Use of 3D Printing and Modular Microfluidics to Integrate Cell Culture, Injections and Electrochemical Analysis

Microfluidics-assisted Tumor Cell Separation Approaches for Clinical Applications: An Overview on Emerging Devices

Recent advances in science and technology have led to revolutions in many scientific and industrial fields. The term lab on a chip, or in other words, performing a variety of complex analyses in just a short time and a minimal space, is a term that has become very common in recent years, and what used to be a dream has now come to life in practice. In this paper, we tried to investigate a specific type of lab technology on a chip, which is, of course, one of the most common, namely the knowledge and technology of cell separation by using a microfluidic technique that can be separated based on size and deformation, adhesion and electrical properties. The tissue of the human body is degraded due to injury or aging. It is often tried to treat this tissue disorder by using drugs, but they are not always enough. Stem cell-based medicine is a novel form that promises the restoration or regeneration of tissues and functioning organs. Although many models of microfluidic systems have been designed for cell separation, choosing the appropriate device to achieve a reliable result is a challenge. Therefore, in this study, Fluorescence Activated Cell Sorting (FACS), Dielectrophoresis (DEP), Magnetic Activated Cell Sorting (MACS), and Acoustic microfluidic system are four distinct categories of active microfluidic systems explored. Also, the advantages, disadvantages, and the current status of the devices mentioned in these methods are reviewed.

Modular microfluidics for life sciences

The advancement of microfluidics has enabled numerous discoveries and technologies in life sciences. However, due to the lack of industry standards and configurability, the design and fabrication of microfluidic devices require highly skilled technicians. The diversity of microfluidic devices discourages biologists and chemists from applying this technique in their laboratories. Modular microfluidics, which integrates the standardized microfluidic modules into a whole, complex platform, brings the capability of configurability to conventional microfluidics. The exciting features, including portability, on-site deployability, and high customization motivate us to review the state-of-the-art modular microfluidics and discuss future perspectives. In this review, we first introduce the working mechanisms of the basic microfluidic modules and evaluate their feasibility as modular microfluidic components. Next, we explain the connection approaches among these microfluidic modules, and summarize the advantages of modular microfluidics over integrated microfluidics in biological applications. Finally, we discuss the challenge and future perspectives of modular microfluidics.

Noncovalent reversible binding-enabled facile fabrication of leak-free PDMS microfluidic devices without plasma treatment for convenient cell loading and retrieval

The conventional approach for fabricating polydimethylsiloxane (PDMS) microfluidic devices is a lengthy and inconvenient procedure and may require a clean-room microfabrication facility often not readily available. Furthermore, living cells can't survive the oxygen-plasma and high-temperature-baking treatments required for covalent bonding to assemble multiple PDMS parts into a leak-free device, and it is difficult to disassemble the devices because of the irreversible covalent bonding. As a result, seeding/loading cells into and retrieving cells from the devices are challenging. Here, we discovered that decreasing the curing agent for crosslinking the PDMS prepolymer increases the noncovalent binding energy of the resultant PDMS surfaces without plasma or any other treatment. This enables convenient fabrication of leak-free microfluidic devices by noncovalent binding for various biomedical applications that require high pressure/flow rates and/or long-term cell culture, by simply hand-pressing the PDMS parts without plasma or any other treatment to bind/assemble. With this method, multiple types of cells can be conveniently loaded into specific areas of the PDMS parts before assembly and due to the reversible nature of the noncovalent bonding, the assembled device can be easily disassembled by hand peeling for retrieving cells. Combining with 3D printers that are widely available for making masters to eliminate the need of photolithography, this facile yet rigorous fabrication approach is much faster and more convenient for making PDMS microfluidic devices than the conventional oxygen plasma-baking-based irreversible covalent bonding method.

Modular Microfluidics: Current Status and Future Prospects

This review mainly studies the development status, limitations, and future directions of modular microfluidic systems. Microfluidic technology is an important tool platform for scientific research and plays an important role in various fields. With the continuous development of microfluidic applications, conventional monolithic microfluidic chips show more and more limitations. A modular microfluidic system is a system composed of interconnected, independent modular microfluidic chips, which are easy to use, highly customizable, and on-site deployable. In this paper, the current forms of modular microfluidic systems are classified and studied. The popular fabrication techniques for modular blocks, the major application scenarios of modular microfluidics, and the limitations of modular techniques are also discussed. Lastly, this review provides prospects for the future direction of modular microfluidic technologies.

3D printed microfluidic device for automated, pressure-driven, valve-injected microchip electrophoresis of preterm birth biomarkers

A 3D printed, automated, pressure-driven injection microfluidic system for microchip electrophoresis (µCE) of preterm birth (PTB)-related peptides and proteins has been developed. Functional microvalves were formed, either with a membrane thickness of 5 µm and a layer exposure time of 450 ms or with a membrane thickness of 10 µm and layer exposure times of 300-350 ms. These valves allowed for control of fluid flow in device microchannels during sample injection for µCE separation. Device design and µCE conditions using fluorescently labeled amino acids were optimized. A sample injection time of 0.5 s and a separation voltage of 450 V (460 V/cm) yielded the best separation efficiency and resolution. We demonstrated the first µCE separation with pressure-driven injection in a 3D printed microfluidic device using fluorescently labeled PTB biomarkers and 532 nm laser excitation. Detection limits for two PTB biomarkers, peptide 1 and peptide 2, for an injection time of 1.5 s were 400 pM and 15 nM, respectively, and the linear detection range for peptide 2 was 50-400 nM. This 3D printed microfluidic system holds promise for future integration of on-chip sample preparation processes with µCE, offering promising possibilities for PTB risk assessment.

Microfluidics for Electrochemical Energy Conversion

Electrochemical energy conversion is an important supplement for storage and on-demand use of renewable energy. In this regard, microfluidics offers prospects to raise the efficiency and rate of electrochemical energy conversion through enhanced mass transport, flexible cell design, and ability to eliminate the physical ion-exchange membrane, an essential yet costly element in conventional electrochemical cells. Since the 2002 invention of the microfluidic fuel cell, the research field of microfluidics for electrochemical energy conversion has expanded into a great variety of cell designs, fabrication techniques, and device functions with a wide range of utility and applications. The present review aims to comprehensively synthesize the best practices in this field over the past 20 years. The underlying fundamentals and research methods are first summarized, followed by a complete assessment of all research contributions wherein microfluidics was proactively utilized to facilitate energy conversion in conjunction with electrochemical cells, such as fuel cells, flow batteries, electrolysis cells, hybrid cells, and photoelectrochemical cells. Moreover, emerging technologies and analytical tools enabled by microfluidics are also discussed. Lastly, opportunities for future research directions and technology advances are proposed.

Configurable 3D Printed Microfluidic Multiport Valves with Axial Compression

In the last decade, the fabrication of microfluidic chips was revolutionized by 3D printing. It is not only used for rapid prototyping of molds, but also for manufacturing of complex chips and even integrated active parts like pumps and valves, which are essential for many microfluidic applications. The manufacturing of multiport injection valves is of special interest for analytical microfluidic systems, as they can reduce the injection to detection dead volume and thus enhance the resolution and decrease the detection limit. Designs reported so far use radial compression of rotor and stator. However, commercially available nonprinted valves usually feature axial compression, as this allows for adjustable compression and the possibility to integrate additional sealing elements. In this paper, we transfer the axial approach to 3D-printed valves and compare two different printing techniques, as well as six different sealing configurations. The tightness of the system is evaluated with optical examination, weighing, and flow measurements. The developed system shows similar performance to commercial or other 3D-printed valves with no measurable leakage for the static case and leakages below 0.5% in the dynamic case, can be turned automatically with a stepper motor, is easy to scale up, and is transferable to other printing methods and materials without design changes.

Recent innovations in cost-effective polymer and paper hybrid microfluidic devices

Hybrid microfluidic systems that are composed of multiple different types of substrates have been recognized as a versatile and superior platform, which can draw benefits from different substrates while avoiding their limitations. This review article introduces the recent innovations of different types of low-cost hybrid microfluidic devices, particularly focusing on cost-effective polymer- and paper-based hybrid microfluidic devices. In this article, the fabrication of these hybrid microfluidic devices is briefly described and summarized. We then highlight various hybrid microfluidic systems, including polydimethylsiloxane (PDMS)-based, thermoplastic-based, paper/polymer hybrid systems, as well as other emerging hybrid systems (such as thread-based). The special benefits of using these hybrid systems have been summarized accordingly. A broad range of biological and biomedical applications using these hybrid microfluidic devices are discussed in detail, including nucleic acid analysis, protein analysis, cellular analysis, 3D cell culture, organ-on-a-chip, and tissue engineering. The perspective trends of hybrid microfluidic systems involving the improvement of fabrication techniques and broader applications are also discussed at the end of the review.

Flexible Materials for High-Resolution 3D Printing of Microfluidic Devices with Integrated Droplet Size Regulation

We develop resins for high-resolution additive manufacturing of flexible micromaterials via projection microstereolithography (PμSL) screening formulations made from monomer 2-phenoxyethyl acrylate, the cross-linkers Ebecryl 8413, tri(propyleneglycol) diacrylate or 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, the photoabsorber Sudan 1, and the photoinitiator diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide. PμSL-printed polymer micromaterials made from this resin library are characterized regarding achievable layer thickness depending on UV exposure energy, and for mechanical as well as optical properties. The best-candidate resin from this screening approach allows for 3D-printing transparent microchannels with a minimum cross section of approximately 35 × 46 μm², which exhibit proper solvent resistance against water, isopropanol, ethanol, n-hexane, and HFE-7500. The mechanical properties are predestined for 3D-printing microfluidic devices with integrated functional units that require high material flexibility. Exemplarily, we design flexible microchannels for on-demand regulation of microdroplet sizes in microemulsion formation. Our two outlines of integrated droplet regulators operate by injecting defined volumes of air, which deform the droplet-forming microchannel cross-junction, and change the droplet size therein. With this study, we expand the library of functional resins for PμSL printing toward flexible materials with micrometer resolution and provide the basis for further exploration of these materials, e.g., as microstructured cell-culturing substrates with defined mechanics.

Electrospun Microfibers Modulate Intracellular Amino Acids in Liver Cells via Integrin β1

Although numerous recent studies have shown the importance of polymeric microfibrous extracellular matrices (ECMs) in maintaining cell behaviors and functions, the mechanistic nexus between ECMs and intracellular activities is largely unknown. Nevertheless, this knowledge will be critical in understanding and treating diseases with ECM remodeling. Therefore, we present our findings that ECM microstructures could regulate intracellular amino acid levels in liver cells mechanistically through integrin β1. Amino acids were studied because they are the fundamental blocks for protein synthesis and metabolism, two vital functions of liver cells. Two ECM conditions, flat and microfibrous, were prepared and studied. In addition to characterizing cell growth, albumin production, urea synthesis, and cytochrome p450 activity, we found that the microfibrous ECM generally upregulated the intracellular amino acid levels. Further explorations showed that cells on the flat substrate expressed more integrin β1 than cells on the microfibers. Moreover, after partially blocking integrin β1 in cells on the flat substrate, the intracellular amino acid levels were restored, strongly supporting integrin β1 as the linking mechanism. This is the first study to report that a non-biological polymer matrix could regulate intracellular amino acid patterns through integrin. The results will help with future therapy development for liver diseases with ECM changes (e.g., fibrosis).

Making quantitative biomicrofluidics from microbore tubing and 3D-printed adapters

Microfluidic technology has tremendously facilitated the development of in vitro cell cultures and studies. Conventionally, microfluidic devices are fabricated with extensive facilities by well-trained researchers, which hinder the widespread adoption of the technology for broader applications. Enlightened by the fact that low-cost microbore tubing is a natural microfluidic channel, we developed a series of adaptors in a toolkit that can twine, connect, organize, and configure the tubing to produce functional microfluidic units. Three subsets of the toolkit were thoroughly developed: the tubing and scoring tools, the flow adaptors, and the 3D cell culture suite. To demonstrate the usefulness and versatility of the toolkit, we assembled a microfluidic device and successfully applied it for 3D macrophage cultures, flow-based stimulation, and automated near real-time quantitation with new knowledge generated. Overall, we present a new technology that allows simple, fast, and robust assembly of customizable and scalable microfluidic devices with minimal facilities, which is broadly applicable to research that needs or could be enhanced by microfluidics.

3D-Printed, Modular, and Parallelized Microfluidic System with Customizable Scaffold Integration to Investigate the Roles of Basement Membrane Topography on Endothelial Cells

Because dysfunctions of endothelial cells are involved in many pathologies, in vitro endothelial cell models for pathophysiological and pharmaceutical studies have been a valuable research tool. Although numerous microfluidic-based endothelial models have been reported, they had the cells cultured on a flat surface without considering the possible three-dimensional (3D) structure of the native extracellular matrix (ECM). Endothelial cells rest on the basement membrane in vivo, which contains an aligned microfibrous topography. To better understand and model the cells, it is necessary to know if and how the fibrous topography can affect endothelial functions. With conventional fully integrated microfluidic apparatus, it is difficult to include additional topographies in a microchannel. Therefore, we developed a modular microfluidic system by 3D-printing and electrospinning, which enabled easy integration and switching of desired ECM topographies. Also, with standardized designs, the system allowed for high flow rates up to 4000 μL/min, which encompassed the full shear stress range for endothelial studies. We found that the aligned fibrous topography on the ECM altered arginine metabolism in endothelial cells and thus increased nitric oxide production. There has not been an endothelial model like this, and the new knowledge generated thereby lays a groundwork for future endothelial research and modeling.

Devices and techniques used to obtain and analyze three-dimensional cell cultures

Cell cultures are indispensable for both basic and applied research. Advancements in cell culture and analysis increase their utility for basic research and translational applications. A marked development in this direction is advent of three-dimensional (3D) cultures. The extent of advancement in 3D cell culture methods over the past decade has warranted referring to a single cell type being cultured as an aggregate or spheroid using simple scaffolds as "traditional." In recent years, the development of "next-generation" devices has enabled cultured cells to mimic their natural environments much better than the traditional 3D culture systems. Automated platforms like chip-based devices, magnetic- and acoustics-based assembly devices, di-electrophoresis (DEP), micro pocket cultures (MPoC), and 3D bio-printing provide a dynamic environment compared to the rather static conditions of the traditional simple scaffold-based 3D cultures. Chip-based technologies, which are centered on principles of microfluidics, are revolutionizing the ways in which cell culture and analysis can be compacted into table-top instruments. A parallel evolution in analytical devices enabled efficient assessment of various complex physiological and pathological endpoints. This is augmented by concurrent development of software enabling rapid large-scale automated data acquisition and analysis like image cytometry, elastography, optical coherence tomography, surface-enhanced Raman scattering (SERS), and biosensors. The techniques and devices utilized for the purpose of 3D cell culture and subsequent analysis depend primarily on the requirement of the study. We present here an in-depth account of the devices for obtaining and analyzing 3D cell cultures.

Microfibrous Extracellular Matrix Changes the Liver Hepatocyte Energy Metabolism via Integrins

Cell line-based liver models are critical tools for liver-related studies. However, the conventional monolayer culture of hepatocytes, the most widely used in vitro model, does not have the extracellular matrix (ECM), which contributes to the three-dimensional (3D) arrangement of the hepatocytes in the liver. As a result, the metabolic properties of the hepatocytes in the monolayer tissue culture may not accurately reflect those of the hepatocytes in the liver. Here, we developed a modular platform for 3D hepatocyte cultures on fibrous ECMs produced by electrospinning, a technique that can turn a polymer solution to the micro/nanofibers and has been widely used to produce scaffolds for 3D cell cultures. Metabolomics quantitation by liquid chromatography-mass spectrometry (LC-MS) indicated that Huh7 hepatocytes grown in microfibers electrospun from silk fibroin exhibited reduced glycolysis and tricarboxylic acid (TCA) cycle, as compared to the cells cultured as a monolayer. Further mechanistic studies suggested that integrins were correlated to the ECM's effects. This is the first time to report how an ECM scaffold could affect the fundamental metabolism of the hepatocytes via integrins.

Sealing 3D-printed parts to poly(dimethylsiloxane) for simple fabrication of Microfluidic devices

Microfluidics has revolutionized the fields of bioanalytical chemistry, cellular biology, and molecular biology. Advancements in microfluidic technologies, however, are often limited by labor, time, and resource-intensive fabrication methods, most commonly a form of photolithography. The advent of 3D printing has helped researchers fabricate proof-of-concept microfluidics more rapidly and at lower costs but suffers from poor resolution and tedious post-processing to remove uncured resin from enclosed channels. Additionally, custom resins and printers are often needed to create entirely enclosed channels, which increases cost and complexity of fabrication. In this work we demonstrate the ability to create microfluidic devices by covalently sealing 3D-printed parts with open-faced channels to polydimethylsiloxane (PDMS). Open-faced channels are easier to print than fully enclosed channels and can be printed using an inexpensive and commercially available stereolithography 3D printer and resin. The 3D-printed parts are sealed to PDMS, a common substrate used in traditional microfluidic fabrication, using two different techniques. The first involves coating the part with a commercially available silicone spray before sealing to PDMS via plasma treatment. In the second technique, the cured methacrylate resin is silanized with (3-Aminopropyl)triethoxysilane (APTES) before binding to PDMS with plasma treatment. Both methods create a strong seal between the two substrates, which is demonstrated with several types of microfluidic devices including droplet and gradient generators.

Development of an on-chip sample injection system with a 6-port valve incorporated in a microchip

Micro-flow-injection analysis (μFIA) is amenable to high-throughput systems with lower consumption of sample and reagent volumes. On-chip sample injection methods are important to prevent reduced analytical performance associated with dead volumes and diffusion of sample solutions. In this study, we have developed an on-chip sample injection system with a small-sized 6-port valve incorporated on a microchip. The valve is made with a 3D printer and is a simple structure that can be easily operated manually. A sample solution in a loading channel can be injected by switching the valve from the load to injection position. Sample injection tests using resorufin solutions revealed that samples can be injected below 100 μL min⁻¹, and the performance of the sample injection system is comparable to that of a commercially available injector. In addition, the sample injection system was successfully applied to a flow-based assay for hydrogen peroxide. The detection limit (3σ) of hydrogen peroxide was estimated to be 0.5 μM, and the assay time after sample injection was approximately 100 s. The developed sample injection system will be useful for various microfluidic-based analyses including μFIA.

We demonstrate on-chip sample injection using a 6-port valve incorporated in a microchip.

Automated Determination of As(III) in Waters with an Electrochemical Sensor Integrated into a Modular Microfluidic System

The presence of high levels of arsenic in waters poses a threat to the human health in many countries all over the world. Effective surveillance programs of water quality require the implementation of in-field tests to assess early the presence of this metal ion and other contaminants. To date, there exist few market-available analytical approaches that suffer from important limitations related to cost, in addition to complex reactions, very long analysis times, and/or high limits of detection. This work describes a robust electrochemical sensor integrated into a modular microfluidic system that shows a clear potential to be deployed for the on-site monitoring of inorganic As(III) species. Flexible and transparent microfluidic modules are fabricated by rapid prototyping techniques and include different microfluidic components among them, flow cells where electrochemical sensors can be easily and reversibly inserted. The electrochemical sensor comprises a gold nanoparticle (AuNP)-modified gold thin-film electrode that is readily applied to the sensitive detection of As(III) by anodic stripping linear sweep voltammetry. The microfluidic system enables the automatic sensor calibration, sample uptake, and preconditioning as well as As(III) detection. The system response to As(III) is linear in a concentration range of 1-150 μg L^-1, with a detection limit of 0.42 μg L^-1, which is well below the threshold value of 10 μg L^-1 set by the World Health Organization. Analysis of tap water and two water samples from two Argentinean aquifers, spiked with different As(III) concentrations, demonstrates the excellent performance of the system.

Review of 3D Cell Culture with Analysis in Microfluidic Systems

A review with 105 references that analyzes the emerging research area of 3D cell culture in microfluidic platforms with integrated detection schemes. Over the last several decades a central focus of cell culture has been the development of better in vivo mimics. This has led to the evolution from planar cell culture to cell culture on 3D scaffolds, and the incorporation of cell scaffolds into microfluidic devices. Specifically, this review explores the incorporation of suspension culture, hydrogels scaffolds, paper-based scaffolds, and fiber-based scaffolds into microfluidic platforms. In order to decrease analysis time, simplify sample preparation, monitor key signaling pathways involved in cell-to-cell communication or cell growth, and combat the limitations of sample volume/ dilution seen in traditional assays, researchers have also started to focus on integrating detection schemes into the cell culture devices. This review will highlight the work that has been performed towards combining these techniques and will discuss potential future directions. It is clear that microfluidic-based 3D cell culture coupled with quantitative analysis can greatly improve our ability to mimic and understand in vivo systems.