| Summary: | 博士 === 國立成功大學 === 化學工程學系 === 88 === Polyurethanes have been used as biomaterials due to their excellent bicompatibility and physical properties. To date, most of the commercial biomedical-grade polyurethanes are composed of aromatic diisocyanate, which might be decomposed to a carcinogenic substance. In contrast, polyurethanes which are composed of aliphatic diisocyanate are shown to be not degrading to a toxic substance.
Commercially available biomedical-grade polyurethane still seems not quite appropriate for use in high demanding blood-contacting applications such as small diameter vascular grafts and the artificial heart. In this study, aliphatic polyurethanes were introduced by ionic groups (such as carboxylate or sulfonate), polydimethylsiloxane (PDMS), or fluorocarbon chains to improve their blood compatibility. The effects of ion incorporation, soft segment type, and fluorocarbon chains on the polymer’s buck, surface, and platelet-contacting properties were studied using Fourier transform infrared spectrophotometry, differential scanning calorimetry, Instron, water absorption analysis, electron spectroscopy for chemical analysis (ESCA), static contact angle analysis, and in vitro platelet adhesion experiments.
For polyurethanes containing a pure polytetramethylene oxide (PTMO) soft segment, the ionic polyurethanes exhibited a smaller fraction of hydrogen-bonded carbonyl groups, poorer microphase separation, smaller fraction of PTMO residing at the surface, and smaller contact angle; however, significant higher water absorption value than nonionic polyurethanes. The in vitro platelet adhesion experiments indicated that ion incorporation, especially for carboxylate, significantly reduced the number and the degree of activation of the adherent platelets.
For polyurethaneureas containing PDMS soft segment, the ionic polyurethaneureas exhibited poor microphase separation, a smaller fraction of PTMO present at the surface, and smaller contact angle. On the other hand, it also showed a larger fraction of PDMS present at the surface and higher water absorption value than its nonionic counterpart. The in vitro platelet adhesion experiments indicated that the ionic groups, especially for carboxylate, and surface enrichment PDMS soft segment could effectively inhibit platelet adhesion.
The fluorinated polyurethanes exhibited weaker hydrogen bonding, a smaller fraction of hydrogen-bonded carbonyls, poorer microphase separated soft segment domains, lower melting temperature and smaller heat of fusion of crystalline hard segment domains than the nonfluorinated polyurethanes. Moreover, the longer length of fluorocarbon chains enhances the effect. The increase of the average length of hard segments also affects the properties mentioned, especially as the average length of hard segments increases from 1 to 2. The surface hard segment content and fluorine/carbon atomic ratio increases as the bulk average length of hard segments increases. In vitro platelet adhesion studies indicated that the addition of fluorocarbon chains into aliphatic polyurethanes lead to a reduction in platelet adhesion and activation.
In this study, the aliphatic polyurethanes containing a pure hard segment were also synthesized to study the effects of fluorocarbon chains on the solubility behavior, microstructure, thermal transition property, crystal morphology, and crystallization behaviors. The fluorinated polyurethane exhibits a lower viscosity, higher solubility in organic solvents, a small fraction of ordered hydrogen-bonded carbonyls, and lower transition temperatures than the corresponding fluorine-free polyurethane. The wide-angle X-ray diffraction measurements reflects change of crystal structure with the (CF2)2 moieties in place of (CH2)2 moieties. Polarized light microscopy also reveals that the polyurethanes exhibit a variety of spherulitic texture. The isothermal crystallization kinetics of these polyurethanes has been investigated by means of differential scanning calorimetry. The Avrami exponent (n) for the polyurethanes is around 2.5, which indicates the growth of crystal might be spherulite growth corresponding to homogeneous (thermal) nucleation and diffusion control. The crystallization activation energy is estimated to be -130.9 kJ/mol for the fluorinated polyurethane and -276.9 kJ/mol for the fluorine-free polyurethane from Arrhonius form.
The incorporation of ionic groups or fluorocarbon chains into polyurethanes significantly reduced platelet deposition and activation. However, they also reduced the polymer’s physical properties. The surface modification modifies only the surface characteristics without changing the bulk properties of the substrates. In this study, the surfaces of aliphatic polyurethane films were modified by grafting fluorocarbon oligomers to improve their blood compatibility. The fluorocarbon oligomers on polyurethane surfaces were terminated with trifluorocarbon or carboxylic acid functionality. The ESCA results demonstrated the fluorocarbon enrichment at the outmost layer in fluorocarbon oligomer grafted polyurethanes. The fluorocarbon oligomer grafted polyurethanes exhibit highly hydrophobic surfaces. The in vitro platelet adhesion experiments indicated that the fluorocarbon oligomer and carboxylic acid functionality significantly reduced the number and the degree of activation of the adherent platelets.
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