Degradation studies on segmented polyurethanes prepared with HMDI, PCL and different chain extenders
Introduction
The various applications of segmented polyurethanes (SPUs) in the biomaterials field are based on their good physicochemical and mechanical properties while exhibiting an acceptable biological performance. These polymers are particularly useful in blood contact applications such as vascular grafts, hearth valves and pacemakers [1]. In spite of these properties, SPUs exhibit poor fatigue properties and long-term oxidative degradation in their various forms (metal ion induced, auto-oxidation, environmental stress cracking, macrophages/phagocytic cell mediated, etc.), especially those based on polyether type polyols [2]. In order to improve SPU properties, the soft segments have received a great deal of attention as they are the most vulnerable part of the polymer. Resistance to oxidative degradation has been achieved by using ether and ester-free polyurethanes through the incorporation of polycarbonate [3], [4] or butadiene diol terminated as soft segments [5], [6], [7], [8]. However, hydrolysis of the carbonate group is still possible in addition to long-term calcification [9], whereas butadiene diol polyurethanes render highly hydrophobic and segregated structures [6].
Recently, polyester-based polyurethanes such as those based on polycaprolactone have been investigated for tissue engineering purposes. This is motivated by the fact that the ester group is susceptible to chemical and enzymatic hydrolysis, a condition that is easily achieved in vivo. In this way, segmented polyurethanes containing polycaprolactone as a soft segment have been prepared with aliphatic or aromatic diisocyanates such as hexamethylene diisocyanate [10], 1,4-butane diisocyanate [11], 4,4′-methylene bis (cyclohexyl isocyanate) [12], l-lysine diisocyanate [13], toluene diisocyanate [14], 4,4′-diphenylmethane diisocyanate [15], [16] and isophorone diisocyanate [17].
In this paper, the synthesis of a biodegradable segmented polyurethanes (BSPUs) based on a low-molecular-weight polycaprolactone (PCL), 4,4′-methylene bis (cyclohexyl isocyanate) (HMDI) and either butanediol (BD) or dithioerythritol (DTE) as a chain extender is reported. HMDI is proposed not only as it renders a highly amorphous segmented polyurethane at low rigid segment content, but also because it leads to harder and stronger elastomers that can be used to prepare composites with low modulus and high strength [18]. Furthermore, HMDI has been shown to be less hydrolytically stable than MDI and will render products which are less toxic [19]. DTE was used as thiol-bearing chain extender since this type of chemical group is reported to have the ability to exchange endogenous nitric oxide [20]. PCL of low molecular weight was used as a biodegradable soft segment as it is known that it can be degraded faster than highly crystalline high-molecular-weight PCL and because its degradation products are non-toxic to cells.
The synthesized polyurethanes were characterized physicochemically and also in terms of their hemocompatibility. We also report on the properties of the degraded polymer after being exposed to acidic, alkaline and oxidative conditions. Although there are several reports on the in vitro [21], [22] and in vivo [23], [24], [25] degradation of polyurethanes, to our knowledge there is no reference on the degradative behaviour of both HMDI:BD:PCL and HMDI:DTE:PCL segmented polyurethanes for biomedical purposes.
Section snippets
Polyurethane synthesis
Materials for polyurethane synthesis were purchased from Aldrich (Milwaukee, WI, USA). BSPUs with a molar ratio of 2.05:1:1 (HMDI:BD or DTE:PCL) were prepared in dimethylformamide solution by a two-step procedure. In the first stage, diol-terminated PCL (Mn = 1250 as provided by the supplier) was mixed, in a glass reactor at 60 °C under nitrogen atmosphere, with a molar excess of 4,4′-methylene bis (cyclohexyl isocyanate) in the presence of 0.15 wt.% stannous octoate in order to form an
Spectroscopic studies
Fig. 2 shows the 1H NMR spectra for BSPU1 and BSPU2 where peak assignation is shown in the figure. In general, similar spectra were observed and only SPUB2 showed additional peaks between 3.6 and 2.6 ppm.
Fig. 3a and b shows the infrared spectra for BSPU1 and BSPU2. The main infrared absorptions were similar for both SPUs and they are in good agreement with those previously reported [10], [18]. In addition, a band at 1632 cm−1 was observed and assigned to the formation of ureas, due to water or
Discussion
The successful use of SPUs in the biomedical field is based not only on the range of physicochemical properties they exhibit but also on their good interaction with the host tissue. There are currently two general types of use for segmented polyurethanes. First, they are used as permanent devices such as pacemaker leads and ventricular assist devices, where resistance to long-term oxidation (metal ion oxidation (MIO) and environmental stress cracking (ESC)) is important [1]. Second, they can be
Conclusion
Partially biodegradable aliphatic segmented polyurethanes were successfully prepared with polycaprolactone diol and 4,4′-methylene bis (cyclohexyl isocyanate), with either butanediol or dithioerythritol as a chain extender. BSPU1 was a non-crystalline elastomeric polyurethane that was easily degraded under acidic and alkaline conditions but only moderately degraded in an oxidative medium. DSC and XRD showed that the acidic and alkaline hydrolysis mainly degrade polycaprolactone while an oxidant
Acknowledgements
This work was supported by CONACYT (México) Grant SEP-2003-CO2-43175. The authors thank Daniel Aguilar for his technical assistance on the XRD experiments.
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