While the achievement of chemical autonomy in cellular functions remains a key goal of protocell research, cellular functions can potentially also be complemented by a microsystem with a comparative physical size. Complementation means that certain chemical functions are supported or taken over by the microsystem. One simple example is containment, where electric fields generated by electrodes can potentially restrain the dilution of genetic material or nutrients to the environment. Another is using micro-flows to create directed nutrient and waste fluxes which simplify for the cell the task of accumulating resources and removing wastes. As we shall see later, the actual range of possibilities is much larger, including catalytically active complementation and separation technologies. Part of the richness of microsystem complementation comes from the possibilities of creating multi-compartmental cells, with dedicated chemistries in separate locations, and directed transport mediating the exchange of material between the compartments. Complementation brings the additional advantage of providing a means of programming artificial cells : the choice of complementation can direct the spatial structure, the resource structure, the metabolism and/or the replication cycle in specific directions.
The complementation of protocells can be regarded analogously to the complementation by “music minus one” electronic media that provides soloists with a complete orchestral accompaniment with the solo part missing. A programmable complementation system can consecutively reduce the level of support, increasing the degree of autonomous orchestration of the chemical system. In principle, a smooth succession of environmental challenges can be provided, easing the combinatorial search for viable artificial cells down to manageable steps, starting from a fully supported system in which all aspects of the protocell are under microfluidic control. Of course, the difficulty of the individual steps is dependent not only on the innate evolutionary potential of the chemical system at each stage, but also on the aptness of engineered changes to its composition. In addition, and perhaps less obviously, a combinatorial succession or array of different environments can be used to enhance the effective evolvability of the chemical system, especially at early stages.