To conclude, the assembly of actin filaments clearly produces a self-assembled supramolecular material that can be formed in a non-equilibrium steady state with unusual and interesting properties. However, it is hard to draw strong, general conclusions about the potential properties and applications of synthetic systems based on this example. For instance, it is not automatic that treadmilling or similar processes will occur in all such materials. The rapid breakdown of disassembling actin filaments might be more general, but we must reemphasize that this is not because of the trapping of "high energy" monomer units within the structure. Similarly, there is no simple link between the free energy of ATP hydrolysis and any free energy "stored" in the self-assembly (see below). Moreover, the in vivo behavior of actin is far more complicated than our simple model; proteins regulate every aspect of the process including nucleation, elongation, severing and even cross-linking filaments. 33 Ambiguity in the analysis of artificial supramolecular materials proposed to achieve a non-equilibrium steady state. Returning to artificial systems, many examples have been described in which, broadly speaking, a species that does not self-assemble, reacts to give a monomer unit that does. A subsequent reaction then destroys the monomer units to regenerate the dispersed starting material. In one example to stand for all, Van Esch and co-workers reported 37 an ester-based supramolecular gelator system that relies on the presence of MeI (Figure ). Di-carboxylate 14 is freely soluble under the reaction conditions but in the presence of MeI, 14 is first converted to monoester 15, which is also freely soluble, and then diester 16, which spontaneously self-assembles into a gel structure. Hydrolysis of 16, first to 15 and then to 14, returns the system to its original, fluid state. (a) The experiment reported, whose results can be explained by two extreme versions of the reaction network in which (b) assembly is not coupled to the hydrolysis of MeI (hydrolysis of 16 takes place in solution) or (c) it is strongly coupled (hydrolysis of 16 takes place exclusively in the gel state); the real network is likely to be intermediate between these extremes. Rate constants k+/-1 and k+/-2 refer to the reaction of MeI to generate an ester and hydrolysis respectively. MeI, I -, -OH and MeOH are omitted from the majority of the network for clarity.