Lisen Wang

Mechanics of adhesion in MEMS—a review

A review is presented of the mechanics of microscale adhesion in microelectromechanical systems (MEMS). Some governing dimensionless numbers such as Tabor number, adhesion parameter and peel number for microscale elastic adhesion contact are discussed in detail. The peel number is modified for the elastic contact between a rough surface in contact with a smooth plane. Roughness ratio is introduced to characterize the relative importance of surface roughness for microscale adhesion contact, and three kinds of asperity height distributions are discussed: Gaussian, fractal, and exponential distributions. Both Gaussian and exponential distributions are found to be special cases of fractal distribution. Casimir force induced adhesion in MEMS, and adhesion of carbon nanotubes to a substrate are also discussed. Finally, microscale plastic adhesion contact theory is briefly reviewed, and it is found that the dimensionless number, plasticity index of various forms, can be expressed by the roughness ratio.

Unique Dielectric Properties Distinguish Stem Cells and Their Differentiated Progeny

The relatively new field of stem cell biology is hampered by a lack of sufficient means to accurately determine the phenotype of cells. Cell‐type‐specific markers, such as cell surface proteins used for flow cytometry or fluorescence‐activated cell sorting, are limited and often recognize multiple members of a stem cell lineage. We sought to develop a complementary approach that would be less dependent on the identification of particular markers for the subpopulations of cells and would instead measure their overall character. We tested whether a microfluidic system using dielectrophoresis (DEP), which induces a frequency‐dependent dipole in cells, would be useful for characterizing stem cells and their differentiated progeny. We found that populations of mouse neural stem/precursor cells (NSPCs), differentiated neurons, and differentiated astrocytes had different dielectric properties revealed by DEP. By isolating NSPCs from developmental ages at which they are more likely to generate neurons, or astrocytes, we were able to show that a shift in dielectric property reflecting their fate bias precedes detectable marker expression in these cells and identifies specific progenitor populations. In addition, experimental data and mathematical modeling suggest that DEP curve parameters can indicate cell heterogeneity in mixed cultures. These findings provide evidence for a whole cell property that reflects stem cell fate bias and establish DEP as a tool with unique capabilities for interrogating, characterizing, and sorting stem cells.

Dual frequency dielectrophoresis with interdigitated sidewall electrodes for microfluidic flow‐through separation of beads and cells

This paper presents a novel design and separation strategy for lateral flow‐through separation of cells/particles in microfluidics by dual frequency coupled dielectrophoresis (DEP) forces enabled by vertical interdigitated electrodes embedded in the channel sidewalls. Unlike field‐flow‐fractionation‐DEP separations in microfluidics, which utilize planar electrodes on the microchannel floor to generate a DEP force to balance the gravitational force and separate objects at different height locations, lateral separation is enabled by sidewall interdigitated electrodes that are used to generate non‐uniform electric fields and balanced DEP forces along the width of the microchannel. In the current design, two separate AC electric fields are applied to two sets of independent interdigitated electrode arrays fabricated in the sidewalls of the microchannel to generate differential DEP forces that act on the cells/particles flowing through. Individual particles (cells or beads) will experience DEP forces differently due to the difference in their dielectric properties. The balance of the differential DEP forces from the electrode arrays will position dissimilar particles at distinct equilibrium planes across the width of the channel. When coupled with fluid flow, this results in lateral separation along the width of the microchannel and the separated particles can thus be automatically directed into branched channel outlets leading to different reservoirs for downstream processing. In this paper, we present the design and analysis of lateral separation enabled by dual frequency coupled DEP, and cell/bead and cell/cell separations are demonstrated with this lateral separation strategy. With vertical interdigitated electrodes on the sidewall, the height of the microchannel can be increased without losing the electric field strength in contrast to other multiple frequency DEP devices with planar electrodes. As a result, populations of cells can be separated simultaneously instead of one by one to enable high‐throughput sorting microfluidic devices.

Side-Wall Vertical Electrodes for Lateral Field Microfluidic Applications

This paper describes the design, fabrication, and testing of microfluidic devices enabled by electrodes embedded vertically in the side walls of SU-8 microchannels. With vertical electrodes on the side walls, one can generate higher lateral electrical fields uniform along the vertical direction in the channel (perpendicular to the substrate). By designing the electrode shapes and configurations, uniform and nonuniform electrical fields in the lateral (planar) directions can be applied to manipulate flow or particles in microchannels for switching or sorting applications. The uniform field is demonstrated in a magnetohydrodynamic (MHD) microfluidic device for directing cells to different channel outlets while the nonuniform field is demonstrated in the generation of dielectrophoresis (DEP) forces for microbead focusing. Metal electrodes are fabricated by electroplating to form vertical electrodes aligned with the channel walls. The multilayer SU-8 lithography technique enables the four walls of the channel to be all SU-8. The thin precoated SU-8 layer on the substrate improves structure integrity of the SU-8 microchannels. The mechanical flexibility of PDMS compensates for the surface nonuniformity from the previous patterning steps to conformably cap the channel. The ability to integrate versatile electrodes design broadens the realm of electrical control and sensing for microfluidic applications

A magnetohydrodynamic (MHD) microfluidic platform for cell switching

We present for the first time the demonstration of magnetohydrodynamic (MHD) micropump based microfluidic platform for cell switching. MHD micropumps were fabricated with a novel microfabrication process that can make vertical electrodes embedded inside microchannels to provide higher efficiency of pumping of the fluid. Multiple micropumps can be integrated on the same platform to have switching function for cell sorting. We have successfully demonstrated the pumping of two types of cells: mouse neural stem cells and neuroblastoma cells.

A Microfabrication Process for Polymer Microchannel With Embedded Vertical Electrodes for Microfluidic Applications

In this paper, we will report the use of SU-8 to make an all polymer microchannel with vertical platinum electrodes, which is fabricated by a multilayer SU-8 coating and electroplating process. The first layer is spun to increase the adhesion of the channel and the substrate, the second layer is the microchannel structure. The third layer is spun on another substrate and then capped onto the second channel layer to make a completely sealed SU-8 Channel. The PDMS substrate we used to spin the third layer on is flexible to enable conformably sealing to the microchannel substrate. A process that makes vertical microelectrodes inside the channels is also developed, which makes it a powerful process to make a complicate microfluidic network.Copyright

Design and Fabrication of Vertical Electrodes in Microchannels for Particles/cells Sorting by Dielectrophoresis

We have developed a process to fabricate vertical electrodes in the side walls of the microchannel. With appropriate electrodes design, DEP force along the lateral direction of the channel can be generated so that one can position participates along the width dimension of the channel. The effect of different electrode configurations on the efficacy of generating non-uniform electric field distribution has been studied by FEM simulation with CFD-ACE. Electrodes with various length ratios and interdigited electrodes have been designed and studied on the generation of non-uniformity of the electric field. Compared to electrodes with different length ratios, side-wall interdigited electrodes provide more consistent direction of DEP force. Experiments on the manipulation of polystyrene microbeads and HEK293 cells with DEP have been demonstrated to verify the performance of the design. The particles/cells can be focused in the middle of the channel, trapped on the side walls, and sorted to different outlets by the flow