Conventional and emerging strategies for the fabrication and functionalization of PDMS-based microfluidic devices

A Preliminary Experimental Study of Polydimethylsiloxane (PDMS)-To-PDMS Bonding Using Oxygen Plasma Treatment Incorporating Isopropyl Alcohol

Polydimethylsiloxane (PDMS) is a widely used material for soft lithography and microfabrication. PDMS exhibits some promising properties suitable for building microfluidic devices; however, bonding PDMS to PDMS and PDMS to other materials for multilayer structures in microfluidic devices is still challenging due to the hydrophobic nature of the surface of PDMS. This paper presents a simple yet effective method to increase the bonding strength for PDMS-to-PDMS using isopropyl alcohol (IPA). The experiment was carried out to evaluate the bonding strength for both the natural-cured and the heat-cured PDMS layer. The results show the effectiveness of our approach in terms of the improved irreversible bonding strength, up to 3.060 MPa, for the natural-cured PDMS and 1.373 MPa for the heat-cured PDMS, while the best bonding strength with the existing method in literature is 1.9 MPa. The work is preliminary because the underlying mechanism is only speculative and open for future research.

Covalently Attached Slippery Surface Coatings to Reduce Protein Adsorptions on Poly(dimethylsiloxane) Planar Surfaces and 3D Microfluidic Channels

Silicone elastomers, such as poly(dimethylsiloxane) (PDMS), have a broad range of applications in basic biomedical research and clinical medicine, ranging from the preparation of microfluidic devices for organs-on-chips and ventriculoperitoneal shunts for the treatment of hydrocephalus to implantable neural probes for neuropharmacology. Despite the importance, the protein adsorptions on silicone elastomers in these application environments represent a significant challenge. Surface coatings with slippery lubricants, inspired by the Nepenthes pitcher plants, have recently received much attention for reducing protein adsorptions. Nevertheless, the depletion of the physically infused lubricants limits their broad applications. In this study, we report a covalently attached slippery surface coating to reduce protein adsorptions on PDMS surfaces. As demonstrations, we show that the adsorption of serum proteins, human fibrinogen and albumin, can be significantly reduced by the slippery surface coating in both planar PDMS surfaces and 3D microfluidic channels. The preparation of slippery surface coatings relies on the acid-catalyzed polycondensation reaction of dimethyldimethoxysilane, which utilizes a low-cost and scalable dip-coating method. Furthermore, cell metabolic activity and viability studies demonstrate the biocompatibility of the surface coating. These results suggest the potential applications of slippery surface coatings to reduce protein adsorptions for implantable medical devices, organs-on-chips, and many others.

Combinatorial Drug Screening Based on Massive 3D Tumor Cultures Using Micropatterned Array Chips

The establishment and application of a generalizable three-dimensional (3D) tumor device for high-throughput screening plays an important role in drug discovery and cancer therapeutics. In this study, we introduce a facile microplatform for considerable 3D tumor generation and combinatorial drug screening evaluation. High fidelity of chip fabrication was achieved depending on the simple and well-improved microcontact printing. We demonstrated the high stability and repeatability of the established tumor-on-a-chip system for controllable and massive production of 3D tumors with high size uniformity. Importantly, we accomplished the screening-like chemotherapy investigation involving individual and combinatorial drugs and validated the high accessibility and applicability of the system in 3D tumor-based manipulation and analysis on a large scale. This achievement in tumor-on-a-chip has potential applications in plenty of biomedical fields such as tumor biology, pharmacology, and tissue microengineering. It offers an insight into the development of the popularized microplatform with easy-to-fabricate and easy-to-operate properties for cancer exploration and therapy.

The Role of Microsphere Structures in Bottom-Up Bone Tissue Engineering

Bone defects have caused immense healthcare concerns and economic burdens throughout the world. Traditional autologous allogeneic bone grafts have many drawbacks, so the emergence of bone tissue engineering brings new hope. Bone tissue engineering is an interdisciplinary biomedical engineering method that involves scaffold materials, seed cells, and "growth factors". However, the traditional construction approach is not flexible and is unable to adapt to the specific shape of the defect, causing the cells inside the bone to be unable to receive adequate nourishment. Therefore, a simple but effective solution using the "bottom-up" method is proposed. Microspheres are structures with diameters ranging from 1 to 1000 µm that can be used as supports for cell growth, either in the form of a scaffold or in the form of a drug delivery system. Herein, we address a variety of strategies for the production of microspheres, the classification of raw materials, and drug loading, as well as analyze new strategies for the use of microspheres in bone tissue engineering. We also consider new perspectives and possible directions for future development.

Microfluidics for diagnosis and treatment of cardiovascular disease

Cardiovascular disease (CVD), a type of circulatory system disease related to the lesions of the cardiovascular system, has become one of the main diseases that endanger human health. Currently, the clinical diagnosis of most CVDs relies on a combination of imaging technology and blood biochemical test. However, the existing technologies for diagnosis of CVDs still have limitations in terms of specificity, detection range, and cost. In order to break through the current bottleneck, microfluidic with the advantages of low cost, simple instruments and easy integration, has been developed to play an important role in the early prevention, diagnosis and treatment of CVDs. Here, we have reviewed the recent various applications of microfluidic in the clinical diagnosis and treatment of CVDs, including microfluidic devices for detecting CVD markers, the cardiovascular models based on microfluidic, and the microfluidic used for CVDs drug screening and delivery. In addition, we have briefly looked forward to the prospects and challenges of microfluidics in diagnosis and treatment of CVDs.

Binary Graft of Poly(acrylic acid) and Poly(vinyl pyrrolidone) onto PDMS Films for Load and Release of Ciprofloxacin

Polymers are versatile compounds which physical and chemical properties can be taken advantage of in wide applications. Particularly, in the biomedical field, polydimethylsiloxane (PDMS) is one of the most used for its high biocompatibility, easy manipulation, thermal, and chemical stability. Nonetheless, its hydrophobic nature makes it susceptible to bacterial pollution, which represents a disadvantage in this field. A potential solution to this is through the graft of stimuli-sensitive polymers that, besides providing hydrophilicity, allow the creation of a drug delivery system. In this research, PDMS was grafted with acrylic acid (AAc) and vinyl pyrrolidone (VP) in two steps using gamma radiation. The resulting material was analyzed by several characterization techniques such as infrared spectroscopy (FTIR), swelling, contact angle, critical pH, and thermogravimetric analysis (TGA), demonstrating the presence of both polymers onto PDMS films and showing hydrophilic and pH-response properties. Among the performed methods to graft, the loading and release of ciprofloxacin were successful in those samples obtained by direct irradiation method. Furthermore, the antimicrobial assays showed zones of inhibition for microorganisms such as Staphylococcus aureus and Escherichia coli.

Breaking the clean room barrier: exploring low-cost alternatives for microfluidic devices

Microfluidics is an interdisciplinary field that encompasses both science and engineering, which aims to design and fabricate devices capable of manipulating extremely low volumes of fluids on a microscale level. The central objective of microfluidics is to provide high precision and accuracy while using minimal reagents and equipment. The benefits of this approach include greater control over experimental conditions, faster analysis, and improved experimental reproducibility. Microfluidic devices, also known as labs-on-a-chip (LOCs), have emerged as potential instruments for optimizing operations and decreasing costs in various of industries, including pharmaceutical, medical, food, and cosmetics. However, the high price of conventional prototypes for LOCs devices, generated in clean room facilities, has increased the demand for inexpensive alternatives. Polymers, paper, and hydrogels are some of the materials that can be utilized to create the inexpensive microfluidic devices covered in this article. In addition, we highlighted different manufacturing techniques, such as soft lithography, laser plotting, and 3D printing, that are suitable for creating LOCs. The selection of materials and fabrication techniques will depend on the specific requirements and applications of each individual LOC. This article aims to provide a comprehensive overview of the numerous alternatives for the development of low-cost LOCs to service industries such as pharmaceuticals, chemicals, food, and biomedicine.

Highly stretchable and sensitive strain sensors based on modified PDMS and hybrid particles of AgNWs/graphene

High-performance strain sensors have received extensive attention due to their wide range of applications in pulsebeat detection, speech recognition, motion detection, and blood pressure monitoring. However, it is difficult to simultaneously attain high sensitivity and excellent stretchability. In this work, a strain sensor based on modified polydimethylsiloxane (PDMS) and conductive hybrid particles of silver nanowires (AgNWs)/graphene was successfully fabricated. A facile solvothermal polymerization process was used to change the structure of cross-linking networks and to obtain the PDMS elastomer with excellent stretchability. The application of the modified PDMS endows the strain sensor with a large strain range (∼20%), which is 100% higher than that of the strain sensor with unmodified PDMS. The AgNWs/graphene hybrid particles were prepared by a simple coprecipitation, reduction, and drying method. AgNWs serve as bridges between graphene sheets, endowing the strain sensor with a large gauge factor (GF = 400). The stability of the strain sensor was also verified. Besides, the strain sensor was successfully used in fields such as finger bending and speech recognition. Considering its high sensitivity, excellent stretchability, and high working stability, the sensor has great potential in health monitoring and motion detection.

Ferrocene-Based Terpolyamides and Their PDMS-Containing Block Copolymers: Synthesis and Physical Properties

Aromatic polyamides are well-known as high-performance materials due to their outstanding properties making them useful in a wide range of applications. However, their limited solubility in common organic solvents restricts their processability and becomes a hurdle in their applicability. This study is focused on the synthesis of processable ferrocene-based terpolyamides and their polydimethylsiloxane (PDMS)-containing block copolymers, using low-temperature solution polycondensation methodology. All the synthesized materials were structurally characterized using FTIR and ¹H NMR spectroscopic techniques. The ferrocene-based terpolymers and block copolymers were soluble in common organic solvents, while the organic analogs were found only soluble in sulfuric acid. WXRD analysis showed the amorphous nature of the materials, while the SEM analysis exposed the modified surface of the ferrocene-based block copolymers. The structure-property relationship of the materials was further elucidated by their water absorption and thermal behavior. These materials showed low to no water absorption along with their high limiting oxygen index (LOI) values depicting their good flame-retardant behavior. DFT studies also supported the role of various monomers in the polycondensation reaction where the electron pair donation from HOMO of diamine monomer to the LUMO of acyl chloride was predicted, along with the calculation of various other parameters of the representative terpolymers and block copolymers.

Recent Trends of Microfluidics in Food Science and Technology: Fabrications and Applications

The development of novel materials with microstructures is now a trend in food science and technology. These microscale materials may be applied across all steps in food manufacturing, from raw materials to the final food products, as well as in the packaging, transport, and storage processes. Microfluidics is an advanced technology for controlling fluids in a microscale channel (1~100 μm), which integrates engineering, physics, chemistry, nanotechnology, etc. This technology allows unit operations to occur in devices that are closer in size to the expected structural elements. Therefore, microfluidics is considered a promising technology to develop micro/nanostructures for delivery purposes to improve the quality and safety of foods. This review concentrates on the recent developments of microfluidic systems and their novel applications in food science and technology, including microfibers/films via microfluidic spinning technology for food packaging, droplet microfluidics for food micro-/nanoemulsifications and encapsulations, etc.

Magnetically Targeted, Water-Triggered, Self-Healing Microcapsules Based on Microfluidic Techniques Enabling Targeted Healing of Water Tree Damage in Epoxy Resins

Repairing the micro-scale damage of insulating materials under a strong electric field has long been a highly desired but challenging task. Among all kinds of damage, water tree damage in the insulating materials of electrical equipment and electronic devices working in humid environments has long been considered irreparable. The main challenge is that residual water prevents the healing agent from filling the water tree branch channel. To solve this problem, this work reports a magnetically targeted, water-triggered, self-healing microcapsule (MTWTSH-MC) that makes a breakthrough against water tree damage based on microfluidic techniques. Targeted microcapsules driven by a directional magnetic field are concentrated to the vulnerable area of the insulating materials, exerting very limited effects on the dielectric. When damage breaks the microcapsules, the healing agent releases and quickly fills the damage channel and then reacts with water in the air or in the branch channel of the water tree, achieving solidification of the healing agent and self-healing of the damage channels. In this way, we can realize self-perception, self-triggering, and self-healing for both mechanical damage and water tree damage in insulation materials without any external stimulation.

Development of a transferable coarse-grained model of polydimethylsiloxane

Polydimethylsiloxane (PDMS) is a popular silicon-based polymer with advanced applications in microfluidics and nanocomposites. The slow dynamics of polymer chains in such complex systems hinders molecular dynamics investigations based on all atom force fields. This limitation can be overcome by exploiting finely tuned coarse-grained (CG) models. This paper develops a transferable CG model of PDMS, compatible with the recent Martini 3 force field, using structural and thermodynamic properties as targets in the parametrization, including a vast set of experimental free energies of transfer. We validate the model transferability by reproducing the correct scaling laws for the PDMS gyration radius in the melt and good and bad solvents. We successfully test the model by reproducing the wetting behavior of water and acetonitrile on PDMS and the phase behavior of a PDMS-peptide triblock copolymer system. This work sets the stage for computational studies involving the interaction between PDMS and many synthetic and biological molecules modeled within the Martini framework.

The CaT stretcher: An open-source system for delivering uniaxial strain to cells and tissues (CaT)

Integration of mechanical cues in conventional 2D or 3D cell culture platforms is an important consideration for in vivo and ex vivo models of lung health and disease. Available commercial and published custom-made devices are frequently limited in breadth of applications, scalability, and customization. Herein we present a technical report on an open-source, cell and tissue (CaT) stretcher, with modularity for different in vitro and ex vivo systems, that includes the following features: 1) Programmability for modeling different breathing patterns, 2) scalability to support low to high-throughput experimentation, and 3) modularity for submerged cell culture, organ-on-chips, hydrogels, and live tissues. The strategy for connecting the experimental cell or tissue samples to the stretching device were designed to ensure that traditional biomedical outcome measurements including, but not limited to microscopy, soluble mediator measurement, and gene and protein expression remained possible. Lastly, to increase the uptake of the device within the community, the system was built with economically feasible and available components. To accommodate diverse in vitro and ex vivo model systems we developed a variety of chips made of compliant polydimethylsiloxane (PDMS) and optimized coating strategies to increase cell adherence and viability during stretch. The CaT stretcher was validated for studying mechanotransduction pathways in lung cells and tissues, with an increase in alpha smooth muscle actin protein following stretch for 24 h observed in independent submerged monolayer, 3D hydrogel, and live lung tissue experiments. We anticipate that the open-source CaT stretcher design will increase accessibility to studies of the dynamic lung microenvironment through direct implementation by other research groups or custom iterations on our designs.

An Asymmetric Microfluidic/Chitosan Device for Sustained Drug Release in Guided Bone Regeneration Applications

One of the major challenges of guided bone regeneration (GBR) is infections caused by pathogen colonization at wound sites. In this paper, an asymmetric microfluidic/chitosan device was developed to release drugs to inhibit infections and to ensure that guided bone regeneration can be realized. The microfluidic technique was introduced into the GBR membrane for the first time, which demonstrated more controllable drug release, more flexible clinical use and had a lower cost compared with surface treatments and embedded nanoparticles. Based on the theory of diffusion and Fick’s first law, the contact area and concentration gradient were adjusted to realize sustained drug release. The standard deviation of minocycline release over 5 days was only 12.7%, which was lower than the joint effect of porous chitosan discs and nanospheres. The in vitro experiments against E. coli and Streptococcus mutans showed the excellent antibacterial performance of the device (>95%). The in vitro experiments for fibroblasts at the microfluidic side and osteoblasts at the chitosan side showed the satisfactory biocompatibility and the ability of the device to enhance bone regeneration. Therefore, this microfluidic/chitosan device is a promising therapeutic approach to prevent infection and guide bone regeneration.

Blood-brain barrier (BBB)-on-a-chip: a promising breakthrough in brain disease research

The blood-brain barrier (BBB) represents a key challenge in developing brain-penetrating therapeutic molecules. BBB dysfunction is also associated with the onset and progression of various brain diseases. The BBB-on-a-chip (μBBB), an organ-on-chip technology, has emerged as a powerful in vitro platform that closely mimics the human BBB microenvironments. While the μBBB technology has seen wide application in the study of brain cancer, its utility in other brain disease models ("μBBB+") is less appreciated. Based on the advances of the μBBB technology and the evolution of in vitro models for brain diseases over the last decade, we propose the concept of a "μBBB+" system and summarize its major promising applications in pathological studies, personalized medical research, drug development, and multi-organ-on-chip approaches. We believe that such a sophisticated "μBBB+" system is a highly tunable and promising in vitro platform for further advancement of the understanding of brain diseases.

The Fabrication and Bonding of Thermoplastic Microfluidics: A Review

Various fields within biomedical engineering have been afforded rapid scientific advancement through the incorporation of microfluidics. As literature surrounding biological systems become more comprehensive and many microfluidic platforms show potential for commercialization, the development of representative fluidic systems has become more intricate. This has brought increased scrutiny of the material properties of microfluidic substrates. Thermoplastics have been highlighted as a promising material, given their material adaptability and commercial compatibility. This review provides a comprehensive discussion surrounding recent developments pertaining to thermoplastic microfluidic device fabrication. Existing and emerging approaches related to both microchannel fabrication and device assembly are highlighted, with consideration toward how specific approaches induce physical and/or chemical properties that are optimally suited for relevant real-world applications.

A Protocol for Fabrication and on-Chip Cell Culture to Recreate PAH-Afflicted Pulmonary Artery on a Microfluidic Device

Pulmonary arterial hypertension (PAH) is a rare pulmonary vascular disease that affects people of all ethnic origins and age groups including newborns. In PAH, pulmonary arteries and arterioles undergo a series of pathological changes including remodeling of the entire pulmonary vasculatures and extracellular matrices, mis-localized growth of pulmonary arterial cells, and development of glomeruloid-like lesions called plexiform lesions. Traditionally, various animal and cellular models have been used to understand PAH pathophysiology, investigate sex-disparity in PAH and monitor therapeutic efficacy of PAH medications. However, traditional models can only partially capture various pathological features of PAH, and they are not adaptable to combinatorial study design for deciphering intricately intertwined complex cellular processes implicated in PAH pathogenesis. While many microfluidic chip-based models are currently available for major diseases, no such disease-on-a-device model is available for PAH, an under investigated disease. In the absence of any chip-based models of PAH, we recently proposed a five-channel polydimethylsiloxane (PDMS)-based microfluidic device that can emulate major pathological features of PAH. However, our proposed model can make a bigger impact on the PAH field only when the larger scientific community engaged in PAH research can fabricate the device and develop the model in their laboratory settings. With this goal in mind, in this study, we have described the detailed methodologies for fabrication and development of the PAH chip model including a thorough explanation of scientific principles for various steps for chip fabrication, a detailed list of reagents, tools and equipment along with their source and catalogue numbers, description of laboratory setup, and cautionary notes. Finally, we explained the methodologies for on-chip cell seeding and application of this model for studying PAH pathophysiology. We believe investigators with little or no training in microfluidic chip fabrication can fabricate this eminently novel PAH-on-a-chip model. As such, this study will have a far-reaching impact on understanding PAH pathophysiology, unravelling the biological mystery associated with sexual dimorphism in PAH, and developing PAH therapy based on patient sex and age.

High sensitivity detection of SARS-CoV-2 by an optofluidic hollow eccentric core fiber

Since the outbreak of coronavirus disease 2019 (COVID-19), efficient real-time monitoring has become one of the challenges faced in SARS-CoV-2 virus detection. A compact all-fiber Mach-Zehnder interferometer optofluidic sensor based on a hollow eccentric core fiber (HECF) for the detection and real-time monitoring of SARS-CoV-2 spike glycoprotein (SARS-CoV-2 S2) is proposed, analyzed and demonstrated. The sensor is comprised of fusion splicing single mode fiber (SMF), hollow core fiber (HCF) and HECF. After the incident light passes through the HCF from the SMF, it uniformly enters the air hole and the suspended micrometer-scale fiber core of the HECF to form a compact all-fiber Mach-Zehnder interferometer (MZI). HECF is side polished to remove part of the cladding that the suspended fiber core can contact the external environment. Subsequently, the mouse anti SARS-CoV-2 S2 antibody is fixed on the surface of the suspended-core for the sake of achieving high sensitivity and specific sensing of SARS-CoV-2 S2. The limit of detection (LOD) of the sensor is 26.8 pM. The proposed sensor has high sensitivity, satisfactory selectivity, and can be fabricated at low cost making it highly suitable for point-of-care testing and high-throughput detection of early stage of COVID-19 infection.

Organ-On-A-Chip Models of the Blood-Brain Barrier: Recent Advances and Future Prospects

The human brain and central nervous system (CNS) present unique challenges in drug development for neurological diseases. One major obstacle is the blood-brain barrier (BBB), which hampers the effective delivery of therapeutic molecules into the brain while protecting it from blood-born neurotoxic substances and maintaining CNS homeostasis. For BBB research, traditional in vitro models rely upon Petri dishes or Transwell systems. However, these static models lack essential microenvironmental factors such as shear stress and proper cell-cell interactions. To this end, organ-on-a-chip (OoC) technology has emerged as a new in vitro modeling approach to better recapitulate the highly dynamic in vivo human brain microenvironment so-called the neural vascular unit (NVU). Such BBB-on-a-chip models have made substantial progress over the last decade, and concurrently there has been increasing interest in modeling various neurological diseases such as Alzheimer's disease and Parkinson's disease using OoC technology. In addition, with recent advances in other scientific technologies, several new opportunities to improve the BBB-on-a-chip platform via multidisciplinary approaches are available. In this review, an overview of the NVU and OoC technology is provided, recent progress and applications of BBB-on-a-chip for personalized medicine and drug discovery are discussed, and current challenges and future directions are delineated.

Contamination and carryover free handling of complex fluids using lubricant-infused pipette tips

Cross-contamination of biological samples during handling and preparation, is a major issue in laboratory setups, leading to false-positives or false-negatives. Sample carryover residue in pipette tips contributes greatly to this issue. Most pipette tips on the market are manufactured with hydrophobic polymers that are able to repel high surface tension liquids, yet they lack in performance when low surface tension liquids and viscous fluids are involved. Moreover, hydrophobicity of pipette tips can result in hydrophobic adsorption of biomolecules, causing inaccuracies and loss in precision during pipetting. Here we propose the use of lubricant-infused surface (LIS) technology to achieve omniphobic properties in pipette tips. Using a versatile and simple design, the inner lumen of commercially available pipette tips was coated with a fluorosilane (FS) layer using chemical vapor deposition (CVD). The presence of FS groups on the tips is confirmed by x-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) tests. After lubrication of the tips through a fluorinated lubricant, the omniphobicity and repellent behaviour of the tips drastically enhanced which are revealed via static and hysteresis contact angle measurements. The repellency of the lubricant-infused pipette tips against physical adsorption is investigated through pipetting a food coloring dye as well as human blood samples and are compared to the untreated tips. The results show significantly less amount carryover residue when the lubricant-infused tips are utilized compared to commercially available ones. We also demonstrate the lubricant-infused tips reduce bacteria contamination of the inner lumen by 3 to 6-log (over 99%, depending on the tip size) after pipetting up and down the bacteria solution.

Emerging 3D printing technologies and methodologies for microfluidic development

This review paper examines recent (mostly 2018 or later) advancements in 3D printed microfluidics. Microfluidic devices are widely applied in various fields such as drug delivery, point-of-care diagnosis, and bioanalytical research. In addition to soft lithography, 3D printing has become an appealing technology to develop microfluidics recently. In this work, three main 3D printing technologies, stereolithography, fused filament deposition, and polyjet, which are commonly used to fabricate microfluidic devices, are thoroughly discussed. The advantages, limitations, and recent microfluidic applications are analyzed. New technical advancements within these technology frameworks are also summarized, which are especially suitable for microfluidic development. Next, new emerging 3D-printing technologies are introduced, including the direct printing of polydimethylsiloxane (PDMS), glass, and biopolymers. Although limited microfluidic applications based on these technologies can be found in the literature, they show high potential to revolutionize the next generation of 3D-printed microfluidic apparatus.

Advances in organ-on-a-chip systems for modelling joint tissue and osteoarthritic diseases

Joint-on-a-chip (JOC) models are powerful tools that aid in osteoarthritis (OA) research. These microfluidic devices apply emerging organ-on-a-chip technology to recapitulate a multifaceted joint tissue microenvironment. JOCs address the need for advanced, dynamic in vitro models that can mimic the in vivo tissue environment through joint-relevant biomechanical or fluidic integration, an aspect that existing in vitro OA models lack. There are existing review articles on OA models that focus on animal, tissue explant, and two-dimensional and three-dimensional (3D) culture systems, including microbioreactors and 3D printing technology, but there has been limited discussion of JOC models. The aim of this article is to review recent developments in human JOC technology and identify gaps for future advancements. Specifically, mechanical stimulation systems that mimic articular movement, multi-joint tissue cultures that enable crosstalk, and systems that aim to capture aspects of OA inflammation by incorporating immune cells are covered. The development of an advanced JOC model that captures the dynamic joint microenvironment will improve testing and translation of potential OA therapeutics.

Versatile and Low-Cost Fabrication of Modular Lock-and-Key Microfluidics for Integrated Connector Mixer Using a Stereolithography 3D Printing

We present a low-cost and simple method to fabricate a novel lock-and-key mixer microfluidics using an economic stereolithography (SLA) three-dimensional (3D) printer, which costs less than USD 400 for the investment. The proposed study is promising for a high throughput fabrication module, typically limited by conventional microfluidics fabrications, such as photolithography and polymer-casting methods. We demonstrate the novel modular lock-and-key mixer for the connector and its chamber modules with optimized parameters, such as exposure condition and printing orientation. In addition, the optimization of post-processing was performed to investigate the reliability of the fabricated hollow structures, which are fundamental to creating a fluidic channel or chamber. We found out that by using an inexpensive 3D printer, the fabricated resolution can be pushed down to 850 µm and 550 µm size for squared- and circled-shapes, respectively, by the gradual hollow structure, applying vertical printing orientation. These strategies opened up the possibility of developing straightforward microfluidics platforms that could replace conventional microfluidics mold fabrication methods, such as photolithography and milling, which are costly and time consuming. Considerably cheap commercial resin and its tiny volume employed for a single printing procedure significantly cut down the estimated fabrication cost to less than 50 cents USD/module. The simulation study unravels the prominent properties of the fabricated devices for biological fluid mixers, such as PBS, urine and plasma blood. This study is eminently prospective toward microfluidics application in clinical biosensing, where disposable, low-cost, high-throughput, and reproducible chips are highly required.

Surface Wettability for Skin-Interfaced Sensors and Devices

The practical applications of skin-interfaced sensors and devices in daily life hinge on the rational design of surface wettability to maintain device integrity and achieve improved sensing performance under complex hydrated conditions. Various bio-inspired strategies have been implemented to engineer desired surface wettability for varying hydrated conditions. Although the bodily fluids can negatively affect the device performance, they also provide a rich reservoir of health-relevant information and sustained energy for next-generation stretchable self-powered devices. As a result, the design and manipulation of the surface wettability are critical to effectively control the liquid behavior on the device surface for enhanced performance. The sensors and devices with engineered surface wettability can collect and analyze health biomarkers while being minimally affected by bodily fluids or ambient humid environments. The energy harvesters also benefit from surface wettability design to achieve enhanced performance for powering on-body electronics. In this review, we first summarize the commonly used approaches to tune the surface wettability for target applications toward stretchable self-powered devices. By considering the existing challenges, we also discuss the opportunities as a small fraction of potential future developments, which can lead to a new class of skin-interfaced devices for use in digital health and personalized medicine.

A Novel Fluidic Platform for Semi-Automated Cell Culture into Multiwell-like Bioreactors

In this work, we developed and characterized a novel fluidic platform that enables long-term in vitro cell culture in a semi-automated fashion. The system is constituted by a control unit provided with a piezoelectric pump, miniaturized valves, and a microfluidic network for management and fine control of reagents’ flow, connected to a disposable polymeric culture unit resembling the traditional multiwell-like design. As a proof of principle, Human Umbilical Vein Endothelial Cells (HUVEC) and Human Mesenchymal Stem Cells (hMSC) were seeded and cultured into the cell culture unit. The proliferation rate of HUVEC and the osteogenic differentiation of hMSC were assessed and compared to standard culture in Petri dishes. The results obtained demonstrated that our approach is suitable to perform semi-automated cell culture protocols, minimizing the contribution of human operators and allowing the standardization and reproducibility of the procedures. We believe that the proposed system constitutes a promising solution for the realization of user-friendly automated control systems that will favor the standardization of cell culture processes for cell factories, drug testing, and biomedical research.

Live single cell imaging assays in glass microwells produced by laser-induced deep etching

Miniaturization of cell culture substrates enables controlled analysis of living cells in confined micro-scale environments. This is particularly suitable for imaging individual cells over time, as they can be monitored without escaping the imaging field-of-view (FoV). Glass materials are ideal for most microscopy applications. However, with current methods used in life sciences, glass microfabrication is limited in terms of either freedom of design, quality, or throughput. In this work, we introduce laser-induced deep etching (LIDE) as a method for producing glass microwell arrays for live single cell imaging assays. We demonstrate novel microwell arrays with deep, high-aspect ratio wells that have rounded, dimpled or flat bottom profiles in either single-layer or double-layer glass chips. The microwells are evaluated for microscopy-based analysis of long-term cell culture, clonal expansion, laterally organized cell seeding, subcellular mechanics during migration and immune cell cytotoxicity assays of both adherent and suspension cells. It is shown that all types of microwells can support viable cell cultures and imaging with single cell resolution, and we highlight specific benefits of each microwell design for different applications. We believe that high-quality glass microwell arrays enabled by LIDE provide a great option for high-content and high-resolution imaging-based live cell assays with a broad range of potential applications within life sciences.

Bio-functionalization of microfluidic platforms made of thermoplastic materials: A review

As a result of their favorable physical and chemical characteristics, thermoplastics have garnered significant interest in the area of microfluidics. The moldable nature of these inexpensive polymers enables easy fabrication, while their durability and chemical stability allow for resistance to high shear stress conditions and functionalization, respectively. This review provides a comprehensive examination several commonly used thermoplastic polymers in the microfluidics space including poly(methyl methacrylate) (PMMA), cyclic olefin polymer (COP) and copolymer (COC), polycarbonates (PC), poly(ethylene terephthalate) (PET), polystyrene (PS), poly(ethylene glycol) (PEG), polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), and polyester. We describe various biofunctionalization strategies applied within thermoplastic microfluidic platforms and their resultant applications. Lastly, emerging technologies with a focus on applying recently developed microfluidic and biofunctionalization strategies into thermoplastic systems are discussed.

An integrated plastic microchip for enhancing electrophoretic separation using tunable pressure-driven backflows

Microfluidic CE (MCE) is an effective solution for rapid and sensitive determination of multiple analytes. Herein, a dynamic coated cyclic olefin copolymer microchip was developed having an on-chip micropump for fluid velocity adjusting in electrophoretic separations. This micropump was fabricated by constructing a polyacrylamide gel membrane at one channel terminal. Once applying electric field across the membrane, a pressure-driven flow generated automatically to balance the electroosmotic flow (EOF) mismatch at the channel-membrane interface. The influence of gel precursor concentration and operating voltages on the fluid velocity was carefully evaluated. Moreover, the highly integration of injection, separation, and pumping units of the MCE system minimized the dead volume and provides satisfied column efficiency. Experiments showed that by adjusting of pumping voltage reduced the fluid velocity by a factor of 6, resulting six- and threefold resolving power enhancements of rhodamine dye mixture and amino acid mixture, respectively. Furthermore, the developed MCE method was applied for rhodamines and amino acids quantitation in food and cosmetics, with standard addition recoveries of 87.3-106.9% and 89.9-117.4%, respectively. These results were also confirmed by standard HPLC method, revealing the application potential in fast and onsite analysis of complex samples.

Skin-on-a-Chip Technology: Microengineering Physiologically Relevant In Vitro Skin Models

The increased demand for physiologically relevant in vitro human skin models for testing pharmaceutical drugs has led to significant advancements in skin engineering. One of the most promising approaches is the use of in vitro microfluidic systems to generate advanced skin models, commonly known as skin-on-a-chip (SoC) devices. These devices allow the simulation of key mechanical, functional and structural features of the human skin, better mimicking the native microenvironment. Importantly, contrary to conventional cell culture techniques, SoC devices can perfuse the skin tissue, either by the inclusion of perfusable lumens or by the use of microfluidic channels acting as engineered vasculature. Moreover, integrating sensors on the SoC device allows real-time, non-destructive monitoring of skin function and the effect of topically and systemically applied drugs. In this Review, the major challenges and key prerequisites for the creation of physiologically relevant SoC devices for drug testing are considered. Technical (e.g., SoC fabrication and sensor integration) and biological (e.g., cell sourcing and scaffold materials) aspects are discussed. Recent advancements in SoC devices are here presented, and their main achievements and drawbacks are compared and discussed. Finally, this review highlights the current challenges that need to be overcome for the clinical translation of SoC devices.

Advantages of optical fibers for facile and enhanced detection in droplet microfluidics

Droplet microfluidics offers a unique opportunity for ultrahigh-throughput experimentation with minimal sample consumption and thus has obtained increasing attention, particularly for biological applications. Detection and measurements of analytes or biomarkers in tiny droplets are essential for proper analysis of biological and chemical assays like single-cell studies, cytometry, nucleic acid detection, protein quantification, environmental monitoring, drug discovery, and point-of-care diagnostics. Current detection setups widely use microscopes as a central device and other free-space optical components. However, microscopic setups are bulky, complicated, not flexible, and expensive. Furthermore, they require precise optical alignments, specialized optical and technical knowledge, and cumbersome maintenance. The establishment of efficient, simple, and cheap detection methods is one of the bottlenecks for adopting microfluidic strategies for diverse bioanalytical applications and widespread laboratory use. Together with great advances in optofluidic components, the integration of optical fibers as a light guiding medium into microfluidic chips has recently revolutionized analytical possibilities. Optical fibers embedded in a microfluidic platform provide a simpler, more flexible, lower-cost, and sensitive setup for the detection of several parameters from biological and chemical samples and enable widespread, hands-on application much beyond thriving point-of-care developments. In this review, we examine recent developments in droplet microfluidic systems using optical fiber as a light guiding medium, primarily focusing on different optical detection methods such as fluorescence, absorbance, light scattering, and Raman scattering and the potential applications in biochemistry and biotechnology that are and will be arising from this.

A pump-free microfluidic device for fast magnetic labeling of ischemic stroke biomarkers

This research proposes a low-cost and simple operation microfluidic chip to enhance the magnetic labeling efficiency of two ischemic stroke biomarkers: cellular fibronectin (c-Fn) and matrix metallopeptidase 9 (MMP9). This fully portable and pump-free microfluidic chip is operated based on capillary attractions without any external power source and battery. It uses an integrated cellulose sponge to absorb the samples. At the same time, a magnetic field is aligned to hold the target labeled by the magnetic nanoparticles (MNPs) in the pre-concentrated chamber. By using this approach, the specific targets are labeled from the beginning of the sampling process without preliminary sample purification. The proposed study enhanced the labeling efficiency from 1 h to 15 min. The dynamic interactions occur in the serpentine channel, while the crescent formation of MNPs in the pre-concentrated chamber, acting as a magnetic filter, improves the biomarker-MNP interaction. The labeling optimization by the proposed device influences the dynamic range by optimizing the MNP ratio to fit the linear range across the clinical cutoff value. The limits of detection (LODs) of 2.8 ng/mL and 54.6 ng/mL of c-Fn measurement were achieved for undiluted and four times dilutions of MNP, respectively. While for MMP9, the LODs were 11.5 ng/mL for undiluted functionalized MNP and 132 ng/mL for four times dilutions of functionalized MNP. The results highlight the potential use of this device for clinical sample preparation and specific magnetic target labeling. When combined with a detection system, it could also be used as an integrated component of a point-of-care platform.

Micro-cavities on PDMS microchannel replicated from sandpaper templates trap cells to enhance cell adhesion and proliferation

Polydimethylsiloxane (PDMS) launched its fame in constructing micro-devices for studying cell growth, cell-cell interaction, assembling organ-on-chip models because of its excellency in optical transparency, gas permeability, nontoxicity, elastics, and well-developed...

A biomimetic anti-biofouling coating in nanofluidic channels

A biomimetic coating using a tailored phosphorylcholine-containing monomer enables to suppress non-specific protein adsorption in nanofluidic channels, paving a way to explore a new anti-biofouling strategy using monomer-based materials for nanodevices.

Rapid Prototyping of Organ-on-a-Chip Devices Using Maskless Photolithography

Organ-on-a-chip (OoC) and microfluidic devices are conventionally produced using microfabrication procedures that require cleanrooms, silicon wafers, and photomasks. The prototyping stage often requires multiple iterations of design steps. A simplified prototyping process could therefore offer major advantages. Here, we describe a rapid and cleanroom-free microfabrication method using maskless photolithography. The approach utilizes a commercial digital micromirror device (DMD)-based setup using 375 nm UV light for backside exposure of an epoxy-based negative photoresist (SU-8) on glass coverslips. We show that microstructures of various geometries and dimensions, microgrooves, and microchannels of different heights can be fabricated. New SU-8 molds and soft lithography-based polydimethylsiloxane (PDMS) chips can thus be produced within hours. We further show that backside UV exposure and grayscale photolithography allow structures of different heights or structures with height gradients to be developed using a single-step fabrication process. Using this approach: (1) digital photomasks can be designed, projected, and quickly adjusted if needed; and (2) SU-8 molds can be fabricated without cleanroom availability, which in turn (3) reduces microfabrication time and costs and (4) expedites prototyping of new OoC devices.

Properties and Applications of PDMS for Biomedical Engineering: A Review

Polydimethylsiloxane (PDMS) is an elastomer with excellent optical, electrical and mechanical properties, which makes it well-suited for several engineering applications. Due to its biocompatibility, PDMS is widely used for biomedical purposes. This widespread use has also led to the massification of the soft-lithography technique, introduced for facilitating the rapid prototyping of micro and nanostructures using elastomeric materials, most notably PDMS. This technique has allowed advances in microfluidic, electronic and biomedical fields. In this review, an overview of the properties of PDMS and some of its commonly used treatments, aiming at the suitability to those fields' needs, are presented. Applications such as microchips in the biomedical field, replication of cardiovascular flow and medical implants are also reviewed.

Super-Hydrophilic Zwitterionic Polymer Surface Modification Facilitates Liquid Transportation of Microfluidic Sweat Sensors

The transportation of sweat in an epidermal sweat sensor is critical for the monitoring of biochemical compositions of human sweat. However, it is still a challenge to engineer microfluidic devices with super-wetting channels for such epidermal sweat sensors. Herein, a zwitterionic poly (2-methacryloyloxyethyl phosphorylcholine) (PMPC) modified microfluidic device with super-wetting and good liquid transport ability via an azo coupling reaction of PMPC onto the surface of polydimethylsiloxane microfluidic devices is reported. The obtained PMPC-modified microfluidic device can be integrated with flexible electrochemical sensor to measure the ion compositions of human sweat in real-time. The super-hydrophilic zwitterionic polymer surface modification can greatly facilitate the transportation of body fluids in microfluidic sensors for the detection of various biomarkers. Such microfluidic sensors have great potential for next-generation personalized healthcare.

PDMS Microfabrication and Design for Microfluidics and Sustainable Energy Application: Review

The polydimethylsiloxane (PDMS) is popular for wide application in various fields of microfluidics, microneedles, biology, medicine, chemistry, optics, electronics, architecture, and emerging sustainable energy due to the intrinsic non-toxic, transparent, flexible, stretchable, biocompatible, hydrophobic, insulating, and negative triboelectric properties that meet different requirements. For example, the flexibility, biocompatibility, non-toxicity, good stability, and high transparency make PDMS a good candidate for the material selection of microfluidics, microneedles, biomedical, and chemistry microchips as well as for optical examination and wearable electronics. However, the hydrophobic surface and post-surface-treatment hydrophobic recovery impede the development of self-driven capillary microchips. How to develop a long-term hydrophilicity treatment for PDMS is crucial for capillary-driven microfluidics-based application. The dual-tone PDMS-to-PDMS casting for concave-and-convex microstructure without stiction is important for simplifying the process integration. The emerging triboelectric nanogenerator (TENG) uses the transparent flexible PDMS as the high negative triboelectric material to make friction with metals or other positive-triboelectric material for harvesting sustainably mechanical energy. The morphology of PDMS is related to TENG performance. This review will address the above issues in terms of PDMS microfabrication and design for the efficient micromixer, microreactor, capillary pump, microneedles, and TENG for more practical applications in the future.

Microfluidic Preparation of Janus Microparticles With Temperature and pH Triggered Degradation Properties

Based on the phase separation phenomenon in micro-droplets, polymer-lipid Janus particles were prepared on a microfluidic flow focusing chip. Phase separation of droplets was caused by solvent volatilization and Janus morphology was formed under the action of interfacial tension. Because phase change from solid to liquid of the lipid hemisphere could be triggered by physiological temperature, the lipid hemisphere could be used for rapid release of drugs. While the polymer we selected was pH sensitive that the polymer hemisphere could degrade under acidic conditions, making it possible to release drugs in a specific pH environment, such as tumor tissues. Janus particles with different structures were obtained by changing the experimental conditions. To widen the application range of the particles, fatty alcohol and fatty acid-based phase change materials were also employed to prepare the particles, such as 1-tetradecanol, 1-hexadecanol and lauric acid. The melting points of these substances are higher than the physiological temperature, which can be applied in fever triggered drug release or in thermotherapy. The introduction of poly (lactic-co-glycolic acid) enabled the formation of multicompartment particles with three distinct materials. With different degradation properties of each compartment, the particles generated in this work may find applications in programmed and sequential drug release triggered by multiple stimuli.