Nanoliter droplet method

The immobilized spots used in protein microarrays may contain protein molecules or smaller molecules such as inhibitors or ligands with which a protein in solution may interact. Very small spots are advantageous in that higher detection throughput can be achieved with minimal amounts of printed material and sample. Spots are typically deposited either by contact methods, such as printing arrayers that use needles to deposit droplets directly on the surface, or by non-contact methods, including the use of capillaries or ink jet dispensers to deposit droplets with nanoliter-picoliter volumes. 

The majority of microfluidic methods produce droplet using passive devices generating a uniform, evenly spaced, continuous stream of droplet, whose volume ranges from femtoliters to nanoliters. Their operational modes take advantage of the characteristics of the flow field to deform the interface and promote the natural growth of interfacial instabilities, avoiding in this way the necessity of any local external actuation. Droplet polydispersity, defined as the ratio between the standard deviation of the size distribution and the mean droplet size, can be as small as l%-3%. 

Dielectrophoretic forces, though, can be induced by means other than an applied electric signal through electrodes. Optical tools can be implemented to modify an applied electric field, making these methods more susceptible for dynamic as opposed to static manipulation of electric fields with surface electrodes. Dielectrophoresis applications are not limited to particulate manipulation either. With properly configured surface-electrode geometry, it is possible to induce fluid motion and create nanoliter-sized droplets. Additionally, dielectrophoretic forces can be utilized to manipulate particles to buUd micro- and nanostructures such as wires. 

A sensing method for detecting, counting, or characterizing of the nanoliter droplets, cells, or other microparticles by measuring the capacitance change between a pair of electrically charged and isolated conductors. 

Ellson et al. [7] used the method of focused acoustic energy to eject a pL droplet from a nanoliter-scale well the droplet diameter being also measured by a digital stroboscope. In their experiment, 20 pL droplets were ejected from 20 nL liquids. The accuracy of liquid volume measurement is determined by the accuracy of... 

Hjelt et al. [10] proposed an integrated, noninva-sive method to measure sub-nanoliter volume droplets. Microelectrodes were integrated on the bottom of a reactor and covered with a layer of SiN. According to the impedance characteristic between electrodes, the size of droplet can be obtained. The volumes of the reactor fabricated in the experiment are 60-40 pL. The relation between liquid volumes and electrode voltages... 

Other devices for extraction with trapping of droplets using an ultrasonic nanoliter liquid droplet ejector [41] and multiple opening rectangular recesses [42] have also been developed. As another method of trapping fluids. Sun et al. developed a micro structured device for extraction with stopped-fiow manipulation [43]. 

Hjelt et al. [10] proposed an integrated, noninvasive method to measure sub-nanoliter volume droplets. Microelectrodes were integrated on the bottom of a reactor and covered with a layer of SiN. According to the impedance characteristic between electrodes, the size of droplet can be obtained. The volumes of the reactor fabricated in the experiment are 60 - 40 pL. The relation between liquid volumes and electrodes voltages was calibrated, and the accuracy of voltage measurement was dz5%, and the resolution of volume measurement could be up to 1 pL. To calibrate the measurement results of the impedance method, the authors used a fluorescence microscope to inspect the liquid volume inside the reactor, and they concluded an accuracy could be achieved of dz5%. The minimum measuring volume in the experiment is 30 pL, and the measurement accuracy is 1.5 pL. 

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