A method of producing a visual marking on the exterior of a human or animal body comprising implanting a composition comprising a polymerisable precursor and a photo-initiator to a human or animal body and photo-polymerising the composition to form a polymer within the human or animal body visible on the exterior of a human or animal body. Also disclosed is a method of forming a polymer within a human or animal body comprising implanting a composition of polymerisable precursor and a photo-initiator, photo-polymerising the composition within the human or animal body, to form a polymer within the human or animal body, wherein the resultant polymer has an electrical conductivity of 10-10 S/cm and an injectable composition comprising a polymerisable precursor and a photo-initiator, photo-polymerising the composition, wherein the resultant polymer has an electrical conductivity of 10-10 S/cm.
