CA2073781A1 - Process for forming bioactive composite coatings on implantable devices - Google Patents
Process for forming bioactive composite coatings on implantable devicesInfo
- Publication number
- CA2073781A1 CA2073781A1 CA 2073781 CA2073781A CA2073781A1 CA 2073781 A1 CA2073781 A1 CA 2073781A1 CA 2073781 CA2073781 CA 2073781 CA 2073781 A CA2073781 A CA 2073781A CA 2073781 A1 CA2073781 A1 CA 2073781A1
- Authority
- CA
- Canada
- Prior art keywords
- electrolyte
- implantable device
- calcium
- calcium phosphate
- hydroxyapatite
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D9/00—Electrolytic coating other than with metals
- C25D9/04—Electrolytic coating other than with metals with inorganic materials
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
- A61L27/00—Materials for grafts or prostheses or for coating grafts or prostheses
- A61L27/28—Materials for coating prostheses
- A61L27/30—Inorganic materials
- A61L27/32—Phosphorus-containing materials, e.g. apatite
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61F—FILTERS IMPLANTABLE INTO BLOOD VESSELS; PROSTHESES; DEVICES PROVIDING PATENCY TO, OR PREVENTING COLLAPSING OF, TUBULAR STRUCTURES OF THE BODY, e.g. STENTS; ORTHOPAEDIC, NURSING OR CONTRACEPTIVE DEVICES; FOMENTATION; TREATMENT OR PROTECTION OF EYES OR EARS; BANDAGES, DRESSINGS OR ABSORBENT PADS; FIRST-AID KITS
- A61F2310/00—Prostheses classified in A61F2/28 or A61F2/30 - A61F2/44 being constructed from or coated with a particular material
- A61F2310/00389—The prosthesis being coated or covered with a particular material
- A61F2310/00592—Coating or prosthesis-covering structure made of ceramics or of ceramic-like compounds
- A61F2310/00796—Coating or prosthesis-covering structure made of a phosphorus-containing compound, e.g. hydroxy(l)apatite
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- Chemical & Material Sciences (AREA)
- Health & Medical Sciences (AREA)
- Inorganic Chemistry (AREA)
- General Health & Medical Sciences (AREA)
- Veterinary Medicine (AREA)
- Oral & Maxillofacial Surgery (AREA)
- Transplantation (AREA)
- Epidemiology (AREA)
- Life Sciences & Earth Sciences (AREA)
- Animal Behavior & Ethology (AREA)
- Dermatology (AREA)
- Public Health (AREA)
- Medicinal Chemistry (AREA)
- Engineering & Computer Science (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Materials For Medical Uses (AREA)
Abstract
Abstract An electrochemical process for forming uniform, bioactive composite coatings on porous and non-porous conductive substrates such as implantable devices is disclosed. Bioactive composite coatings obtained by the process of this invention consist of a crystalline oxide base layer and a bioactive calcium phosphate outer layer such as hydroxyapatite.
Bioactive composite coatings obtained by this process are effective in minimizing metal-ion release from the substrate, as well as, improving the bioactivity of the implantable devices. The process of this invention allows the production of such coatings in a commercially viable manner.
Bioactive composite coatings obtained by this process are effective in minimizing metal-ion release from the substrate, as well as, improving the bioactivity of the implantable devices. The process of this invention allows the production of such coatings in a commercially viable manner.
Description
2~7~
Field of the Invention:
This invention relates to a conductive substrate and, in particular, to a conductive implantable device coated with a bioactive composite coating and a process for production thereof. The composite coating is comprised of two distinct layers: a base layer composed of an oxide to minimize metal-ion release from the substrate and a top layer which is essentially composed of a calcium phosphate ceramic material to enhance surface bioactivity. The article coated with this composite coating is useful as an implantable device such as artificial hip joints and tooth roots.
Background of the Invention:
Medical devices such as joint prostheses have been commonly implanted into the skeletal structure of humans to replace missing or damaged skeletal parts. It is often intended that these implants become a permanent part of the body. In such cases, it is important that the prosthesis be strongly fixed to the skeletal bone structure. Cementless fixation of permanent implants has become a widespread surgical procedure which aids in avoiding some of the late complications of cemented prostheses. In principal, cementless fixation can be achieved by bone tissue ingrowth in porous coatings or by bone tissue apposition on surface structured prostheses. In recent years, the concept of biological attachment of load-bearing implants, using bioactive calcium phosphate coatings, has also been developed as an alternative solution to the difficulties associated with the mechanical fixation and acrylic cements.
It is known that bioactive coatings such as calcium phosphate ceramics provide direct bone contact as the implant-bone interface and provide chemical bonding with the bone structure.
Much of the prior art, however, teaches the use of plasma spray techniques to form bioactive ceramic coatings. Limitations of the plasma spray technique include: possible clogging of the 2~37$~
surface porosity, thereby obstructing bone tissue ingrowth and possible damage to the ceramic coating and the substrate as a result of intense heat. Furthermore, the plasma spray technique is a line-of-sight process that produces a non-uniform coating when applied to porous and non-regular implants. Thus, a significant portion of the implant surface remains uncovered and prone to metal-ion release. The metal-ion release is particularly of concern in the case of porous coated implants with relatively large surface areas. For such implants, the chemical and cellular response of the tissues to metal-ion release may be critical in determining the extent of new bone formation and its subsequent bonding to the implant. The inhibition of apatite formation by various metal-ions including Ti, V and Al ions and their side effects have been documented.
It is clear, therefore, that there is need for a uniform bio-active coating with optimum characteristics which, in addition to enhancing bioactivity, can effectively minimize the metal-ion release from the substrate. There is also need for a process which will allow uniform formation of such coatings in a commercially viable manner.
Brief Statement of Invention:
By one aspect of this invention, there is provided a process for forming a two layer composite coating on a conductive implantable device from a single electrolyte bath containing Ca- and P- containing ions, which significantly improves the corrosion resistance and bioactivity of the device. The two layer coating, prepared by the process of the invention, is comprised of a base layer essentially composed on an oxide, and an outer layer essentially composed of a calcium phosphate compound selected from Alpha- and Beta- tricalcium phosphate, calcium hydroxyapatite, calcium deficient hydroxyapatite, carbonate- containing hydroxyapatite, fluro-apatite or mixtures of these compounds.
2~37~1 The process of the invention includes the steps of:
a) immersing the implantable device in an electrolyte containing Ca- and P- bearing ions;
b) passing an electrical charge of anodic current through the electrolyte to the implantable device, the duration of the anodic charge and the magnitude of the current being effective to permit the growth of an oxide film on the implantable device with a desirable thickness;
c) passing the second electrical charge of current through the electrolyte to the implantable device, the second electrical charge being a charge of cathodic current having a waveform, duration, and magnitude effective to allow electrodeposition of a calcium phosphate coating of desired thickness on the base layer, and d) repeating steps (b) and (c) a plurality of times to form a composite coating on the implantable device comprising an oxide base layer and a calcium phosphate outer layer of desirable thickness.
By another aspect of this invention, there is provided an implantable device having a composite coating formed on, at least, part of its surface. The composite coating consists of two layers: a base layer essentially consisting of a metal oxide;
and an outer layer essentially consisting of calcium phosphate selected from the group consisting of Alpha- and Beta-tricalcium~-phosphate, hydroxyapatite, Ca- deficient hydroxy-apatite, C03- containing hydroxyapatite, Fluro-apatite and a mixture thereof.
A further objective of the invention is to provide a conductive substrate having a composite coating formed on at least part of its surface. The composite coating consists of two layers:
a base layer essentially consisting of a metal oxide to improve corrosion resistance of the substrate; and an outer layer 2~7378~
essentially consisting of Alpha- and Beta- tricalcium phosphate, hydroxyapatite, Ca- deficient hydroxyapatite, C03- containing hydroxyapatite, Fluro-apatite, or a mixture thereof.
Detailed Description of Preferred Embodiments of Invention:
The process of coating implantable devices, implants, and the like, is conducted according to the present invention, in an electrolyte solution containing Ca- and P- bearing ions. The pH of the electrolyte is adjusted between 2 and 8 and most preferably between 4 and 5. The electrolyte may be prepared by dissolving calcium phosphate compounds such as Alpha-tricalcium phosphate or calcium hydroxyapatite in an acidic solution. The acid may beselected from hydrochloric acid and/or nitric acid. The electrolyte may also be prepared by dissolving Ca- and P- containing compounds in water. Examples of Ca- containing compounds are calcium nitrate and calcium chloride and examples of P- containing compounds are NH4H2P04 and Na2HP04. The pH of the electrolyte may be adjusted, preferably between 4 and 5, by the addition of an acid or base.
It is desirable that Ca/P molar ratio of the electrolyte be maintained between 1 to 2 and most preferably between 1.5 to 1.67. In one preferred embodiment of the present invention, the electrolyte has a calcium ion concentration of less than 0.1 mole per litre and the Ca/P molar ratio of the electrolyte is 1.67. The electrolyte may additionally contain ions such as F-, C03--, HC03-, N03-, Cl-, Mg++, A13+ and ions of platinum group family to produce coatings with specific chemical compos-itions. The electrolyte may also contain organic substances such as proteins and biologically active material such as collagen and bone morphogenic protein. The electrolyte, further, may contain dissolved gases such as C2~N2~-~co2` and oxidizing~agents and wetting agents to improve the quality of the coating. The temperature of the electrolyte during the process may vary from about room temperature up to a point reasonably below the boiling point of the electrolyte, depending on the operating :2Q~3~1 pressure. For example, the process may be conducted in an electrolytic autoclave under the saturation vapour pressures of the electrolyte between room temperature and 250 degrees Centigrade.
The process of the present invention can be carried out using a conventional electrolytic cell, having at least one counter electrode made of, for example, platinum or graphite and equipped with a programmable power supply. The programmable power supply is preferably capable of producing constant voltages and constant currents of a selectably programmable polarity and intensity, and programmable "on/off" currents and voltage signals, pulsed waveform and other desirable waveforms with selectably programmable frequencies and peak intensities. A
preferred programmable waveform for this process is shown in Fig.l. The horizontal axis, in this figure, corresponds to time in arbitrary units and the vertical axis corresponds to the voltage applied to the electrolytic cell during the process, in arbitrary units. The voltage waveform, in Fig.l, consists of a constant voltage, Vl, applied to the cell for a period, tl, to form the oxide base layer, followed by an "on/off" cathodic voltage. The on/off cathodic voltage has a peak value, V2, an "on-period", t2, and an "off-period", t3. During the period, tl, a charge of anodic current is passed through the electrolyte to the implantable device. The duration period, tl, and the voltage, Vl, are selected such that it results in an anodic charge effective to permit the growth of an oxide with desirable thickness on the implantable device. It is preferred that the anodic voltage, Vl, to be about 10 volts and most preferably, about 5 volts. The time period, tl, is also preferably about 30 minutes.
Referring to Fig.l, the "on/off" cathodic voltage results in passing a series of electrical charges of a cathodic current through the electrolyte to the implantable device, which results 2~7378 ~
in electrodeposition and crystallization of calcium phosphate compounds on top of the oxide base layer. The peak voltage, V2, and the time periods, t2, and t3, are selected such that the average cathodic current density flowing is about 5 mA/Cm2.
The entire period, during which a series of cathodic charges is passing to the implantable device, should be sufficient to permit formation of calcium phosphate coating with a desirable thickness. Roughly, a time period of 30 minutes is required to obtain a calcium phosphate coating of about 30 micro-metres.
The process of this invention is applicable both to pure titanium and to titanium-based alloys, e.g., those containing alloying constituents such as aluminum &vanadium. Other film forming metals such as Co-Ni alloys and zirconium based alloys may also be used as the substrate in this invention. The article to be coated, according to the process of this invention, may be suitably cleaned or given a cleaning pre-treatment through various means using conventional processes.
The article may also be activated using conventional methods.
In one preferred embodiment of the present process, the article to be coated is first treated with a colloidal titanium-based surface adjustment agent prior to immersing the article in the electrolyte and effecting electrolysis. Titanium based colloidal solutions containing between 10 to 200 ppm of titanium ions may be employed in conjunctionwith the present invention. The article to be coated may have a porous surface or may be roughened by, for example, sand blasting, etching by chemical or electrochemical means. The surface of the article may also have micro or macro textures.
In one preferred embodiment of the present invention, the composite coating obtained by the process of this invention is treated with an alkaline solution containing calcium ions to increase the Ca/P molar ratio of the coating. A preferred solution is 0.001 mole per litre of calcium hydroxide maintained 2~7378~
at a temperature in the range of 30 -to 80 degrees Centigrade.
The coated article may be simply immersed in this solution for about 1 hour to obtain a coating with a desirable Ca/P ratio and to improve the crystal structure of the calcium phosphate coating.
In another preferred embodiment of the present invention, the coating obtained by the process of the present invention is treated with a biologically active substance such as collagen, bone morphogenic protein or an antibiotic.
In yet another preferred embodiment of the present invention, the coating obtained by the process of the present invention is densified and sintered by a heat treatment, for example, in a vacuum at a temperature between ~00 to 800 degrees Centigrade.
Alternatively, the coating may be densified using hydrothermal treatment. Desirably, the hydrothermal treatment is conducted in an autoclave at a temperature between 100 to 200 degrees Centigrade, under a pressure of about 10 kg/Cm2.
Example #1:
An electrolyte was prepared by mixing 1 litre 0.042 mole per litre CaC12 2H20 and 1 litre 0.025 mole per litre NH4H2P04 solutions. The pH of the mixed solution, measured at room temperature, was 4.1. The electrolyte was then transferred to a conventional electrolytic cell having a capacity of 2.5 litres and maintained at 65 degrees Centigrade. The cell was fitted with a graphite electrode acting as a counter electrode of the cell. The surface of a commercially pure titanium sample 5 cm long, 1 cm wide and 1 mm thick was mechanically abraded and then cleaned with methanol, washed with distilled water and dried in a stream of air. The sample was then immersed in the electrolyte and an electrical charge of anodic current was passed through the electrolyte to the sample for 20 minutes. For this purpose, a DC voltage of 5 volts was applied between the graphite electrode 2Q~3~
and the titanium sample. A base layer composed of a corrosion resistant oxide film was thus developed on the surface of the titanium sample. At the end of this period, the polarity of the cell was reversed and a charge of cathodic current was passed through the electrolyte to the titanium sample for 30 minutes to form an outer layer of calcium phosphate compound on the base layer. The cathodic current used had a pulsed - on/off waveform with a pulse width of 20 msec and a pulse spacing of 20 msec. The average current density was 2 mA / Cm2. A
composite coating was thus formed on the titanium substrate having two distinct layers consisting of an oxide base layer and a calcium phosphate outer layer. The coated sample was then removed from the cell, washed with distilled water and then dried in a stream of hot air for 5 minutes. Electron micro-scopic examinations of the coated substrate were carried out using a JEOL - scanning electron microscope. At a-rel-a~ively high magnification ( x 3000), it was observed that the calcium phosphate outer layer was composed of an interlocking network of non-oriented plate-like crystals with the largest dimensions having an average value of 2 - 3 micrometers. The dimensions of the micropores present in the coating also range from about 2 to 3 micrometers. The results obtained by wet chemical analysis of calcium phosphate scraped from the sample indicated that the calcium phosphate outer layer contained about 2 to 3 %
CO3 by weight. The Ca/P molar ratio of the calcium phosphate outer layer was also found to be slightly less than 1.67. The X-ray diffraction pattern of the calcium phosphate scraped from the sample confirmed that the calcium phosphate outer layer had a crystalline structure with apatitic characteristics similar to bone apatite. The infrared spectra of the calcium phosphate coating further supported the X-ray diffraction results. The IR - spectra was generally similar to those reported for non-stoichiometric, carbonate-containing hydroxyapatite and bone apatite. The IR - spectrum indicated relatively small hydroxyl peaks at 630 and 3570 Cm~l. These peaks are characteristics 2 ~ 7 .~
for hydroxyapatite. The bands at 1040 and 1090 cm-l specific for P04 modes of hydroxyapatite were also apparent in the spectra. The IR - spectra also indicated bands at 1400 - 1450 and 872 cm-l, which are assigned to carbonate ions. There was also a broad band in the range of 3000 - 3600 Cm-l, indicative of adsorbed water.
Example #2:
A composite coating, having an oxide base layer and a calcium phosphate outer layer, was formed on a titanium substrate using a procedure identical to the one in example #1. The coated sample was then immersed in 0.001 mole per litre of calcium hydroxide solution at 70 degrees centigrade for 1 hour.
The sample was then removed from this solution and dired in a stream of hot air for 5 minutes. IR - spectra of the calcium phosphate outer layer of the composite coating obtained, indicated intense hydroxyl peaks at 630 and 3570 Cm-l.
Example #3:
A composite coating, similar to one in example #2, was prepared on a titanium substrate. The coated sample was then heat treated at 400 degrees Fahrenheit for 30 minutes. The IR - spectra of the calcium phosphate outer layer, subsequent to heat treatment, indicated very intense OH - peaks and a weak broad band in the 3000 - 3600 Cm-l range.
Example #4:
A composite coating was prepared on a titanium substrate using an identical procedure as described in example #3 except that Nitrogen gas was continuously sparged into the electrolyte and calcium hydroxide solution during the preparation of the coating.
The IR - spectra of calcium phosphate outer layer, in this case, indicated no significant bands at 1400-1450 Cm-l assigned to carbonate ions. The IR - spectra, however, indicated intense hydroxyl peaks at 630 and 3570 Cm-l, characteristics of hydroxy-apatite. The IR - spectra results, together with the X-ray data 2~7~
indicted that the calcium phosphate outer layer was well-crystallized, stoichiometric hydroxyapatite with no significant carbonate content.
Example #5:
A composite coating, having an oxide base layer and a calcium phosphate outer layer, was prepared using an identical procedure as described in example #4, except that a porous titanium substrate was used, in this case. Titanium wire (99.9% purity, 0.25 mm diameter) was used to form a porous substrate with an average pore diameter of approximately 300 micrometres. The composite coating obtained on this substrate was dense, uniform, and well adhered to the substrate without clogging the pores
Field of the Invention:
This invention relates to a conductive substrate and, in particular, to a conductive implantable device coated with a bioactive composite coating and a process for production thereof. The composite coating is comprised of two distinct layers: a base layer composed of an oxide to minimize metal-ion release from the substrate and a top layer which is essentially composed of a calcium phosphate ceramic material to enhance surface bioactivity. The article coated with this composite coating is useful as an implantable device such as artificial hip joints and tooth roots.
Background of the Invention:
Medical devices such as joint prostheses have been commonly implanted into the skeletal structure of humans to replace missing or damaged skeletal parts. It is often intended that these implants become a permanent part of the body. In such cases, it is important that the prosthesis be strongly fixed to the skeletal bone structure. Cementless fixation of permanent implants has become a widespread surgical procedure which aids in avoiding some of the late complications of cemented prostheses. In principal, cementless fixation can be achieved by bone tissue ingrowth in porous coatings or by bone tissue apposition on surface structured prostheses. In recent years, the concept of biological attachment of load-bearing implants, using bioactive calcium phosphate coatings, has also been developed as an alternative solution to the difficulties associated with the mechanical fixation and acrylic cements.
It is known that bioactive coatings such as calcium phosphate ceramics provide direct bone contact as the implant-bone interface and provide chemical bonding with the bone structure.
Much of the prior art, however, teaches the use of plasma spray techniques to form bioactive ceramic coatings. Limitations of the plasma spray technique include: possible clogging of the 2~37$~
surface porosity, thereby obstructing bone tissue ingrowth and possible damage to the ceramic coating and the substrate as a result of intense heat. Furthermore, the plasma spray technique is a line-of-sight process that produces a non-uniform coating when applied to porous and non-regular implants. Thus, a significant portion of the implant surface remains uncovered and prone to metal-ion release. The metal-ion release is particularly of concern in the case of porous coated implants with relatively large surface areas. For such implants, the chemical and cellular response of the tissues to metal-ion release may be critical in determining the extent of new bone formation and its subsequent bonding to the implant. The inhibition of apatite formation by various metal-ions including Ti, V and Al ions and their side effects have been documented.
It is clear, therefore, that there is need for a uniform bio-active coating with optimum characteristics which, in addition to enhancing bioactivity, can effectively minimize the metal-ion release from the substrate. There is also need for a process which will allow uniform formation of such coatings in a commercially viable manner.
Brief Statement of Invention:
By one aspect of this invention, there is provided a process for forming a two layer composite coating on a conductive implantable device from a single electrolyte bath containing Ca- and P- containing ions, which significantly improves the corrosion resistance and bioactivity of the device. The two layer coating, prepared by the process of the invention, is comprised of a base layer essentially composed on an oxide, and an outer layer essentially composed of a calcium phosphate compound selected from Alpha- and Beta- tricalcium phosphate, calcium hydroxyapatite, calcium deficient hydroxyapatite, carbonate- containing hydroxyapatite, fluro-apatite or mixtures of these compounds.
2~37~1 The process of the invention includes the steps of:
a) immersing the implantable device in an electrolyte containing Ca- and P- bearing ions;
b) passing an electrical charge of anodic current through the electrolyte to the implantable device, the duration of the anodic charge and the magnitude of the current being effective to permit the growth of an oxide film on the implantable device with a desirable thickness;
c) passing the second electrical charge of current through the electrolyte to the implantable device, the second electrical charge being a charge of cathodic current having a waveform, duration, and magnitude effective to allow electrodeposition of a calcium phosphate coating of desired thickness on the base layer, and d) repeating steps (b) and (c) a plurality of times to form a composite coating on the implantable device comprising an oxide base layer and a calcium phosphate outer layer of desirable thickness.
By another aspect of this invention, there is provided an implantable device having a composite coating formed on, at least, part of its surface. The composite coating consists of two layers: a base layer essentially consisting of a metal oxide;
and an outer layer essentially consisting of calcium phosphate selected from the group consisting of Alpha- and Beta-tricalcium~-phosphate, hydroxyapatite, Ca- deficient hydroxy-apatite, C03- containing hydroxyapatite, Fluro-apatite and a mixture thereof.
A further objective of the invention is to provide a conductive substrate having a composite coating formed on at least part of its surface. The composite coating consists of two layers:
a base layer essentially consisting of a metal oxide to improve corrosion resistance of the substrate; and an outer layer 2~7378~
essentially consisting of Alpha- and Beta- tricalcium phosphate, hydroxyapatite, Ca- deficient hydroxyapatite, C03- containing hydroxyapatite, Fluro-apatite, or a mixture thereof.
Detailed Description of Preferred Embodiments of Invention:
The process of coating implantable devices, implants, and the like, is conducted according to the present invention, in an electrolyte solution containing Ca- and P- bearing ions. The pH of the electrolyte is adjusted between 2 and 8 and most preferably between 4 and 5. The electrolyte may be prepared by dissolving calcium phosphate compounds such as Alpha-tricalcium phosphate or calcium hydroxyapatite in an acidic solution. The acid may beselected from hydrochloric acid and/or nitric acid. The electrolyte may also be prepared by dissolving Ca- and P- containing compounds in water. Examples of Ca- containing compounds are calcium nitrate and calcium chloride and examples of P- containing compounds are NH4H2P04 and Na2HP04. The pH of the electrolyte may be adjusted, preferably between 4 and 5, by the addition of an acid or base.
It is desirable that Ca/P molar ratio of the electrolyte be maintained between 1 to 2 and most preferably between 1.5 to 1.67. In one preferred embodiment of the present invention, the electrolyte has a calcium ion concentration of less than 0.1 mole per litre and the Ca/P molar ratio of the electrolyte is 1.67. The electrolyte may additionally contain ions such as F-, C03--, HC03-, N03-, Cl-, Mg++, A13+ and ions of platinum group family to produce coatings with specific chemical compos-itions. The electrolyte may also contain organic substances such as proteins and biologically active material such as collagen and bone morphogenic protein. The electrolyte, further, may contain dissolved gases such as C2~N2~-~co2` and oxidizing~agents and wetting agents to improve the quality of the coating. The temperature of the electrolyte during the process may vary from about room temperature up to a point reasonably below the boiling point of the electrolyte, depending on the operating :2Q~3~1 pressure. For example, the process may be conducted in an electrolytic autoclave under the saturation vapour pressures of the electrolyte between room temperature and 250 degrees Centigrade.
The process of the present invention can be carried out using a conventional electrolytic cell, having at least one counter electrode made of, for example, platinum or graphite and equipped with a programmable power supply. The programmable power supply is preferably capable of producing constant voltages and constant currents of a selectably programmable polarity and intensity, and programmable "on/off" currents and voltage signals, pulsed waveform and other desirable waveforms with selectably programmable frequencies and peak intensities. A
preferred programmable waveform for this process is shown in Fig.l. The horizontal axis, in this figure, corresponds to time in arbitrary units and the vertical axis corresponds to the voltage applied to the electrolytic cell during the process, in arbitrary units. The voltage waveform, in Fig.l, consists of a constant voltage, Vl, applied to the cell for a period, tl, to form the oxide base layer, followed by an "on/off" cathodic voltage. The on/off cathodic voltage has a peak value, V2, an "on-period", t2, and an "off-period", t3. During the period, tl, a charge of anodic current is passed through the electrolyte to the implantable device. The duration period, tl, and the voltage, Vl, are selected such that it results in an anodic charge effective to permit the growth of an oxide with desirable thickness on the implantable device. It is preferred that the anodic voltage, Vl, to be about 10 volts and most preferably, about 5 volts. The time period, tl, is also preferably about 30 minutes.
Referring to Fig.l, the "on/off" cathodic voltage results in passing a series of electrical charges of a cathodic current through the electrolyte to the implantable device, which results 2~7378 ~
in electrodeposition and crystallization of calcium phosphate compounds on top of the oxide base layer. The peak voltage, V2, and the time periods, t2, and t3, are selected such that the average cathodic current density flowing is about 5 mA/Cm2.
The entire period, during which a series of cathodic charges is passing to the implantable device, should be sufficient to permit formation of calcium phosphate coating with a desirable thickness. Roughly, a time period of 30 minutes is required to obtain a calcium phosphate coating of about 30 micro-metres.
The process of this invention is applicable both to pure titanium and to titanium-based alloys, e.g., those containing alloying constituents such as aluminum &vanadium. Other film forming metals such as Co-Ni alloys and zirconium based alloys may also be used as the substrate in this invention. The article to be coated, according to the process of this invention, may be suitably cleaned or given a cleaning pre-treatment through various means using conventional processes.
The article may also be activated using conventional methods.
In one preferred embodiment of the present process, the article to be coated is first treated with a colloidal titanium-based surface adjustment agent prior to immersing the article in the electrolyte and effecting electrolysis. Titanium based colloidal solutions containing between 10 to 200 ppm of titanium ions may be employed in conjunctionwith the present invention. The article to be coated may have a porous surface or may be roughened by, for example, sand blasting, etching by chemical or electrochemical means. The surface of the article may also have micro or macro textures.
In one preferred embodiment of the present invention, the composite coating obtained by the process of this invention is treated with an alkaline solution containing calcium ions to increase the Ca/P molar ratio of the coating. A preferred solution is 0.001 mole per litre of calcium hydroxide maintained 2~7378~
at a temperature in the range of 30 -to 80 degrees Centigrade.
The coated article may be simply immersed in this solution for about 1 hour to obtain a coating with a desirable Ca/P ratio and to improve the crystal structure of the calcium phosphate coating.
In another preferred embodiment of the present invention, the coating obtained by the process of the present invention is treated with a biologically active substance such as collagen, bone morphogenic protein or an antibiotic.
In yet another preferred embodiment of the present invention, the coating obtained by the process of the present invention is densified and sintered by a heat treatment, for example, in a vacuum at a temperature between ~00 to 800 degrees Centigrade.
Alternatively, the coating may be densified using hydrothermal treatment. Desirably, the hydrothermal treatment is conducted in an autoclave at a temperature between 100 to 200 degrees Centigrade, under a pressure of about 10 kg/Cm2.
Example #1:
An electrolyte was prepared by mixing 1 litre 0.042 mole per litre CaC12 2H20 and 1 litre 0.025 mole per litre NH4H2P04 solutions. The pH of the mixed solution, measured at room temperature, was 4.1. The electrolyte was then transferred to a conventional electrolytic cell having a capacity of 2.5 litres and maintained at 65 degrees Centigrade. The cell was fitted with a graphite electrode acting as a counter electrode of the cell. The surface of a commercially pure titanium sample 5 cm long, 1 cm wide and 1 mm thick was mechanically abraded and then cleaned with methanol, washed with distilled water and dried in a stream of air. The sample was then immersed in the electrolyte and an electrical charge of anodic current was passed through the electrolyte to the sample for 20 minutes. For this purpose, a DC voltage of 5 volts was applied between the graphite electrode 2Q~3~
and the titanium sample. A base layer composed of a corrosion resistant oxide film was thus developed on the surface of the titanium sample. At the end of this period, the polarity of the cell was reversed and a charge of cathodic current was passed through the electrolyte to the titanium sample for 30 minutes to form an outer layer of calcium phosphate compound on the base layer. The cathodic current used had a pulsed - on/off waveform with a pulse width of 20 msec and a pulse spacing of 20 msec. The average current density was 2 mA / Cm2. A
composite coating was thus formed on the titanium substrate having two distinct layers consisting of an oxide base layer and a calcium phosphate outer layer. The coated sample was then removed from the cell, washed with distilled water and then dried in a stream of hot air for 5 minutes. Electron micro-scopic examinations of the coated substrate were carried out using a JEOL - scanning electron microscope. At a-rel-a~ively high magnification ( x 3000), it was observed that the calcium phosphate outer layer was composed of an interlocking network of non-oriented plate-like crystals with the largest dimensions having an average value of 2 - 3 micrometers. The dimensions of the micropores present in the coating also range from about 2 to 3 micrometers. The results obtained by wet chemical analysis of calcium phosphate scraped from the sample indicated that the calcium phosphate outer layer contained about 2 to 3 %
CO3 by weight. The Ca/P molar ratio of the calcium phosphate outer layer was also found to be slightly less than 1.67. The X-ray diffraction pattern of the calcium phosphate scraped from the sample confirmed that the calcium phosphate outer layer had a crystalline structure with apatitic characteristics similar to bone apatite. The infrared spectra of the calcium phosphate coating further supported the X-ray diffraction results. The IR - spectra was generally similar to those reported for non-stoichiometric, carbonate-containing hydroxyapatite and bone apatite. The IR - spectrum indicated relatively small hydroxyl peaks at 630 and 3570 Cm~l. These peaks are characteristics 2 ~ 7 .~
for hydroxyapatite. The bands at 1040 and 1090 cm-l specific for P04 modes of hydroxyapatite were also apparent in the spectra. The IR - spectra also indicated bands at 1400 - 1450 and 872 cm-l, which are assigned to carbonate ions. There was also a broad band in the range of 3000 - 3600 Cm-l, indicative of adsorbed water.
Example #2:
A composite coating, having an oxide base layer and a calcium phosphate outer layer, was formed on a titanium substrate using a procedure identical to the one in example #1. The coated sample was then immersed in 0.001 mole per litre of calcium hydroxide solution at 70 degrees centigrade for 1 hour.
The sample was then removed from this solution and dired in a stream of hot air for 5 minutes. IR - spectra of the calcium phosphate outer layer of the composite coating obtained, indicated intense hydroxyl peaks at 630 and 3570 Cm-l.
Example #3:
A composite coating, similar to one in example #2, was prepared on a titanium substrate. The coated sample was then heat treated at 400 degrees Fahrenheit for 30 minutes. The IR - spectra of the calcium phosphate outer layer, subsequent to heat treatment, indicated very intense OH - peaks and a weak broad band in the 3000 - 3600 Cm-l range.
Example #4:
A composite coating was prepared on a titanium substrate using an identical procedure as described in example #3 except that Nitrogen gas was continuously sparged into the electrolyte and calcium hydroxide solution during the preparation of the coating.
The IR - spectra of calcium phosphate outer layer, in this case, indicated no significant bands at 1400-1450 Cm-l assigned to carbonate ions. The IR - spectra, however, indicated intense hydroxyl peaks at 630 and 3570 Cm-l, characteristics of hydroxy-apatite. The IR - spectra results, together with the X-ray data 2~7~
indicted that the calcium phosphate outer layer was well-crystallized, stoichiometric hydroxyapatite with no significant carbonate content.
Example #5:
A composite coating, having an oxide base layer and a calcium phosphate outer layer, was prepared using an identical procedure as described in example #4, except that a porous titanium substrate was used, in this case. Titanium wire (99.9% purity, 0.25 mm diameter) was used to form a porous substrate with an average pore diameter of approximately 300 micrometres. The composite coating obtained on this substrate was dense, uniform, and well adhered to the substrate without clogging the pores
Claims (21)
1. A process for forming a corrosion resistant and bioactive composite coating on a conductive implantable device using a single electrolyte bath containing Ca- and P- bearing ions, the coating comprising a base layer essentially comprised of a crystalline oxide, and an outer layer essentially composed of a calcium phosphate compound selected from the group consisting of Alpha- and Beta- tricalcium phosphate, calcium hydroxyapatite, calcium deficient hydroxyapatite, carbonate-containing hydroxy-apatite, fluro-apatite and mixtures thereof, the process comprising the steps of:
a) immersing the implantable device in an electrolyte containing Ca- and P- bearing ions;
b) passing an electrical charge of anodic current through the electrolyte to the implantable device, the duration of the anodic charge and the magnitude of the current being effective to permit the growth of an oxide film on the implantable device with a desirable thickness;
c) passing the second electrical charge of current through the electrolyte to the implantable device, the second electrical charge being a charge of cathodic current, having a waveform, duration, and magnitude effective to allow electrodeposition of a calcium phosphate coating of desired thickness on the base layer, and d) repeating steps (b) and (c) a plurality of times to form a composite coating on the implantable device comprising an oxide base layer and a calcium phosphate outer layer of desirable thicknesses.
a) immersing the implantable device in an electrolyte containing Ca- and P- bearing ions;
b) passing an electrical charge of anodic current through the electrolyte to the implantable device, the duration of the anodic charge and the magnitude of the current being effective to permit the growth of an oxide film on the implantable device with a desirable thickness;
c) passing the second electrical charge of current through the electrolyte to the implantable device, the second electrical charge being a charge of cathodic current, having a waveform, duration, and magnitude effective to allow electrodeposition of a calcium phosphate coating of desired thickness on the base layer, and d) repeating steps (b) and (c) a plurality of times to form a composite coating on the implantable device comprising an oxide base layer and a calcium phosphate outer layer of desirable thicknesses.
2. A process as claimed in claim 1, wherein, said electrolyte has a pH ranging from 3 to 8 and a Ca/P molar ratio ranging from 1 to 2.
3. A process as claimed in claim 1, wherein, said electrolyte has a pH ranging from 3 to 5.
4. A process as claimed in claim 1, wherein, said electrolyte has a Ca/P molar ratio close to 1.67.
5. A process as claimed in claim 1, wherein, the concentration of calcium ions in said electrolyte is less than 0.1 mole per litre.
6. A process as claimed in claim 1, wherein, the temperature of said electrolyte is maintained in the range of from about 20 to 90 degrees Centigrade.
7. A process as claimed in claim 1, wherein, said electrolyte contains ions selected from the group consisting of F-, C03--, HC03-, N03-, Cl-, Mg++, Al3+, and ions of platinum group family.
8. A process as claimed in claim 1, wherein, said electrolyte contains biologically active substances selected from the group consisting of collagen, proteins, bone morphogenic protein and antibiotics.
9. A process as claimed in claim 1, wherein, said electrolyte contains oxidizing agents and wetting agents.
10. A process as claimed in claim 1, wherein, said implantable device is made of titanium or titanium-based alloys.
11. A process as claimed in claim 1, wherein, said implantable device has at least one roughened or porous region on its surface.
12. A process as claimed in claim 1, wherein, the second electrical charge being a charge of cathodic current having a pulsed - on/off waveform, a sine waveform or a constant current waveform.
13. A process as claimed in claim 1, wherein, the second electrical charge being a charge of cathodic current having a pulsed -on/off waveform with a pulse width in the range of from about 1 millisecond to about 100 milliseconds, and a pulse spacing in the range of from about 1 millisecond to about 100 milli-seconds.
14. A process as claimed in claim 1, wherein, the second electrical charge being a charge of cathodic current of about 5 mA/Cm2.
15. A process as claimed in claim 1, wherein, said implantable device is treated with a colloidal titanium-based surface adjustment agent prior to immersing said implantable device into said electrolyte and effecting electrolysis.
16. The process of claim 1, further comprising the step of contacting the coated implantable device with an alkaline solution containing calcium ions for a sufficient time to obtain a composite coating with a calcium phosphate outer layer having a Ca/P molar ratio close to 1.67.
17. A process as claimed in claim 16, wherein, said alkaline solution is a diluted calcium hydroxide solution containing about 0.001 moles per litre calcium ions, maintained at about 80 degrees Centigrade.
18. The process of claim 1, further comprising the step of treating the coated implantable device with a biologically active substance selected from the group consisting of collagen, bone morphogenic protein and antibiotics.
19. The process of claim 1, further comprising the step of sintering the coated implantable device at a temperature between about 200 to 1300 degrees Centigrade.
20. An implantable device comprising an implantable member, having at least part of its surface coated with a composite coating produced by the process of claim 1, said coating having a base layer essentially consisting of a crystalline metal oxide, and an outer layer essentially consisting of calcium phosphate selected from the group consisting of Alpha- and Beta- tri-calcium phosphate, calcium hydroxyapatite, calcium deficient hydroxyapatite, carbonate-containing hydroxyapatite, fluro-apatite and mixtures thereof.
21. A conductive substrate having at least part of its surface coated with a composite coating produced by the-process of claim 1, said coating having a base layer essentially consisting of a crystalline metal oxide, and an outer layer essentially consisting of calcium phosphate selected from the group consisting of Alpha-and Beta- tricalcium phosphate, calcium hydroxyapatite, calcium deficient hydroxyapatite, carbonate-containing hydroxyapatate, fluro-apatite and mixtures thereof.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2073781 CA2073781A1 (en) | 1992-07-13 | 1992-07-13 | Process for forming bioactive composite coatings on implantable devices |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2073781 CA2073781A1 (en) | 1992-07-13 | 1992-07-13 | Process for forming bioactive composite coatings on implantable devices |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2073781A1 true CA2073781A1 (en) | 1994-01-14 |
Family
ID=4150150
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2073781 Abandoned CA2073781A1 (en) | 1992-07-13 | 1992-07-13 | Process for forming bioactive composite coatings on implantable devices |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2073781A1 (en) |
Cited By (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1996007438A1 (en) * | 1994-09-07 | 1996-03-14 | Dot Dünnschicht- Und Oberflächentechnologie Gmbh | Method for the electrodeposition of hydroxyapatite layers |
| WO1996024391A1 (en) * | 1995-02-10 | 1996-08-15 | Technische Universität Dresden | Process for producing a graduated coating of calcium phosphate phases and metallic oxide phases on metal implants |
| DE102004022768A1 (en) * | 2004-05-05 | 2005-12-08 | Heraeus Kulzer Gmbh | Medical implants with glass-ceramic-like multi-element surface layer |
| EP1618904A1 (en) * | 2004-06-30 | 2006-01-25 | Friadent GmbH | Implant with a biofunctionalized surface and method for its production |
| WO2006007730A1 (en) * | 2004-07-21 | 2006-01-26 | The University Of British Columbia | Method of electrolytically depositing a pharmaceutical coating onto a conductive osteal implant |
| WO2008025122A1 (en) * | 2006-08-30 | 2008-03-06 | The University Of British Columbia | Bioceramic composite coatings and process for making same |
| WO2008146113A2 (en) * | 2007-05-28 | 2008-12-04 | Fin-Ceramica Faenza S.P.A. | Process for coating a surface of a metal element to increase osteointegration thereof and prosthetic device including said element |
| ITMI20102071A1 (en) * | 2010-11-09 | 2012-05-10 | Chemical Ct S R L | ELECTRODEPTISITION OF NANOMETRIC HYDROXYAPATITIS ON PROSTHETIC PLANTS AND ELECTROLYTIC PROCESS FOR ITS REALIZATION |
| EP3072534A1 (en) * | 2013-03-11 | 2016-09-28 | DOT GmbH | Method for the preparation of a functionalised implant and functionalised implant |
-
1992
- 1992-07-13 CA CA 2073781 patent/CA2073781A1/en not_active Abandoned
Cited By (12)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1996007438A1 (en) * | 1994-09-07 | 1996-03-14 | Dot Dünnschicht- Und Oberflächentechnologie Gmbh | Method for the electrodeposition of hydroxyapatite layers |
| US5759376A (en) * | 1994-09-07 | 1998-06-02 | Dot Dunnschicht- Und Oberflaechen-Technologie Gmbh | Method for the electrodeposition of hydroxyapatite layers |
| WO1996024391A1 (en) * | 1995-02-10 | 1996-08-15 | Technische Universität Dresden | Process for producing a graduated coating of calcium phosphate phases and metallic oxide phases on metal implants |
| US5723038A (en) * | 1995-02-10 | 1998-03-03 | Jurgen Hofinger | Process for producing a gradient coating made of calcium phosphate phases and metal oxide phase on metallic implants |
| DE102004022768A1 (en) * | 2004-05-05 | 2005-12-08 | Heraeus Kulzer Gmbh | Medical implants with glass-ceramic-like multi-element surface layer |
| EP1618904A1 (en) * | 2004-06-30 | 2006-01-25 | Friadent GmbH | Implant with a biofunctionalized surface and method for its production |
| WO2006007730A1 (en) * | 2004-07-21 | 2006-01-26 | The University Of British Columbia | Method of electrolytically depositing a pharmaceutical coating onto a conductive osteal implant |
| WO2008025122A1 (en) * | 2006-08-30 | 2008-03-06 | The University Of British Columbia | Bioceramic composite coatings and process for making same |
| WO2008146113A2 (en) * | 2007-05-28 | 2008-12-04 | Fin-Ceramica Faenza S.P.A. | Process for coating a surface of a metal element to increase osteointegration thereof and prosthetic device including said element |
| WO2008146113A3 (en) * | 2007-05-28 | 2009-02-05 | Fin Ceramica Faenza Spa | Process for coating a surface of a metal element to increase osteointegration thereof and prosthetic device including said element |
| ITMI20102071A1 (en) * | 2010-11-09 | 2012-05-10 | Chemical Ct S R L | ELECTRODEPTISITION OF NANOMETRIC HYDROXYAPATITIS ON PROSTHETIC PLANTS AND ELECTROLYTIC PROCESS FOR ITS REALIZATION |
| EP3072534A1 (en) * | 2013-03-11 | 2016-09-28 | DOT GmbH | Method for the preparation of a functionalised implant and functionalised implant |
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