Dry Film Resist Laminated Microfluidic System for Electrical Impedance Measurements

In micro-electrical-mechanical systems (MEMS), thick structures with high aspect ratios are often required. Dry film photoresist (DFR) in various thicknesses can be easily laminated and patterned using standard UV lithography. Here, we present a three-level DFR lamination process of SUEX for a microfluidic chip with embedded, vertically arranged microelectrodes for electrical impedance measurements. To trap and fix the object under test to the electrodes, an aperture is formed in the center of the ring-shaped electrodes in combination with a microfluidic suction channel underneath. In a proof-of-concept, the setup is characterized by electrical impedance measurements with polystyrene and ZrO2 spheres. The electrical impedance is most sensitive at approximately 2 kHz, and its magnitudes reveal around 200% higher values when a sphere is trapped. The magnitude values depend on the sizes of the spheres. Electrical equivalent circuits are applied to simulate the experimental results with a close match.

Paper Millifluidics Lab: Using a Library of Color Tests to Find Adulterated Antibiotics

A two to three period analytical chemistry experiment has been developed which allows second year students to explore chemical color tests used to detect adulterated pharmaceuticals. Students prepare several paper analytical devices (PADs) to generate positive and negative controls antibiotics, along with cutting agents such as starch and chalk. These PADs are used to identify the active ingredients and excipients in mystery tablets prepared by their classmates. In the second part of the lab, the students select an individual color test and design an experiment to quantify their mystery pill's active pharmaceutical ingredient (API). Nearly all of the student groups were able to successfully identify adulterants present in their mystery tablets. The quantification of the mystery tablets was also successful with all but one group calculating the correct concentration within 6%. In a postlab assessment, the students identified their largest gains in their ability to analyze data and other information, skill in science writing, and learning of laboratory techniques.

Microfluidic single-cell analysis of oxidative stress in Dictyostelium discoideum

Microfluidic chemical cytometry is a powerful technique for examining chemical contents of individual cells, but applications have focused on cells from multicellular organisms, especially mammals. We demonstrate the first use of microfluidic chemical cytometry to examine a unicellular organism, the social amoeba Dictyostelium discoideum. We used the reactive oxygen species indicator dichlorodihydrofluorescein diacetate to report on oxidative stress and controlled for variations in indicator loading and retention using carboxyfluorescein diacetate as an internal standard. After optimizing indicator concentration, we investigated the effect of peroxide treatment through single-cell measurements of 353 individual cells. The peak area ratio of dichlorofluorescein to carboxyfluorescein increased from 1.69 ± 0.89 for untreated cells to 5.19 ± 2.72 for cells treated with 40 mM hydrogen peroxide. Interestingly, the variance of the data also increased with oxidative stress. While preliminary, these results are consistent with the hypothesis that heterogeneous stress responses in unicellular organisms may be adaptive.

Microfluidic Chemical Cytometry for Enzyme Assays of Single Cells

Cellular heterogeneity occurs, and should be probed, at multiple levels of cellular structure and physiology from the genome to enzyme activity. In particular, single-cell measures of protein levels are complemented by single-cell measurements of the activity of these proteins. Microfluidic assays of enzyme activity at the single-cell level combine moderate to high throughput with low dead volumes and the potential for automation. Herein, we describe the steps required to fabricate and operate a microfluidic device for chemical cytometry of fluorescent or fluorogenic reporters of enzyme activity in individual cells.

Response of single leukemic cells to peptidase inhibitor therapy across time and dose using a microfluidic device

Single-cell methodologies are revealing cellular heterogeneity in numerous biological processes and pathologies. For example, cancer cells are characterized by substantial heterogeneity in basal signaling and in response to perturbations, such as drug treatment. In this work, we examined the response of 678 individual U937 (human acute myeloid leukemia) cells to an aminopeptidase-inhibiting chemotherapeutic drug (Tosedostat) over the course of 95 days. Using a fluorescent reporter peptide and a microfluidic device, we quantified the rate of reporter degradation as a function of dose. While the single-cell measurements reflected ensemble results, they added a layer of detail by revealing unique degradation patterns and outliers within the larger population. Regression modeling of the data allowed us to quantitatively explore the relationships between reporter loading, incubation time, and drug dose on peptidase activity in individual cells. Incubation time was negatively correlated with the number of peptide fragment peaks observed, while peak area (which was proportional to reporter loading) was positively correlated with both the number of fragment peaks observed and the degradation rate. Notably, a statistically significant change in the number of peaks observed was identified as dose increased from 2 to 4 μM. Similarly, a significant difference in degradation rate as a function of reporter loading was observed for doses ≥2 μM compared to the 1 μM dose. These results suggest that additional enzymes may become inhibited at doses >1 μM and >2 μM, demonstrating the utility of single-cell data to yield novel biological hypotheses.

Microfluidic chemical cytometry of peptide degradation in single drug-treated acute myeloid leukemia cells

Microfluidic systems show great promise for single-cell analysis; however, as these technologies mature, their utility must be validated by studies of biologically relevant processes. An important biomedical application of these systems is characterization of tumor cell heterogeneity. In this work, we used a robust microfluidic platform to explore the heterogeneity of enzyme activity in single cells treated with a chemotherapeutic drug. Using chemical cytometry, we measured peptide degradation in the U937 acute myeloid leukemia (AML) cell line in the presence and absence of the aminopeptidase inhibitor Tosedostat (CHR-2797). The analysis of 99 untreated cells revealed rapid and consistent degradation of the peptide reporter within 20 min of loading. Results from drug-treated cells showed inhibited, but ongoing degradation of the reporter. Because the device operates at an average sustained throughput of 37 ± 7 cells/h, we were able to sample cells over the course of this time-dependent degradation. In data from 498 individual drug-treated cells, we found a linear dependence of degradation rate on amount of substrate loaded superimposed upon substantial heterogeneity in peptide processing in response to inhibitor treatment. Importantly, these data demonstrated the potential of microfluidic systems to sample biologically relevant analytes and time-dependent processes in large numbers of single cells.

Analysis of intracellular reducing levels in human hepatocytes on three-dimensional focusing microchip

A novel three-dimensional hydrodynamic focusing microfluidic device integrated with high-throughput cell sampling and detection of intracellular contents is presented. It has a pivotal role in maintaining the reducing environment in cells. Intracellular reducing species such as vitamin C and glutathione in normal and tumor cells were labeled by a newly synthesized 2,2,6,6-tetramethyl-piperidine-1-oxyl-based fluorescent probe. Hepatocytes are adherent cells, which are prone to attaching to the channel surface. To avoid the attachment of cells on the channel surface, a single channel microchip with three sheath-flow channels located on both sides of and below the sampling channel was developed. Hydrostatic pressure generated by emptying the sample waste reservoir was used as driving force of fluid on the microchip. Owing to the difference between the liquid levels of the reservoirs, the labeled cells were three-dimensional hydrodynamically focused and transported from the sample reservoir to the sample waste reservoir. Hydrostatic pressure takes advantage of its ease of generation on a microfluidic chip without any external pressure pump, which drives three sheath-flow streams to constrain a sample flow stream into a narrow stream to avoid blockage of the sampling channel by adhered cells. The intracellular reducing levels of HepG2 cells and L02 cells were detected by home-built laser-induced fluorescence detector. The analysis throughput achieved in this microfluidic system was about 59-68 cells/min.