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.

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.

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.

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.

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.

Uncovering the Contribution of Microchannel Deformation to Impedance-Based Flow Rate Measurements.

Changes in electrical impedance have previously been used to measure fluid flow rate in microfluidic channels. Ionic redistribution within the electrical double layer by fluid flow has been considered to be the primary mechanism underlying such impedance based microflow sensors. Here we describe a previously unappreciated contribution of microchannel deformation to such measurements. We found that flow-induced microchannel deformation contributes significantly to the change in electrical impedance of solutions, in particular to those solutions producing an electrical double layer in the order of a few tens of nanometers (i.e., containing relatively high ionic strength). Since the flow velocity at the measurement surface is near zero, due to the laminar nature of the flow, the contribution of the double layer under the conditions mentioned above should be negligible. In contrast, an increase in the fluid flow rate results in an increase in the microchannel cross-sectional area (because of higher local pressure), therefore, producing a decrease in solution resistance between the two electrodes. Our results suggest that microflow sensors based on the concept of elastic deformation could be designed for in situ monitoring and fine control of fluid flow in flexible microfluidics. Finally, we show that purposefully engineering a larger deformability of the microchannel, by changing the geometry and the Young's modulus of the microchannel, enhances the sensitivity of this flow rate measurement.

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.

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.