Li-Yen Chen

The formation mechanism of membranes prepared from the nonsolvent-solvent-crystalline polymer systems

Abstract Two different types of membranes were prepared by immersion–precipitation process in two nonsolvent–DMSO–poly(ethylene-co-vinyl alcohol) (EVAL) systems. The effect of nonsolvent on the resulting membrane structure was studied via scanning electron microscopy. On using 2-propanol as the nonsolvent, the membrane showed a homogeneous particulate morphology. If the nonsolvent was changed to water, the membrane structure was a typically asymmetric structure while the particulate morphology was suppressed. In order to understand the change of the obtained membrane structures, the phase diagrams of water–DMSO–EVAL and 2-propanol–DMSO–EVAL systems were contrasted at 25°C. Both crystallization-induced gelation and liquid–liquid demixing were observed and equilibrium crystallization lines are always positioned above the binodal boundaries. Moreover, the precipitation rate of the EVAL solution in 2-propanol and water were examined by the light transmission experiments and the local composition profiles of membrane solution during membrane formation were analyzed by using a ternary mass transfer model. Based on both thermodynamic behaviors and kinetic properties, the membrane structures obtained were discussed in terms of the sequence and mechanism of phase transformations during membrane formation.

Comparison of epoxides on grafting collagen to polyurethane and their effects on cellular growth

The current study investigated the effects of vary epoxides on linking capacity of collagen to carboxyl-group-enriched polyurethane (PU) and the consequent effects on the growth of endothelial cells. Epoxides of EX-810, 1,4BDE, DER732, DER331, and DER332 were initially reacted with the carboxyl groups of PU substrates at 110 degrees C for 20 h. Free epoxy rings of epoxide-PU substrates, characterized by Fourier transform infrared spectroscopy and quantified by titration with HCl and NaOH, were available for collagen grafting. The amounts of collagen grafted were in accordance with the amounts of free epoxy rings detected and correlated with the growth of endothelial cells on the substrates. Our results indicated that epoxides with shorter aliphatic intermediate chain can graft more collagen to the epoxide-PU substrates than epoxides with longer intermediate chain or with aromatic groups. Epoxides were also demonstrated to be nontoxic linking agents for biomaterials.

Human monocyte adhesion and activation on crystalline polymers with different morphology and wettability in vitro

This study evaluated the effects of crystalline polyamide (Nylon-66), poly(ethylene-co-vinyl alcohol) (PEVA), and poly(vinylidene fluoride) (PVDF) polymers with nonporous and porous morphologies on the ability of monocytes to adhere and subsequently activate to produce IL-1beta, IL-6, and tumor necrosis factor alpha. The results indicated monocyte adhesion and activation on a material might differ to a great extent, depending on the surface morphology and wettability. As the polymer wettability increases, the ability of monocytes to adhere increases but the ability to produce cytokines decreases. Similarly, these polymers, when prepared with porous surfaces, enhance monocyte adhesion but suppress monocyte release of cytokines. Therefore, the hydrophobic PVDF with a nonporous surface stimulates the most activity in adherent monocytes but shows the greatest inhibition of monocyte adhesion when compared with all of the other membranes. In contrast, the hydrophilic Nylon-66, which has a porous surface, is a relatively better substrate for this work. Therefore, monocyte behavior on a biomaterial may be influenced by a specific surface property. Based on this result, we propose that monocyte adhesion is regulated by a different mechanism than monocyte activation. Consequently, the generation of cytokines by monocytes is not proportional to the number of cells adherent to the surface.

The formation mechanism of membranes prepared from the crystalline EVAL polymer–water (nonsolvent)–2-propanol (nonsolvent) system

Crystalline poly(ethylene‐co‐vinyl alcohol) (EVAL) membranes from the water‐2-propanol cosolvent were prepared by immersion‐precipitation at 608C. The use of two nonsolvents serving as a cosolvent system, replacing the traditional solvent‐ nonsolvent pair, for the membrane formation was investigated. The two-nonsolvent proportion in which they are mixed to produce the cosolvent system and the prediction of the membrane structures were based on the complex ternary phase diagram at 608C. It was attempted to relate the influence of liquid‐liquid demixing and solid‐liquid demixing on the membrane morphologies to the ternary phase diagram. Two different morphologies and two different demixing rates could be obtained by using water and 2-propanol, respectively, to precipitate the cosolvent system. When water was used as the precipitation medium, it showed a rapidly precipitating system and cellular morphologies were obtained due to liquid‐liquid demixing. In contrast, when 2-propanol was used as the precipitation medium, it showed delayed demixing and crystallization-induced particulate morphologies were formed. Trends expected on the basis of the phase diagram was in reasonable agreement with the observed membrane morphology. Therefore, the principles of membrane formation established for the ternary systems with nonsolvent‐solvent‐polymer can be extended to a nonsolvent‐nonsolvent‐polymer system. # 1999 Elsevier Science B.V. All rights reserved.

Prediction of EVAL membrane morphologies using the phase diagram of water-DMSO-EVAL at different temperatures

Abstract The effects of precipitation temperature on the structures of the EVAL membranes formed in a wet phase inversion process were studied. As the temperature was changed, the phase behavior (liquid–liquid demixing and crystallization boundaries) of the membrane formation system changed accordingly. Therefore, a wide variety of morphologies of the EVAL membranes could be synthesized at different temperatures. At low temperatures (e.g., 25°C), the membrane solution precipitated into a particulate morphology that was governed by the polymer crystallization mechanism, whereas at elevated temperatures (e.g., 65°C) liquid–liquid demixing process dominated the precipitation process and the membrane became cellular in its morphology. In the intermediate cases, the membrane exhibited a structure containing features from both types of phase separations. For the current system, it was found that there existed a good correlation between the phase behavior and the morphology of the membranes.

Membranes with a particulate morphology prepared by a dry-wet casting process

Abstract The effect of the evaporation step on the occurrence of particles in poly(ethylene-co-vinyl alcohol) (EVAL) membranes cast from DMSO solutions via the dry/wet process were studied. The structure of the EVAL membranes can be changed from an asymmetrical structure consisting of a dense skin layer and finger-like macrovoids in the sublayer to a skinless and symmetric structure by constituent particles bonded to each other. From the analysis of the membrane formation mechanism, the competition of different phase separation process during membrane formation is most important. The onset of the phase separation may be either liquid–liquid demixing or solid–liquid demixing, which determines the resulting membrane properties. Directly immersing the casting solution into a water bath, liquid–liquid demixing is of considerable importance during the phase separation of the solutions. As a result of the lower activation energy for nucleation, liquid–liquid demixing can precede solid–liquid demixing even in cases where solid–liquid demixing is favored thermodynamically. By using the evaporation process, the phase separation proceeds slowly via solid–liquid demixing and thus leads to a particulate morphology in the membrane. This suggests that the evaporation step cause crystallization of EVAL molecules from the casting solution to inhibit the macrovoid formation. In addition, the duration of the evaporation step is shown to have a strong influence on the disappearance of particles. The results presented here offer a qualitative basis for the development of membranes with a particulate morphology.

Effect of forms of collagen linked to polyurethane on endothelial cell growth

Collagen has been widely coated or grafted onto polymer surfaces to improve the biocompatibility of materials. To better support the growth of endothelial cells on polyurethane (PU), collagen was grafted to the carboxyl group enriched PU through 1,2-bis(2,3-epoxypropoxy)ethane linking. Our results demonstrated that collagen in various conditions may result in different forms being grafted to the PU substrate, which subsequently affected the growth of endothelial cells. Collagen predialyzed against physiological phosphate buffered saline (PBS) could be reconstituted into native type fibrils with a bigger diameter at 37 degrees C than could collagen neutralized by titration with NaOH. At low temperature, titrated collagen formed floss-like fibrils packed in a ball with cobblestone-like morphology. The amount of collagen grafted was related to the condition of the collagen used, which in consequence affected the diameter of the collagen fibril formed and the growth of endothelial cells. In conclusion, reconstituted collagen fibrils formed from collagen in PBS at 37 degrees C grafted in the highest amounts to an epoxy-PU substrate and that optimally supported the growth of endothelial cells. Such prepared materials may be potentially good vascular bioprosthetic materials and may provide a wide range of biological applications.

Analysis of ultrahigh molecular weight polyethylene failure in artificial knee joints: thermal effect on long-term performance.

The mechanism resulting in damage to and failure of ultrahigh molecular weight polyethylene (UHMWPE) tibial inserts was investigated on clinically retrieved components. The severity of the subsurface damage increased with the length of time that the component had been implanted. A theoretical analysis was developed to account for the generation of subsurface damage based on a heat transfer model. Friction generates surface heat during articulation of total knee systems. Due to the cooling effect of body fluid on the surface, the rise in temperature on the UHMWPE surface is lower than that below the surface. The peak temperature was estimated to occur on a plane positioned about 1 to 2 mm below the surface. This result was similar to the bulk temperature variation observed during in vivo and in vitro studies by other investigators. Although the difference in temperature on and below the surface is only a few degrees, the thermal effect becomes apparent after a long time and may be explained by the viscoelastic behavior of polymers: the temperature-time equivalence. It is therefore suggested that this thermal effect is another contributory factor to material damage, in addition to high stress and oxidative degradation (in appropriate cases). Therefore, any technological efforts aimed at improving the performance of artificial joint prostheses should minimize the thermal effects at the subsurface of the articular components.

Crystallization of polypropylene II. Non-isothermal kinetics

Abstract With the aid of differential scanning calorimetry (DSC), we first analyzed the isothermal crystallization kinetics of polypropylene (PP) on the basis of the Avrami equation, the Turnbull-Fisher nucleation theory and the Hoffman-Lauritzen growth theory. It is shown that the nucleation rate, the average radial growth rate of spherulites and the overall crystallization rate constant first increase and then decrease as the supercooling temperature increases.

A Three-Dimensional Registration Method for MicroUS/MicroPET Multimodality Small-Animal Imaging:

Small-animal models are used extensively in disease research, genomics research, drug development and developmental biology. The development of noninvasive small-animal imaging techniques with adequate spatial resolution and sensitivity is therefore of prime importance. In particular, multimodality small-animal imaging can provide complementary information. This paper presents a method for registering high-frequency ultrasonic (microUS) images with small-animal positron-emission tomography (microPET) images. Registration is performed using six external multimodality markers, each being a glass bead with a diameter of 0.43–0.60 mm, with 0.1 μl of [18F]FDG placed in each marker holder. A small-animal holder is used to transfer mice between the microPET and microUS systems. Multimodality imaging was performed on C57BL/6J black mice bearing WF-3 ovary cancer cells in the second week after tumor implantation and rigid-body image registration of the six markers was also performed. The average registration error was 0.31 mm when all six markers were used and increased as the number of markers decreased. After image registration, image segmentation and fusion are performed on the tumor. Our multimodality small-animal imaging method allows structural information from microUS to be combined with functional information from microPET, with the preliminary results showing it to be an effective tool for cancer research.