Recommendations on fit-for-purpose criteria to establish quality management for microphysiological systems and for monitoring their reproducibility

Cell culture technology has evolved, moving from single-cell and monolayer methods to 3D models like reaggregates, spheroids, and organoids, improved with bioengineering like microfabrication and bioprinting. These advancements, termed microphysiological systems (MPSs), closely replicate tissue environments and human physiology, enhancing research and biomedical uses. However, MPS complexity introduces standardization challenges, impacting reproducibility and trust. We offer guidelines for quality management and control criteria specific to MPSs, facilitating reliable outcomes without stifling innovation. Our fit-for-purpose recommendations provide actionable advice for achieving consistent MPS performance.

Heart-on-a-chip systems: disease modeling and drug screening applications

Cardiovascular disease (CVD) is the leading cause of death worldwide, casting a substantial economic footprint and burdening the global healthcare system. Historically, pre-clinical CVD modeling and therapeutic screening have been performed using animal models. Unfortunately, animal models oftentimes fail to adequately mimic human physiology, leading to a poor translation of therapeutics from pre-clinical trials to consumers. Even those that make it to market can be removed due to unforeseen side effects. As such, there exists a clinical, technological, and economical need for systems that faithfully capture human (patho)physiology for modeling CVD, assessing cardiotoxicity, and evaluating drug efficacy. Heart-on-a-chip (HoC) systems are a part of the broader organ-on-a-chip paradigm that leverages microfluidics, tissue engineering, microfabrication, electronics, and gene editing to create human-relevant models for studying disease, drug-induced side effects, and therapeutic efficacy. These compact systems can be capable of real-time measurements and on-demand characterization of tissue behavior and could revolutionize the drug development process. In this review, we highlight the key components that comprise a HoC system followed by a review of contemporary reports of their use in disease modeling, drug toxicity and efficacy assessment, and as part of multi-organ-on-a-chip platforms. We also discuss future perspectives and challenges facing the field, including a discussion on the role that standardization is expected to play in accelerating the widespread adoption of these platforms.

From animal testing to in vitro systems: advancing standardization in microphysiological systems

Limitations with cell cultures and experimental animal-based studies have had the scientific and industrial communities searching for new approaches that can provide reliable human models for applications such as drug development, toxicological assessment, and in vitro pre-clinical evaluation. This has resulted in the development of microfluidic-based cultures that may better represent organs and organ systems in vivo than conventional monolayer cell cultures. Although there is considerable interest from industry and regulatory bodies in this technology, several challenges need to be addressed for it to reach its full potential. Among those is a lack of guidelines and standards. Therefore, a multidisciplinary team of stakeholders was formed, with members from the US Food and Drug Administration (FDA), the National Institute of Standards and Technology (NIST), European Union, academia, and industry, to provide a framework for future development of guidelines/standards governing engineering concepts of organ-on-a-chip models. The result of this work is presented here for interested parties, stakeholders, and other standards development organizations (SDOs) to foster further discussion and enhance the impact and benefits of these efforts.

Microfluidic Blood Separation: Key Technologies and Critical Figures of Merit

Blood is a complex sample comprised mostly of plasma, red blood cells (RBCs), and other cells whose concentrations correlate to physiological or pathological health conditions. There are also many blood-circulating biomarkers, such as circulating tumor cells (CTCs) and various pathogens, that can be used as measurands to diagnose certain diseases. Microfluidic devices are attractive analytical tools for separating blood components in point-of-care (POC) applications. These platforms have the potential advantage of, among other features, being compact and portable. These features can eventually be exploited in clinics and rapid tests performed in households and low-income scenarios. Microfluidic systems have the added benefit of only needing small volumes of blood drawn from patients (from nanoliters to milliliters) while integrating (within the devices) the steps required before detecting analytes. Hence, these systems will reduce the associated costs of purifying blood components of interest (e.g., specific groups of cells or blood biomarkers) for studying and quantifying collected blood fractions. The microfluidic blood separation field has grown since the 2000s, and important advances have been reported in the last few years. Nonetheless, real POC microfluidic blood separation platforms are still elusive. A widespread consensus on what key figures of merit should be reported to assess the quality and yield of these platforms has not been achieved. Knowing what parameters should be reported for microfluidic blood separations will help achieve that consensus and establish a clear road map to promote further commercialization of these devices and attain real POC applications. This review provides an overview of the separation techniques currently used to separate blood components for higher throughput separations (number of cells or particles per minute). We present a summary of the critical parameters that should be considered when designing such devices and the figures of merit that should be explicitly reported when presenting a device's separation capabilities. Ultimately, reporting the relevant figures of merit will benefit this growing community and help pave the road toward commercialization of these microfluidic systems.

Overcoming technological barriers in microfluidics: Leakage testing

The miniaturization of laboratory procedures for Lab-on-Chip (LoC) devices and translation to various platforms such as single cell analysis or Organ-on-Chip (OoC) systems are revolutionizing the life sciences and biomedical fields. As a result, microfluidics is becoming a viable technology for improving the quality and sensitivity of critical processes. Yet, standard test methods have not yet been established to validate basic manufacturing steps, performance, and safety of microfluidic devices. The successful development and widespread use of microfluidic technologies are greatly dependent on the community’s success in establishing widely supported test protocols. A key area that requires consensus guidelines is leakage testing. There are unique challenges in preventing and detecting leaks in microfluidic systems because of their small dimensions, high surface-area to volume ratios, low flow rates, limited volumes, and relatively high-pressure differentials over short distances. Also, microfluidic devices often employ heterogenous components, including unique connectors and fluid-contacting materials, which potentially make them more susceptible to mechanical integrity failures. The differences between microfluidic systems and traditional macroscale technologies can exacerbate the impact of a leak on the performance and safety on the microscale. To support the microfluidics community efforts in product development and commercialization, it is critical to identify common aspects of leakage in microfluidic devices and standardize the corresponding safety and performance metrics. There is a need for quantitative metrics to provide quality assurance during or after the manufacturing process. It is also necessary to implement application-specific test methods to effectively characterize leakage in microfluidic systems. In this review, different methods for assessing microfluidics leaks, the benefits of using different test media and materials, and the utility of leakage testing throughout the product life cycle are discussed. Current leakage testing protocols and standard test methods that can be leveraged for characterizing leaks in microfluidic devices and potential classification strategies are also discussed. We hope that this review article will stimulate more discussions around the development of gas and liquid leakage test standards in academia and industry to facilitate device commercialization in the emerging field of microfluidics.

Amontons-Coulomb-like slip dynamics in acousto-microfluidics

Acousto-microfluidics uses acoustic waves to manipulate and sense particles and fluids, and its integration into biomedical technologies has grown substantially in recent years. Fluid manipulation and measurement with surface acoustic waves rely on the efficient transmission of acoustic energy from the device to the fluid. Acoustic transmission into the fluid can be reduced significantly by slip at the fluid-solid interface, but, up until now, this phenomenon has been widely neglected during the design of acousto-microfluidic devices. Here our interpretation supports that the slip dynamics at the liquid-solid interface in acousto-microfluidics are highly analogous to the Amontons-Coulomb laws for dry friction between solids. In particular, there is a relationship between the local fluid pressure and shear stress, where we show that pressure-shear stress conditions can be divided into slip and no-slip regions, similar to the cone of friction found in dry friction. This improved understanding of slip will enable more reliable and predictable acousto-microfluidic technologies, thus expanding their use in new applications in biology and medicine.

Acoustic waves can be used to manipulate particles and fluids in biomedical applications. The authors show that slip at the fluid-solid interface, characterized by a lower acoustic transmission into the fluid, is similar to Amontons-Coulomb friction, as found between solids. 

Broadband Dielectric Spectroscopy as a Potential Label-Free Method to Rapidly Verify Ultraviolet-C Radiation Disinfection

Microwave (MW) sensing offers noninvasive, real-time detection of the electromagnetic properties of biological materials via the highly concentrated electromagnetic fields, for which advantages include wide bandwidth, small size, and cost-effective fabrication. In this paper, we present the application of MW broadband dielectric spectroscopy (BDS) coupled to a fabricated biological thin film for evaluating ultraviolet-C (UV-C) exposure effects. The BDS thin film technique could be deployed as a biological indicator for assessing whole-room UV-C surface disinfection. The disinfection process is monitored by BDS as changes in the electrical properties of surface-confined biological thin films photodegraded with UV-C radiation. Fetal bovine serum (FBS, a surrogate for protein) and bacteriophage lambda double-stranded deoxyribonucleic acid (dsDNA) were continuously monitored with BDS during UV-C radiation exposure. The electrical resistance of FBS films yielded promising yet imprecise readings, whereas the resistance of dsDNA films discernibly decreased with UV-C exposure. The observations are consistent with the expected photo-oxidation and photodecomposition of protein and DNA. While further research is needed to characterize these measurements, this study presents the first application of BDS to evaluate the electrical properties of solid-state biological thin films. This technique shows promise toward the development of a test method and a standard biological test to determine the efficacy of UV-C disinfection. Such a test with biological indicators could easily be applied to hospital rooms between patient occupancy for a multipoint evaluation to determine if a room meets a disinfection threshold set for new patients.

Accelerating innovation and commercialization through standardization of microfluidic-based medical devices

Worldwide, the microfluidics industry has grown steadily over the last 5 years, with the market for microfluidic medical devices experiencing a compound growth rate of 22%. The number of submissions of microfluidic-based devices to regulatory agencies such as the U.S. Food & Drug Administration (FDA) has also steadily increased, creating a strong demand for the development of consistent and accessible tools for evaluating microfluidics-based devices. The microfluidics community has been slow, or even reluctant, to adopt standards and guidelines, which are needed for harmonization and for assisting academia, researchers, designers, and industry across all stages of product development. Appropriate assessments of device performance also remain a bottleneck for microfluidic devices. Standards reside at the core of mature supply chains generating economies of scale and forging a consistent pathway to match stakeholder expectations, thus creating a foundation for successful commercialization. This article provides a unique perspective on the need for the development of standards specific to the emerging biomedical field of microfluidics. Our aim is to facilitate innovation by encouraging the microfluidics community to work together to help bridge knowledge gaps and improve efficiency in getting high-quality microfluidic medical devices to market faster. We start by acknowledging the progress that has been made in various areas over the past decade. We then describe the existing gaps in the standardization of flow control, interconnections, component integration, manufacturing, assembly, packaging, reliability, performance of microfluidic elements and safety testing of microfluidic devices throughout the entire product life cycle.

Lens-Free Imaging as a Sensor for Dynamic Cell Viability Detection Using the Neutral Red Uptake Assay

Neutral red is a low-cost supravital stain for determining cell viability. The standard protocol relies on a destructive extraction process to release the accumulated dye for endpoint spectrophotometric quantification. We report a non-destructive, live-cell quantification of neutral red uptake using a compact lens-free system. Two light sources indentify the cell perimeter and quantify neutral red uptake. The quantification occurs during staining, thus eliminating the destructive extraction process and reducing assay time. Our system enables live quantification for continuous high-throughput screening of cell viability within confined spaces such as incubators.

Generating Multiscale Gold Nanostructures on Glass without Sidewall Deposits Using Minimal Dry Etching Steps

The advent of recent technologies in the nanoscience arena requires new and improved methods for the fabrication of multiscale features ( e.g., from micro- to nanometer scales). Specifically, biological applications generally demand the use of transparent substrates to allow for the optical monitoring of processes of interest in cells and other biological materials. Whereas wet etching methods commonly fail to produce essential nanometer scale features, plasma-based dry etching can produce features down to tens of nanometers. However, dry etching methods routinely require extreme conditions and extra steps to obtain features without residual materials such as sidewall deposits (veils). This work presents the development of a gold etching process with gases that are commonly used to etch glass. Our method can etch gold films using reactive ion etching (RIE) at room temperature and mild pressure in a trifluoromethane (CHF₃)/oxygen (O₂) environment, producing features down to 50 nm. Aspect ratios of 2 are obtainable in one single step and without sidewall veils by controlling the oxygen present during the RIE process. This method generates surfaces completely flat and ready for the deposition of other materials. The gold features that were produced by this method exhibited high conductivity when carbon nanotubes were deposited on top of patterned features (gold nanoelectrodes), hence demonstrating an electrically functional gold after the dry etching process. The production of gold nanofeatures on glass substrates would serve as biocompatible, highly conductive, and chemically stable materials in biological/biomedical applications.

Proceedings of the First Workshop on Standards for Microfluidics

In the last two decades, the microfluidics/lab-on-a-chip field has evolved from the concept of micro total analysis systems, where systems with integrated pretreatment and analysis of chemicals were envisioned, to what is known today as lab-on-a-chip, which is expected to be modular. This field has shown great potential for the development of technologies that can make, and to some extent are making, a big difference in areas such as in vitro diagnostics, point of care testing, organ on a chip, and many more. Microfluidics plays an essential role in these systems, and determining the standards needed in this area is critical for enabling new markets and products, and to advance research and development. Our goal was to bring together stakeholders from industry, academia, and government to discuss and define the needs within the field for the development of standards. This publication contains a summary of the workshop, abstracts from each presentation, and a summary of the breakout sessions from the National Institute of Standards and Technology Workshop on Standards for Microfluidics, held on June 1-2, 2017. The workshop was attended by 46 persons from 26 organizations and 11 countries. This was a unique and exciting opportunity for stakeholders from all over the world to join in the discussion of future developments towards standardization in the microfluidics arena.

The art in science of MicroTAS: 10 years of miniaturized art

Maria Cristina Letizia (center) from EPFL, Switzerland, was the winner of the 2017 MicroTAS Art in Science competition. She received the award certificate from Maria Southall (right) from Lab on a Chip and Darwin Reyes (left) from NIST.

The 2016 edition of the Art in Science of MicroTAS

Vaibhav Jain (center) from Purdue University wins the MicroTAS 2016 Art in Science award, presented by Sam Keltie (right) from Lab on a Chip/The Royal Society of Chemistry and Darwin Reyes (left) from NIST.

Fibronectin in Layer-by-Layer Assembled Films Switches Tumor Cells between 2D and 3D Morphology

Tumor cells showing a 3D morphology and in coculture with endothelial cells are a valuable in vitro model for studying cell-cell interactions and for the development of pharmaceuticals. Here, we found that HepG2 cells, unlike endothelial cells, show differences in adhesion to fibronectin alone, or in combination with poly(allylamine hydrochloride). This response allowed us to engineer micropatterned heterotypic cultures of the two cell types using microfluidics to pattern cell adhesion. The resulting cocultures exhibit spatially encoded and physiologically relevant cell function. Further, we found that the protrusive, migratory and 3D morphological responses of HepG2 are synergistically modulated by the constituents of the hybrid extracellular matrix. Treating the hybrid material with the cross-linking enzyme transglutaminase inhibited 3D morphogenesis of tumor cells. Our results extend previous work on the role of fibronectin in layer-by-layer assembled films, and demonstrate that cell-specific differences in adhesion to fibronectin can be used to engineer tumor cell cocultures.

From microgalaxies to micro-oceans to Warhol's cellular images: the art in science of MicroTAS 2015

Matteo Cornaglia (center) from École Polytechnique Fédérale de Lausanne (EPFL) receives the MicroTAS 2015 Art in Science award presented by Darwin Reyes (left) from NIST and Sarah Ruthven (right) from Lab on a Chip/The Royal Society of Chemistry.

The art in science of MicroTAS: the 2014 issue

Shilpa Sivashankar (representing David Castro) of KAUST receives the 7th Annual MicroTAS Art in Science Award presented by Darwin Reyes of NIST, Harpal Minhas of LOC/RSC and Albert Folch of the University of Washington.

The Art in Science of MicroTAS: the 2013 edition

The 6th annual μTAS Art in Science Award was presented to Ye Wang of Eindhoven University of Technology at the 17th International Conference of Miniaturized Systems for Chemistry and Life Sciences held in Freiburg, Germany, on October 27-31, 2013. The winning image captivated the hearts and minds of the judges and is featured on the front cover of this issue. "Artificial Life", as the author of the work that produced this image named it, was taken with a scanning electron microscope (SEM) and shows cilia-like structures (or "microhairs") generated from PDMS (polydimethylsiloxane) and magnetic nanoparticles (Y. Wang, Y. Gao, H. M. Wyss, P. D. Anderson and J. M. J. den Toonder, "Out of Cleanroom, Self-assembled Magnetic Artificial Cilia", micro-TAS, 2013, 787-789). To produce the structure the author used a glass mold produced by femtolaser modification and hydrofluoric acid etching. As the title implies, the features produced by this procedure resemble cilia-like structures as seen in a number of eukaryotic cells.

Dielectrophoretic cell capture on polyester membranes

A new system for dielectrophoretic cell capture on permeable polyester membranes is presented. Conventional photolithographic techniques were used to fabricate gold microelectrodes on a polyester membrane. The characterization of the microelectrodes showed that there were no differences regarding roughness, permeability, and hydrophilicity of the membrane before and after processing. Finally, dielectrophoretic cell capture and viability in a microfluidic device was demonstrated on the patterned membrane. These membranes could ultimately be combined with multilayer microfluidic devices to form a powerful tool for studies of cell-cell interactions in coculture, whereby spatial separation of different cell types and/or microenvironments are required.

Hybrid cell adhesive material for instant dielectrophoretic cell trapping and long-term cell function assessment

Dielectrophoresis (DEP) for cell manipulation has focused, for the most part, on approaches for separation/enrichment of cells of interest. Advancements in cell positioning and immobilization onto substrates for cell culture, either as single cells or as cell aggregates, has benefited from the intensified research efforts in DEP (electrokinetic) manipulation. However, there has yet to be a DEP approach that provides the conditions for cell manipulation while promoting cell function processes such as cell differentiation. Here we present the first demonstration of a system that combines DEP with a hybrid cell adhesive material (hCAM) to allow for cell entrapment and cell function, as demonstrated by cell differentiation into neuronlike cells (NLCs). The hCAM, comprised of polyelectrolytes and fibronectin, was engineered to function as an instantaneous cell adhesive surface after DEP manipulation and to support long-term cell function (cell proliferation, induction, and differentiation). Pluripotent P19 mouse embryonal carcinoma cells flowing within a microchannel were attracted to the DEP electrode surface and remained adhered onto the hCAM coating under a fluid flow field after the DEP forces were removed. Cells remained viable after DEP manipulation for up to 8 d, during which time the P19 cells were induced to differentiate into NLCs. This approach could have further applications in areas such as cell-cell communication, three-dimensional cell aggregates to create cell microenvironments, and cell cocultures.

Microfluidic based contactless dielectrophoretic device: Modeling and analysis

While there have been many attempts at patterning cells onto substrates, a reliable method for trapping cell clusters and forming cell arrays in a predefined geometry remains to be demonstrated. We intend to develop a multielectrode array platform to initially trap cells via dielectrophoresis (DEP) and to later measure their electrical activity. As a first step toward that objective, here we present an interdigitated microfabricated comb structure. We designed an optimal insulation layer via finite element modeling for maximum dielectrophoretic field strength in solution and minimal cell damage. The microfabricated structure was combined with a microfluidic channel to vertically constrain cell position. With the objective of capturing cells onto the substrate, we here show that there is an optimal thickness of dielectric which limits electrolysis in solution and still allows for sufficient dielectrophoretic force on the cells to pull them onto the surface.

Polyelectrolyte Multilayer-Treated Electrodes for Real-Time Electronic Sensing of Cell Proliferation

We report on the use of polyelectrolyte multilayer (PEM) coatings as a non-biological surface preparation to facilitate uniform cell attachment and growth on patterned thin-film gold (Au) electrodes on glass for impedance-based measurements. Extracellular matrix (ECM) proteins are commonly utilized as cell adhesion promoters for electrodes; however, they exhibit degradation over time, thereby imposing limitations on the duration of conductance-based biosensor experiments. The motivation for the use of PEM coatings arises from their long-term surface stability as promoters for cell attachment, patterning, and culture. In this work, a cell proliferation monitoring device was fabricated. It consisted of thin-film Au electrodes deposited with a titanium-tungsten (TiW) adhesion layer that were patterned on a glass substrate and passivated to create active electrode areas. The electrode surfaces were then treated with a poly(ethyleneimine) (PEI) anchoring layer and subsequent bilayers of sodium poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH). NIH-3T3 mouse embryonic fibroblast cells were cultured on the device, observed by optical microscopy, and showed uniform growth characteristics similar to those observed on a traditional polystyrene cell culture dish. The optical observations were correlated to electrical measurements on the PEM-treated electrodes, which exhibited a rise in impedance with cell proliferation and stabilized to an approximate 15 % increase as the culture approached confluency. In conclusion, cells proliferate uniformly over gold and glass PEM-treated surfaces, making them useful for continuous impedance-based, real-time monitoring of cell proliferation and for the determination of cell growth rate in cellular assays.

Cellular Immobilization within Microfluidic Microenvironments: Dielectrophoresis with Polyelectrolyte Multilayers

The development of biomimetic microenvironments will improve cell culture techniques by enabling in vitro cell cultures that mimic in vivo behavior; however, experimental control over attachment, cellular position, or intercellular distances within such microenvironments remains challenging. We report here the rapid and controllable immobilization of suspended mammalian cells within microfabricated environments using a combination of electronic (dielectrophoresis, DEP) and chemical (polyelectrolyte multilayers, PEMS) forces. While cellular position within the microsystem is rapidly patterned via intermittent DEP trapping, persistent adhesion after removal of electronic forces is enabled by surface treatment with PEMS that are amenable to cellular attachment. In contrast to DEP trapping alone, persistent adhesion enables the soluble microenvironment to be systematically varied, facilitating the use of soluble probes of cell state and enabling cellular characterization in response to various soluble stimuli.

Facilitating the culture of mammalian nerve cells with polyelectrolyte multilayers

When neuron-like cells (NLCs) derived from pluripotent embryonal carcinoma cells (P19) are cultured on bare tissue culture substrates, they require a monolayer of fibroblast cells to exhibit normal neurite outgrowth, behavior typical of neuronal cultures. However, substrate treatment with polyelectrolyte multilayers (PEMs) composed of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonic acid) (PSS) significantly improved these cultures. Cell morphology was more spread, indicative of healthy cells, and direct attachment of neuronal cell bodies to the treated surface was observed. Neuronal outgrowth across the surface was not dependent on an underlying fibroblast monolayer with the PEMs surface treatment. Additionally, the PEMs surface treatment can be used to condition various surfaces, facilitating neuronal cultures on surfaces which are natively hydrophilic (tissue culture polystyrene) or hydrophobic (poly(dimethylsiloxane), PDMS). Microfluidic networks were used to micropattern the PEMs onto PDMS, resulting in confined regions of cellular attachment and directed neuronal outgrowth. The ability of PEMs to encourage NLC attachment without supporting cells to a variety of surfaces and surface geometries greatly simplifies neuronal culture methodology and enables neuronal investigations in new environments.

Micropatterning neuronal cells on polyelectrolyte multilayers

This paper describes an approach to adhere retinal cells on micropatterned polyelectrolyte multilayer (PEM) lines adsorbed on poly(dimethylsiloxane) (PDMS) surfaces using microfluidic networks. PEMs were patterned on flat, oxidized PDMS surfaces by sequentially flowing polyions through a microchannel network that was placed in contact with the PDMS surface. Polyethyleneimine (PEI) and poly(allylamine hydrochloride) (PAH) were the polyions used as the top layer cellular adhesion material. The microfluidic network was lifted off after the patterning was completed and retinal cells were seeded on the PEM/PDMS surfaces. The traditional practice of using blocking agents to prevent the adhesion of cells on unpatterned areas was avoided by allowing the PDMS surface to return to its uncharged state after the patterning was completed. The adhesion of rat retinal cells on the patterned PEMs was observed 5 h after seeding. Cell viability and morphology on the patterned PEMs were assayed. These materials proved to be nontoxic to the cells used in this study regardless of the number of stacked PEM layers. Phalloidin staining of the cytoskeleton revealed no apparent morphological differences in retinal cells compared with those plated on polystyrene or the larger regions of PEI and PAH; however, cells were relatively more elongated when cultured on the PEM lines. Cell-to-cell communication between cells on adjacent PEM lines was observed as interconnecting tubes containing actin that were a few hundred nanometers in diameter and up to 55 microm in length. This approach provides a simple, fast, and inexpensive method of patterning cells onto micrometer-scale features.

Guiding DC glow discharge in microchannels

This work describes the conditions, in terms of dimensions and geometry, to guide a dc glow discharge preferentially through a microchannel in simple networks. Two- and three-channel microfluidic structures were studied. A preference towards a wider channel, in a two-channel network, was observed when the difference in width was at least 18% and the length was at least 10 mm. In a three-channel structure, a change in glow discharge intensity was observed when the network was downscaled from a pathlength of 2 to 0.5 cm. While the intensity within the path with fewer turns decreased with a reduction in size, the intensity of the path with greater number of turns increased.

Glow discharge in microfluidic chips for visible analog computing

Here we present a novel visible analog computing approach for solving a wide class of shortest path problems. Using a microfluidic chip for computation, based on the lighting up of a glow discharge, the solution to maze search problems, the solution of a network shortest path and k-shortest paths problems and the practical application of finding the shortest paths between several landmarks from a street map are presented. The solution and visible display (in real time) for these problems shows only a small difference in practical problem solving time among problems with varying differences in size.

Toxic evaluation of organic extracts from airborne particulate matter in Puerto Rico

In recent years, several hypotheses have emerged to explain the toxicologic activity of particulate matter. Organic compounds, ultrafine particles, biologic components, and transition metals are some of the constituents that reportedly exert some type of adverse effect on human health. A considerable fraction of the urban particulate matter consists of carbon compounds, which originate mostly from anthropogenic sources. The toxicity of organic fractions from particulate matter have been mainly evaluated by considering their mutagenic activity. This research expands on the toxicologic profile of organic compounds adsorbed to particulate matter, specifically in Puerto Rico, by using the cytotoxic neutral red bioassay (NRB). The NRB uses normal human epidermal keratinocytes or other types of cells to measure the effect on cell viability when exposed to organic compounds associated to the particles in the air. We validated the NRB for particulate matter by using a standard reference material (SRM 1649). We used the NRB to determine toxicologic differences of extracts between an urban industrialized site with anthropogenic activity versus a coastal region with less human activity. The cytotoxicity associated with organic compounds in particulate matter collected at the urban industrialized site was detected in both the particulate matter (3/4) 10 microm in aerodynamic diameter (PM(10)) and particulate matter (3/4) 100 microm in aerodynamic diameter (PM(100)). Greater toxic effects were observed in PM(10) extracts than in PM(100) extracts, but PM(10) toxic effects were not significantly different from those in PM(100). The extracts from the industrialized site were more cytotoxic than the extracts from coastal reference site, although in the summer, extracts from both sites were significantly cytotoxic to normal human epidermal keratinocytes. In addition, the nonpolar extracts of both PM(10) and PM(100) exerted the greatest cytotoxicity, followed by the polar, and, finally, the moderately polar extract. This study demonstrates that extracts from the Guaynabo industrialized site were more toxic than similar extracts obtained from a reference coastal site in Fajardo, Puerto Rico.