Miniaturized chemical processes have many advantages, such as reduced use of chemical reagents and solvents, precisely controlled reaction conditions, much shortened reaction time, and the ability to integrate into a digital device. [1][2][3][4] They are very useful for high-throughput analyses and purifi cations in chemical and biological processes, such as drug discovery, [ 5 ] DNA analysis, [ 6,7 ] protein crystallization, [ 8 ] and the synthesis of molecules or particles. [9][10][11] Indeed, considerable efforts have been made to miniaturize chemical processes using various principles. For example, oil/water emulsions have been used to perform chemical reactions in parallel in a large number of emulsifi ed tiny droplets. [ 10 ] Microfl uidic "lab-on-a-chip" devices have been designed to manipulate chemical processes within micro-channels either in continuous fl uid or disconnected fl uid segments separated by an immiscible fl uid or gas. [ 10 , 12 ] Although the channel-based micro-reactors show great potential in high-throughput reaction and integratability to external analysis facilities, the reaction-specifi c device preparations, the channel-associated cross-contaminations, and the need for external pumps to actuate the fl uid motion make them neither universal nor miniature. Also, microfl uidics is hard to use to handle a single droplet. Recently, free aqueous droplets in an immiscible fl uid (channel-free microfl uidics) [ 9 ] has also been proposed as an alternative strategy, but cross-contamination still remains a concern.
In living systems, cells are well-known as the basic structural and functional units [ 13 ] that mediate various biochemical reactions (e.g., catabolism and anabolism) for sustaining the growth and division of cells and hence living systems. Although their structure looks very simple, cells communicate with each other in a highly intelligent way, throughout the whole living system. In this regard, how to mimic living cells, both in structure and function, is the ultimate challenge toward the development of artifi cial miniature reactors.