EP2212261A2

EP2212261A2 - Method of fabricating porous ceramic structures based on calcium phosphates, alumina or zirconia

Info

Publication number: EP2212261A2
Application number: EP08842450A
Authority: EP
Inventors: Claudia Marina Souto Ranito Lourenço
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-10-25
Filing date: 2008-10-27
Publication date: 2010-08-04
Also published as: WO2009053835A3; WO2009053835A2; WO2009053835A4; PT103866A

Abstract

The present invention relates to a method of fabricating porous ceramic structures using the replication method and ceramics based on calcium phosphates (hydroxyapatite, tricalcium phosphate) alumina or zirconia as raw materials. The process is based on the selection of a polymeric substrate (polyurethane foam or another of the same type), to be replicated. The substrate is impregnated in a ceramic suspension (consisting of ceramic particles dispersed in a dispersant solution) and dried. The dried sponge is then taken to an initial sinterization in order to eliminate the polymeric substrate and then to a final sinterization, at high temperatures, for the densification of the ceramic network and con¬ sequently obtain mechanical resistance of the final structure. The proposed method allows the fabrication of porous three-dimensional structures with universal geometries, or specific geometries for a certain application, since this is a flexible method based on the identical replica of the polymeric substrate. The method also allows the fab¬ rication of structures with adequate raw materials for the intended application, by changing the ceramic powder composition. In this manner, the present invention can be used in ceramic composite industry, applying the proposed method in prostheses, orthodontia, bone implants, water filters, ion exchanges, catalyser supports, humidity sensors, etc.

Description

Method of fabricating porous ceramic structures based on calcium phosphates, alumina or zirconia Field of the invention

[ 1 ] The present invention relates to a method of producing a porous ceramic structure, using the replication method and ceramic phosphates (hydroxyapatite, tricalcium phosphate), alumina or zirconia as raw materials.

[2] The proposed method allows the fabrication of porous three-dimensional structures with universal geometries, or specific geometries for a certain application, since this is a flexible method based on the identical replica of the polymeric substrate. The method also allows the fabrication of structures with adequate raw materials for the intended application, by altering the ceramic powder composition. In this manner, the proposed method can be used in the ceramic industry namely in prostheses, orthodontia, bone implants, water filters, ion exchanges, catalyser supports, humidity sensors, etc.

Background of the invention

[3] Due to the aging factor in the world population, especially in the developed countries, it is very likely that the number of bone fractures will grow. With the aim of improving the life conditions of people and animals that suffer from bone illnesses, degenerative or accidental, it is necessary to invest in the orthopaedic area. The capacity to walk, to move and to be self mobile must to the aim of near future scientific developments and the near future technologies. In this way we can guaranty the wellbeing of the populations.

[4] The growing search for new materials for orthopaedic or maxi facial surgery, lead to the development, in the last two decades, of ceramics based on hydroxyapatite (HAP, Caio(P0₄)6(OH)₂) and tricalcium phosphate (TCP, Ca₃(PO_t)₂). These materials were used for the repair or replacement of bone defects, owing to it's biocompatibility, bioactivity and osseoconductive behaviour, being similar to natural bone. Depending on the properties needed, zirconia (Zr₂O₃) and alumina (Al₂O₃) have been widely used in locations that are subjected to wear and tear.

[5] Efforts in fabricating porous ceramics with interconnected porous have been made in order to enhance tissue growth. An interconnected structure allows new tissue to penetrate the porous substrate stimulating the growth of new bone tissue.

[6] The porous ceramic structure with interconnected porosity possesses a wide range of applications, as in the environmental applications, namely as water purifiers, ion exchanges, as adsorbents of organic species, catalyser supports and humid sensors.

[7] The document GB2365423 reports the fabrication of artificial ceramic bones based on a slip-cast method, containing ceramic powder, a binding and a pore forming agent. The pore forming agent acts on the slurry forming porosities. This mixture is sintered to eliminate the binding agent and the pore forming agent, enhancing the mechanical stability of the final structure. This method differs from the present invention since the present inventions uses a polymeric substrate and replicates the three dimensional structure into a ceramic substrate, instead of a slip-casting method that uses a pore forming agent. The pore forming agent will promote a non-uniform distribution, which consequently result in un-stable mechanical properties. The final structure of the method used in the present invention possesses a uniform porosity distribution, in a ceramic matrix, originating a high mechanical resistance structure.

[8] The document US20070210493 reports the fabrication of porous ceramics based on a slurry containing particles what will be responsible for the pore forming after the contraction or transformation of the particles during sintering. This invention consists of a mixture of glass, silica and ceramic particles. This mixture is set in a mould, dried and sintered in order to obtain a hard and resistant ceramic. This method differs from the present invention since the present inventions uses a polymeric substrate and replicates the three dimensional structure into a ceramic substrate, instead of using a slurry containing glass and silica particles to be slip casted into a mould that after sintering, the particles will originate porosities due to their contraction. The particles after contraction originate the porosities, will do so in a non-uniform distribution, which consequently result in un-stable mechanical properties. The final structure of the method used in the present invention possesses a uniform porosity distribution, in a ceramic matrix, originating a high mechanical resistance structure.

[9] The document WO2007096102 reports the fabrication of porous ceramic structure based on the casting of a gel containing a dispersed polymer into a ceramic slurry dispersed following in-situ polymerization. This method differs from the present invention, since the present inventions uses a polymeric substrate and replicates the three dimensional structure into a ceramic substrate, instead of using a casting process of a gel containing a dispersed polymer into a ceramic suspension following polymerization in-situ. The polymerization in-situ will not guarantee a uniform porosity distribution as the final structure of the method proposed in the present invention possesses, resulting a high mechanical resistance structure.

[10] The process described in the background of the invention does not guarantee a homogeneous porosity distribution, nor a interconnected porosity. Theses heterogeneities in the structure of an object will put in doubt the mechanical resistance of the object.

[11] In this manner, the possible applications of the porous ceramic structure will be reduced due to mechanical instability. This will not happen with the present invention.

[12] In order to have a good mechanical resistance of a porous ceramic structure, it is necessary to guarantee a homogeneous pore distribution in the entire structure, avoiding internal tensions that will lead cracking of the object, when a certain force is applied.

[13] If a certain force is applied on a structure, with a homogeneous porosity distribution, the applied force will be distributed constantly in the internal structure, avoiding the occurrence of internal tensions.

[14] The presented invention is based on a concept that guaranties a uniform porosity distribution, using a polymeric subtract. Depending on the application of the structure, we can choose a polymeric substrate with a constant pore diameter distribution or a structure with various pore sizes diameter distributions.

[15] The document Key Eng. Mater. VoIs. 284-286 (2005) pp. 341-344, refers to the preliminary development in the fabrication of hydroxyapatite foams for bone replacement, base on the replication method. This method differs from the present invention, since the invention uses an impregnation step and an excess removal step, in a controlled manner, having the polymeric substrate pass through a device with two compressions roles, instead of using a manual role. This manual role will not guarantee a uniform excess removal of the slurry in the polymeric structure. Another distinct aspect between the document and the present invention refers to the step that only the present invention possesses which is the controlled drying step of the impregnated foam, using a humidity chamber at 40⁰C and 60%RH, instead of using an oven at 100⁰C. In result of this, the sedimentation of the ceramic slurry is minimized and I can guaranty a uniformed ring of the impregnated substrate, even in the interior. This will not happen with a simple drying step in a normal oven, accordingly to the report in the referred document.

[16] Another important aspect reported in the present invention and the referred document, relates the usage of a controlled sintering atmosphere of the substrates, accordingly to the present invention, instead of a simple air sintering, according to the referred document. The controlled atmosphere will prevent the substrate from oxidation processes, from phase and composition variations, and other phenomena's that may occur due to the sintering atmosphere not being controlled.

[ 17] Another point that the present invention defends, is the usage of different geometries, instead of using simple squares according to the referred document. The proposed invention is a more flexible process with the advantage that we can fabricate specific geometries for a certain application.

[18] In this way, the differences appointed between the two mentioned processes, result in a ceramic structure with enhanced mechanical properties, and for that reason, the processes has a wide variety of applications, not known or used up to date. Disclosure of Invention

Detailed description of the invention

[ 19] The present invention reports the fabrication method of porous ceramic structures, based on the replication method, using ceramic raw materials that consist of calcium phosphate (hydroxyapatite, HAP, Ca,o(P0₄)6(OH)2), tricalcium phosphate, TCP, Ca₃ (PO₄^), alumina (Al₂O₃)or ZiTCOiUa(Zr₂O₃).

[20] This method consists on a selection of a polymeric substrate (polyurethane substrate), which is then impregnated in a ceramic suspension, that consists of ceramic particles dispersed in a dispersant solution and set to dry. After drying, the impregnated substrate undergoes and initial sintering processes, in order to eliminate the polymeric substrate, and a final sintering, at high temperatures to generate mechanical resistance in the structure.

[21 ] Polymeric substrates, used in the content of the present invention, can be selected from other materials, like the polyurethane foam or other of the same type.

[22] The ceramic suspensions used in the content of the present invention contain ceramic particles disperse in a dispersant solution. The choice of the ceramic powder used as raw materials is determined by the aim of the final product, i.e. the application of the actual ceramic in question. In this manner, when the application refers to, for example, implants where mechanical force is a crucial factor, the recommended ceramic raw material should be alumina or zirconia. In case the implants are used to substitute the natural bone temporally, the recommended raw material should be calcium phosphates.

[23] The dispersant solutions used in the content of the present invention can be, for example non limitative, based on ammonium polycarbonate or other similar compound.

[24] The removal of the excess ceramic suspension is done by passing, the impregnated foam, two or three times through compression rolls.

[25] The impregnated substrate is then dried in a humid chamber in order to gradually dry the impregnated suspension homogeneously.

[26] After drying, the substrate is sintered in an initial stage in order to eliminate the polymeric substrate, leaving behind the ceramic matrix. The polymeric substrate is eliminated, due to high temperature around 350⁰C. After the polymeric substrate removal, the sintering phase continues until the density of the ceramic is around 0.6 and 0.8 g/cm3, which generates mechanical resistance of the ceramic product. Depending on the ceramic and the mixture of ceramics used, the sintering temperature reaches a maximum value of 1200⁰C to 1400⁰C. The sintering temperature is determined using the density of the raw material, which after sintering reaches a value around 0.6 and 0.8 g/cm3.

[27] 1. Substrate selection

[28] The fist stage consists of selecting a polymeric foam with an open pore structure that will be used as a substrate. Once impregnated with the suspension, the polymeric substrate will be eliminated in the sintering stage.

[29] In this manner, the polymeric substrate should agree with the following requirements: (1) Contain an open pore structure with different mean cell diameters; (2) Recover the original shape after impregnation; (3) Be easily and totally eliminated at temperatures below the sintering temperature of the ceramic. Since the medical and environmental requirements need materials with different pore sizes, it is possible to use polymeric foams that are not toxic, with adequate properties. Their are different types of foams that satisfy the above requirements, for example the polyvinyl chloride

(PVC), polyurethane (PU) and other similar materials that posses a density around 21 kg/m³ and permeability > 60 1/min. [30] 2. Preparation of the suspension

[31] The ceramic suspension consists of a 40-60 vol% of solid loading, in a 0.4% dispersant solution (ammonium polycarbonate or similar). The suspension is then stabilised under rotation for 4h whilst the ceramic powder is slowly introduced in the dispersant solution. [32] The suspension is homogenized, with the use of a mixer and a rotation velocity of

350 rpm during 2h-4h to guaranty a perfect homogenizing suspension. [33] After the suspension has been homogenized, the suspension is left to rest, at room temperature during 15-30 minutes, in order to eliminate possible air bubbles. [34] 3. Substrate impregnation

[35] Once the ceramic suspension is prepared, the foams are impregnated in the suspension. The foam is compressed to remove air, immersed in the slurry and then allowed to expand. After impregnation, 25-75% of the slurry is removed from the substrate using compression roles. [36] The excess slurry is removed by passing the substrate through compression roles

(preferentially two roles). The operation is repeated until 25-75% of the impregnated suspension is removed from the polymeric substrate. [37] The suspension has a sedimentary tendency through gravity force, so this stage is crucial for the obtaining of homogeneous structures. If this stage is not successful, porous structures will be obtained with a very fine and variable thickness. [38] The impregnated substrate is dried in a humid chamber at 40⁰C and 60%RH during

24 hours. These drying conditions allow a gradual and uniform drying of the entire foam structure. [39] After drying, the impregnated substrate undergoes thermal treatment in order to eliminate the polymeric substrate and structure densification for mechanical resistance. [40] The thermal treatment consists of a slow warming rate of l°C/min to 500⁰C, soaked of 1 hour, a new warming rate of 5°C/min to 1200-1400⁰C, depending of the type of ceramic used, soaked of 1 hour, and finally a slow cool down in the furnace. [41] During thermal treatment, an inert and controlled atmosphere avoids phase transformation and any oxidation that may occur. The temperature and time of sintering should be done until the product reaches densification around 0.6 to 0.8 g/cm3 and a porosity of around 4% in the ceramic walls of the structure. [42] The porous ceramic structures may be obtained in the pretended shape, using previously moulded polymeric foam, and consequently less finishing procedures, for example machining. [43] The method proposed in the present invention allows the fabrication of hard ceramic structures to be used in various applications, for example in the medical field where these products may be used as bone implants or eye prostheses, in the environmental field for water purification, for ion exchange, for organic species adsorbent, for catalysts support and humidity sensors.

[44] hi case of fabricating bone implants or eye prostheses, it is possible to obtain a structure with similar properties of the natural bone, namely with an interconnected porosity of 65%-80%, mean pore diameter above 150 micrometers, a compression resistance around 1-3 MPa and an elastic moduli around 2-3 GPa. The obtained porous structure possesses a microstructure similar to that of natural bone.

[45] The proposed method is a flexible process that allows the fabrication of universal geometries or specific geometries for each patient, since the method consists of an identical replica of the polymeric substrate. The proposed method also allows the fabrication of implants with different ceramic raw materials depending on the final application of the product, by altering the composition of the raw material.

[46] hi case the manufactured product is to be used in the environmental field, it is possible to use these products in different applications, where it is possible to manufacture a product with a specific pore size necessary for the exact application. Example of carrying out the invention

[47] Production of porous ceramic structures based on hydroxyapatite (Cai₀(PO₄)6(OH)

₂) and tricalcium phosphate (Ca₃(PO₄^).

[48] The fist step consisted on the selection of a polyurethane polymeric substrate with a density around 21 kg/m³ and permeability > 60 1/min. The mean opened pore diameters were around 700 micrometers.

[49] A ceramic suspension was then prepared using the following ingredients:

[50] 60 grammas of hydroxyapatite powder

[51] 20 grammas of tricalcium phosphate powder

[52] 34 grammas of deionised water

[53] 0.3 grammas of ammonium polycarbonate

[54] The dispersant solution, containing ammonium polycarbonate, was agitated with a

350 rpm velocity, and the ceramic powder was slowly added . After adding all the ceramic powder, the suspension was agitated during 3h using the same velocity, until a homogeneous suspension, with no powder agglomerates was found.

[55] The suspension was then left to rest, at room temperature, during 15-30 minutes, in order to eliminate possible air bubbles.

[56] The next step consists on the impregnation of the polyurethane foam in the ceramic suspension. The immersed polymeric foam was compressed and let to expand and withdrawn from the suspension. The next step consist of removing the excess suspension so, the impregnated foam was passed, two times, through a compression roll device.

[57] The substrate was then dried in a humid chamber at 40⁰C and 60%RH for 24 hours.

[58] The dry foam was then slowly heated to 500⁰C at l°C/min and soaked for 1 hour, and then heated to 1300⁰C at 5°C/min for 1 hour. This thermal treatment was held in an inert atmosphere.

[59] After thermal treatment, the porous ceramic structure presented a porosity of 80% and a compression resistance of 2.5 MPa.

Claims

[ 1 ] The fabrication method of the porous ceramicstructures, comprising the following steps: a) selection of the reticulated polymeric substrate from materials that have a density around 21 kg/m³ and permeability superior to 60 1/min; b) preparation of the ceramic suspension from ceramic powders and a dispersant solution; c) impregnation of the polymeric substrate in the ceramic suspension through the compression of the substrate in the suspension; d) removal of the excess suspension from the substrate through compression of the polymeric foam; e) drying the impregnated substrate in a humid chamber; f) initial sintering of the impregnated substrate at temperatures around 350⁰C; g) final sintering of the ceramic skeleton at temperatures around 1200⁰C and 1400⁰C.

[2] Method, according to claim 1, characterized by the reticulated substrate or porous foam which density is around 21 kg/m³, and permeability superior to 60 1/ min.

[3] Method, according to claim 1, characterized by the preparation of a ceramic suspension with 40 to 60 vol% in solids, where a solution with around 0.4% of dispersant is homogenously mixed and the ceramic powder is introduced and slowly dissolved in the solution and the mixture becomes homogeneous.

[4] Method, according to claim 3, characterized by ceramic materials that are chosen from calcium phosphates, for example hydroxyapatite and tricalcium phosphate, alumina, zirconia and mixtures of the referred ceramics.

[5] Method, according to claim 4, characterized by a homogeneous suspension obtained with a mixer at a rotational velocity of 350 rpm during 2h to 4h, and then left to rest, at room temperature, during 15-30 minutes.

[6] Method, according to claim 1, characterized by the removal of excess suspension, performed by the passing of the impregnated substrate through the compression rolls, and repeating the procedure two to four times until 25-75% of the ceramic suspension is removed.

[7] Method, according to claim 1, characterized by the drying step of the impregnated substrate to be held in a humid chamber at 40⁰C and 60%RH during 24 hours.

[8] Method, according to claim 1, characterized by the initial sintering stage consisting of a thermal cycle of a very slow warming rate, around l°C/min until reaching 500⁰C, then soaked of 1 hour, and a new slow warming rate of 5°C/min, until reaching 1200⁰C- 1400⁰C, and soaked of 1 hour and then a slow cool down in the furnace.

[9] Method, according to claim 8, characterized by a controlled atmosphere, where the sintering is performed with an inert gas, for example nitrogen.

[10] The use of the method, according to the referred claims, characterized by the production of ceramic composites, with a porous rigid structure, with a compression strength around 1-3 MPa, with a elastic moduli of 2-3 GPa, destined to be used as prostheses, in orthodontia, as bone implants, as water filters, as catalyze supports, as ion exchange, as humidity sensors, and as other products of the same type.