A series of hydrogel microspheres having different polystyrene (PSt) contents was prepared by the precipitation polymerization of acrylamide (AAm) and comonomers followed by the seeded polymerization of styrene (St). The resulting AAm-St microspheres had different morphologies and surface properties dependent on the amount of St charged in the seeded polymerization. These AAm-St microspheres were contacted with granulocytes in phosphatebuffered saline (PBS) or plasma at 37 °C to clarify the effect of surface structure of the microspheres and of blood components on phagocytosis. Phagocytosis of the microspheres by granulocytes in PBS was enhanced with increasing PSt content in the microspheres. This suggested that the hydrophobicity of the microspheres is one of the major factors deciding the extent of phagocytosis in PBS. On the other hand, the extent of phagocytosis in plasma depended on the PSt content in the microspheres in an unexpected manner. The microspheres with a small content of PSt were less susceptible to phagocytosis than the microspheres containing no St. Phagocytosis in inactivated serum was slight, regardless of the type of microspheres. These results suggested that complement and the complement-immunogloblin G complex are responsible for the enhancement of phagocytosis of hydrophilic and hydrophobic microspheres, respectively.
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