Microfluidic Fabrication of Bioinspired Cavity-Microfibers for 3D Scaffolds

Microfiber Fabricated via Microfluidic Spinning toward Tissue Engineering Applications

Microfibers, a type of long, thin, and flexible material, can be assembled into functional 3D structures by folding, binding, and weaving. As a novel spinning method, combining microfluidic technology and wet spinning, microfluidic spinning technology can precisely control the size, morphology, structure, and composition of the microfibers. Particularly, the process is mild and rapid, which is suitable for preparing microfibers using biocompatible materials and without affecting the viability of cells encapsulated. Furthermore, owing to the controllability of microfluidic spinning, microfibers with well-defined structures (such as hollow structures) will contribute to the exchange of nutrients or guide cell orientation. Thus, this method is often used to fabricate microfibers as cell scaffolds for cell encapsulation or adhesion and can be further applied to biomimetic fibrous tissues. In this review, the focus is on different fiber structures prepared by microfluidic spinning technology, including solid, hollow, and heterogeneous structures, generated from three essential elements: spinning platform, fiber composition, and solidification methods. Furthermore, the application of microfibers is described with different structures in tissue engineering, such as blood vessels, skeletal muscle, bone, nerves, and lung bronchi. Finally, the challenges and future development prospects of microfluidic spinning technology in tissue engineering applications are discussed.

Alginate microfibers as therapeutic delivery scaffolds and tissue mimics

Alginate, a naturally occurring polysaccharide, has been widely used in cell encapsulation, 3D culture, cell therapy, tissue engineering, and regenerative medicine. Alginate's frequent use comes from its biocompatibility and ability to easily form hydrogel in a variety of forms (e.g. microcapsules, microfibers, and porous scaffolds), which can provide immunoprotection for cell therapy and mimic the extracellular matrix for tissue engineering. During the past 15 years, alginate hydrogel microfibers have attracted more and more attention due to its continuous thin tubular structures (diameter or shell thickness ⩽ 200 µm), high-density cell growth, high handleability and retrievability, and scalability. This review article provides a concise overview of alginate and its resultant hydrogel microfibers for the purpose of promoting multidisciplinary, collaborative, and convergent research in the field. It starts with a historical review of alginate as biomaterials and provides basics about alginate structure, properties, and mechanisms of hydrogel formation, followed by current challenges in effective cell delivery and functional tissue engineering. In particular, this work discusses how alginate microfiber technology could provide solutions to unmet needs with a focus on the current state of the art of alginate microfiber technology and its applications in 3D cell culture, cell delivery, and tissue engineering. At last, we discuss future directions in the perspective of alginate-based advanced technology development in biology and medicine.

Microfluidics-Enabled Soft Manufacture of Materials with Tailorable Wettability

Microfluidics and wettability are interrelated and mutually reinforcing fields, experiencing synergistic growth. Surface wettability is paramount in regulating microfluidic flows for processing and manipulating fluids at the microscale. Microfluidics, in turn, has emerged as a versatile platform for tailoring the wettability of materials. We present a critical review on the microfluidics-enabled soft manufacture (MESM) of materials with well-controlled wettability and their multidisciplinary applications. Microfluidics provides a variety of liquid templates for engineering materials with exquisite composition and morphology, laying the foundation for precisely controlling the wettability. Depending on the degree of ordering, liquid templates are divided into individual droplets, one-dimensional (1D) arrays, and two-dimensional (2D) or three-dimensional (3D) assemblies for the modular fabrication of microparticles, microfibers, and monolithic porous materials, respectively. Future exploration of MESM will enrich the diversity of chemical composition and physical structure for wettability control and thus markedly broaden the application horizons across engineering, physics, chemistry, biology, and medicine. This review aims to systematize this emerging yet robust technology, with the hope of aiding the realization of its full potential.

Recent Advances in Microfluidically Spun Microfibers for Tissue Engineering and Drug Delivery Applications

In recent years, the unique and tunable properties of microfluidically spun microfibers have led to tremendous advancements for the field of biomedical engineering, which have been applied to areas such as tissue engineering, wound dressing, and drug delivery, as well as cell encapsulation and cell seeding. In this article, we analyze the most recent advances in microfluidics and microfluidically spun microfibers, with an emphasis on biomedical applications. We explore in detail these new and innovative experiments, how microfibers are made, the experimental purpose of making microfibers, and the future work that can be done as a result of these new types of microfibers. We also focus on the applications of various materials used to fabricate microfibers, as well as their many promises and limitations.

Microfluidic-directed biomimetic Bulbine torta-like microfibers based on inhomogeneous viscosity rope-coil effect

Helical structures are attracting increasing attention owing to their unique typical physical and chemical properties. However, it remains a challenge to construct atypical helical structures at the microscale. This paper proposes a continuous spinning method with a microfluidic-chip-based spinning device to engineer atypical helical microfibers. The strategy causes polymer fluid to form the biomimetic Bulbine torta (BT)-like shape with the aid of the inhomogeneous viscosity rope-coil effect. In particular, the structure parameter of the BT microfibers could be optimized through the synchronous regulation of the microfluidic flow and reaction kinetics, and the obtained microfibers exhibit ultrahigh strain sensitivity, indicating great promise as exceptional candidates for constructing ideal strain sensors. In addition, single- and double-hollow BT microfibers are also prepared by introducing the core flow channel into the microfluidic chip and demonstrate high structural similarity to irregular blood vessels (e.g. varicose veins), which is promising for the actual application of blood vessel tissue engineering.

Bioinspired Fibers with Controlled Wettability: From Spinning to Application

Our knowledge on spider silks shows the importance of joining heterogeneous structures and surface chemical compositions in preparing fibers, fibrous surfaces, and 3D materials with a controllable wettability. We start our review with spider silk and proceed to the historical development of nature-inspired spinning processes, their products, and their advantages and disadvantages. Relevant wetting states are then summarized in fiber-based systems. Recent applications are reviewed, including one-dimensional spindle-knotted fibers for highly efficient fog harvesting, long-distance transport, and stimulus-responsive wettability and two-dimensional spindle-knotted fibrous systems for water collection, functional surfaces, and filtration. Finally, we offer some perspective on future research trends regarding biomimetic fibers for wetting-controlled engineering.

4D Printed Cardiac Construct with Aligned Myofibers and Adjustable Curvature for Myocardial Regeneration

As an innovative additive manufacturing process, 4D printing can be utilized to generate predesigned, self-assembly structures which can actuate time-dependent, and dynamic shape-changes. Compared to other manufacturing techniques used for tissue engineering purposes, 4D printing has the advantage of being able to fabricate reprogrammable dynamic tissue constructs that can promote uniform cellular growth and distribution. For this study, a digital light processing (DLP)-based printing technique was developed to fabricate 4D near-infrared (NIR) light-sensitive cardiac constructs with highly aligned microstructure and adjustable curvature. As the curvature of the heart is varied across its surface, the 4D cardiac constructs can change their shape on-demand to mimic and recreate the curved topology of myocardial tissue for seamless integration. To mimic the aligned structure of the human myocardium and to achieve the 4D shape change, a NIR light-sensitive 4D ink material, consisting of a shape memory polymer and graphene, was created to fabricate microgroove arrays with different widths. The results of our study illustrate that our innovative NIR-responsive 4D constructs exhibit the capacity to actuate a dynamic and remotely controllable spatiotemporal transformation. Furthermore, the optimal microgroove width was discovered via culturing human induced pluripotent stem cell-derived cardiomyocytes and mesenchymal stem cells onto the constructs' surface and analyzing both their cellular morphology and alignment. The cell proliferation profiles and differentiation of tricultured human-induced pluripotent stem cell-derived cardiomyocytes, mesenchymal stem cells, and endothelial cells, on the printed constructs, were also studied using a Cell Counting Kit-8 and immunostaining. Our results demonstrate a uniform distribution of aligned cells and excellent myocardial maturation on our 4D curved cardiac constructs. This study not only provides an efficient method for manufacturing curved tissue architectures with uniform cell distributions, but also extends the potential applications of 4D printing for tissue regeneration.

Polymorphic calcium alginate microfibers assembled using a programmable microfluidic field for cell regulation

Effectively guiding and accurately controlling cell adhesion and growth on the surfaces of specific morphological materials are key issues and hot research topics for optimizing biomaterials. Herein, novel polymorphic alginate microfibers formed through microfluidic spinning technology in a single microchip are presented. Through programming the flow and reaction kinetics in microchannels, other than self-modified micromorphic channel geometry, polymorphic microfibers with precisely tuned curvature-adjustable morphology can be obtained. Finite element (FE) simulations of the flow field (unidirectional fluid-solid coupling) proved the efficacy of the proposed control strategy. Moreover, the specific disordered-ordered cell arrangements showed a linear relationship between bioinspired alginate microfibers with different curvatures and the orientation angle of L929 cells, and diversified growth morphologies, including oblate ellipse, star, tree and strip shapes, occurred on the customizable interface curvature of the calcium alginate microfibers, providing a paradigm for using specific structured natural biomedical materials for cell regulation. This work represents a new design concept for manufacturing polymorphic fibrous biomedical materials through a unique marriage of the fields of green chemistry, hydromechanics, and biomaterials, which should be very useful for guiding the controllable construction of alginate materials for use in structural materials for biomedical and engineering purposes.

Hourglass-Shaped Microfibers

Heterotypic microfibers have been recognized as promising building blocks for the multifunctionality demanded in various fields, such as environmental and biomedical engineering. We present a novel microfluidics-based technique to generate bio-inspired microfibers with hourglass-shaped knots (named hourglass-shaped microfibers) via the integration of a non-solvent-induced phase separation (NIPS) process. The microfibers with spindle knots (named spindle-microfibers) are generated as templates at a large scale. The morphologies of spindle-microfibers can be precisely regulated by controlling the flow rates of the constituent fluids. After post-treatment of the partially gelled spindle-microfibers in ethanol, the encapsulated oil cores leak from knots, and the fibers morph into an hourglass shape. By controlling the oil core spillage and the template's configurations, a variety of hourglass-shaped microfibers can be obtained with adjustable morphologies and densities ranging from those of cavity-microfibers to those of spindle-microfibers. The hourglass-shaped microfibers preponderate spindle-microfibers in terms of changeable weight, adjustable morphologies, high specific surface areas, and enhanced surface roughness. Their unique macroscale topographies and properties lead to enhanced dehumidification and water collection abilities. This NIPS-integrated microfluidic technique offers a promising and novel way to manufacture microfibers by design, tailoring their structures and properties to suit a desired application.

Microfluidic Approach to Generate a Tadpole-Egg-Shaped Alginate Fiber and Its Application in Tissue Engineering

Herein, we introduce a facile microfluidic technique to produce a hybrid alginate fiber with a tadpole-egg shape. A triple-flow polydimethylsiloxane microfluidic device was constructed to allow the formation of oil droplets inside the alginate stream and was instantaneously gelated with the coaxially adjacent CaCl₂. The fiber entrapping the uniform oil droplets was dehydrated, leading to the formation of a distinct tadpole-egg-shaped structure. A series of diverse fiber architectures was fabricated in a controlled manner based on the flow rates of the relevant flows. The tadpole-egg-shaped alginate fibers were employed as building blocks to create a three-dimensional microwell template for cell cultures. First, the tadpole-egg-shaped alginate fibers containing the oil droplets were half-dipped into a melted agarose solution. After the solidification of the agarose gel, the alginate fibers were degraded by an ethylenediaminetetraacetic acid (EDTA) solution to generate the hemispherical microwells. Mesenchymal stem cells (MSCs) were cultured in the microwells to generate spheroids, which were induced into chondrocytes using transforming growth factor-β3. The formed MSC spheroids exhibited a relatively high ratio of cell viability with more than 95% live cells after 14 days of culture. The success of the chondrogenic differentiation was proven based on staining (Safranin O) and the glycosaminoglycan levels. The latter was significantly higher in spheroids that were induced to form chondrocytes compared to those that were not induced after 21 days of differentiation. Second, we investigated the potential of the tadpole-egg-shaped alginate fibers as microcarriers for applications in drug delivery and implantable technologies. It was revealed that the degradation of the Ca-alginate wall of the hybrid fibers to release the oil droplets required an EDTA solution with a concentration of 500 mM for a 15 min period. This result can be used to further develop the tadpole-egg-shaped alginate fibers as uniform microcarriers with multiple compartments.

Multifunctional Micro/Nanoscale Fibers Based on Microfluidic Spinning Technology

Superfine multifunctional micro/nanoscale fibrous materials with high surface area and ordered structure have attracted intensive attention for widespread applications in recent years. Microfluidic spinning technology (MST) has emerged as a powerful and versatile platform because of its various advantages such as high surface-area-to-volume ratio, effective heat transfer, and enhanced reaction rate. The resultant well-defined micro/nanoscale fibers exhibit controllable compositions, advanced structures, and new physical/chemical properties. The latest developments and achievements in microfluidic spun fiber materials are summarized in terms of the underlying preparation principles, geometric configurations, and functionalization. Variously architected structures and shapes by MST, including cylindrical, grooved, flat, anisotropic, hollow, core-shell, Janus, heterogeneous, helical, and knotted fibers, are emphasized. In particular, fiber-spinning chemistry in MST for achieving functionalization of fiber materials by in situ chemical reactions inside fibers is introduced. Additionally, the applications of the fabricated functional fibers are highlighted in sensors, microactuators, photoelectric devices, flexible electronics, tissue engineering, drug delivery, and water collection. Finally, recent progress, challenges, and future perspectives are discussed.

Hierarchical fibers for water collection inspired by spider silk

Water scarcity plagues two-thirds of the global population. Interestingly, researchers have found that spider silk exhibits excellent water-collection ability owing to its unique structure and chemical components. Based on this characteristic, numerous bioinspired fibers have been fabricated for water collection. Herein, we review the water-collection process for spider silk and recent vital advances in bioinspired fibriform materials, focusing on the water-collection mechanisms of spindle-knot fibers, which exhibit directional droplet transport, hanging mechanism and hanging ability. Also, we evaluated their water-collection abilities on a micro- and macro-scale, which gave a better view for the design of bioinspired water-collection materials. These advances enable the significant use of bioinspired fibers in water collection, which may be applied in several other fields, such as directional transport, tissue engineering, oil-water separation and biosensors.

Electric Field-Induced Cutting of Hydrogel Microfibers with Precise Length Control for Micromotors and Building Blocks

Microfiber modules with controllable lengths emerged as novel biomimetic platforms and are significant for many tissue engineering applications. However, accurately controlling the length of microfibers on the scale of millimeter or even micrometer still remains challenging. Here, a novel and scalable strategy to generate microfiber modules with precisely tunable lengths ranging from 100 to 3500 μm via an alternating current (AC) electric field is presented. To control the microfiber length, double-emulsion droplets containing a chelating agent (sodium citrate) as a spacing node are first uniformly embedded in the microfibers in a controllable spatial arrangement. This process is precisely tuned by adjusting the flow rates, thus, tailoring the resulting multicompartmental microfiber structure. Next, an AC voltage signal is used to trigger the electric field-induced cutting process, where the time-averaged electrical force acting on the induced bipolar charge from the Maxwell-Wagner structural polarization mechanism breaks the stress balance at the interfaces, rupturing the double-emulsion droplets, and resulting in the burst release of the encapsulated chelating agents into the hydrogel cavity. The outer hydrogel shell is quickly dissolved by a chemical reaction, cutting the long fiber into a series of microfiber units of given length. Furthermore, adding magnetic nanoparticles endows magnetic functionality with these microfiber modules, which are allowed to serve as micromotors and building blocks. This electro-induced cutting method provides a facile strategy for the fabrication of microfibers with desired lengths, showing considerable promise for various chemical and biological applications.