Final Activity Report Summary - POLYSELF (Functionalization of polysaccharide-based biomaterials and development of supramolecular materials using self-assembling peptides) Within the scope of POLYSELF project a remarkable discovery was made on self-assembly of macromolecules and small molecules across the scales. Under specific conditions, instant self-assembly between two components, a viscous liquid containing charged macromolecules, such as hyaluronic acid (HA) and a second liquid containing small peptide molecules of opposite charge, occurred at the liquid-liquid interface and could result in the formation of self-sealing sacs and membranes of arbitrary size and shape. The two liquids should mix rapidly upon contact but, surprisingly, solid materials with structural order formed instantly upon contact at their interface. The self-assembled structures grew further to macroscopic dimensions with a high degree of hierarchical order and could take many interesting forms. In one example, the fast initiation of the ordered structures allowed for the formation of films taking the arbitrary shape of the cross section at the liquid-liquid interface. In another example, the initially formed films could buckle and sink into one liquid refreshing the interface and leading to the formation of a self-closing sac. These hybrid materials were mechanically robust and could even be sutured to tissues, were permeable to proteins and could support cell viability and function. Furthermore, in these sacs, a large defect could be instantly repaired by exposure to solutions of the small molecules. Their unique structural and physical characteristics offered significant potential in cell therapies, drug diagnostics and regenerative medicine applications. Such sacs might be used as controlled environments for cell expansion, stem cell differentiation or bio-signalling studies, acting as a kind of mini cell biology lab. A major advantage was the potential ability to organise cells into unique tissue structures.