Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis
Reexamination Certificate
1998-06-29
2001-02-27
Isabella, David J. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
C623S001460, C604S266000
Reexamination Certificate
active
06193752
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a urological implant, in particular a vascular wall support for the urinary tract, such as a urethral, prostate or ureter stent, comprising a substantially tubular, inherently stable carrier. Vascular wall supports of the generic type, which are usually designated as “stents” in technical terminology, are as a rule used for dilating and keeping open pathologically closed or stenotic body vessels. Further examples of application for the urological implants specified are urinary catheters, artificial ureters, artificial urinary diversion systems correspondingly completed by a bladder and so forth.
2. Background Art
To simplify matters, the stents mentioned above will be dealt with below by way of example in the discussion of the prior art and the invention:
Urethral, prostate and ureter stents are indispensable implants for patients with disorders of the urinary tract, if these patients are not in the shape and condition for corresponding surgery. A great variety of stent designs and materials are available today. As a rule, design and material depend on the place where the stent is used and on the implantation technique used. For instance, tubular carriers made from an expansible lattice material are customary. The stent is inserted in its contracted condition via a catheter into the corresponding vessel, where it is expanded for instance by the action of a bubble, its lattice structure extending and the vessel thus being dilated. The stent remains at the place of implantation, serving to support the vascular wall against renewed stenosis.
A serious problem resides in that with all the materials used and tested so far for urinary tract stents, crystallization processes and bacterial colonization will occur on the surface of the stent after several weeks of contact with urine. This leads to serious malfunction of the implants, which strongly jeopardizes the medical success of stent implantation. Numerous studies have dealt with various materials of medicinal technology such as titanium, stainless steel and polymers. However, really durable resistance to crystallize has not been found in any material.
An example of corresponding prior art measures can be seen from U.S. Pat. No. 5 328 954. This document discloses an encrustation and bacterial resistant coating for use on medical devices. The coating includes a reaction product formed by covalent linkage of a hydrophilic polyurethane pre-polymer and aminopolycarboxylic acid. A urease inhibitor and/or antibacterial agent may also be added to the coating.
The aim of this coating—namely to render surfaces resistant to encrustations—is pursued, using antibacterial substances which are bonded on the surface, there preventing a pH increase as produced by bacterial colonization. In this case, encrustations are to be blocked by precipitation of the urine components no longer taking place due to an increased pH.
The substances specified for immobilization are among others ethylene-diamine-tetra-acetate complexes, antibiotics and silver ions, which are ionically or covalently bonded to polyurethanes. As has however been found, the blocking of encrustations achievable by this prior art is still in need of improvement. It is an object of the invention to solve these problems.
SUMMARY OF THE INVENTION
The invention is based on, and proceeds from, natural biomedical processes for stopping crystalline growth in the urinary tract:
Most ionic components of the human urine are present in oversaturated concentrations. Nevertheless, precipitation or formation of greater crystallites does not occur in sound urine. This is prevented by the presence of low- or high-molecular inhibitors. Sodium, potassium, magnesium and citrate ions among others belong to the low-molecular inhibitors, the effectiveness of which is to be rated rather low. The decisive part as inhibitors is played by the macromolecules contained in urine, which can be divided into two groups. They are glycosamino glycans (GAG) and proteins. Heparin, heparene sulfate and various chondroitin sulfates belong to the first group, nephrocalcin, osteopontin and Tamm-Horsfall protein belong to the second. These mentioned macromolecules work through adsorption on molecules and crystal surfaces, thus preventing the growth of greater crystal structures. The action of an inhibitor is mostly restricted to one or few sorts of particles. For instance, nephrocalcin bonds to potassium ions and heparin to oxalate ions and crystallites as substances to be inhibited.
If the inhibitor and the to-be-inhibited substances are present, dissolved in the urine, then there is active effectiveness. If however the urine components under regard adsorb on surfaces, the inhibitors cannot become effective. Owing to the geometric structure of the macromolecules, the linkage between the inhibitor and the substance to be inhibited becomes impossible. The to-be-inhibited substances which adsorb on the surface can virtually serve as crystal germs or seeds from which proceeds a crystallization process. The inhibitors cannot bond to the adsorbed substances either, as a result of which the crystallization process proceeds continuously.
The prevention of chemical linkage as a result of the spatial arrangement of molecules is designated as steric hindrance. Since this steric hindrance occurs on all surfaces, crystalline growth is largely independent of material and surface properties. Appropriate linkage of inhibitors to the substrate however produces a layer on which urine components are bonded as in the urine to be designated as an electrolytic solution. No further components can deposit on urine components thus bonded so that crystalline growth is blocked.
Before the background of these biomedical fundamentals and with a view to solving the problems mentioned at the outset, the invention proposes a urological implant, in particular a vascular wall support for the urinary tract, in which, a spacer molecule film and an inhibitor film bonded thereto of a glycosamino glycan is bonded on the surface of the preferably substantially tubular, inherently stable carrier.
As stated within the scope of the invention, any direct linkage of inhibitors on the carrier surface is not sufficient. Direct linkage will lead to deformations of the macro-molecular inhibitors which will prevent bonding between the inhibitor and the substances to be inhibited, i.e. the crystallizing urine components. Inserting the claimed spacer between the carrier surface and the inhibitor film increases the distance between the carrier surface and inhibitor, thus preventing the deformation, explained above, of the molecules and the steric hindrance.
In accordance with preferred embodiments, the carrier may consist of a polymer such as silicone, polyurethane or the like on the one hand, or of tantalum, a titanium alloy, medical steel or pyrolytic carbon on the other hand, in each case with an amorphous silicon carbide coating as an active substrate surface. A configuration of a metal structure with a polymer coating is feasible too. Also the selection of the chemical components of the spacer film is made in dependence on the material properties of the carrier and its substrate surface. In the case of a silicone substrate, the spacer film can be formed on the basis of a propyl-siloxyl compound, in particular a partially substituted 3-(adipinic-acid-amido-)propyl-siloxyl compound. When carriers are used having an amorphous silicon carbide coating as a activated substrate surface, a spacer film on the basis of a photoactive benzophenone compound, in particular a Fmoc-p-Bz-Phe-OH solution in N-N′-dimethylformamide, has proved suitable.
Preferably an immobilized heparin layer is used as an inhibitor film on the spacer film. The heparin layer is deposited and immobilized on the substrate surface for instance by means of a covalent peptide bond. This type of bond is found in particular when the spacer film is realized, as mentioned above, on the basis of a photoactive benzophenone compound.
Su
Browdy and Neimark
Isabella David J.
Koh Choon P.
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