Coating processes – Medical or dental purpose product; parts; subcombinations;... – Implantable permanent prosthesis
Reexamination Certificate
1999-12-02
2003-07-29
Chen, Bret (Department: 3731)
Coating processes
Medical or dental purpose product; parts; subcombinations;...
Implantable permanent prosthesis
C427S002100, C427S002280, C427S407100, C427S409000, C427S301000, C427S302000, C427S327000, C427S340000
Reexamination Certificate
active
06599558
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of treating metal surfaces to enhance the bio-compatibility and/or physical characteristics of said surfaces. The invention also relates to bio-compatible metal articles. The invention is particularly relevant to surfaces of medical devices.
BACKGROUND OF THE INVENTION
Many medical techniques are known in which human or animal blood is brought into contact with foreign surfaces, either within the body or outside the body. In some situations, usually due to mechanical characteristics, it is necessary to use metallic surfaces, as required by coronary stents (vascular endoprostheses) located within arteries or, for example, within heat exchanger assemblies external to the body. Thus, in the first application, the mechanical strength of the metal object is required whereas in the external application it is the heat transfer characteristics that are required. However, in both applications, blood or related blood products are brought into contact with metal surfaces, which may in turn have detrimental effects upon the blood itself.
The Problem of Clotting
When presented to foreign surfaces, blood has a tendency to clot. It is known that blood activation in response to contact with a foreign surface occurs by the intrinsic pathway (see FIG. 2 of Johan Riesenfeld et al, Surface Modification with Functionally Active Heparin, Medical Device Technology, March 1995 pages 24-31, the disclosure of which is incorporated herein by reference), triggered by the conversion of Hageman (F)XII to an active enzyme, FXIIa. This then initiates the sequential activation of coagulation factors FXI, FIX and FX and finally FXa converts prothrombin into enzymatically active thrombin, which precipitates the soluble plasma protein fibrinogen into a solid fibrin clot. The coagulation system is under the control of a series of regulatory mechanisms in the blood and the vascular wall, the most important being the plasma coagulation inhibitor, antithrombin (III).
Heparin is a naturally occurring substance that consists of a polysaccharide with a heterogeneous structure and a molecular weight ranging from approximately 6000 to 30000 Dalton (atomic mass units). It prevents uncontrolled clotting by suppressing the activity of the coagulation system through complexing with antithrombin (III), whose activity it powerfully enhances. Approximately one in three heparin molecules contains a sequence of highly specific structures to which antithrombin binds with high affinity. When bound to the specific sequence, the coagulation enzymes are inhibited at a rate that is several order of magnitude higher than in the absence of Heparin. Thus, the heparin molecule is not in itself an inhibitor but acts as a catalyst for natural control mechanisms without being consumed during the anticoagulation process. The catalytic nature of heparin is a desirable property for the creation of a bio-active surface, because the immobilised heparin is not functionally exhausted during exposure to blood but remains a stable active catalyst on the surface.
A method for making nonthrombogenic surfaces is disclosed in U.S. Pat. No. 3,634,123. A method for reducing thrombosis of blood, induced by contact with foreign surfaces, is shown in which the surface are treated with a cationic surface active agent and a conventional anticoagulant such as heparin. The technique disclosed in this patent is appropriate for plastic surfaces but cannot be extended to metal surfaces.
The Benestent II Group at the Department of Cardiology, University Hospital Rotterdam have developed a heparin coated Palmaz-Schatz stent, in which an end point of the heparin molecule is covalently coupled to an underlying polymer matrix, similar to the type manufactured under the trade mark Carmeda Bioactive Surface by CBAS Carmeda Inc, Sweden. The process consists of four stages:
etch the metal surface
introduce a poly-amino layer which is ionically attached to the surface
covalently bond the functional amino groups to the aldehyde groups of partially degraded heparin molecules
chemically stabilise the bonded heparin by use of a reducing agent.
An advantage of this known approach is that it allows heparin molecules to be attached to the poly-amino layer in a relatively friendly chemical environment. However, the poly-amino layer is only physically attached to the conditioned metal surface and as such the strength of the attachment is somewhat dubious. Thus, in continuous use within the body, there is a risk of heparin or similar molecules becoming detached thereby reducing the effectiveness of the stent, which in turn may require further surgery. Similarly, in external applications, the effectiveness of the device may degrade and this degradation may be accelerated if the device has to be cleaned under particularly harsh conditions. Finally anti-coagulant coating methods generally incur relatively high manufacturing costs.
U.S. Pat. No. 5,356,433 discloses the treatment of a stent or other medical device by the alleged formation of covalent linkages between a biologically active agent and a metallic surface. In one example tantalum stents were primed with a solution in ethanol of N-(2-aminoethyl-3-aminopropyltrimethoxysilane so that a bond was formed between the tantalum oxide layer on the surface of the stents and the silicon of the silane on curing at 110° C. Heparin is then coupled to the amino groups using 1,3-ethyldimethyl-aminopropyl carbodiimide (EDC). In a second example, an ethanolic solution of an amiofunctional polymeric silane, trimethylsilylpropyl substituted polyethyleneimine is bonded to the surface of tantalum stents, also with curing at 110° C., after which heparin was coupled to the coating using EDC. Other examples use stainless steel wire, platinum tungsten wire and aminopropyl-trimethoxysilane as primer. However, priming has to be carried out with heating. The present applicants consider that covalent bonds to the metal surface are not formed under the conditions described. The reason is that the water which is inevitably present in the ethanol hydrolyses the linkages between the methoxy groups and silicon and because the reaction between the trimethoxysilane groups and surface oxide requires a catalyst which is absent. Furthermore, the heparin is coupled to the priming layer directly and not by polymeric or oligomeric spacer arms, is not sterically available, and will therefore not exhibit its full anti-coagulant activity. U.S. Pat. No. 5,607,475 reports that the use of aminosilanes in coatings on metal or glass surfaces has not been good at producing a surface with a high level of both bio-effectiveness and stability.
U.S. Pat. No. 5,607,475 discloses an endoprosthesis having a metal surface for contact with body fluids, the metal surface having a coating thereon comprising:
(a) a silane which includes a vinyl functionality, the silane being adherent to the metal surface so that the vinyl functionality is pendant from the surface;
(b) a graft polymer, the graft polymer being covalently bonded with the pendant vinyl functionality of the adherent silane, the graft polymer being simultaneously formed and bonded to the pendant vinyl functionality by free radical reaction initiated by an oxidising metal with at least one ethylenically unsaturated monomer selected from the group consisting of acrylamide and acrylic acid;
(c) a polyamine spacer covalently attached to the graft polymer; and
(d) a biomolecule covalently attached to the spacer.
The preferred primer is trichlorovinylsilane which is applied in xylene. However, under these conditions, the primer is merely physically held to the metal surface and does not form a chemical bond with oxide on the metallic surface. The procedure for subsequent attachment of heparin is lengthy and complex. The method described is neither effective nor practical, and the information and belief of the present applicants is that it has not been put into practice.
WO 97/07834 acknowledges that in order to obtain truly anti-thrombogenic surfaces, proper immobilisation
Al-Lamee Kadam Gayad
Taktak Yousef Samih
Baker & Botts LLP
Chen Bret
Kolb Michener Jennifer
Polybiomed Limited
LandOfFree
Treating metal surfaces to enhance bio-compatibility and/or... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Treating metal surfaces to enhance bio-compatibility and/or..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Treating metal surfaces to enhance bio-compatibility and/or... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3068995