The present invention addresses the problem of providing a copolymer having sufficient biocompatibility for use in medical material applications, and also providing a method for forming a crosslinked body, wherein said copolymer is used to modify a substrate surface so as to be biocompatible. More specifically, the present invention imparts a protein adsorption suppressing effect and a cell adhesion suppressing effect, which are features of a phosphorylcholine group, to a substrate surface. A copolymer containing, at a specific ratio, a phosphorylcholine structural unit, a hydrophobic structural unit, and a photoreactive structural unit was found to solve the above problem, thus completing the present invention.本發明提供一種用於醫療材料用途的具有充分的生物相容性的共聚物、以及用該共聚物將基材表面改變為生物相容性的交聯體形成方法，具體為賦予基材表面以作為磷酸膽鹼基特徵的蛋白質吸附抑制效果和細胞黏附抑制效果。本發明的發明人發現以特定比例含有磷酸膽鹼結構單元、疏水性結構單元及光反應性結構單元的共聚物能夠解決上述問題，從而完成了本發明。
