WO2017146622A1

WO2017146622A1 - A method of fabricating translucent nano-ceramics

Info

Publication number: WO2017146622A1
Application number: PCT/SE2016/050147
Authority: WO
Inventors: Wei Xia; Xixiao CAI
Original assignee: Wei Xia; Cai Xixiao
Priority date: 2016-02-26
Filing date: 2016-02-26
Publication date: 2017-08-31

Abstract

The present invention relates to a method of preparing translucent ceramic material where the method comprises the formation of a well dispersed aqueous solution of the ceramic material and filtering the solution. The filtering results in a pressure exerted in the filtrate making the obtained filter cake translucent.

Description

A METHOD OF FABRICATING TRANSLUCENT NANO-CERAMICS

FIELD OF THE INVENTION

The present invention relates to a method of preparing translucent ceramic materials by the use of a filtering method. The translucent ceramic materials may be used in optical devices, tissue engineering or as an implant.

BACKGROUND

Transparent polycrystalline ceramics exhibit improved heat resistant and strength compared to single crystals, even though they have lower visible transmittance . There is a growing interest in applying such ceramic-based materials in biomedicine and biomaterials, particularly in the application of direct observation of bone matrix in vitro on bioceramics and percutaneous devices. Today cells and tissue needs to be dyed in order to be studied with an optical microscope. A transparent ceramic should have a compact and pore-less microstructure since pores often contribute to opacity in a ceramic. However, it is a challenge to achieve full densification of polycrystalline ceramics since both a good vacuum level, as well as high temperature and pressure are required during fabrication. There is a need to minimize light scattering to achieve transparency in a ceramic. In the field of optics or material sciences, transparency is the physical property describing a material's ability to allows light to pass through the material without being scattered. Hence a translucent material can be defined as a material allowing light to pass through, but objects on the other side cannot be seen clearly or entirely.

Hydroxyapatite (HA), considered as a polycrystalline ceramic biomaterial having bioactive and biocompatible properties, has been widely used in interdisciplinary fields of science including physics, chemistry, biology, and medicine. HA exhibits osteo-conductive, non-toxic and non-immunogenic properties especially in the field of repairing bones and teeth. However, it is difficult to directly observe HA-cell or HA-tissue interactions with conventional light microscopy methods because HA is normally opaque. Making an HA ceramic transparent will therefore extend its biomedical applications. Previous studies have observed bone matrix formation on transparent HA ceramics simply and dynamically using light microscopy, and studies have been done to investigate bone remodeling on transparent HA ceramic. Furthermore, a transparent HA ceramic can be a good candidate for percutaneous devices since it can work as a window for observation of changes inside the body. Also, several research groups are continuously investigating the growth of osteoblast or osteoblast-like cells on transparent hydroxyapatite ceramics. HA devices for long-term blood pressure and deep body temperature detection have also be studied. In fact, for better detection by modern bio-imaging optical techniques, the transparent HA ceramics can play significant roles here for its excellent biocompatibility. Osteogenic differentiation cascade of living stem cells on transparent HA ceramics have been detected. Thus, transparent HA ceramics can function as windows to track instant biological changes. Generally, to achieve transparent HA ceramics, densification is essential to reduce the porosity. Recently, spark plasma sintering (SPS), hot isostatic pressing (HIP), and pressure-less sintering (PLS) are examples of hydrothermal methods that have been recently used. For instance, transparent hydroxyapatite ceramics have been prepared at ambient-pressure pressure by microwave processing as well as by conventional sintering, or transparent HA have been prepared by pulse electric current sintering method.

In this invention, we describe the fabrication of translucent HA, strontium substituted HA (SrHA) nano-ceramics and AI₂O3 nano-ceramics by a simple filtration system (schematic map of the whole process shown in Fig.10.) and characterize them with a series of spectroscopic, microscopic and mechanical methods.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome the drawbacks of the prior art and provide a straightforward method for providing translucent ceramic materials. The method may be used to prepare any ceramic material and does not require any expensive equipment and is not a multi-step method. The method provides good control with good stability and reproducibility. The present invention provides a new method of fabricating translucent ceramics, such as HA and strontium substituted HA nano- ceramics, via a simple filtration system, by controlling the morphology and size of nanoparticles. The obtained material also has high mechanical strength. The present invention can be widely applied as intermediate products and final products (ceramics), with the focus of bio-medicine or bio-industry. Potential applications will include detection of HA and tissue/cell interactions, translucent teeth strips and ophthalmic biomimetic materials.

In a first aspect the present invention relates to a method as defined in claim 1. In a second aspect the present invention relates to a material obtained by the method according to claim 1.

In a third aspect the present invention relates to a ceramic material obtained by the method according to the present invention.

In a fourth aspect the present invention relates to a cell culture plate comprising the material obtained by the method according to the present invention.

In a fifth aspect the present invention relates to a dental implant comprising the material obtained by the method according to the present invention.

In a sixth aspect the present invention relates to an optical device comprising the material obtained by the method according to the present invention, In a seventh aspect the present invention relates to a window comprising the material obtained by the method according to the present invention.

All the embodiments disclosed in the present application are applicable to all the aspects of the present invention.

Description of drawings Figure 1, schematic view of the filtering system.

Figure 2, SEM images of the intermediate product as revealed by SEM (A) HA (B) 005SrHA. Figure 3, SEM images of the final products as revealed by SEM (A) low magnification HA, (B) High magnification HA, (C) low magnification 005SrHA, (D) high magnification 005SrHA.

Figure 4, transmission electron microscopy (TEM) images at different magnification of HA ceramics as final products.

Figure 5, hardness (HIT) and elastic modulus (EIT) of the HA and 005SrHA ceramic specimen as final products.

Figure 6, transmission of intermediate products and final products: the nanocrystalline HA (A) and calcined 005SrHA ceramics (B) as a function of the wavelength of visible light.

Figure 7, XRD pattern of the ceramics before and after calcination (as intermediate product and final product) for HA and 005SrHA.

Figure 8, FTIR spectra of HA and 005SrHA (as intermediate and final products) in powder and ceramic specimen. Figure 9, optical images (at different magnification) of cell growth under translucent HA ceramic material. Rat osteoblast-like cells were firstly cultured on the tissue culture plate, and then said cells were covered with HA. The optical images were taken by a Carl Zeiss light microscope.

Figure 10, schematic view of the process according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the present application the term "well dispersed" means that particles are evenly distributed in a solution after mixing and that they do not precipitate straight away. In one embodiment well dispersed nanoparticles stays in the solution for several days, preferably weeks, before precipitation.

In the present application the term "essentially spherical" means having a spherical shape but does not have to be a perfect sphere. In the present application the term "precursor material" means the obtained material or cake after filtration.

In the present application the term "intermediate product" means the product obtained after drying the precursor material. In the present application the term "final product" means sintered or calcined material.

In the present application the terms "powder" and "nanoparticles" denotes the same thing if nothing else is stated.

The present invention relates to a method of preparing translucent ceramic materials of for example HA or SrHA. The method is straightforward and does not involve any complicated or tedious steps. The obtained material may be used for a lot of different applications.

This invention can be realized in the following steps.

Synthesis of HA and SrHA nanoparticles The synthesis of HA and SrHA nanoparticles can be simplified and described in the general reaction scheme shown below (Eqn. 1), which is one of the standard routes of precipitating HA based on the reaction between calcium nitrate and ammonium dihydrogen phosphate.

5Ca(N0₃)₂ + 3(NH₄)₂HP0₄ » Ca₅(P04)₃(OH) + 3H₂0 + 10NH₄NO₃ (1) After the nanoparticles are synthesized the particles are isolated by filtration.

According to the present invention a simple filtration system may be used to achieve translucent HA and SrHA precursor material. The obtained material may then be dried and optionally treated by calcination or sintering.

The present invention is based on that the material during the filtration is transformed into a translucent material. This is result of a pressure difference between the pressure above the filter and below the filter where the pressure is measured in the air above and below the filter. In order to obtain translucent material the nanoparticles in the solution needs to be dispersed or preferably well dispersed in the solution. This may be obtained by controlling the process parameters during the formation of the particles in the solution or by adding small amounts of a base or an acid in order to minimize the agglomeration of the nanoparticles. When forming the nanoparticles in the solution, especially HA and SrHA particles, the pH should be maintained at 10 or above in the solutions comprising the reactants and in the reaction solution formed when mixing the solutions comprising the reactants. During the preparation of the particles the solutions comprising the reactants are preferably added intermittently, such as drop wise, to water or to each other during which the pH of the formed aqueous suspension is maintained at 10 or above. The mixing is preferably done by stirring or shaking. An aqueous solution of ammonia or sodium hydroxide may be used to control the pH of the aqueous suspension.

The particle size of the nanoparticles in the solution should preferably be 10-500nm such as 30-300nm or 70-200nm. In one embodiment the particle size is around 20- lOOnm. The nanoparticles are preferably essentially spherical or spherical. In order to keep the particles well dispersed in the solution the particle size should preferably be within these ranges. A more narrow size distribution is also believed to affect the dispersion of the particles. The particles in the solution aggregate to larger clusters which allow them to stay on the filter. The morphology of the particles is preferably spherical.

Prior to filtration the suspension may be sonicated or ultrasonicated (>20kHz) in order to make sure the particles are well dispersed and a homogenous suspension.

Referring now to Figure 1 disclosing a schematic view of the filtering system. The aqueous suspension is filtered using any suitable filtering system 1 and means for creating a reduced pressure 3 for example a pump. The filtering system may comprise a container 5 into which the aqueous suspension 7 is added and where the container has a connection or is connected to a filter 9. The system further comprises a collecting container 11 on the other side of the filter in order to collect the permeate, the liquid passing through the filter. The collecting container 11 may be connected to a means for reducing the pressure 3. The means for reducing the pressure should create a pressure difference between the pressure above the filter (PI) and below the filter (P2) so that the pressure difference (ΔΡ) is 1.1 to lObar.

By having a pressure difference of 1.1 to lObar the nanoparticles and the agglomerated particles in the solution will form a filter cake on the filter and form a densified material (precursor material) with good mechanical properties and translucency. The pressure difference may be 1.5 bar or higher, or 2 bar or higher, or 2.5 bar or higher, or 3 bar or higher, or 9 bar or lower, or 8 bar or lower, or 6 bar or lower, or 4 bar or lower. In order to have a time and cost efficient method the ranges may be 1.5 to 5 bar, or 2-4 bar, or 2.4 to 5 bar, or 2.5 to 3 bar. However the pressure difference may also be dependent on the size of the containers 5 and 1 1.

The filter may have a pore size of not more than 0.8 μπι, preferably 0.1 to 0.8μπι such as 0.15 to 0.5μπι, or 0.2 to 0.4 μπι. Too large pores will result in a high loss of particles while too small pores demands more power full pumps and the method becomes inefficient and expensive. Using a filter having pores of 0.2 to 0.4μπι have shown to be the most efficient. Results have shown a filtration time of 2- 10 minutes. The obtained translucent filter cake may be washed using water or any suitable aqueous media.

The obtained filter cake, or intermediate material, may be air dried or dried in a furnace at elevated temperature to obtain the intermediate product. The dryness of the intermediate product may be 80 wt% or higher, such as 90 wt% or higher, or 95wt% or higher, or 99wt% or higher. The pressure may be reduced or a normal atmosphere may be used. The temperature during drying may be 25- 100°C such as 30-80°C or 40-60°C. The intermediate material may then be sintered or treated by calcination to obtain the final product. The temperature during the sintering may be 500- 1600°C for example 800-1300°C, or 900- 1200°C depending on the material. During the sintering the material becomes denser and the number or the size of the pores or holes in the material are preferably reduced or eliminated. AI₂O3 are preferably sintered at higher temperatures for example at 1 100- 1600°C while HA materials are preferably sintered at lower temperatures. The heating rate may be l- 10°C/min such as 5°C/min. The sample is kept at the end temperature during a sufficient period of time for example 1-5 hours, preferably 2-3 hours. The atmosphere may be normal pressure. After sintering the samples may be cooled at the same rate to room temperature. The grains of the final product may be quadrangular or hexagonal and the grain size may be from approximately 100 nm to 1 μπι, such as 100-300nm, for HA and SrHA nano-ceramic material.

Any suitable substances may be used to prepare the ceramic material according to the present invention, also pre-made or purchased particles may be used. Non- limiting examples are calcium nitrate, strontium nitrate, diammonium hydrogen phosphate and ammonia. A non-limiting list of particle materials is hydroxyapatite (HA), strontium substituted hydroxyapatite (SrHA) and alumina oxide (AI2O3) . Strontium substituted hydroxyapatite may be substituted from more than 0 mol% up to and including 20 mol% such as 0.01mol% or more, or 0.05mol% or more, or 0.1mol% or more, or lmol% or more, or 5mol% or more, or 15 mol% or less, or 10 mol% or less, or 6 mol% or less. A preferred range is 0.01mol% to 3mol%, such as 0.05mol% to lmol%. The obtained HA and SrHA intermediate material has a translucency of 50-65% for HA and 40-50% for SrHA over the wavelengths of 400- 700 nm. The transmittance of the sintered HA nano-ceramic material is 30-50%, such as around 40%, over the wavelengths of 400-700 nm, whereas SrHA nano- ceramic material is approximately 20-40% such as around 30%. The hardness of the sintered HA nano-ceramic material is around 9.0 to 10.0 GPa and the elastic modulus is around 140 to 160 GPa, whereas the SrHA ceramic material is around 8.0 to 12.0 GPa, such as 9- 10GPa, and the elastic modulus is around 130 to 180 GPa, such as 140- 160GPa.

The obtained HA as SrHA ceramic material exhibits a carbonate peak, Figure 8, when analyzed using FTIR which to the knowledge of the present inventors have not been seen before.

Preparation of nanoparticles

The nanoparticles may be synthesized by a typical precipitation procedure.

The nanoparticles are prepared by mixing the reactants and the reactants may be prepared in two or more separate solutions. When preparing HA or SrHA nanoparticles phosphate and calcium reactants are used. The phosphate reactant may be diammonium hydrogen phosphate, ammonium hydrogen phosphate, monosodium phosphate, disodium phosphate or a combination thereof. In one embodiment the phosphate reagent is diammonium hydrogen phosphate. The calcium reagent may be selected from calcium nitrate or calcium chloride. The reactants are mixed in water preferably purified water such as deionized water or distilled water. When preparing strontium substituted HA (SrHA) a strontium reactant is used for example strontium nitrate or strontium chloride.

The concentration of phosphate reagent in the reactant solution may be 0.05-0.5M such as 0.1-0.3M, or 0.2M. The concentration of calcium reagent in the reactant solution may be 0.1-0.7M such as 0.2-0.4M, or 0.3M. The concentration of strontium reagent is dependent on the degree of substitution wanted in the SrHA. In one embodiment the concentration of strontium reagent is 0.001-0.05M such as 0.005-0.03M, or 0.01-0.02M. The pH of the reactant solutions or the reaction solution (the solution formed when the reactants are mixed) is preferably 10 or more, such as 1 1 or more, or 12 or more.

The reactant solutions may be added to water or to each other to form an aqueous suspension. For example the strontium reagent solution may be added to the phosphate reagent solution. In order to avoid rapid growth of crystalline particles and thereby not obtain well dispersed particles the addition of the reactant solution should be done intermittently such as drop wise. The addition is preferably done during stirring or shaking, preferably vigorously. The mixing may be left for 1 hour or longer, such as 5 hours or longer or 15 hours or longer, preferably around 24 hours.

During addition the pH is preferably monitored and kept at 10 or more, such as 1 1 or more, or 12 or more. The pH may be adjusted by addition of an aqueous solution of ammonia or sodium hydroxide. The Ca/P ratio in the aqueous suspension is 1.60- 1.80, such as 1.65- 1.70, or 1.67.

The formation of the particles may be done at a temperature of 20-100°C for example 25-40°C. Applications for the products

Transparent materials or transparent ceramic materials provide opportunities in the tissue-engineering field. The material obtained according to the present invention may be a translucent HA or calcium replaced strontium doped nano- ceramic materials that can be used as culture substrates for example a cell culture plate. After cell seeding on the surfaces of such translucent HA and calcium replaced strontium doped nano-ceramic materials, one can observe the cell attachment, proliferation, and differentiation on such culture substrates. More specifically, it becomes possible to use an optical microscope to monitor the cascade of the same samples throughout the cell culture period. In other words, it will be of great interest to conduct real time monitoring of different cells or neurons cultured on ceramics.

Bioceramics with high translucency and mechanical strength can be used to improve the aesthetics of a tooth. The material obtained by the present method may be used as an implant such as a dental implant or for repairing teeth. The present HA and SrHA ceramics have good translucency and high hardness, equal and even higher than earlier reported data. The final product may be used as the obtained sheets or flakes or it may be cut or crushed into smaller pieces.

The translucent material, especially AI₂O3, may also be used in optical devices or windows and is specifically suitable for devices or windows in high temperature environments since the material may withstand such high temperatures.

EXAMPLES

Example 1. From starting material to intermediate product All the experiments were conducted at room temperature however the reaction temperature may be varied from for example RT to 100°C. The first step was to synthesize HA and SrHA nanoparticles.

For the HA nanoparticles, diammonium hydrogen phosphate was mixed in deionized water to form a clear solution with a constant concentration of 0.05 - 0.5 M. Calcium nitrate solution was prepared in deionized water with a constant stoichiometric Ca/P molar ratio of 1.67 for the formation of Cas(P0₄)30H. The molecular formula usually can be written as Caio(P0₄)6(OH)2 to denote that the crystal unit cell comprises two entities. The initial pH of each solution was adjusted to 10 or higher. The strontium substituted HA nanoparticles were prepared in a similar manner as for the HA by the following procedure. Diammonium hydrogen phosphate was prepared and mixed with deionized water to give a concentration of 0.05 - 0.52 M. The substitution of Ca by Sr was varied between 0 - 20%. For the formation of 5% strontium substituted calcium hydroxyapatite, ((Cag.95Sro.o5(P04)6(OH) ) written as 005SrHA in the following), the calculated concentration of calcium nitrate and strontium nitrate when mixed with deionized water was 0.317 M and 0.013 M , the mixing was done during stirring. In order to avoid rapid growth of crystalline particles, the solutions of pure calcium nitrate and mixed calcium nitrate containing strontium nitrate were added to diammonium hydrogen phosphate solution drop-wise. The whole procedure was performed under vigorous stirring overnight. The precipitate was then kept stationary in the mother liquor for another 24 h.

In order to study if the method was applicable to other ceramic materials AI2O3 nanoparticles were purchased from US Research Nanomaterials, Inc and dissolved in deionized water to form a solution with a constant concentration of 0.05 - 0.3 M.

The second step was to use a vacuum pump system (D-79112) connected with a filter unit from KNF Neuberger GmbH, Freiburg, Germany and a polycarbonate filter paper (Whatman® Nuclepore™ Track-Etched Membranes) with a diameter of 47 mm and pore size 0.2 - 0.4 m. A 20 mL suspension was taken out from each sample and ultrasonicated to ensure a homogeneous suspension of well dispersed spherical particles. The particle size was around 20- 100nm.

The pressure difference between above and under the formed filter cake was 2.4 bar. The samples were dried in air at RT to 60° C after filtration.

From intermediate material to final product HA and strontium doped HA precursor materials (HA, 005SrHA) were dried and then placed in an ordinary non-vacuum furnace (Nabertherm GmbH, Germany) for calcination. The samples for calcination were heated at a rate of 5°C/min up to 1000°C and kept at 1000°C for 2 hours. Then, the samples were cooled at the same rate to room temperature and the final product was then collected.

Results - Intermediate product The first feature seen is that the particles in intermediate products of HA and strontium doped HA materials had spherical morphologies, which are displayed in Figure 2. The average diameter of the spherical particles was around 100 to 300 nm for HA precursor material and 200 to 400 nm for 005SrHA precursor material.

The second feature is the UV-vis transmission spectrum of HA and 005SrHA precursor materials having a thickness of 1 mm, as shown in Figure 6(A-B). The obtained spectrum indicates that the transmittance of such HA precursor material is approaching 60% over the wavelengths of 400-700 nm, whereas for the 005SrHA precursor material it is approximately 45%.

The third feature is the XRD diffraction peaks of HA and calcium replaced strontium doped precursor materials were broad and crystal size range from 10 to 20 nm, as shown in Figure 7.

The fourth feature is the FTIR spectrum of HA and strontium doped precursor materials, which revealed the main infrared band positions and their assignments, as shown in Figure 8. The spectra indicate that both HA and strontium doped precursor materials are carbonate substituted types. The peaks at 2344 cm ¹ and 2362 cm ¹ are attributed to the absorption of atmospheric CO₂ during the precipitation of HA particles. The bands at 1029 cm ¹ and the bands in the range from 565 cm ¹ to 630 cm ¹ are attributed to the presence of orthophosphate ions.

Final product For both final products, HA and strontium doped nano-ceramic materials, the first feature seen is the compact structure for each material and that the inter-granular pores were difficult to detect. The grain morphology is mainly between quadrangular to hexagonal and ranged from 100 to 300 nm.

The second feature is the highly compact structure with grain sizes from approximately 200 nm to 1 μπι for HA and SrHA nano-ceramic material, as shown in Figure 3. Besides, SAED pattern confirmed the poly crystalline nature of the material, as shown in Figure 4.

The third feature is the hardness of HA nano-ceramic material which is around 8.5 to 9.5 GPa and the elastic modulus which is around 140 to 150 GPa, whereas the 005SrHA ceramic material is around 8.5 to 9.5 GPa and 150 to 160 GPa respectively, as shown in Figure 5.

The fourth feature is the UV-vis transmission spectrum of HA and 005SrHA nano- ceramic material in a thickness of 1 mm, as shown in Figure 6(A-B). The obtained spectrum indicates that the transmittance of such HA nano-ceramic material is approaching 40% over the wavelengths of 400-700 nm, whereas 005SrHA nano- ceramic material is approximately 30%.

The fifth feature is that the XRD diffraction peaks of HA and strontium doped nano- ceramic materials were sharp and crystal size range from 100 to 200 nm, as shown in Figure 7. The sixth feature is the FTIR spectrum of HA and calcium replaced strontium doped nano-ceramic materials, which revealed the main infrared band positions and their assignments, as shown in Figure 8. The spectra indicate that both HA and strontium doped nano-ceramic materials are carbonate substituted types. The peaks at 2344 cm ¹ and 2362 cm ¹ are attributed to the absorption of atmospheric CO during the precipitation of HA particles. The bands at 1029 cm ¹ and in the range from 565 cm ¹ to 630 cm ¹ are attributed to the presence of orthophosphate ions. The peaks of phosphate groups at 630 cm ¹, 961 cm ¹, 1095 cm ¹ become better resolved in HA and calcium replaced strontium doped nano-ceramic materials. EXAMPLE 2: Fabrication of HA translucent nano-ceramics Preparation of HA nanoparticles

HA nanoparticles were synthesized by a typical precipitation procedure. Diammonium hydrogen phosphate was individually prepared in deionized water to form a clear solution with a constant concentration of 0.2 M. Calcium nitrate was prepared in deionized water with a constant stoichiometric Ca/P molar ratio of 1.67 for the formation of Cas(P0₄)30H. The molecular formula usually can be written as Caio(P0₄)6(OH) to denote that the crystal unit cell comprises two entities. The initial pH of each solution was adjusted to 10.

In order to avoid the rapid growth of crystalline particles, the solution of pure calcium was added to diammonium hydrogen phosphate solution drop-wise. The whole procedure was performed under vigorous stirring overnight. The precipitate was then kept stationary in the mother liquor for another 24 h.

Preparation of translucent HA nano-ceramic precursor materials

A 20 mL suspension was taken out from the mother liquor and ultrasonicated to ensure a homogeneous suspension of well dispersed spherical particles. The particle size was around 20-100nm. The suspension was made into a precursor material through a simple glassware based filtration system induced by a laboratory pump (Pmax= 2.4 bar). The obtained filter cake was dried in air at room temperature after filtration. The vacuum pump systems (D-791 12) were introduced here. It was connected with a filter unit was purchased from KNF Neuberger GmbH, Freiburg, Germany and the polycarbonate filter paper (Whatman® Nuclepore™ Track-Etched Membranes) with a diameter of 47 mm and pore size 0.4 pm was used throughout the whole procedure. Preparation of translucent HA nano-ceramics

HA precursor materials were completely dried and then placed in an ordinary non- vacuum furnace (Nabertherm GmbH, Germany) for calcination. The samples for calcination were heated at a rate of 5°C/min up to 1000°C and kept at 1000°C for 2 hours. Then, the samples were cooled at the same rate to room temperature. HA translucent nano-ceramics are obtained.

EXAMPLE 3: Fabrication of translucent SrH A nano-ceramics

Preparation of SrHA nanoparticles

SrHA nanoparticles were synthesized by a typical precipitation procedure. Diammonium hydrogen phosphate was individually prepared in deionized water to form a clear solution with a constant concentration of 0.2 M. For the formation of 0.05% strontium substituted calcium hydroxyapatite, (Cag.95Sro.o5(P04)6(OH) ) written as 005SrHA in the following), the calculated concentration of calcium nitrate and strontium nitrate is 0.317 M and 0.013 M and both chemicals were mixed in deionized water under stirring.

In order to avoid the rapid growth of crystalline particles, the solutions of pure calcium nitrate and mixed calcium nitrate containing strontium nitrate were added to diammonium hydrogen phosphate solution drop -wise. The whole procedure was performed under vigorous stirring overnight. The precipitate was then kept stationary in the mother solution for another 24 h.

Preparation of translucent SrHA precursor materials

A 20 mL suspension was taken out from the mother liquor and ultrasonicated to ensure a homogeneous suspension of well dispersed of spherical particles. The particle size was around 20-100nm. The suspension was made into a precursor material through a simple glassware based filtration system induced by a laboratory pump (Pmax= 2.4 bar). The obtained filter cake were dried in air at room temperature after filtration.

The vacuum pump systems (D-791 12) connected with a filter unit was purchased from KNF Neuberger GmbH, Freiburg, Germany and the polycarbonate filter paper (Whatman® Nuclepore™ Track-Etched Membranes) with a diameter of 47 mm and pore size 0.4 pm was used throughout the whole procedure.

Preparation of translucent SrHA ceramics

SrHA precursor materials were completely dried and then placed in an ordinary non-vacuum furnace (Nabertherm GmbH, Germany) for calcination. The samples for calcination were heated at a rate of 5°C/min up to 1000°C and kept at 1000°C for 2 hours. Then, the samples were cooled at the same rate to room temperature. Translucent SrHA nano-ceramics were thus obtained.

EXAMPLE 4: Fabrication of translucent AI2O3 nano-ceramics

AI2O3 nanoparticles, spherical, were purchased from be US Research Nanomaterials, Inc and suspended in deionized water to form a solution with a constant concentration of 0.2 M. A 20 mL suspension was taken out from the mother liquor and ultrasonicated to ensure a homogeneous suspension. The particle size was around 20-100nm. The suspension was made into a precursor material through a simple glassware based filtration system as described above induced by a laboratory pump (Pmax=2.4 bar). The obtained filter cake was dried in air at room temperature after filtration.

AI₂O3 precursor materials were completely dried and then placed in an ordinary non-vacuum furnace (Nabertherm GmbH, Germany) for calcination. The samples for calcination were heated at a rate of 5°C/min up to 1000°C and kept at 1000°C for 2 hours. Then, the samples were cooled at the same rate to room temperature. Translucent AI₂O3 nano-ceramics were thus obtained.

Claims

1. A method of preparing translucent ceramics wherein the method comprises the steps of:

- preparing an aqueous suspension of dispersed essentially spherical nanoparticles of ceramic material;

-filtering the suspension to obtain a filter cake using a filter having a pore size of 0.2-0.4μπι; and wherein the pressure difference between above the filter and below the filter is 2- lObar.

2. The method according to claim 1 wherein the pressure difference is 2.4-5bar.

3. The method according to claim 1 or 2 wherein the filter cake is sintered.

4. The method according to any one of the preceding claims wherein the filter cake is dried at a temperature of 25- 100°C prior to sintering and wherein the sintering is done at a temperature of 500 to 1600°C.

5. The method according to any one of the preceding claims wherein the ceramic material is hydroxyapatite, ion substituted hydroxyapatite wherein the substitution ion may be Sr, or alumina oxide.

6. The method according to any one of the preceding claims wherein the size of the nanoparticles is 10-500nm and wherein the particles are essentially spherical.

7. The method according to any one of the preceding claims wherein the sintering is done at atmospheric pressure.

8. The method according to any one of the preceding claims wherein the nanoparticles are HA or SrHA nanoparticles and are prepared by adding reactants intermittently to water during which the pH of the aqueous suspension is maintained at 10 or above;

-allowing the reactants to form the nanoparticles during which the pH of the aqueous suspension is maintained at 10 or above by adding a suitable base; and -wherein the temperature during the formation of the nanoparticles in the aqueous suspension is 20- 100°C.

9. The method according to claim 8 wherein an aqueous solution of ammonia or sodium hydroxide is used to control the pH of the aqueous suspension.

10. The method according to any one of the preceding claims wherein the suspension is ultrasonicated prior to filtration.

11. A ceramic material obtained by the method according to any one of claim 1 to 10.

12. A cell culture plate comprising the material according to claim 11.

13. A dental implant comprising the material according to claim 1 1.

14. An optical device comprising the material according to claim 1 1.

15. A window comprising the material according to claim 1 1.