WO2012126904A1

WO2012126904A1 - Method of formation of a nanostructured biocompatible coating on implants for replacement of bone tissues

Info

Publication number: WO2012126904A1
Application number: PCT/EP2012/054887
Authority: WO
Inventors: Ara Arshavirovich Abramyan; Elena Romualdovna Andreeva; Viacheslav Ivanovich Beklemyshev; Liudmila Borisovna Buravkova; Anatoly Ivanovich Grigoriev; Igor Ivanovich Makhonin; Umberto Orazio Giuseppe Maugeri; Irina Mihailovna Meshkova; Vladimir Aleksandrovich Solodovnikov
Original assignee: Closed Stock Company "Institute Of Applied Nanotechnology"; Fondazione Salvatore Maugeri Clinica Del Lavoro E Della Riabilitazione; Sib Laboratories Limited
Priority date: 2011-03-24
Filing date: 2012-03-20
Publication date: 2012-09-27
Also published as: RU2448741C1

Abstract

The invention relates to medicine and medical technology, namely to a method of formation of a biocompatible coating on the surface of implants used in reconstructive surgery for replacement of bone tissues. The disclosed method of formation of a nanostructured biocompatible coating on surfaces of implants consists in deposition of a film of polysilicon on the surface of the implant in a reactor. The obtained film of polysilicon is etched chemically for formation of a nanostructured near-surface layer of porous polysilicon. Etching of the film of polysilicon is carried out by immersion of the implant into a water solution containing a mix of tetrafluoroboric acid, nitric acid and an ammonium salt surfactant, with subsequent washing of the implant with deionized water and drying. Technological applications of the method of formation of a biocompatible coating by obtaining a nanostructured layer of porous polysilicon on surfaces of implants, without dependence from materials used for their manufacturing and from their design features, are extended by the present invention.

Description

METHOD OF FORMATION OF A NANOSTRUCTURED BIOCOMPATIBLE COATING ON IMPLANTS FOR REPLACEMENT OF BONE TISSUES

Field of the invention

The invention relates to medicine and medical technology, namely to a method of formation of a biocompatible coating on the surface of implants used in reconstructive surgery, with replacement of bone tissues with implants.

Such operations are characteristic for traumatology, orthopedics and stomatology.

Dozens of companies in the world are engaged in development and manufacture of implants.

For the reliability of osseointegration, i.e. biological ingrowth of the implant into the bone without inflammatory reactions and decrease in rejection of implants, the contact surface of implants are modified by deposition of elements of macroretention i.e. dents, grooves for integration of the implant with the bone tissue and to form a special biocompatible coating for the germination of bone on the implant surface.

State of the art

Various technological processes for formation of biocompatible coatings on surfaces of im plants are known.

The most widespread biocompatible coatings for implants are coatings on the basis of calcium-phosphatic compounds - hydroxyapatites, which are similar to the composition of a bone tissue.

So, for example, in the technical solutions given in the Russian patents RU No. 2291918, pub. from 2007; No. 2221904, pub. from 2004; No. 2154463, pub. from 2000, a process of formation of biocoatings containing calcium-phosphatic compounds was carried out by placing an implant manufactured from titanium or its alloys into a water solution of acid electrolyte on the basis of orthophosphoric acid and a powder of hydroxyapatite; the formation of a coating was performed by excitation of microarc discharges.

Common shortcoming of the given methods consists in application of orthophosphoric acid as the basis for electrolyte; this acid is a hostile environment.

In the technical solution according to the patent RU No. 2206642, a process of formation of biocoatings containing calcium-phosphatic compounds was carried out by placing a product (implant) in a water solution of electrolyte and then excitating microarc discharges on the surface of the product by superposition of impulses of a current at voltage up to 1000 V. Durability of impulses is 30-400 micro-seconds and a pause between them is not less than 100 micro- seconds. In the variant described in the example 12 of the patent, deposition of a coating on titanium VT 1-00 was carried out within 10 minutes in a water solution of electrolyte with pH 12-14 and with contents of KOH, hydroxyapatite and AI₃O₃, with a the ratio of anodic and cathodic currents equal to 6,7. According to the resulted data the thickness of the obtained coating is roughly 41 microns and porosity is 24%. However significant thickness of the coating decreases durability what can lead to osteoporosis and other complications.

The convenience of using the given solution for obtaining microcoatings on tiny implants, not admitting significant thickness of a coating on the surface, is technologically limited.

It follows from the available art that the required physical-mechanical properties of calcium-phosphatic coatings on titanium implants (thickness, microhardness, porosity, roughness) are different essentially in traumatology, orthopedics or stomatology and they depend on the set of the following conditions:

- purpose of implants (for implantation in an organism, or in a role of artificial limbs - imitations of absent bodies);

- duration of the presence in an organism, necessity of extraction at certain time as a result of restoration of natural bodies, or necessities of replacement, prospective loadings, etc.

Shortcomings of known technical solutions for formation of biocompatible calcium-phosphatic coatings on surfaces of implants, are the following:

- impossibility of regulation of their physical-mechanical properties, basically strength;

- low crack resistance and small fatigue strength in physiological conditions;

- porous surfaces of coatings have irregular structure; cells of pores are various in the sizes; completely monolithic zones are possible in a coating, what decreases efficiency of germination, in particular of bone tissues, and leads to osteoporosis and other complications;

- low stimulating influence on growth of new bone tissues; - the significant amount of phosphorus in compound Caio-_x(HP0₄)_x(P0₄)6-x(OH)2-_x (0<x<l) does not provide reproduction of morphology of bone tissues (see magazine «Nanotechnology Ecology Manufacture^ N. 1 (8), January, 2011, p.102-103).

At present implants with modified surfaces on the basis of coatings of polycrystalline silicon with porous structure, in a near-surface layer, are the most promising.

Biocompatibility of porous polysilicon was described in 1995 (Canham and others, «Bioactive Silicon Structure Through Nanoetching Techniques», magazine «Advanced Materials», 1995, Vol. 7, No. 12, pp. 1033- 1037).

The technical solution related to the use of porous polysilicon as a biomaterial compatible with tissues of a live organism was described in the patent GB No. 2317885, pub. from 08.04.1998. The possibility of using polycrystalline silicon with a nano structured layer of porous polysilicon as a coating on surfaces of implants manufactured basically from titanium or steels is confirmed by the patent US No. 7186267, pub. from 03.06.2007 (application US No. 2004/000313, pub. from 15.01.2004). According to the given technical solution it was confirmed that the presence of a coating of polysilicon with a nano structured layer of porous polysilicon on the surface of implants promotes formation of bone tissues on its surface.

The technical solution according to the patent US No. 7186267 is chosen as the nearest analogue of the present invention.

In the named technical solution, formation of a biocompatible coating on surfaces of implants was carried out by deposition of a film of polysilicon on the surface of the implant in a reactor, and with subsequent etching with formation of a nano structured near- surface layer of porous polysilicon.

As it follows from the given technical solution, the process of formation of a biocompatible covering on the basis of porous polysilicon on surfaces of implants was carried out with use of known technology. According to it a film of polysilicon was deposited on a sublayer of silicomethane (SiH₄) by pyrolysis in reactors working at lowered pressure and with subsequent electrochemical etching of the film with use of an electrolyte containing hydrofluoric acid (HF) (see patent US No. 5348618, pub. from 20.09.1994). The used method of etching is applied for nanostructuring a near-surface layer of a film of polysilicon deposited on the surface of implants manufactured with electrochemically active materials, including titanium and steel, which are widely used in implantology.

The share of titanium and its alloys which are the best for implantology, in regard to biocompatibility and specific strength, amounts approximately to 5% of the metals which are used for implantation. This datum is explained by the low workability of titanium.

At present a significant attention is given to creation of implants from dielectrics, in particular from glass ceramics. Application of implants from the said materials provides reliability of their fixing, accelerates the process of engraftment, reduces the risk of postoperative complications, excludes screw thread cutting in a bone, and facilitates preparation for implantation. The basic technological process of manufacture of the given implants, is the rather cheap casting. What essentially reduces cost of prosthesis.

However dielectric properties of glass ceramic materials exclude the possibility of realization of a technological process on nanostructuring a near-surface layer made of a film of polycrystalline silicon, by the method of electrochemical etching.

Possibility of using the known technical solution according to the patent US No. 7186267 is also technologically limited in case of etching a film of polysilicon, on a relief surface of implants, with the purpose of obtaining a nanostructured layer of porous polysilicon homogeneous in thickness. That is explained by heterogeneity of density of a current in various sectors of a relief surface of implants.

Summary of the invention

The technical advantageous result of the present invention consists in expanding the possibilities of applying the method of formation of a biocompatible coating, by obtaining a nanostructured layer of porous polysilicon on surfaces of implants, without dependence from materials used for their manufacturing and from design features of implants.

A method of formation of a nanostructured biocompatible coating on implants is described as a solution of the given technical problem. It consists essentially in deposition of a film of polysilicon on the surface of the implant in a reactor and subsequent etching with formation of a nanostructured near- surface layer of porous polysilicon. Etching of the film of polysilicon is carried out by immersion of the implant into a mix containing 50- 55% water solution of tetrafluoroboric acid (HBF₄), 70-90% water solution of nitric acid (HNO₃) and anionic surfactants on the basis of ammonium salts of a perfluoro sulfonic acid R_fS0₃NH₄ in amount of 5- 10³ to 5-10^"2 (% b.w.), where R_f is C_sFi₇ or C₂F₅OC₃F₆OC₂F₄ or C₆F₁₃CH₂CH₂. Water solutions of acids are used at the following ratio of their parts by volume: HBF₄: HNO₃ as (100 - 800) : (1 - 1,1) with subsequent washing of the implant with deionized water and drying.

A coating consisting of a film of polysilicon with a thickness equal to 0,4-0,6 micron and with formation of a nanostructured near-surface layer of porous polysilicon with a thickness of no more than 200 nanometri is formed on the surface of the implant according to the present invention.

At realization of the present invention, technological opportunities of the method of formation of a biocompatible coating on surfaces of implants, with obtaining of a nanostructured layer of porous polysilicon are extended as follows:

- the process of chemical etching of a film of polycrystalline silicon is carried out without dependence from used materials of implants of the film of polycrystalline silicon generated on their surfaces, at their processing with a water solution of concentrated tetrafluoroboric acid (HBF₄) and nitric acid (HNO₃);

- the process of oxidation of a polycrystalline film in the presence of nitric acid in a water solution, is accompanied by formation of nitrous acid and ions of nitrosonium (NO⁺) which is the active oxidizing agent. Its dot influence on local zones of crystal structure of polysilicon realizes a process of selective etching in these zones with formation of porous silicon. Use of concentrated nitric acid in a chemical solution, at presence of tetrafluoroboric acid (HBF₄), increases concentration of ions of nitrosonium in the solution. What stabilizes the process of formation of the nanoporous structure of a near- surface layer of polysilicon;

- the specified solution of acids provides a process of formation of a near-surface layer of nanoporous polysilicon of homogeneous thickness on the surface of implants with a complex relief, due to use of anionic surfactant on the basis of ammonium salt of perfluoro sulfonic acid in a water solution of acids;

- use of concentrated tetrafluoroboric acid (HBF₄) in the mixture allows the realization of the process of chemical formation of a nanostructured layer of porous polysilicon on surfaces of implants modified by a film of polycrystalline silicon, without dependence from used materials for implants (metal, ceramics).

From the analysis of the state of the art, no technical solution was revealed with a set of characteristics corresponding to the declared invention and realizing the above described result.

The analysis of available art testifies about conformity of the declared technical solution to criteria of "novelty" and "degree of inventiveness".

The declared invention can be industrially realized with use of known technological processes, equipments and materials intended for manufacturing implants from titanium, its alloys, steel and dielectric materials, for example on the basis of glass ceramic.

Brief description of the drawing

Fig. 1 represents a fragment (according to axis X, Y, Z,) of the corss-section of a film of polysilicon with a nano structured layer of porous polysilicon, obtained with chemical etching of a film of polycrystalline silicon generated on a surface to be protected, according to the invention.

Detailed description of the invention

The following known equipment and materials are used for realization of the invention:

1. A reactor for gas-phase deposition of a film of polycrystalline silicon on the surface of products used, for example in electronic industry, for manufacturing semiconductor devices. A process of deposition of a film of polysilicon on the surface of a product, in a reactor, is carried out with a known method and preferably by pyrolysis of silicomethane (SiFL_t) at a temperature of 600-650°C and at a lowered pressure (25-130 Pa). The following chemical reaction takes place: SiH₄ → Si + 2H₂ and the film of polycrystalline silicon is formed as a result of the process.

Implants with various relief surfaces and made either from metals (titanium, alloys of titanium, steel) or from metal ceramics and glass ceramics, can be used for the given technological process.

2. A 50-55% water solution of tetrafluoroboric acid (HBF₄).

It is preferable for realization of the present invention to use a concentrated solution at 50% of tetrafluoroboric acid (HBF₄) with the coefficient of the rate of mass fractions of fluorine and boron of 3,85 - 4, 05, according to TR 301-14-92 (Russia). 3. Water solution of nitric acid (HNO₃) at 70-90% concentration, (see reference book «Aldrich», 2006, p. 1821, [7697-37-2]).

It is preferable to use 70% concentrated water solution of nitric acid. The given reagent is preferable due to its sanitary-hygienic properties.

4. Anionic surfactants on the basis of ammonium salt of perfluoro sulfonic acid

R_fS0₃ ^"NH₄ ⁺, where R_f is C₈F₁₇ or C₂F₅OC₃F₆OC₂F₄ or C₆F₁₃CH₂CH₂. Anionic organofluorine surfactants are manufactured by the company «Dupont» (USA) and by the company 3M (USA). Anionic fluorine-containing surfactants of the company 3M (USA) has the trade mark Novec 4300.

The anionic organofluorine surfactant chosen for realization of the present invention has high resistance to a hostile environment, increases moistening surfaces and stabilizes the processes of oxidation appearing at formation of the nano structured layer of porous polysilicon.

The concentration of the specified anionic organofluorine surfactant in a water solution of the named acids, specified in the present invention is optimal for conditions of moistening surfaces, for influence on processes of oxidation and for costs due to its use.

Anionic fluorine-containing surfactant of type FS-62, chemical formula C₆Fi₃CH₂CH₂S03^_NH₄ ⁺, produced by the company «Dupont» was chosen for realization of the present invention (see http://www.dupont.com/).

The process of formation of a biocompatible coating on surfaces of implants was carried out by realizing the following steps:

Step 1

Disks from steel - 3 pieces, from titanium - 3 pieces and a glass ceramic plate - 1 piece, were used as samples for realization of the present invention. Diameter of disks - 10 cm, thickness - 4 mm , size of the plate - 2x3 cm, thickness- 4 mm.

The films of polycrystalline silicon were generated on the disks and on the plate by the method of gas-phase deposition. The process of formation of the film of polycrystalline silicon on the disks from steel was carried out up to the thickness of 0,5 microns, and the thickness of the film on the disks from titanium and on the plate was 0,4 microns.

The minimum value of thickness (0,4 micron) of the film of polycrystalline silicon specified in the given invention, is optimal for formation of biocompatible coatings on surfaces of implants and preferably of tiny implants used in stomatology. The possibility of obtaining a film of polycrystalline silicon on surfaces of implants of less than 0,4 micron, with use of known methods, is extremely complex.

Step 2

The concentrated water solution of acids was prepared for realization of the present invention. The following components were used for preparation of the solution:

50% water solution of tetrafluorobonc acid (HBF₄), with the coefficient of the ratio of mass fractions of fluorine and boron for the given acid 3,85 - 4,05, and 70% water solution of nitric acid (HN0₃).

The ratio of volumetric parts of used acids specified in the present invention is the optimum.

At increase in the ratio of volumetric parts of nitric acid (HNO₃) and tetrafluoroboric acid (HBF₄) in the composition, polishing influence of the solution on crystals of polysilicon takes place while, at decrease in the ratio, efficiency of the process of selective etching of local zones of the film of polycrystalline silicon decreases. As a whole change of the ratio of volumetric parts of nitric acid (HNO₃) and tetrafluoroboric acid (HBF₄) in the solution worsens the process of nano structuring the film of polysilicon.

Water solutions of acids were prepared for realization of the invention according to the following examples:

Example 1.

100 ml [700 (vol. parts)] - 50% solution of tetrafluoroboric acid;

0,15 ml [~1 vol. parts] - 70% solution of nitric acid.

The mix was intermingled with addition of 0,008 mg of an anionic fluorine- containing surfactant of type FS-62.

Example 2 (control).

100 ml [700 (vol. parts)] - 50% solution of tetrafluoroboric acid;

0,15 ml [~ 1 vol. parts] - 70% solution of nitric acid.

The mix was intermingled.

Step 3

Steel and titanium disks were placed in a water solution obtained according to the Example 1. In total 2 disks from each material and a plate from glass ceramics were used.

Steel and titanium disks were placed in a water solution obtained according to the Example 2. One disk from each material was used. At wet etching disks, time of processing was determined by a visual estimation of change of color of the modified surfaces of disks from grey color, which is characteristic for a film of polycrystalline silicon, to bluish-violet color.

Surfaces of used samples with the film of polycrystalline silicon deposited on them developed a uniform bluish-violet color in 0,5 minutes, in liquid etchant according to the Example 1.

Time of processing of the implants in a water solution of compositions used according to the invention, depends basically on thickness of the film of polycrystalline silicon on its surface.

Non-uniform bluish-violet color of the surface was obtained in a similar time of processing of the surface of disks (with the film of polycrystalline silicon deposited on them) by processing in liquid etchant according to the Example 2. That demonstrates inefficiency of the given solution for obtaining a nanostructured near-surface layer of poly silicon, uniform on thickness.

The samples of porous polysilicon obtained as a result of wet etching were washed out with deionized water and were exposed to drying.

During washing of the samples, the surfaces of porous polysilicon were exposed to oxidation with preferential formation of superficial bonds of the type Si-OH, which are necessary for increase in biocompatibility of a material.

The titanium disk processed with wet etching in a water solution of acids according to the Example 1 was investigated with use of the atomic-force microscope Solver P-47 (manufacturer NT-MDT, Moscow, Russia) for determination and estimation of the nanostructured near-surface layer of porous polysilicon, obtained on the film of polycrystalline silicon.

In the submitted Figure 1, the image of a fragment (according to axis: X, Y, Z) of a film of polysilicon (1) with a nanostructured near-surface layer of porous polysilicon (2) obtained by chemical etching of a film according to the invention is shown.

It follows from the figure that the formation of a homogeneous in composition and in thickness nanostructured near-surface layer of porous silicon (2) is provided on a film of polycrystalline silicon (1) at realization of the invention.

The thickness of the obtained nanostructured near-surface layer of porous silicon does not exceed 200 nm. Biological compatibility of porous polysilicon, presence of adhesion of bone tissues, formation of calcium-phosphatic compounds on its surface and absence of cytotoxicity have proved to be true with a lot of research works, including those indicated below:

- Bayliss S.C., Harris PJ. and other. Phosphate and cell growth on nano structured semiconductors. Journal of Materials Science Letters. 1997:16:737-740;

- Yangyang Lu, Fan Yang, Linto Cai. Osteoblast adhesion on porous silicon. Bulletin of Advanced Technology Research. 2009, Jan., Vol.3, No.l, 25-28.

In particular, influence of porous polysilicon on viability and proliferation of cells were examined in vitro on cultures of cells - osteoblasts and fibroblasts. The cells (a nutrient medium) were labeled preliminary with the indicator (probe) CellTracker™ (Green CMFDA, Molecular Probes) for their tracking. The probes CellTracker™ are fluorescent chloromethyl derivatives. The labeled cells were researched with the help of the upright microscope Leica intended for high contrast epifluorescence and equipped with digital imaging software to capture the images.

Results of researches show that a nanostructured layer of porous polysilicon promotes adhesion of cellular structures on the surface of a material. The researches confirm that porous polysilicon supports growth of osteoblasts and fibroblasts and is a nontoxic and biocompatible material.

Claims

1. Method of formation of a nano structured biocompatible coating on implants for replacement of bone tissues, characterized in that a film of polycrystalline silicons is formed on the implant, this film is etched with an aqueous solution containing as essential components tetrafluoroboric acid (HBF₄), nitric acid (HNO₃) and an anionic surfactant which is an ammonium salt of a perfluoro sulfonic acid (R_fS0₃NH₄) and finally the modified implant is washed with deionized water and dried.

2. Method according to claim 1, wherein the etching treatment causes the formation of a nano structured near-surface layer of porous polysilicon.

3. Method according to claim 1, wherein the etching solution consists of a mix containing 50-55% water solution of tetrafluoroboric acid (HBF₄), 70-90% water solution of nitric acid (HN0₃), 5-10^"3 / 5-10^"2 % of ammonium salt of a perfluoro sulfonic acid of formula R_fS0₃NH₄, where R_f is selected in the group comprising C₈F₁₄, C₂F₅OC₃F₆OC₂F₄ (and C₆F₁₃CH₂CH₂)

4. Method according to claim 1, wherein the water solutions of the acids are used in a ratio of the volumes between them of HBF₄ : HN0₃ as (100-800) : (1-1,1).

5. Method according to claim 1, wherein the film of polycrystalline silicon, formed on the implant, has a thickness of 0,4-0,6 micron.

6. Method according to claim 2, wherein the nanostructered near-surface layer of porous polysilicon has a thickness of no more than 200nm.

7. Method according to claim 1, wherein the film of polycrystalline silicon is produced by pyrolysis of silicomethane (SiH₄) at a temperature of 600-650°C and at a pressure of 25-130Pa.

8. Method according to claim 1, wherein the implants for replacement of bone tissues are of metals, of metal ceramics or of glass ceramics.

9. Method according to claim 8, wherein the implants are of titanium, alloys of titanium, steel or dielectric materials.

10. Implants of metals, of metal ceramics or of glass ceramics, suitable for replacement of bone tissues, characterized in that comprise a nano structured near-surface layer of porous polysilicon, having a thickness of no more than 200nm.