Interplay between materials and microfluidics

Controllable lubricant-infused wrinkled surface for light-manipulated droplet climbing/pinning on inclined surfaces

Stimuli-responsive droplet transport on solid surfaces holds significant importance in various engineering domains; however, achieving accurate and robust manipulation of droplets, particularly their climbing and pinning on inclined surfaces, remains an unresolved challenge. This study proposes a novel light-responsive surface that integrates wrinkle structure with a lubricant-infused photothermal film, enabling flexible light-controlled movement of droplets even on inclined surfaces. The key to constructing this surface lies in fine control of the lubricant infusion amount onto a wrinkled Fe3O4/polydimethylsiloxane (PDMS) composite film, where wrinkles were "half covered, half exposed". Thus, a droplet placed on this controllable lubricant-infused wrinkled surface (CLWS) comes into contact with both the lubricant and the raised parts of wrinkles. In the absence of light, the droplet pins onto the inclined surface, displaying a large sliding angle up to 50°. Upon exposure to external light, the droplet exhibits climbing ability on inclined surfaces with a tilt angle larger than 15°. This behavior is primarily attributed to the Marangoni effect generated by photothermal conversion, which not only provides the driving force for climbing but also alters the distribution of lubricant to mitigate the resistance. The proposed CLWS demonstrates its suitability for various droplets including water, glycol and glycerol, while enabling complex operations such as directional movement, Z-shape turning, and multi-droplet fusion on inclined or curved surfaces. We believe that our proposed CLWS, designed for light-induced droplet climbing/pinning on inclined surfaces, significantly augments the versatility and application potential in the realm of droplet manipulation techniques.

Taking the 3Rs to a higher level: replacement and reduction of animal testing in life sciences in space research

Human settlements on the Moon, crewed missions to Mars and space tourism will become a reality in the next few decades. Human presence in space, especially for extended periods of time, will therefore steeply increase. However, despite more than 60 years of spaceflight, the mechanisms underlying the effects of the space environment on human physiology are still not fully understood. Animals, ranging in complexity from flies to monkeys, have played a pioneering role in understanding the (patho)physiological outcome of critical environmental factors in space, in particular altered gravity and cosmic radiation. The use of animals in biomedical research is increasingly being criticized because of ethical reasons and limited human relevance. Driven by the 3Rs concept, calling for replacement, reduction and refinement of animal experimentation, major efforts have been focused in the past decades on the development of alternative methods that fully bypass animal testing or so-called new approach methodologies. These new approach methodologies range from simple monolayer cultures of individual primary or stem cells all up to bioprinted 3D organoids and microfluidic chips that recapitulate the complex cellular architecture of organs. Other approaches applied in life sciences in space research contribute to the reduction of animal experimentation. These include methods to mimic space conditions on Earth, such as microgravity and radiation simulators, as well as tools to support the processing, analysis or application of testing results obtained in life sciences in space research, including systems biology, live-cell, high-content and real-time analysis, high-throughput analysis, artificial intelligence and digital twins. The present paper provides an in-depth overview of such methods to replace or reduce animal testing in life sciences in space research.

Organs on chips: fundamentals, bioengineering and applications

Human body constitutes unique biological system containing specific fluid mechanics and biomechanics. Traditional cell culture techniques of 2D and 3D do not recapitulate these specific natures of the human system. In addition, they lack the spatiotemporal conditions of representing the cells. Moreover, they do not enable the study of cell-cell interactions in multiple cell culture platforms. Therefore, establishing biological system of dynamic cell culture was of great interest. Organs on chips systems were fabricated proving their concept to mimic specific organs functions. Therefore, it paves the way for validating new drugs and establishes mechanisms of emerging diseases. It has played a key role in validating suitable vaccines for Coronavirus disease (COVID-19). Herein, the concept of organs on chips, fabrication methodology and their applications are discussed.

Interfacial engineering for biomolecule immobilisation in microfluidic devices

Microfluidic devices are used for various applications in biology and medicine. From on-chip modelling of human organs for drug screening and fast and straightforward point-of-care (POC) detection of diseases to sensitive biochemical analysis, these devices can be custom-engineered using low-cost techniques. The microchannel interface is essential for these applications, as it is the interface of immobilised biomolecules that promote cell capture, attachment and proliferation, sense analytes and metabolites or provide enzymatic reaction readouts. However, common microfluidic materials do not facilitate the stable immobilisation of biomolecules required for relevant applications, making interfacial engineering necessary to attach biomolecules to the microfluidic surfaces. Interfacial engineering is performed through various immobilisation mechanisms and surface treatment techniques, which suitably modify the surface properties like chemistry and energy to obtain robust biomolecule immobilisation and long-term storage stability suitable for the final application. In this review, we provide an overview of the status of interfacial engineering in microfluidic devices, covering applications, the role of biomolecules, their immobilisation pathways and the influence of microfluidic materials. We then propose treatment techniques to optimise performance for various biological and medical applications and highlight future areas of development.

Anisotropic Microcarriers: Fabrication Strategies and Biomedical Applications

Anisotropic microcarriers (AMs) have attracted increasing attention. Although significant efforts have been made to explore AMs with various morphologies, their full potential is yet to be realized, as most studies have primarily focused on materials or fabrication methods. A thorough analysis of the interactional and interdependent relationships between these factors is required, along with proposed countermeasures tailored for researchers from various backgrounds. These countermeasures include specific fabrication strategies for various morphologies and guidelines for selecting the most suitable AM for certain biomedical applications. In this review, a comprehensive summary of AMs, ranging from their fabrication methods to biomedical applications, based on the past two decades of research, is provided. The fabrication of various morphologies is investigated using different strategies and their corresponding biomedical applications. By systematically examining these morphology-dependent effects, a better utilization of AMs with diverse morphologies can be achieved and clear strategies for breakthroughs in the biomedical field are established. Additionally, certain challenges are identified, new frontiers are opened, and promising and exciting opportunities are provided for fabricating functional AMs with broad implications across various fields that must be addressed in biomaterials and biotechnology.

3D bioprinting of collagen-based high-resolution internally perfusable scaffolds for engineering fully biologic tissue systems

Organ-on-a-chip and microfluidic systems have improved the translational relevance of in vitro systems; however, current manufacturing approaches impart limitations on materials selection, non-native mechanical properties, geometric complexity, and cell-driven remodeling into functional tissues. Here, we three-dimensionally (3D) bioprint extracellular matrix (ECM) and cells into collagen-based high-resolution internally perfusable scaffolds (CHIPS) that integrate with a vascular and perfusion organ-on-a-chip reactor (VAPOR) to form a complete tissue engineering platform. We improve the fidelity of freeform reversible embedding of suspended hydrogels (FRESH) bioprinting to produce a range of CHIPS designs fabricated in a one-step process. CHIPS exhibit size-dependent permeability of perfused molecules into the surrounding scaffold to support cell viability and migration. Lastly, we implemented multi-material bioprinting to control 3D spatial patterning, ECM composition, cellularization, and material properties to create a glucose-responsive, insulin-secreting pancreatic-like CHIPS with vascular endothelial cadherin+ vascular-like networks. Together, CHIPS and VAPOR form a platform technology toward engineering full organ-scale function for disease modeling and cell replacement therapy.

Advances and Applications of Micro- and Mesofluidic Systems

Microfabrication technology has advanced scientific understanding and expanded our molecular control capabilities, enabling the development of 3D models in micrometer structures. The sizes of the fluidic channels are arranged in descending order, starting with the macro-, followed by the meso-, micro-, and nanoscale. These advances bring advantages and speed up biological and chemical experimental processes. Such miniaturized systems show significant advances, particularly in meso- and microreactors, through high-throughput screening. This work proposes a bibliometric analysis of the advances and applications of the Web of Science (WoS) database, analyzing the main highlights of the publications, indicators, and impact on knowledge production. In the past 20 years, approximately 3,934 documents published and cited, mainly by major world powers on micro- and mesofluidic systems, are increasingly expanding in the academic and industrial sectors.

Advances in Microfluidics-Enabled Dimensional Design of Micro-/Nanomaterials for Biomedical Applications: A Review

Biomedical materials are of great significance for preventing and treating major diseases and protecting human health. At present, more stringent requirements have been put forward for the preparation methods and dimension control of biomedical materials based on the urgent demand for high-performance biomedical materials, especially the existence of various physiological size thresholds in vitro/in vivo. Microfluidic platforms break the limitations of traditional micro-/nanomaterial synthesis, which provide a miniaturized and highly controlled environment for size-dependent biomaterials. In this review, the basic conceptions and technical characteristics of microfluidics are first described. Then the syntheses of biomedical materials with different dimensions (0D, 1D, 2D, 3D) driven by microfluidics have been systematically summarized. Meanwhile, the applications of microfluidics-driven biomedical materials, including diagnosis, anti-inflammatory, drug delivery, antibacterial, and disease therapy, are discussed. Furthermore, the challenges and developments in the research field are further proposed. This work is expected to facilitate the convergence between the bioscience and engineering communities and continue to contribute to this emerging field.

Anti-smudge superhard transparent coatings via ultra-small nanoparticle pattern surfaces

Anti-smudge coating materials have a broad prospect, but they are susceptible to wear from nails and sand. Therefore, the potential application of such coatings on glass substrates needs coating features such as superhardness and high transparency. However, realizing these key properties combined with anti-smudge function is significantly challenging. In this work, we show a conceptional nanoparticle pattern designing strategy of materials, inspired by stepping on cobblestone roads with the foot feeling of only the hardness of stones. Realize the nanoparticle pattern surface of "cobblestone roads" via facile and scalable interfacial reactions within a molecular compatible system, to successfully achieve the desired coating material properties including anti-smudge, superhardness, and high transparency. The coating was composed of tensely crosslinked sub-10 nm building blocks that bear an anti-smudge molecular layer, exhibiting undistinguished inorganic phase behavior when it was subjected to external forces within the contact point of micro- or above 10 nm nanoscale.

Dynamically adaptive soft metamaterial for wearable human-machine interfaces

Our bodies continuously change their shape. Wearable devices made of hard materials, such as prosthetic limbs worn by millions of amputees every day, cannot adapt to fluctuations in the shape and volume of the body caused by daily activities, weight gain or muscle atrophy. We report a meta-material (Roliner) that is a dynamically adaptive human-machine interface for wearable devices. In this work, we focus on prosthetic limbs as the first application of Roliner. Roliner is made of silicone elastomers with embedded millifluidic channels that can be pneumatically pressurized. Roliner can reconfigure its material properties (behave like silicone or polyurethane with different shore hardness in different areas and times) and volume/shape based on the preference of the amputee in real-time, acting as a spatiotemporally adaptive meta-material. Preclinical studies of Roliner have demonstrated non-inferiority in operation and improved comfort for amputees.

Innovative Applications and Perspectives of Surface-Enhanced Raman Spectroscopy Technology in Biomedicine

Surface-enhanced Raman spectroscopy (SERS) has become a revolutionary technique in the biomedical field, providing unparalleled sensitivity for the detection and characterization of biological samples. In this review, recent SERS innovations are comprehensively discussed, including advanced substrate materials, different SERS detection strategies, and multimodal approaches that combine SERS with other biotechnologies. Among them, the role of SERS in the accurate diagnosis of tumors is highlighted, which has promoted accurate molecular analysis and real-time monitoring of treatment effects. In addition, the growing potential of SERS in the treatment of chronic diseases such as cardiovascular disease, diabetes, and neurodegenerative diseases is discussed. Moreover, the integration with microfluidic chip systems for precise single-cell analysis is presented. To give a forward-looking view, the key challenges faced by SERS technology are also proposed, and possible solutions to overcome these obstacles are provided.

Synergizing microfluidics and plasmonics: advances, applications, and future directions

In the past decade, interest in nanoplasmonic structures has experienced significant growth, owing to rapid advancements in materials science and the evolution of novel nanofabrication techniques. The activities in the area are not only leading to remarkable progress in specific applications in photonics, but also permeating to and synergizing with other fields. This review delves into the symbiosis between nanoplasmonics and microfluidics, elucidating fundamental principles on nanophotonics centered on surface plasmon-polaritons, and key achievements arising from the intricate interplay between light and fluids at small scales. This review underscores the unparalleled capabilities of subwavelength plasmonic structures to manipulate light beyond the diffraction limit, concurrently serving as fluidic entities or synergistically combining with micro- and nanofluidic structures. Noteworthy phenomena, techniques and applications arising from this synergy are explored, including the manipulation of fluids at nanoscopic dimensions, the trapping of individual nanoscopic entities like molecules or nanoparticles, and the harnessing of light within a fluidic environment. Additionally, it discusses light-driven fabrication methodologies for microfluidic platforms and, contrariwise, the use of microfluidics in the fabrication of plasmonic nanostructures. Pondering future prospects, this review offers insights into potential future developments, particularly focusing on the integration of two-dimensional materials endowed with exceptional optical, structural and electrical properties, such as goldene and borophene, which enable higher carrier densities and higher plasmonic frequencies. Such advancements could catalyze innovations in diverse applications, including energy harvesting, advanced photothermal cancer therapies, and catalytic processes for hydrogen generation and CO2 conversion.

Armored Regenerable Cilia

Flexible cilia of natural species are well-known for their capabilities to transport objects by their collective motions. Therefore, well-ordered, flexible, and stimuli-responsive artificial cilia have been developed to render similar functionalities. However, flexibility and stimuli-responsiveness of a microcilium are inherently incompatible with durability/robustness against mechanical damage, limiting the artificial cilia to applications with only gentle operating conditions. The critical (but long neglected in surface engineering) property of natural hairs is that they are rooted under the skin, allowing the regeneration of the damaged hairs from their undamaged roots (hair follicles). To integrate the functionalities of cilia and hair, we developed a fabrication strategy called stencil-assisted self-alignment of iron-laden aerosols to produce a surface termed armored regenerable cilia. This surface contains well-ordered, appropriately packed, flexible, and magneto-responsive artificial wires rooted within pores. The wall of the pore serves as the armor to protect the bottom part of the wires from mechanical damage, allowing the remaining wires to regrow when the self-alignment of iron-laden aerosols repeats. The armored regenerable cilia with functionalities such as water repellency, object manipulation, and impurity removal are expected to guide the design and fabrication of smart surfaces serving real-life applications.

Engineered Living Systems Based on Gelatin: Design, Manufacturing, and Applications

Engineered living systems (ELSs) represent purpose-driven assemblies of living components, encompassing cells, biomaterials, and active agents, intricately designed to fulfill diverse biomedical applications. Gelatin and its derivatives have been used extensively in ELSs owing to their mature translational pathways, favorable biological properties, and adjustable physicochemical characteristics. This review explores the intersection of gelatin and its derivatives with fabrication techniques, offering a comprehensive examination of their synergistic potential in creating ELSs for various applications in biomedicine. It offers a deep dive into gelatin, including its structures and production, sources, processing, and properties. Additionally, the review explores various fabrication techniques employing gelatin and its derivatives, including generic fabrication techniques, microfluidics, and various 3D printing methods. Furthermore, it discusses the applications of ELSs based on gelatin in regenerative engineering as well as in cell therapies, bioadhesives, biorobots, and biosensors. Future directions and challenges in gelatin fabrication are also examined, highlighting emerging trends and potential areas for improvements and innovations. In summary, this comprehensive review underscores the significance of gelatin-based ELSs in advancing biomedical engineering and lays the groundwork for guiding future research and developments within the field.

Liquid-solid composites with confined interface behaviors

In the evolving landscape of materials science, the journey from traditional composite materials to liquid-solid composites has marked a significant shift. Composite materials, typically solid state, have long been the cornerstone of many applications due to their structural stability and mechanical properties. However, the emergence of liquid-solid composites has introduced a new paradigm, leveraging the dynamic composite interfaces and fluidic nature of liquids. Recent years have witnessed the rapid development of liquid-solid composites, distinguishing themselves by their defect-free, molecularly smooth surfaces and adaptive features. In this review, we introduce liquid-based confined interface materials, which represent a cutting-edge advancement, integrating confined liquids within solid frameworks at mesoscopic scales. Characterized by their confined competitive multiphase interfacial interactions, these materials offer practical functionalities like anti-fouling, multiphase flow control and drag reduction. We summarize the development of the materials, and showcase important applications based on the controllable motions of confined liquids and solid frameworks. We also discuss their design and preparation and address future challenges and outlooks, such as artificial intelligence, in advancing functionalities.

Bioinspired Liquid Pockets with Externally Induced Internal Microscale Flow

The gastric mucosal barrier, through its gastric pits, serves as a pathway for secretions, ensuring that mucus produced by the gastric glands is transferred to the gastric lumen, providing stable protection. Here a bioinspired liquid pockets material is shown, composed of a thermo-driven hydrogel that acts as an external activation unit to release interflowing liquid responsively, and porous matrices that serve as interconnected pockets to transfer it, enabling controlled internal flow and adaptive barrier functionality. Experiments and theoretical analysis demonstrate the stability and regulatory mechanisms of these liquid pockets, based on the interconnected pockets between the external activation unit and internal fluid flow. It exhibits a new pathway for regulating microscale flow at responsive material interfaces, enabling applications from sequential drug release, self-cleaning, and antifouling to anti-swelling. These unique capabilities address long-standing challenges in microscale flow control, with potential impacts in diverse fields including microfluidics, drug delivery, medical devices, 3D printing, and beyond.

Microfluidic-based electrically driven particle manipulation techniques for biomedical applications

Microfluidic chips exhibit unique advantages in both economy and rapidity, particularly for the separation and detection of biomolecules. In this review, we first introduced the mechanisms of several electrically driven methods, such as electrophoresis, dielectrophoresis, electro-wetting and electro-rotation. We then discussed in detail the application of these methods in nucleic acid analysis, protein manipulation and cell treatment. In addition, we outlined the considerations for material selection, manufacturing processes and structural design of microfluidic chips based on electrically driven mechanisms.

Upcycling biowaste into advanced carbon materials via low-temperature plasma hybrid system: applications, mechanisms, strategies and future prospects

This review focuses on the recent advances in the sustainable conversion of biowaste to valuable carbonaceous materials. This study summarizes the significant progress in biowaste-derived carbon materials (BCMs) via a plasma hybrid system. This includes systematic studies like AI-based multi-coupling systems, promising synthesis strategies from an economic point of view, and their potential applications towards energy, environment, and biomedicine. Plasma modified BCM has a new transition lattice phase and exhibits high resilience, while fabrication and formation mechanisms of BCMs are reviewed in plasma hybrid system. A unique 2D structure can be designed and formulated from the biowaste with fascinating physicochemical properties like high surface area, unique defect sites, and excellent conductivity. The structure of BCMs offers various activated sites for element doping and it shows satisfactory adsorption capability, and dynamic performance in the field of electrochemistry. In recent years, many studies have been reported on the biowaste conversion into valuable materials for various applications. Synthesis methods are an indispensable factor that directly affects the structure and properties of BCMs. Therefore, it is imperative to review the facile synthesis methods and the mechanisms behind the formation of BCMs derived from the low-temperature plasma hybrid system, which is the necessity to obtain BCMs having desirable structure and properties by choosing a suitable synthesis process. Advanced carbon-neutral materials could be widely synthesized as catalysts for application in environmental remediation, energy conversion and storage, and biotechnology.

Thermoelectric Materials and Devices for Advanced Biomedical Applications

Thermoelectrics (TEs), enabling the direct conversion between heat and electrical energy, have demonstrated extensive application potential in biomedical fields. Herein, the mechanism of the TE effect, recent developments in TE materials, and the biocompatibility assessment of TE materials are provided. In addition to the fundamentals of TEs, a timely and comprehensive review of the recent progress of advanced TE materials and their applications is presented, including wearable power generation, personal thermal management, and biosensing. In addition, the new-emerged medical applications of TE materials in wound healing, disease treatment, antimicrobial therapy, and anti-cancer therapy are thoroughly reviewed. Finally, the main challenges and future possibilities are outlined for TEs in biomedical fields, as well as their material selection criteria for specific application scenarios. Together, these advancements can provide innovative insights into the development of TEs for broader applications in biomedical fields.

Boosting Microfluidic Enzymatic Cascade Reactions with pH-Responsive Polymersomes by Spatio-Chemical Activity Control

Microfluidic flow reactors permit the implementation of sensitive biocatalysts in polymeric environments (e.g., hydrogel dots), mimicking nature through the use of diverse microstructures within defined confinements. However, establishing complex hybrid structures to mimic biological processes and functions under continuous flow with optimal utilization of all components involved in the reaction process represents a significant scientific challenge. To achieve spatial, chemical, and temporal control for any microfluidic application, compartmentalization is required, as well as the unification of different sensitive compartments in the reaction chamber for the microfluidic flow design. This study presents a self-regulating microfluidic system fabricated by a sequential photostructuring process with an intermediate chemical process step to realize pH-sensitive hybrid structures for the fabrication of a microfluidic double chamber reactor for controlled enzymatic cascade reaction (ECR). The key point is the adaptation and retention of the function of pH-responsive horseradish peroxidase-loaded polymersomes in a microfluidic chip under continuous flow. ECR is successfully triggered and controlled by an interplay between glucose oxidase-converted glucose, the membrane state of pH-responsive polymersomes, and other parameters (e.g., flow rate and fluid composition). This study establishes a promising noninvasive regulatory platform for extended spatio-chemical control of current and future ECR and other cascade reaction systems.

Biomimetic Liver Lobules from Multi-Compartmental Microfluidics

Engineered liver lobule is highly practical in hepatic disease treatment, while constructing a 3D biomimetic lobule with a heterogeneous architecture on a large scale is challenging. Here, inspired by the natural architectural construction of hepatic lobules, biomimetic hepatic lobules are proposed with coaxially through-pores for nutrient exchange via microfluidic technology. This multi-channel microfluidic chip is made by parallelly installing capillaries. Sodium alginate (Alg) is pumped through its central channel, while Ca2+-loaded gelatin methacrylate (GelMA) solutions encapsulating hepatocytes, mesenchymal stem cells, and endothelia cells are pumped through surrounding channels, respectively. The rapid gelation of Alg and Ca2+ brings about an in situ formation of Alg fiber, with heterogeneous multi-cell-laden GelMA microcarriers forming around it. The peeled-off microcarriers each featured with a coaxially through pore, simulating the cord-like structure of hepatic lobule and facilitating nutrients exchange. Meanwhile, the spatially anisotropic arrangement of cells highly simulates the hepatic architecture. It is demonstrated that by transplanting these biomimetic microparticles into liver in situ, the failed liver in rat shows increased regeneration and decreased necrosis. These results indicated that the microfluidic multi-compartmental microcarriers provide a new strategy to engineer 3D artificial livers for clinical translation.

Antigravity Autonomous Superwettable Pumps for Spontaneous Separation of Oil-Water Emulsions

Oil-water separation based on superwettable materials offers a promising way for the treatment of oil-water mixtures and emulsions. Nevertheless, such separation techniques often require complex devices and external energy input. Therefore, it remains a great challenge to separate oil-water mixtures and emulsions through an energy-efficient, economical, and sustainable way. Here, a novel approach demonstrating the successful separation of oil-water emulsions using antigravity-driven autonomous superwettable pumps is presented. By transitioning from traditional gravity-driven to antigravity-driven separation, the study showcases the unprecedented success in purifying oil/water from emulsions by capillary/siphon-driven superwettable autonomous pumps. These pumps, composed of self-organized interconnected channels formed by the packing of superhydrophobic and superhydrophilic sand particles, exhibit outstanding separation flux, efficiency, and recyclability. The findings of this study not only open up a new avenue for oil-water emulsion separation but also hold promise for profound impacts in the field.

Overcoming bubble formation in polydimethylsiloxane-made PCR chips: mechanism and elimination with a high-pressure liquid seal

The thermal expansion of gas and the air permeability of polydimethylsiloxane (PDMS) were previously thought to be the main causes of bubbles and water loss during polymerase chain reaction (PCR), resulting in a very complex chip design and operation. Here, by calculating and characterizing bubble formation, we discovered that water vapor is the main cause of bubbling. During PCR, heat increases the volume of the bubble by a factor of only ~0.2 in the absence of water vapor but by a factor of ~6.4 in the presence of water vapor. In addition, the phenomenon of "respiration" due to the repeated evaporation and condensation of water vapor accelerates the expansion of bubbles and the loss of water. A water seal above 109 kPa can effectively prevent bubbles in a bare PDMS chip with a simple structure, which is significant for the wide application of PDMS chips.

Recent Advances in Polymer Science and Fabrication Processes for Enhanced Microfluidic Applications: An Overview

This review explores significant advancements in polymer science and fabrication processes that have enhanced the performance and broadened the application scope of microfluidic devices. Microfluidics, essential in biotechnology, medicine, and chemical engineering, relies on precise fluid manipulation in micrometer-sized channels. Recent innovations in polymer materials, such as flexible, biocompatible, and structurally robust polymers, have been pivotal in developing advanced microfluidic systems. Techniques like replica molding, microcontact printing, solvent-assisted molding, injection molding, and 3D printing are examined, highlighting their advantages and recent developments. Additionally, the review discusses the diverse applications of polymer-based microfluidic devices in biomedical diagnostics, drug delivery, organ-on-chip models, environmental monitoring, and industrial processes. This paper also addresses future challenges, including enhancing chemical resistance, achieving multifunctionality, ensuring biocompatibility, and scaling up production. By overcoming these challenges, the potential for widespread adoption and impactful use of polymer-based microfluidic technologies can be realized.

A 3D-printed tumor-on-chip: user-friendly platform for the culture of breast cancer spheroids and the evaluation of anti-cancer drugs

Tumor-on-chips (ToCs) are useful platforms for studying the physiology of tumors and evaluating the efficacy and toxicity of anti-cancer drugs. However, the design and fabrication of a ToC system is not a trivial venture. We introduce a user-friendly, flexible, 3D-printed microfluidic device that can be used to culture cancer cells or cancer-derived spheroids embedded in hydrogels under well-controlled environments. The system consists of two lateral flow compartments (left and right sides), each with two inlets and two outlets to deliver cell culture media as continuous liquid streams. The central compartment was designed to host a hydrogel in which cells and microtissues can be confined and cultured. We performed tracer experiments with colored inks and 40 kDa fluorescein isothiocyanate dextran to characterize the transport/mixing performances of the system. We also cultured homotypic (MCF7) and heterotypic (MCF7-BJ) spheroids embedded in gelatin methacryloyl hydrogels to illustrate the use of this microfluidic device in sustaining long-term micro-tissue culture experiments. We further demonstrated the use of this platform in anticancer drug testing by continuous perfusion of doxorubicin, a commonly used anti-cancer drug for breast cancer. In these experiments, we evaluated drug transport, viability, glucose consumption, cell death (apoptosis), and cytotoxicity. In summary, we introduce a robust and friendly ToC system capable of recapitulating relevant aspects of the tumor microenvironment for the study of cancer physiology, anti-cancer drug transport, efficacy, and safety. We anticipate that this flexible 3D-printed microfluidic device may facilitate cancer research and the development and screening of strategies for personalized medicine.

Prototyping in Polymethylpentene to Enable Oxygen-Permeable On-a-Chip Cell Culture and Organ-on-a-Chip Devices Suitable for Microscopy

With the rapid development and commercial interest in the organ-on-a-chip (OoC) field, there is a need for materials addressing key experimental demands and enabling both prototyping and large-scale production. Here, we utilized the gas-permeable, thermoplastic material polymethylpentene (PMP). Three methods were tested to prototype transparent PMP films suitable for transmission light microscopy: hot-press molding, extrusion, and polishing of a commercial, hazy extruded film. The transparent films (thickness 20, 125, 133, 356, and 653 µm) were assembled as the cell-adhering layer in sealed culture chamber devices, to assess resulting oxygen concentration after 4 days of A549 cell culture (cancerous lung epithelial cells). Oxygen concentrations stabilized between 15.6% and 11.6%, where the thicker the film, the lower the oxygen concentration. Cell adherence, proliferation, and viability were comparable to glass for all PMP films (coated with poly-L-lysine), and transparency was adequate for transmission light microscopy of adherent cells. Hot-press molding was concluded as the preferred film prototyping method, due to excellent and reproducible film transparency, the possibility to easily vary film thickness, and the equipment being commonly available. The molecular orientation in the PMP films was characterized by IR dichroism. As expected, the extruded films showed clear orientation, but a novel result was that hot-press molding may also induce some orientation. It has been reported that orientation affects the permeability, but with the films in this study, we conclude that the orientation is not a critical factor. With the obtained results, we find it likely that OoC models with relevant in vivo oxygen concentrations may be facilitated by PMP. Combined with established large-scale production methods for thermoplastics, we foresee a useful role for PMP within the OoC field.

Critical review and recent advances of emerging real-time and non-destructive strategies for meat spoilage monitoring

Monitoring and evaluating food quality, especially meat quality, has received a growing interest to ensure human health and decrease waste of raw materials. Standard analytical approaches used for meat spoilage assessment suffer from time consumption, being labor-intensive, operation complexity, and destructiveness. To overcome shortfalls of these traditional methods and monitor spoilage microorganisms or related metabolites of meat products across the supply chain, emerging analysis devices/systems with higher sensitivity, better portability, on-line/in-line, non-destructive and cost-effective property are urgently needed. Herein, we first overview the basic concepts, causes, and critical monitoring indicators associated with meat spoilage. Then, the conventional detection methods for meat spoilage are outlined objectively in their strengths and weaknesses. In addition, we place the focus on the recent research advances of emerging non-destructive devices and systems for assessing meat spoilage. These novel strategies demonstrate their powerful potential in the real-time evaluation of meat spoilage.

Biomimicking TRPM8: A Conversely Temperature-Dependent Nonionic Retrorse Nanochannel for Ion Flow Control

Ion channels play a crucial role in the transmembrane transport and signal transmission of substances. In animals, transient receptor potential vanilloid 1 (TRPV1) and transient receptor potential melastatin 8 (TRPM8) serve as temperature-sensing units in sensory nerve endings. TRPV1 allows cells to sense heat, while TRPM8 enables them to detect cold, both serving to protect living organisms from harmful substances and environments. However, almost all studies on artificial nanochannels have mainly focused on TRPV1-like "forward nanochannels" thus far, which are incapable of "backward" responding to heat. So, we constructed an innovational TRPM8-inspired "retrorse nanochannel" through internal modification of poly(acrylamide-co-acrylonitrile) [P(AAm-co-AN)] with an upper critical solution temperature (UCST). Our results demonstrated that the internally modified nanochannels exhibited rapid, stable, and reversible heat-closing capability and converse temperature dependence within the typical temperature range of 25-40 °C. The biomimetic ion channel can effectively function as a facile, precise, and reversible thermal gate for controlling the transport of ions and substances. It also offers a promising microscopic technology for managing thermal effects on the substance, fluid, energy, and even signal delivery.

Catalase Detection via Membrane-Based Pressure Sensors

Membrane-based sensors (MePSs) exhibit remarkable precision and sensitivity in detecting pressure changes. MePSs are commonly used to monitor catalytic reactions in solution, generating gas products crucial for signal amplification in bioassays. They also allow for catalyst quantification by indirectly measuring the pressure generated by the gaseous products. This is particularly interesting for detecting enzymes in biofluids associated with disease onset. To enhance the performance of a MePS, various structural factors influence membrane flexibility and response time, ultimately dictating the device's pressure sensitivity. In this study, we fabricated MePSs using polydimethylsiloxane (PDMS) and investigated how structural modifications affect the Young's modulus (E) and residual stress (σ0) of the membranes. These modifications have a direct impact on the sensors' sensitivity to pressure variations, observed as a function of the volume of the chamber (Σ) or of the mechanical properties of the membrane itself (S). MePSs exhibiting the highest sensitivities were then employed to detect catalyst quantities inducing the dismutation of hydrogen peroxide, producing dioxygen as a gaseous product. As a result, a catalase enzyme was successfully detected using these optimized MePSs, achieving a remarkable sensitivity of (22.7 ± 1.2) µm/nM and a limit of detection (LoD) of 396 pM.

Visualization-enhanced under-oil open microfluidic system for in situ characterization of multi-phase chemical reactions

Under-oil open microfluidic system, utilizing liquid-liquid boundaries for confinements, offers inherent advantages including clogging-free flow channels, flexible access to samples, and adjustable gas permeation, making it well-suited for studying multi-phase chemical reactions that are challenging for closed microfluidics. However, reports on the novel system have primarily focused on device fabrication and functionality demonstrations within biology, leaving their application in broader chemical analysis underexplored. Here, we present a visualization-enhanced under-oil open microfluidic system for in situ characterization of multi-phase chemical reactions with Raman spectroscopy. The enhanced system utilizes a semi-transparent silicon (Si) nanolayer over the substrate to enhance visualization in both inverted and upright microscope setups while reducing Raman noise from the substrate. We validated the system's chemical stability and capability to monitor gas evolution and gas-liquid reactions in situ. The enhanced under-oil open microfluidic system, integrating Raman spectroscopy, offers a robust open-microfluidic platform for label-free molecular sensing and real-time chemical/biochemical process monitoring in multi-phase systems.

3D Bioprinting of Collagen-based Microfluidics for Engineering Fully-biologic Tissue Systems

Microfluidic and organ-on-a-chip devices have improved the physiologic and translational relevance of in vitro systems in applications ranging from disease modeling to drug discovery and pharmacology. However, current manufacturing approaches have limitations in terms of materials used, non-native mechanical properties, patterning of extracellular matrix (ECM) and cells in 3D, and remodeling by cells into more complex tissues. We present a method to 3D bioprint ECM and cells into microfluidic collagen-based high-resolution internally perfusable scaffolds (CHIPS) that address these limitations, expand design complexity, and simplify fabrication. Additionally, CHIPS enable size-dependent diffusion of molecules out of perfusable channels into the surrounding device to support cell migration and remodeling, formation of capillary-like networks, and integration of secretory cell types to form a glucose-responsive, insulin-secreting pancreatic-like microphysiological system.

Liquid-infused interfacial floatable porous membrane as movable gate for ultrafast immiscible oil/water separation

Liquid separation methods are widely used in industrial and everyday applications, however, their applicability is often constrained by low efficiency, membrane fouling, and poor energy efficiency. Herein, a conceptually novel liquid-infused interfacial floatable porous membrane (LIIFPM) system for high-performance oil/water separation is proposed. The system functions by allowing a liquid to wet and fill a superamphiphilic porous membrane, thereby creating a stable liquid-infused interface that floats at the oil/water interface and prevents the passage of immiscible liquids. The lower-layer liquid can outflow directly, while the flow of the upper-layer liquid is stopped by the membrane. Remarkably, the efficiency of the LIIFPM system is independent of the membrane pore size, enabling ultrafast immiscible oil/water separation in an energy-saving and antifouling manner.

Turning catalytically active pores into active pumps

We develop a semi-analytical model of self-diffusioosmotic transport in active pores, which includes advective transport and the inverse chemical reaction that consumes solute. In previous work [Antunes et al., Phys. Rev. Lett. 129, 188003 (2022)], we have demonstrated the existence of a spontaneous symmetry breaking in fore-aft symmetric pores that enables them to function as a micropump. We now show that this pumping transition is controlled by three timescales. Two timescales characterize advective and diffusive transport. The third timescale corresponds to how long a solute molecule resides in the pore before being consumed. Introducing asymmetry to the pore (either via the shape or the catalytic coating) reveals a second type of advection-enabled transition. In asymmetric pores, the flow rate exhibits discontinuous jumps and hysteresis loops upon tuning the parameters that control the asymmetry. This work demonstrates the interconnected roles of shape and catalytic patterning in the dynamics of active pores and shows how to design a pump for optimum performance.

Artificial Intelligence in Regenerative Medicine: Applications and Implications

The field of regenerative medicine is constantly advancing and aims to repair, regenerate, or substitute impaired or unhealthy tissues and organs using cutting-edge approaches such as stem cell-based therapies, gene therapy, and tissue engineering. Nevertheless, incorporating artificial intelligence (AI) technologies has opened new doors for research in this field. AI refers to the ability of machines to perform tasks that typically require human intelligence in ways such as learning the patterns in the data and applying that to the new data without being explicitly programmed. AI has the potential to improve and accelerate various aspects of regenerative medicine research and development, particularly, although not exclusively, when complex patterns are involved. This review paper provides an overview of AI in the context of regenerative medicine, discusses its potential applications with a focus on personalized medicine, and highlights the challenges and opportunities in this field.

Bio-inspired microfluidics: A review

Biomicrofluidics, a subdomain of microfluidics, has been inspired by several ideas from nature. However, while the basic inspiration for the same may be drawn from the living world, the translation of all relevant essential functionalities to an artificially engineered framework does not remain trivial. Here, we review the recent progress in bio-inspired microfluidic systems via harnessing the integration of experimental and simulation tools delving into the interface of engineering and biology. Development of "on-chip" technologies as well as their multifarious applications is subsequently discussed, accompanying the relevant advancements in materials and fabrication technology. Pointers toward new directions in research, including an amalgamated fusion of data-driven modeling (such as artificial intelligence and machine learning) and physics-based paradigm, to come up with a human physiological replica on a synthetic bio-chip with due accounting of personalized features, are suggested. These are likely to facilitate physiologically replicating disease modeling on an artificially engineered biochip as well as advance drug development and screening in an expedited route with the minimization of animal and human trials.

A review on inertial microfluidic fabrication methods

In recent decades, there has been significant interest in inertial microfluidics due to its high throughput, ease of fabrication, and no need for external forces. The focusing efficiency of inertial microfluidic systems relies entirely on the geometrical features of microchannels because hydrodynamic forces (inertial lift forces and Dean drag forces) are the main driving forces in inertial microfluidic devices. In the past few years, novel microchannel structures have been propounded to improve particle manipulation efficiency. However, the fabrication of these unconventional structures has remained a serious challenge. Although researchers have pushed forward the frontiers of microfabrication technologies, the fabrication techniques employed for inertial microfluidics have not been discussed comprehensively. This review introduces the microfabrication approaches used for creating inertial microchannels, including photolithography, xurography, laser cutting, micromachining, microwire technique, etching, hot embossing, 3D printing, and injection molding. The advantages and disadvantages of these methods have also been discussed. Then, the techniques are reviewed regarding resolution, structures, cost, and materials. This review provides a thorough insight into the manufacturing methods of inertial microchannels, which could be helpful for future studies to improve the harvesting yield and resolution by choosing a proper fabrication technique.

A Microfluidic Approach for Synthesis of Silver Nanoparticles as a Potential Antimicrobial Agent in Alginate-Hyaluronic Acid-Based Wound Dressings

The recognized antimicrobial activity of silver nanoparticles is a well-studied property, especially when designing and developing biomaterials with medical applications. As biological activity is closely related to the physicochemical characteristics of a material, aspects such as particle morphology and dimension should be considered. Microfluidic systems in continuous flow represent a promising method to control the size, shape, and size distribution of synthesized nanoparticles. Moreover, using microfluidics widens the synthesis options by creating and controlling parameters that are otherwise difficult to maintain in conventional batch procedures. This study used a microfluidic platform with a cross-shape design as an innovative method for synthesizing silver nanoparticles and varied the precursor concentration and the purging speed as experimental parameters. The compositional and microstructural characterization of the obtained samples was carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and dynamic light scattering (DLS). Four formulations of alginate-based hydrogels with the addition of hyaluronic acid and silver nanoparticles were obtained to highlight the antimicrobial activity of silver nanoparticles and the efficiency of such a composite in wound treatment. The porous structure, swelling capacity, and biological properties were evaluated through physicochemical analysis (FT-IR and SEM) and through contact with prokaryotic and eukaryotic cells. The results of the physicochemical and biological investigations revealed desirable characteristics for performant wound dressings (i.e., biocompatibility, appropriate porous structure, swelling rate, and degradation rate, ability to inhibit biofilm formation, and cell growth stimulation capacity), and the obtained materials are thus recommended for treating chronic and infected wounds.

Formation of Protein Nanoparticles in Microdroplet Flow Reactors

Nanoparticles are increasingly being used for biological applications, such as drug delivery and gene transfection. Different biological and bioinspired building blocks have been used for generating such particles, including lipids and synthetic polymers. Proteins are an attractive class of material for such applications due to their excellent biocompatibility, low immunogenicity, and self-assembly characteristics. Stable, controllable, and homogeneous formation of protein nanoparticles, which is key to successfully delivering cargo intracellularly, has been challenging to achieve using conventional methods. In order to address this issue, we employed droplet microfluidics and utilized the characteristic of rapid and continuous mixing within microdroplets in order to produce highly monodisperse protein nanoparticles. We exploit the naturally occurring vortex flows within microdroplets to prevent nanoparticle aggregation following nucleation, resulting in systematic control over the particle size and monodispersity. Through combination of simulation and experiment, we find that the internal vortex velocity within microdroplets determines the uniformity of the protein nanoparticles, and by varying parameters such as protein concentration and flow rates, we are able to finely tune nanoparticle dimensional properties. Finally, we show that our nanoparticles are highly biocompatible with HEK-293 cells, and through confocal microscopy, we determine that the nanoparticles fully enter into the cell with almost all cells containing them. Due to the high throughput of the method of production and the level of control afforded, we believe that the approach described in this study for generating monodisperse protein-based nanoparticles has the potential for intracellular drug delivery or for gene transfection in the future.

The applications and techniques of organoids in head and neck cancer therapy

Head and neck cancer (HNC) is one of the most common cancers on the planet, with approximately 600,000 new cases diagnosed and 300,000 deaths every year. Research into the biological basis of HNC has advanced slowly over the past decades, which has made it difficult to develop new, more effective treatments. The patient-derived organoids (PDOs) are made from patient tumor cells, resembling the features of their tumors, which are high-fidelity models for studying cancer biology and designing new precision medicine therapies. In recent years, considerable effort has been focused on improving "organoids" technologies and identifying tumor-specific medicine using head and neck samples and a variety of organoids. A review of improved techniques and conclusions reported in publications describing the application of these techniques to HNC organoids is presented here. Additionally, we discuss the potential application of organoids in head and neck cancer research as well as the limitations associated with these models. As a result of the integration of organoid models into future precision medicine research and therapeutic profiling programs, the use of organoids will be extremely significant in the future.

Deep learning with microfluidics for on-chip droplet generation, control, and analysis

Droplet microfluidics has gained widespread attention in recent years due to its advantages of high throughput, high integration, high sensitivity and low power consumption in droplet-based micro-reaction. Meanwhile, with the rapid development of computer technology over the past decade, deep learning architectures have been able to process vast amounts of data from various research fields. Nowadays, interdisciplinarity plays an increasingly important role in modern research, and deep learning has contributed greatly to the advancement of many professions. Consequently, intelligent microfluidics has emerged as the times require, and possesses broad prospects in the development of automated and intelligent devices for integrating the merits of microfluidic technology and artificial intelligence. In this article, we provide a general review of the evolution of intelligent microfluidics and some applications related to deep learning, mainly in droplet generation, control, and analysis. We also present the challenges and emerging opportunities in this field.

Ultrasound-trigged micro/nanorobots for biomedical applications

Micro- and nanorobots (MNRs) propelled by external actuations have broad potential in biomedical applications. Among the numerous external excitations, ultrasound (US) features outstanding practical significance with merits of its noninvasiveness, tunability, penetrability, and biocompatibility. Attributing to various physiochemical effects of US, it can propel the MNRs with sophisticated structures through asymmetric acoustic streaming, bubble oscillation, and so on. In this review, we introduce several advanced and representative US-propelled MNRs with inhomogeneous density distribution, asymmetric shape, hollow cavity, etc. The potential biomedical applications of these cutting-edge MNRs are also presented, including intracellular delivery, harmful substances collection, and so on. Furthermore, we conclude the advantages and limitations of US-propelled MNRs and prospect their future developments in multidisciplinary fields.

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.

On-Chip Liquid Manipulation via a Flexible Dual-Layered Channel Possessing Hydrophilic/Hydrophobic Dichotomy

The hydrophilic/hydrophobic cooperative interface provides a smart platform to control liquid distribution and delivery. Through the fusion of flexibility and complex structure, we present a manipulable, open, and dual-layered liquid channel (MODLC) for on-demand mechanical control of fluid delivery. Driven by anisotropic Laplace pressure, the mechano-controllable asymmetric channel of MODLC can propel the directional slipping of liquid located between the paired tracks. Upon a single press, the longest transport distance can reach 10 cm with an average speed of ∼3 cm/s. The liquid on the MODLC can be immediately manipulated by pressing or dragging processes, and versatile liquid-manipulating processes on hierarchical MODLC chips have been achieved, including remote droplet magneto-control, continuous liquid distributor, and gas-producing chip. The flexible hydrophilic/hydrophobic interface and its assembly can extend the function and applications of the wettability-patterned interface, which should update our understanding of complex systems for sophisticated liquid transport.

Improved Liquid Collection on a Dual-Asymmetric Superhydrophilic Origami

Manipulating fluid with an open channel provides a promising strategy to simplify the current systems. Nevertheless, spontaneous on-surface fluid transport with large flux, high speed, and long distance remains challenging. Inspired by scallop shells, here a shell-like superhydrophilic origami (S-SLO) with multiple-paratactic and dual-asymmetric channels is presented to improve fluid collection. The origami channel can capture various types of liquids, including droplets, flow, and steam, and then transport collected liquid unidirectionally. The S-SLO with 2 mm depth can reach maximum flux of 450 mL h^-1 , which is five times the capacity of a flat patterned surface with similar dimension. To diversify the function of such interface, the SLO is further integrated with a superhydrophobic zirconium carbide/silicone coating for enhanced condensation via the collaboration of directional fluid manipulation and a radiative cooling layer. Compared with the unmodified parallel origami, the shell-like origami with a radiative cooling layer shows a 56% improvement in condensate efficiency as well as the directional liquid drainage. This work demonstrates a more accessible design for the optimization of on-surface fluid control, and the improved performance of liquid transport should extend the applications of bioinspired fluid-manipulating interfaces.

Lithographic Microneedle-Motors from Multimodal Microfluidics for Cargo Delivery

Micromotors have led to an unprecedented revolution in the field of cargo delivery. Attempts in this area trend toward enriching their structures and improving their functions to promote their further applications. Herein, novel microneedle-motors (MNMs) for active drug delivery through a flexible multimodal microfluidic lithographic approach are presented. The multimodal microfluidics is composed of a co-flow geometry-derived droplet fluid and an active cargo mixed laminar flow in a triangular microchannel. The MNMs with sharp tips and spherical fuel-loading cavities are obtained continuously from microfluidics with the assistance of flow lithography. The structural parameters of the MNMs could be precisely tailored by simply choosing the flow speed or the shape of the photomask. As the actives are mixed into the phase solution during the generation, the resultant MNMs are loaded with cargoes for direct applications without any extra complex operation. Based on these features, it is demonstrated that with sharp tips and autonomous movement, the MNMs can efficiently penetrate the tissue-like substrates, indicating the potential in overcoming physiological barriers for cargo release. These results indicate that the proposed multimodal microfluidic lithographic MNMs are valuable for practical active cargo delivery in biomedical and other relative areas.

Microfluidic solutions for biofluids handling in on-skin wearable systems

On-skin wearable systems for biofluid sampling and biomarker sensing can revolutionize the current practices in healthcare monitoring and personalized medicine. However, there is still a long path toward complete market adoption and acceptance of this fascinating technology. Accordingly, microfluidic science and technology can provide excellent solutions for bridging the gap between basic research and clinical research. The research gap has led to the emerging field of epidermal microfluidics. Moreover, recent advances in the fabrication of highly flexible and stretchable microfluidic systems have revived the concept of micro elastofluidics, which can provide viable solutions for on-skin wearable biofluid handling. In this context, this review highlights the current state-of-the-art platforms in this field and discusses the potential technologies that can be used for on-skin wearable devices. Toward this aim, we first compare various microfluidic platforms that could be used for on-skin wearable devices. These platforms include semiconductor-based, polymer-based, liquid metal-based, paper-based, and textile-based microfluidics. Next, we discuss how these platforms can enhance the stretchability of on-skin wearable biosensors at the device level. Next, potential microfluidic solutions for collecting, transporting, and controlling the biofluids are discussed. The application of finger-powered micropumps as a viable solution for precise and on-demand biofluid pumping is highlighted. Finally, we present the future directions of this field by emphasizing the applications of droplet-based microfluidics, stretchable continuous-flow micro elastofluidics, stretchable superhydrophobic surfaces, liquid beads as a form of digital micro elastofluidics, and topological liquid diodes that received less attention but have enormous potential to be integrated into on-skin wearable devices.

Microwell array chip-based single-cell analysis

Single-cell profiling is key to uncover the cellular heterogeneity and drives deep understanding of cell fate. In recent years, microfluidics has become an ideal tool for single-cell profiling owing to its benefits of high throughput and automation. Among various microfluidic platforms, microwell has the advantages of simple operation and easy integration with in situ analysis ability, making it an ideal technique for single-cell studies. Herein, recent advances of single-cell analysis based on microwell array chips are summarized. We first introduce the design and preparation of different microwell chips. Then microwell-based cell capture and lysis strategies are discussed. We finally focus on advanced microwell-based analysis of single-cell proteins, nucleic acids, and metabolites. The challenges and opportunities for the development of microwell-based single-cell analysis are also presented.

Lab-on-a-Chip Electrochemical Biosensors for Foodborne Pathogen Detection: A Review of Common Standards and Recent Progress

Foodborne pathogens are an important diagnostic target for the food, beverage, and health care industries due to their prevalence and the adverse effects they can cause to public health, food safety, and the economy. The standards that determine whether a given type of food is fit for consumption are set by governments and must be taken into account when designing a new diagnostic tool such as a biosensor platform. In order to meet these stringent detection limits, cost, and reliability standards, recent research has been focused on developing lab-on-a-chip-based approaches for detection devices that use microfluidic channels and platforms. The microfluidics-based devices are designed, developed, and used in different ways to achieve the established common standards for food pathogen testing that enable high throughput, rapid detection, low sample volume, and minimal pretreatment procedures. Combining microfluidic approaches with electrochemical biosensing could offer affordable, portable, and easy to use devices for food pathogen diagnostics. This review presents an analysis of the established common standards and the recent progress made in electrochemical sensors toward the development of future lab-on-a-chip devices that will aid 'collection-to-detection' using a single method and platform.

Microfluidic preparation of optical sensors for biomedical applications

Optical biosensors are platforms that translate biological information into detectable optical signals, and have extensive applications in various fields due to their characteristics of high sensitivity, high specificity, dynamic sensing, etc. The development of optical sensing materials is an important part of optical sensors. In this review, we emphasize the role of microfluidic technology in the preparation of optical sensing materials and the application of the derived optical sensors in the biomedical field. We first present some common optical sensing mechanisms and the functional responsive materials involved. Then, we describe the preparation of these sensing materials by microfluidics. Afterward, we enumerate the biomedical applications of these optical materials as biosensors in disease diagnosis, drug evaluation, and organ-on-a-chip. Finally, we discuss the challenges and prospects in this field.

A review on the recent progress, opportunities, and challenges of 4D printing and bioprinting in regenerative medicine

Four-dimensional (4 D) printing is a novel emerging technology, which can be defined as the ability of 3 D printed materials to change their form and functions. The term 'time' is added to 3 D printing as the fourth dimension, in which materials can respond to a stimulus after finishing the manufacturing process. 4 D printing provides more versatility in terms of size, shape, and structure after printing the construct. Complex material programmability, multi-material printing, and precise structure design are the essential requirements of 4 D printing systems. The utilization of stimuli-responsive polymers has increasingly taken the place of cell traction force-dependent methods and manual folding, offering a more advanced technique to affect a construct's adjusted shape transformation. The present review highlights the concept of 4 D printing and the responsive bioinks used in 4 D printing, such as water-responsive, pH-responsive, thermo-responsive, and light-responsive materials used in tissue regeneration. Cell traction force methods are described as well. Finally, this paper aims to introduce the limitations and future trends of 4 D printing in biomedical applications based on selected key references from the last decade.