Volume 13, Issue 5 -

A practical label-free method for counting CD4+ T cells is proposed on the basis of a microfluidic chip with fluidic electrodes. With the help of hydrodynamic focusing, two sheath flows of KCl solution, serving as electric conductors to replace solid metal electrodes, are used to squeeze the cell suspension. By measuring the electrical impedances between the fluidic electrodes, a linear relationship is found between the logarithmic value of cell concentration and the impedance value (R 2  = 0.97). The detection limit of CD4+ T lymphocytes reaches as low as ∼10 cells·µL −1 , Both the CD4+ T cell concentration of the nude mice group and that of the normal group were counted to further test the function of the chip system, which indicated that there existed significant differences between these groups, showing the approach is valid for identifying immunodeficiency.

We have developed a microfluidic resistor element exhibiting a hydrodynamic resistance that is strongly dependent on the pressure at the inlet of the device. Such an element can be used to construct microfluidic circuits that possess a highly non-linear dependence between the pressure and the flow rate, in contrast to conventional microfluidic resistors. We show that this effect can be exploited to create valves which do not require an external control line, but are actuated directly through the pressure difference between the single inlet and outlet of the device. Furthermore, we show that the flux to each branch of the circuit controlled by the valve can be fixed for high or low flow rates by modifying the fixed resistances internal to the device.

Thermo-responsive multicompartment microcapsules for co-encapsulation and controlled release of different lipophilic components are developed from multicomponent-double-emulsion templates by microfluidics. Each of the microcapsule comprises a thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) shell and different oil cores. The different oil cores can be used as separate compartments for co-encapsulation of distinct lipophilic components. The precise manipulation of different oil cores afforded by our microfluidic device enables the fabrication of microcapsules with select number of each core, for optimizing the encapsulation of different lipophilic components. Because of the shell shrinking during heating process due to the thermo-responsive PNIPAM network, different lipophilic components co-encapsulated in the microcapsule can be simultaneously released via an external thermal trigger. The thermo-responsive multicompartment microcapsule provides an efficient system for co-encapsulation and controlled release of multiple lipophilic components. The microfluidic preparation approach presented here shows exciting potential in the design and fabrication of functional multicompartment microcapsules for co-encapsulation techniques.

Microchip ITP (isotachophoresis) is getting popularity as a preparative technique for preconcentration and separation of chemical species and/or ions in liquid phase. In constant voltage mode ITP, generally a high electric potential difference is applied to a discontinuous buffer for faster and higher resolution separation. However, the higher current from the applied electric field induces Joule heating in the buffer which modifies the mobility and diffusion coefficient of analytes in the system. This change in mobility and diffusion coefficient strongly influences the transient separation process in ITP. In this study the effect of Joule heating on separation behavior of analyte compounds has been presented in a constant voltage mode ITP where two chemical species are separated from an initial mixture. The model is based on mass, energy, and charge conservation and electroneutrality condition in the system. A set of nonlinear governing equations are solved numerically for temperature dependent properties such as diffusion coefficient, effective electrophoretic mobility, and thermal conductivity using a finite volume based model. Numerical results suggest that for temperature dependent properties of control parameters, the separation speed of analytes is significantly different from that of constant temperature case. In constant voltage mode ITP, the temperature peak forms at the location of trailing electrolyte, and its influence propagates toward the direction of band movement as separation proceeds.

Microcapillary devices with tapered constrictions were fabricated for the study of the features of the cavitation. The cavitation was induced by applying negative pressure at the outlet with opening the inlet to atmospheric pressure, and the experimental results show that the cavitation bubbles generated at the center of the tapered tip flow upstream and adhere on the wall near the orifice, which is explained by the recirculation flow according to the numerical simulation results. The desinent cavitation number keeps constant for a microcapillary device, and the device works in a negative-pressure system is more susceptible to the cavitation than that works in a positive-pressure system. In addition, the effect of the orifice size on the incipient cavitation number and the desinent cavitation number has been studied with five devices with different sizes of the orifices from 50 to 120 microns. Both the incipient and desinent cavitation numbers increase monotonely with the size of the orifice. These results may be helpful to the design of the geometries of the microfluidic devices for various applications.

We study numerically the dynamics of jets and drops in a microcapillary co-flow device. The co-flow stream encounters different flow regimes, including dripping, jetting. Using a level-set/finite element axysimmetric numerical simulation, we study the dynamics of breakup of a jet subject to flow rate perturbations. A scaling law for the width of the unperturbed jet is presented and compared to existing experimental results as well as numerical measurements. Then, we show that the introduction of a sinusoidal perturbation of the inner fluid flow rate can facilitate breakup of the fluid in regimes where a jet is usually observed. Moreover, the flow rate perturbation leads to a good control over the size and the frequency of the resulting droplets. Using theoretical modelling, we provide a criterion to determine the optimal frequency to break up the jet. We also derive scaling laws to determine the volume of inner fluid encapsulated in the emulsion droplets as a function of the frequency and to estimate the distance for the jet to break up as a function of the amplitude of perturbation. These scaling laws are in good agreement with results of numerical simulations. Our work suggests a novel approach and offers guiding principles to break up liquid jets in cases where dripping is difficult to achieve.