Seaweed needs no farmland, no freshwater and no fertiliser. That single fact underpins Notpla — short for “not plastic” — founded in 2014 by two Imperial College London and Royal College of Art students who wanted packaging that vanishes. The chemistry begins with alginate, a polysaccharide that can account for up to 40% of the dry mass of brown seaweed. In living kelp it is a structural gel, letting fronds flex in surf without tearing. Extracted as sodium alginate, it dissolves into a viscous solution. Introduce calcium and the transformation is immediate. Alginate is a linear copolymer of two sugar acids: mannuronic (M) and guluronic (G). Only the G-blocks have the buckled geometry needed to cradle a divalent calcium ion between two adjacent chains. Materials scientists named it the egg-box model in 1973. - Sodium alginate solution meets a calcium bath - Ca2+ displaces Na+ and bridges neighbouring chains - The network crosslinks from the outside inwards - A coherent, watertight me...