Small Caliber Vascular Grafts. Part II: Polyurethanes Revisited

Abstract

Polyurethanes are considered to be one of the most bio- and blood-compatible biomaterials known today. By intelligent utilization of principles governing the structure/property relationship of these polymers, one can generate systems which resemble, in principle, the physical-mechanical behavior of living tissue. Thus, it is not surprising that these materials played a major role in development of small caliber vascular grafts targeted for vascular access, peripheral and coronary artery bypass indications. Numerous technologies, often esoteric in nature, were and are utilized to generate porous, potentially multilayered conduits possessing some or many characteristics of natural blood vessels. Properties such as durability, elasticity, compliance, pulsatility, and propensity for healing became attainable via polyurethanes. Furthermore, additional surface and/or bulk modification via attachments of biologically active species such as anticoagulants, cell proliferation suppressants, anti-infective compounds or biorecognizable groups are possible due to reactive groups which are part of the polyurethane structure. These modifications are designed to control or mediate host acceptance and healing of the graft. Finally, a myriad of practical processing technologies are used to fabricate functional grafts. Among those, casting, electrostatic and wet spinning of fibers and monofilaments, extrusion, dip coating or spraying of mandrels with polymer/additive solutions are often coupled with chemical-potential-difference-driven coagulation and phase inversion leading to grafts feeling and often behaving like natural vessels. Historically, the first polyurethanes utilized were hydrolytically unstable polyester polyurethanes containing hydrolysis-prone polyester polyols as soft segments, followed by hydrolytically stable but oxidation sensitive polyether polyols based polyurethanes. Polyether-based polyurethanes and their clones containing silicone and other modifying polymeric intermediates represented significant progress. Many viable technologies were discovered and developed using polyether-based polyurethanes. Chronic in vivo instability observed on prolonged implantation became, however, a major roadblock. The path led finally to the use of hydrolytically and oxidatively stable polycarbonate polyols as the soft segment to generate biodurable materials with resistance to biodegradation adequate for vascular access or perhaps peripheral graft indications. This biodurability needs to be further increased in order to utilize the full potential of polyurethanes in development of patent small caliber graft. Modification of both the soft and hard segments needs to be considered in order to maximize biodurability of both basic building blocks of the polyurethane. This paper reviews the achievements, discusses trends, and offers the view of the future in this exciting area of material/device combination.

Synthesis of Six-Membered Cyclic Carbonate Monomers by Disproportionation of 1,3-Bis(alkoxycarbonyloxy)propanes and Their Polymerization

“…32, No. 5,2000 receiver. 18 g (yield 30%, calculated on TMP) of the distillate, which comprises 3fe to the extent 95%, were obtained.…”

“…After evaporation of an excess of diethyl carbonate the flask content [ mainly tetrakis ( ethoxycarbonyloxymethyl) methane (2ge)] was subjected to the thermal disproportionation (200-220 °C) and subsequent distillative depolymerization (220-270°C) collecting, under vacuum (0.1 mmHg) 3 g (yield 12%) of biscyclic carbonate of pentaerythritol (2,4,8,10-tetraoksa [5,5] …”

Aliphatic polycarbonate-based thermoplastic polyurethane elastomers containing diphenyl sulfide units

“…TPUs based on polycarbonate diols (PCDs) combine excellent tensile strength with good oxidation and hydrolytic stability. Because of these advantageous properties, polycarbonate TPUs are widely used in many biomedical applications [2][3][4][5].…”

Fabrication of Small Caliber Stent-grafts Using Electrospinning and Balloon Expandable Bare Metal Stents

“…Small caliber vascular grafts made of PU have been studied to promote healing by using modifications such as surface coatings, anti-coagulants, and cell proliferation suppressants. All these modifications are designed to mediate host acceptance and promote graft healing 23 .…”