An implantable medical device includes a device body at least partially formed of a polymeric material including a base polymer and a block copolymer. The block copolymer includes at least one polyethylene oxide (PEO) block and at least one polyisobutylene (PIB) block. The PEO and PIB blocks may be coupled together by a urethane or urea linkage. The block copolymer may be a triblock copolymer, PEO-PIB-PEO, and the base polymer may be a polystyrene-polyisobutylene-polystyrene triblock copolymer.