Discontinuous Coating Method Using A Bioabsorbable And Bioactive Biomaterial Applied To Solid Substrates, Discontinuous Coating And Use Thereof

Abstract

The invention relates to the development of a discontinuous coating using a bioabsorbable and bioactive biomaterial (which can be a bioceramic, a biopolymer or a bioactive composite) applied to solid (smooth, rough or porous) substrates. This discontinuous coating should be totally consumed in up to 10 days after implantation, so that in the end only the interface of the newly formed tissue with the implant remains. The proposed discontinuous coating can be obtained by any method that allows depositing a continuous layer, once the process parameters are duly adjusted. The preferred method for applying this discontinuous layer is airbrushing or air atomizing. The preferred biomaterial is bioglass.

Claims

1. A process of discontinuous coating using a bioabsorbable and bioactive material on solid substrates comprising: providing particles of biomaterial; applying the particles of biomaterial to a surface of a solid substrate; and fixing the particles of biomaterial to the surface of the solid substrate.

2. The process of discontinuous coating according to claim 1, wherein the biomaterial is selected from the group consisting of a bioceramic, a biopolymer, a composite, and a bioglass.

3. The process of discontinuous coating, according to claim 1, wherein the biomaterial is a bioglass.

4. The process of discontinuous coating according to claim 1, wherein applying the particles of biomaterial to the surface of the solid substrate comprises applying the particles of biomaterial in an island format with size distribution of from 1 to 1200 m.

5. The process of discontinuous coating according to claim 1, wherein applying the particles of biomaterial to the surface of the solid substrate comprises applying the particles of biomaterial to between 2 and 80% of the surface of the solid substrate.

6. The process of discontinuous coating according to claim 1, wherein applying the particles of biomaterial to the surface of the solid substrate comprises an application method selected from the group consisting of thermal spray, airbrushing, air atomizing, drip coating, dip coating, and brush coating.

7. The process of discontinuous coating according to claim 6, wherein applying the particles of biomaterial particles to the surface of the solid substrate is by airbrushing or air atomizing.

8. The process of discontinuous coating according to claim 7, wherein applying the particles of biomaterial particles to the surface of the solid substrate is by airbrushing with the following parameters: TABLE-US-00002 Parameter Range Stroke Width (mm) 1-40 Stroke Density (%) 1-5 Work Density For Four Coatings (%) 25-30 Work Density For Five Coatings (%) 45-50 Work Density For Six Coatings (%) 50-60 Air Pressure During Application (Pascal) 0.5-3.sup. Distance Of Airbrush Nozzle To The 10-40 Surface Of The Solid Substrate During Application (mm) Nozzle Needle Diameter (m) 60-120 Application Speed (mm/s) 5-100 Relative Humidity During Application (%) 44-49 Temperature During Application ( C.) 19-28

9. The process of discontinuous coating according to claim 1, wherein at initiation of applying the particles of biomaterial to the surface of the solid substrate, the solid substrate has a higher melting point than the particles of biomaterial.

10. The process of discontinuous coating according to claim 9 further comprising elevating the temperature of the solid substrate to a level that is higher than the vitreous transition temperature of the particles of biomaterial.

11. RIM The process of discontinuous coating according to claim 1, wherein at initiation of applying the particles of biomaterial to the surface of the solid substrate, the solid substrate has a lower melting point than the particles of biomaterial.

12. The process of discontinuous coating according to claim 11 further comprising mixing the particles of biomaterial into a polymer-based bioabsorbable gel.

13. The process of discontinuous coating according to claim 12, wherein the particles of biomaterial are dispersed in the gel in a concentration of from 0.001 to 0.6 g/ml.

14. The discontinuous coating obtained through the process of claim 1.

15. The discontinuous coating according to claim 14, wherein the coating is totally consumed in up to 10 days after in vivo implantation of the solid substrate.

16. The discontinuous coating according to claim 15, wherein the coating promotes interface between newly formed tissue and the solid substrate and local stimulation for growth of newly formed tissue.

17.-18. (canceled)

19. The process of discontinuous coating according to claim 7, wherein applying the particles of biomaterial particles to the surface of the solid substrate is by airbrushing; wherein the solid substrate is symmetric; wherein the coating is done in rotation from 0.5 to 4 rpm; and wherein the coating is done at an angle between 0 and 45 horizontally.

20. The process of discontinuous coating according to claim 8, wherein the air pressure is 1 Pascal; and wherein the distance of airbrush nozzle to the surface of the solid substrate is 20 mm.

21. The process of discontinuous coating according to claim 12, wherein the bioabsorbable gel is selected from the group consisting of HPMC (hydroxypropyl methylcellulose) and collagen.

22. The process of discontinuous coating according to claim 21, wherein the bioabsorbable gel has a concentration of 0.05 to 1.2 g for each liter of the mixture containing 30 to 50% of water and 50 to 70% of ethyl alcohol.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 shows a schematic illustration of the layers described in the state of the art. 1Biomaterial particlesBioglass or hydroxyapatite, over the continuous layer of adhesive material; 2Continuous adhesive layer which will join the implant to the biomaterial; 3Layer of newly formed tissue over the surface of the composite layer composed of elastomeric material and biomaterial (hydroxyapatite or bioglass); 4Biomaterial particlesbioglass or hydroxyapatite, dispersed in the volume of the continuous layer of adhesive material (elastomeric); 5Biomaterial particlesbioglass, dispersed in the volume of a ceramic material (alumina); 6Layer of newly formed tissue over the surface of the solid substrate ceramic composite with the biomaterial (bioglass) dispersed in its volume.

[0023] FIG. 2 shows a schematic illustration of the discontinuous layer in the two possible situations: substrate with higher melting point than the biomaterial particles and substrate with lower melting point than the biomaterial particles. 1Original bioglass particles; 2Bioabsorbable polymer layer used as initial adhesive; 3Bioglass particles softened by thermal treatment and directly adhered to the solid substrate with high melting point (metal, ceramic or composite); 4Layer of newly formed tissue in interface with the solid substrate.

[0024] FIG. 3 shows images obtained through scanning electron microscopy showing the range of precipitation of hydroxyapatite from the F18 bioglass particles after a bioactivity test.

[0025] FIG. 4 graphically represents the distribution of the fraction of the area occupied by the particles on the surface (%) as an x and y function of the stroke (mm) after 5 coatings.

[0026] FIG. 5 shows the fraction of coated area as a function of the number of coatings in the airbrushing coating method.

[0027] FIG. 6 shows the water-titanium contact angle variation according to the surface condition of titanium.

[0028] FIG. 7 shows particles adhered to the titanium surface after a micro-indentation adhesion test.

[0029] FIG. 7a shows the presence of cracks caused by a 200 gf vickers micro-indentation.

[0030] FIG. 7b shows the same particle from (a), emphasizing that the cracks generated did not affect the adhesion of the particles even after 24 hours of bioactivity testing.

[0031] FIG. 8 shows the osteoblastic cells adhered to the smooth titanium surfaces after three days of cultivation. The dark grey and light grey fluorescent sections indicate, respectively, an actin cytoskeleton, marked with phalloidin combined with Alexa Fluor, and cell nucleus, marked with DAPI ((4,6-diamidino-2-phenylindole).

[0032] FIG. 8a shows titanium with no coating.

[0033] FIG. 8b shows titanium coated with 45S5 bioglass.

[0034] FIG. 8c shows titanium coated with F18 bioglass.

[0035] FIG. 9 graphically represents the viability of osteoblastic cells over time, over the titanium surfaces, with and without coating.

[0036] FIG. 10a shows the aspect of the smooth titanium implant before thermal treatment.

[0037] FIG. 10b shows the aspect of the smooth titanium implant after thermal treatment with an average coating fraction of 8%.

[0038] FIG. 10c shows the aspect of the rough titanium implant after thermal treatment with an average coating fraction of 8%.

[0039] FIG. 11 shows the surface of a coated dental implant with an average coating fraction of 25%.

[0040] FIG. 12 shows the top of the thread crest of an implant coated after the insertion and removal test.

DETAILED DESCRIPTION OF THE INVENTION

[0041] The invention relates to a discontinuous coating composed of a biomaterial (which can be a bioceramic, biopolymer or bioactive composite), bioabsorbable and bioactive applied over a solid substrate. It refers, therefore, to a discontinuous layer which will be absorbed by the body during the tissue scarring process which ensures a lack of interfaces (potential cause of flaws), and direct contact of the solid substrate (implant) with the newly formed tissue as shown schematically on FIG. 2.

Fixation of Particles to the SurfaceLayer Formation, Discontinuous

[0042] Material used: biomaterial in particle form, preferably bioglass.

[0043] Fixation method: temperature increase for situations in which the substrates have higher melting point than the biomaterial to be deposited and use of bioabsorbable polymeric gel as an adhesive substance for situations in which the substrates have lower melting point than the biomaterial to be deposited.

Coating Parameters

Powder Quantity

[0044] The definition of the amount of powder to be fixed to the surface is based on the answer to the following question: what is the minimum fraction of particles that produce, after a bioactivity test, a hydroxyapatite layer that coats 100% of the surface.

[0045] The results of these tests have shown that the range of precipitation of hydroxyapatite from the edge of the particles depends on the particle size distribution of the bioglass powder and its composition. For the bioglass used, the F18 bioglass, with a particle distribution between 10 and 100 um, this distance was approximately equal to the particle diameter, as seen in FIG. 3. This result indicated that if 25% of the surface is homogeneously covered by particles, it will be 100% covered by hydroxyapatite after the bioactivity test. The bioactivity tests were carried out based on the ISO 23.317 (2007) standard Implants for surgeryin vitro evaluation for apatite-forming ability of implant materials.

[0046] Usage of other types of bioglass with various powder grain sizes changes the coated fraction, which leads to maximum formation of hydroxyapatite in the bioactivity test due to differences in particle speed dissolution. Thus, it was possible to determine that, if regions of bioactivity test with average diameter between 1 and 1200 m coat surface fractions between 2 and 80%, the aforementioned condition (surface 100% coated by hydroxyapatite after bioactivity test) will be met.

Coating Method

[0047] Based on previously established criteria, five methods of transferring biomaterial to the solid substrate may be used:

[0048] 1. Brush painting.

[0049] 2. Drip coating

[0050] 3. Dip Coating

[0051] 4. Airbrushing or air atomizing.

[0052] 5. Thermal spray.

Example of Coating Process of Titanium Discs Through Airbrushing

[0053] Titanium discs with 10 mm diameter and 3 mm thickness were used in this example. The transfer of biomaterial to the surface of the solid substrate, or coating, used biomaterial particles with particle size distribution between 5 and 500 m.

[0054] There is no information in the literature of the invention regarding the airbrushing method in the field of embodiment of this invention. For this reason, some parameters were necessary for this process in order to make it repeatable. Initially, the term stroke was established. In the graphic and art fields, in which the airbrush is extensively used, stroke refers to the pattern of the deposited paintmore or less dense, more or less wide, left by the airbrush in the work surface. In the current situation, the stroke shall be the particle deposition pattern over the titanium surface, characterized by the particle distribution and fraction delivered to the surface by the airbrush. This deposition pattern shall be determined by the following parameters:

[0055] 1. Stroke densityaverage fraction of the surface area covered by the particles delivered to the surface during the first coating.

[0056] 2. Final Stroke Density or work densityfinal average fraction of the surface area covered by the particles delivered to the surface. As many coatings as needed may be used to reach the final density.

[0057] 3. Stroke Widthregion in which the area fraction range covered by the particles delivered to the surface has the same values as the final stroke density.

[0058] The characteristics of the stroke used in this example are graphically presented in FIG. 4. Its main characteristics were:

[0059] Stroke Width: 1 to 40 mm

[0060] Stroke density: 1 to 5%

[0061] Work density for 4 coatings: 25 to 30%

[0062] Work density for 5 coatings: 45 to 50%

[0063] Work density for 6 coatings: 50 to 60%

[0064] Through FIG. 4, it is already possible to note that, after approximately 18 mm, the stroke already yielded the work density, which ranges from 40 to 50% for 5 coatings. From 25 mm, the stroke already has approximately 10 mm, which is the diameter of the titanium discs to be coated, and larger than the 3 mm average diameter of the real implants used in the subsequent application example in this work.

[0065] FIG. 5 shows that the amount of particles delivered to the surface is not linear and varies from 3.5% of area fraction for one coating up to 60% for six coatings.

[0066] The other parameters used in airbrushing were:

[0067] Air pressure between 0.5 and 3 Pascal, preferably 1 Pascal,

[0068] Distance from the nozzle to the surface between 10 and 40 mm, preferably 20 mm,

[0069] Nozzle needle between 60 and 120 m,

[0070] Applying speed between 5 and 100 mm/s,

[0071] For situations of dental implant coating, said implants must be positioned at a 0 to 45 horizontal angle and under rotation from 0.5 to 4 rpm,

[0072] Relative air humidity between 44 and 49%,

[0073] Temperature between 19 and 28 C.

[0074] The study of this example shows that it is possible to adjust a continuous layer production technique, such as airbrushing, in order to generate a discontinuous layer. Furthermore, it also shows that it is possible to project discontinuous layers with any biomaterial fraction. In this example, if generating discontinuous layers with 25% of the surface coated with bioglass is needed, 4 coatings should be applied.

Characterization of Titanium Disc Coatings Contact Angle (Wetting)

[0075] The ability of bone-integrated dental implants to withstand loads depends on the quality and quantity of bone tissue connected to the implant that, in turn, is strongly influenced by the implant surface characteristics. Hydrophilic surfaces improve bone apposition and provide a quicker bone-implant contact.

[0076] Countless in vitro, in vivo and clinical results have shown the importance of this wetting or contact angle test for the dental implant field.

[0077] Table 1 shows the results of contact angle measurements for various conditions of the titanium surface.

[0078] From the samples coated with the F18 bioglass, the rough surface obtained through double acid etching featured the lowest contact angle. The angle value obtained was 52. FIG. 6 shows the variation of the contact angle due to variation of some parameters. It was possible to evidence that the coating of surfaces with bioglasses significantly decreased the contact angle of the titanium surfaces.

[0079] Most implant surfaces available at the market have contact angles higher than 10. A few examples, including one national version, have contact angles around 5. Nevertheless, it is possible to find in the market an implant with zero contact angle, in other words, with full wetting and that leads to better bone integration results than surfaces with higher wetting angles. These implants are manufactured by a worldwide leading company in the manufacture of dental implants.

TABLE-US-00001 TABLE 1 Results of contact angle assessments between distilled water and the various conditions of the titanium surface Contact Bioglass Sample Angle () Fraction (%) Smooth, coated with F18 bioglass 8.9 1 9.9 1 Smooth, coated with F18 bioglass 11.0 4 25.0 1.sup. Smooth, not coated 54.5 8 Rough, not coated 25.0 1 Rough, coated with F18 bioglass 5.3 2 7 1 Smooth, coated with 45S5 bioglass 3.1 1 23 6 Rough, coated with 45S5 bioglass .sup.0.8 0.6 17 5

Adhesion Evaluation of Bioglass Islands to the Substrate

[0080] Micro-indentations with 200 gf loads were generated on the surfaces of the bioglass islands fixed to the solid substrates. The objective was to quantitatively evaluate interface adhesion, an important parameter for placement dental implants. On FIG. 7, no cracks were noticed on the titanium-bioglass interface, which enabled partial or full particle detachment. All cracks observed were perpendicular to the surface. This behavior is an indication of good adherence between titanium and bioglass.

In Vitro Tests

Cell Adhesion and Cytotoxicity

[0081] The objective of in vitro tests is to evaluate the biocompatibility of the biomaterial in a biological environment. Biocompatibility is related to the acceptance of a biomaterial by the nearby tissue and by the body as a whole, in other words, it determines if the performance of the biomaterial is compatible with the expectations and does not harm the patient/user.

Samples

[0082] Smooth titanium discs were coated by airbrushing as described in the section Example of coating process of titanium discs through airbrushing to be tested in vitro. Smooth titanium discs with no coating were tested for control purposes.

Testing Conditions

[0083] For in vitro tests, a strain of tumor cells established from human osteosarcoma, named SaOS-2 was used (origin: Banco de Celulas do Rio de JaneiroBCBRJ).

[0084] The cytotoxicity evaluation was carried out through the MTT method, or metabolic oxidization of 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT of Sigma companySaint LouisMOEUA) for 1, 3 and 5 days. The experiment was performed in triplicate.

[0085] The cell adhesion test evaluated the viability of cells adhered to the surface after 4 hours of incubation. After this time, samples were taken from the incubator and the cells were enzymatically detached from the titanium surfaces.

Cell Adhesion

[0086] FIG. 8 shows images obtained through confocal laser scanning microscopy. It includes cells adhered to the titanium surfaces after three days of cultivation. Note that the cells exhibit cytoplasmic extensions, which seek anchoring in various directions, mainly in the direction of surface lines. In (c), cell alignment with parallel features of superficial ridges is more clearly noted, and also highlights the set of various cells adhered to the surface emphasizing the joining of various cytoplasmic extensions and, consequently, of various cells. This hints at a good cell spread and adhesion, and evidences surface biocompatibility. It should be noted that there is virtually no morphological differences, or cell amount or distribution between the three surfaces.

Cytotoxicity

[0087] According to the results produced by the MTT test (FIG. 9), it was observed that the coated surfaces sustained the viability of the osteoblastic cells, SaOS-2, throughout the cultivation time.

[0088] No significant statistical differences were observed between the tested samplescoated or not coated. For this reason, the coated surfaces analyzed have no cytotoxic effect, since according to the ISO 10993-5 standard Biological evaluation of medical devices Part 5: Tests for in vitro cytotoxicity, the cytotoxic effect is verified where there is reduction of at least 30% in cell viability in relation to the known patternwhich is pure titanium in this case.

Example of Dental Implant Coating by Airbrushing

[0089] The option for testing airbrushing coating on dental implants was due to its higher geometric complexity and reduced sizes in comparison to orthopedic implants, for example.

[0090] FIG. 10 shows two surfaces covered with F18 bioglass with an average coating fraction of 8% and FIG. 11 shows the surface of an implant with an average coating fraction of 25%.

[0091] The implant coatings used the same parameters used for coating the aforementioned titanium discs.

Characterization of Implants Coated with Airbrushing Surface of the Implants

[0092] It should be highlighted that, aside from smooth surfaced implants, implants with double acid-etched surfacesor roughwere coated. This surface is the best surface currently available commercially. Rough surfaces coated with bioglass, as already presented, have excellent wetting (low contact angle), which is very desirable for a bone integrating implant.

Homogeneity and Area Fraction

[0093] The homogeneity analysis was based in approximately 200 images. Coated dental implants featured an average coating area of 25%.

Adhesion Evaluation of Bioglass Islands to the Dental Implant

[0094] As the dental implant is a bone screw, the ASTM F-543:2007 standardAttachment A2 (Test Method for Driving Torque of Medical Bone Screws) was used as test reference. Based on this standard, the implant insertion was simulated, but under more severe test conditions. These conditions were: (1) test performed in both ways, in other words, insertion and removal of the implant; (2) the process was carried out on a dry basis and (3) on a material that emulates the cortical bonedenser, and generally present in just part of the insertion depth of the implant. Thus, it is expected that, if the coating withstands these test conditions, it will certainly withstand the real insertion process.

[0095] The parameters of this test, based on the ASTM F-543:2007 standardAttachment A2, basically relate to the standardization of the test block. Thus, the testing parameters were:

[0096] Test block material: 40 degree polyurethane with equivalent density and properties to a cortical bone.

[0097] Block dimensions: 100 mm height, 40 mm diameter. The standard indicates that the lowest dimension must be higher than 10 the nominal diameter of the screw to be tested, and minimum 4.8 mm height.

[0098] Screw insertion hole: conditions as provided by the manufacturerfor this test, 2 mm up to a 9 mm depth and 2.8 mm up to a 7 mm depth, for a 3.5 mm diameter implant.

[0099] As evidenced on FIG. 12, no detachments or cracks of the coating features were found. In addition, based on these results, it can be stated that the coating adhesion over the implants shall not be affected by the actual use conditions, since the conditions of this test are much more severe than the real conditions.

CONCLUSIONS

About the Coating Example on Titanium Discs

[0100] The airbrushing coating process is viable for obtaining discontinuous coatings with any desired final fractions, as it was possible to project and produce coatings in an accurate and reproducible manner.

[0101] By this example of suitability of a method for obtaining a continuous layerairbrushingit may be stated that it is possible to obtain a discontinuous layer through any continuous layer deposition method, upon correct adjustment of the respective process parameters.

[0102] The addition of the F18 bioglass layer has generated a surface with a contact angle on par with the best implants currently on the world market, in other words, 0<5.

[0103] The results obtained on in vitro tests show that the discontinuous layer obtained may provide significant improvements to the general performance of implants (specially orthopedic and dental implants).

About the Coating Example on Titanium Dental Implants

[0104] The bioglass layer applied over the titanium dental implant surface was found to be very homogenous. The adhesion of particles to the metallic substrate was suitable, as the dental implant, although tested under more severe conditions than the real use conditionsfrom the viewpoint of shearing forces, showed no flaws on its layer.