Abstract: A method of making an insert comprising an elongate core having a plurality of loops disposed longitudinally therealong and angularly thereabout by taking such an insert wherein a portion of each loop lies in close proximity to a first conceptual enveloping surface and deforming the loops or displacing the loops so as to be inclined to the longitudinal axis of the core so that a greater proportion of each loop periphery is in close proximity to a second conceptual surface of smaller cross-section than said first surface. Also disclosed are inserts made in the above way and vessels incorporating such inserts.