Microfluidic sheath-flow

Howell PB Jr, Golden JP, Hilliard LR, Erickson IS, Mott DR, Ligler FS (2008) Two simple and rugged designs for creating microfluidic sheath flow. Lab Chip 8 1097-1103... 

In laminar flow, the viscous force of the fluid dominates over the inertial force so that in microfluidics, hydrodynamic focusing techniques can be used to increase the sensitivity of the system. Rodriguez-Trujillo et al. [61, 62] used a sheath flow with a lower conductivity than the central sample flow to concentrate the electric field lines into the impedance sensing region. A 3D hydrodynamic focusing scheme was proposed by Scott et al. 

To overcome these limitations imposed on conventional and microfluidic methods for size separation, hydrophoretic methods have been developed. Here we provide a review of the methods for continuous size separation of microparticles, blood cells and cell-cycle synchrony, and for sheathless focusing of cells without external fields and sheath flows in microfluidic devices. We describe details of the separation mechanism and its application to particle and cell manipulation, comparing its advantages and disadvantages with other microfluidic methods. Finally, we present some challenges of the hydrophoretic technology. 

Similar to capillaries, the most common detection method for high-speed separations on microdevices is fluorescence, yet few methods have matched the limits of detection found in sheath-flow formats routinely used in capillary systems. This is due in part to the microfluidic substrate being in a planar format and therefore difficult to reproduce the three-dimensional Taylor cone found in sheath-flow capillary systems. The most simple and common method for high-sensitivity detection in microdevices is the use of a confocal detection scheme. ... 

The microfluidic device design and the relative flow rate of sheath and sample play important roles in hydrodynamic focusing. Lee et al. [6] proposed a theoretical model to predict the width of focused center flow inside a microfabricated flow cytometer [6]. Based on potential flow theory, they derived the equation for flow inside a planar microfabricated flow cytometer under the two-dimensional situation shown in Fig. 3a. The flow is considered laminar, and the diffusion and mixing between focused stream and sheath flows is assumed negligible. With these assumptions, conservation of mass yields... 

In addition to the MFFD, another flow focusing device was first reported by Nisisako et al. [7] for the production of polymer particles. In that device, the hydrodynamic focusing of the dispersed phase is achieved thanks to two sheath fluids coming from both sides of the microchannel in which flows the dispersed phase (Figure 18.13, top). This sheath-flow microfluidic device (SFMD), etched in a quartz glass slab, was used in conjunction with a Y-junction to emulsify Janus droplets (Figure 18.13, middle). Two differently colored solutions of isobornyl acrylate (IBA) admixed with a small amount of a thermal initiator were fed to the... 

Muthard RW, Diamond SL. Rapid on-chip recalcification and drug dosing of citrated whole blood using microfluidic buffer sheath flow. Biorheology 2014 51 227-37. 

Figure 6.6 Different microfluidic junctions for droplet generation and manipulation (a) T-junction at inlet (Thorsen et al., 2001) (b) T-junction at outlet (Link et al., 2004 Tan et al., 2004) (c) Sheath-flow junction (Xu and Nakajima, 2004 Nisisako et al., 2004) (d) Y-junction (Nisisako et al., 2004, 2005).

The process of 3D focusing in this device can be divided into two steps. First, the sample stream is focused in the vertical direction using microfluidic drifting. The lateral drift of the sample flow is caused by the effect of the Dean vortices induced by the centrifugal effect of the curve, which transports the fluid in the opposite side of the channel. Second, classic horizontal focusing is obtained using two horizontal sheath streams. The result of these two steps is a stream focused in both the vertical and horizontal directions. 

---