Hydrophilic copolymer and medical device

A method of treating a lesion, the method comprising:

• inserting a medical device into a living body;
• advancing the medical device in the body toward the lesion so that a part of the medical device is positioned at the lesion;
• increasing the temperature of the medical device after the part of the medical device is positioned at the lesion;
• and
• reducing the temperature of the medical device prior to removing the medical device from the living body.

A hydrophilic copolymer comprising:

• more than 50% by mol of a structural unit derived from a polymerizable monomer (A) from which a homopolymer having a lower critical solution temperature is formed;
• a structural unit derived from a polymerizable monomer (B) having at least one group selected from the group consisting of a sulfonic acid group (—SO3H), a sulfate group (—OSO3H), a sulfurous acid group (—OSO2H), and a group of salts thereof;
• and
• a structural unit derived from a polymerizable monomer (C) having a photoreactive group.