Actuating mechanisms thermal active

Microfluidic Control Sequential and combinatorial delivery of signals to cells or tissue in microfluidic devices can be accomplished by using built-in control systems. Several microfluidic tools including valves, pumps, mixers, fluidic oscillators, fluidic diodes, etc. have been developed to accomplish fluidic control in these devices. These components can either be passive or active. Examples of passive elements include one-way valves (flap, ball) and hydrophobic patohes which take advantage of the interactiOTi between the chemical surface properties of the substrate and Uquid. Active elements, on the other hand, typically require some type of actuation mechanism. Several mechanisms for force transduction in microfluidic devices include mechanical, thermal, electrical, magnetic, and chemical actuation systems as well as the use of biological transducers. There has been a significant amount of work in this area that has been presented in a review by Erickson and Li [5]. 

As shown in this section, organic SMP composites have been extensively used for their enhanced mechanical, electrical, and thermal properties. With speculative uses in self-healing applications [24], moisture-activated networks [40, 41], and other thermally and electrically actuated networks [42, 43], organic SMPCs demonstrate the capability to adapt neatly to the challenges presented by niche problems. However, the prime focus of this review is the loading of various inorganic fillers into SMP networks to tackle issues that are currently imattainable because of both the physical and cost limitations of organic fillers. 

Microfluidic valves are essential for flow control of sample and reagents on a microfluidic CD. There are two categories of valves, namely passive valves, and active valves. Passive valves have no moving parts and work on the principle of the capillary effect [6]. Active valves on the other hand require moving parts such as a membrane or plunger that requires external mechanical, pneumatic, electric or thermal force for actuation. 

As it has been just aforementioned, for fast azobenzene-based artificial muscle-like actuators to be achieved, it is essential that the azo-chromophore used returns to its thermodynamically stable trans form in the dark as fast as possible. In this way, azobenzenes that undergo their thermal isomerisation through the rotational mechanism, of which push-pull azoderivatives are the most well-known example, will be valuable candidates for this aim since they are endowed with high thermal isomerisation rates at room temperature. In fact, the two only polysiloxane-based photoactive artificial muscle-like actuators exhibiting low relaxation times published so far are based on this strategy. Indeed, these systems contain the well-known push-pull azo-dyes 4-amino- (Camacho Lopez et al. 2004) and A-N, W-dimethylamino-4 -nitroazobenzene (Harvey and Terentjev 2007) as photo-active molecules, which are doped into host elastomeric networks but not covalently bonded to the polymeric structure decreasing thereby the stability of the final photo-actuator. 

---