Forming multi-layer 3D structures involving the joining of at least two structural elements, at least one of which is formed as a multi-layer 3D structure, wherein the joining occurs via one of: (1) elastic deformation and elastic recovery, (2) relative deformation of an initial portion of at least one element relative to another portion of the at least one element until the at least two elements are in a desired retention position after which the deformation is reduced or eliminated, or (3) moving a retention region of one element into the retention region of the other element, without deformation of either element, along a path including a loading region of the other element and wherein during normal use the first and second elements are configured relative to one another so that the loading region of the second element is not accessible to the retention region of the first element.