Liquefied Microcapsules as Dual-Microcarriers for 3D+3D Bottom-Up Tissue Engineering

Strategies to decouple cell micro-scale and macro-scale environments for designing multifunctional biomimetic tissues

The regulation of cellular behavior within a three-dimensional (3D) environment to execute a specific function remains a challenge in the field of tissue engineering. In native tissues, cells and matrices are arranged into 3D modular units, comprising biochemical and biophysical signals that orchestrate specific cellular activities. Modular tissue engineering aims to emulate this natural complexity through the utilization of functional building blocks with unique stimulation features. By adopting a modular approach and using well-designed biomaterials, cellular microenvironments can be effectively decoupled from their macro-scale surroundings, enabling the development of engineered tissues with enhanced multifunctionality and heterogeneity. We overview recent advancements in decoupling the cellular micro-scale niches from their macroenvironment and evaluate the implications of this strategy on cellular and tissue functionality.

Biofabrication of engineered blood vessels for biomedical applications

To successfully engineer large-sized tissues, establishing vascular structures is essential for providing oxygen, nutrients, growth factors and cells to prevent necrosis at the core of the tissue. The diameter scale of the biofabricated vasculatures should range from 100 to 1,000 µm to support the mm-size tissue while being controllably aligned and spaced within the diffusion limit of oxygen. In this review, insights regarding biofabrication considerations and techniques for engineered blood vessels will be presented. Initially, polymers of natural and synthetic origins can be selected, modified, and combined with each other to support maturation of vascular tissue while also being biocompatible. After they are shaped into scaffold structures by different fabrication techniques, surface properties such as physical topography, stiffness, and surface chemistry play a major role in the endothelialization process after transplantation. Furthermore, biological cues such as growth factors (GFs) and endothelial cells (ECs) can be incorporated into the fabricated structures. As variously reported, fabrication techniques, especially 3D printing by extrusion and 3D printing by photopolymerization, allow the construction of vessels at a high resolution with diameters in the desired range. Strategies to fabricate of stable tubular structures with defined channels will also be discussed. This paper provides an overview of the many advances in blood vessel engineering and combinations of different fabrication techniques up to the present time.

Single-Step Biofabrication of In Situ Spheroid-Forming Compartmentalized Hydrogel for Clinical-Sized Cartilage Tissue Formation

3D cellular spheroids offer more biomimetic microenvironments than conventional 2D cell culture technologies, which has proven value for many tissue engineering applications. Despite beneficiary effects of 3D cell culture, clinical translation of spheroid tissue engineering is challenged by limited scalability of current spheroid formation methods. Although recent adoption of droplet microfluidics can provide a continuous production process, use of oils and surfactants, generally low throughput, and requirement of additional biofabrication steps hinder clinical translation of spheroid culture. Here, the use of clean (e.g., oil-free and surfactant-free), ultra-high throughput (e.g., 8.5 mL min-1 , 10 000 spheroids s-1 ), single-step, in-air microfluidic biofabrication of spheroid forming compartmentalized hydrogels is reported. This novel technique can reliably produce 1D fibers, 2D planes, and 3D volumes compartmentalized hydrogel constructs, which each allows for distinct (an)isotropic orientation of hollow spheroid-forming compartments. Spheroids produced within ink-jet bioprinted compartmentalized hydrogels outperform 2D cell cultures in terms of chondrogenic behavior. Moreover, the cellular spheroids can be harvested from compartmentalized hydrogels and used to build shape-stable centimeter-sized biomaterial-free living tissues in a bottom-up manner. Consequently, it is anticipated that in-air microfluidic production of spheroid-forming compartmentalized hydrogels can advance production and use of cellular spheroids for various biomedical applications.

Mass production of lumenogenic human embryoid bodies and functional cardiospheres using in-air-generated microcapsules

Organoids are engineered 3D miniature tissues that are defined by their organ-like structures, which drive a fundamental understanding of human development. However, current organoid generation methods are associated with low production throughputs and poor control over size and function including due to organoid merging, which limits their clinical and industrial translation. Here, we present a microfluidic platform for the mass production of lumenogenic embryoid bodies and functional cardiospheres. Specifically, we apply triple-jet in-air microfluidics for the ultra-high-throughput generation of hollow, thin-shelled, hydrogel microcapsules that can act as spheroid-forming bioreactors in a cytocompatible, oil-free, surfactant-free, and size-controlled manner. Uniquely, we show that microcapsules generated by in-air microfluidics provide a lumenogenic microenvironment with near 100% efficient cavitation of spheroids. We demonstrate that upon chemical stimulation, human pluripotent stem cell-derived spheroids undergo cardiomyogenic differentiation, effectively resulting in the mass production of homogeneous and functional cardiospheres that are responsive to external electrical stimulation. These findings drive clinical and industrial adaption of stem cell technology in tissue engineering and drug testing.

Polymer Complex Multilayers for Drug Delivery and Medical Devices

Polymer complex multilayers (PCMs) can be engineered into various structures with tunable properties via layer-by-layer (LBL) assembly driven by noncovalent forces. Due to their ease of preparation, capability of integrating multiple functional components, and excellent substrate compliance, biocompatible PCMs as coating materials or individual entities have attracted extensive attention in biomedical applications. This Spotlight on Applications presents recent progress on PCMs applied for drug delivery and medical devices. We provide several examples to address the importance of using PCM platforms to achieve controlled drug delivery including stimuli-triggered release, sustained release, and spatiotemporal sequential release. The effects of PCM coatings on the bioresponse regulation and performance enhancement of implantable devices are also highlighted. Moreover, the design and fabrication of flexible electrical and optical elements modified with LBL PCMs have been discussed, which demonstrates the great potential to advance emerging wearable devices for disease monitoring and health management.

Marine-origin polysaccharides-based free-standing multilayered membranes as sustainable nanoreservoirs for controlled drug delivery

The layer-by-layer (LbL) assembly technology has been widely used to functionalise surfaces and precisely engineer robust multilayered bioarchitectures with tunable structures, compositions, properties, and functions at the nanoscale by resorting to a myriad of building blocks exhibiting complementary interactions. Among them, marine-origin polysaccharides are a sustainable renewable resource for the fabrication of nanostructured biomaterials for biomedical applications owing to their wide bioavailability, biocompatibility, biodegradability, non-cytotoxicity, and non-immunogenic properties. Chitosan (CHT) and alginate (ALG) have been widely employed as LbL ingredients to shape a wide repertoire of size- and shape-tunable electrostatic-driven multilayered assemblies by exploring their opposite charge nature. However, the insolubility of CHT in physiological conditions intrinsically limits the range of bioapplications of the as-developed CHT-based LbL structures. Herein, we report the preparation of free-standing (FS) multilayered membranes made of water-soluble quaternised CHT and ALG biopolymers for controlled release of model drug molecules. The influence of the film structure in the drug release rate is studied by assembling two distinct set-ups of FS membranes, having the model hydrophilic drug fluorescein isothiocyanate-labelled bovine serum albumin (FITC-BSA) either as an intrinsic building block or added as an outer layer after the LbL assembly process. Both FS membranes are characterised for their thickness, morphology, in vitro cytocompatibility, and release profile, with those having FITC-BSA as an intrinsic LbL ingredient denoting a more sustained release rate. This work opens up new avenues for the design and development of a wide array of CHT-based devices for biomedical applications, overcoming the limitations associated with the insolubility of native CHT under physiological conditions.

Fabrication of cell-laden microbeads and microcapsules composed of bacterial polyglucuronic acid

In the past decades, the microencapsulation of mammalian cells into microparticles has been extensively studied for various in vitro and in vivo applications. The aim of this study was to demonstrate the viability of bacterial polyglucuronic acid (PGU), an exopolysaccharide derived from bacteria and composed of glucuronic acid units, as an effective material for cell microencapsulation. Using the method of dropping an aqueous solution of PGU-containing cells into a Ca²⁺-loaded solution, we produced spherical PGU microbeads with >93 % viability in the encapsulated human hepatoma HepG2 cells. Hollow-core microcapsules were formed via polyelectrolyte complex layer formation of PGU and poly-l-lysine, after which Ca²⁺, a cross-linker of PGU, was chelated, and this was accomplished by sequential immersion of microbeads in aqueous solutions of poly-l-lysine and sodium citrate. The encapsulated HepG2 cells proliferated and formed cell aggregates within the microparticles over a 14-day culture, with significantly larger aggregates forming within the microcapsules. Our results provide evidence for the viability of PGU for cell microencapsulation for the first time, thereby contributing to advancements in tissue engineering.

Viscous Microcapsules as Microbioreactors to Study Mesenchymal Stem/Stromal Cells Osteolineage Commitment

It is essential to design a multifunctional well-controlled platform to transfer mechanical cues to the cells in different magnitudes. This study introduces a platform, a miniaturized bioreactor, which enables to study the effect of shear stress in microsized compartmentalized structures. In this system, the well-established cell encapsulation system of liquefied capsules (LCs) is used as microbioreactors in which the encapsulated cells are exposed to variable core viscosities to experience different mechanical forces under a 3D dynamic culture. The LC technology is joined with electrospraying to produce such microbioreactors at high rates, thus allowing the application of microcapsules for high-throughput screening. Using this platform for osteogenic differentiation as an example, shows that microbioreactors with higher core viscosity which produce higher shear stress lead to significantly higher osteogenic characteristics. Moreover, in this system the forces experienced by cells in each LC are simulated by computational modeling. The maximum wall shear stress applied to the cells inside the bioreactor with low, and high core viscosity environment is estimated to be 297 and 1367 mPa, respectively, for the experimental setup employed. This work outlines the potential of LC microbioreactors as a reliable in vitro customizable platform with a wide range of applications.

Preparation and Properties of Bimetallic Chitosan Spherical Microgels

The aim of this work was to prepare bimetallic chitosan microgels with high sphericity and investigate the influences of metal-ion type and content on the size, morphology, swelling, degradation and biological properties of microgels. Amino and hydroxyl groups of chitosan (deacetylation degree, DD, of 83.2% and 96.9%) served as ligands in the Cu2+–Zn2+/chitosan complexes with various contents of cupric and zinc ions. The electrohydrodynamic atomization process was used to produce highly spherical microgels with a narrow size distribution and with surface morphology changing from wrinkled to smooth by increasing Cu2+ ions’ quantity in bimetallic systems for both used chitosans. The size of the bimetallic chitosan particles was estimated to be between 60 and 110 µm for both used chitosans, and FTIR spectroscopy indicated the formation of complexes through physical interactions between the chitosans’ functional groups and metal ions. The swelling capacity of bimetallic chitosan particles decreases as the DD and copper (II) ion content increase as a result of stronger complexation with respect to zinc (II) ions. Bimetallic chitosan microgels showed good stability during four weeks of enzymatic degradation, and bimetallic systems with smaller amounts of Cu2+ ions showed good cytocompatibility for both used chitosans.

Chitosan-Based Biomaterials: Insights into Chemistry, Properties, Devices, and Their Biomedical Applications

Chitosan is a marine-origin polysaccharide obtained from the deacetylation of chitin, the main component of crustaceans’ exoskeleton, and the second most abundant in nature. Although this biopolymer has received limited attention for several decades right after its discovery, since the new millennium chitosan has emerged owing to its physicochemical, structural and biological properties, multifunctionalities and applications in several sectors. This review aims at providing an overview of chitosan properties, chemical functionalization, and the innovative biomaterials obtained thereof. Firstly, the chemical functionalization of chitosan backbone in the amino and hydroxyl groups will be addressed. Then, the review will focus on the bottom-up strategies to process a wide array of chitosan-based biomaterials. In particular, the preparation of chitosan-based hydrogels, organic–inorganic hybrids, layer-by-layer assemblies, (bio)inks and their use in the biomedical field will be covered aiming to elucidate and inspire the community to keep on exploring the unique features and properties imparted by chitosan to develop advanced biomedical devices. Given the wide body of literature that has appeared in past years, this review is far from being exhaustive. Selected works in the last 10 years will be considered.

Scalable Production of Size-Controlled Cholangiocyte and Cholangiocarcinoma Organoids within Liver Extracellular Matrix-Containing Microcapsules

Advances in biomaterials, particularly in combination with encapsulation strategies, have provided excellent opportunities to increase reproducibility and standardization for cell culture applications. Herein, hybrid microcapsules are produced in a flow-focusing microfluidic droplet generator combined with enzymatic outside-in crosslinking of dextran-tyramine, enriched with human liver extracellular matrix (ECM). The microcapsules provide a physiologically relevant microenvironment for the culture of intrahepatic cholangiocyte organoids (ICO) and patient-derived cholangiocarcinoma organoids (CCAO). Micro-encapsulation allowed for the scalable and size-standardized production of organoids with sustained proliferation for at least 21 days in vitro. Healthy ICO (n = 5) expressed cholangiocyte markers, including KRT7 and KRT19, similar to standard basement membrane extract cultures. The CCAO microcapsules (n = 3) showed retention of stem cell phenotype and expressed LGR5 and PROM1. Furthermore, ITGB1 was upregulated, indicative of increased cell adhesion to ECM in microcapsules. Encapsulated CCAO were amendable to drug screening assays, showing a dose-response response to the clinically relevant anti-cancer drugs gemcitabine and cisplatin. High-throughput drug testing identified both pan-effective drugs as well as patient-specific resistance patterns. The results described herein show the feasibility of this one-step encapsulation approach to create size-standardized organoids for scalable production. The liver extracellular matrix-containing microcapsules can provide a powerful platform to build mini healthy and tumor tissues for potential future transplantation or personalized medicine applications.

Liquefied Microcapsules Compartmentalizing Macrophages and Umbilical Cord-Derived Cells for Bone Tissue Engineering

Extraordinary capabilities underlie the potential use of immune cells, particularly macrophages, in bone tissue engineering. Indeed, the depletion of macrophages during bone repair often culminates in disease scenarios. Inspired by the native dynamics between immune and skeletal systems, this work proposes a straightforward in vitro method to bioengineer biomimetic bone niches using biological waste. For that, liquefied and semipermeable reservoirs generated by electrohydrodynamic atomization and layer-by-layer techniques are developed to coculture umbilical cord-derived human cells, namely monocyte-derived macrophages, mesenchymal-derived stromal cells (MSCs), and human umbilical vein endothelial cells (HUVECs). Poly(ε-caprolactone) microparticles are also added to the liquefied core to act as cell carriers. The fabricated microcapsules grant the successful development of viable microtissues, ensuring the high diffusion of bioactive factors. Interestingly, macrophages within the bioengineered microcapsules increase the release of osteocalcin, osteoprotegerin, and vascular endothelial growth factor. The cytokines profile variation indicates macrophages' polarization into a prohealing phenotype. Altogether, the incorporation of macrophages within the fabricated microcapsules allows to recreate an appropriate bone microenvironment for developing new bone mineralized microtissues. The proposed bioencapsulation protocol is a powerful self-regulated system, which might find great applicability in bone tissue engineering based on bottom-up approaches or disease modeling.

Bioengineering the human bone marrow microenvironment in liquefied compartments: A promising approach for the recapitulation of osteovascular niches

Recreating the biological complexity of living bone marrow (BM) in a single in vitro strategy has faced many challenges. Most bioengineered strategies propose the co-culture of BM cellular components entrapped in different matrices limiting their migration and self-organization capacity or in open scaffolds enabling their escaping. We propose a methodology for fabricating a "human bone marrow-in-a-liquefied-capsule" to overcome these challenges, embracing the most important BM components in a single platform. Since free dispersion of the cells within the BM is an essential feature to maintain their in vivo properties, this platform provides a liquefied environment for the encapsulated cells to move freely and self-organize. Inside liquefied capsules, an engineered endosteal niche (eEN) is co-cultured with human umbilical cord cells, including endothelial cells and hematopoietic stem and progenitor cells (HSPCs). Two different human-like BM niches were recreated under static and dynamic systems. Although the culture of the engineered BM capsules (eBMC) in these different environments did not change the structural and compositional features of the BM niches, the biophysical stimulation potentiated the cellular intercommunication and the biomolecules secretion, demonstrating an enhanced in vitro bio performance. Moreover, while the eBMC without HSPCs provided the secretion of hematopoietic supportive factors, the presence of these cells recapitulated more closely the biological complexity of the native BM niches. This functional eBMC approach is an innovative platform capable of investigating several components and interactions of BM niches and how they regulate BM homeostasis and hematopoiesis. STATEMENT OF SIGNIFICANCE: The recapitulation of the multifaceted bone marrow (BM) microenvironment under in vitro conditions has gained intensive recognition to understand the intrinsic complexity of the native BM. While conventional strategies do not recapitulate the BM osteovascular niches nor give the cellular components a free movement, we report for the first time the development of human bone marrow-in-a-liquefied-capsule to overcome such limitations. Our engineered BM capsules (eBMC) partially mimic the complex structure, composition, and spatial organization of the native osteovascular niches present in the BM. This strategy offers a platform with physiological relevance to exploit the niches' components/networks and how they regulate the hematopoiesis and the initiation/progression of various BM-related pathologies.

High-Throughput Production of Microsponges from Platelet Lysate for Tissue Engineering Applications

Cell-based therapies require a large number of cells, as well as appropriate methods to deliver the cells to damaged tissue. Microcarriers provide an optimal platform for large-scale cell culture while also improving cell retention during cell delivery. However, this technology still presents significant challenges due to low-throughput fabrication methods and an inability of the microcarriers to recreate the properties of human tissue. This work proposes, for the first time, the use of methacryloyl platelet lysates (PLMA), a photocrosslinkable material derived from human platelet lysates, to produce porous microcarriers. Initially, high quantities of PLMA/alginate core-shell microcapsules are produced using coaxial electrospray. Subsequently, the microcapsules are collected, irradiated with ultraviolet light, washed, and freeze dried yielding PLMA microsponges. These microsponges are able to support the adhesion and proliferation of human adipose-derived stem cells, while also displaying potential in the assembly of autologous microtissues. Cell-laden microsponges were shown to self-organize into aggregates, suggesting possible applications in bottom-up tissue engineering applications. Impact Statement Microcarriers have increasingly been used as delivery platforms in cell therapy. Herein, the encapsulation of human-derived proteins in alginate microcapsules is proposed as a method to produce microcarriers from photopolymerizable materials. The capsules function as a template structure, which is then processed into spherical microparticles, which can be used in cell culture, cell delivery, and bottom-up assembly. As a proof of concept, this method was combined with lyophilization to process methacryloyl platelet lysates into injectable microsponges for cell delivery.

Bioengineered Hierarchical Bonelike Compartmentalized Microconstructs Using Nanogrooved Microdiscs

Fabrication of vascularized large-scale constructs for regenerative medicine remains elusive since most strategies rely solely on cell self-organization or overly control cell positioning, failing to address nutrient diffusion limitations. We propose a modular and hierarchical tissue-engineering strategy to produce bonelike tissues carrying signals to promote prevascularization. In these 3D systems, disc-shaped microcarriers featuring nanogrooved topographical cues guide cell behavior by harnessing mechanotransduction mechanisms. A sequential seeding strategy of adipose-derived stromal cells and endothelial cells is implemented within compartmentalized, liquefied-core macrocapsules in a self-organizing and dynamic system. Importantly, our system autonomously promotes osteogenesis and construct's mineralization while promoting a favorable environment for prevascular-like endothelial organization. Given its modular and self-organizing nature, our strategy may be applied for the fabrication of larger constructs with a highly controlled starting point to be used for local regeneration upon implantation or as drug-screening platforms.

Hipster microcarriers: exploring geometrical and topographical cues of non-spherical microcarriers in biomedical applications

Structure and organisation are key aspects of the native tissue environment, which ultimately condition cell fate via a myriad of processes, including the activation of mechanotransduction pathways. By modulating the formation of integrin-mediated adhesions and consequently impacting cell contractility, engineered geometrical and topographical cues may be introduced to activate downstream signalling and ultimately control cell morphology, proliferation, and differentiation. Microcarriers appear as attractive vehicles for cell-based tissue engineering strategies aiming to modulate this 3D environment, but also as vehicles for cell-free applications, given the ease in tuning their chemical and physical properties. In this review, geometry and topography are highlighted as two preponderant features in actively regulating interactions between cells and the extracellular matrix. While most studies focus on the 2D environment, we focus on how the incorporation of these strategies in 3D systems could be beneficial. The techniques applied to design 3D microcarriers with unique geometries and surface topographical cues are covered, as well as specific tissue engineering approaches employing these microcarriers. In fact, successfully achieving a functional histoarchitecture may depend on a combination of fine-tuned geometrically shaped microcarriers presenting intricately tailored topographical cues. Lastly, we pinpoint microcarrier geometry as a key player in cell-free biomaterial-based strategies, and its impact on drug release kinetics, the production of steerable microcarriers to target tumour cells, and as protein or antibody biosensors.

Core-Shell Microcapsules: Biofabrication and Potential Applications in Tissue Engineering and Regenerative Medicine

The construction of biomaterial scaffolds that accurately recreate the architecture of living tissues in vitro is a major challenge in the field of tissue engineering and regenerative medicine. Core-shell microcapsules...

Induction of mesenchymal stem cell differentiation by co-culturing with mature cells in double-layered 2-methacryloyloxyethyl phosphorylcholine polymer hydrogel matrices

The effects of differentiated cells on stem cell differentiation were analyzed via co-culturing using a cell-encapsulated double-layered hydrogel system. As a polymer hydrogel matrix, a water-soluble zwitterionic polymer having both a 2-methacryloyloxyethyl phosphorylcholine unit and a p-vinylphenylboronic acid unit (PMBV), was complexed spontaneously with poly(vinyl alcohol) (PVA) under mild cell culture conditions. The creep modulus of the hydrogel was controlled by changing the composition of the polymer in the solution. Mouse mesenchymal stem cells (MSCs), C3H10T1/2 cells, were encapsulated into PMBV/PVA hydrogels and cultured. In the PMBV/PVA hydrogel with a lower creep modulus (0.40 kPa), proliferation of C3H10T1/2 cells occurred, and the formation of cell aggregates was observed. On the other hand, a higher creep modulus (1.7 kPa) of the hydrogel matrix prevented cell proliferation. Culturing C3H10T1/2 cells encapsulated in the PMBV/PVA hydrogel in the presence of bone morphogenetic protein-2 increased the activity of intracellular alkaline phosphatase (ALP). This indicated that C3H10T1/2 cells differentiated into mature osteoblasts. When the C3H10T1/2 cells encapsulated in the PMBV/PVA hydrogel were cultured in combination with the mature osteoblasts in the hydrogel by a close contacting double-layered hydrogel structure, higher ALP activity was observed compared with the cells cultured separately. It was considered that the differentiation of C3H10T1/2 cells in the hydrogel layer was induced by cytokines diffused from mature osteoblasts encapsulated in another hydrogel layer. It could be concluded that the PMBV/PVA hydrogel system provides a good way to observe the effects of the surrounding cells on cell function in three-dimensional culture.

Design of Protein-Based Liquefied Cell-Laden Capsules with Bioinspired Adhesion for Tissue Engineering

Platforms with liquid cores are extensively explored as cell delivery vehicles for cell-based therapies and tissue engineering. However, the recurrence of synthetic materials can impair its translation into the clinic. Inspired by the adhesive proteins secreted by mussels, liquefied capsule is developed using gelatin modified with hydroxypyridinones (Gel-HOPO), a catechol analogue with oxidant-resistant properties. The protein-based liquefied macrocapsule permitted the compartmentalization of living cells by an approachable and non-time-consuming methodology resorting to i) superhydrophobic surfaces as a processing platform of hydrogel beads, ii) gelation of gelatin at temperatures < 25 °C, iii) iron coordination of the hydroxypyridinone (HOPO) moieties at physiological pH, and iv) core liquefaction at 37 °C. With the design of a proteolytically degradable shell, the possibility of encapsulating human adipose-derived mesenchymal stem cells (hASC) with and without the presence of polycaprolactone microparticles (μPCL) is evaluated. Showing prevalence toward adhesion to the inner shell wall, hASC formed a monolayer evidencing the biocompatibility and adequate mechanical properties of these platforms for proliferation, diminishing the need for μPCL as a supporting substrate. This new protein-based liquefied platform can provide biofactories devices of both fundamental and practical importance for tissue engineering and regenerative medicine or in other biotechnology fields.

Bioinstructive Layer-by-Layer-Coated Customizable 3D Printed Perfusable Microchannels Embedded in Photocrosslinkable Hydrogels for Vascular Tissue Engineering

The development of complex and large 3D vascularized tissue constructs remains the major goal of tissue engineering and regenerative medicine (TERM). To date, several strategies have been proposed to build functional and perfusable vascular networks in 3D tissue-engineered constructs to ensure the long-term cell survival and the functionality of the assembled tissues after implantation. However, none of them have been entirely successful in attaining a fully functional vascular network. Herein, we report an alternative approach to bioengineer 3D vascularized constructs by embedding bioinstructive 3D multilayered microchannels, developed by combining 3D printing with the layer-by-layer (LbL) assembly technology, in photopolymerizable hydrogels. Alginate (ALG) was chosen as the ink to produce customizable 3D sacrificial microstructures owing to its biocompatibility and structural similarity to the extracellular matrices of native tissues. ALG structures were further LbL coated with bioinstructive chitosan and arginine–glycine–aspartic acid-coupled ALG multilayers, embedded in shear-thinning photocrosslinkable xanthan gum hydrogels and exposed to a calcium-chelating solution to form perfusable multilayered microchannels, mimicking the biological barriers, such as the basement membrane, in which the endothelial cells were seeded, denoting an enhanced cell adhesion. The 3D constructs hold great promise for engineering a wide array of large-scale 3D vascularized tissue constructs for modular TERM strategies.

Minimalist Tissue Engineering Approaches Using Low Material-Based Bioengineered Systems

From an "over-engineering" era in which biomaterials played a central role, now it is observed to the emergence of "developmental" tissue engineering (TE) strategies which rely on an integrative cell-material perspective that paves the way for cell self-organization. The current challenge is to engineer the microenvironment without hampering the spontaneous collective arrangement ability of cells, while simultaneously providing biochemical, geometrical, and biophysical cues that positively influence tissue healing. These efforts have resulted in the development of low-material based TE strategies focused on minimizing the amount of biomaterial provided to the living key players of the regenerative process. Through a "minimalist-engineering" approach, the main idea is to fine-tune the spatial balance occupied by the inanimate region of the regenerative niche toward maximum actuation of the key living components during the healing process.

Electrohydrodynamic 3D Printing Scaffolds for Repair of Achilles Tendon Defect in Rats

Tissue engineering (TE) studies for Achilles tendon (AT) defects are a difficult and popular field in orthopedic medical practice. In this study, we applied electrohydrodynamic three-dimensional (3D) printing technology to construct scaffolds made of poly-(ɛ-ɛ-caprolactone) (PCL) and Pluronic F127 (F127) with different mass-volume ratios. The fibers and porous capabilities of the scaffolds were controlled using this technology. We found that F127 improved the hydrophilicity and degradation of PCL in vitro. The PCL scaffolds with 5% F127 were mostly favorable for cell adhesion and growth, suggesting that the scaffolds had good biocompatibility in vitro. Scaffolds with 5% F127 seeded with C3H10T1/2 cells were transplanted into AT defects in rats. A histological analysis indicated that the TE scaffolds were beneficial for the accumulation and arrangement of collagen fibers. Thus, this study provides fundamental experimental data for future clinical applications regarding TE for ATs.

Biopolymer-Based Microcarriers for Three-Dimensional Cell Culture and Engineered Tissue Formation

The concept of three-dimensional (3D) cell culture has been proposed to maintain cellular morphology and function as in vivo. Among different approaches for 3D cell culture, microcarrier technology provides a promising tool for cell adhesion, proliferation, and cellular interactions in 3D space mimicking the in vivo microenvironment. In particular, microcarriers based on biopolymers have been widely investigated because of their superior biocompatibility and biodegradability. Moreover, through bottom-up assembly, microcarriers have opened a bright door for fabricating engineered tissues, which is one of the cutting-edge topics in tissue engineering and regeneration medicine. This review takes an in-depth look into the recent advancements of microcarriers based on biopolymers-especially polysaccharides such as chitosan, chitin, cellulose, hyaluronic acid, alginate, and laminarin-for 3D cell culture and the fabrication of engineered tissues based on them. The current limitations and potential strategies were also discussed to shed some light on future directions.

Enzymatic outside-in cross-linking enables single-step microcapsule production for high-throughput three-dimensional cell microaggregate formation

Cell-laden hydrogel microcapsules enable the high-throughput production of cell aggregates, which are relevant for three-dimensional tissue engineering and drug screening applications. However, current microcapsule production strategies are limited by their throughput, multistep protocols, and limited amount of compatible biomaterials. We here present a single-step process for the controlled microfluidic production of single-core microcapsules using enzymatic outside-in cross-linking of tyramine-conjugated polymers. It was hypothesized that a physically, instead of the conventionally explored biochemically, controlled enzymatic cross-linking process would improve the reproducibility, operational window, and throughput of shell formation. Droplets were flown through a silicone delay line, which allowed for highly controlled diffusion of the enzymatic cross-linking initiator. The microcapsules' cross-linking density and shell thickness is strictly depended on the droplet's retention time in the delay line, which is predictably controlled by flow rate. The here presented hydrogel cross-linking method allows for facile and cytocompatible production of cell-laden microcapsules compatible with the formation and biorthogonal isolation of long-term viable cellular spheroids for tissue engineering and drug screening applications.

One-Step Rapid Fabrication of Cell-Only Living Fibers

Cellular aggregates are used as relevant regenerative building blocks, tissue models, and cell delivery platforms. Biomaterial-free structures are often assembled either as 2D cell sheets or spherical microaggregates, both incompatible with free-form deposition, and dependent on challenging processes for macroscale 3D upscaling. The continuous and elongated nature of fiber-shaped materials enables their deposition in unrestricted multiple directions. Cellular fiber fabrication has often required exogenously provided support proteins and/or the use of biomaterial-based sacrificial templates. Here, the rapid (<24 h) assembly of fiberoids is reported: living centimeter-long scaffold-free fibers of cells produced in the absence of exogenous materials or supplements. Adipose-derived mesenchymal stem cell fiberoids can be easily modulated into complex multidimensional geometries and show tissue-invasive properties while keeping the secretion of trophic factors. Proangiogenic properties studied on a chick chorioallantoic membrane in an ovo model are observed for heterotypic fiberoids containing endothelial cells. These micro-to-macrotissues may find application as morphogenic therapeutic and tissue-mimetic building blocks, with the ability to integrate 3D and 4D full biological materials.