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The purpose of this study is to clarify the main factor causing excellent blood compatibility of poly(2-methoxyethyl acrylate)(PMEA) by the comparison between PMEA and seven PMEA analogous polymers. The polymers have a typical functional group as ester side chains such as methoxyethyl, hydroxyethyl, phenoxyethyl, and alkyl groups. The properties of the polymers relating to water were investigated in terms of contact angle, equilibrium water content (EWC), and thermal analysis by differential scanning calorimetry. The water in PMEA could be classified into three types: nonfreezing water, freezing bound water, and free water while the water in the analogous polymers was classified into just two types: free and nonfreezing waters, regardless of the chemical structure of the side chain. The surface property represented by the contact angle of water corresponded to the content of the bound water (nonfreezing water + freezing bound water). The platelet compatibility in vitro did not depend on the contents of these waters, or on the contact angle. On the basis of the results of this work and the previous work on the platelet compatibility of poly(MEA-co-HEMA) (Tanaka et al. Biomacromolecules 2002;3;36-41), the main factor causing the excellent compatibility of PMEA is discussed.

Previously, we reported that poly(2-methoxyethylacrylate) (PMEA) showed excellent blood compatibility and implied that the water structure in PMEA contributed to the blood compatibility. In this study, the relationship between the water structure and the blood compatibility is clarified by studying the influence of the monomer composition of poly(MEA-co-HEMA) on the water structure and the blood compatibility of the copolymers. The water in the polymer was classified into three types: free water, freezing bound water, and nonfreezing water. The polymers with 0-30 mol % of HEMA content had a significant amount of freezing bound water, and the amount decreases greatly when the composition of HEMA exceeded 30 mol %. On the other hand, the amount of other water increased simply with an increase of HEMA content. The evaluation of the platelet compatibility of poly(MEA-co-HEMA) revealed that the adhesion number and the morphological change of platelet on the copolymer surface were least when the HEMA content was 0-20 mol %. These two results strongly suggest that the freezing bound water relates to the platelet compatibility of the polymers.

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The structure of water sorbed into poly(2-methoxyethyl acrylate) (PMEA), poly(2-hydroxyethyl methacrylate) (PHEMA), and their copolymers (p(MEA/HEMA)) was investigated by attenuated total reflection infrared (ATR-IR) spectroscopy. The extinction coefficient of the OH stretching band of sorbed water ( OH) was calculated from the band area obtained by IR measurement and the amount of sorbed water obtained by thermogravimetric analysis. When the polymers contacted with water vapor (relative humidity ) ∼55%), the OH values were quite similar in all polymers. On the other hand, when the polymers contacted with liquid water, the OH values were drastically changed by the content of 2-methoxyethyl acrylate (MEA). When the MEA content of the polymers was low (<60 mol %), the OH value of the water sorbed into polymers in contact with liquid water was equal to that in contact with water vapor. In the higher MEA content (70-100 mol %), on the other hand, the OH values of the water sorbed into polymers in contact with liquid water were 5-8 times larger than that in contact with water vapor. These results seemed to indicate that the interaction between the primary hydration water around the MEA-containing polymer chain and water molecules surrounding the primarily hydrating water is very weak. Such water with a large OH value seemed to correspond to "cold-crystallizable" water, which has been observed by DSC as anomalous water other than intermediate and nonfreezable waters. Taking both the experimental results obtained in this work and those thermodynamically obtained previously into consideration, it was strongly suggested that the cold crystallization of water is generated by caging water molecules in a small space by the polymer chains with a small hydration region. The correlation between the OH value and the blood compatibility of the copolymer was also discussed.

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The resistance of random copolymers of BMA and CMB against biofouling was evaluated. The amount of proteins adsorbed onto the CMB copolymers was smaller than that onto other polymers (non-ionic polymers and copolymers of ordinary ionic monomers and BMA) and decreased with an increase in the content of CMB residues. Furthermore, there was a dramatic decrease in the number of cells (platelets and fibroblasts) that adhered to the CMB copolymers compared with that to other polymers. In contrast with this, CMB copolymers were slightly perturbative to both complement and coagulation systems. However, the overall results suggest that zwitterionic moieties are effective for making polymer materials biocompatible due to their excellent anti-biofouling property.

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