An implantable medical device for electrical stimulation or sensing includes a body supporting at least one flexible elongate conductor element. The body includes an insulating structure that protects the flexible conductor element(s). The insulating structure is realized from multiple polymer layers wherein at least one of the polymer layers is formed from a polymer blend of a thermoplastic polyurethane material and an isobutylene block copolymer. In one particular embodiment, the insulating structure of the body includes at least a first polymer layer, a second polymer layer and a third polymer layer, where the second polymer layer covers and interfaces to the first polymer layer, and the third polymer layer covers and interfaces to the second polymer layer. The first polymer layer is formed from a thermoplastic polyurethane material. The third polymer layer is formed from an isobutylene block copolymer. The intermediate second polymer layer is compatible with the particular polymers of the first and third polymer layers and is formed from a polymer blend of a thermoplastic polyurethane material and an isobutylene block copolymer.
