Patent Publication Number: US-2009220675-A1

Title: Apparatus for producing a biomimetic coating on a medical implant

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of co-pending U.S. patent application Ser. No. 10/795,758, filed on Mar. 8, 2004, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention pertains to a coating for an implantable article, and more particularly, to an apparatus and method for producing a coating on an implantable article. 
     BACKGROUND OF THE INVENTION 
     Mineralized and/or ceramic coatings are applied to a variety of articles. For example, such coatings are frequently applied to biological implants (e.g., medical implants). In the case of biological implants, the coating is usually applied to a metal or plastic substrate, but the coating can be applied to other substrates as well such as ceramic and silicon. 
     Biological implants, such as joint and dental prostheses, usually must be permanently affixed or anchored within bone. In some instances it is acceptable to use a bone cement to affix the prosthesis within bone. In the case of many joint prostheses, however, it is now more common to affix the joint prosthesis by encouraging natural bone ingrowth in and around the prosthesis. The bone-to-implant interfaces that result from natural bone ingrowth tend to be stronger over time and more permanent than are bone cement-prosthesis bonds. 
     Although various materials, including titanium alloys, are biocompatible, they are not necessarily bioactive because they can neither induce bone formation nor form chemical bonds with bone. However, providing irregular beaded or porous surfaces on the implant can enhance bone ingrowth and prosthesis fixation. For example, coating the implant with a bioactive mineralized and/or ceramic material can attain enhanced fixation of implants within bone. Such coatings have been shown to encourage more rapid bone ingrowth in and around the prosthesis. The ability of such coatings to induce bone formation can be improved by incorporating a biological agent into the bores of the ceramic coating. 
     Various techniques are used to apply such mineralized and/or ceramic coatings to bioimplantable substrates including using plasma spraying, ion implantation and sol-gel processing. However, each of these methods has some drawbacks. For instance, the applied coatings can be relatively thick and brittle and may not adhere well to the implant. Moreover, it can be difficult to apply the coatings on surfaces having complex geometries using these techniques resulting in uneven coating coverage and even uncoated areas. Using low temperature aqueous solutions can alleviate the problems associated with applying such coatings to implants having complex geometries. For example, an aqueous solution can be used to form a hydroapatite coating on a substrate. However, currently known low temperature processes typically require pretreatment of the substrate. These processes are also difficult to control so as to achieve a consistent uniform coating on the implant. 
     Accordingly, despite the existence of a variety of apparatuses and processes for producing biological agent containing coatings on implantable articles, there remains a need for an improved apparatus and process that can reliably achieve a consistent uniform coating on the implant. The invention provides such an apparatus and process. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides an apparatus for producing a biomimetic coating on an implantable article. The apparatus includes an outer reactor for containing a biomimetic coating solution and an inner reactor arranged within the outer reactor. The inner reactor is smaller than the outer reactor so as to define a space between the inner and outer reactors. The apparatus also includes a pump system for circulating the biomimetic coating solution. The pump system is arranged to operate in the space between the inner and outer reactors. The inner reactor includes a flow control system that is selectively operable to control the flow of biomimetic coating solution between the outer and inner reactors. 
     The invention further provides a method for producing a biomimetic coating on an implantable article. The method includes the step of filling an outer reactor and an inner reactor arranged within the outer reactor with a biomimetic coating solution. A flow control system associated with the inner reactor is moved to an open position so as to permit unrestrained flow of biomimetic coating solution between the outer and inner reactors. The biomimetic coating solution is mixed and heated to a desired homogeneity and temperature through a pump system that operates in a space between the outer and inner reactors. An implantable article is inserted in the inner reactor and the flow control system associated with the inner reactor is moved to a closed position so as to limit flow of biomimetic between the outer and inner reactors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a perspective view of an illustrative apparatus for applying a biomimetic coating on an implantable article in accordance with the invention. 
         FIG. 2  is a side view of the coating apparatus of  FIG. 1 . 
         FIG. 3  is another side view of the coating apparatus of  FIG. 1 . 
         FIG. 4  is a top view of the coating apparatus of  FIG. 1 . 
         FIG. 5  is a perspective view of the coating apparatus of  FIG. 1  showing some of the flow control apertures closed by stoppers. 
         FIG. 6  is a perspective view of an alternative embodiment of a coating apparatus according to the present invention. 
         FIG. 7  is a perspective view of the coating apparatus of  FIG. 6  showing one of the inner reactor walls partially removed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     Referring more particularly to  FIGS. 1-3  of the drawings, there is shown an illustrative apparatus  10  for producing a coating on a substrate, in this case an implantable article, in accordance with the present invention. More specifically, and as described in greater detail below, the illustrative apparatus  10  is capable of producing biomimetic coatings, such as for example a biomimetic apatite coating, on implantable articles. Specific details regarding such coatings and methods for preparing such coatings can be found in commonly owned U.S. application Ser. No. 10/355,827, the disclosure of which is incorporated herein by reference. While the present invention is described in connection with the illustrative process of producing a biomimetic apatite coating on an implantable article, it will be readily appreciated that the invention is equally applicable to other biomimetic surface coating processes or other processes for producing bioactive surface or ceramic coatings with or without a biological agent. Moreover, the present invention could be applied in contexts such as coating processes for substrates other than implantable articles. 
     The composition used to form the biomimetic coating is in solution form. The solution can comprise the various chemical components suspended or dissolved in a liquid carrier. In addition, the chemical components could include a biological agent that would improve the ability of the ability of the coating to induce or conduct bone formation. For example, the composition could include a biological agent, calcium ions, phosphate ions and a liquid carrier. The liquid carrier can be any suitable aqueous or non-aqueous liquid in which the necessary chemical components for forming the bioactive surface or ceramic coating can be suspended or dissolved for delivery to the implantable article or other substrate. Some examples of suitable liquid carriers include, water, tris-buffered saline, phosphate-buffered saline, and the like. 
     The process for producing the biomimetic coating generally includes a solution preparation phase and a coating formation phase. In the solution preparation phase, the various chemicals that make up the particular composition that will be used to form biomimetic coating are mixed together and heated to the desired temperature. The substrates to be coated, in this case implantable articles, are then added. The implantable articles are then incubated in the solution for a period of time. The incubation time can be any suitable or desired period of time, including up to 24 hours or more or even 72 hours or more. Similarly, the incubation can take place at any suitable temperature necessary for properly preparing the biomimetic coating. For example, the incubation could take place at between about 30° C. and about 50° C. Once the coating formation phase is completed, the implantable articles can be removed from the solution. Additional processing steps could include rinsing and/or drying of the implantable articles. As noted above, specific details regarding the composition of the solution and the coating process can be found in commonly owned U.S. application Ser. No. 10/355,827. 
     To ensure that the coating process achieves a consistent uniform coating on the implantable articles, the illustrated apparatus  10  permits the circulation and temperature of the biomimetic coating material, such as biomimetic apatite coating material, to be tightly controlled during both the solution preparation and coating formation stages of the process. Without such tight control, the coating formed on the implantable articles can be very uneven with areas where the coating is too thin or nonexistent and areas where the coating is too thick and blocks the porous structure. 
     In the illustrated embodiment, the coating apparatus  10  includes inner and outer reactors  12 ,  14  and an associated pump system  16 . The outer reactor  14  protects the integrity of the coating solution and helps maintain the temperature of the solution. In the illustrated embodiment, the outer reactor  14  has a generally rectangular configuration including two pairs of opposing sidewalls  18 , a bottom wall  20  and an open top (see, e.g.,  FIG. 1 ). However, the outer reactor  14  can have any suitable or desired configuration. As can be appreciated, the outer reactor should be large enough to hold the desired amount of solution. For example, in one preferred embodiment, the outer reactor has a 110 liter capacity. 
     The open top of the outer reactor  14  can be large enough to allow for removal of the inner reactor  12  and/or to provide access to the inner reactor  12  for cleaning. The opening in the top of the outer reactor  14  can also be large enough to allow for manual placement of the medical implants in the inner reactor  12 . The open top of the outer reactor can be covered by a removable lid  22  that sits firmly on the top of the outer reactor  14  in order to prevent excess evaporation of the solution. To accommodate external devices such as electrodes and temperature probes, the removable outer reactor lid  22  can have one or more apertures  24  therein (see, e.g.,  FIG. 4 ). The apertures  24  in the lid  22  can also be used to introduce or remove solution from the reactors  12 ,  14  without removing the entire lid. 
     The outer reactor  14  can be constructed of any suitable material including polymers or metals that can withstand the required temperatures for an extended period of time. Moreover, the material used to construct the outer reactor  14  should be strong enough to support the weight of the solution and inner reactor  12 . Similarly, the lid  22  can be made of any suitable material (polymers, metals, etc.) that is capable of withstanding the required temperature for a prolonged period of time without warping. 
     For removal of the solution from the reactors, the outer reactor  14  can be equipped with a drainage system including a drain and a drain valve  28 . The drain valve  28  can be manually operable and when opened would allow the solution to drain from the outer reactor  14  through a drain line to a sink or other receptacle for the used solution. The drain should be large enough to empty the reactors at a reasonable rate. However, the drain should be small enough to ensure that excess liquid does not collect in the tubing between the drain and the drain valve  28 . All the liquid in the reactors and the drain line either from the coating process or a cleaning operation should be drained completely to prevent future coating processes from becoming contaminated. 
     The pump system  16  essentially controls the apparatus and the coating process by controlling the temperature and circulation rate of the biomimetic coating solution throughout the solution preparation phase and the coating formation phase. In this case, the pump system  16  is provided on the outer reactor  14 . More particularly, the illustrated coating apparatus  10  includes two pump units  29  each of which is mounted on a corresponding bridge  30  that rests on the top of the outer reactor  14 . These pump units  29  control the circulation of the biomimetic coating solution within the apparatus  10  so as to ensure that the chemicals are thoroughly mixed during the solution preparation phase and the homogeneity and temperature of the coating solution is maintained during the coating formation phase. 
     In the illustrated embodiment, each of the pump units  29  comprises an electronically controlled immersion thermostat having an integrated pump. However, it will be appreciated that separate thermostats and pumps can be provided. The illustrated pump units  29  can be setup so as to maintain the biomimetic coating solution at a set temperature and circulation rate during the solution preparation phase and again during the coating preparation phase. Each pump unit  29  has a heating coil  32  that extends downward into the outer reactor and a pump housing  34  which is also disposed inside the outer reactor  14  (see  FIGS. 1 and 2 ). The heating coils  32  heat the biomimetic coating solution in the apparatus while the pump housings  34  provide the circulation of the solution in the apparatus  10 . 
     In this case, a discharge tube  36  is also connected to the pump housing  34  of each of the pump units  29 . These discharge tubes  36  extend downward inside the outer reactor  14  to weighted blocks  38  arranged at the bottom of the outer reactor  14  so as to direct the pump discharge along one of the sidewalls  18  of the outer reactor  14 . As will be appreciated, the pump system could be setup so that the pump units discharge in any particular direction so long as sufficient circulation of the biomimetic coating solution is maintained. The illustrated pump units  29  also include outlet lines  42  that extend into the outer reactor  14  and that can be used to introduce gases into the system via corresponding inlet lines. One example of a suitable pump unit for use in the apparatus of the present invention is the Ecoline E 100 immersion thermostat with integrated pump available from Lauda of Germany. Of course, any suitable pump and thermostat could be used. 
     The inner reactor  12  sits within the outer reactor  14  and receives and holds the implantable articles during the coating process. For example, a positioner or platform (not shown) can be provided in the inner reactor  12  to support the implantable articles. During the coating process, the inner reactor  12  serves as a buffer to protect the implantable articles being coated from turbulence caused by the pump system  16 . Specifically, the individual pump units  29  are arranged so as to operate within the space between in the inner and outer reactors  12 ,  14 . In particular, the heating coil  32 , pump housing  34 , discharge tube  36  and outlet line  42  associated with each of the pump units  29  is arranged in and operates in the area between the sidewalls of the inner and outer reactors  12 ,  14 . This arrangement helps insulate the implantable articles from turbulence or other forces caused by operation of the pump system  16 . 
     In the illustrated embodiment, the inner reactor  12  has a generally rectangular configuration consisting of two pairs of opposing sidewalls  44 , a bottom wall  46  and an open top that can be closed with a lid  47  (see  FIGS. 1-3 ). Each of the sidewalls  44  of the inner reactor  12  extends parallel to and is shorter in length than a corresponding sidewall  18  of the outer reactor  14 . The inner reactor  12  is shorter in height than the outer reactor  14  and is supported on legs  48  that space the bottom wall  46  of the inner reactor  12  above the bottom wall  20  of the outer reactor  14 . As a result, space is provided between the inner and outer reactors  12 ,  14  on all sides of the inner reactor including above and below the inner reactor. This allows for a steady flow of the biomimetic coating solution around all sides of the inner reactor, ensuring homogeneity of the solution and a stable temperature. It will be appreciated that the inner reactor can have any desired configuration and that the space between the inner and outer reactors can also have any desired configuration. 
     In order to have access to the inside of the inner reactor  12  for placement of the implantable articles, the lid  47  of the inner reactor  12  can be selectively removable. The lid  47  fits firmly on the top of the inner reactor  12  so that the lid will not moved as a result of circulation of the biomimetic coating solution during the coating process. In addition, in the illustrated embodiment, the inner reactor lid  47  has a plurality of apertures  50  formed therein (see  FIG. 1 ). These apertures  50  allow circulation of the biomimetic coating solution between the inner and outer reactors  12 ,  14 . Moreover, these apertures  50  can be made large enough to allow external devices such as electrodes or temperature probes to have access to the solution in the inner reactor  12 . The apertures  50  in the inner reactor lid  47  also provide access for removal and/or addition of biomimetic coating solution to and/or from the inner reactor  12  without disturbing the lid during the coating process. 
     As with the outer reactor  14 , the inner reactor  12  can be constructed of any suitable material including polymers or metals that can withstand the required temperatures for an extended period of time. Moreover, the material used to construct the inner reactor should be strong enough to support the weight of the solution. Likewise, the lid  47  of the inner reactor  12  can be made of any suitable material (polymers, metals, etc.) that is capable of withstanding the required temperature for a prolonged period of time without warping. 
     To allow the circulation of the biomimetic coating solution within the inner reactor  12  to be controlled, the inner reactor  12  can be configured with a selectively adjustable or closable flow control system that permits the flow between the inner and outer reactors  12 ,  14  to be selectively adjusted. It is generally preferable that there be more circulation of the biomimetic coating solution in the inner reactor  12  in the solution preparation phase and early coating formation phase than in the later stages of the coating formation phase. Thus, the inner reactor  12  is configured so that the flow between the inner and outer reactors  12 ,  14  is reduced during the late stages of coating formation. This reduces the circulation of the coating solution in the inner reactor  12  thereby enhancing late stage coating formation. However, even during these later stages of the coating process, there is some circulation in the inner reactor  12  in order to maintain homogeneity of the solution and a stable temperature. 
     In the embodiment illustrated in  FIGS. 1-5 , the flow control between the inner and outer reactors  12 ,  14  is achieved by providing selectively closable apertures  52  in the walls of the inner reactor. In particular, one or more apertures  52  can be provided in one or more of the sidewalls  44  of the inner reactor  12 . Each of these apertures  52  can be selectively opened or closed by a closure device such as a tapered stopper  53  as shown in  FIG. 5 . In this case, a plurality of apertures  52  are provided in each of the sidewalls  44  of the inner reactor  12  and in the bottom wall  46  of the reactor. The apertures in the bottom wall  46  of the inner rector are also used in draining the system. As noted previously, the lid  47  of the inner reactor  12  also has a plurality of apertures  50  therein. The number, size and configuration of the apertures, of course, can vary depending upon the degree of control over the flow between the reactors that is desired. 
     During the solution preparation phase, the majority if not all of the flow control apertures  52  in the inner reactor  12  are left open to allow for sufficient mixing of the chemicals that make up the biomimetic coating solution (see  FIGS. 1-3 ). Later in the coating process, these flow control apertures  52  can be selectively closed so as to limit the flow between the outer and inner reactor  14 ,  12  and thereby reduce the circulation of the biomimetic coating solution in the inner reactor  12 . The flow control apertures  52  can be closed using separate stoppers  53  that can be plugged into the apertures  52  as desired (see  FIG. 5 ). The number of apertures  52  that are plugged closed can vary depending on the desired circulation in the inner reactor  12 . Generally, at least some of the flow control apertures  52  in the inner reactor  12  are left open so as to ensure sufficient circulation to maintain the temperature and homogeneity of the solution. In the case of the illustrated embodiment, the apertures in the lid  47  and bottom wall  46  of the inner reactor  12  are typically always left open. 
     As will be appreciated the flow between the inner and outer reactors  12 ,  14  can be controlled in different ways and the present invention is not limited to any particular flow control system or type of flow control element. For example, an alternative embodiment of an inner reactor  12  having a different flow control system is illustrated in  FIGS. 6 and 7 . In this embodiment, the inner reactor  12  has a modular construction in which the sidewalls  54  can be easily removed and replaced from the reactor. In particular, the inner reactor  12  can be constructed with corner elements that define slots  56  for receiving the contiguous edges of the walls  54  defining that corner. These slots allow the walls  54  to be selectively slid into and out of place. For example, in the solution preparation phase, the walls  54  would be completely removed or slid partially up into an open position so as to maximize mixing of the solution. The solid walls  54  would then be slid into place or closed during the coating formation stage in order to limit the flow between the inner and outer reactors  12 ,  15 . Some openings or other means would still have to be provided in the inner reactor  12 , in this case openings in the lid and bottom wall, to permit sufficient circulation to maintain the temperature and homogeneity of the solution when the walls  54  were closed. 
     In use, to begin the solution preparation phase, the lids  22 ,  47  of the inner and outer reactors  12 ,  14  would be removed, and the chemicals making up the biomimetic coating solution would then be added to the reactors. This initial addition could be done by hand or as a part of an automated process. The pump system  16  would then be used to mix and heat the chemicals to prepare the biomimetic coating solution. During the preparation of the solution, all of the flow control apertures  52  in the inner reactor  12  would be open or, in the case of the  FIGS. 6 and 7  embodiment, the walls  54  would be raised or removed. Once the solution preparation was completed, the implantable articles can be arranged in the inner reactor  12 . Either at or some time after the initiation of coating formation, some or all of the flow control apertures  52  in the inner reactor  12  can be closed to limit the flow between the inner and outer reactors  12 ,  14 . Alternatively, with the  FIGS. 6 and 7  embodiment, the walls  54  could be slid into place. During coating formation, the pump system  16  would continue to operate and at least some of the flow control apertures  52  in the inner reactor  12  would remain open to maintain the temperature and homogeneity of the coating solution in the inner reactor  12 . Upon completion of the coating formation phase, the implantable articles can be removed from the inner reactor  12  and subjected to any needed subsequent processing steps such as rinsing or drying. In turn, the coating solution can be drained from the inner and outer reactors, which can then be cleaned for reuse. 
     The term “bioactive” as used herein means having the ability to effect local tissue activity, for instance, by improving local bone formation or by preventing the onset and proliferation of microbial species. 
     The term “implantable article” as used herein refers to any object or device that can be inserted or embedded into or grafted onto a body, or any part thereof, and that is designed for biomedical use. The implantable article, for example, can be a bone substitute, a joint prosthesis, a dental implant (prosthodontics), a maxillofacial implant, a vertebral surgery aid, a transcutaneous device (stoma or the like), or other medical or cosmetic device. Such implantable articles can serve as a bone replacement or bone reinforcement, as well as a means of fixing a device to a particular bone. 
     By “biocompatible substrate” is meant any object or device that is compatible with the body into which the object or device is inserted or embedded or with the body onto which the object or device is grafted, such that the object or device will not cause an adverse immune response in the body. The biocompatible substrate can comprise any suitable material(s), such as silicon, metals, ceramics, or polymers. Biocompatible metals include titanium, tantalum, niobium, zirconium, and alloys thereof (e.g., titanium alloys and tantalum alloys), as well as cobalt-chromium alloys and stainless steel. Biocompatible polymers can be natural or synthetic polymers, such as polyethylene (e.g., ultrahigh molecular weight polyethylene or polyethylene oxide), polypropylene, polytetrafluoroethylene, polyglycolic acid, polylactic acid, other polysaccharides, and copolymers of any of the foregoing (e.g., copolymers of polylactic acid and polyglycol acid). Preferably, the biocompatible substrate comprises, consists essentially of, or consists of a biocompatible metal. More preferably, the biocompatible substrate comprises, consists essentially of, or consists of titanium. The biocompatible substrate can be any suitable portion of the implantable article, preferably a component of a prosthesis (particularly a joint prosthesis). 
     All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.