The effect of average segment length on morphology and properties of a series of polyurethane elastomers. 1. Characterisation of the series

A series of eight thermoplastic polyurethane elastomers were synthesized from 4,4′-methylene diphenyl diisocyanate (MDI) and 1,4-butanediol (BDO) chain extender, with poly(hexamethylene oxide) (PHMO) macrodiol soft segments. The soft segment molecular weights employed ranged from 433 g/mol to 1180 g/mol. All materials contained 60% (w/w) of the soft segment macrodiol. Differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), wide angle x-ray diffraction (WAXD), and small angle x-ray scattering (SAXS) techniques were employed to characterize morphology. Tensile and Shore hardness tests were also performed. Materials were tested in the annealed state. It was found that an increase in segment length was accompanied by an increase in the degree of microphase separation, average interdomain spacing, hard domain order, hardness, stiffness, and opacity. DSC experiments showed the existence of several hard segment melting regions that were postulated to result from the disordering or melting of various hard segment length populations. For the system and composition ratio employed, it was found that optimum tensile properties (UTS and breaking strain) were achieved when a PHMO molecular weight of between 650 and 850 was utilized

New methods for the assessment of in vitro and in vivo stress cracking in biomedical polyurethanes

This article describes a new test method for the assessment of the severity of environmental stress cracking of biomedical polyurethanes in a manner that minimizes the degree of subjectivity involved. The effect of applied strain and acetone pre-treatment on degradation of Pellethane 2363 80A and Pellethane 2363 55D polyurethanes under in vitro and in vivo conditions is studied. The results are presented using a magnification-weighted image rating system that allows the semi-quantitative rating of degradation based on distribution and severity of surface damage. Devices for applying controlled strain to both flat sheet and tubing samples are described. The new rating system consistently discriminated between. the effects of acetone pre-treatments, strain and exposure times in both in vitro and in vivo experiments. As expected, P80A underwent considerable stress cracking compared with P55D. P80A produced similar stress crack ratings in both in vivo and in vitro experiments, however P55D performed worse under in vitro conditions compared with in vivo. This result indicated that care must be taken when interpreting in vitro results in the absence of in vivo data. (C) 2001 Elsevier Science Ltd. All rights reserved

High modulus biodegradable polyurethanes for vascular stents: Evaluation of accelerated in vitro degradation and cell viability of degradation products

We have recently reported the mechanical properties and hydrolytic degradation behavior of a series of NovoSorb™ biodegradable polyurethanes (PUs) prepared by varying the hard segment (HS) weight percentage from 60 to 100. In this study, the in vitro degradation behavior of these PUs with and without extracellular matrix (ECM) coating was investigated under accelerated hydrolytic degradation (phosphate buffer saline; PBS/70°C) conditions. The mass loss at different time intervals and the effect of aqueous degradation products on the viability and growth of human umbilical vein endothelial cells (HUVEC) were examined. The results showed that PUs with HS 80% and below completely disintegrated leaving no visual polymer residue at 18 weeks and the degradation medium turned acidic due to the accumulation of products from the soft segment (SS) degradation. As expected the PU with the lowest HS was the fastest to degrade. The accumulated degradation products, when tested undiluted, showed viability of about 40% for HUVEC cells. However, the viability was over 80% when the solution was diluted to 50% and below. The growth of HUVEC cells is similar to but not identical to that observed with tissue culture polystyrene standard (TCPS). The results from this in vitro study suggested that the PUs in the series degraded primarily due to the SS degradation and the cell viability of the accumulated acidic degradation products showed poor viability to HUVEC cells when tested undiluted, however particles released to the degradation medium showed cell viability over 80%