RU2741918C1 - Method of producing biocompatible porous zirconium dioxide ceramics for endoprosthesis replacement - Google Patents

Abstract

FIELD: medicine.

SUBSTANCE: invention refers to medicine, more specifically to the technology of producing biocompatible composite materials which can be used for making bone implants in restorative surgery. Liquid-phase directed synthesis of precursor powders in ZrO₂-Y₂O₃-Al₂O₃ system is carried out by selecting as initial reagents salts ZrO(NO₃)₂⋅2H₂O, Y(NO₃)₃⋅5H₂O and Al(NO₃)₃⋅9H₂O, from which diluted solutions are prepared and two-stage precipitation of hydroxides from corresponding salts is carried out with aqueous solution of ammonia NH₄OH until complete precipitation of all hydroxides, mixing and obtaining sediment with high degree of homogeneity. Gel-like precipitate is filtered and subjected to rapid freezing at -25°C for 24 hours, after which the synthesized powder composition [(ZrO₂)_0.97(Y₂O₃)_0.03]_0.8(Al₂O₃)_0.2 dried and thermally treated at 800°C. Synthesized powder is added with optimum composition of pore-forming additive with content of 10 wt% of calcium hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ and 30 wt.% (NH₄)₂CO₃, after which obtained nanopowder mixtures are formed into compacts by cold uniaxial pressing at pressure of 100 MPa and sintered in air at 1300°C with isothermal holding for 2 hours and heating rate of 350-400°/h.

EFFECT: disclosed method provides synthesis and production of composition based on nanopowders [(ZrO₂)_0.97(Y₂O₃)_0.03]_0.8(Al₂O₃)_0.2 highly porous strong ceramics for medical purposes for replacement surgery and transplantology.

1 cl, 1 tbl, 4 dwg

Description

The invention relates to medicine, specifically to a technology for producing biocompatible composite materials that can be used for the manufacture of bone implants in reconstructive surgery.

Ceramic materials currently occupy a priority position in the world market and are widely demanded in various fields of industry and medicine. This is due to their high corrosion, chemical, radiation resistance, thermal stability, low thermal conductivity, which makes it possible to operate ceramic elements for a long time under the influence of chemically aggressive media and elevated temperatures without degradation of properties. The best properties are found for ceramic materials made of partially stabilized zirconium dioxide (t-ZrO ₂ ), which is obtained by adding stabilizing additives such as yttrium, cerium oxide, etc. at the stage of synthesis of the precursor powder [1-3]. The attractiveness of ceramics based on t-ZrO ₂ for medical use in comparison with metals and polymers is due to the exceptional chemical inertness, high strength and good compatibility with the human body.

Wear-resistant ceramics, metals and alloys, and various polymers are used for the manufacture of endoprostheses. They can be easily processed to achieve a good fit between the parts of the prosthesis structure. The most popular material for creating prostheses is stainless steel alloys due to the low cost of the product. However, metal structures often cause allergic reactions and are rejected by the body, which reduces their service life and leads to the need for repeated surgical intervention and replacement of the prosthesis. Polymeric materials are also not always compatible with the body. In addition, they have low wear resistance and are capable of being destroyed due to the effect of biological fluids in the human body [4, 5].

In recent years, more and more attention has been paid to the development of biocompatible ceramics for replacement surgery and endoprosthetics. The creation of domestic competitive ceramic compositions based on zirconium dioxide is associated with the use of nanodispersed powders. In recent years, to produce nanosized powders based on t-ZrO ₂ is widely used the so-called method of "soft" chemistry based on the synthesis of nanoparticles from aqueous solutions of corresponding metal salts at relatively low (~300 ° C) temperatures. The ability to monitor and control the conditions of the process makes it possible to obtain precursor powders of a given chemical, phase, and granulometric composition [2, 6-8].

When creating a composite for implants with characteristics close to those of bone tissue, it is optimal to combine materials with the required mechanical and biological properties. The developed pore structure of the composite provides strong adhesion of the implant to the tissues, which is an important condition for ensuring high strength characteristics and durability of the structure, for example, in hip joint prosthetics. Ceramics based on t-ZrO ₂ are used in medicine for reconstruction and replacement of bone tissue, because it does not have a toxic effect on the body and is capable of maintaining mechanical characteristics for a long time, being in a biological environment [9].

Porous ceramic materials are a special class of materials whose performance is determined by pore volume and pore size. The structure of the material should allow bone tissue to grow deeper, which requires a developed pore system. Modern technological methods for increasing the porosity of ceramics are quite diverse [8, 10-12].

Highly porous ceramics are usually prepared by a simple and efficient solid-phase sintering method with the introduction of pore-forming additives. Activated carbon, alcohols, amines, as well as ammonium carbonates and nitrates and other compounds are used as pore-forming agents, which decompose when heated to form gaseous products, forming a porous structure during sintering of the material. One of the highly efficient and safe blowing agents is ammonium carbonate (NH ₄ ) ₂ CO ₃ , the decomposition of one mole of which releases three moles of the gaseous phase, which contributes to the formation of a developed pore structure in the ceramic matrix [13-14].

When the porosity of ceramics exceeds 40%, its strength drops sharply; therefore, such additives that can have both pore-forming and reinforcing properties, for example, aluminum hydroxide, are of interest.

The open porosity of human tubular bones is 40-50%, the modulus of elasticity is ~ 100 GPa. The use of ceramic endoprostheses implies constantly occurring elastic stresses / relaxation in the place of contact of the implant with human bone and muscle tissues. For the most durable use of the implant, the elastic properties of bone and ceramic must be comparable to prevent possible mutual destruction. From a medical point of view, one of the most promising materials for the manufacture of implants is calcium hydroxyapatite Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ (HAP) due to its similarity to bone tissue [15].

In the modern scientific literature, there are works devoted to the creation of biomaterials, both from zirconium dioxide and from HAP. However, it is difficult to make durable all-ceramic constructions from these materials due to insufficiently strong adhesion of the t-ZrCO ₂ implant to bone and muscle tissue and low strength and fracture toughness of HAP. Therefore, in recent years, special attention has been paid to the creation and study of combined materials from a mixture of t-ZrO ₂ and HAP. Preparation of native t-ZrO ₂ / HA composites for reconstructive medicine, combining high strength and biocompatibility, simple and effective method is currently an urgent problem [4, 16-18].

The disadvantages of the known methods for producing biocompatible porous ceramics based on zirconium dioxide are as follows.

The use of a complex multistage technology for the synthesis of oxide nanopowders-precursors, including several stages of grinding, entails the need to clean the powders from unwanted impurities, which increases energy consumption and, as a consequence, the cost of the material. In addition, the use of zirconium and yttrium chlorides as starting reagents leads to a negative effect of chlorine ions on the crystalline structure of ceramics, which also entails the introduction of an additional purification step.

Often, organic substances (alcohols, amines, etc.) are used as pore-forming and sintering additives, which can have a negative effect on the human body. In addition, pore-forming agents help to loosen the material and reduce its strength, which leads to the need to increase the sintering temperature or introduce additional sintering additives that contaminate the final material.

In view of the peculiarities of the mechanical properties of ceramics made of zirconium dioxide and hydroxyapatite, their use separately can lead to the production of a final material with low strength and fracture toughness with porosity above 40%.

The objective of the invention is to develop a full-stage method for producing durable highly porous ceramics based on zirconium dioxide with open porosity of at least 45%, modulus of elasticity of at least 90 GPa with the possibility of varying these parameters, promising for use as a material for endoprosthetics.

According to the invention, a method for producing biocompatible porous ceramics based on zirconium dioxide for endoprosthetics includes several stages: liquid-phase directed synthesis of precursor powders, introduction of pore-forming additives, consolidation of nanopowders to obtain final ceramics of a given chemical composition, while liquid-phase directed synthesis of precursor powders in the ZrO ₂ system -Y ₂ O ₃ -Al ₂ O _{3 is} carried out by choosing salts ZrO (NO ₃ ) ₂ ⋅2H ₂ O, Y (NO ₃ ) ₃ ⋅5H ₂ O and Al (NO ₃ ) ₃ ⋅9H ₂ O as starting reagents , from which dilute solutions are prepared and a two-stage precipitation of hydroxides from the corresponding salts with an aqueous solution of ammonia NH ₄ OH is carried out until all hydroxides are completely precipitated, mixing and a precipitate with a high degree of homogeneity is obtained, then the gel-like precipitate is filtered and subjected to rapid freezing at -25 ° C for 24 hours, after which the synthesized powder of the composition [(ZrO ₂ ) _0.97 (Y ₂ O ₃ ) _0.03 ] _0.8 (Al ₂ O ₃ ) _0.2 dried and subjected to heat treatment at 800 ° C, then the introduction of pore-forming additives is carried out, for which, using mechanical mixing, an optimal composition of a pore-forming additive with a content of 10 wt. % calcium hydroxyapatite Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ HAP and 30 wt. % (NH ₄ ) ₂ CO ₃ , after which, to obtain the final ceramics of a given chemical composition, the resulting mixtures are molded into compacts by cold uniaxial pressing at a pressure of 100 MPa and sintered in air at 1300 ° C with isothermal holding for 2 h and a heating rate of 350 -400 ° / h.

The technical result achieved by using the claimed set of essential features is as follows.

The use of dilute solutions of nitric acid salts during precipitation is more expedient than concentrated solutions, since this allows a more dispersed and homogeneous precipitation product to be obtained. By controlling the rate of their entry into the mother liquor, a uniform pH is achieved throughout the entire volume and the remoteness of nucleation centers is ensured.

Parallel two-stage precipitation ensures that all hydroxides are completely precipitated and a highly homogeneous precipitate is obtained. Rapid freezing of the gel weakens the forces of interaction between the crystalline particles of the sediment and helps to reduce the degree of its agglomeration.

Solid-phase sintering with the addition of pore-forming burnout additives to the charge to obtain highly porous ceramics is affordable and effective. To obtain porous ceramics for medical use, it is necessary to take into account that sintering additives and pore-forming agents must be non-toxic and safe for humans, not cause allergies and rejection of the body.

The essence of the invention is illustrated by drawings, where FIG. 1 shows a scheme for the synthesis of powders based on t-ZrO ₂ - Al ₂ O ₃ , Fig. 2 - the results of differential thermal analysis of a xerogel with a composition of 80 mol.% (ZrO ₂ ) _0.97 (Y ₂ O ₃ ) _0.03 - 20 mol. % Al ₂ O _3; in fig. 3 - sequence of the formation of a ceramic nanocomposite in the system ZrO ₂ -Y ₂ O ₃ -Al ₂ O ₃ , obtained by the method of co-precipitation: 1 - xerogel after freezing at -25 ° C and drying at 110 ° C; 2 - powder obtained after firing at 800 ° C; 3 - ceramics sintered at 1300 ° C. Designations: ■ - tetragonal solid solution based on ZrO ₂ , • - cubic solid solution based on ZrO ₂ fluorite type, in Fig. 4 - differential pore size distribution of the porous structure of ceramics based on t-ZrO ₂ with the addition of pore-forming compositions (1300 ° C, 2 h).

The claimed method is carried out as follows.

The starting reagents are nitric acid salts ZrO (NO ₃ ) ₂ ⋅2H ₂ O, Y (NO ₃ ) ₃ ⋅5H ₂ O and Al (NO ₃ ) ₃ ⋅9H ₂ O, from which diluted solutions (~ 0.1 M) are prepared ... The use of dilute solutions during precipitation is more expedient than concentrated solutions, since this allows a more dispersed precipitation product to be obtained. The deposition of zirconium oxyhydroxide occurs at pH 2.3-4.2, yttrium hydroxide - 7.0-7.4. The pH of the precipitation of aluminum hydroxide lies in the range of 4.0–5.2; upon reaching the pH of 7.8, the process of dissolution of the precipitate of aluminum hydroxide begins, up to complete dissolution at pH 10.8 [19]. Therefore, the deposition process is carried out in two parallel stages: 1st stage - coprecipitation of nitric acid salts of zirconium and yttrium, 2nd stage - precipitation of aluminum nitrate. After precipitation of a mixture of zirconium and yttrium hydroxides, the precipitate is washed by decantation until the pH of the solution reaches ~ 7.0, freshly precipitated aluminum hydroxide is added and stirred. Zirconium hydroxide is the main component of the coprecipitation product in the system [(ZrO ₂ ) _0.97 (Y ₂ O ₃ ) _0.03 ] _0.8 (Al ₂ O ₃ ) _0.2 and it is characterized by hydrolysis and polymerization reactions with the formation tetrameric hydroxyl complexes [Zr (OH) _2⋅ nH ₂ O] ₄ ⁸⁺ , which lead to the formation of "hard" agglomerates. Therefore, it is necessary that the residence time of the precipitate in the mother liquor be minimal [20, 21].

The gel-like precipitate is filtered and then frozen at -25 ° C (24 h), resulting in an X-ray amorphous xerogel, FIG. 3, curve 1, with a specific surface area of 150 m ² / g and an average pore size of ~ 3-4 nm. Rapid freezing of the gel weakens the forces of interaction between the crystalline particles of the sediment and helps to reduce the degree of its agglomeration.

The processes of thermolysis of the obtained xerogel were investigated by the DTA method, Fig. 2. On the thermogram in the temperature range 110-150 ° C, there is an endothermic effect corresponding to the main stage of sediment dehydration. Xerogels contain a significant amount of adsorbed water in the form of H ₂ O molecules and structural water in the form of OH ^- groups. In addition, powders based on the ZrO ₂ (Y ₂ O ₃ ) - Al ₂ O _{3 system are} characterized by an increased content of structurally bound water as compared to powders based on ZrO ₂ (Y ₂ O ₃ ). To increase the efficiency of dehydration, the sediment is subjected to low-temperature treatment at -25 ° C (24 h), which promotes freezing of most of the water and intensifies the dehydration process at the drying stage. This allows the weight loss in the xerogel to be reduced by up to 10%. Exothermic peak at 435 ° C indicates the onset of crystallization of the metastable cubic solid solution based on zirconium dioxide (c-ZrO _2). The exo effect at 610 ° C corresponds to the crystallization of γ-Al ₂ O ₃ . The broad exothermic effect at 840 ° C indicates the occurrence of the γ-Al ₂ O ₃ → δ-Al ₂ O ₃ phase transition. The average crystallite size in the synthesized powder of the composition [(ZrO ₂ ) _0.97 (Y ₂ O ₃ ) _0.03 ] _0.8 (Al ₂ O ₃ ) _0.2 is 8-10 nm.

To obtain the pore structure of ceramics based on synthesized xerogels and powders, non-toxic and available pore-forming additives are chosen: ammonium carbonate (NH ₄ ) ₂ CO _3, grade "h" and aluminum hydroxide Al (OH) ₃ synthesized by us and calcium hydroxyapatite Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ , as well as mixtures thereof.

To obtain porous ceramics for medical purposes, it is necessary to take into account that sintering additives and pore-forming agents must be safe for humans, not cause allergies and rejection of the body, which interfere with the osseointegration of the structure.

It is known that when a porosity of more than 40% is reached due to pore-forming additives, the strength of the material drops sharply; therefore, aluminum hydroxide is of greatest interest not only as a pore-forming agent, but also as a sintering additive.

In addition, ammonium carbonate is one of the safe and highly effective blowing agents. One mole of (NH ₄ ) ₂ CO ₃ upon decomposition gives three moles of the gaseous phase, which contributes to the formation of a developed porous structure.

From a medical point of view, one of the most promising materials for the manufacture of implants is calcium hydroxyapatite (Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ ) due to its biocompatibility with the human body.

The initial powders are mixed with the selected (optimal) compositions of pore-forming additives based on Al (OH) ₃ , (NH ₄ ) ₂ CO ₃ and HAP in a Fritsch Pulverisette planetary mill in corundum nozzles with corundum grinding bodies for 2 hours. The resulting mixtures are formed by dry uniaxial pressing on a hydraulic press PGR-400 in a steel mold and sintered in air on corundum substrates in a SNOL 6.7 / 1300 furnace at 1300 ° С with isothermal holding for 2 h, heating rate - 350-400 ° С / h A sufficiently high heating rate is selected to reduce the rate of crystallite growth.

Based on the study of the effect of adding individual pore-forming agents (NH ₄ ) ₂ CO ₃ and Al (OH) ₃ to the initial powders, it was not possible to obtain high porosity of ceramics while maintaining its strength. Analyzing the results obtained, pore-forming additives were selected in the form of compositions Al (OH) ₃ + (NH ₄ ) ₂ CO ₃ and HAP + (NH ₄ ) ₂ CO ₃ - table. one.

The introduction of a pore-forming composition with a composition of 15 wt. % Al (OH) ₃ + 20 wt. % (NH ₄ ) ₂ CO ₃ into the initial powders I and II showed that the properties of ceramics largely depend on the composition of the zirconium-containing matrix. Alumina contributes to the inhibition of the growth of zirconium dioxide grains, which makes it possible to increase the strength of the material due to the process of transformation hardening, which is characteristic of nanosized ceramics made of t-ZrO2 _.

In connection with the above, the study of the effect of the composition of pore-forming compositions on the properties of ceramics was carried out using the initial powder II.

In a blowing agent based on a mixture of Al (OH) ₃ and (NH ₄ ) ₂ CO _3, an increase in the amount of ammonium carbonate to 30 wt. % leads to a slight increase in the open porosity of the ceramic (composition 3), and a decrease in the amount of aluminum hydroxide to 10 wt. % has no negative effect on the value of the elastic modulus.

To study the effect of calcium hydroxyapatite on the properties of ceramics, a pore-forming composition is used, in which aluminum hydroxide is replaced with the same amount of calcium hydroxyapatite (10 wt%), the content of ammonium carbonate is also reduced to 10 wt. %. After sintering, the modulus of elasticity of ceramic samples (composition 4) increases to the values required when using ceramics for endoprosthetics. An increase in the elastic modulus is due to the formation of a Ca _0.10 Zr _0.90 O ₂ solid solution, which is formed as a result of the complete decomposition of calcium hydroxyapatite after firing at 1300 ° C and partial thermal diffusion of Ca ^{2+ ions} into the ZrO ₂ lattice [16]. The decrease in the value of open porosity is associated with a significant decrease in the content of (NH ₄ ) ₂ CO ₃ .

An increase in the mixture of pore-forming agents in the amount of ammonium carbonate up to 30 wt. % while maintaining the amount of calcium hydroxyapatite (composition 5) makes it possible to obtain after sintering ceramics with properties closest to the parameters of human tubular bone tissue: open porosity - 48%) and modulus of elasticity - 98 GPa. The closed porosity of all obtained ceramic samples was insignificant and did not exceed 0.16%.

Mercury porosimetry was used to obtain data on the size and distribution of pores in a sample of the obtained ceramic material (Fig. 4).

From FIG. 4 it follows that this ceramic has a bimodal distribution of meso- and macropores in size in the range of 40-800 nm. The first maximum is due to mesopores less than 100 nm, the size of which is commensurate with the grain size. The second maximum is formed by macropores with a size of 100 nm and more. Such large voids in ceramics are due to the formation of a framework of agglomerates of the powders used during pressing, the sizes of which determine the size of the macropores between them. A significant proportion of the pores have a radius of less than 200 nm. According to the data obtained, the average pore size was ~ 180 nm.

The obtained highly porous strong ceramics based on t-ZrO ₂ / HAP in terms of their mechanical (open porosity 48%, modulus of elasticity 94 GPa) and medico-biological (biocompatibility with tissues of the human body) properties are promising as a material for endoprosthetics and can be recommended for use. in transplantology [22].

When creating new medical materials for modern substitution surgery and endoprosthetics, the key issue is the development of technologies for producing high-quality oxide nanocomposites with specified physicochemical and mechanical properties. The claimed invention is a technology for the liquid-phase synthesis of a multicomponent biocompatible ceramic material in the ZrO ₂ -Y ₂ O ₃ -Al ₂ O _{3 system} for creating a ceramic implant for endoprosthetics.

The ratio of oxides and optimal conditions for obtaining a nanocrystalline powder based on zirconium dioxide were selected, and the conditions for its consolidation were identified. Selected complex pore-forming additives for the production of highly porous ceramics based on zirconium dioxide: ammonium carbonate, aluminum hydroxide, calcium hydroxyapatite and their mixture in various percentages. This choice is due to the fact that ammonium carbonate and aluminum hydroxide are available, environmentally friendly and do not pollute the final product, they are safe for the human body, and calcium hydroxyapatite (HAP) is an inorganic component of human bone tissue. The optimal composition of the pore-forming additive with a content of 10 wt. % HAP and 30 wt. % (NH ₄ ) ₂ CO ₃ . Based on the initial powders of the composition [(ZrO ₂ ) _0.97 (Y ₂ O ₃ ) _0.03 ] _0.8 (Al ₂ O ₃ ) _0.2 with pore-forming additives, a highly porous strong ceramic with an open porosity of 48%, elastic modulus was obtained 94 GPa, promising for use as a material for endoprosthetics. It was found that by varying the content of different components of the complex pore-forming agent: (NH ₄ ) ₂ CO ₃ and HAP, the porosity and strength of the material can be varied over a wide range.

All of the above indicates that the proposed technology for the synthesis of nanopowders with the composition [(ZrO ₂ ) _0.97 (Y ₂ O ₃ ) _0.03 ] _0.8 (Al ₂ O ₃ ) _{0.2 is} promising and based on them, a highly porous, durable ceramics medical appointments for substitution surgery and transplantology.

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Claims (1)

The method of producing biocompatible porous ceramics based on zirconium dioxide for endoprosthetics includes several stages: liquid-phase directed synthesis of precursor powders, introduction of pore-forming additives, consolidation of nanopowders to obtain final ceramics of a given chemical composition, while liquid-phase directed synthesis of precursor powders in the ZrO ₂ -Y system ₂ O ₃ -Al ₂ O _{3 is} carried out by selecting the salts ZrO (NO ₃ ) ₂ ⋅2H ₂ O, Y (NO ₃ ) ₃ ⋅5H ₂ O and Al (NO ₃ ) ₃ ⋅9H ₂ O as starting reagents, from which prepare dilute solutions and carry out a two-stage precipitation of hydroxides from the corresponding salts with an aqueous solution of ammonia NH ₄ OH until all hydroxides are completely precipitated, mixing and obtaining a precipitate with a high degree of homogeneity, then the gel-like precipitate is filtered and subjected to rapid freezing at -25 ° C for 24 hours , after which the synthesized powder of the composition [ZrO ₂ ) _0.97 (Y ₂ O ₃ ) _0.03 ] _0.8 (Al ₂ O ₃ ) _{0.2 is} dried and subjected to heat treatment at 800 ° C, then the introduction of pore-forming additives is carried out, for which, using mechanical mixing, an optimal composition of a pore-forming additive is added to the synthesized powder containing 10 wt% calcium hydroxyapatite Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ HAP and 30 wt .% (NH ₄ ) ₂ CO ₃ , after which, to obtain the final ceramics of a given chemical composition, the resulting mixtures of nanopowders are molded into compacts by cold uniaxial pressing at a pressure of 100 MPa and sintered in air at 1300 ° C with isothermal holding for 2 h and a heating rate - 350-400 ° / h.

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