Actuators in microfluidics

Electrophoresis provides an important method of actuation in microfluidics. Many of the different types of species that need to be transported on a microfluidic device are charged, and electrophoresis thus provides a convenient method for moving them to different locations on a chip. Among the most important examples are colloidal particles, such as latex spheres, and biomolecules, such as proteins and DNA. Under certain circumstances, electrophoresis can also be used to separate these particles by zeta potential or size, which is an important part of biological analyses. 

Richter, T., Shultz-Lockyear, L.L., Oleschuk, R.D., Bilitewski, U., Harrison, D.J., Bi-enzymatic and capillary electrophoretic analysis of non-fluorescent compounds in microfluidic devices Determination of xanthine. Sensors Actuators B 2002, 81, 369-376. 

Conventional solid-state actuators and valves such as peristaltic pumps and solenoid valves require external power and complex fabrication schemes which limit their use in many microfluidic applications. Of particular interest at the moment is the development of low-cost, efficient, polymer-based actuators and valves for sample handling in microfluidics systems. 

Several research groups have proposed using electric helds for controlling flow in microfluidic systems. The two techniques discussed here include electroosmotic valves [290] and electrocapillary force actuation [296]. Electroosmotic valving uses critical dimensions so that the valve is open to EOF flows and closed to pressure-driven flows. The electric held is turned off in channels where how is not desired, and the capillary forces prevent the huids from progressing. Electric held is applied to channels where how is desired. This systems appears to be a fairly simple method for valving in microHuidics. 

Although polymerization of NIPAAm has been extensively investigated in the past using free radical initiator, this method can not be used for the preparation of thin films. Thin hydrogels films can find applications as microfluidic devices for sensors and actuators in case patterning is possible. This problem can be solved by preparing gel films from narrowly distributed microgel particles (Zhou and Wu... 

Kim JH, Lau KT, Diamond D (2008) Fabrication of microfluidic pump using conducting polymer actuator. In Proceedings IEEE international conference on sensor networks, ubiquitous, and trustworthy computing, 11-13 June 2008, Taichung, Taiwan, pp 457-463 Laser DJ, Santiago JG (2004) J Micromech Microeng 14 35-64... 

FIGURE 40.7 Characteristic frequencies could be shifted by an order of magnitude by locally altering PDMS membrane thicknesses ( fluidic capacitors ). Different curves represent different combinations of the thicknesses of two fluidic capacitor components in the same fluidic network. These fluidic bandpass filters provide the proof-of-concept of a new paradigm in microfluidic flow control, where actuation frequency could be used to passively control relative flow rates. 

Lilliehorn T., Nilsson M., Simu U., Johansson S., Ahnqvist M., Nilsson J., and Laurell T., Dynamic arraying of microbeads for bioassays in microfluidic channels. Sensors and Actuators B, 106, 851, 2005. 

Microfluidic generation of droplets is a method of droplet formation in microfluidic channels. It works by combining two or more streams of immiscible fluids and generating a shear force on the discontinuous phase causing it to break up into discrete droplets. In contrast to piezoelectric, pneumatic and acoustic forms of droplet generation, in this method, there is no need for an actuator to impose instabilities on the liquid jet. In the absence of an actuator, the size and polydisper-sity of the droplets are determined by the dimensions of microchannels, the flow rates of liquids, wetting properties of microchannels, etc. 

Thermally sensitive polymers have also been used in microfluidic devices. One such example is a sensor using N-isopropylacrylamide on a fixed poly(dimethylsiloxane) (PDMS) substrate as an actuator. The fixed substrate caused swelling response to be anisotropic with an actuating motion perpendicular to the substrate. ... 

The anisotropic wet etching of silicon is a unique fabrication process in the MEMS field. The need to develop new processes to fabricate functional 3D microstructures in various materials is urgent for progress in microfluidic systems, microsensors, micro-actuators, and microinstrumentation. At present, to integrate surface micromachined devices and standard IC devices with bulk micromachined structures to demonstrate a new functional MEMS application is still a challenge for researchers who work in this field. The cooperation of multidisciplinary researchers will be required to develop miniature systems with the most appropriate building philosophy and the best operation performance. 

Hu N, Yang J, Joo SW, Banerjee AN, Qian S (2012) Cell electrofusion in microfluidic devices a review. Sens Actuators B 178 63-85... 

The abUity to control the wettability of a liquid, ideally without mechanically moving parts, is paramount in the handling and actuation of fluids in microfluidic devices. This has prompted a recent resurgence in electrowetting studies, which allows a rapid, reversible, and precise... 

Electrowetting, Applications, Fig. 2 Schematic depiction of a typical static electrowetting setup for drop actuation in a microfluidic device... 

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