Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From reactant having at least one -n=c=x group as well as...
Patent
1993-02-23
1995-02-28
Welsh, Maurice J.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From reactant having at least one -n=c=x group as well as...
528 59, 528 76, C08G 1830
Patent
active
053938585
DESCRIPTION:
BRIEF SUMMARY
The invention relates to polyurethane or polyurethane-urea elastomeric compositions which are particularly suitable for use in devices that contact living tissue or bodily fluids.
The applications of polyurethane elastomers are limited by their susceptibility to degradation, particularly hydrolysis and oxidation. For example, degradation problems have arisen with artificial leathers, shoe soles and cable sheathing. This problem has been partially overcome by the use of polyether macrodiols. The problem is particularly acute in biomedical applications where because of their good mechanical performance (e.g. high tensile strength, good tear and abrasion resistance), inherent biocompatibility and nonthrombogenicity, polyurethanes are the materials of choice for many applications and have found use in pacemakers leads, various types of catheters, implantable prostheses, cardiac assist devices, heart valves, sutures, and vascular grafts, as well as in extra-corporeal blood contacting applications.
Polyurethane elastomers are usually prepared by reacting excess diisocyanate with a polyol "soft segment" to form a prepolymer having terminally reactive isocyanate groups, which is then reacted with a diol or diamine chain extender. Although the diisocyanate plays an important role, many of the properties associated with the polyurethane elastomer are derived from the soft segment portion of the chain. The soft segments of most commercial polyurethane elastomers are derived from polyether macrodiols, for example poly(ethylene oxide), poly(propylene oxide) and poly(tetramethylene oxide) and polyester macrodiols (poly(ethylene adipate) and polycaprolactone glycols. Polyurethanes that have polyether soft segments have better resistance to hydrolysis than those with polyester soft segments and are preferred as biomaterials. The most widely accepted commercial medical-grade polyurethanes are Pellethane (Registered Trade Mark) and Biomer (Registered Trade Mark), although Tecoflex (Registered Trade Mark), Vialon (Registered Trade Mark) and Mitrathane (Registered Trade Mark) have found some acceptance. These materials all have in common the use of poly(tetramethylene oxide) as the macrodiol soft segment. S. Gogolewski in Colloid and Polymer Science, volume 267, pp 757-785 (1989) summarizes the prior art commercial and experimental biomedical polyurethanes which have been disclosed. There are no reports of biomedical polyurethanes having the composition described below.
The biostability of polyurethanes is reviewed by Michael Szycher, a recognized leader in the field, in Journal of Biomaterials Applications, volume 3, pp 297-402 (October, 19&8). Degradation can be manifested in terms of surface or deep cracking, stiffening, erosion, or the deterioration of mechanical properties, such as flex life. The deterioration ultimately leads to failure of the device. Degradation can also cause the leaching of cytotoxic agents, resulting in tissue necrosis or in some cases, the formation of tumors. The inadequate biostability of polyurethanes is generally recognised as a severe limitation to the successful development of long term artificial hearts and synthetic polyurethane small bore vascular grafts.
The biologically-induced degradation of polyurethanes has been attributed to several factors and some of these are summarized in the review by Michael Szycher cited above. Although there is still some controversy, it is widely held that the following mechanisms of degradation are important:
Other agents (e.g. fungi) have also been implicated by some workers. Of these degradative pathways, environmental stress cracking is arguably the most complex and depends on a combination of a chemical agent and either residual internal stress (e.g. from processing), or externally applied stress (e.g. from flexing of an implant during use). Calcification has been reviewed by R. J. Thoma and coworkers in Journal of Biomaterials Applications, volume 3, pp 180-206 (October, 1988) and is a problem in certain applications, such as in the artificial heart an
REFERENCES:
patent: 3252943 (1966-05-01), Dankert
Brandwood Arthur
Gunatillake Pathiraja
Meijs Gordon F.
Rizzardo Ezio
Schindhelm Klaus H.
Commonwealth Scientific and Industrial Research Organisation
Unisearch Limited
Welsh Maurice J.
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