According to the present disclosure there is provided a method of forming a first structure (150), comprising the steps of: providing one or more slots (124) in a material, thereby to define a plurality of strips (122) of the material, the slots (124) defining a series of connecting elements (128) connecting adjacent strips (122) of the material, the connecting elements (128) being spaced apart along a first direction (130), forming the material to provide: an array of oppositely oriented channels (152), each channel (152) extending in a second direction perpendicular to the first direction, and the array itself extending in the first direction, each channel comprising a base, and sides extending from the base; and a first, non-continuous, outer plane; and a second, non-continuous, outer plane, such that the first structure is foldable along each of the series of connecting elements about the first direction to bring the first outer plane of a first strip toward, and optionally into contact with, the first outer plane of a second, adjacent strip.