Networks in nature

Synthetic networks, their structure and mechanical properties have been studied since the beginning of polymer science, while the considerable variety of networks in nature has been largely neglected, though use has been made of them as long as human culture has existed. In principle synthetic and natural networks are equivalent, but there are significant differences. (1) The main difference consists in the high polarity of the polymeric components and the related capability of binding large quantities of water. (2) The biosynthesis of covalently linked, macroscopic networks is rare and often too slow a process. In many cases the networks are formed via physical forces between various polymeric components. (3) Often the network formation can be reversed by changing the temperature, or pH, or by adding urea or detergents. (4) The viscoelastic behaviour is often governed by the rigidity of the macromolecular substructures. Examples are given for reversible gels with some charged polysaccharides and with fibrin. Special interactions are demonstrated with the network of connective tissue. The important role of comb‐like macromolecules with protein backbone and polysaccharide side chains is outlined. Special interactions are crucial also for immunologic response, i.e. for cell recognition and antibody‐antigen complex formation. The latter is one of the best examples of a successful application of the Flory‐Stockmayer theory of branching and gelation. Most of the reversible gels are not yet fully understood in their properties, and theory is lacking. This is particularly true for products like gelatin and starch.