A system comprising: an implant for implantation at an implantation site within a lumen of the body of a human or animal, the implant (24, 62) having a matrix with a luminal surface, an abluminal surface and a wall thickness separating said surfaces, the implant having capability to expand and contract radially through a range of diameters, with corresponding changes in the cross-sectional area of the lumen (34, 60) defined by the matrix, said implant exhibiting a remembered configuration in which the diameter of the implant is at the low end of said range of diameters, such that the implant is prompted to change its configuration towards said remembered configuration, that is, towards a reduced diameter configuration, upon the imposition on the implant of outside stimulus, and said implant being characterised by a molecule distributed within its bulk volume that preferentially responds to an applied field that permeates said volume by absorbing energy at a particular wavelength from said applied field, wherein said applied field constitutes the outside stimulus, and the preferential response of the molecule raises the temperature of the implant, the temperature rise serving as the prompt on the implant to change to said remembered configuration; and a self-shrinkage activation catheter (32, 54) equipped to impose on the implant said applied field thereby to stimulate the implant to contract radially from a first deployed configuration towards its remembered configuration, wherein the activation catheter (32, 54) is a balloon catheter with means to manage the temperature of the surface of the balloon, whereby said balloon surface is effective to conduct heat energy to the luminal surface of the implant matrix. an implant for implantation at an implantation site within a lumen of the body of a human or animal, the implant (24, 62) having a matrix with a luminal surface, an abluminal surface and a wall thickness separating said surfaces, the implant having capability to expand and contract radially through a range of diameters, with corresponding changes in the cross-sectional area of the lumen (34, 60) defined by the matrix, said implant exhibiting a remembered configuration in which the diameter of the implant is at the low end of said range of diameters, such that the implant is prompted to change its configuration towards said remembered configuration, that is, towards a reduced diameter configuration, upon the imposition on the implant of outside stimulus, and said implant being characterised by a molecule distributed within its bulk volume that preferentially responds to an applied field that permeates said volume by absorbing energy at a particular wavelength from said applied field, wherein said applied field constitutes the outside stimulus, and the preferential response of the molecule raises the temperature of the implant, the temperature rise serving as the prompt on the implant to change to said remembered configuration; and said implant exhibiting a remembered configuration in which the diameter of the implant is at the low end of said range of diameters, such that the implant is prompted to change its configuration towards said remembered configuration, that is, towards a reduced diameter configuration, upon the imposition on the implant of outside stimulus, and said implant being characterised by a molecule distributed within its bulk volume that preferentially responds to an applied field that permeates said volume by absorbing energy at a particular wavelength from said applied field, wherein said applied field constitutes the outside stimulus, and the preferential response of the molecule raises the temperature of the implant, the temperature rise serving as the prompt on the implant to change to said remembered configuration; and a self-shrinkage activation catheter (32, 54) equipped to impose on the implant said applied field thereby to stimulate the implant to contract radially from a first deployed configuration towards its remembered configuration, wherein the activation catheter (32, 54) is a balloon catheter with means to manage the temperature of the surface of the balloon, whereby said balloon surface is effective to conduct heat energy to the luminal surface of the implant matrix.