Microfluidics-Based Fabrication of Cell-Laden Hydrogel Microfibers for Potential Applications in Tissue Engineering

Engineered biomimetic micro/nano-materials for tissue regeneration

The incidence of tissue and organ damage caused by various diseases is increasing worldwide. Tissue engineering is a promising strategy of tackling this problem because of its potential to regenerate or replace damaged tissues and organs. The biochemical and biophysical cues of biomaterials can stimulate and induce biological activities such as cell adhesion, proliferation and differentiation, and ultimately achieve tissue repair and regeneration. Micro/nano materials are a special type of biomaterial that can mimic the microstructure of tissues on a microscopic scale due to its precise construction, further providing scaffolds with specific three-dimensional structures to guide the activities of cells. The study and application of biomimetic micro/nano-materials have greatly promoted the development of tissue engineering. This review aims to provide an overview of the different types of micro/nanomaterials, their preparation methods and their application in tissue regeneration.

Microfluidic Fabrication of Natural Polymer-Based Scaffolds for Tissue Engineering Applications: A Review

Natural polymers, thanks to their intrinsic biocompatibility and biomimicry, have been largely investigated as scaffold materials for tissue engineering applications. Traditional scaffold fabrication methods present several limitations, such as the use of organic solvents, the obtainment of a non-homogeneous structure, the variability in pore size and the lack of pore interconnectivity. These drawbacks can be overcome using innovative and more advanced production techniques based on the use of microfluidic platforms. Droplet microfluidics and microfluidic spinning techniques have recently found applications in the field of tissue engineering to produce microparticles and microfibers that can be used as scaffolds or as building blocks for three-dimensional structures. Compared to standard fabrication technologies, microfluidics-based ones offer several advantages, such as the possibility of obtaining particles and fibers with uniform dimensions. Thus, scaffolds with extremely precise geometry, pore distribution, pore interconnectivity and a uniform pores size can be obtained. Microfluidics can also represent a cheaper manufacturing technique. In this review, the microfluidic fabrication of microparticles, microfibers and three-dimensional scaffolds based on natural polymers will be illustrated. An overview of their applications in different tissue engineering fields will also be provided.

Microfluidic-assisted fiber production: Potentials, limitations, and prospects

Besides the conventional fiber production methods, microfluidics has emerged as a promising approach for the engineered spinning of fibrous materials and offers excellent potential for fiber manufacturing in a controlled and straightforward manner. This method facilitates low-speed prototype synthesis of fibers for diverse applications while providing superior control over reaction conditions, efficient use of precursor solutions, reagent mixing, and process parameters. This article reviews recent advances in microfluidic technology for the fabrication of fibrous materials with different morphologies and a variety of properties aimed at various applications. First, the basic principles, as well as the latest developments and achievements of microfluidic-based techniques for fiber production, are introduced. Specifically, microfluidic platforms made of glass, polymers, and/or metals, including but not limited to microfluidic chips, capillary-based devices, and three-dimensional printed devices are summarized. Then, fiber production from various materials, such as alginate, gelatin, silk, collagen, and chitosan, using different microfluidic platforms with a broad range of cross-linking agents and mechanisms is described. Therefore, microfluidic spun fibers with diverse diameters ranging from submicrometer scales to hundreds of micrometers and structures, such as cylindrical, hollow, grooved, flat, core-shell, heterogeneous, helical, and peapod-like morphologies, with tunable sizes and mechanical properties are discussed in detail. Subsequently, the practical applications of microfluidic spun fibers are highlighted in sensors for biomedical or optical purposes, scaffolds for culture or encapsulation of cells in tissue engineering, and drug delivery. Finally, different limitations and challenges of the current microfluidic technologies, as well as the future perspectives and concluding remarks, are presented.

Microfluidic Formulation of Topological Hydrogels for Microtissue Engineering

Microfluidics has recently emerged as a powerful tool in generation of submillimeter-sized cell aggregates capable of performing tissue-specific functions, so-called microtissues, for applications in drug testing, regenerative medicine, and cell therapies. In this work, we review the most recent advances in the field, with particular focus on the formulation of cell-encapsulating microgels of small "dimensionalities": "0D" (particles), "1D" (fibers), "2D" (sheets), etc., and with nontrivial internal topologies, typically consisting of multiple compartments loaded with different types of cells and/or biopolymers. Such structures, which we refer to as topological hydrogels or topological microgels (examples including core-shell or Janus microbeads and microfibers, hollow or porous microstructures, or granular hydrogels) can be precisely tailored with high reproducibility and throughput by using microfluidics and used to provide controlled "initial conditions" for cell proliferation and maturation into functional tissue-like microstructures. Microfluidic methods of formulation of topological biomaterials have enabled significant progress in engineering of miniature tissues and organs, such as pancreas, liver, muscle, bone, heart, neural tissue, or vasculature, as well as in fabrication of tailored microenvironments for stem-cell expansion and differentiation, or in cancer modeling, including generation of vascularized tumors for personalized drug testing. We review the available microfluidic fabrication methods by exploiting various cross-linking mechanisms and various routes toward compartmentalization and critically discuss the available tissue-specific applications. Finally, we list the remaining challenges such as simplification of the microfluidic workflow for its widespread use in biomedical research, bench-to-bedside transition including production upscaling, further in vivo validation, generation of more precise organ-like models, as well as incorporation of induced pluripotent stem cells as a step toward clinical applications.

Organ-on-a-chip: A new tool for in vitro research

Organ-on-a-chip (OOC, organ chip) technology can closely simulate the human microenvironment, synthesize organ-like functional units on a fluidic chip substrate, and simulate the physiology of tissues and organs. It will become an increasingly important platform for in vitro drug development and screening. Most importantly, organ-on-a-chip technology, incorporating 3D cell cultures, overcomes the traditional drawbacks of 2D (flat) cell-culture technology in vitro and in vivo animal trials, neither of which generate completely reliable results when it comes to the actual human subject. It is expected that organ chips will allow huge reductions in the incidence of failure in late-stage human trials, thus slashing the cost of drug development and speeding up the introduction of drugs that are effective. There have been three key enabling technologies that have made organ chip technology possible: 3D bioprinting, fluidic chips, and 3D cell culture, of which the last has allowed cells to be cultivated under more physiologically realistic growth conditions than 2D culture. The fusion of these advanced technologies and the addition of new research methods and algorithms has enabled the construction of chip types with different structures and different uses, providing a wide range of controllable microenvironments, both for research at the cellular level and for more reliable analysis of the action of drugs on the human body. This paper summarizes some research progress of organ-on-a-chip in recent years, outlines the key technologies used and the achievements in drug screening, and makes some suggestions concerning the current challenges and future development of organ-on-a-chip technology.

Upregulation of biochemical and biophysical properties of cell-laden microfiber, silk-hyaluronic acid composite

Cell-laden filament-like hydrogels are advantageous for many applications including drug screening, tissue engineering, and regenerative medicine. However, most of the designed filament vehicles hold weak mechanical properties, which hinder their applications in specific tissue engineering. We present a binary hybrid silk and hyaluronic acid hydrogel microfiber generated through a microfluidic system to encapsulate cells with superior mechanical properties and biocompatibility. Cell-laden hydrogel microfibers were continuously produced through coaxial double orifice microfluidic device and horseradish peroxidase mediated crosslinking, which conjugated introduce phenolic moieties in the backbone of silk fibroin and HA derivatives (Silk-Ph and HA-Ph, respectively). The iterative hybrid Silk-Ph + HA-Ph fibers were fabricated in tunable size distribution between 195 and 680 μm through control of outer flow velocity. Tensile strength and maximum stain of prepared Silk-Ph + HA-Ph sample upregulated more than three times higher than the single HA-Ph sample, which demonstrated significant impacts of synthesized silk derivative in hydrogel fiber composition. The proteolytic degradation of microfibers manipulated by hyaluronidase and collagenase treatment. Encapsulation process and crosslinking did not insert any harmful effect on cell viability (> 90%) and the cells maintained their growth ability after encapsulation process. Cellular filament-like tissue fabricated from proliferation of cells in Silk-Ph + HA-Ph microfiber.

Endothelialized microvessels fabricated by microfluidics facilitate osteogenic differentiation and promote bone repair

In bone tissue engineering, vascularization is one of the critical factors that limit the effect of biomaterials for bone repair. While various approaches have been tried to build vascular networks in bone grafts, lack of endothelialization still constitutes a major technical hurdle. In this study, we have developed a facile technique to fabricate endothelialized biomimetic microvessels (BMVs) from alginate-collagen composite hydrogels within a single step using microfluidic technology. BMVs with different sizes could be readily prepared by adjusting the flow rate of microfluids. All BMVs supported perfusion and outward penetration of substances in the tube. Endothelial cells could adhere and proliferate on the inner wall of tubes. It was also found that the expression of CD31 and secretion of BMP-2 and PDGF-BB were higher in the rat umbilical vein endothelial cells (RUVECs) in BMVs than those cultured on hydrogel. When co-cultured with bone marrow mesenchymal stem cells (BMSCs), endothelialized BMVs promoted the osteogenic differentiation of BMSCs compared to those in acellular BMV group. In vivo, markedly enhanced new bone formation was achieved by endothelialized BMVs in a rat critical-sized calvarial defect model compared to those with non-endothelialized BMVs or without BMVs. Together, findings from both in vitro and in vivo studies have proven that endothelialized BMVs function to facilitate osteogenesis and promote bone regeneration, and therefore might present an effective strategy in bone tissue engineering. STATEMENT OF SIGNIFICANCE: In bone tissue engineering, limited vascularization is one of the critical factors that limit the effect of biomaterials for bone repair. In this study, we developed a facile technique to fabricate endothelialized biomimetic microvessels (BMVs) from alginate-collagen composite hydrogels within a single step using microfluidic technology. Both in vitro and in vivo studies have proven that endothelialized BMVs function to facilitate osteogenesis and promote bone regeneration, and therefore might present an effective strategy in bone tissue engineering.

Controlled Fabrication of Bioactive Microtubes for Screening Anti-Tongue Squamous Cell Migration Drugs

The treatment of tongue squamous cell carcinoma (TSCC) faces challenges because TSCC has an aggressive biological behavior and manifests usually as widespread metastatic disease. Therefore, it is particularly important to screen out and develop drugs that inhibit tumor invasion and metastasis. Two-dimensional (2D) cell culture has been used as in vitro models to study cellular biological behavior, but growing evidence now shows that the 2D systems can result in cell bioactivities that deviate appreciably the in vivo response. It is urgent to develop a novel 3D cell migration model in vitro to simulate the tumor microenvironment as much as possible and screen out effective anti-migration drugs. Sodium alginate, has a widely used cell encapsulation material, as significant advantages. We have designed a microfluidic device to fabricate a hollow alginate hydrogel microtube model. Based on the difference in liquid flow rate, TSCC cells (Cal27) were able to be evenly distributed in the hollow microtubes, which was confirmed though fluorescence microscope and laser scanning confocal microscope (LSCM). Our microfluidic device was cheap, and commercially available and could be assembled in a modular way, which are composed of a coaxial needle, silicone hose, and syringes. It was proved that the cells grow well in artificial microtubes with extracellular matrix (ECM) proteins by LSCM and flow cytometry. Periodic motility conferred a different motor state to the cells in the microtubes, more closely resembling the environment in vivo. The quantitative analysis of tumor cell migration could be achieved simply by determining the position of the cell in the microtube cross-section. We verified the anti-migration effects of three NSAIDs drugs (aspirin, indomethacin, and nimesulide) with artificial microtubes, obtaining the same results as conventional migration experiments. The results showed that among the three NSAIDs, nimesulide showed great anti-migration potential against TSCC cells. Our method holds great potential for application in the more efficient screening of anti-migration tumor drugs.

A Microfluidic Device to Fabricate One-Step Cell Bead-Laden Hydrogel Struts for Tissue Engineering

Cell-laden structures are widely applied for a variety of tissue engineering applications, including tissue restoration. Cell-to-cell interactions in bioprinted structures are important for successful tissue restoration, because cell-cell signaling pathways can regulate tissue development and stem cell fate. However, the low degree of cell-cell interaction in conventional cell-laden bioprinted structures is challenging for the therapeutic application of this modality. Herein, a microfluidic device with cell-laden methacrylated gelatin (GelMa) bioink and alginate as a matrix hydrogel is used to fabricate a functional hybrid structure laden with cell-aggregated microbeads. This approach effectively increases the degree of cell-to-cell interaction to a level comparable to cell spheroids. The hybrid structure is obtained using a one-step process without the exhausting procedure. It consists of cell bead fabrication and an extrusion process for the cell-bead laden structure. Different flow rates are appropriately selected to develop cell-laden struts with homogeneously distributed cell beads for each hydrogel in the process. The hybrid struts exhibit significantly higher cellular activities than those of conventional alginate/GelMa struts, which are bioprinted using similar cell densities and bioink formulations. Furthermore, hybrid struts with adipose stem cells are implanted into mice, resulting in significantly higher myogenesis in comparison to normally bioprinted struts.

Facile Cell-Friendly Hollow-Core Fiber Diffusion-Limited Photofabrication

Bioprinting emerges as a powerful flexible approach for tissue engineering with prospective capability to produce tissue on demand, including biomimetic hollow-core fiber structures. In spite of significance for tissue engineering, hollow-core structures proved difficult to fabricate, with the existing methods limited to multistage, time-consuming, and cumbersome procedures. Here, we report a versatile cell-friendly photopolymerization approach that enables single-step prototyping of hollow-core as well as solid-core hydrogel fibers initially loaded with living cells. This approach was implemented by extruding cell-laden hyaluronic acid glycidyl methacrylate hydrogel directly into aqueous solution containing free radicals generated by continuous blue light photoexcitation of the flavin mononucleotide/triethanolamine photoinitiator. Diffusion of free radicals from the solution to the extruded structure initiated cross-linking of the hydrogel, progressing from the structure surface inwards. Thus, the cross-linked wall is formed and its thickness is limited by penetration of free radicals in the hydrogel volume. After developing in water, the hollow-core fiber is formed with centimeter range of lengths. Amazingly, HaCaT cells embedded in the hydrogel successfully go through the fabrication procedure. The broad size ranges have been demonstrated: from solid core to 6% wall thickness of the outer diameter, which was variable from sub-millimeter to 6 mm, and Young's modulus ∼1.6 ± 0.4 MPa. This new proof-of-concept fibers photofabrication approach opens lucrative opportunities for facile three-dimensional fabrication of hollow-core biostructures with controllable geometry.

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.

One-Step Generation of Aqueous-Droplet-Filled Hydrogel Fibers as Organoid Carriers Using an All-in-Water Microfluidic System

Hydrogel fibers are promising carriers for biological applications due to their flexible mechanical properties, well-defined spatial distribution, and excellent biocompatibility. In particular, the droplet-filled hydrogel fibers with the controllable dimension and location of droplets display great advantages to enhance the loading capacity of multiple components and biofunctions. In this work, we proposed a new all-in-water microfluidic system that allows for one-step fabrication of aqueous-droplet-filled hydrogel fibers (ADHFs) with unique morphology and tunable configurations. In the system, the aqueous droplets with equidistance are successfully arranged within the alginate calcium fibers, relying on the design of the pump valve cycle and the select of two immiscible liquids with a stable aqueous interface. The architecture of the ADHF can be flexibly controlled by adjusting the three phase flow rates and the valve switch cycle. The produced ADHFs exhibit high controllability, uniformity, biocompatibility, and stability. The established system enabled the formation of functional human islet organoids in situ through encapsulating pancreatic endocrine progenitor cells within microfibers. The generated islet organoids within droplets exhibit high cell viability and islet-specific function of insulin secretion. The proposed approach provides a new way to fabricate multifunctional hydrogel fibers for materials sciences, tissue engineering, and regenerative medicine.

Rapid Fabrication of Cell-Laden Microfibers for Construction of Aligned Biomimetic Tissue

Bottom-up engineering of tissue constructs is being rapidly developed and broadly applied in biomanufacturing. As one type of building block, cell-laden microfibers are promising for reconstruction of oriented structures and functions of linear tissues, such as skeletal muscles, myocardia, and spinal cord tissues. Herein, we propose wet-spinning method with agitating collection, wherein alginate-based material is extruded into an agitated CaCl2 bath with a magnetic rotor acting as the microfiber collector. By applying this method, we achieve rapid fabrication and oriented collection of hydrogel microfibers with diameters ranging from 100 to 400 μm. In addition, we encapsulate myoblasts in the hydrogel to form cell-laden microfibers, which show a high viability (more than 94%) during in vitro culture. Moreover, the method allows to fabricate of cell-laden core-sheath microfibers and hollow microfibers. We also fabricate 3D constructs using various methods of microfiber assembly like weaving and braiding. The assembling results suggest that the proposed method is a promising technology for bottom-up engineering of aligned biomimetic tissue constructs.

Cell-cell interaction in a coculture system consisting of CRISPR/Cas9 mediated GFP knock-in HUVECs and MG-63 cells in alginate-GelMA based nanocomposites hydrogel as a 3D scaffold

The interaction between osteogenic and angiogenic cells through a coculturing system in biocompatible materials has been considered for successfully engineering vascularized bone tissue equivalents. In this study, we developed a hydrogel-blended scaffold consisted of gelatin methacryloyl (GelMA) and alginate enriched with hydroxyapatite nanoparticles (HAP) to model an in vitro prevascularized bone construct. The hydrogel-based scaffold revealed a higher mechanical stiffness than those of pure (GelMA), alginate, and (GelMA+ HAP) hydrogels. In the present study, we generated a green fluorescent protein (GFP) knock-in umbilical vein endothelial cells (HUVECs) cell line using the CRISPR/Cas9 technology. The GFP was inserted into the human-like ROSA locus of HUVECs genome. HUVECs expressing GFP were cocultured with OB-like cells (MG-63) within three-dimensionally (3D) fabricated hydrogel to investigate the response of cocultured osteoblasts and endothelial cells in a 3D structure. Cell viability under the 3D cocultured gel was higher than the 3D monocultured. Compared to the 3D monocultured condition, the cells were aligned and developed into the vessel-like structures. During 14 days of culture periods, the cells displayed actin protrusions by the formation of spike-like filopodia in the 3D cocultured model. Angiogenic and osteogenic-related genes such as CD31, vWF, and osteocalcin showed higher expression in the cocultured versus the monocultured. These results have collectively indicated that the 3D cocultured hydrogel facilitates interaction among cells, thereby having a greater effect on angiogenic and osteogenic properties in the absence of induction media.

Simple Fabrication of Multicomponent Heterogeneous Fibers for Cell Co-Culture via Microfluidic Spinning

Microfluidic spinning, as a combination of wet spinning and microfluidic technology, has been used to develop microfibers with special structures to facilitate cell 3D culture/co-culture and microtissue formation in vitro. In this study, a simple microchip-based microfluidic spinning strategy is presented for the fabrication of multicomponent heterogeneous calcium alginate microfibers. The use of two kinds of microchip enables the one-step preparation of multicomponent heterogeneous microfibers with various arrangement patterns, including the preparation of one-, two-, and three-component microfibers by a two-layer microchip and preparation of four component microfibers with different arrangement by a membrane-sandwiched three-layer microchip. The obtained microfibers could be used to encapsulate various kinds of cells, such as the human non-small cell lung cancer cell NCI-H1650, the human fetal lung fibroblast HFL1, the normal pulmonary bronchial epithelial cell 16HBE, and human umbilical vein endothelial cells. By adding chitosan to the medium to keep the fibers stable, 3D long-term in vitro cell co-culture has been carried out up to 21 days. This method is very simple and easy to operate, continuously produces spatially well-defined heterogeneous microfibers, has important applications for composite functional biomaterials, and shows great potential in organs-on-a-chip and biomimetic systems.

A Novel pH- and Salt-Responsive N-Succinyl-Chitosan Hydrogel via a One-Step Hydrothermal Process

In this study, we synthesized a series of pH-sensitive and salt-sensitive N-succinyl-chitosan hydrogels with N-succinyl-chitosan (NSCS) and the crosslinker glycidoxypropyltrimethoxysilane (GPTMS) via a one-step hydrothermal process. The structure and morphology analysis of the NSCS and glycidoxypropyltrimethoxysilane-N-succinyl chitosan hydrogel (GNCH) revealed the close relation between the swelling behavior of hydrogels and the content of crosslinker GPTMS. The high GPTMS content could weaken the swelling capacity of hydrogels and improve their mechanical properties. The hydrogels show high pH sensitivity and reversibility in the range of pH 1.0 to 9.0, and exhibit on-off switching behavior between acidic and alkaline environments. In addition, the hydrogels perform smart swelling behaviors in NaCl, CaCl2, and FeCl3 solutions. These hydrogels may have great potential in medical applications.