MICROWAVE TECHNOLOGY

Inventors:

ULRIKA BROHEDE MIRJAM LILJA

RELATED APPLICATION DATA

This application is a PCT International Application claiming priority of U.S. Provisional Application No. 61/678,457, filed August 1, 2012.

TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY

[0001] The present embodiment relates to a process using microwave technology for coating implants with a biomimetically grown hydroxyapatite coating.

SUMMARY

[0002] In one embodiment, a process for producing a hydroxyapatite coated substrate comprising the steps of providing a substrate, contacting the substrate with a liquid solution, and heating the substrate and solution with microwave energy to biomimetically grow a hydroxyapatite coating on a surface of the substrate.

[0003] In another embodiment, an implant includes a substrate and a hydroxyapatite coating grown on the substrate. The hydroxyapatite coating is directly grown on the substrate without any post-treatment.

[0004] These and other objects, features, aspects, and advantages of the present disclosure will become more apparent from the following detailed description of the preferred embodiments relative to the accompanied drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Fig. 1 is a flow diagram of the process of the present invention.

[0006] Figs. 2A-2D are electron microscope images of hydroxyapatite formation on an edge of surfaces coated by microwave technique. [0007] Figs. 3A-3D are electron microscope images of hydroxyapatite formation in the middle of surfaces coated by microwave technique.

[0008] Figs. 4A and 4B are electron microscope images of hydroxyapatite coatings formed by conventional oven heating and ultrasound methods.

[0009] Fig. 5 is an electron microscope image of a hydroxyapatite coating made by the process of the present invention.

[0010] Figs. 6A-6B are electron microscope images of hydroxyapatite coatings formed at high temperatures according to the process of the present invention.

DETAILED DESCRIPTION

[0011] Hydroxyapatite (HA) coated implants for orthopedic and dental

reconstruction are widely used. Hydroxyapatite occurs naturally in bone and teeth.

Synthetic HA (Caio(P0 ₄ )6(OH) ₂ ) therefore is rapidly integrated into natural bone and tissue to increase bonding properties with the implant. Implants made from Ti6A14V and other biocompatible metals alloys and ceramics are particularly compatible with HA coatings.

[0012] Numerous techniques are known for coating implant substrates with HA including plasma spraying, dip coating, sputter deposition, electrophoretic deposition and sol-gel synthesis. All these conventional techniques have numerous disadvantages. For example, plasma sprayed HA, the only viable commercially available HA coating process on the market, is a high temperature process resulting in a thick and dense coating with non-uniform coverage and poor substrate adhesion.

[0013] The biomimetic method consists of soaking the implant in a simulated body fluid at an appropriate temperature and pH. For obtaining a biomimetic HA coating on a substrate it requires soaking the substrate. The biomimetic method is used and the temperature of the solution can be, for example, 20-70°C. The method used today is to heat the solution in a conventional oven whereby a conformed coating will be formed after several days, about 4-7. Such a prolonged amount of time it takes to grow the coating in a large scale production could cause a bottle-neck during this process step.

[0014] Researchers have put effort into finding ways to increase the number of nucleation sites and/or to change HA composition, not with the focus of speeding up the process, but to increase the coating adhesion and/or to change the texture and

morphology in the coating. The use of microwave irradiation to form HA nanoparticles in a biomimetic solution has been explored. However, the particles are not capable of being used as a coating because there is no adheration possible without post treatment. There are numerous possible deposition methods for nanoparticles. If nanoparticles would be used to coat an implant surface, which may have a very complicated geometry, the coating process itself would need to be a multi-step process consisting of coating deposition, consisting of several steps, and post-treatment in case of wet-chemical deposition routes. This post-treatment may require quite high temperatures in order to obtain a crystalline HA coating, which also may impact the porosity. High temperatures may lead to changes in the microstructure due to i.e. grain growth, and phase

transformation. See Lilja M. et al. "Drug Loading and Release of Tobramycin from Hydroxyapatite Coated Fixation Pins," J Mater Sci Mater Med in press 10.1007/sl0556- 013-4979-1 (2013).

[0015] A different approach could be to apply high local temperature to the nanoparticles and melt them in order to form a coating, such as it is used in plasma spraying or laser coatings. In that case, coating geometry is a limiting factor and such coatings are often characterized by a poor structural porosity.

[0016] In order to overcome the prior art HA coating methodologies, the present disclosure employs microwave technology for achieving a faster, more controllable and stable/reliable process for biomimetically grown hyroxyapatite coatings than what are currently available and furthermore allows for tailoring of the HA coating structure. The biomimetic coating process of the present embodiments only requires a bioactive surface and the buffer solution to form a coating. No post-treatment is required to achieve a crystalline, uniform coating. A biomimetically grown HA coating contributes towards an enhanced bone bonding capability and increases bone in-growth towards the implant surface. The microwave heating reduces the process significantly. Process time and cost are reduced and coating quality and flexibility are increased.

[0017] Referring to Fig. 1, the present embodiment provides a process 10 for producing a biomimetic HA coated implant. In a first step 12, a substrate or base is provided. For example, the substrate can be a ceramic, titanium or a titanium alloy, stainless steel, Cobalt-chrome alloys, wollastonite or bioglass material. Substrates having crystalline, bioactive oxide surfaces selected from the group of Ti0 ₂ , Si0 ₂ , Mg0 ₂ , A10 ₂ , and Cr0 ₂ are good candidates. It should also be appreciated that other materials can be chosen depending upon the end product implant and use.

[0018] In the next step 14, the substrate is immersed in a simulated body fluid, such as a phosphate buffer saline (PBS) solution. The solution is prepared with various ion concentrations to mimic the chemical composition of human body fluids, such as blood plasma. The solution can contain calcium and phosphate ions. Moreover, the coating can be a substituted HA, where the substitution ions can be F, Sr, Mg, Si. Optimally, the PBS has a pH of 7.1-7.5 at room temperature. Changing the pH value can be a way to address the affinity of doping elements and antibiotics towards the HA. Also, the ion composition and concentrations can be varied in order to achieve different HA structures.

[0019] As discussed supra, the present embodiment uses microwave energy to heat the phosphate buffer saline (PBS) solution instead of conventional oven heating. The microwave catalyzes the HA nucleation and result in faster HA crystal formation. The negatively charged Ti-0 groups of the Ti0 ₂ attract the positively charged Ca ²⁺ ions from the body fluid. As a result of this ionic interaction, an amorphous calcium titante layer is built on the surface with a slightly positive charge due to the Ca ²⁺ ions. This layer attracts the negatively charged phosphate ions towards the surface, resulting in a metastable calcium phosphate layer, which transforms into a thermodynamically more favorable crystalline structure. Increased kinetic energy from the microwave process results in a faster binding occurrence between the ions and possibly an increased number of binding sites, both from the solution and the substrate surface.

[0020] As shown in step 16, the substrate soaking in the ionic solution is irradiated in a microwave cell. Preferably, the irradiation temperature is in the range of about 40 to about 250 C°, although the optimal coating temperature is within the range of about 40 to about 90°C. The temperature is a variable for the desired coating structure and allows tuning coating porosity as well as morphology and may thus be varied in cycles, with dynamic or linear ramping. The microwave process allows for quick temperature changes within seconds. [0021] In addition, HA coatings have shown promising potential to be used as a drug vehicle for local drug delivery at the implantation site. Co-precipitation of HA and drug on the surface can be obtained when the temperature is below about 90 °C when chemical structures, i.e. carboxyl groups in antibiotics, bind to calcium ions in the HA, or when ions from both PBS and the drug simultaneously are incorporated by co-precipitation. The coating thickness, porosity and morphology can be varied to achieve differently designed drug delivery profiles, such as; different releasing times with initial burst effect and/or controlled long term release and/or cycled release.

[0022] Initial experiments show a monolayer of HA forming within about 1 to about 4 hours on the substrate surface, where the conventional method needed 1-3 days for full substrate coverage. The substrates are PVD coated Titanium grade 5 turned discs, where the coating is an anatase dominated Ti0 ₂ with a coating thickness of about 500nm. Figs. 2A- 2D are electron microscope images of HA formation on the edge of the disc after being in a microwave temperature controlled 3 ml Dulbecco's PBS (D8662, Sigma Aldrich) bath at 60°C for 1 hour (Fig. 2A); 60°C for 4 hours (Fig. 2B); 60°C for 96 hours (Fig. 2C) and 120°C for 1 hour (Fig. 2D). The HA crystals form on the Ti0 ₂ surface as a result of the interaction between the slightly negative charged anatase surface and the ions in the phosphate buffered saline (PBS) solution, as described above. Longer immersion time of the Ti0 ₂ coated substrates in PBS at 60°C does not lead to increased HA crystals formation on the Ti0 ₂ surface due to saturation of the HA, i.e., there are no additional binding sites available.

[0023] Figs. 3 A- 3D are electron microscope images of HA formation in the middle of the disc in an microwave temperature controlled 3 ml PBS bath at 60°C for 1 hour (Fig. 3A); 60°C for 4 hours (Fig. 3B); 60°C for 96 hours (Fig. 3C) and 120°C for 1 hour (Fig. 3D). Formation of the HA crystals is described above. The amount of HA crystals observed in the middle of the disc is decreased compared to the edge as a result of too few ions in the solution. Nucleation starts at preferred sites close to the edge.

[0024] Fig. 4A illustrates a rolled Ti0 ₂ surface soaked in a PBS bath and heated in an oven for lhour at 60 °C. The sample was placed in plastic tubes containing 40 ml of Dulbecco's Phosphate Buffered Saline (PBS), which was kept in a controlled temperature of 60 °C in a Termak's (Termak, Norway) laboratory oven for 1 hour. After removal from the PBS, the samples were carefully rinsed in deionized water and left to dry in air. Formation of the HA crystals on the Ti02 surface is described above.

[0025] Fig. 4B illustrates a rolled Ti0 ₂ surface soaked in ultrasound heated PBS for 1 hour at 60°C. As described above, the sample was kept in a plastic tube filled with 40ml PBS and kept for 1 hour at 60°C in a Branson (Danbury, CT) ultrasonic bath. The temperature of the PBS solution was measured with a thermometer. As it can be seen from the SEM image, the Ti0 ₂ surface is coated with a network of HA crystals, having a much smaller diameter and distance to each other, i.e. a denser structure, compared to the samples presented in Fig. 2 and 3. Nucleation and growth of HA crystals, and thus coatings, can obviously be tuned by selecting the energy and frequency to heat the PBS.

[0026] Fig. 5 illustrates a turned Ti0 ₂ surface heated for 1 hour at 60°C in a microwave according to the process of the present invention. As can be seen from the electron microscope image, a high density of HA crystals on the Ti0 ₂ surface are visible, compared to Fig. 4a. Applying microwave for heating the PBS contributes towards catalyzing HA nucleation and growth of the crystals, as described above. The surface topography on the rolled and turned disc does not affect the HA nucleation time in the same order of time as the effect of sonication does, i.e. on the present time scale the topography has no or only a minor contribution to HA formation time.

[0027] Figs. 6A and 6B illustrate HA formation at high temperature (150°C) and for long periods of time, for example, 15 hours. This experiment was made to investigate if there could be seen a significant difference on nucleation or to extract more ions from the PBS solution. The result instead showed that there is probably an optimum regarding temperature and ionic concentration for the set-up. The image shows a cluster of formed HA on the first nucleation site close to the edge of the disc. The structure changes more by variation of wave-length for example, within the range of 5 x 10 8 - 5 x 1011 Hz for microwave and 20 x 10 ³ - 20 x 10 ⁷ Hz for ultrasound, than time and temperature.

[0028] Referring again to Figs 4A-4C, at 60°C HA is nucleated at the surface.

However, a limited amount of HA is formed. This can be due to the lack of ions available in the amount of solution. [0029] Referring again to Figs 2D, 3D, 6A and 6B, at the higher temperatures spheres are formed on the surface, due to preferred growth at initial nucleation sites. More HA is nucleated at the center of the disc as compared to the edge.

[0030] In summary, compared to the standard HA coating methods, HA formation according to the process of the present invention occurs earlier and faster.

[0031] Itemized list of embodiments:

1. A process for producing a hydroxyapatite coated substrate, comprising the steps of:

providing a substrate;

contacting the substrate with a liquid solution;

heating the substrate and solution with energy means for growing a hydroxyapatite coating directly on a surface of the substrate.

2. The process according to item 1, characterized in that the energy means comprises microwave energy.

3. The process according to item 1 or 2, characterized in that the substrate is a metal selected from the group of Ti0 ₂ , Si0 ₂ , Mg0 ₂ , A10 ₂ , and Cr0 ₂ .

4. The process according to any one of items 1 to 3, characterized in that the coating is an ion substituted hydroxyapatite.

5. The process according to any one of items 1 to 4, characterized in that the substituted ions are selected from the group of F, Sr, Si and Mg.

6. The process according to any one of items 1 to 5, characterized in that the step of contacting the article with a solution comprises soaking the article in a phosphate buffer saline solution. 7. The process according to any one of items 1 to 6, characterized in that the solution contains calcium and phosphate ions.

8. The process according to any one of items 1 to 7, characterized in that the step of heating the substrate and solution comprises heating to a temperature of about 40 to about 250°C.

9. The process according to any one of items 1 to 8, characterized in that the heating temperature is within the range of about 40to about 90°C.

10. The process according to any one of items 1 to 9, comprising the step of altering the heating temperature to control porosity and morphology of the

hydroxyapatite coating.

11. The process according to any one of items 1 to 10, characterized in that the liquid solution is a biomimetic solution and the step of growing the coating comprises biomimetically growing the hydroxyapatite on the substrate.

12. The process according to any one of items 1 to 11, characterized in that the biomimetic coating is grown directly on the substrate without any post treatment.

13. An implant made according to the process of any one of items 1-12.

14. An implant comprising:

a substrate; and

a hydroxyapatite coating grown on said substrate, wherein said hydroxyapatite is directly grown on said substrate without any post-treatment.

15. The implant according to item 14, characterized in that the substrate is a metal selected from the group of Ti0 ₂ , Si0 ₂ , Mg0 ₂ , A10 ₂ , and Cr0 ₂ . 16. The implant according to item 14 or 15, characterized in that the coating is an ion substituted hydroxyapatite.

17. The implant according to any one of items 14-16, characterized in that the substituted ions are selected from the group of F, Sr, Si and Mg.

18. The implant according to any one of items 14-17, characterized in that the coating is a biomimetic hydroxyapatite coating.

[0032] Although the present disclosure has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred therefore, that the present embodiment be limited not by the specific disclosure herein, but only by the appended claims.