An implantable medical device obtained by means of two-photon laser polymerisation of a resin to form a three-dimensional matrix, wherein: said three-dimensional matrix comprises a number of levels distributed in height; and said three-dimensional matrix comprises reference means designed to uniquely identify the height of each level from a pre-set reference, by means of a multi-photon fluorescence-excitation microscope; said implantable medical device being characterised in that said reference means comprise a solid having a cross section that varies with height.