Variable-height channels for microparticle characterization and display.

Characterizing and isolating microparticles of different sizes is often desirable and essential for biological analysis. In this work, we present a new and straightforward technique to fabricate variable-height glass microchannels for size-based passive trapping of microparticles. The fabrication technique uses controlled non-uniform exposure to an etchant solution to create channels of arbitrary height that vary in a predetermined way from the inlet to the outlet. Channels that vary from 1 µm to over 20 µm in height along a length of approximately 6 cm are shown to effectively and reproducibly separate particles by size including particles whose diameters differ by less than 100 nm when the standard deviation in size is less than 0.66 µm. Additionally, healthy red blood cells and red blood cells chemically modified with glutaraldehyde to reduce their deformability were introduced into different channels. The healthy cells can flow into shallower heights, while the less deformable ones are trapped at deeper heights. The macroscopic visualization of microparticle separation in these devices in addition to their ease of use, simple fabrication, low cost, and small size suggest their viability in the final detection step of many bead-based assay protocols.

A droplet-based microfluidic viscometer for the measurement of blood coagulation.

A continuous microfluidic viscometer is used to measure blood coagulation. The viscometer operates by flowing oil and blood into a cross section where droplets are generated. At a set pressure, the length of the droplets is inversely proportional to the viscosity of the blood sample being delivered. Because blood viscosity increases during coagulation as the blood changes from a liquid to a solid gel, the device allows to monitor coagulation by simply measuring the drop length. Experiments with swine blood were carried out in its native state and with the addition of coagulation activators and inhibitors. The microfluidic viscometer detected an earlier initiation of the coagulation process with the activator and a later initiation with the inhibitor compared to their corresponding controls. The results from the viscometer were also compared with the clinical method of thromboelastography (TEG), which was performed concurrently for the same samples. The time to initiation of coagulation in the microfluidic viscometer was correlated with the reaction time in TEG. Additionally, the total time for the measurement of clot strengthening in TEG correlated with the time for the maximum viscosity observed in the microfluidic viscometer. The microfluidic viscometer measured changes in viscosity due to coagulation faster than TEG detected the clot formation. The present viscometer is a simple technology that can be used to further study the entire coagulation process.

Micro-Particle Operations Using Asymmetric Traps.

Micro-particle operations in many lab-on-a-chip devices require active-type techniques that are accompanied by complex fabrication and operation. The present study describes an alternative method using a passive microfluidic scheme that allows for simpler operation and, therefore, potentially less expensive devices. We present three practical micro-particle operations using our previously developed passive mechanical trap, the asymmetric trap, in a non-acoustic oscillatory flow field. First, we demonstrate size-based segregation of both binary and ternary micro-particle mixtures using size-dependent trap-particle interactions to induce different transport speeds for each particle type. The degree of segregation, yield, and purity of the binary segregations are 0.97 ± 0.02, 0.96 ± 0.06, and 0.95 ± 0.05, respectively. Next, we perform a solution exchange by displacing particles from one solution into another in a trap array. Lastly, we focus and split groups of micro-particles by exploiting the transport polarity of asymmetric traps. These operations can be implemented in any closed fluidic circuit containing asymmetric traps using non-acoustic oscillatory flow, and they open new opportunities to flexibly control micro-particles in integrated lab-on-a-chip platforms with minimal external equipment.