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

Abstract

Nowadays, precise flow control, provision of exact reagent amounts, contamination prevention between reagents, autonomy, disposability and low-cost manufacture are factors that can not be found together for micro-fluidic valves.Valves made using photo-responsive materials are of great interest as functional materials within micro-fluidic systems since actuation can be controlled by simple light irradiation, without physical contact, offering improvements in versatility during manifold fabrication, and control of actuation. Nevertheless, their poor versatility, slow response times and limited robustness render them currently as scientific curiosities rather than ideally functioning devices.[1]The incorporation of photo-responsive gels with ionic liquids (ILs) produces hybrid ionogels with 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. [2] We have investigated the potential use of spiropyran-modified ionogels for controlling fluid within micro-fluidic channels. This remote ability could greatly simplify specific device design and cost. We have effectively incorporated ionic liquids into hydrogels to attain specific material contraction performance.[3] An advantage of using ionic liquids is the plethora of synthetic and material design options that can be used to design devices optimized for certain performance parameters or behaviours.

Manipulation of fluid flow direction in microfluidic paper-based analytical devices with an ionogel negative passive pump

“…"Lab-on-a-Chip" (LOC) or "micro-Total Analysis Systems" (μTAS) have the greatest potential of integrating multiple functional elements into a small device to produce truly samplein/answer-out systems 23 . The design, fabrication, flow control, analysis and connection techniques are under continuous development improving among others, the throughput and the automation while at the same time leading to reduced costs 24 . However, the development of fully integrated microfluidic devices is still facing some significant obstacles, including the lack of robust fundamental building blocks for fluid control, the miniaturisation or elimination of external fluidic control elements 23,25 .…”

“…In the case of active valves and pumps, the actuation depends on external power supplies and they require relatively complex procedures for integration into the microfluidic devices which mean a drawback for the device. On the other hand, passive valves/pumps have recently received huge amount of interest since they do not need any external actuation components and are easy to fabricate 24 . They can be seen as a response to the need of simple and effective fluid control elements for building low cost and sophisticated lab-on-a-chip devices.…”

“…They can be seen as a response to the need of simple and effective fluid control elements for building low cost and sophisticated lab-on-a-chip devices. These valves are essentially fabricated from stimuli responsive polymer gels 24 .Gels consisting of either physical or chemical cross-links can undergo controlled and reversible shape changes in response to an applied field 26 . In other words, they demonstrate substantial and reversible changes in equilibrium degree of swelling in response to weak changes in their surroundings (solvent composition, temperature, pH, and supply of electric field, light, etc.)…”

“…In particular ionogels can also be used as stimuli responsive gels. They have many advantages over conventional materials since their robustness, acid/ base character, viscosity and other critical operational characteristics can be finely modified through the tailoring of chemical and physical properties of the ILs 24 . The characteristics of the ionogels can be tuned by simply changing the IL and so the actuation behaviour when used as microvalves in microfluidic devices can be more precisely controlled 24,32 .…”

“…They have many advantages over conventional materials since their robustness, acid/ base character, viscosity and other critical operational characteristics can be finely modified through the tailoring of chemical and physical properties of the ILs 24 . The characteristics of the ionogels can be tuned by simply changing the IL and so the actuation behaviour when used as microvalves in microfluidic devices can be more precisely controlled 24,32 . For instance, two interesting approaches developed by us to control fluid movement in microfluidic devices are the use of photo-responsive ionogels controlled by light 33 and reversible thermoresponsive ionogel 34 .…”