Naoshi Kawamoto

Blood compatibility of polypropylene surfaces in relation to the crystalline-amorphous microstructure.

Blood-contacting properties of polypropylene surfaces with different crystalline states at the surface layer were examined in terms of plasma protein adsorption and changes in cytoplasmic free Ca2+ levels in platelets. Though the wettability of polypropylene surfaces was almost constantly independent from the surface layer crystallinity and interlamellar spacing, an increase in adhesiveness was observed with decreasing surface layer crystallinity and interlamellar spacing. It is suggested that the surface properties of the sheets varied in relation to the crystalline-amorphous microstructure. Minimum magnitudes in albumin and fibrinogen adsorption were observed on the polypropylene surface with a particular surface layer crystallinity (c. 55 wt%). A decrease in interlamellar spacing resulted in enhancing albumin adsorption and diminishing fibrinogen adsorption. Transient phenomena in plasma protein adsorption were observed on their surfaces with a plasma concentration. It is considered that the polypropylene surface with a particular crystalline-amorphous microstructure reduces the denaturation of adsorbed proteins. An increase in cytoplasmic free Ca2+ levels in platelets was prevented at the polypropylene surface with a surface layer crystallinity of 55 wt%: the particular crystalline-amorphous microstructure of such apolar surfaces as polypropylenes acts to reduce platelet activation. Thus, it is concluded that the blood compatibility of polypropylene surfaces is greatly improved by controlling a crystalline-amorphous microstructure at the surface layer.

Mechanistic aspects of blood-contacting properties of polypropylene surfaces -from the viewpoint of macromolecular entanglement and hydrophobic interaction via water molecules

Polypropylene surfaces with a particular crystalline-amorphous microstructure have been demonstrated to reduce protein adsorption and platelet activation. Such blood-contacting properties may be affected by the crystalline-amorphous microstructure of the surfaces, although wettability such as dynamic contact angles and surface free energy components were almost constant, being independent from the variation in the microstructure. In order to clarify the mechanistic aspects on their blood-contacting properties, the physicochemical properties of the surfaces were evaluated for a series of compression-molded polypropylene sheets in terms of the work of adhesion and the structure of sorbed water. The work of adhesion of the compression-molded sheets increased with decreasing surface layer crystallinity, presumably due to macromolecular entanglement with a polymeric glue used. The work of adhesion involving macromolecular entanglement may occur between proteins and the surfaces. Thus, a decrease in the surface layer crystallinity is considered to cause an increase in the protein adsorption. The structure of water sorbed into the sheets changed--it was more gaseous (isolated) at the surfaces with a higher crystallinity. This suggests that the hydrophobic interaction via water molecules increased with surface layer crystallinity, resulting in increasing protein adsorption and denaturation. Thus, it is considered that both macromolecular entanglement and hydrophobic interaction are important on the mechanistic aspects of blood-contacting properties of polypropylene surfaces. In order to confirm this hypothesis, the evaluation of the physicochemical properties and blood-contacting properties was also performed on a series of uniaxially drawn polypropylene films. A decrease in the work of adhesion and the hydrophobic interaction at the surfaces was observed with increasing draw ratio, and the protein adsorption and platelet activation were effectively prevented with increasing draw ratio. This result supports our hypothesis. Therefore, it is concluded that the excellent blood-contacting properties of polypropylene surfaces can be achieved by reducing the macromolecular entanglement and the hydrophobic interaction with proteins.

Microstructural characterization of polypropene surfaces using grazing incidence X-ray diffraction

The grazing incidence X-ray diffraction was applied for characterizing the crystalline structure of the outermost layer of polypropene sheets. Even in the outermost surface layer within about 5 nm, the crystalline structure of the a form was confirmed by the X-ray diffraction patterns. The values of a and c for the crystal lattice dimension were almost constant in spite of the variation of surface layer crystallinity, whereas the value of b for the surface layer decreased with increasing crystallinity or decreasing comonomer content of polypropene. This suggests that the density of the crystal increased as a function of crystallinity. Additionally, the value of b for the surface layer was smaller than that of the bulk. It was concluded that the lattice distortion can be ascribed to the residual stress caused by the molding pressure under the higher super-cooling rate.

Crystalline structure at surfaces of uniaxially drawn polypropene films

A series of uniaxially drawn films of isotactic polypropene and ethene-propene random copolymer was prepared. The differences in the crystal orientation between the bulk and the surface layer were examined using wide-angle X-ray diffraction and grazing incidence X-ray diffraction techniques. It was found that the c-axis orientation of the surface layer is almost identical with that of the bulk. The magnitude in the b-axis of the polypropene films decreased with increasing draw ratio. The changes in the b-axis at the bulk were found to be greater than those at the surface layer.

Blood-contacting properties of polypropylene surfaces

Blood-contacting properties of polypropylene varied with changes in crystalline-amorphous microstructure such as the crystallinity and the degree of orientation. The results from our evaluation of surface adhesiveness using polymeric glue and water structure which was sorbed into the surfaces indicate that variations in the microstructure may cause differences in macromolecular entanglement with the glue and hydrophobic interaction at the surfaces. Therefore, the prevention of these interactions may help to attain better blood-contacting properties.