Patent Application
20130330537
1. Field of the Invention
The present invention relates generally to materials used as scaffolds for facilitating the growth of biological tissues. The present invention relates more specifically to a sintered ceramic material with interconnecting pores a method for making the material of a predefined shape suitable for use as scaffolding for the regeneration of bone tissues.
2. Description of the Related Art
In tissue engineering for bone regeneration, a polymeric or ceramic scaffold is often a key component that serves as a platform for cell interactions and guide for bone formation while also providing structural support to the newly formed tissue. To perform this function, the scaffold for bone regeneration should meet certain criteria, including, but not limited to, biocompatibility, resorbability, osteoconductivity, permeability to allow for fluid exchange and pore size to account for cellular infiltration.
Much research has been reported in recent years in the use of polymeric and ceramic biomaterials for producing scaffolds for bone tissue regeneration. However, no single material or fabrication technique optimal for bone tissue regeneration has been identified. Current materials and techniques have met with varying success, yet each has inherent limitations that are still to be addressed.
As mentioned previously, scaffolds for bone tissue regeneration should be biocompatible, bioresorbable, contain an open-pore architecture and be mechanically similar to the bone repair site. Restoration of natural bone function is dependent on establishing conditions where materials and cells are combined to create regenerative environment. This can be accomplished, in part, by closely matching the composition, structure, chemistry, and mechanical properties of the implant to that of natural bone. The inorganic portion of natural bone is composed of biological apatite, rich in calcium and phosphate. The architecture of the scaffold is similar to that of trabecular bone and when using a calcium phosphate ceramic, the composition resembles the inorganic phase of bone tissue. Additionally, the architecture of the scaffolds (pore size, porosity, interconnectivity and permeability) should be adequate to allow for favorable transport/diffusion of ions, nutrients and wastes, which is important for osteoconduction and tissue growth.
Thus far, a number of manufacturing methods for the production of porous materials have been developed. Among these methods, the polymeric foam replication method has received particular attention because it can provide predictable structure with very high porosity and good interconnections between pores. In this method, a fully reticulated foam is used as a template to produce scaffolds with a highly controlled and precise pore size distribution. These properties would be expected to promote bone in-growth and the vascularization of newly formed tissue, but also to result in a decrease in the strength of the materials.
Various ceramics can be used for the preparation of such scaffolds. Among these varieties are included multiple types of calcium phosphates (such as hydroxyapatite (HAp), tricalcium phosphate (TCP), amorphous calcium phosphate (ACP), tetracalcium phosphate (TTCP), monocalcium phosphate (MCP), and octacalcium phosphate (OCP)) and other ceramics such as calcium sulfate, aluminum oxide, silicon dioxide, and zirconium dioxide. Among the various forms of calcium phosphate ceramics, HAp has gained attention because of usage in bone grafting, resulting from excellent osteoconductive and bioactive properties. HAp is thermodynamically the most stable crystalline phase of calcium phosphate in physiological conditions and encourages attachment of extracellular proteins and cells. The various ceramics each have a unique resorption rates, which can be tuned by blending multiples forms into the scaffold, such as tricalcium phosphate and hydroxyapatite Current laboratory mechanisms only allow for the manual coating of the foam pieces, leaving inconsistencies in the slurry preparation and coating process. A method is necessary that can easily be reproduced as well as be scaled to meet industry production needs. This method should also be able to be tuned to meet the specific needs of the scaffold, to include pore size, strut thickness, and ceramic chosen.
The present invention therefore provides a template coating method to create porous sintered ceramic materials of a predefined size and shape which contains interconnected pores and maintains high porosity. This method creates porous ceramic material in a way that is repeatable, scalable, and tunable to the specific needs of the scaffold. In this method, polymeric foam is immersed in sodium hydroxide/de-ionized water, compressed to remove air bubbles, and sonicated to treat the foam material for ceramic coating. The foam is thoroughly rinsed, compressed, and dried to produce the treated foam.
In order, an aqueous polymer solution, a ceramic powder, and a dispersant are combined in a mixing cup. The slurry is thoroughly mixed to a homogeneous consistency and then sonicated in a bath-style sonicator. An organic solvent (drying agent) is added to the slurry and it is mixed again. This ceramic slurry is sonicated and mixed again to ensure homogenous consistency. The first slurry is then ready for the first coating step.
In a first coating application, treated foam pieces and ceramic slurry are mixed twice until the coatings are uniform on the treated foam. Blocked pores are cleared with air until the coated foam is fully reticulated. The coated foam is dried overnight then processed with a specific sintering cycle, removed from the furnace and stored until the second coating application.
A second ceramic slurry is prepared which is less viscous than the first slurry. As in the first slurry preparation, an aqueous polymer solution and a sifted ceramic powder are combined in a mixing cup with a dispersant and mixed to create a homogeneous slurry. A drying agent is added, the slurry is mixed again and sonicated. The slurry is mixed once again and is then ready for the second coating step.
In a second coating application, the dry, sintered ceramic material from the first coating are placed in a sieve and small amounts of slurry are poured on top of them, lightly and carefully shaken to allow the slurry to pass through the pores, and blocked pores are cleared with compressed air. The coated ceramic foam material is dried overnight then processed with a specific sintering cycle, removed from the furnace and stored in a dry location. The purpose of the second coating step is to fill any holes in the surface of the structure, to round the strut surface, and to improve the mechanical integrity of the granules.
The ceramic foam is then used as bone-like scaffolds similar in composition and structure to the inorganic portion of trabecular bone. These scaffolds mimic the natural architecture of trabecular bone, may be prepared to fit into any size and shape for variety of uses, and promote regeneration of functional bone tissues.
Reference is made first to
As
As the overall construction process of the present invention is notably a two coating manufacturing method, the first coating sub-process is followed by a second similar, but not identical coating sub-process. A second slurry is prepared at sub-process Step 110. This is followed at sub-process Step 112 with a second coating process. A second sintering is carried out at sub-process Step 114. Once the second sintering is complete, the overall porous sintered ceramic material construction process is complete at Step 116, and the product resulting from the manufacturing method may be packaged and used for its intended purpose.
Step 122 involves sizing the foam material into predetermined shapes and sizes. For example, for a granular material, the foam is sized to approximately 2×2×2 mm (alternatively in the range of 1-3 mm) pieces for the granule templates. This foam material is then immersed at Step 124 in a 4% (w/v) NaOH/DI solution. Any air bubbles that are released as part of the immersion process may be gently displaced from the foam material and allowed to escape the solution.
The sodium hydroxide/distilled water solution (NaOH/DI) serves to effectively clean and roughen the foam material in preparation for the first coating. The cleaning solution comprises an aqueous solution of pH 9 to pH 14 and may include sodium hydroxide, ammonium hydroxide, and potassium hydroxide.
Step 126 involves sonicating the foams in a beaker for 15 minutes while ensuring full immersion. This step cleans and fully removes the particles that were etched out. The foam material templates are then rinsed with continuously flowing deionized water (DI) for approximately 2 hours at Step 128.
At Step 130 the foams are removed from the distilled water and compressed between drying sheets to remove the excess water. As the foam material remains resilient at this stage of the process, the compression and drying of the foam material does not alter their structural characteristics. Once again, the process for initially cleaning the foam material facilitates the subsequent adhesion of the slurry to all surfaces of the foam.
At Step 132 the foam material is placed in an open container and dried in a 45° C. oven for approximately 18 hours. Finally, at Step 134 the foam material preparation is complete and the material is now ready for a first coating.
Reference is next made to
By using alternate ceramic powders with differing densities, diameters, and surface areas, the dissolved polymers and powder-to-liquid ratios must be altered to accompany these changes. For example, a scaffold constructed from alumina powder of the same diameter and surface area as the currently used hydroxyapatite powder, which has a higher density than hydroxyapatite, would require an alteration in the powder-to-liquid ratio of the slurry, which uses mass as its unit of measure. To achieve a slurry with the same solids content by volume, the amount of alumina powder would be increased, resulting in a need to increase the dissolved polymers and drying agent. The drying agent may be dimethylformamide or dimethylsulfoxide. In addition, increasing the carboxymethylcellulose would increase the viscosity of the slurry. This, in turn, would create thicker coatings on the foam surface.
Referring to
This is followed at Step 144 by the addition of the sifted hydroxyapatite (HAp) powder. The HAp powder is added to the solution at a 1.4:1.0 w/w powder to solution ratio. The HAp powder preferably comprises a spherical particulate having a 20-40 nm diameter.
At Step 146, Darvan 821A (an aqueous solution containing 39.5-40.5% ammonium polyacrylate dispersant) is added at a rate of 3% by weight of HAp. The dispersant may be ammonium polyacrylate or ammonium polymethacrylate. At Step 148, the slurry is mixed in a dual action mixer (such as a FlackTek SpeedMixer or similar) for 20 seconds at 2500 rpm. At Step 150, a quantity of dimethylformamide (DMF) is added at a 10% by weight of HAp. At Step 152, the slurry is mixed again at 2500 rpm for 20 seconds. The DMF provides a drying agent for the slurry. At Step 154, the slurry is sonicated for 20 minutes in a bath style sonicator in order to break up all of the particles. This is followed by Step 156 where the slurry is again mixed at 2500 rpm for 20 seconds. Finally, at Step 158, the first slurry is ready for the first coating step.
Referring to
At Step 168, the coated foam material is removed and placed on a porous surface. The material is then subjected to a flow of pressurized air to help separate the granules from each other and to clear the pores of the granule templates, in order to once again become fully reticulated. At Step 170, the material is allowed to dry for approximately 18 hours at 21°-24° C. in an environment having a relative humidity of less than 50%. After drying occurs, the first coating process is complete at Step 172.
Reference is next made ahead to
The trays must be able to withstand the high temperatures of the sintering process and not become chemically involved in the reactions initiated at such high temperatures. The alumina trays containing the ceramic coated foam material is placed and positioned within a programmable oven. Initially, the temperature is raised at Step 224 to approximately 240° C. at a rate of 3° C. per minute. This is followed at Step 226 by a period of increasing the temperature from 240° C. to 290° C. at a rate of 1° C. per minute. At Step 228, the temperature is raised from 290° C. to 410° C. at a rate of 1° C. per minute. Subsequently, the temperature is raised at Step 230 from 410° C. to 600° C. at a rate of 2.5° C. per minute. The temperature is then held at Step 232 at 600° C. for approximately one hour.
After a temperature hold at 600° C., the temperature is again raised at Step 234 from 600° C. to 1250° C. at a rate of 3° C. per minute. A second hold at 1250° C. occurs at Step 236 for approximately two hours. The temperature may be held between 1200 and 1600° C. for 2 to 5 hours. The heating steps require holding at a temperature equal to or greater than the transition temperature of the ceramic powder. Sintering occurs, and the particles of hydroxyapatite fuse to form a stable block. Then, at Step 238, the oven and the porous hydroxyapatite material is allowed to cool to room temperature at a rate of 5° C. per minute. This sintering cycle provides the optimum schedule for the process by slowly burning off the binders, the dispersant and eventually the polymeric foam.
The resulting ceramic foam is a replica of the polymeric foam. Once at room temperature, the porous sintered ceramic material may be stored at Step 240, preferably at an elevated temperature of approximately 45° C. temperature until the second coating process is ready to be carried out. This elevated temperature is used to prevent the collection of moisture from the atmosphere.
Reference is now made back to
DMF is added at Step 190 at the rate of 3% by weight of HAp. A smaller quantity of DMF is required in the second slurry compared to the first step because the prevention of cracks in the coating are not as crucial as the first coating step. The slurry is again mixed at 2500 rpm for 20 seconds at Step 192. The slurry is sonicated at Step 194 for 20 minutes in a bath style sonicator. The slurry is again mixed at Step 196 at 2500 rpm for 20 seconds. Step 190 represents the completed preparation of the second slurry ready for the second coating step.
Step 208 involves once again subjecting the coated ceramic material (while on the sieve or other porous surface) to low pressure air to help separate the granules and clear the pores. Step 210 then involves drying the granules for approximately 18 hours at 21°-24° C. in an environment having a relative humidity of less than 50%. Step 212 thereby completes the second coating process allowing the manufacturing process to proceed once again to a sintering cycle. The finalized porous sintered ceramic structures range in size from 0.5 mm to 2000 mm. and the shape comprises one or more shapes selected from the group consisting of spherical, cuboidal, star-shaped, egg-shaped, cylindrical, plates, and screws. The pores of the finalized porous sintered ceramic structures range in size from 100 to 500 microns. The detailed sintering cycle process shown in
Reference is next made to
The mixer 22 utilized in the method of the present invention is preferably a dual action mixer that provides two rotational motions to the container 24 (preferably a closed mixing container) so as to facilitate the smooth and complete mixing of the material. It is preferable that no mixing blade or other invasive device be utilized in the mixer in order to prevent loss of slurry and physical damage of the granules. The mixing is achieved by the rotational forces associated with movement of the mixing container within the mixer according to two different rotational paths. The parameters associated with the dual action mixer include both a time variable 26 and a rotations per minute, or rpm variable 28.
Various types of drying structures are utilized for a specific period of time 44 in the present invention, including porous surfaces 32 that allow excess slurry material to drain away from the granules, and sieves 30 that similarly allow excess slurry material to drain away, and allow a flow of air to facilitate the excess slurry separation. A further high temperature tray 34 (preferably made of alumina) is utilized in the sintering cycle process of the method of the present invention. A programmable sintering oven 36 is utilized that is capable of not only achieving the elevated temperatures required for the sintering process, but also controlling the temperature and the rate of change of the temperature in an accurate manner. The parameters associated with the oven are therefore the temperature 38 within the oven, the time duration 40 of the maintenance of the temperature within the oven, and a carefully controlled rate of change of temperature 42 within the oven (both increasing and decreasing in temperature).
Although the present invention has been described in terms of the foregoing preferred embodiments, this description has been provided by way of explanation only, and is not intended to be construed as a limitation of the invention. Those skilled in the art will recognize modifications of the present invention that might accommodate specific applications and tissue requirements. Those skilled in the art will further recognize additional methods for modifying the composition and construction to accommodate these variations in tissue requirements. Such modifications, as to size structure, orientation, geometry, and even composition and construction techniques, where such modifications are coincidental to the type of product material required, do not necessarily depart from the spirit and scope of the invention.
