WO2014162167A1

WO2014162167A1 - Method for producing synthetic hydroxyapatite nanoplates and nanopowder containing synthetic hydroxyapatite nanoplate

Info

Publication number: WO2014162167A1
Application number: PCT/IB2013/052575
Authority: WO
Inventors: Dariusz SMOLEŃ; Aleksandra KĘDZIERSKA; Elżbieta PIETRZYKOWSKA; Tadeusz Chudoba; Witold ŁOJKOWSKI; Wojciech ŚWIĘSZKOWSKI; Krzysztof KURZYDŁOWSKI
Original assignee: Instytut Wysokich Cisnien Polskiej Akademii Nauk
Priority date: 2013-03-30
Filing date: 2013-03-30
Publication date: 2014-10-09

Abstract

The method according to the invention relates to hydroxyapatite with a hexagonal structure. It includes a step of the precursor preparation in a form of homogeneous mixture of the suspension of calcium hydroxide in water and phosphoric acid, and a step of forming of crystalline nanostructures in such precursor. During the step of crystalline nanostructures formation the prepared precursor is subjected to microwave radiation having a power density in the range from 1 to 10 watts per 1 cm3 of the precursor for a period of 10 to 900 seconds. The nanopowder featured in the invention has the average size of nanonoplates, determined by the longest chord of the outline as a result of the designation of the particle size distribution during the dark-field imagining in the transmission electron microscope and its fitting to the resulting Gaussian distribution graph, in the range of 3 to 30 nm.

Description

METHOD FOR PRODUCING SYNTHETIC

HYDROXYAPATITE NANOPLATES AND NANOPOWDER CONTAINING SYNTHETIC HYDROXYAPATITE NANOPLATE

Technical field

The invention relates to a method of producing nanoplates having hexagonal structure from synthetic hydroxyapatite and to nanopowder containing nanoplates of such hydroxyapatite.

Background art

There is a high demand worldwide for materials enabling the regeneration of bone loss caused by accidents, cancer, dental procedures or surgeries, as such wastage often cannot be self-regenerated by the human body. Research is carried out seeking new technologies for repair and regeneration of bone loss. Human bone in 75% of its weight consists of nonorganic substance called bioapatite, which gives the stiffness to the bone and its resistance to mechanic injuries. The use of apatite from human bone material in a bigger scale meets a psychological barrier and carries the risk of transferring the donors' pathogens to the receiver. Therefore, for many years trials have been made to produce synthetic hydroxyapatite having a composition according to the formula Cai₀(PO₄)6(OH)2 and being an exact equivalent of bioapatite. A usefulness of powdered synthetic hydroxyapatite for medical purposes is related to the proportion of calcium and phosphor and the size of such powder grains. Bioapatite forms flat elongated grains with a hexagonal structure, the length of which does not exceed the diagonal of 50 nm. The majority of the produced synthetic hydroxyapatite crystallizes in the monoclinic system, although synthetic hexagonal crystallites are also known. The research results show that the size of hydroxyapatite grains has a decisive influence on the level of osteoblasts development and size of cells apoptosis; the smaller the grain size of hydroxyapatite, the more intense and fast the development of osteoblast is, and the apoptosis less intense. One of the known methods for preparing synthetic hydroxyapatite is the formation of the crystal structures in the mixture of calcium hydroxide solution in water and phosphoric acid. The character of the invention is the closest to the solution disclosed in European Patent number EP1296888. This publication describes the method for preparing the nanopowder containing crystallites of pure hydroxyapatite with a hexagonal structure from the precursor formed by the mixture of calcium hydroxide and phosphoric acid suspension, where the precursor is cyclically and repeatedly placed in three subsequent reaction chambers. As a result of the above motioned process the nanopowder is produced consisting of nanoplates, i.e. structures having one dimension significantly smaller than the other two is produced.

Disclosure of Invention

The aim of the invention was to obtain powdered synthetic hydroxyapatite with a hexagonal structure, being the best equivalent of bioapaptite found in human bones.

A method according to the invention comprises a step of precursor preparation, and a step of nanostructures formation in the form of platelet crystals of this precursor. The precursor is a homogeneous mixture of calcium hydroxide solution in water and phosphoric acid. The invention nature lies in the fact that when nanocrystal structures are being formed the precursor is subjected to microwave radiation with power density in a range of 1 to 10 watts per 1 cm³ of the precursor for a period of 10 to 900 seconds.

In one embodiment of the method according to the invention the amount of microwave energy delivered to the precursor in a stage of crystalline nanostructures formation is at least 10 J per 1 cm³ of the precursor's volume.

In another embodiment of the method according to the invention at the stage of crystalline nanostructures formation the microwave radiation at a frequency in the range of 2.4 to 2.5 GHz is applied.

In another embodiment of the method according to the invention the crystalline phase of the formation of nanostructures is carried out in a reaction vessel of a dielectric material, preferably in a Teflon vessel.

In further embodiments of the method according to the invention the crystallization phase of the formation of nanostructures is carried out in a sealed reaction vessel and ends at the stage when a given pressure in the vessel occurs, not higher than 2 atm. The step of the crystalline nanostructures formation can be also ended after a certain temperature is archived in the vessel, not higher than 150 °C.

In yet another embodiment of method according to the invention in the stage of precursor preparation to a stirred suspension of calcium hydroxide in water, phosphoric acid is dosed, a single dose of acid does not exceeding 0.5 milliliters, and the interval between subsequent doses being not shorter than 0.1 seconds.

A nanopowder according to the invention is characterized by the average size of the nanoplates, determined by the longest chord of the outline as a result of the designation of the particle size distribution of the image when viewed in the dark- field transmission electron microscope and its fitting to the resulting Gaussian distribution graph, ranges from 3 to 30 nm.

In one of the embodiments of the nanopowder according to the invention it has nanoplates with the average size in the range of 3 to 15 nm.

In another embodiment of the nanopowder according to the invention the shape index of the nanoplates, defined as a ratio of nanoplate length and width, is from 2 to 5.

The further embodiment of the nanopowder according to the invention is characterized by a molar ratio of calcium to phosphorus (Ca/P) in nanoplates in the range of 1.55 to 1.65.

In another embodiment of the nanopowder according to the the invention a specific surface area determined by the BET method is greater than 180 m²/g.

In yet another embodiment of the nanopowder according to invention the nanopowder solubility determined according to ISO 10993-14 norm ranges from 5 to 35 mg per 1 dm³.

The nanopowder according to the invention, for example obtained according to the method by the invention, may be used for the treatment of fractures and bone defects and dental cavities.

As a result of the invention realization hydroxyapatite nanopowder was obtained having the characteristics much closer to bioapatite than any other formulas of this type known to date. Brief Description of Drawings

The invention has been described in the exemplary embodiments in the attached drawings, in which Figure 1 shows a TEM image of the nanaoplates group featured in the invention, and Figure 2 shows an enlarged image of one nanoplate. Figure 3 shows an exemplary diffractogram XRD of the nanopowder featured in the invention, and Figure 4 presents an exemplary size distribution of nanoplates in such nanopowder.

Mode for Carrying out Invention

The nanoplates according to the method proposed in the invention are prepared in two stages. First, the precursor is prepared as a homogeneous mixture of a suspension of calcium hydroxide in water and phosphoric acid, afterwards the precursor is subjected to microwave radiation, which results in a process of crystalline nanostructures formation in the form of plates.

Example 1

Into a beaker with 75 ml of water placed on a magnetic stirrer, operating at a speed of 800 rpm, 1.7243 g (0.0233 mol) of calcium hydroxide was added, followed by fifteen minutes of stirring until a homogenic suspension was formed. Then, into the beaker with the resulting suspension 0.95 cm³ of 85% phosphoric acid solution was dispensed in 0.01 ml doses at three second intervals each. The amount of acid added was stoichiometric. When dosing was ended the resulting precursor was left for thirty minutes of stirring. Then, the precursor was transferred to a Teflon reaction vessel, which was sealed and placed in a microwave reactor Magnum II by Ertec and the phase of crystalline nanostructures formation began. In this stage, the precursor was subjected to the microwave radiation at frequency of 2.45 GHz (± 50 kHz) and the power density 8 W/1 cm³ of the precursor. In the same time the pressure in the reaction vessel was controlled by measuring of the force exerted on the inside of the lid of the reaction vessel, and the step of crystalline nanostructures formation was terminated when the pressure in the reaction vessel reached 2 atm. Thereafter, the reactor with the reaction vessel was cooled for five minutes by means of the water cooler installed in the reactor. After opening of the reactor vessel the reaction liquid was decanted from the resulting precipitte. The obtained precipitate was transferred to a filter and washed with 150 ml of anhydrous ethanol, and then dried in air sterilizer in air flow at 30°C for approximately sixteen hours. As a result of this procedure dry nanopowder was obtained, and its examination by TEM (transmission electron microscope) proved it to be pure- phase hydroxyapatite with hexagonal structure having following parameters of the network: a: 9.424 (4) A, and c: 6.879 (4) A, and a molar ratio of Ca/P equal to 1.647. The average size of obtained nanoplates was determined by means of TEM imaging registered using the dark-field technique where the arithmetic mean of the longest chord of the outlines of at least 200 nanoplates was calculated. Thus the determined average size of obtained nanoplates was 5.8 nm, wherein the shape index of the nanoplates, defined as a ratio of nanoplate length and width, ranged from 2 to 3. A specific surface area of the obtained nanopowder was 188.6 m²/g.

Example 2

The preparation of the precursor was the same as in Example 1 , but the stirring time after the dosing of phosphoric acid was extended to thirty-five minutes. Then similarly as in Example 1 the step of nanostructures formation was started in the same microwave reactor, however instead of the pressure the temperature was measured in the reaction vessel. This step was completed when the temperature in the vessel reached 112°C. Similarly to previously presented embodiment dry nanopowder was isolated, which turned out to be pure-phase hydroxyapatite with a hexagonal structure, and its molar ratio of Ca/P was 1.582. The average size of nanoplates, determined using the dark-field technique described above, was 5.6 nm. Figure 4 shows the size distribution of nanoplates obtained in this example, with the shape ratio varying in the range of 4 to 5. A specific surface area of the nanopowder obtained was 223.5 m²/g.

Example 3

The precursor was prepared as described in the first example. Later, similarly as in the first example forming of the nano-structures in a microwave reactor was carried out, the microwave radiation was administered to the precursor for ninety seconds and the dry powdered hexagonal hydroxyapatite was isolated in same manner, and its molar ratio of Ca/P was 1.572. Average size of nanoplates obtained, determined by dark-field technique, was 5.4 nm, and the shape ratio ranged from 4 to 5. A specific surface area of obtained nanopowder was 226.8 m²/g. Figure 1 shows a TEM image of the nanoplates obtained in this example. In the picture, the outline of the nanoplate and the section 5 nm long is designated by the white line.

Example 4

Similarly as in the first example, to a suspension of calcium hydroxide phosphoric acid was added, and then the precursor was intensively stirred until a stable pH within the range from 9 to 10 was obtained. Then the precursor was placed in a Teflon vessel tightly closed and in the same as previously microwave reactor for the step of crystalline nanostructures formation, the microwave energy being provided to the precursor until the temperature of 100°C was achieved. As in the previous examples dry hydroxyapatite powder was extracted, having a molar ratio of Ca/P 1.596. A specific surface area of the nanopowder obtained was 236.4 m²/g. The average size of the nanoplates obtained determined using the previously described technique of the dark-field was 5.4 nm, and the shape ratio ranged from 4 to 5.

The examination using XRD (X-ray Diffraction) confirmed the presence of only one phase of hydroxyapatite, which by the shape and location of the diffraction peaks is mostly similar to the characteristics of natural bone mineral. The example XRD diffraction pattern of nanopowder embodying the invention is shown in Fig. 3. The characteristic feature of such nanopowder is a highly developed specific surface area, which, measured by the BET method (the isotherm method of Brunauer, Em- mett, Teller), greatly exceeds the value of 180 m²/g. Confirmed by ICP method (detection of elements using Inductively Coupled Plasma), the ratio of calcium to phosphorus is in the range of 1.55 to 1.65.

The nanopowder produced by the method according to the invention has features making it ideal for use as material for biocompatible, resorbable bone implants due to the highly developed specific surface area (above 180 m²/g), high homogeneity of the material, crystalic structure, morphology and ratio of calcium to phosphorus corresponding to the natural bioapatite, all this being a guarantee that biological impacts will be similar to that of the natural component of bone material. The rapid and controlled crystallization enabled by the use of microwave technique, ensures a sufficiently rapid and uniform absorption of the material in the body, which is not possible in the case of products which are a mixture of hydroxyapatite in several phases or with morphology of a different shape and a large variation of grain size.

Claims

1. A method for obtaining nanoplates of synthetic hydroxyapatite with a hexagonal structure, comprising a step of precursor preparation, which includes preparation of homogeneous suspension of calcium hydroxide in water and phosphoric acid, and a step of formation from the precursor of the nanostructures in the form of crystalline plates, characterized in that during the step of the crystalline nanostructures formation the prepared precursor is subjected to microwave radiation, with a density of power in the range of 1 to 10 watts per 1 cm³ of the precursor for a period of 10 to 900 seconds.

2. The method for obtaining nanoplates according to Claim 1 , characterized in that the amount of microwave energy delivered to the precursor at the step of nano-crystalline structure formation amounts to at least 10 J per 1 cm³ of the precursor volume.

3. The method for obtaining nanoplates according to Claim 1 or 2, characterized in that during the step of crystalline nanostructures formation the microwave radiation is used having the frequency ranging from 2.4 to 2.5 GHz.

4. The method for obtaining nanoplates according to any of the Claims from 1 to 3, characterized in that the step of crystalline nanostructures formation is carried out in a reaction vessel of a dielectric material, preferably a Teflon vessel.

5. The method for obtaining nanoplates according to Claim 4, characterized in that the step of crystalline nanostructures formation is carried out in a tightly sealed reaction vessel and is terminated when the assumed pressure in the reaction vessel is reached, not higher than 2 atm.

6. The method for obtaining nanoplates according to Claim 4, characterized in that the step of crystalline nanostructures formation is terminated when the assumed temperature in the reaction vessel is reached, not higher than 150°C.

7. The method according to any of the Claims from 1 to 6, characterized in that at the step of precursor preparation to a stirred suspension of calcium hydroxide in water phosphoric acid is dispensed, wherein the single dose does not exceed 0.5 ml and intervals between subsequent doses are not shorter than 0.1 second.

8. A nanopowder comprising of nanoplates of synthetic hydroxyapatite with hexagonal structure, characterized in that the average size of the nanoplates, determined by the method of the outline longest chord based on the assessment of particles size distribution using the dark-field technique in transmission electron microscope and its fitting to the obtained Gauss distribution graph, ranges from 3 to 30 nm.

9. The nanopowder according to the Claim 8, characterized by the average size of the nanoplates in the range of 3 to 15 nm.

10. The nanopowder according to the Claim 8 or 9, characterized by the shape ratio of the nanoplates, defined as a ratio of nanoplate length to its width, in the range from 2 to 5.

11. The nanopowder according to any of Claims from 8 to 11 , characterized by a molar ratio of calcium to phosphorus (Ca P) in nanoplates in the range from 1.55 to 1.65.

12. The nanopowder according to any of Claims from 8 to 12, characterized by a specific surface measured by the BET method being greater than 180 m²/g.

13. Tye nanopowder according to any of Claims from 8 to 12, characterized by the solubility, determined according to ISO 10993-14 norm, from 5 to 35 mg per 1 dm³.

14. The nanopowder according to any of Claims from 8 to 13, characterized in that it was obtained using the method described in any of the Claims from 1 to 6.

15. The use of nanopowder according to any of Claims from 8 to 14, for the treatment of fractures and bone defects, and dental cavities.