometer scale coatings are referred to as a “nanomask.”
As those skilled in the art will appreciate, the nanomask, e.g., Al2O3 clusters 112 may be deposited using a source of the mask material or may be deposited reactively by, for example, sputtering a metal in a chamber containing oxygen (O2), nitrogen (N2), or some other compound forming gas. Any number of well-known means, such as sputtering, cathodic arc evaporation, thermal evaporation and chemical vapor deposition can deposit discontinuous clusters 112. As mentioned previously, the deposition conditions strongly affect clusters 112 size and spacing, and conditions are chosen which produce the desired results.
For the purposes of bone growth, nucleation characteristics resulting in a discontinuous coating of clusters 112 having diameters from about several nanometers to about several hundreds of nanometers, and heights from about several nanometers to about several hundreds of nanometers, have been found to be particularly advantageous. The dimensions of resulting structures 120 of course still depend on the ratio of the etch rate of substrate 114 to the etch rate of clusters 112. Although the aforementioned embodiment is described in terms of preferentially bonding to bone, one of ordinary skill in the art will recognize that a substrate have clusters of different dimensions than previously set forth will preferentially bond to other types of cells, and such variations are within the spirit and scope of the claimed invention. In a preferred embodiment, resulting structures 120 have lateral dimensions, i.e., diameters 122, from approximately ten (10) to several hundreds of nanometers across and heights 124 from approximately ten (10) to ten thousand (10,000) nanometers.
The height H of a given resulting structure 120 will be:
H=R×h,
Where h is the height of the initial cluster 112 that produced structure 120 and R is the ratio of the etch rate of substrate 114 to the etch rate of cluster 112. Of course, a given cluster 112 will not have a single height, but will be domed or otherwise irregular, and therefore, the resulting structure 120 may also be irregularly shaped. For example, as is well known from published sputter yields for Al2O3 and Ti, an Al2O3 nanomask deposited on a Ti substrate and sputtered using 500 electron volts (eV) under Argon (Ar) will result in a ratio R of approximately 17. Therefore, if a nanomask cluster of atoms had a height h of 10 nanometers, the height H of the resulting structure would be approximately 170 nanometers.
In order to control the nucleation characteristics of the nanomask coating, it is possible to change the chemical binding energy between substrate 114 and the coating material, e.g., Al2O3. For example, a very thin layer of a material having weak chemical bonding with the nanomask material, such as a hydrocarbon, may be deposited onto the substrate prior to the deposition of the coating material. Such a low energy coating, as it is known, will result in fewer, larger nuclei of the nanomask material, clusters 112. Alternatively, it is possible to use plasma cleaning as an integral part of the coating process to change the nucleation characteristics. In that case, an initial high voltage can be applied to substrate 114 in order to clean substrate 114 and remove any residual contamination. This cleaning may be done with the deposition source off or it may be carried out during the initial stages of deposition. Times for such cleaning may range from less than a minute to several minutes.
For purposes of cell attachment, coated substrate 116 may not require etching in order to form preferred sites for cell growth. In certain cases, it is possible that material boundaries formed between substrate 114 and clusters 112 will produce enough of discontinuity in surface characteristics to stimulate the attachment of cells at the locations of clusters 112 and/or therebetween clusters 112. It has been found, for example, that material boundaries on such scales may result in relatively large local electric fields, which may enhance the attachment of biological materials at those locations. For example, a discontinuous coating of Gold (Au) on Ti may result in large chemical potentials at the boundaries of the two materials that stimulate biological materials, such as proteins, to locate preferentially at those boundaries. As one of ordinary skill in the art will appreciate, other types of dissimilar materials are also candidates for such nanoscale coating clusters, and such variations are within the scope of the claimed invention.
Clusters 112 may be deposited on otherwise smooth portions of substrate 114 or it is also possible to form clusters 112 on the surfaces of a sintered powder, thereby creating a surface with two roughness scales. In addition, if clusters 112 are porous they may be infused with bioactive materials, such as superoxide dismutuse to inhibit inflammation or proteins to promote bone growth.
As described supra, once clusters 112 are deposited on substrate 114, thereby forming coated substrate 116, structures 118 can be produced by etching coated substrate 116. Any etching known in the art may be used, such as reactive or non-reactive ion etching. For example, introducing an inert gas such as Argon at a pressure from approximately one (1) mTorr to one hundred (100) Torr, and applying a voltage to coated substrate 116 that is high enough to cause physical sputtering, typically between one hundred (100) and one thousand (1000) volts (V), will result in the desired etching. The sputtering voltage may be direct current (DC), pulsed DC, radio frequencies (RF) in the megahertz range, or an intermediate frequency, i.e., alternating current (AC), and such voltage should be applied under conditions that produce a glow discharge. The gas used may be inert, such as Ar, or can be chosen to accentuate the difference in sputtering rates between clusters 112 and substrate 114. For example, if clusters 112 are a metal oxide and substrate 114 is a polymer, it is known in the art that a plasma containing O2 will etch the polymer very quickly while etching the metal oxide slowly. Such a process is known as reactive ion etching and relies on chemical processes as well as physical bombardment to remove material.
The above described etching processes are common in the electronics industry, where etch masks are routinely used to produce specific desired patterns in integrated circuits, for example. However, in those cases the patterns that define the final structure are made using lithography, which is an expensive process. In the method of the instant invention, the patterns are formed on the surfaces of implantable devices by choosing deposition conditions that form a random pattern of clusters of atoms, and therefore is far more cost effective and simple to perform than lithography processes.
The deposition of clusters 112 and subsequent etching of coated substrate 116 may be done in one continuous operation, or may be performed sequentially. An example of a continuous operation is depositing Al2O3 clusters 112 onto Ti substrate 114 using RF sputtering. During deposition of clusters 112, a voltage may also be applied to substrate 114. The voltage should be kept low enough that it will not cause clusters 112 to be removed faster than they are deposited. However, once clusters 112 are properly deposited on substrate 114, the voltage may be increased to cause sputtering of both clusters 112 and substrate 114 in such a way that there is a net removal of material, and the formation of nanostructures 120 as described above. It has been found that using RF sputtering to deposit clusters 112 is a relatively inefficient deposition process. That is, a relatively intense RF plasma is needed to produce even a small deposition rate of a nanomask material such as Al2O3. However, because the nanomask material is so thin on average, a low deposition rate is often acceptable. The advantage of using RF sputtering arises once the nanomask is deposited. By leaving the RF power on and applying a DC voltage to coated substrate 116, the intense RF plasma provides a dense source of ions which are available to etch coated substrate 116. In other words, applying a DC voltage to coated substrate 116 in the presence of RF plasma will produce a far greater etch rate than applying the same voltage in the absence of RF plasma. Even though there are still sputtered atoms arriving at coated substrate 116, they are removed as quickly as they arrived by the combined effect of the dense plasma and high substrate voltage.
Alternatively, the deposition and etching steps may be sequential. If both steps are accomplished using sputtering, this may be accomplished by simply turning off the power to the deposition source of clusters 112 and turning on the power to substrate 114. Or alternatively, the deposition and etching steps may take place in separate chambers.
It should be appreciated the above described sputtering of the hollow tube and medical device contained therein may occur simultaneously, and an example of such is shown in
Furthermore, the present invention method allows for the creation of different surfaces on the inside and outside of medical devices, e.g., stents, which serve different purposes. For example, it may be possible to first deposit a material only on the outside of the medical device that enhances the biocompatibility of that surface with respect to a lumen wall. This could be done using conventional deposition techniques such as sputtering, evaporation, spray coating, plasma polymerization or others while using a mandrel to prevent coating on the interior surface of the device. In a separate operation, the present invention method could be used to create another surface on the inside of the medical device that serves an alternative purpose, for example, biocompatibility with blood rather than tissue or promotion of endothelial cell growth via a rough surface or inhomogeneous surface.
In some instances, it may be useful to use a drug that prevents cell growth for a period of time in combination with a medical device whose inner surface has been altered so that it promotes endothelial cell growth. In these instances, the textured inner surface may cause platelet attachment, which is undesirable, during the period of time when the drug is preventing cell growth. It has been found that this issue can be addressed by coating at least the inner surface of the medical device with a biodegradable polymer. The smooth surface of the polymer suppresses platelet attachment while the drug acts to prevent cell growth. When the polymer is gone, i.e., has degraded, and the drug no longer acts to prevent cell growth, the surface of the medical device that promotes endothelial cell growth is then exposed and becomes effective.
A further advantage of the present invention relates to controlling the temperature of medical devices during their coating or treatment. For example, if the inside diameter of the hollow cathode or discharge tube is slightly smaller than the outside diameter of the device, the device will remain in intimate contact with the tube during processing. Therefore, if the tube is cooled, for example by a circulating liquid, the medical device can also be cooled during processing. This is particularly important for medical devices made of a nickel/titanium alloy known as Nitinol. Nitinol has the unusual properties of superelasticity and shape memory which result from the fact that Nitinol exists in a martensitic phase below a first transition temperature, known as Mf, and an austenitic phase above a second transition temperature, known as Af. Both Mf and Af can be manipulated by altering the ratio of nickel to titanium in the alloy as well as changing the thermal processing of the material. In the martensitic phase, Nitinol is very ductile and easily deformed, while in the austenitic phase Nitinol has a high elastic modulus. Applying stresses to materials at temperatures above Af produces some martensitic materials, however when the stresses are removed, the material returns to its original shape. This results in a very springy behavior for Nitinol, referred to as superelasticity or pseudoelasticity. Furthermore, if the temperature is lowered below Mf and the Nitinol is deformed, raising the temperature above Af will cause the Nitinol to recover its original shape. This property is described as shape memory.
It is well known that if Nitinol is raised to too high a temperature for too long of a period of time, the Af value will rise. Additionally, sustained temperatures above 300-400 degrees Centigrade will adversely affect typical Af values used in medical devices. Likewise, if stainless steel is raised to too high a temperature, it can lose its temper, while other materials would also be adversely affected by exposure to such conditions. Therefore, the time-temperature history of a medical device during a coating operation is critical. In view of the foregoing, the present invention allows the temperature of a device to be controlled directly while uniformly treating or coating its interior surface.
It should also be appreciated that the present invention method can also be used to selectively remove material from the interior surfaces of medical devices. For example, many polymer deposition processes used to coat devices are conformal, i.e., a process of spraying a dielectric material onto a device to protect it from moisture, fungus, dust, corrosion, abrasion, and other environmental stresses. Parylene, which is widely used as a coating material, is deposited by polymerizing a monomer vapor, and thereby coating parylene on all exposed surfaces. As has been discussed above, it may be desirable to remove such a polymer coating from the interior surface while leaving it on the exterior surface. Thus, the present method can be used to plasma etch a polymer using an oxygen containing plasma, thereby removing it from the interior surface while leaving it on the exterior surface as desired.
Thus, it is seen that the objects of the present invention are efficiently obtained, although modifications and changes to the invention should be readily apparent to those having ordinary skill in the art, which modifications are intended to be within the spirit and scope of the invention as claimed. It also is understood that the foregoing description is illustrative of the present invention and should not be considered as limiting. Therefore, other embodiments of the present invention are possible without departing from the spirit and scope of the present invention.
This application is a Continuation-in-Part of application Ser. No. 11/704,650, filed on Feb. 9, 2007, which application claims the benefit of Provisional Application Ser. No. 60/771,834, filed Feb. 9, 2006, which applications are each incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60771834 | Feb 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11704650 | Feb 2007 | US |
| Child | 12221612 | US |
