Materials science: the key to revolutionary breakthroughs in micro-fluidic devices

In microfluidics, valves and pumps that can combine specifications like precise flow control, provision of precise reagent quantities, minimal sample carryover, and low-cost manufacture, while also being inherently compatible with microfluidic system fabrication, are beyond the current state of the art. Actuators in micro-fluidics made using stimuli-responsive materials are therefore of great interest as functional materials since actuation can be controlled without physical contact, offering improvements in versatility during manifold fabrication, and control of the actuation mechanism. Herein we review the potential use of novel approaches to valving and pumping based on stimuli-responsive polymers for controlling fluid movement within micro-fluidic channels. This has the potential to dramatically simplify the design, fabrication and cost of microfluidic systems. In particular, stimuli-responsive gels incorporating ionic liquids (ILs) produce so-called 'ionogels' that have many advantages over conventional materials. For example, through the tailoring of chemical and physical properties of ILs, robustness, acid/base character, viscosity and other critical operational characteristics can be finely adjusted. Therefore, the characteristics of the ionogels can be tuned by simply changing the IL and so the actuation behaviour of micro-valves made from these novel materials can be more closely controlled.