Previous Research

During my doctoral studies at the University of Illinois, I developed microfluidic systems for biochemical and medical diagnostics.  I also designed and tested, with the Mirkin and Barron Groups at Northwestern University, disposable devices for ultra-sensitive detection of protein cancer markers.

Functionalized nanoparticles hold great promise in realizing highly sensitive and selective biodetection. I developed a single disposable chip which is capable of carrying out a multi-step process that employs nanoparticles—a bio-barcode assay (BCA) for single protein marker detection. To illustrate the capability of the system, I tested for the presence of prostate specific antigen (PSA) in buffer solution and goat serum. Detection was accomplished at PSA concentrations as low as 500 aM. This corresponds to only 300 copies of protein analytes using 1 µL total sample volume. I established that the on-chip BCA for PSA detection offers four orders of magnitude higher sensitivity compared to commercially available ELISA-based PSA tests.

The realization of a BCA in a microfluidic format presents unique opportunities and challenges. I also developed a modified form of the BCA called the surface immobilized biobarcode assay (SI-BCA). The SI-BCA employs microchannel walls functionalized with antibodies that bind with the intended targets. Compared with the conventional BCA, it reduces the system complexity and results in shortened process time, which is attributed to significantly reduced diffusion times in the micro-scale channels. Raw serum samples, without any pretreatment, were evaluated with this technique. Prostate specific antigen in the samples was detected at concentrations ranging from 40 fM to 40 pM. The detection limit of the assay using buffer samples was 10 fM. The entire assay, from sample injection to final data analysis was completed in 80 min.

I developed, with the Sligar Group at the University of Illinois, a microfluidic method for precisely patterning lipid bilayers and a multiplexed assay to examine the interaction between the lipids and protein analytes. The lipids were packaged into nanoscale lipid bilayer particles known as Nanodiscs and delivered to surfaces using microfluidic channels. Two types of lipids were used in this study: biontinylated lipids and phosphoserine lipids. The deposition of biotinylated lipids on a glass surface was confirmed by attaching streptavidin coated quantum dots to the lipids, followed by fluorescent imaging. Using this multiplexed grid assay, I examined binding of annexin to phosphoserine lipids, and compared these results to similar analysis performed by surface plasmon resonance.

In collaboration with the Mrksich Group at the University of Chicago, we investigated the response of lamellipodia to local and global geometric cues was investigated. Substrates were patterned with shapes having well-defined geometric cues to characterize the influence of curvature on the polarization of highly metastatic B16F10 rat melanoma cells. Substrates were patterned using microcontact printing to define adhesive islands of defined shape and size on a background that otherwise prevents cell adhesion. Cells adherent to these surfaces responded to local curvature at the perimeter of the adhesive islands; convex features promoted the assembly of lamellipodia and concave features promoted the assembly of stress filaments. Cells adherent to rectangular shapes displayed a polarized cytoskeleton that increased with the aspect ratio of the shapes. Shapes that combined local geometric cues, by way of concave or convex edges, with aspect ratio were used to understand the additive effects of shape on polarization. The dependence of cell polarity on shape was determined in the presence of small molecules that alter actomyosin contractility and revealed a stronger dependence on contractility for shapes having straight edges, in contrast to those having curved edges. This study demonstrates that the cytoskeleton modulates cell polarity in response to multiple geometric cues in the extracellular environment.

I have also been involved in various efforts to create microfluidic systems and components.

I helped design microfluidic laboratory-on-a-chip (LOC) systems based on a modular architecture. The architecture was conceptualized on two levels: a single-chip level and a multiple-chip module (MCM) system level. At the individual chip level, a multilayer approach segregates components belonging to two fundamental categories: passive fluidic components (channels and reaction chambers) and active electromechanical control structures (sensors and actuators). This distinction is explicitly made to simplify the development process and minimize cost. Components belonging to these two categories were built separately on different physical layers and communicated fluidically via cross-layer interconnects. The chip that hosts the electromechanical control structures is called the microfluidic breadboard (FBB). A single LOC module was constructed by attaching a chip comprised of a custom arrangement of fluid routing channels and reactors (passive chip) to the FBB. Many different LOC functions can be achieved by using different passive chips on an FBB with a standard resource configuration. Multiple modules can be interconnected to form a larger LOC system (MCM level). We demonstrated the utility of this architecture by developing systems for two separate biochemical applications: one for detection of protein markers of cancer and another for detection of metal ions. In the first case, free prostate-specific antigen was detected at 500 aM concentration by using a nanoparticle-based bio-bar-code protocol on a parallel MCM system. In the second case, we used a DNAzyme-based biosensor to identify the presence of Pb²⁺ (lead) at a sensitivity of 500 nM in <1 nL of solution.

One of the major hurdles in the Dip-Pen Nanolithography (DPN) is supplying ink to the writing probes.  During my Ph.D., I helped develop an architecture for a microfluidic chip that dresses (inks) multiple nanolithography tips in a high-density array in a parallel and multiplexed fashion. The microfluidic chip consists of multiple precision patterned thin-film polydimethyl-siloxane (PDMS) patches serving as porous inking pads. Inking chemicals are supplied from loading reservoirs to the inking pads through microfluidic channels. The gas-permeable thin PDMS membranes allow ink molecules to diffuse through while preventing bulk liquid from overflowing or evaporating. The inking chip provides high-density inking, easy loading of inks, and reduced evaporation losses. We tested by chip by inking scanning probe contact printing probes and commercial nitride probes.

I assisted in the optimization of a micro magnetic stir-bar mixer driven by an external rotating magnetic field. The original design employed polydimethyl-siloxane (PDMS) channels. The PDMS piece with embedded fluid channels were manually aligned to a glass substrate and assembled. In the new design, a micro magnetic stir-bar was monolithically integrated in parylene surface-micromachined channels with improved design features, including small tolerance of the stir-bar to channel wall (10 µm). The new parylene based microchannels provided improved mixing and also eliminated certain problems associated with PDMS-based channels. For example, the porosity of PDMS causes evaporation and absorption of chemicals and thus channels made of PDMS are prone to cross-contamination. We also demonstrated that the magnetic stir-bar can be used to pump liquid in micro channels.