Over the last decade, microfluidics has become increasingly popular in biology and bioengineering. While lab-on-a-chip fabrication costs have continued to decrease, the hardware required for delivering controllable fluid flows to the microfluidic devices themselves remains expensive and often cost prohibitive for researchers interested in starting a microfluidics project. Typically, microfluidic experiments require precise and tunable flow rates from a system that is simple to operate. While many labs use commercial platforms or syringe pumps, these solutions can cost thousands of dollars and can be cost prohibitive. Here, we present an inexpensive and easy-to-use constant pressure system for delivering flows to microfluidic devices. The controller costs less than half the price of a single syringe pump but can independently switch and deliver fluid through up to four separate fluidic inlets at known flow rates with significantly faster fluid response times. It is constructed of readily available pressure regulators, gauges, plastic connectors and adapters, and tubing. Flow rate is easily predicted and calibrated using hydraulic circuit analysis and capillary tubing resistors. Finally, we demonstrate the capabilities of the flow system by performing well-known microfluidic experiments for chemical gradient generation and emulsion droplet production.

We present a new type of free-flow electrophoresis (FFE) device for performing on-chip microfluidic isotachophoresis and zone electrophoresis. FFE is performed using metal gallium electrodes, which are isolated from a main microfluidic flow channel using thin micron-scale polydimethylsiloxane/carbon black (PDMS/CB) composite membranes integrated directly into the sidewalls of the microfluidic channel. The thin membrane allows for field penetration and effective electrophoresis, but serves to prevent bubble generation at the electrodes from electrolysis. We experimentally demonstrate the ability to use this platform to perform on-chip free-flow electrophoretic separation and isotachophoretic concentration. Due to the small size and simple fabrication procedure, this PDMS/CB platform could be used as a part of an on-chip upstream sample preparation toolkit for portable microfluidic diagnostic applications.

The ability to transport and store a large human blood inventory for transfusions is an essential requirement for medical institutions. Thus, there is an important need for rapid and low‐cost characterization tools for analyzing the properties of human red blood cells (RBCs) while in storage. In this study, we investigate the ability to use dielectrophoresis (DEP) for measuring the storage‐induced changes in RBC electrical properties. Fresh human blood was collected, suspended in K2‐EDTA anticoagulant, and stored in a blood bank refrigerator for a period of 20 days. Cells were removed from storage at 5‐day intervals and subjected to a glutaraldehyde crosslinking reaction to “freeze” cells at their ionic equilibrium at that point in time and prevent ion leakage during DEP analysis. The DEP behavior of RBCs was analyzed in a high permittivity DEP buffer using a three‐dimensional DEP chip (3DEP) and also compared to measurements taken with a 2D quadrupole electrode array. The DEP analysis confirms that RBC electrical property changes occur during storage and are only discernable with the use of the cell crosslinking reaction above a glutaraldehyde fixation concentration of 1.0 w/v%. In particular, cytoplasm conductivity was observed to decrease by more than 75% while the RBC membrane conductance was observed to increase by more than 1000% over a period of 20 days. These results show that the presented combination of chemical crosslinking and DEP can be used as rapid characterization tool for monitoring electrical properties changes of human RBCs while subjected to refrigeration in blood bank storage.

Biosensors require a biorecognition element that specifically binds to a target analyte, and a signal transducer, which converts this targeted binding event into a measurable signal. While current biosensing methods are capable of sensitively detecting a variety of target analytes in a laboratory setting, there are inherent difficulties in developing low-cost portable biosensors for point-of-care diagnostics using traditional optical, mass, or electroanalytical-based signal transducers. It is therefore important to develop new biosensing transducer elements for recognizing binding events at low cost and in portable environments. Here, we demonstrate a novel electrokinetic liquid biosensing method for the sensitive label-free detection of a model biomolecule against a background of serum protein. The biosensor is based on the motion of a microfluidic-generated electrical liquid interface when subjected to an external alternating current electrical field. We demonstrate that the electric field-induced motion of the interface can be used as a sensitive and specific transducer for the detection of avidin at femtomolar concentrations in solution. This new detection strategy does not require surface functionalization or fluorescent labels, and has the potential to serve as a sensitive low-cost method for portable biomarker detection.

We present a novel label-free electrokinetic method for detecting biomolecular binding on nanoparticles suspended in the vicinity of a laminar microfluidic interface. The sensor is based on the deflection of the liquid interface in an external AC electric field. Biomolecular binding on particles is shown to increase the interfacial electrical conductivity of the suspending electrolyte, which is sensitively transduced as a change in electrokinetic interfacial deflection. Using this approach, we detect the binding of biotin on streptavidin-functionalized nanoparticles at concentrations as low as 500 aM and perform detection of human IgG at clinically relevant concentrations (1.25-12 mg/mL) without labels. The interface response is only influenced by specific binding on nanoparticle binding sites and decreases when the particle concentration of reactive particles is reduced. Furthermore, we show that control experiments with both nonreactive particles and lack of a specific target analyte do not produce interface deflection. This work provides a promising method for quantifying bead-based binding kinetics for sensitive and specific biosensing in solution without labels.

A microfluidic device (channels <70 μm) was utilized to create micro-scale bubbles to significantly increase mass transfer efficiency at low flow rates. The convergence of one gas and two liquid channels at a Y-junction generates bubbles via cyclic changes in pressure. At low flow rates, the bubbles had an average diameter of 110 μm, corresponding to a volumetric mass transfer KL a of 1.43 h(-1) . Values of KL a normalized per flow rate showed that the microbubbler had a 100-fold increased transfer efficiency compared to four other commonly used bubblers. The calculated percentage of oxygen transferred was approximately 90%, which was consistent with a separate off-gas analysis. The improved mass transfer was also tested in an algae bioreactor in which the microbubbler absorbed approximately 90% of the CO2 feed compared to 2% in the culture with an alternative needle bubbling method. The microbubbler yielded a cell density 82% of the cell density for the alternative needle tip with an 800-fold lower flow rate (0.5 mL/min versus 400 mL/min) and a 700-fold higher ratio of biomass to fed carbon dioxide. The application of microfluidics may transform interfacial processing in order to increase mass transfer efficiencies, minimize gas feeding, and provide for more sustainable multiphase processes. Biotechnol. Bioeng. 2016;113: 1924-1933. © 2016 Wiley Periodicals, Inc.

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