Bacteria and cancer cell pearl chain under dielectrophoresis.

In this work, we aim to observe and study the physics of bacteria and cancer cells pearl chain formation under dielectrophoresis (DEP). Experimentally, we visualized the formation of Bacillus subtilis bacterial pearl chain and human breast cancer cell (MCF-7) chain under positive and negative dielectrophoretic force, respectively. Through a simple simulation with creeping flow, AC/DC electric fields, and particle tracing modules in COMSOL, we examined the mechanism by which bacteria self-organize into a pearl chain across the gap between two electrodes via DEP. Our simulation results reveal that the region of greatest positive DEP force shifts from the electrode edge to the leading edge of the pearl chain, thus guiding the trajectories of free-flowing particles toward the leading edge via positive DEP. Our findings additionally highlight the mechanism why the free-flowing particles are more likely to join the existing pearl chain rather than starting a new pearl chain. This phenomenon is primarily due to the increase in magnitude of electric field gradient, and hence DEP force exerted, with the shortening gap between the pearl chain leading edge and the adjacent electrode. The findings shed light on the observed behavior of preferential pearl chain formation across electrode gaps.

Characterization of Shrink Film Properties for Rapid Microfluidics Lab-on-Chip Fabrication.

Shrink film is a thin sheet of polystyrene plastic that shrinks to 25-40% of its original size when heated. This study investigated the shrinkage factor of the film at different temperatures and baking times to determine the optimal fabrication recipe for shrink film microfluidic device production. Additionally, this study characterized the properties of shrink film, including minimum possible feature size and cross-section geometries, using manual engraving and the CAMEO 4 automated cutting machine. The optimal shrinkage factor ranged from 1.7 to 2.9 at 150 °C and a baking time of 4 min, producing the ideal size for microfluidic device fabrication. The X- and Y-axes shrank ~2.5 times, while Z-axis thickened by a factor of ~5.8 times. This study achieved a minimum feature size of 200 microns, limited by the collapsing of channel sidewalls when shrunk, leading to blockages in the microchannel. These findings demonstrate the feasibility and versatility of using shrink film as a cost-effective and efficient material for the rapid fabrication of microfluidic devices. The potential applications of this material in various fields such as the medical and biomedical industries, bacteria and algae culture and enumeration are noteworthy.

Urine osmolality assessment through the integration of urea hydrolysis and impedance measurement.

We present the development and validation of an impedance-based urine osmometer for accurate and portable measurement of urine osmolality. The urine osmolality of a urine sample can be estimated by determining the concentrations of the conductive solutes and urea, which make up approximately 94% of the urine composition. Our method utilizes impedance measurements to determine the conductive solutes and urea after hydrolysis with urease enzyme. We built an impedance model using sodium chloride (NaCl) and urea at various known concentrations. In this work, we validated the accuracy of the impedance-based urine osmometer by developing a proof-of-concept first prototype and an integrated urine dipstick second prototype, where both prototypes exhibit an average accuracy of 95.5 ± 2.4% and 89.9 ± 9.1%, respectively in comparison to a clinical freezing point osmometer in the hospital laboratory. While the integrated dipstick design exhibited a slightly lower accuracy than the first prototype, it eliminated the need for pre-mixing or manual pipetting. Impedance calibration curves for conductive and non-conductive solutes consistently yielded results for NaCl but underscored challenges in achieving uniform urease enzyme coating on the dipstick. We also investigated the impact of storing urine at room temperature for 24 hours, demonstrating negligible differences in osmolality values. Overall, our impedance-based urine osmometer presents a promising tool for point-of-care urine osmolality measurements, addressing the demand for a portable, accurate, and user-friendly device with potential applications in clinical and home settings.

A Technique for Rapid Bacterial-Density Enumeration through Membrane Filtration and Differential Pressure Measurements.

In this article, we present a microfluidic technique for the rapid enumeration of bacterial density with a syringe filter to trap bacteria and the quantification of the bacterial density through pressure difference measurement across the membrane. First, we established the baseline differential pressure and hydraulic resistance for a filtration membrane by fully wetting the filter with DI water. Subsequently, when bacteria were infused and trapped at the pores of the membrane, the differential pressure and hydraulic resistance also increased. We characterized the infusion time required for the bacterial sample to achieve a normalized hydraulic resistance of 1.5. An equivalent electric-circuit model and calibration data sets from parametric studies were used to determine the general form of a calibration curve for the prediction of the bacterial density of a bacterial sample. As a proof of concept, we demonstrated through blind tests with Escherichia coli that the device is capable of determining the bacterial density of a sample ranging from 7.3 × 106 to 2.2 × 108 CFU/mL with mean and median accuracies of 87.21% and 91.33%, respectively. The sample-to-result time is 19 min for a sample with lower detection threshold, while for higher-bacterial-density samples the measurement time is further shortened to merely 8 min.

Disposable flow cytometer with high efficiency in particle counting and sizing using an optofluidic lens.

Flow cytometers are widely applied to environmental monitoring, industrial testing, and biochemical studies. Integrating a flow cytometer into microfluidic networks helps to miniaturize the system and make it portable for field use. The integration of optical components, such as lenses, further improves the compactness and thus has been intensively studied recently. However, the current designs suffer from severe light scattering due to the roughness of the solid-based lens interface. In this Letter, we propose a flow cytometer using an optofluidic lens to focus the light beam. Benefiting from the smooth liquid-liquid lens interface and the refractive-index matching liquid as cladding streams, a light beam can be well focused without scattering. The variations of the signal peak values are reduced, owing to the small beam width at the beam waist. The device presents an efficient and accurate performance on both the counting and sizing of particles.

Dielectrophoretic trapping and impedance detection of Escherichia coli, Vibrio cholera, and Enterococci bacteria.

In this work, a dielectrophoretic impedance measurement (DEPIM) lab-on-chip device for bacteria trapping and detection of Escherichia coli, Vibrio cholerae, and Enterococcus is presented. Through the integration of SU-8 negative photoresist as a microchannel and the precise alignment of the SU-8 microchannel with the on-chip gold interdigitated microelectrodes, bacteria trapping efficiencies of up to 97.4%, 97.7%, and 37.7% were achieved for E. coli, V. cholerae, and Enterococcus, respectively. The DEPIM device enables a high detection sensitivity, which requires only a total number of 69 ± 33 E. coli cells, 9 ± 2 Vibrio cholera cells, and 36 ± 13 Enterococcus cells to observe a discernible change in system impedance for detection. Nonetheless, the corrected limit of detection for Enterococcus is 95 ± 34 after taking into consideration the lower trapping efficiency. In addition, a theoretical model is developed to allow for the direct estimation of the number of bacteria through a linear relationship with the change in the reciprocal of the overall system absolute impedance.

Deterministic sequential isolation of floating cancer cells under continuous flow.

Isolation of rare cells, such as circulating tumor cells, has been challenging because of their low abundance and limited timeframes of expressions of relevant cell characteristics. In this work, we devise a novel hydrodynamic mechanism to sequentially trap and isolate floating cells in biosamples. We develop a microfluidic device for the sequential isolation of floating cancer cells through a series of microsieves to obtain up to 100% trapping yield and >95% sequential isolation efficiency. We optimize the trappers' dimensions and locations through both computational and experimental analyses using microbeads and cells. Furthermore, we investigated the functional range of flow rates for effective sequential cell isolation by taking the cell deformability into account. We verify the cell isolation ability using the human breast cancer cell line MDA-MB-231 with perfect agreement with the microbead results. The viability of the isolated cells can be maintained for direct identification of any cell characteristics within the device. We further demonstrate that this device can be applied to isolate the largest particles from a sample containing multiple sizes of particles, revealing its possible applicability in isolation of circulating tumor cells in cancer patients' blood. Our study provides a promising sequential cell isolation strategy with high potential for rapid detection and analysis of general floating cells, including circulating tumor cells and other rare cell types.

Enhancing malaria diagnosis through microfluidic cell enrichment and magnetic resonance relaxometry detection.

Despite significant advancements over the years, there remains an urgent need for low cost diagnostic approaches that allow for rapid, reliable and sensitive detection of malaria parasites in clinical samples. Our previous work has shown that magnetic resonance relaxometry (MRR) is a potentially highly sensitive tool for malaria diagnosis. A key challenge for making MRR based malaria diagnostics suitable for clinical testing is the fact that MRR baseline fluctuation exists between individuals, making it difficult to detect low level parasitemia. To overcome this problem, it is important to establish the MRR baseline of each individual while having the ability to reliably determine any changes that are caused by the infection of malaria parasite. Here we show that an approach that combines the use of microfluidic cell enrichment with a saponin lysis before MRR detection can overcome these challenges and provide the basis for a highly sensitive and reliable diagnostic approach of malaria parasites. Importantly, as little as 0.0005% of ring stage parasites can be detected reliably, making this ideally suited for the detection of malaria parasites in peripheral blood obtained from patients. The approaches used here are envisaged to provide a new malaria diagnosis solution in the near future.

Micromagnetic resonance relaxometry for rapid label-free malaria diagnosis.

We report a new technique for sensitive, quantitative and rapid detection of Plasmodium spp.-infected red blood cells (RBCs) by means of magnetic resonance relaxometry (MRR). During the intraerythrocytic cycle, malaria parasites metabolize large amounts of cellular hemoglobin and convert it into hemozoin crystallites. We exploit the relatively large paramagnetic susceptibility of these hemozoin particles, which induce substantial changes in the transverse relaxation rate of proton nuclear magnetic resonance of RBCs, to infer the 'parasite load' in blood. Using an inexpensive benchtop 0.5-Tesla MRR system, we show that with minimal sample preparatory steps and without any chemical or immunolabeling, a parasitemia level of fewer than ten parasites per microliter in a volume below 10 µl of whole blood is detected in a few minutes. We demonstrate this method both for cultured Plasmodium falciparum parasites and in vivo with Plasmodium berghei-infected mice.

Adhesive-based liquid metal radio-frequency microcoil for magnetic resonance relaxometry measurement.

This paper reports the fabrication and characterization of an adhesive-based liquid-metal microcoil for magnetic resonance relaxometry (MRR). Conventionally, microcoils are fabricated by various techniques such as electroplating, microcontact printing and focused ion beam milling. These techniques require considerable fabrication efforts and incur high cost. In this paper, we demonstrate a novel technique to fabricate three-dimensional multilayer liquid-metal microcoils together with the microfluidic network by lamination of dry adhesive sheets. One of the unique features of the adhesive-based technique is that the detachable sample chamber can be disposed after each experiment and the microcoil can be reused without cross-contamination multiple times. The integrated microcoil has a low direct-current (DC) resistance of 0.3 Ω and a relatively high inductance of 67.5 nH leading to a high quality factor of approximately 30 at 21.65 MHz. The microcoil was characterized for ∼0.5 T proton MRR measurements. The optimal pulse duration, amplitude, and frequency for the 90° pulse were 131 µs, -30 dB (1.56 W) and 21.6553 MHz, respectively. In addition, we used the liquid-metal microcoil to perform a parametric study on the transverse relaxation rate of human red blood cells at different hematocrit levels. The transverse relaxation rate increases quadratically with the hematocrit level. The results from the liquid-metal microcoil were verified by measurements with a conventional solenoid coil.