From droplets and particles to hierarchical spatial organization-nanotechnology challenges for microfluidics
The compartimentation of fluids in the microliter, nanoliter and picoliter range leads recently to many applications of microfluidics in material development, in diagnostics and biological screenings. Droplet-based microfluidics allows the improvement of nanoparticle homogeneity and the tuning of particle properties. It supports combinatorial synthesis of inorganic as well as organic substances and can be applied for the cultivation and screening of bacteria, eucaryotic cells and fish embryos. The well-ordered handling and the addressing of microfluidssegments improves the information transfer between chemical, biological and electronic systems. Despite this remarkable technical progress, there is a particular importance of microfluidics for future nanotechnological solutions. The hierarchical spatial organization of liquids, particles and gels in microfluidics represents a fundamental biomimetic principle which overcomes the limits of planar technology and opens the gate for realizing complex structured threedimensional nanoarchitectures. Recent applications of microstructured fluids in chemistry and biology and concepts for future developments will be discussed.
Fluid compartimentation is a basic principle in living nature. Many cells and cell organelles represent fluid compartments in the volume range between the subfemtoliter and the nanoliter level. Liquid droplets in the femtoliter and picoliter range are technically used in a lot of different emulsions. Small droplets are essentially for ink-jet printing and in other recent technologies. But, there is a principle difference between the droplets in emulsion on one side and cells and ink-jet droplets on the other side: Emulsions are statistical ensembles of large numbers of droplets with no exact definition of position and speed and no individuality in their composition. In contrast, cells and cell organelles have a life history, can be distinguished one from each other carrying individual information. The droplets of an inkjet printer are information carrying objects, too. They are marked by a certain time of formation and place for their deposition on paper or and other substrate. The individuality of cells is mainly driven by their molecularbiological components. They control cell metabolism, cellular motion, the communication between cells and their reproduction. Interaction of cells leads to the formation of larger or smaller cell aggregates up to the building of tissues, organs and multicellular organisms. Each multicellular system represents a hierarchy of liquid compartments. Obviously, the compartimenation of liquids by hierarchically organized assemblies is one of the most fundamental principles in the organization of higher life. Is the liquid compartmentation a suited natural example for technical systems? The developments in the last years in the field of droplet-based microfluidics  and on micro segmented-flow [2-4] suggest a formerly unrecognized power of liquid microcompartmentation in technology. It is related to the possibility to generate and manipulate small droplets and the increasing ability to control and change their composition. Meanwhile, micro devices allow to generate and to transport droplets in the lower microliter, in the nanoliter and the upper picoliter range with high reproducibility. The formation of droplets and their transport behavior through microtubes and micro channels is comparatively well understood
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