Base material and copolymer

A base material that comes into contact with water, comprising a base material main body and a surface layer provided on at least a part of a surface that comes into contact with water of the base material main body,

• wherein the surface layer is composed of a cured product of a composition containing a compound having a biocompatible moiety and a reactive silyl group,
• the biocompatible moiety is composed of at least one selected from the group consisting of a structure represented by the following formula 1, a structure represented by the following formula 2 and a structure represented by the following formula 3,
• the composition has a content of the biocompatible moiety being 25 to 83% by mass and a content of the reactive silyl group being 2 to 70% by mass, in a solid component, and
• in the case where the biocompatible moiety has the structure represented by the following formula 1, 50 to 100% by mol of the structure represented by the following formula 1 is the structure represented by the following formula 1 in a structure represented by the following formula 4:
• wherein, in the formula 1, n is an integer of 1 to 300;
• in the formula 2, R1 to R3 are each independently an alkyl group having a carbon number of 1 to 5, and a is an integer of 1 to 5;
• and
• in the formula 3, R4 and R5 are each independently an alkyl group having a carbon number of 1 to 5, X− is a group represented by the following formula 3-1 or a group represented by the following formula 3-2, and b is an integer of 1 to 5:
• wherein, in the formula 4, n is an integer of 1 to 300 and R6 is a hydrogen atom or an alkyl group having a carbon number of 1 to 5.