Title:  Biomimetic calcium phosphate implant coatings and methods for making the same

United States Patent:  6,129,928

Assignee:  ICET, Inc. (Norwood, MA)

Filed:  September 4, 1998

This invention encompasses porous, nanocrystalline, biomimetic calcium phosphate coatings of the order of 2-30 microns that can be grown on metal implants. The chemical surface treatments and methods for making the calcium phosphate coatings are disclosed. Post treatment with dilute hydrogels such as phema reinforce the inorganic structure and enhance the mechanical strength of the coatings. Methods are also disclosed for adsorbing or covalently attaching growth factor proteins to derivatives of the hydrogel coated calcium phosphate coatings. Such hydrogel reinforced calcium phosphate coatings show equivalent bone tissue growth as the currently used implants and are easily resorbed. This property in combination with the immobilized growth factors is expected to enhance the process of osseointegration of the disclosed coatings.

DETAILED DESCRIPTION OF THE INVENTION

Currently, plasma sprayed hydroxyapatite coatings are promoted as highly dense, resistant to resorption (dissolves slowly), highly crystalline (69%) and less than 5% porosity with pure 98.3%-99% hydroxyapatite content. The controversy over the importance of these parameters can be seen throughout the hydroxyapatite implant literature. For example, with respect to crystallinity, Bruijn et al. (Cells Mater. 3:115-127 (1993)) have recently shown that the crystallinity of the plasma sprayed hydroxyapatite coating is an important feature influencing cell response at the implant surface. According to Bruijn et al., the lower the crystallinity of a hydroxyapatite coating, the greater is the calcium resorption. This study reported that the implant surface formed a favorable interlocking between the bone forming cells and the coating. Maxian et al. (Biomaterials Society Meeting, 1993) have also shown that the greater surface activity of resorbable calcium phosphate coatings (with a high rate of dissolution) enhanced cell attachment and cell spreading. Particulate debris from highly crystalline bioactive materials is undesirable whereas chemical dissolution is desirable and may actually be a prerequisite for enhanced bone attachment and initiation of a carbonated hydroxyapatite layer. Active cellular resorption of hydroxyapatite coatings followed by phagocytosis of resorbed particles of multinucleated cells has been reported (Maxian et al., J. Biomed. Mat. Res. 27:717-728 (1993)), possibly altering the chemical interfacial bond between bone and implant in the long term.

Porous materials (pore size greater than 100 microns) allow bone cells to penetrate the coating and to grow in pores onto the metal implant, thus strengthening the union between the implant and the bone. (Langer and Vacanti, Science 260:920 (1996)). There is therefore a need to develop a resorbable, high area, porous calcium phosphate coatings with immobilized growth factors for bone fixation as an alternative to nonresorbable calcium phosphate coatings.

This invention discloses a solution-based method of rapidly producing thin (2-30 micron), porous, nanocrystalline (therefore high area) calcium phosphate crystallites on metal implants with strong interfacial chemical bonds between the metal and the initial calcium phosphate layers through a bridging nucleating agent. The oxidized implant surface is pretreated with a nucleating agent and is exposed to a coating solution comprising a trace amount of the nucleating agent and calcium and phosphate in a sufficient concentration to form calcium phosphate crystals.

The newly discovered method is comparable to a biomimetic mechanism that occurs in nature. Examples of natural biomimetic processes include the formation of shells, bones, and eggshells where layer organic macromolecules, such as proteins with anionic acidic functionalities, act as a template to control the nucleation and, hence, the growth of calcium containing minerals. Advantages of the novel biomimetic calcium phosphate coating include the fine control of crystallinity, density, surface roughness, and thinness.

Earlier efforts to coat implant materials by crystallization of hydroxyapatite from solution have had marginal success. These methods have encountered problems in that the hydroxyapatite coatings grow slowly, are very thin, adhere poorly, and are very soft. (Bunker et al., Science 264:48 (1994).) One possible answer to improve the speed of deposition and thickness of layers is to attach a nucleating agent to the metal surface. Accordingly, this invention has identified certain nucleating agents for inducing the biomimetic calcium phosphate coating onto the implant surface.

Strength and toughness are issues nor only for plasma spray calcium phosphate coatings, but also for biomimetic calcium phosphate coatings. Plasma sprayed coatings need to be strong to work successfully in vivo over time. The biomimetic coatings must also be tough in order to resist handling and the actual implant process. There is therefore a need to improve the strength of a calcium phosphate coating. Accordingly, the invention covers a method of reinforcing a calcium phosphate coating with a with hydrogel polymer or copolymerized hydrogel. The reinforcement enhances the mechanical strength of the coatings.

The hydrogel polymers form a strong, energy-absorbing bond with the calcium phosphate crystals. Depending upon the type of hydrogel polymer used, the polymer may be derivatized to allow simple adsorption or direct chemical attachment of growth factor proteins onto the surface of the implants.

The present invention covers methods for chemisorbing a nucleating agent onto the surface of a metal implant. The method preferably comprises the steps of oxidizing the implant by soaking the implant in a hot solution of hydrogen peroxide; and soaking the implant in a high concentration solution of the nucleating agent. The metal may be selected from tantalum, cobalt, chromium, titanium, a cobalt alloy, a chromium alloy, or a titanium alloy. The metal alloy is preferably a titanium alloy of 6% aluminum and 4% vanadium. The nucleating agent may be wherein the nucleating agent is a synthetic or natural compound or polymer that contains a phosphate, carboxyl, sulfonate, phosphonate, amino, or other acidic functionality. The nucleating agent is preferably phosphoserine, polyvinylphosphonic acid, polyvinylsulphonic acid, or phosphoric acid, most preferably phosphoserine.

The method, in another preferred embodiment, can include a roughening step prior to the oxidation step.

The invention also encompasses methods making a biomimetic calcium phosphate coating on the surface of a metal implant. Other preferred embodiments include a coating solution for preparing biomimetic calcium phosphate coating. The preferred coating solution comprises 3.5 mM calcium chloride, 2.6 mM potassium dihydrogen phosphate and 5-50 ppm phosphoserine, preferably 50 ppm phosphoserine, at pH 6.5. The coating solution may be used at 37-50^oC., preferably at 50^oC. Biomimetic coatings prepared with this solution and/or method have a thickness of 2-30 microns, preferably 5-15 microns, most preferably 15 microns.

Yet another preferred embodiment is a method of reinforcing the strength of a calcium phosphate coated metal implant with a hydrogel polymer. The method for reinforcing the calcium phosphate coatings comprises a heat treating step followed by a soaking step of the implant in a hydrogel polymer and drying the implant overnight in an oven. The hydrogel polymer is preferably hydroxyalkylacrylate or hydroxyalkylmethacrylate, more preferably polyhydroxyethylmethacrylate, polyhydroxypropylmethacrylate polyhydroxytetrafurfurylmethacrylate, polyhydroxyethylacrylate, polyhydroxypropylacrylate, polyhydroxytetrafurfurylacrylate. The most preferred hydrogel polymer is polyhydroxyethylmethacrylate.

Metal implants with other types of calcium phosphate coatings may be reinforced using the inventive methods. The other calcium phosphate coatings which fall within the scope of the invention include hydroxyapatite, tetracalcium phosphate, octacalcium phosphate, and mixed calcium phosphate phases. The calcium phosphate coatings may be plasma sprayed coatings or biomimetic coatings, such as the type described and claimed herein.

Another preferred method of reinforcing the strength of the calcium phosphate coatings on the metal implant is with a copolymerized hydrogel. The steps of this method include making a copolymer of a hydrogel by convention free radical polymerization in solvent; purifying the copolymerized hydrogel in a non-solvent; drying the copolymerized hydrogel as a powder; heating the calcium phosphate coated metal implant at 350^oC.; soaking the metal implant in a solution of the copolymerized hydrogel; and drying the metal implant at 50-60^oC. overnight.

A preferred method according to the instant invention is to make activated hydrogel polymers for coupling growth factor proteins, wherein the hydrogel reinforces calcium phosphate coatings on implants. The activated hydrogel polymers may, alternatively, be directly treated onto an implant without reinforcing a calcium phosphate coating. The activating method follows a first step of activating the hydrogel polymer by copolymerizing the hydrogel polymer with a space group molecule. The spacer group molecule has a protein reactive functional group. Next, a growth factor protein is coupled to the spacer group molecule at the protein reactive functional group. Preferentially, the spacer group molecule is a polyethyleneglycol acrylate or polyethyleneglycol methacrylate. The protein reactive functional group may be a n-hydroxy succinimide, tresylate, aldehyde, epoxide, pnp carbonate, cyanuric chloride, isocyanate, carbonyl imidazole, vinyl sulfone, maleimide, and dithioorthopyridine. Other protein reactive functional groups may include tresylate, aldehyde, epoxide, pnp carbonate, cyanuric chloride, isocyanate, carbonyl imidazole, vinyl sulfone, maleimide, dithioorthopyridine, cyanogen bromide, cyclic carbonate, chloroalkyl formate, cyclic azide, nitrophenylchloroformate, dialdehyde, isocyanate, diisocyanate. The most preferred protein reactive functional group is n-hydroxy succinimide. The spacer group molecule is most preferentially polyethyleneglycol acrylate n-hydroxy succinimide. According to this and other methods described below, the growth factor protein may be a transforming growth factor, an insulin-like growth factor, or a bone morphogenic growth factor. The growth factor protein most preferred is TGF-.beta..

Yet another inventive method is one for imbibing a growth factor protein into the reinforcing hydrogel polymer by soaking a calcium phosphate hydrogel reinforced coated implant in a dilute solution of growth factor.

Kits also fall within the claimed invention. These kits are useful for in situ coupling or absorption of growth factor proteins onto a calcium phosphate coated metal implant surface prior to implantation surgery. In a preferred kit, the kit may contain a calcium phosphate coated metal implant that is to be surgically implanted into a patient; a reactive hydrogel polymer dissolved in a suitable solvent; and a solution of growth factor proteins. A dentist or physician would soak the calcium phosphate coated metal implant first in the reactive hydrogel polymer solution and then in the growth factor solution. Other kits would include containers of a reactive hydrogel polymer that is first activated at a protein reactive functional group so as to covalently couple a growth factor protein.

Claim 1 of 87 Claims

What is claimed is:

1. A method of making a biomimetic calcium phosphate coating on the surface of a metal implant comprising the sequential steps of

(1) chemisorbing a nucleating agent onto the surface of the metal implant comprising the steps of

(a) oxidizing the implant by soaking the implant in a solution of hydrogen peroxide; and

(b) soaking the implant in a solution comprising a nucleating agent;

(2) decanting the excess solution comprising the nucleating solution;

(3) soaking the metal implant in a solution comprising calcium chloride;

(4) washing and drying the metal implant;

(5) for a period of 5-7 days soaking the metal implant in a sterile, deaerated coating solution comprising calcium and phosphate in concentrations sufficient to form calcium phosphate crystals on the metal implant and a trace amount of the nucleating agent, wherein the solution is stirred and changed daily;

(6) rinsing the metal implant; and

(7) drying the metal implant.

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