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.

Polydimethylsiloxane/polyether-mixed macrodiol-based polyurethane elastomers: biostability.

A series of four thermoplastic polyurethane elastomers were synthesized with varying proportions of poly(hexamethylene oxide) (PHMO) and poly(dimethylsiloxane) (PDMS) macrodiols. The macrodiol ratios (by weight) employed were (% PDMS:% PHMO) 100:0, 80:20, 50:50 and 20:80. The weight fraction of macrodiol in each polymer was fixed at 60%. The mixed macrodiols were reacted with 4,4'-methylenediphenyl diisocyanate (MDI) and 1,4-butanediol (BDO) chain extender. The biostability of these polymers was assessed by strained subcutaneous implantation in sheep for three months followed by microscopic examination. Pellethane 2363-80A and 2363-55D were employed as control materials. The mechanical properties of the polymers were tested and discussed along with the biostability results. The results showed that soft, flexible PDMS-based polyurethanes with very promising biostability can be successfully produced using the mixed macrodiol approach. The formulation with 80% PDMS macrodiol produced the best result in terms of a combination of flexibility, strength and biostability.

In-vivo degradation of polyurethanes: transmission-FTIR microscopic characterization of polyurethanes sectioned by cryomicrotomy.

A combination of cryomicrotomy and transmission Fourier transform infrared (FTIR) microscopy was used to investigate chemical changes in unstrained sheets of Pellethane 2363-80A, Tecoflex EG80A and Biomer caused by biodegradation (18 month subcutaneous ovine implant). Cryomicrotomy was used to obtain thin sections (ca. 2.5 microm) from the surface into the bulk, parallel to the plane of the surface. FTIR microscopy was then used to obtain infrared absorbance spectra in the range 4000-600 cm(-1). Comparisons between the infrared spectra (by spectral subtraction) from implant surface, implant interior and non-implanted controls were used to detect chemical changes. Scanning electron microscopy was used to assess microstructural changes owing to biodegradation. Biodegradation in Biomer was observed as uniform pitting and superficial fissuring (<2.0 microm depth) over the implant surface. Biodegradation in Pellethane 2363-80A and Tecoflex EG 80A was observed as severe localized embrittlement of the surface with fissures infiltrating up to 40 microm into the bulk. The chemical changes associated with biodegradation were observed as localized oxidation of the soft segment and hydrolysis of the urethane bonds joining hard and soft segments. Tecoflex EG80A was also found to be susceptible to localized hydrolysis of the urethane bond within the aliphatic hard segment. Biomer showed evidence of a significant non-specific degradation in the non-implanted wet control (37 degrees C phosphate buffered saline at pH 7.3) samples and in the implant bulk.

Degradation of medical-grade polyurethane elastomers: the effect of hydrogen peroxide in vitro.

Treatment of Pellethane 2363-80A--a medical-grade poly(tetramethylene oxide)-based polyurethane elastomer--with 25% (w/w) hydrogen peroxide at 100 degrees C for times ranging from 24 h to 336 h led to significant decreases in ultimate tensile properties and decreases in molecular weight, both at the surface and in the bulk. IR spectral changes were similar to those observed after degradation in vivo. Differential scanning calorimetry showed that hydrogen-peroxide-induced degradation was associated with greater order in the hard domain and greater mobility in the soft domain. Studies conducted with low-molecular-weight model compounds for the hard and soft segments confirmed that methylene groups adjacent to oxygen were susceptible toward oxidation. The extent of degradation of a series of commercial polyurethanes on treatment with hydrogen peroxide (25%, 24 h, 100 degrees C) correlated well with their reported susceptibility to environmental stress cracking in vivo.