Microfluidics, a technology characterized by the engineered manipulation of fluids at the submillimetre scale, has shown considerable promise for improving diagnostics and biology research. Certain properties of microfluidic technologies, such as rapid sample processing and the precise control of fluids in an assay, have made them attractive candidates to replace traditional experimental approaches. Here we analyse the progress made by lab-on-a-chip microtechnologies in recent years, and discuss the clinical and research areas in which they have made the greatest impact. We also suggest directions that biologists, engineers and clinicians can take to help this technology live up to its potential.

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The present and future role of microfluidics in biomedical research

Cell culture has become an indispensable tool to help uncover fundamental biophysical and biomolecular mechanisms by which cells assemble into tissues and organs, how these tissues function, and how that function becomes disrupted in disease. Cell culture is now widely used in biomedical research, tissue engineering, regenerative medicine, and industrial practices. Although flat, two-dimensional (2D) cell culture has predominated, recent research has shifted toward culture using three-dimensional (3D) structures, and more realistic biochemical and biomechanical microenvironments. Nevertheless, in 3D cell culture, many challenges remain, including the tissue-tissue interface, the mechanical microenvironment, and the spatiotemporal distributions of oxygen, nutrients, and metabolic wastes. Here, we review 2D and 3D cell culture methods, discuss advantages and limitations of these techniques in modeling physiologically and pathologically relevant processes, and suggest directions for future research.

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Modeling Physiological Events in 2D vs. 3D Cell Culture.

Droplet microfluidics generates and manipulates discrete droplets through immiscible multiphase flows inside microchannels Due to its remarkable advantages, droplet microfluidics bears significant value in an extremely wide range of area In this review, we provide a comprehensive and in-depth insight into droplet microfluidics, covering fundamental research from microfluidic chip fabrication and droplet generation to the applications of droplets in bio(chemical) analysis and materials generation The purpose of this review is to convey the fundamentals of droplet microfluidics, a critical analysis on its current status and challenges, and opinions on its future development We believe this review will promote communications among biology, chemistry, physics, and materials science

Emerging Droplet Microfluidics

Microengineered cell culture systems are becoming sufficiently sophisticated that they can recapitulate many of the phenomena observed in tissues and organisms. Here, Huh and colleagues discuss the advances made in these 'organs-on-chips' and how they could be used in drug development, including target identification and validation, toxicity screening and stratified medicine.

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Organs-on-chips at the frontiers of drug discovery

Accurate and high throughput cell sorting is a critical enabling technology in molecular and cellular biology, biotechnology, and medicine While conventional methods can provide high efficiency sorting in short timescales, advances in microfluidics have enabled the realization of miniaturized devices offering similar capabilities that exploit a variety of physical principles We classify these technologies as either active or passive Active systems generally use external fields (eg, acoustic, electric, magnetic, and optical) to impose forces to displace cells for sorting, whereas passive systems use inertial forces, filters, and adhesion mechanisms to purify cell populations Cell sorting on microchips provides numerous advantages over conventional methods by reducing the size of necessary equipment, eliminating potentially biohazardous aerosols, and simplifying the complex protocols commonly associated with cell sorting Additionally, microchip devices are well suited for parallelization, enabling complete lab-on-a-chip devices for cellular isolation, analysis, and experimental processing In this review, we examine the breadth of microfluidic cell sorting technologies, while focusing on those that offer the greatest potential for translation into clinical and industrial practice and that offer multiple, useful functions We organize these sorting technologies by the type of cell preparation required (ie, fluorescent label-based sorting, bead-based sorting, and label-free sorting) as well as by the physical principles underlying each sorting mechanism

Microfluidic cell sorting: a review of the advances in the separation of cells from debulking to rare cell isolation

Microfluidic cell-based systems have enabled the study of cellular phenomena with improved spatiotemporal control of the microenvironment and at increased throughput. While poly(dimethylsiloxane) (PDMS) has emerged as the most popular material in microfluidics research, it has specific limitations that prevent microfluidic platforms from achieving their full potential. We present here a complete process, ranging from mold design to embossing and bonding, that describes the fabrication of polystyrene (PS) microfluidic devices with similar cost and time expenditures as PDMS-based devices. Emphasis was placed on creating methods that can compete with PDMS fabrication methods in terms of robustness, complexity, and time requirements. To achieve this goal, several improvements were made to remove critical bottlenecks in existing PS embossing methods. First, traditional lithographic techniques were adapted to fabricate bulk epoxy molds capable of resisting high temperatures and pressures. Second, a method was d...

Rapid Prototyping of Arrayed Microfluidic Systems in Polystyrene for Cell-Based Assays

Microfluidic kit-on-a-lid: a versatile platform for neutrophil chemotaxis assays.

Asthma is a chronic inflammatory disorder that affects more than 300 million people worldwide. Asthma management would benefit from additional tools that establish biomarkers to identify phenotypes of asthma. We present a microfluidic solution that discriminates asthma from allergic rhinitis based on a patient's neutrophil chemotactic function. The handheld diagnostic device sorts neutrophils from whole blood within 5 min, and generates a gradient of chemoattractant in the microchannels by placing a lid with chemoattractant onto the base of the device. This technology was used in a clinical setting to assay 34 asthmatic (n = 23) and nonasthmatic, allergic rhinitis (n = 11) patients to establish domains for asthma diagnosis based on neutrophil chemotaxis. We determined that neutrophils from asthmatic patients migrate significantly more slowly toward the chemoattractant compared with nonasthmatic patients (P = 0.002). Analysis of the receiver operator characteristics of the patient data revealed that using a chemotaxis velocity of 1.55 μm/min for asthma yields a diagnostic sensitivity and specificity of 96% and 73%, respectively. This study identifies neutrophil chemotaxis velocity as a potential biomarker for asthma, and we demonstrate a microfluidic technology that was used in a clinical setting to perform these measurements.

https://www.pnas.org/content/pnas/early/2014/04/03/1324043111.full.pdf

Characterizing asthma from a drop of blood using neutrophil chemotaxis

Improvements in neutrophil chemotaxis assays have advanced our understanding of the mechanisms of neutrophil recruitment; however, traditional methods limit biologic inquiry in important areas. We report a microfluidic technology that enables neutrophil purification and chemotaxis on-chip within minutes, using nanoliters of whole blood, and only requires a micropipette to operate. The low sample volume requirements and novel lid-based method for initiating the gradient of chemoattractant enabled the measurement of human neutrophil migration on a cell monolayer to probe the adherent and migratory states of neutrophils under inflammatory conditions; mouse neutrophil chemotaxis without sacrificing the animal; and both 2D and 3D neutrophil chemotaxis. First, the neutrophil chemotaxis on endothelial cells revealed 2 distinct neutrophil phenotypes, showing that endothelial cell-neutrophil interactions influence neutrophil chemotactic behavior. Second, we validated the mouse neutrophil chemotaxis assay by comparing the adhesion and chemotaxis of neutrophils from chronically inflamed and wild-type mice; we observed significantly higher neutrophil adhesion in blood obtained from chronically inflamed mice. Third, we show that 2D and 3D neutrophil chemotaxis can be directly compared using our technique. These methods allow for new avenues of research while reducing the complexity, time, and sample volume requirements to perform neutrophil chemotaxis assays.

Eric K. Sackmann

Papers

The present and future role of microfluidics in biomedical research

Rapid Prototyping of Arrayed Microfluidic Systems in Polystyrene for Cell-Based Assays

Microfluidic kit-on-a-lid: a versatile platform for neutrophil chemotaxis assays.

Characterizing asthma from a drop of blood using neutrophil chemotaxis

Microfluidic KIT-ON-A-LID: A versatile platform for neutrophil chemotaxis assays and asthma diagnostics