The third decade of microfluidics

Perhaps long overdue, but we are two years into the ‘‘third decade of microfluidics’’. In 1990, Andreas Manz published the seminal paper ‘‘Microchemical Total Analysis Systems’’, which described a vision of integrating and automating the whole range of chemical analysis process steps by microfluidic components, primarily based on IC-like microfabrication processes and materials. In 1999 and 2000, two papers by George Whitesides and Steve Quake ignited the fields of ‘‘PDMS microfluidics’’ and ‘‘soft lithography’’. The simplicity of this microfabrication process offered researchers an unprecedented versatile tool to study microscale fluid flow with the subsequent inclusion of biological and chemical constituents. Researchers from diverse fields of science and engineering flocked to this field and in a sense revived many ‘‘mature’’ and ‘‘saturated’’ disciplines with newfound creativity and innovative problems to study, build and solve. These researchers can now be found in traditional academic departments, such as chemistry, physics, mechanical engineering, and chemical engineering. Recently, commercial prospects are being heavily exploited again after a ‘‘biochip’’ bubble burst shortly after the turn of the millennium. Looking back, these two microfluidic eras are much defined by the manufacturing (or microfabrication) processes and materials that provided ‘‘easy access’’ to non-experts of microfabrication the ability to build according to their interest, expertise, and requirements. For the third decade, would there be another microfluidic paradigm based on a new process, material, or platform? Or have we reached the point of maturation or saturation since every idea with game-changing potential has been exploited? Is there is a new broad reaching process brewing before us in the dawn of the third decade? What would that possibly be? What does it look like? While it may not be clear yet, there are a few processes that are starting to be adopted by academicians and industry alike. One process, is deemed ‘‘droplet microfluidics’’ or ‘‘emulsion microfluidics’’ that utilizes two immiscible phase fluids to establish compartmentalization and interfaces that enable a broad range of biological and chemical applications not previously feasible. This is not a new substrate material, but the fabrication of precise microfluidic reactor volumes and concentrations, which create a ‘‘self-assembled’’ bottom-up fabrication process within a ‘‘lithography-based’’ top-down fabrication process by known substrate materials (polymers, silicon, glass, etc.). In a sense, the ‘‘fluids within’’ have become part of the microfabrication process and structure that can be reconfigured according to the application. Another process, coined ‘‘paper microfluidics’’ replaces hollow, freeflow microchannels with the weaved microfibers of paper that wick fluids without the need for additional pumps. This process envisions microfluidic chips being ‘‘printed’’ on paper much like newspapers and the versatility and creativity of these platforms have once again enabled a wide range of biological and chemical ‘‘read-outs’’ that are now ‘‘in print’’. This printing paradigm can be extended to many other materials, further extending the two-dimensional microfluidics paradigm. For example, most recently there has been ‘‘roll-toroll’’ printing of plastics and printing of electrical components on flexible substrates. Another intriguing technology on the horizon is termed ‘‘3D printing’’ and has been demonstrated in the fabrication of miniaturized fluidic ‘‘reactionware’’ for chemical syntheses. The intriguing process allowed for ‘‘reactants’’ or ‘‘reagents’’ to be introduced/ stored during the fabrication of the devices. It is possible to foresee lamination processes that can combine these ‘‘microfluidic printing’’ sheets for manufacturable large scale integrated multilayer and multi-functional devices with three dimensional features. Could this be the starting point of the third decade of microfluidic revolution?