Automation Highlights from the Literature

Femtoliter Droplet Handling in Nanofluidic Channels: A Laplace Nanovalve

Analytical analysis of samples with ultra-small volume (femtoliter to attoliter) is critical for single-cell and single-molecule studies. Handling of ultra-small-volume samples has been made possible with nanofluidic chips that feature fluidic components with submicron dimensions. However, there have been numerous technical challenges in terms of fluidic control in nanofluidic systems, with one example being the fluid valve. So far, researchers have failed to effectively incorporate micro-electric-mechanical system valves in nanofluidic systems because of fabrication challenges. A nonmechanical valve named Laplace nanovalve developed by Mawatari et al. may provide a solution.

This Laplace nanovalve is a nanopillar array fabricated on the bottom of a nanochannel using a two-step electron beam lithography and dry-etching process. The entire channel, including nanopillars, are chemically treated and rendered hydrophobic. When contacting liquid, a sharp wettability boundary is formed between the flat part of the channel and the more hydrophobic nanopillar surface. The surface tension results in a Laplace pressure at the liquid surface, which prevents the liquid meniscus from entering through the valve. Because of the scaling law, the nanopillar array gives rise to a high-breakthrough pressure for the valve. Actuation of femtoliter volume can be implemented by controlling the driven pressure above or below the valve’s breakthrough pressure. (Mawatari, K., et al., Anal. Chem. 2012, 84, 10812–10816)

Gravity-Driven Deterministic Lateral Displacement for Particle Separation in Microfluidic Devices

Deterministic lateral displacement (DLD) is a popular microfluidic method for microparticle/cell separation. It works by infusing mixed suspended particles/cells through an array of micro-posts. Interactions between particles/cells and posts cause particles/cells to deviate from their trajectories. The accumulative effect of multiple interactions deterministically separates particles/cells based on surface or mechanical characteristics. Advantages of the DLD lie in its continuous nature and potential for high-throughput operation.

Devendra et al. introduce a useful addition to existing DLD technology portfolios. They investigate a two- dimensional continuous size-based separation of suspended particles in gravity-driven deterministic lateral displacement (g-DLD) devices. The suspended particles are driven through a periodic array of cylindrical obstacles under the action of gravity. The entire range of gravitational force orientations with respect to array of obstacles is tested, and force angles leading to vector separation are identified. The authors develop a simple model that accurately predicts the dependence of the migration angle on the forcing direction to provide design guidance for future g-DLD devices. In the experiment, the authors observe a phenomenon named directional locking that indicates strong separation dependence on the size of the particle, and they suggest that relatively small forcing angles are well suited for size-fractionation purposes. Excellent separation resolution is demonstrated for a binary mixture of particles. (Devendra, R., et al., Anal. Chem. 2012, 84, 10621–10627)

Quantifying Analytes in Paper-Based Microfluidic Devices without Using External Electronic Readers

Point-of-care (POC) analysis is of great importance for identifying and measuring the quantity of analytes in a variety of environments that lack access to laboratory infrastructure. Often a disposable chip and an external reader are included in a POC analysis platform, with the chip providing the platform for analytical assay and the external reader recording the assay time and detection signal. There has been interest in further simplifying the external reader of the POC systems to improve the usability of the device. The World Health Organization has indicated that the use of external readers is a challenge that must be overcome when creating ideal POC diagnostic assays for use in the developing world.

Eliminating the external reader, or being “equipment-free,” is one of the most desired attributes for diagnostic tests in these regions. Lewis et al. introduce a paper-based microfluidic platform to tackle this issue. The platform includes two complementary assay strategies that allow quantifying the level of an analyte by simply measuring time, and no additional external electronic reader is required. The strategy involves either (1) tracking the time required for a sample to pass through a hydrophobic detection reagent in a single conduit within a three-dimensional (3D) paper-based microfluidic device (the authors refer to this as the digital assay) or (b) counting the number of bars that become colored after a fixed assay period in a paper-based microfluidic device (the authors refer to this as an analog assay). Both assays work by quantifying the concentration of hydrogen peroxide, a commonly detected species that can be linked to many diagnostic assays. The digital assay detects hydrogen peroxide based on the time required for the sample to flow through the device in the z direction of a 3D microfluidic channel network. Analog assays quantify the level of hydrogen peroxide by counting the number of completed colormetric reactions in a fixed assay time. Both assays are made possible by using a special compound that can modulate the wetting properties of the microfluidic network upon exposure to hydrogen peroxide, which makes it possible to link the hydrogen peroxide concentration to the fluid’s traveling progress through the microfluidic network. The authors indicate that the advantage of this method is that it requires only a timer, the ability to see color, and the ability to count, which can prove to be valuable for many applications in resource-limited areas that lack both laboratory infrastructures and trained personnel. (Lewis, G., et al., Angew. Chem. 2012, 51, 12707–12710)

Microfluidics Separation Reveals the Stem-Cell–Like Deformability of Tumor-Initiating Cells

Study of cell deformability can reveal a great deal of information regarding cell physiology and has been used as a powerful tool for stem cell and cancer research. For example, studies reveal the difference in deformability of the cytoskeleton and nucleoskeleton at various differentiation stages. Studies also show that the increase of deformability can be correlated with an increase of metastatic potential. Better studying of the implication of cell deformability requires a reliable method of isolating cells with differential deformabilities. Until today, this has remained a great challenge.

Zhang et al. tackle this issue with a unique cell purification system designed to leverage cancer cell mechanical properties to cell isolation. The separation device combines microbarriers and hydrodynamic force to separate deformable from stiff cells. The key of the device is the precise placement of the microbarriers to slow down the passage of stiff cells. The gaps between microbarriers range in size from 7 to 15 µm, which allows efficient continuous separation. In their experiments, the authors demonstrate the separation of heterogeneous breast cancer cell lines into two different subpopulations (flexible vs. stiff). The separated cells underwent genome-wide gene-expression analysis and tumorigenicity assays upon the completion of separation, and overexpression of multiple genes involved in cancer cell motility and metastasis was found to be associated with flexible phenotypes. The authors believe this device could become a very useful tool for studying the cell deformability markers for a variety of cancer cells. (Zhang, W., et al., Proc. Natl. Acad. Sci. U. S. A, 2012, 109, 18707–18712)

Energy Extraction from the Biologic Battery in the Inner Ear

Implantable medical devices typically require a large battery to sustain the operation over a long period of time. In clinical applications, because of the size limitation of the human anatomy, a large battery is not ideal or practical. Therefore, patients often need to carry cumbersome external power sources that are connected with the device either via wire or wirelessly. Mercier et al. introduce an innovate approach to harvest energy for implant devices from nearby sources within the human body. In this work, they look into endocochlear potential (EP), a battery-like electrochemical gradient found in and actively maintained by the inner ear. They demonstrate that the mammalian EP can be used as a power source for electronic devices. This was achieved by designing an anatomically sized, ultra-low-quiescent-power energy harvester chip integrated with a wireless sensor capable of monitoring the EP itself. The chip is able to extract a minimum of 1.12 nW from the EP of a guinea pig for up to 5 h, enabling a 2.4-GHz radio to transmit measurement of the EP every 40 to 360 s. With future optimization of electrode design, the authors envision using the biologic battery in the inner ear to power chemical and molecular sensors or drug-delivery actuators for diagnosis and therapy of hearing loss and other disorders. (Mercier, P. P., et al., Nat. Biotech. 2012, 30, 1240–1243)

A Self-Powered Acetaldehyde Sensor Based on Biofuel Cell

Acetaldehyde is among the most popular targets for environment monitoring because of its impact in water and atmosphere pollution and direct threat to human health. Much effort has been made toward developing sensitive, rapid, simple, and low-cost acetaldehyde detection methods. Zhang et al. introduce an interesting self-powered acetaldehyde sensor based on enzymatic biofuel cells (BFCs). In this work, a self-powered device, an ethanol/air enzymatic BFC, is used as the core component, which can generate a maximum power output density of 28.5 µW cm⁻² at 0.34 V and an open circuit potential of 0.64 V.

The acetaldehyde detection strategy, in which the novelty of this work lies, involves the regulation of the BFC performance caused by reversible kinetics suppression of acetaldehyde. In the BFC, the product of ethanol oxidation, acetaldehyde, counteracts the electrocatalysis at the bioanode and causes the decrease of the power output of the BFC. Based on such an interaction, the acetaldehyde can be detected via monitoring of BFC output. Experiments conducted by the authors demonstrate excellent selectivity with a wide linear range (5–200 µM) and low detection limit (1 µM), which satisfy the criteria provided by the World Health Organization. More importantly, the self-powered feature dramatically reduces the complexity of the system, making it an attractive solution for detecting acetaldehyde in aqueous environments, particularly for water quality control and monitoring. (Zhang, L., et al., Anal. Chem., 2012, 84, 10345–10349)

Electrochemical Microdevice for On-Site Determination of Rice Freshness

Koyachi et al. present an interesting work on electrochemical sensors for real-time point-of-use determination of rice freshness. Rice is one of the most important sources of food in the world. Often, as a precaution for supply shortages due to nature disasters, part of harvested rice grains are stored over a long period of time without processing until right before consumed. The quality of rice, in terms of taste, flavor, pasting properties, and protein and lipid contents, does deteriorate during prolonged storage. In addition, aged rice can be illegally sold as fresh rice for higher profits. Therefore, a rice freshness measurement method is needed to improve storage conditions and to prevent misconduct in rice-trading activities.

In this report, the authors develop an electrochemical sensor–based device for real-time point-of-use determination of rice freshness. Because the aging of rice is associated with the reduction of peroxidase activity, the electrochemical sensor in this work is designed to detect peroxide activity in rice as an indicator for freshness. Hydrogen peroxide and hydroquinone are added in the sensor as reaction substrates. The enzymatic reactions, catalyzed by peroxidase, produces benzoquinone, which can be subsequently detected using cyclic voltammograms using a gold working electrode. Distinct differences can be observed in the detected current between fresh rice and aged rice. The sensor is validated against traditional methods such as fluorometric and guaiacol methods, and good correlations are achieved. The device is constructed to detect multiple rice grains simultaneously, which makes it possible to examine a mixture of grains with different freshnesses. (Koyachi, E., et al., Bios. Bioelectron. 2013, 42, 640–645)

Ni Foam: A Novel Three-Dimensional Porous Sensing Platform for Sensitive and Selective Nonenzymatic Glucose Detection

The glucose sensor is the most successful biosensor ever developed. It has become an indispensable part of diabetes control and treatment regimens. The success of the glucose sensor is largely attributed to glucose oxidase, a nearly perfect enzyme with high substrate specificity, high activity, and high stability (compared with other enzymes). However, enzyme-based biosensors face intrinsic disadvantages such as instability of enzymes in less-than-perfect storage conditions and high cost associated with enzyme preparation and purification. Much interest has been focused on the development of an enzyme alternative for glucose sensors.

Lu et al. report a glucose sensor based on low cost, non–noble metal–based electrocatalysts. The authors use commercially available 3D porous Ni foam (NF) as a sensing element for nonenzymatic glucose detection. In this sensor, NF is not only the working electrode but also an effective electrocatalyst for electro-oxidation of glucose. A high selectivity toward glucose is observed in the experiment, and linear range and detection limits of 0.05 to 7.35 mM and 2.2 µM are achieved, respectively. Initial application of glucose sensing in human blood serum is also demonstrated. (Lu, W., et al., Analyst, 2013, 138, 417–420)

High-Throughput Genome Scanning in Constant Tension Fluidic Funnels

In most living organisms, including all cellular and some viral organisms, genetic information is encoded in long, polymeric chains of DNA. Although full DNA sequencing can effectively identify each organism, often lower-resolution scanning of DNA sequence is sufficient for identification. Genome sequence scanning (GSS) is a bacterial identification technology that detects sparse sequence- specific fluorescent tags on long DNA molecules. This requires the stretching or linearization of a long tangled DNA molecule using certain DNA-stretching methods, such as elongation of DNA using shear flow generated in a continuous-flow microfunnel. A successful GSS detection depends on the high-throughput detection of well-stretched DNA molecules and a highly efficient DNA molecule–stretching method.

In a recent work published by Griffis et al., the authors report a refined compound funnel design and demonstrate significantly improved GSS detection throughput. The authors optimize the funnel design in several aspects: First, the relationship between fluid strain rate and molecule tension is explored to generate more efficient funnel design while minimizing the negative impact of high fluid velocities. Second, a constant-strain detection channel is introduced to minimize the shear-induced molecular tumbling and relaxation. This allows the DNA to be maintained at stretched status with constant tension during observation and to achieve efficient detection of fluorescence tags without sacrificing the throughput. With the improved funnel design, the authors demonstrate a 30-fold increase in detection throughput, compared with previously reported similar devices. (Griffis, J. W., et al., Lab Chip, 2013, 13, 240–251)

Label-Free Electrophysiological Cytometry for Stem Cell–Derived Cardiomyocyte Clusters

Today, cell identification and purification remain challenging issues for clinical stem cell applications. Conventional fluorescence and magnetic cell cytometries require reliable exogenous cell surface markers, which are hard to find for many cells types, such as cardiomyocytes. Alternative methods such as genetic modification markers also have a fair share of concerns, such as the possibility of tumorigenesis. More importantly, labeling approaches provide no information about the stimulus-response characteristics of cells, which are important in evaluating cell functionality.

Myers et al. investigate the possibility of exploiting the electrophysiology signal, more specifically, extracellular field potential (FP), as a signal for cytometry. Many types of cells currently being studied for regenerative medicines happen to be electrically excitable (e.g., cardiomyocytes, neurons, and smooth muscle cells). Each cell type has a characteristic FP signal that can provide rich phenotypic information because of the unique profile of many different cell ion channels. In addition, the authors also show FP signal change as a cell matures from an embryonic to an adult phenotype during stem cell differentiation. Therefore, they propose the use of electrophysiology to provide contrast signal for cell cytometry. This FP signal can be detected with a microelectrode incorporated in a microfluidic flow cytometry device. In this work, an electrophysiology-activated cell cytometry (EPACC) is developed, and differentiation of human-induced pluripotent stem cell–derived cardiomyocyte (iPSC-CM) clusters from undifferentiated iPSC clusters is demonstrated. At the current stage, the device is capable of identifying only clusters of cells (100s–1000s of cells), but the authors believe it will be possible to identify single cells and ultimately to sort live single cells for future downstream experiments using EPACC. (Myers, F. B., et al., Lab Chip, 2013, 13, 220–228)

Flexible Normal Force Sensor Skin for Tactile Feedback

Tactile sensing capability on curved surfaces is needed in a wide variety of applications, such as robots. For example, flexible tactile sensors that can wrap around robotic fingers can provide real-time pressure feedback to enhance grip by cushioning impact and increasing the effective contact area during grasp. Most flexible sensors are based on thin film or solid electrical components that are susceptible to failure due to cracking and fatigue.

Wong et al. introduce a conductive fluid-based normal force sensor skin for tactile sensing. In this work, the authors develop a 5 × 5 array of flexible and multipayer capacitive fluidic normal force sensors. The sensor includes liquid metal-filled microfluidic channels as the capacitive plates and conductive interconnect. The fluidic channel is fabricated via a multilayer soft-lithography technique. The channel is filled with the liquid metal alloy Galinstan and air pockets that modify the mechanical and electrical properties of the sensor. Individual sensing elements are tested and calibrated, especially at the lower force range from 0 to 2.5 N. Good repeatability with both static loads and dynamic loads is demonstrated.

Thanks to the density of microfabrication, the sensor array can provide a spatial resolution of 0.5 mm. It can also be easily wrapped around a surface similar to a human finger in terms of curvature (~1.5 cm⁻¹). In addition, the authors have shown that the curvature tolerance can be as high as more than 6 cm⁻¹. The sensor’s sensitivity and range can be easily tuned by adjusting the heterogeneous PDMS-air dielectric medium. With this great sensitivity and high flexibility, the authors envision that this technology will be a good addition to the existing technology portfolio of tactile sensing for robotic applications, especially when light touch is important. (Wong, R. D. P., et al., Sens. Actuators A, 2012, 179, 62–69)