This invention concerns a support structure for releasably clamping onto a body relative to a central axis in use, wherein the support structure comprises a plurality of links connected at a plurality of pivots spaced about the axis and an array of contact members depending from the links at angularly spaced locations about the axis, whereby pivoting of the links causes actuation of the contact members in a substantially radial direction relative to the axis such that said contact members communicate an applied contact force about the body in use so as to clamp the support structure to the body via the array of contact members.