Method of making a compound precursor by coating

Details Method of making a compound precursor by coating Method of making a compound precursor by coating

2002-08-07

2004-04-20

Barr, Michael (Department: 1762)

Coating processes

Particles, flakes, or granules coated or encapsulated

C427S222000, C427S240000, C427S421100, C427S427000, C427S376100, C427S002240, C427S002260, C427S002100

Reexamination Certificate

active

06723380

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of making a compound precursor and, more particularly to a method of making a compound precursor by coating in which a solvent is sprayed onto a polymeric matrix to slightly dissolve the surface of the polymeric matrix, and then a powdered inorganic reinforced filler is coated onto the surface-dissolved matrix, forming the desired compound precursor.
BACKGROUND OF THE INVENTION
When the hard tissue of a living being is damaged or the bond defect caused by injuring disease is happened, it takes a long time to repair the tissue and the bond defect, or the reproduction of the tissue and the bond defect may be not possible. Therefore, many researches use synthesized or processed biomaterials to substitute for damaged hard tissue, recovering the functioning of the tissue. Poly-L-lactide is commonly used as a substitute for human bones.
Polylactide is a biocompatible and biodegradable copolymer. Subject to structure of the polylactide, polylactide includes crystalline poly-L-lactide and poly-D,L-lactide and amorphous poly-D-lactide. Natural lactide in living beings is of L type, thus L type polylactide and D,L type polylactide are commonly used for making biomaterial for use in human beings. Polylactide is degraded to lactic acid by hydrolysis and deesterification, lactic acid is oxidized into pyruvate under the presence of lactate dehydrogenase, and then pyruvate is metabolized into carbon dioxide and water by means of Kreb's cycle, and then discharged out of the body through the lungs and the kidneys (Hollinger & Battistone, 1986; Kulkarni et al., 1966). When using polylactide to make biomaterial for use as a substitute for bones, the mechanical strength must be strong enough to bear external force. In order to enhance the mechanical strength of polylactide-matrix biomaterial, ceramic material of hydroxyapatite may be composed to the polylactide-matrix.
Hydroxyapatite [Ca
10
(PO
4
)
6
(OH)
2
] is the main ingredient of the teeth and bones of the body of a human being, which is intensively used in medical implant materials and chromatography (protein purification and DNA separation) (P. Luo & T. G. Nieh, 1996), and has proper strength and osteoconductivity (Verheyen et al., 1992). Among biomaterials, hydroxyapatite is a calcium phosphate ceramic material commonly used for repairing the hard tissue defects. Hydroxyapatite in living beings will be affected by the pH value of the living beings. Different pH values produce different effects to the dissolution of hydroxyapatite. It will cause the dissolution of hydroxyapatite if the pH value is lower than 4.2 and/or there exists CO
3
−
, Sr
2+
or Mg
2+
. Hydroxyapatite is soluble in acid environment. Hydroxyapatite is a hard but fragile material having a relatively higher Young's modulus and lower extensibility. The hard material property of hydroxyapatite can improve the strength of hydroxyapatite/polylactide composite and extend its strength maintaining time. According to U.S. Pat. No. 6,232,384B1, the initial bending strength of hydroxyapatite/polylactide compound is greater than 250 MPa, which lasts for as long as 3 months in a living being and, starts to degrade and to be absorbed 6 months ~3 years thereafter (Hyon, 2001).
Hydroxyapatite/polylactide composites are commonly made by solution-mixing method. KEMAL KESENC{dot over (I)} et al. made an experiment in 2000 to produce hydroxyapatite/polylactide composites with hydroxyapatite and poly-L-lactide as matrix by means of precipitation in solution. KEMAL KESENC{dot over (I)} et al. obtained poly-L-lactide by means of ring-opening polymerization. Poly-L-lactide was dissolved in chloroform, and mixed with hydroxyapatite at different ratios. When well mixed, mixed solution was dried. The product thus obtained was then treaded through a series of procedures including heat pressing, temperature lowering, storing, etc. to obtain final product of hydroxyapatite/polylactide composites (KEMAL KESENC{dot over (I)} et al., 2000). Except solution-mixing method, Törmälä et al., 1992 mentioned, in U.S. Pat. No. 5,084,051, several composite preparation methods as outlined hereinafter:
A. Cover bioabsorbable polymer membrane on bioceramics (size: 30×10 mm
2
), put a heating board on the polymer membrane and start heat pressing to mount polymer membrane on bioceramics. The thickness of polymer membrane is about 1 mm. The compound material thus obtained is cooled to room temperature under the presence of a pressure.
B. Put reinforced fibers on bioceramics, then cover a layer of polymer membrane (thickness about 2 mm) on reinforced fibers, heat press the layer of polymer membrane, causing reinforced fibers and polymer membrane to be melted and adhered to the inside of bioceramics. The compound material thus obtained is cooled to room temperature under the presence of a pressure.
C. Apply a polymer-dissolved solvent to the surface of bioceramics, and then repeat the procedure after the solvent transited into vapor. The procedure is repeated again and again until the thickness of the coated polymer reached 0.5 mm.
D. Put reinforced fibers onto bioceramics, and then apply a polymer solution to the boundary between reinforced fibers and bioceramics, and then cover a Teflon sheet on the top, and then apply a pressure to the Teflon sheet against reinforced fibers and bioceramics, and then repeat the procedure of adding polymer solution and giving a pressure after solvent of previously applied polymer solution changed into vapor. The procedure is repeated again and again until the thickness of fiber reinforced polymer reached 0.5 mm.
E. Monomer-contained solution, which is capable of polymerization, is applied to the surface of bioceramics until the thickness of polymer thus formed reached 1 mm.
F. Put reinforced fibers on bioceramics, and then apply a monomer-contained solution (solution applicable for polymerization) to the boundary between reinforced fibers and bioceramics until the thickness of fiber-reinforced polymer thus formed reached 1 mm.
U.S. Pat. No. 4,781,183 (Casey et al., 1988) mentioned two systems of composite preparation methods as follows:
A. Bioabsorbable Particulate Filled Systems:
Melt polymer matrix in nitrogen or vacuum, and add stuffing material slowly to the desired concentration.
B. Fiber Reinforced Systems:
It is also called solution impregnation and laminations and melt impregnation and laminations. Solution impregnation and laminations is to impregnate filler in a solution of the biodegradable polymer, enabling the solution to permeate into the filler, and then the impregnated filler is thoroughly dried. The well-dried filler is then heat-pressed into the desired final product. For melt impregnation and laminations, films of the biodegradable polymer are made by solvent casting or melt pressing. Alternatively, fibrous mats are made from polymer by running a solution of the polymer into a non-solvent in a thin stream to form a stringy precipitate, followed by pressing into a mat at room temperature. The films or mats are then laid between yarn or fabric layers in a mold of a predetermined thickness. Vacuum is applied to the lay-up, by vacuum-bagging the mold, and heat and compression are applied to consolidate the laminate.
The prior art methods mentioned above are still not satisfactory in function, and have numerous drawbacks as outlined hereinafter.
A. Complicated Preparation Procedure:
It takes much time to obtain the desired final product through a series of procedures including dissolving, blending, pressure giving, drying, temperature lowering, storing, and etc.
B. High Manufacturing Cost:
Because much amount of solvent and materials and many instrument and equipment are used during the fabrication, the manufacturing cost is high.
C. Not in Conformity With Environment Protection:
A big amount of waste product is produced during the fabrication, and the produced waste product tends to cause pollution.
D. Using a Big Amount of Toxic Solution:
Much solvent harmful to

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