The calcifying solution is preferably filter sterilized through a 0. The molar calcium to phosphorus ratio in the calcifying solution is generally within the range , more preferably between 1. The concentrations of the ions in the calcifying solution are chosen such, that in the absence of the gaseous weak acid, the solution is super-saturated or oversaturated. The molarity of the calcium source will generally be in the range 0.
The phosphate source will generally be from about 0. The concentration of magnesium in the calcifying solutions will usually be within the range 0. The carbonate concentration will range from 0 to 50 mM, more preferably 0 to 42 mM. The ionic strength will be within the range 0. The calcifying solution is preferably stirred to approximately rpm, more usually 50 to rpm.
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The carbon dioxide has a limited solubility in aqueous solutions. In contact with air, a carbonated aqueous solution is free of CO 2 or completely degassed within few hours depending on the surface of solution in contact with air. In the open bioreactor described herein, the complete exchange of dissolved CO 2 gas with atmosphere takes approximately 8 to 48 hours, more preferably between 12 to 24 hours.
The natural release of CO 2 gas causes the pH of the remaining solution to increase figure 2. In others words, saturation in the calcifying solution can increase until the precipitation of the bioactive layers on the surface of implantable materials occurs. Optionally, air can be bubbled through the solution to degas or aerate the solution and accelerate the escape, release or exchange of the gaseous weak acid. The initial and final pH values as well as pH changes with time depend on the amount of carbonate and phosphate salts added to the calcifying solution.
The buffering capability can be adjusted to a desired pH value by adding more or less of phosphate and carbonate salts. The pH can be maintained within the desired range by introducing carbon dioxide gas. In essence, the flow of carbon dioxide can be adjusted by using an electro or selenoid valve piloted by the controller. During the natural release of CO 2 gas out of the calcifying solution, the pH will increase to about , more preferably about 7.
The carbonated calcium phosphate layer will precipitate on the surface of implantable devices at a pH value of within about 6. The said precipitation on the surface of medical implants is related to a heterogeneous nucleation step.
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The carbonated calcium phosphate crystals might subsequently precipitate into the calcifying solution by a crystal growth process. In the invention, the heterogeneous nucleation is favored by the energetic stabilization of nucleus on the substrate. The high density of nucleation ensures a uniform deposition of carbonated calcium phosphate crystals onto the surface of medical implants. The process of bubbling carbon dioxide gas into the aqueous calcifying solution and escape of the carbon dioxide gas from the solution can be repeated to deposit a subsequent layer of carbonated calcium phosphate minerals on the implantable material.
In a method according to the invention, it may be essential to control the pH and thereby the nucleation stage by bubbling CO 2 gas for various time. The bubbling time is usually comprised between a few seconds to minutes, preferably about 1 to seconds. The introduction of carbon dioxide causes a decrease of pH while the pH of calcifying solution has a tendency to increase naturally without bubbling CO 2 gas.
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The increase of pH may be due to the natural exchange of CO 2 gas with atmosphere and the buffering capability of the calcification solution. By adjusting the time and flow of CO 2 gas introduced into the calcifying solution, the pH can oscillate around a value ranging from 6 to 9, more preferably the pH of the calcifying solution can be maintained between 6. This pH oscillation is correlated to the nucleation stage of carbonated calcium phosphate crystals on the surface of medical implants.
A high density of nucleation is thereby provided and carbonated calcium phosphate crystals can nucleate and grow onto the surface of medical implants. Homogeneous layers can uniformly deposit on the implant substrate. The total thickness of layers will preferably be within the range 0. While the layers are thin, usually below 5 microns, the coatings can diffract the natural light forming colored fringes ranging from blue to red colors.
This diffraction of light is similar to the phenomenon that may be observed when a drop of oil is present on water. For higher thickness, the layers give a shiny gray or white coloration. The thin carbonated calcium phosphate layers can induce the precipitation of subsequent layers by immersion into a second calcifying solution.
In other words, the thin carbonated calcium phosphate layers can serve as seed crystals for subsequent layers.
The second calcifying solution is preferably super-saturated with respect to hydroxyapatite. Under the super-saturation conditions, crystal growth may take place, and thick, crystalline and uniform calcium phosphate layers can be produced onto the surface of a medical implant. The second calcifying solution should contain calcium and phosphate salts with only small amounts of, or even without, inhibitors of crystal growth, like magnesium or carbonate.
As the second, or further layer will be deposited on a calcium phosphate coating the first layer , a good attachment is more easily achieved. The second calcifying solution can be prepared in the absence or presence of a gaseous weak acid, such as carbon dioxide. Preferably, the second calcifying solution is buffered at a physiological pH, around 7.
The concentration of calcium ions in the second calcifying solution may range from 0. The concentration of phosphate may range from 0. Magnesium and carbonate ions are preferably present in concentrations below 1 and 5 mM, respectively. More specifically, magnesium might be present in an amount between 0. Sodium chloride, or any suitable salt may be added to maintain the ionic strength of the second calcifying solution at a value of 0. The composition and crystal size of the layers will be strongly dependent on the amount of crystal growth inhibitors in the calcifying solutions.
In a preferred embodiment, the layers will be composed of hydroxyl carbonate apatite with a poor crystallinity or amorphous calcium phosphates containing magnesium and carbonate ions.
Calcium Phosphate Coatings - DOT GmbH
A series of salts having the general formulae I can be coated to medical devices. The chemical composition of the coating can be variable but the layers always contain magnesium, calcium and phosphate ions. If desired carbonate ions may be also incorporated within the coating, additionally the film can include traces of sodium and chloride ions. The amount of calcium and phosphorus will average between 20 to 40 and 10 to 30 weight percent, respectively. The magnesium and carbonate contents in the coating will be within the range of 0.
The present coatings may incorporate a wide variety of biologically active agents, such as peptides, growth factors, bone morphogenetic proteins, combinations thereof, and the like. The growth factors will be co-precipitated within the layers on the surface of implantable devices and may serve as drug delivery systems.
The gradual release of growth factors around the coated article can stimulated osteoblasts cells and enhance bone healing. Furthermore, antibiotics like tobramycin, vancomycin, and the like can be also precipitated within the coatings to prevent infection post-surgically. The coating process described herein can deposit a variety of calcium phosphate compounds containing carbonate and others ions on the surface of an implantable device.
The layers will be similar in composition and crystallinity with bone and teeth minerals and have desired bioresorbability, bone-bonding properties to improve the biological fixation of medical devices to living calcified tissue. It has further been found, that coatings on medical implants, prepared in a biomimetic approach, such as the present process, have osteoinductive properties.
A biomimetic approach concerns a process resulting in a calcium phosphate coating that, to a certain extent, mimics calcium phosphates resulting from biological mineralization processes, such as in bone or sea shells. This means that a biomimetic process often takes place at ambient temperature and results in a calcium phospate that resembles one, or a combination of the numerous naturally occurring calcium phosphate compositions. A biomimetic coating may be prepared employing a solution that is rich in at least calcium and phosphorous ions, either or not in physiologic concentrations, and optionally in the presence of nucleation promoting agents, such as bioactive glass particles.
Examples of biomimetic approaches include the process as described herein, but also those described by Kokubo see EP No. It has now been found that a biomimetic approach leads to a specific reactivity e. It has further been found, that the present process for providing a coating on a substrate leads to a particular morphology and crystal orientation, that increases the osteoinductive character of the biomimetic coating.
Further, certain coatings having specific chemical compositions, such as OCP coatings, lead to even greater osteoinductive effects. This invention is illustrated by the following examples but should not be construed to be limited thereto. In the examples, the percentages are expressed in weight unless specified otherwise. Pieces of titanium alloy are cut from a sheet of commercially available Ti6Al4V foil or rods. Ti6Al4V plates of 10 x 10 x 2 mm and cylinders of 5 mm in diameter and 10 mm in length are used.
Ti6Al4V wires of 1 mm in diameter are also coated with the bioactive carbonated calcium phosphate layers. Prior to coating, the implants are sand- or grit-blasted to increase their surface roughness. The etched Ti6Al4V plates were thoroughly washed with pure water. The calcifying solution is prepared by dissolving The calcifying solution is pumped through a 0. Carbon dioxide gas is introduced into the solution at a pressure of 0.
The pH of the solution is measured with an electrode and continuously monitored. The solution is maintained at pH 5. The calcifying solution is continuously stirred at rpm. After soaking for 24 hours, the pH of calcifying solution is within the range 7. The thickness of the bioactive layers is measured by using Eddy-Current instruments. The coating has a thickness averaging between 1 to 5 microns. The tensile bonding strength of the layers onto the substrate average between 40 to 65 Mpa.
Dense and uniform carbonated calcium phosphate layer are observed on the surface of implants. The FT-IR spectra Figure 7 show featureless and wide carbonate and phosphate bands typical of poorly crystallised hydroxyl cabonate apatite similar to bone mineral. The TF-XRD patterns Figure 8 indicate the diffraction lines of the Ti6Al4V substrate and halo or bump located at around 30 degrees 2 theta characteristic of amorphous calcium phosphate or poorly crystallised hydroxyl carbonate apatite phase.
The implants are then placed into meshed bags and hold into the bioreactor system. After autoclaving, the implants are soaked into a calcifying solution as described in example 1. After coating, the coated devices are ultrasonically rinsed with pure water and sterilized with an autoclave. The SEM observations and EDAX analyses confirm the uniform deposition of a well-attached dense calcium phosphate layer on and into the porous tantalum implants Figures 5 and 6.
Three Ti6Al4V plates were successively cleaned in acetone, ethanol, and demi water. Next, the plates were etched, using a mixture of hydrochloric acid and sulfuric acid, and thoroughly rinsed with demi water. A calcifying solution was prepared by dissolving A calcium phosphate layer was found to partially cover the plates.
As can be seen from Figure 9, the coating was not uniformly deposited on the surface of the substrates.