Automated 3D bioassembly of micro-tissues for biofabrication of hybrid tissue engineered constructs

Engineered 3D mesenchymal stem cell aggregates with multifunctional prowess for bone regeneration: Current status and future prospects

BACKGROUND

Impaired efficacy of in vitro expanded mesenchymal stem cells (MSCs) is a universal and thorny situation, which cast a shadow on further clinical translation of exogenous MSCs. Moreover, the relatively lengthy healing process, host metabolic heterogeneity and the sophisticated cell recognition and crosstalk pose rigorous challenges towards MSC-based bone regeneration strategies. Three-dimensional (3D) cell aggregates facilitate more robust intercellular communications and cell-extracellular matrix (ECM) interactions, providing a better mimicry of microarchitectures and biochemical milieus in vivo, which is conducive for stemness maintenance and downstream bone formation.

AIM OF REVIEW

This review enunciates the phenotypic features of MSCs in aggregates, which deepens the knowledge of the MSC fate determination in 3D microenvironment. By summarizing current empowerment methods and biomaterial-combined techniques for establishing functionalized MSC aggregates, this review aims to spark innovative and promising solutions for exalting the translational value of MSCs and improve their therapeutic applications in bone tissue repair.

KEY SCIENTIFIC CONCEPTS OF REVIEW

3D aggregates optimize regenerative behaviors of in vitro cultured MSCs including cell adhesion, viability, proliferation, pluripotency and immunoregulation capacity, etc. Biomaterials hybridization endows MSC aggregates with tailored mechanical and biological properties, which offers more possibilities in adapting various clinical scenarios.

A platform for Bioengineering Tissue Membranes from cell spheroids

Cell spheroids are essential building blocks for engineering tissues like cartilage, bone, liver, cardiac, pancreatic, and neural tissues, but controlling their fusion and organisation is challenging. Spheroids tend to fuse into a larger mass, impeding nutrient and waste diffusion. To overcome this, we developed a method to assemble spheroids into a thin layer by using two mesh scaffolds to spread them evenly, and a solid frame with grid to secure the assembly. This allows the spheroids to fuse into a thin membrane-like tissue, allowing better medium diffusion during cell culture. We demonstrated this method by producing cartilage tissue membranes from human mesenchymal stem cell spheroids undergoing chondrogenic differentiation, evaluating spheroid sizes, assembly timing, fusion process and membrane thickness. Our method is a versatile platform for producing tissue membranes from cell spheroids, with significant potential in tissue engineering for creating functional tissue constructs from various cell types.

AggreBots: configuring CiliaBots through guided, modular tissue aggregation

Ciliated biobots, or CiliaBots, are a class of engineered multicellular tissues that are capable of self-actuated motility propelled by the motile cilia located on their exterior surface. Correlations have been observed between CiliaBot motility patterns and their morphology and cilia distribution. However, precise control of these structural parameters to generate desired motility patterns predictably remains lacking. Here, we developed a novel Aggregated CiliaBot (AggreBot) platform capable of producing designer motility patterns through spatially controlled aggregation of epithelial spheroids made from human airway cells (referred to as CiliaBot Building Blocks or CBBs), yielding AggreBots with configurable geometry and distribution of active cilia. Guided multi-CBB aggregation led to the production of rod-, triangle-, and diamond-shaped AggreBots, which consistently effected greater motility than traditional single-spheroid CiliaBots. Furthermore, CBBs were found to maintain internal boundaries post-aggregation through the combined action of pathways controlling cellular fluidity and tissue polarity. This boundary fidelity, combined with the use of CBBs with immotile cilia due to mutations in the CCDC39 gene, allowed for the generation of hybrid AggreBots with precision control over the coverage and distribution of active cilia, further empowering control of motility patterns. Our results demonstrate the potential of AggreBots as self-propelling biological tissues through the establishment of morphological "levers" by which alterations to tissue motility can be theoretically planned and experimentally verified.

Strategies to overcome the limitations of current organoid technology - engineered organoids

Organoids, as 3D in vitro models derived from stem cells, have unparalleled advantages over traditional cell and animal models for studying organogenesis, disease mechanisms, drug screening, and personalized diagnosis and treatment. Despite the tremendous progress made in organoid technology, the translational application of organoids still presents enormous challenges due to the complex structure and function of human organs. In this review, the limitations of the translational application of traditional organoid technologies are first described. Next, we explore ways to address many of the limitations of traditional organoid cultures by engineering various dimensions of organoid systems. Finally, we discuss future directions in the field, including potential roles in drug screening, simulated microphysiology system and personalized diagnosis and treatment. We hope that this review inspires future research into organoids and microphysiology system.

Enhancing Fracture Healing with 3D Bioprinted Hif1a-Overexpressing BMSCs Hydrogel: A Novel Approach to Accelerated Bone Repair

Addressing the urgent need for effective fracture treatments, this study investigates the efficacy of a 3D bioprinted biomimetic hydrogel, enriched with bone marrow mesenchymal stem cells (BMSCs) and targeted hypoxia-inducible factor 1 alpha (Hif1a) gene activation, in enhancing fracture healing. A photocross-linkable bioink, gelatin methacryloyl bone matrix anhydride (GBMA) is developed, and selected its 5% concentration for bioink formulation. Rat BMSCs are isolated and combined with GBMA to create the GBMA@BMSCs bioink. This bioink is then used in 3D bioprinting to fabricate a hydrogel for application in a rat femoral fracture model. Through transcriptome sequencing, WGCNA, and Venn analysis, the hypoxia-inducible factor Hif1a is identified as a critical gene in the fracture healing process. In vitro studies showed that Hif1a promoted BMSC proliferation, chondrogenic differentiation, and cartilage matrix stability. The in vivo application of the GBMA@BMSCs hydrogel with Hif1a overexpression significantly accelerated fracture healing, evidenced by early and enhanced cartilage callus formation. The study demonstrates that 3D bioprinting of GBMA@BMSCs hydrogel, particularly with Hif1a-enhanced BMSCs, offers a promising approach for rapid and effective fracture repair.

Spherical Shell Bioprinting to Produce Uniform Spheroids with Controlled Sizes

Conventional cell spheroid production methods are largely manual, leading to variations in size and shape that compromise consistency and reliability for use in cell-based therapeutic applications. To enhance spheroid production, a spherical shell bioprinting system was implemented, enabling the high-throughput generation of uniform cell spheroids with precisely controlled sizes. The system encapsulates cells within thin alginate hydrogel shells formed through bioprinting and ion crosslinking reactions. Alginate-calcium ion crosslinking created alginate shells that contained gelatin-based bioinks with embedded cells, facilitating spontaneous cell aggregation within the shells and eliminating the need for plastic wells. By adjusting cell concentrations in the alginate-gelatin bioink, we achieved precise control over spheroid size, maintaining a sphericity above 0.94 and size deviations within ±10 µm. This method has been successfully applied to various cell types including cancer cells, fibroblasts, chondrocytes, and epithelial cells, demonstrating its versatility. This scalable approach enhances the reliability of cell therapy and drug screening, offering a robust platform for future biomedical applications.

Type II photoinitiators with collagen-based cyanine for cell encapsulation under green-red LED

3D bioprinting with cell-laden materials is an emerging technique for fabricating functional tissue constructs. However, current cell-laden bioinks often lack sufficient cytocompatibility with commonly used UV-light sources. In this study, green to red photoinduced hydrogel crosslinking was obtained by introducing synthesized biosafety photoinitiators and used in light-based direct ink writing (DIW) 3D printing for enabling cell encapsulation successfully. The novel type II photointiators contain iodonium (ONI) and synthesized cyanine dyes CZBIN, TDPABIN, Col-SH-CZ, and Col-SH-TD with strong absorption in the range of 400-600 nm. Collagen-based macromolecule dyes Col-SH-CZ and Col-SH-TD showed excellent cytocompatibility. The photochemistry of these photoinitiators revealed an efficient photoinduced electron transfer (PET) process from the singlet excited states of the dyes to iodonium (ONI), facilitating the crosslinking of the biogels. L929 cells were encapsulated in Gel-MA hydrogels containing various photoinitiating systems and exposed to near-ultraviolet, green, or red LED irradiation. DIW-type 3D printing of Gel-MA bioink with L929 cells was also evaluated. The cell viability achieved with green light encapsulation reached 90 %. This novel approach offers promising prospects for bioprinting functional tissues with enhanced cytocompatibility under visible light conditions.

Bioprinting of Cells, Organoids and Organs-on-a-Chip Together with Hydrogels Improves Structural and Mechanical Cues

The 3D bioprinting technique has made enormous progress in tissue engineering, regenerative medicine and research into diseases such as cancer. Apart from individual cells, a collection of cells, such as organoids, can be printed in combination with various hydrogels. It can be hypothesized that 3D bioprinting will even become a promising tool for mechanobiological analyses of cells, organoids and their matrix environments in highly defined and precisely structured 3D environments, in which the mechanical properties of the cell environment can be individually adjusted. Mechanical obstacles or bead markers can be integrated into bioprinted samples to analyze mechanical deformations and forces within these bioprinted constructs, such as 3D organoids, and to perform biophysical analysis in complex 3D systems, which are still not standard techniques. The review highlights the advances of 3D and 4D printing technologies in integrating mechanobiological cues so that the next step will be a detailed analysis of key future biophysical research directions in organoid generation for the development of disease model systems, tissue regeneration and drug testing from a biophysical perspective. Finally, the review highlights the combination of bioprinted hydrogels, such as pure natural or synthetic hydrogels and mixtures, with organoids, organoid-cell co-cultures, organ-on-a-chip systems and organoid-organ-on-a chip combinations and introduces the use of assembloids to determine the mutual interactions of different cell types and cell-matrix interferences in specific biological and mechanical environments.

Laser-assisted bioprinting of targeted cartilaginous spheroids for high density bottom-up tissue engineering

Multicellular spheroids such as microtissues and organoids have demonstrated great potential for tissue engineering applications in recent years as these 3D cellular units enable improved cell-cell and cell-matrix interactions. Current bioprinting processes that use multicellular spheroids as building blocks have demonstrated limited control on post printing distribution of cell spheroids or moderate throughput and printing efficiency. In this work, we presented a laser-assisted bioprinting approach able to transfer multicellular spheroids as building blocks for larger tissue structures. Cartilaginous multicellular spheroids formed by human periosteum derived cells (hPDCs) were successfully bioprinted possessing high viability and the capacity to undergo chondrogenic differentiation post printing. Smaller hPDC spheroids with diameters ranging from ∼100 to 150µm were successfully bioprinted through the use of laser-induced forward transfer method (LIFT) however larger spheroids constituted a challenge. For this reason a novel alternative approach was developed termed as laser induced propulsion of mesoscopic objects (LIPMO) whereby we were able to bioprint spheroids of up to 300µm. Moreover, we combined the bioprinting process with computer aided image analysis demonstrating the capacity to 'target and shoot', through automated selection, multiple large spheroids in a single sequence. By taking advantage of target and shoot system, multilayered constructs containing high density cell spheroids were fabricated.

Droplet-based microfluidics for engineering shape-controlled hydrogels with stiffness gradient

Current biofabrication strategies are limited in their ability to replicate native shape-to-function relationships, that are dependent on adequate biomimicry of macroscale shape as well as size and microscale spatial heterogeneity, within cell-laden hydrogels. In this study, a novel diffusion-based microfluidics platform is presented that meets these needs in a two-step process. In the first step, a hydrogel-precursor solution is dispersed into a continuous oil phase within the microfluidics tubing. By adjusting the dispersed and oil phase flow rates, the physical architecture of hydrogel-precursor phases can be adjusted to generate spherical and plug-like structures, as well as continuous meter-long hydrogel-precursor phases (up to 1.75 m). The second step involves the controlled introduction a small molecule-containing aqueous phase through a T-shaped tube connector to enable controlled small molecule diffusion across the interface of the aqueous phase and hydrogel-precursor. Application of this system is demonstrated by diffusing co-initiator sodium persulfate (SPS) into hydrogel-precursor solutions, where the controlled SPS diffusion into the hydrogel-precursor and subsequent photo-polymerization allows for the formation of unique radial stiffness patterns across the shape- and size-controlled hydrogels, as well as allowing the formation of hollow hydrogels with controllable internal architectures. Mesenchymal stromal cells are successfully encapsulated within hollow hydrogels and hydrogels containing radial stiffness gradient and found to respond to the heterogeneity in stiffness through the yes-associated protein mechano-regulator. Finally, breast cancer cells are found to phenotypically switch in response to stiffness gradients, causing a shift in their ability to aggregate, which may have implications for metastasis. The diffusion-based microfluidics thus finds application mimicking native shape-to-function relationship in the context of tissue engineering and provides a platform to further study the roles of micro- and macroscale architectural features that exist within native tissues.

Propelling Minimally Invasive Tissue Regeneration With Next-Era Injectable Pre-Formed Scaffolds

The growing aging population, with its associated chronic diseases, underscores the urgency for effective tissue regeneration strategies. Biomaterials play a pivotal role in the realm of tissue reconstruction and regeneration, with a distinct shift toward minimally invasive (MI) treatments. This transition, fueled by engineered biomaterials, steers away from invasive surgical procedures to embrace approaches offering reduced trauma, accelerated recovery, and cost-effectiveness. In the realm of MI tissue repair and cargo delivery, various techniques are explored. While in situ polymerization is prominent, it is not without its challenges. This narrative review explores diverse biomaterials, fabrication methods, and biofunctionalization for injectable pre-formed scaffolds, focusing on their unique advantages. The injectable pre-formed scaffolds, exhibiting compressibility, controlled injection, and maintained mechanical integrity, emerge as promising alternative solutions to in situ polymerization challenges. The conclusion of this review emphasizes the importance of interdisciplinary design facilitated by synergizing fields of materials science, advanced 3D biomanufacturing, mechanobiological studies, and innovative approaches for effective MI tissue regeneration.

Modular formation of in vitro tumor models for oncological research/therapeutic drug screening

In recognition of the lethal nature of cancer, extensive efforts have been made to understand the mechanistic causation while identifying the effective therapy modality in hope to eradicate cancerous cells with minimal damage to healthy cells. In search of such effective therapeutics, establishing pathophysiologically relevant in vitro models would be of importance in empowering our capabilities of truly identifying those potent ones with significantly reduction of the preclinical periods for rapid translation. In this regard, wealthy progresses have been achieved over past decades in establishing various in vitro and in vivo tumor models. Ideally, the tumor models should maximally recapture the key pathophysiological attributes of their native counterparts. Many of the current models have demonstrated their utilities but also showed some noticeable limitations. This book chapter will briefly review some of the mainstream platforms for in vitro tumor models followed by detailed elaboration on the modular strategies to form in vitro tumor models with complex structures and spatial organization of cellular components. Clearly, with the ability to modulate the building modules it becomes a new trend to form in vitro tumor models following a bottom-up approach, which offers a high flexibility to satisfy the needs for pathophysiological study, anticancer drug screening or design of personalized treatment.

Bioprinting Using Organ Building Blocks: Spheroids, Organoids, and Assembloids

Three-dimensional (3D) bioprinting, a promising advancement in tissue engineering technology, involves the robotic, layer-by-layer additive biofabrication of functional 3D tissue and organ constructs. This process utilizes biomaterials, typically hydrogels and living cells, following digital models. Traditional tissue engineering uses a classic triad of living cells, scaffolds, and physicochemical signals in bioreactors. A scaffold is a temporary, often biodegradable, support structure. Tissue engineering primarily falls into two categories: (i) scaffold based and (ii) scaffold free. The latter, scaffold-free 3D bioprinting, is gaining increasing popularity. Organ building blocks (OBB), capable of self-assembly and self-organization, such as tissue spheroids, organoids, and assembloids, have begun to be utilized in scaffold-free bioprinting. This article discusses the expanding range of OBB, presents the rapidly evolving collection of bioprinting and bioassembly methods using these OBB, and finally, outlines the advantages, challenges, and future perspectives of using OBB in organ printing.

Converging bioprinting and organoids to better recapitulate the tumor microenvironment

Bioprinting shows excellent potential for preclinical tumor modeling, with significant advantages over 2D cell cultures in replicating the tumor microenvironment (TME). Recently, the use of tumor organoids in bioprinting models has emerged as a groundbreaking approach to simulate volumetric tumor tissues. This synergetic fabrication method leverages the advantages of the spatial and geometric control of bioprinting to assemble heterogeneous TME components, while tumor organoids maintain collective cell behaviors. In this review, we provide a landscape of the latest progress on the convergence of 3D bioprinting and tumor organoids. Furthermore, we discuss the potential to incorporate organ-on-a-chip with bioprinting tumor organoids to improve the biomimicry and predictability of therapeutic performance. Lastly, we address the challenges to personalized medicine and predictive clinical integration.

Engineered 3D liver-tissue model with minispheroids formed by a bioprinting process supported with in situ electrical stimulation

Three-dimensional (3D) bioprinting, an effective technique for building cell-laden structures providing native extracellular matrix environments, presents challenges, including inadequate cellular interactions. To address these issues, cell spheroids offer a promising solution for improving their biological functions. Particularly, minispheroids with 50-100 μm diameters exhibit enhanced cellular maturation. We propose a one-step minispheroid-forming bioprinting process incorporating electrical stimulation (E-MS-printing). By stimulating the cells, minispheroids with controlled diameters were generated by manipulating the bioink viscosity and stimulation intensity. To validate its feasibility, E-MS-printing process was applied to fabricate an engineered liver model designed to mimic the hepatic lobule unit. E-MS-printing was employed to print the hepatocyte region, followed by bioprinting the central vein using a core-shell nozzle. The resulting constructs displayed native liver-mimetic structures containing minispheroids, which facilitated improved hepatic cell maturation, functional attributes, and vessel formation. Our results demonstrate a new potential 3D liver model that can replicate native liver tissues.

Injectable MSC Spheroid and Microgel Granular Composites for Engineering Tissue

Many cell types require direct cell-cell interactions for differentiation and function; yet, this can be challenging to incorporate into 3-dimensional (3D) structures for the engineering of tissues. Here, a new approach is introduced that combines aggregates of cells (spheroids) with similarly-sized hydrogel particles (microgels) to form granular composites that are injectable, undergo interparticle crosslinking via light for initial stabilization, permit cell-cell contacts for cell signaling, and allow spheroid fusion and growth. One area where this is important is in cartilage tissue engineering, as cell-cell contacts are crucial to chondrogenesis and are missing in many tissue engineering approaches. To address this, granular composites are developed from adult porcine mesenchymal stromal cell (MSC) spheroids and hyaluronic acid microgels and simulations and experimental analyses are used to establish the importance of initial MSC spheroid to microgel volume ratios to balance mechanical support with tissue growth. Long-term chondrogenic cultures of granular composites produce engineered cartilage tissue with extensive matrix deposition and mechanical properties within the range of cartilage, as well as integration with native tissue. Altogether, a new strategy of injectable granular composites is developed that leverages the benefits of cell-cell interactions through spheroids with the mechanical stabilization afforded with engineered hydrogels.

Bioassembly of hemoglobin-loaded photopolymerizable spheroids alleviates hypoxia-induced cell death

The delivery of oxygen within tissue engineered constructs is essential for cell survivability; however, achieving this within larger biofabricated constructs poses a significant challenge. Efforts to overcome this limitation often involve the delivery of synthetic oxygen generating compounds. The application of some of these compounds is problematic for the biofabrication of living tissues due to inherent issues such as cytotoxicity, hyperoxia and limited structural stability due to oxygen inhibition of radical-based crosslinking processes. This study aims to develop an oxygen delivering system relying on natural-derived components which are cytocompatible, allow for photopolymerization and advanced biofabrication processes, and improve cell survivability under hypoxia (1% O2). We explore the binding of human hemoglobin (Hb) as a natural oxygen deposit within photopolymerizable allylated gelatin (GelAGE) hydrogels through the spontaneous complex formation of Hb with negatively charged biomolecules (heparin, hyaluronic acid, and bovine serum albumin). We systematically study the effect of biomolecule inclusion on cytotoxicity, hydrogel network properties, Hb incorporation efficiency, oxygen carrying capacity, cell viability, and compatibility with 3D-bioassembly processes within melt electrowritten (MEW) scaffolds. All biomolecules were successfully incorporated within GelAGE hydrogels, displaying controllable mechanical properties and cytocompatibility. Results demonstrated efficient and tailorable Hb incorporation within GelAGE-Heparin hydrogels. The developed system was compatible with microfluidics and photopolymerization processes, allowing for the production of GelAGE-Heparin-Hb spheres. Hb-loaded spheres were assembled into MEW polycaprolactone scaffolds, significantly increasing the local oxygen levels. Ultimately, cells within Hb-loaded constructs demonstrated good cell survivability under hypoxia. Taken together, we successfully developed a hydrogel system that retains Hb as a natural oxygen deposit post-photopolymerization, protecting Hb from free-radical oxidation while remaining compatible with biofabrication of large constructs. The developed GelAGE-Heparin-Hb system allows for physoxic oxygen delivery and thus possesses a vast potential for use across broad tissue engineering and biofabrication strategies to help eliminate cell death due to hypoxia.

Scaffolded spheroids as building blocks for bottom-up cartilage tissue engineering show enhanced bioassembly dynamics

Due to the capability of cell spheroids (SPH) to assemble into large high cell density constructs, their use as building blocks attracted a lot of attention in the field of biofabrication. Nevertheless, upon maturation, the composition along with the size of such building blocks change, affecting their fusiogenic ability to form a cohesive tissue construct of controllable size. This natural phenomenon remains a limitation for the standardization of spheroid-based therapies in the clinical setting. We recently showed that scaffolded spheroids (S-SPH) can be produced by forming spheroids directly within porous PCL-based microscaffolds fabricated using multiphoton lithography (MPL). In this new study, we compare the bioassembly potential of conventional SPHs versus S-SPHs depending on their degree of maturation. Doublets of both types of building blocks were cultured and their fusiogenicity was compared by measuring the intersphere angle, the length of the fusing spheroid pairs (referred to as doublet length) as well as their spreading behaviour. Finally, the possibility to fabricate macro-sized tissue constructs (i.e. cartilage-like) from both chondrogenic S-SPHs and SPHs was analyzed. This study revealed that, in contrast to conventional SPHs, S-SPHs exhibit robust and stable fusiogenicity, independently from their degree of maturation. In order to understand this behavior, we further analyze the intersection area of doublets, looking at the kinetic of cell migration and at the mechanical stability of the formed tissue using dissection measurements. Our findings indicate that the presence of microscaffolds enhances the ability of spheroids to be used as building blocks for bottom-up tissue engineering, which is an important advantage compared to conventional spheroid-based therapy approaches. STATEMENT OF SIGNIFICANCE: The approach of using SPHs as building blocks for bottom-up tissue engineering offers a variety of advantages. At the same time the self-assembly of large tissues remains challenging due to several intrinsic properties of SPHs, such as for instance the shrinkage of tissues assembled from SPHs, or the reduced fusiogenicity commonly observed with mature SPHs. In this work, we demonstrate the capability of scaffolded spheroids (S-SPH) to fuse and recreate cartilage-like tissue constructs despite their advanced maturation stage. In this regard, the presence of microscaffolds compensates for some of the intrinsic limitations of SPHs and can help to overcome current limitations of spheroid-based tissue engineering.

Temporal Enzymatic Treatment to Enhance the Remodeling of Multiple Cartilage Microtissues into a Structurally Organized Tissue

Scaffold-free tissue engineering aims to recapitulate key aspects of normal developmental processes to generate biomimetic grafts. Although functional cartilaginous tissues are engineered using such approaches, considerable challenges remain. Herein, the benefits of engineering cartilage via the fusion of multiple cartilage microtissues compared to using (millions of) individual cells to generate a cartilaginous graft are demonstrated. Key advantages include the generation of a richer extracellular matrix, more hyaline-like cartilage phenotype, and superior shape fidelity. A major drawback of aggregate engineering is that individual microtissues do not completely (re)model and remnants of their initial architectures remain throughout the macrotissue. To address this, a temporal enzymatic (chondroitinase-ABC) treatment is implemented to accelerate structural (re)modeling and shown to support robust fusion between adjacent microtissues, enhance microtissue (re)modeling, and enable the development of a more biomimetic tissue with a zonally organized collagen network. Additionally, enzymatic treatment is shown to modulate matrix composition, tissue phenotype, and to a lesser extent, tissue mechanics. This work demonstrates that microtissue self-organization is an effective method for engineering scaled-up cartilage grafts with a predefined geometry and near-native levels of matrix accumulation. Importantly, key limitations associated with using biological building blocks can be alleviated by temporal enzymatic treatment during graft development.

Engineering three-dimensional bone macro-tissues by guided fusion of cell spheroids

Introduction

Bioassembly techniques for the application of scaffold-free tissue engineering approaches have evolved in recent years toward producing larger tissue equivalents that structurally and functionally mimic native tissues. This study aims to upscale a 3-dimensional bone in-vitro model through bioassembly of differentiated rat osteoblast (dROb) spheroids with the potential to develop and mature into a bone macrotissue.

Methods

dROb spheroids in control and mineralization media at different seeding densities (1 × 104, 5 × 104, and 1 × 105 cells) were assessed for cell proliferation and viability by trypan blue staining, for necrotic core by hematoxylin and eosin staining, and for extracellular calcium by Alizarin red and Von Kossa staining. Then, a novel approach was developed to bioassemble dROb spheroids in pillar array supports using a customized bioassembly system. Pillar array supports were custom-designed and printed using Formlabs Clear Resin® by Formlabs Form2 printer. These supports were used as temporary frameworks for spheroid bioassembly until fusion occurred. Supports were then removed to allow scaffold-free growth and maturation of fused spheroids. Morphological and molecular analyses were performed to understand their structural and functional aspects.

Results

Spheroids of all seeding densities proliferated till day 14, and mineralization began with the cessation of proliferation. Necrotic core size increased over time with increased spheroid size. After the bioassembly of spheroids, the morphological assessment revealed the fusion of spheroids over time into a single macrotissue of more than 2.5 mm in size with mineral formation. Molecular assessment at different time points revealed osteogenic maturation based on the presence of osteocalcin, downregulation of Runx2 (p < 0.001), and upregulated alkaline phosphatase (p < 0.01).

Discussion

With the novel bioassembly approach used here, 3D bone macrotissues were successfully fabricated which mimicked physiological osteogenesis both morphologically and molecularly. This biofabrication approach has potential applications in bone tissue engineering, contributing to research related to osteoporosis and other recurrent bone ailments.

Advances in tissue engineering and biofabrication for in vitro skin modeling

The global prevalence of skin disease and injury is continually increasing, yet conventional cell-based models used to study these conditions do not accurately reflect the complexity of human skin. The lack of inadequate in vitro modeling has resulted in reliance on animal-based models to test pharmaceuticals, biomedical devices, and industrial and environmental toxins to address clinical needs. These in vivo models are monetarily and morally expensive and are poor predictors of human tissue responses and clinical trial outcomes. The onset of three-dimensional (3D) culture techniques, such as cell-embedded and decellularized approaches, has offered accessible in vitro alternatives, using innovative scaffolds to improve cell-based models' structural and histological authenticity. However, these models lack adequate organizational control and complexity, resulting in variations between structures and the exclusion of physiologically relevant vascular and immunological features. Recently, biofabrication strategies, which combine biology, engineering, and manufacturing capabilities, have emerged as instrumental tools to recreate the heterogeneity of human skin precisely. Bioprinting uses computer-aided design (CAD) to yield robust and reproducible skin prototypes with unprecedented control over tissue design and assembly. As the interdisciplinary nature of biofabrication grows, we look to the promise of next-generation biofabrication technologies, such as organ-on-a-chip (OOAC) and 4D modeling, to simulate human tissue behaviors more reliably for research, pharmaceutical, and regenerative medicine purposes. This review aims to discuss the barriers to developing clinically relevant skin models, describe the evolution of skin-inspired in vitro structures, analyze the current approaches to biofabricating 3D human skin mimetics, and define the opportunities and challenges in biofabricating skin tissue for preclinical and clinical uses.

Volumetric Printing Across Melt Electrowritten Scaffolds Fabricates Multi-Material Living Constructs with Tunable Architecture and Mechanics

Major challenges in biofabrication revolve around capturing the complex, hierarchical composition of native tissues. However, individual 3D printing techniques have limited capacity to produce composite biomaterials with multi-scale resolution. Volumetric bioprinting recently emerged as a paradigm-shift in biofabrication. This ultrafast, light-based technique sculpts cell-laden hydrogel bioresins into 3D structures in a layerless fashion, providing enhanced design freedom over conventional bioprinting. However, it yields prints with low mechanical stability, since soft, cell-friendly hydrogels are used. Herein, the possibility to converge volumetric bioprinting with melt electrowriting, which excels at patterning microfibers, is shown for the fabrication of tubular hydrogel-based composites with enhanced mechanical behavior. Despite including non-transparent melt electrowritten scaffolds in the volumetric printing process, high-resolution bioprinted structures are successfully achieved. Tensile, burst, and bending mechanical properties of printed tubes are tuned altering the electrowritten mesh design, resulting in complex, multi-material tubular constructs with customizable, anisotropic geometries that better mimic intricate biological tubular structures. As a proof-of-concept, engineered tubular structures are obtained by building trilayered cell-laden vessels, and features (valves, branches, fenestrations) that can be rapidly printed using this hybrid approach. This multi-technology convergence offers a new toolbox for manufacturing hierarchical and mechanically tunable multi-material living structures.

Engineered biomaterials to guide spheroid formation, function, and fabrication into 3D tissue constructs

Cellular spheroids are aggregates of cells that are being explored to address fundamental biological questions and as building blocks for engineered tissues. Spheroids possess distinct advantages over cellular monolayers or cell encapsulation in 3D natural and synthetic hydrogels, including direct cell-cell interactions and high cell densities, which better mimic aspects of many tissues. Despite these advantages, spheroid cultures often exhibit uncontrollable growth and may be too simplistic to mimic complex tissue structures. To address this, biomaterials are being leveraged to further expand the use of cellular spheroids for biomedical applications. In this review, we provide an overview of recent studies that utilize engineered biomaterials to guide spheroid formation and function, as well as their fabrication into tissues for use as tissue models and for therapeutic applications. First, we describe biomaterial strategies that allow the high-throughput fabrication of homogeneously-sized spheroids. Next, we summarize how engineered biomaterials are introduced into spheroid cultures either internally as microparticles or externally as hydrogel microenvironments to influence spheroid behavior (e.g., differentiation, fusion). Lastly, we discuss a variety of biofabrication strategies (e.g., 3D bioprinting, melt electrowriting) that have been used to develop macroscale tissue models and implantable constructs through the guided assembly of spheroids. Overall, the goal of this review is to provide a summary of how biomaterials are currently being engineered and leveraged to support spheroids in biomedical applications, as well as to provide a future outlook of the field. STATEMENT OF SIGNIFICANCE: Cellular spheroids are becoming increasingly used as in vitro tissue models or as 'building blocks' for tissue engineering and repair strategies. Engineered biomaterials and their processing through biofabrication approaches are being leveraged to structurally support and guide spheroid processes. This review summarizes current approaches where such biomaterials are being used to guide spheroid formation, function, and fabrication into tissue constructs. As the field is rapidly expanding, we also provide an outlook on future directions and how new engineered biomaterials can be implemented to further the development of biofabricated spheroid-based tissue constructs.

3D Bioprinting of Hyaline Articular Cartilage: Biopolymers, Hydrogels, and Bioinks

The musculoskeletal system, consisting of bones and cartilage of various types, muscles, ligaments, and tendons, is the basis of the human body. However, many pathological conditions caused by aging, lifestyle, disease, or trauma can damage its elements and lead to severe disfunction and significant worsening in the quality of life. Due to its structure and function, articular (hyaline) cartilage is the most susceptible to damage. Articular cartilage is a non-vascular tissue with constrained self-regeneration capabilities. Additionally, treatment methods, which have proven efficacy in stopping its degradation and promoting regeneration, still do not exist. Conservative treatment and physical therapy only relieve the symptoms associated with cartilage destruction, and traditional surgical interventions to repair defects or endoprosthetics are not without serious drawbacks. Thus, articular cartilage damage remains an urgent and actual problem requiring the development of new treatment approaches. The emergence of biofabrication technologies, including three-dimensional (3D) bioprinting, at the end of the 20th century, allowed reconstructive interventions to get a second wind. Three-dimensional bioprinting creates volume constraints that mimic the structure and function of natural tissue due to the combinations of biomaterials, living cells, and signal molecules to create. In our case-hyaline cartilage. Several approaches to articular cartilage biofabrication have been developed to date, including the promising technology of 3D bioprinting. This review represents the main achievements of such research direction and describes the technological processes and the necessary biomaterials, cell cultures, and signal molecules. Special attention is given to the basic materials for 3D bioprinting-hydrogels and bioinks, as well as the biopolymers underlying the indicated products.

Biofabrication of Modular Spheroids as Tumor-Scale Microenvironments for Drug Screening

To streamline the drug discovery pipeline, there is a pressing need for preclinical models which replicate the complexity and scale of native tumors. While there have been advancements in the formation of microscale tumor units, these models are cell-line dependent, time-consuming and have not improved clinical trial success rates. In this study, two methods for generating 3D tumor microenvironments are compared, rapidly fabricated hydrogel microspheres and traditional cell-dense spheroids. These modules are then bioassembled into 3D printed thermoplastic scaffolds, using an automated biofabrication process, to form tumor-scale models. Modules are formed with SKOV3 and HFF cells as monocultures and cocultures, and the fabrication efficiency, cell architecture, and drug response profiles are characterized, both as single modules and as multimodular constructs. Cell-encapsulated Gel-MA microspheres are fabricated with high-reproducibility and dimensions necessary for automated tumor-scale bioassembly regardless of cell type, however, only cocultured spheroids form compact modules suitable for bioassembly. Chemosensitivity assays demonstrate the reduced potency of doxorubicin in coculture bioassembled constructs and a ≈five-fold increase in drug resistance of cocultured cells in 3D modules compared with 2D monolayers. This bioassembly system is efficient and tailorable so that a variety of relevant-sized tumor constructs could be developed to study tumorigenesis and modernize drug discovery.

Hybrid cell constructs consisting of bioprinted cell-spheroids

Bioprinted cell constructs have been investigated for regeneration of various tissues. However, poor cell-cell interactions have limited their utility. Although cell-spheroids offer an alternative for efficient cell-cell interactions, they complicate bioprinting. Here, we introduce a new cell-printing process, fabricating cell-spheroids and cell-loaded constructs together without preparation of cell-spheroids in advance. Cells in mineral oil droplets self-assembled to form cell-spheroids due to the oil-aqueous interaction, exhibiting similar biological functions to the conventionally prepared cell-spheroids. By controlling printing parameters, spheroid diameter and location could be manipulated. To demonstrate the feasibility of this process, we fabricated hybrid cell constructs, consisting of endothelial cell-spheroids and stem cells loaded decellularized extracellular matrix/β-tricalcium phosphate struts for regenerating vascularized bone. The hybrid cell constructs exhibited strong angiogenic/osteogenic activities as a result of increased secretion of signaling molecules and synergistic crosstalk between the cells.

Review on Bioinspired Design of ECM-Mimicking Scaffolds by Computer-Aided Assembly of Cell-Free and Cell Laden Micro-Modules

Tissue engineering needs bioactive drug delivery scaffolds capable of guiding cell biosynthesis and tissue morphogenesis in three dimensions. Several strategies have been developed to design and fabricate ECM-mimicking scaffolds suitable for directing in vitro cell/scaffold interaction, and controlling tissue morphogenesis in vivo. Among these strategies, emerging computer aided design and manufacturing processes, such as modular tissue unit patterning, promise to provide unprecedented control over the generation of biologically and biomechanically competent tissue analogues. This review discusses recent studies and highlights the role of scaffold microstructural properties and their drug release capability in cell fate control and tissue morphogenesis. Furthermore, the work highlights recent advances in the bottom-up fabrication of porous scaffolds and hybrid constructs through the computer-aided assembly of cell-free and/or cell-laden micro-modules. The advantages, current limitations, and future challenges of these strategies are described and discussed.

Controlling Microenvironments with Organs-on-Chips for Osteoarthritis Modelling

Osteoarthritis (OA) remains a prevalent disease affecting more than 20% of the global population, resulting in morbidity and lower quality of life for patients. The study of OA pathophysiology remains predominantly in animal models due to the complexities of mimicking the physiological environment surrounding the joint tissue. Recent development in microfluidic organ-on-chip (OoC) systems have demonstrated various techniques to mimic and modulate tissue physiological environments. Adaptations of these techniques have demonstrated success in capturing a joint tissue's tissue physiology for studying the mechanism of OA. Adapting these techniques and strategies can help create human-specific in vitro models that recapitulate the cellular processes involved in OA. This review aims to comprehensively summarise various demonstrations of microfluidic platforms in mimicking joint microenvironments for future platform design iterations.

Challenges in osteoarthritis treatment

Osteoarthritis (OA) is the most common form of arthritis and a degenerative joint cartilage disease that is the most common cause of disability in the world among the elderly. It leads to social, psychological, and economic costs with financial consequences. The principles of OA treatment are to reduce pain and stiffness as well as maintain function. In recent years, due to a better understanding of the underlying pathophysiology of OA, a number of potential therapeutic advances have been made, which include tissue engineering, immune system manipulation, surgical technique, pharmacological, and non-pharmacological treatments. Despite this, there is still no certain cure for OA, and different OA treatments are usually considered in relation to the stage of the disease. The purpose of the present review is to summarize and discuss the latest results of new treatments for OA and potential targets for future research.

Decellularized ECM hydrogels: prior use considerations, applications, and opportunities in tissue engineering and biofabrication

Tissue development, wound healing, pathogenesis, regeneration, and homeostasis rely upon coordinated and dynamic spatial and temporal remodeling of extracellular matrix (ECM) molecules. ECM reorganization and normal physiological tissue function, require the establishment and maintenance of biological, chemical, and mechanical feedback mechanisms directed by cell-matrix interactions. To replicate the physical and biological environment provided by the ECM in vivo, methods have been developed to decellularize and solubilize tissues which yield organ and tissue-specific bioactive hydrogels. While these biomaterials retain several important traits of the native ECM, the decellularizing process, and subsequent sterilization, and solubilization result in fragmented, cleaved, or partially denatured macromolecules. The final product has decreased viscosity, moduli, and yield strength, when compared to the source tissue, limiting the compatibility of isolated decellularized ECM (dECM) hydrogels with fabrication methods such as extrusion bioprinting. This review describes the physical and bioactive characteristics of dECM hydrogels and their role as biomaterials for biofabrication. In this work, critical variables when selecting the appropriate tissue source and extraction methods are identified. Common manual and automated fabrication techniques compatible with dECM hydrogels are described and compared. Fabrication and post-manufacturing challenges presented by the dECM hydrogels decreased mechanical and structural stability are discussed as well as circumvention strategies. We further highlight and provide examples of the use of dECM hydrogels in tissue engineering and their role in fabricating complex in vitro 3D microenvironments.

Baghdadite: A Novel and Promising Calcium Silicate in Regenerative Dentistry and Medicine

For several years, ceramic biomaterials have been extensively utilized to rebuild and substitute for body tissues. Calcium silicates have been proven to exhibit excellent bioactivity due to apatite formation and cell proliferation stimulation, in addition to degradability at levels adequate for hard tissue formation. These ceramics' excellent biological characteristics have attracted researchers. Baghdadite is a calcium silicate incorporating zirconium ions that enhances human osteoblast multiplication and development, increasing mineralization, and ossification. It has currently received much interest in academic institutions and has been extensively explored in the form of permeable frameworks, varnishes, bone adhesive and gap fillings, microparticles, and nanospheres, particularly in a wide range of biomedical applications. This review article aims to summarize and analyze the most recent research on baghdadite's mechanical characteristics, apatite-forming capability, dissolution pattern, and physiochemical qualities as a scaffold for dentofacial tissuè regeneration purposes.

Design, Fabrication, and Application of Mini-Scaffolds for Cell Components in Tissue Engineering

The concept of "lockyballs" or interlockable mini-scaffolds fabricated by two-photon polymerization from biodegradable polymers for the encagement of tissue spheroids and their delivery into the desired location in the human body has been recently introduced. In order to improve control of delivery, positioning, and assembly of mini-scaffolds with tissue spheroids inside, they must be functionalized. This review describes the design, fabrication, and functionalization of mini-scaffolds as well as perspectives on their application in tissue engineering for precisely controlled cell and mini-tissue delivery and patterning. The development of functionalized mini-scaffolds advances the original concept of "lockyballs" and opens exciting new prospectives for mini-scaffolds' applications in tissue engineering and regenerative medicine and their eventual clinical translation.

Articular Cartilage Regeneration through Bioassembling Spherical Micro-Cartilage Building Blocks

Articular cartilage lesions are prevalent and affect one out of seven American adults and many young patients. Cartilage is not capable of regeneration on its own. Existing therapeutic approaches for articular cartilage lesions have limitations. Cartilage tissue engineering is a promising approach for regenerating articular neocartilage. Bioassembly is an emerging technology that uses microtissues or micro-precursor tissues as building blocks to construct a macro-tissue. We summarize and highlight the application of bioassembly technology in regenerating articular cartilage. We discuss the advantages of bioassembly and present two types of building blocks: multiple cellular scaffold-free spheroids and cell-laden polymer or hydrogel microspheres. We present techniques for generating building blocks and bioassembly methods, including bioprinting and non-bioprinting techniques. Using a data set of 5069 articles from the last 28 years of literature, we analyzed seven categories of related research, and the year trends are presented. The limitations and future directions of this technology are also discussed.

Mathematical and computational models in spheroid-based biofabrication

Ubiquitous in embryonic development, tissue fusion is of interest to tissue engineers who use tissue spheroids or organoids as building blocks of three-dimensional (3D) multicellular constructs. This review presents mathematical models and computer simulations of the fusion of tissue spheroids. The motivation of this study stems from the need to predict the post-printing evolution of 3D bioprinted constructs. First, we provide a brief overview of differential adhesion, the main morphogenetic mechanism involved in post-printing structure formation. It will be shown that clusters of cohesive cells behave as an incompressible viscous fluid on the time scale of hours. The discussion turns then to mathematical models based on the continuum hydrodynamics of highly viscous liquids and on statistical mechanics. Next, we analyze the validity and practical use of computational models of multicellular self-assembly in live constructs created by tissue spheroid bioprinting. Finally, we discuss the perspectives of the field as machine learning starts to reshape experimental design, and modular robotic workstations tend to alleviate the burden of repetitive tasks in biofabrication. STATEMENT OF SIGNIFICANCE: Bioprinted constructs are living systems, which evolve via morphogenetic mechanisms known from developmental biology. This review presents mathematical and computational tools devised for modeling post-printing structure formation. They help achieving a desirable outcome without expensive optimization experiments. While previous reviews mainly focused on assumptions, technical details, strengths, and limitations of computational models of multicellular self-assembly, this article discusses their validity and practical use in biofabrication. It also presents an overview of mathematical models that proved to be useful in the evaluation of experimental data on tissue spheroid fusion, and in the calibration of computational models. Finally, the perspectives of the field are discussed in the advent of robotic biofabrication platforms and bioprinting process optimization by machine learning.

Cell Aggregate Assembly through Microengineering for Functional Tissue Emergence

Compared to cell suspensions or monolayers, 3D cell aggregates provide cellular interactions organized in space and heterogeneity that better resume the real organization of native tissues. They represent powerful tools to narrow down the gap between in vitro and in vivo models, thanks to their self-evolving capabilities. Recent strategies have demonstrated their potential as building blocks to generate microtissues. Developing specific methodologies capable of organizing these cell aggregates into 3D architectures and environments has become essential to convert them into functional microtissues adapted for regenerative medicine or pharmaceutical screening purposes. Although the techniques for producing individual cell aggregates have been on the market for over a decade, the methodology for engineering functional tissues starting from them is still a young and quickly evolving field of research. In this review, we first present a panorama of emerging cell aggregates microfabrication and assembly technologies. We further discuss the perspectives opened in the establishment of functional tissues with a specific focus on controlled architecture and heterogeneity to favor cell differentiation and proliferation.

Tracheal reconstruction with pedicled tandem grafts engineered by a radial stretch bioreactor

The engineering of tracheal substitutes is pivotal in improving tracheal reconstruction. In this study, we aimed to investigate the effects of biomechanical stimulation on tissue engineering tracheal cartilage by mimicking the trachea motion through a novel radial stretching bioreactor, which enables to dynamically change the diameter of the hollow cylindrical implants. Applying our bioreactor, we demonstrated that chondrocytes seeded on the surface of Poly (ε-caprolactone) scaffold respond to mechanical stimulation by improvement of infiltration into implants and upregulation of cartilage-specific genes. Further, the mechanical stimulation enhanced the accumulation of cartilage neo-tissues and cartilage-specific extracellular macromolecules in the muscle flap-remodeled implants and reconstructed trachea. Nevertheless, the invasion of fibrous tissues in the reconstructed trachea was suppressed upon mechanical loading.

GelMA Hydrogel Reinforced with 3D Printed PEGT/PBT Scaffolds for Supporting Epigenetically-Activated Human Bone Marrow Stromal Cells for Bone Repair

Epigenetic approaches using the histone deacetylase 2 and 3 inhibitor-MI192 have been reported to accelerate stem cells to form mineralised tissues. Gelatine methacryloyl (GelMA) hydrogels provide a favourable microenvironment to facilitate cell delivery and support tissue formation. However, their application for bone repair is limited due to their low mechanical strength. This study aimed to investigate a GelMA hydrogel reinforced with a 3D printed scaffold to support MI192-induced human bone marrow stromal cells (hBMSCs) for bone formation. Cell culture: The GelMA (5 wt%) hydrogel supported the proliferation of MI192-pre-treated hBMSCs. MI192-pre-treated hBMSCs within the GelMA in osteogenic culture significantly increased alkaline phosphatase activity (p ≤ 0.001) compared to control. Histology: The MI192-pre-treated group enhanced osteoblast-related extracellular matrix deposition and mineralisation (p ≤ 0.001) compared to control. Mechanical testing: GelMA hydrogels reinforced with 3D printed poly(ethylene glycol)-terephthalate/poly(butylene terephthalate) (PEGT/PBT) scaffolds exhibited a 1000-fold increase in the compressive modulus compared to the GelMA alone. MI192-pre-treated hBMSCs within the GelMA–PEGT/PBT constructs significantly enhanced extracellular matrix collagen production and mineralisation compared to control (p ≤ 0.001). These findings demonstrate that the GelMA–PEGT/PBT construct provides enhanced mechanical strength and facilitates the delivery of epigenetically-activated MSCs for bone augmentation strategies.

Multiparametric Material Functionality of Microtissue-Based In Vitro Models as Alternatives to Animal Testing

With the definition of the 3R principle by Russel and Burch in 1959, the search for an adequate substitute for animal testing has become one of the most important tasks and challenges of this time, not only from an ethical, but also from a scientific, economic, and legal point of view. Microtissue-based in vitro model systems offer a valuable approach to address this issue by accounting for the complexity of natural tissues in a simplified manner. To increase the functionality of these model systems and thus make their use as a substitute for animal testing more likely in the future, the fundamentals need to be continuously improved. Corresponding requirements exist in the development of multifunctional, hydrogel-based materials, whose properties are considered in this review under the aspects of processability, adaptivity, biocompatibility, and stability/degradability.

3D bioassembly of cell-instructive chondrogenic and osteogenic hydrogel microspheres containing allogeneic stem cells for hybrid biofabrication of osteochondral constructs

Recently developed modular bioassembly techniques hold tremendous potential in tissue engineering and regenerative medicine, due to their ability to recreate the complex microarchitecture of native tissue. Here, we developed a novel approach to fabricate hybrid tissue-engineered constructs adopting high-throughput microfluidic and 3D bioassembly strategies. Osteochondral tissue fabrication was adopted as an example in this study, because of the challenges in fabricating load bearing osteochondral tissue constructs with phenotypically distinct zonal architecture. By developing cell-instructive chondrogenic and osteogenic bioink microsphere modules in high-throughput, together with precise manipulation of the 3D bioassembly process, we successfully fabricated hybrid engineered osteochondral tissue in vitro with integrated but distinct cartilage and bone layers. Furthermore, by encapsulating allogeneic umbilical cord blood-derived mesenchymal stromal cells (UCB-MSCs), and demonstrating chondrogenic and osteogenic differentiation, the hybrid biofabrication of hydrogel microspheres in this 3D bioassembly model offers potential for an off-the-shelf, single-surgery strategy for osteochondral tissue repair.

Bubble-based microrobots enable digital assembly of heterogeneous microtissue modules

The specific spatial distribution of tissue generates a heterogeneous micromechanical environment that provides ideal conditions for diverse functions such as regeneration and angiogenesis. However, to manufacture microscale multicellular heterogeneous tissue modules in vitro and then assemble them into specific functional units is still a challenging task. In this study, a novel method for the digital assembly of heterogeneous microtissue modules is proposed. This technique utilizes the flexibility of digital micromirror device-based optical projection lithography and the manipulability of bubble-based microrobots in a liquid environment. The results indicate that multicellular microstructures can be fabricated by increasing the inlets of the microfluidic chip. Upon altering the exposure time, the Young's modulus of the entire module and different regions of each module can be fine-tuned to mimic normal tissue. The surface morphology, mechanical properties, and internal structure of the constructed bionic peritoneum were similar to those of the real peritoneum. Overall, this work demonstrates the potential of this system to produce and control the posture of modules and simulate peritoneal metastasis using reconfigurable manipulation.

Recent approaches towards bone tissue engineering

Bone tissue engineering approaches have evolved towards addressing the challenges of tissue mimetic requirements over the years. Different strategies have been combining scaffolds, cells, and biologically active cues using a wide range of fabrication techniques, envisioning the mimicry of bone tissue. On the one hand, biomimetic scaffold-based strategies have been pursuing different biomaterials to produce scaffolds, combining with diverse and innovative fabrication strategies to mimic bone tissue better, surpassing bone grafts. On the other hand, biomimetic scaffold-free approaches mainly foresee replicating endochondral ossification, replacing hyaline cartilage with new bone. Finally, since bone tissue is highly vascularized, new strategies focused on developing pre-vascularized scaffolds or pre-vascularized cellular aggregates have been a motif of study. The recent biomimetic scaffold-based and scaffold-free approaches in bone tissue engineering, focusing on materials and fabrication methods used, are overviewed herein. The biomimetic vascularized approaches are also discussed, namely the development of pre-vascularized scaffolds and pre-vascularized cellular aggregates.

MI192 induced epigenetic reprogramming enhances the therapeutic efficacy of human bone marrows stromal cells for bone regeneration

Human bone marrow stromal cells (hBMSCs) have been extensively utilised for bone tissue engineering applications. However, they are associated with limitations that hinder their clinical utility for bone regeneration. Cell fate can be modulated via altering their epigenetic functionality. Inhibiting histone deacetylase (HDAC) enzymes have been reported to promote osteogenic differentiation, with HDAC3 activity shown to be causatively associated with osteogenesis. Therefore, this study aimed to investigate the potential of using an HDAC2 & 3 selective inhibitor - MI192 to induce epigenetic reprogramming of hBMSCs and enhance its therapeutic efficacy for bone formation. Treatment with MI192 caused a time-dose dependant reduction in hBMSCs viability. MI192 was also found to substantially alter hBMSCs epigenetic function through reduced HDAC activity and increased histone acetylation. hBMSCs were pre-treated with MI192 (50 μM) for 48 h prior to osteogenic induction. MI192 pre-treatment significantly upregulated osteoblast-related gene/protein expression (Runx2, ALP, Col1a and OCN) and enhanced alkaline phosphatase specific activity (ALPSA) (1.43-fold) (P ≤ 0.001). Moreover, MI192 substantially increased hBMSCs extracellular matrix calcium deposition (1.4-fold) (P ≤ 0.001) and mineralisation when compared to the untreated control. In 3D microtissue culture, MI192 significantly promoted hBMSCs osteoblast-related gene expression and ALPSA (> 2.41-fold) (P ≤ 0.001). Importantly, MI192 substantially enhanced extracellular matrix deposition (ALP, Col1a, OCN) and mineralisation (1.67-fold) (P ≤ 0.001) within the bioassembled-microtissue (BMT) construct. Following 8-week intraperitoneal implantation within nude mice, MI192 treated hBMSCs exhibited enhanced extracellular matrix deposition and mineralisation (2.39-fold) (P ≤ 0.001) within the BMT when compared to the untreated BMT construct. Taken together, these results demonstrate that MI192 effectively altered hBMSCs epigenetic functionality and is capable of promoting hBMSCs osteogenic differentiation in vitro and in vivo, indicating the potential of using epigenetic reprogramming to enhance the therapeutic efficacy of hBMSCs for bone augmentation strategies.

Cell-laden injectable microgels: Current status and future prospects for cartilage regeneration

Injectable hydrogels have been employed extensively as versatile materials for cartilage regeneration due to their excellent biocompatibility, tunable structure, and ability to accommodate bioactive factors, as well as their ability to be locally delivered via minimally invasive injection to fill irregular defects. More recently, in vitro and in vivo studies have revealed that processing these materials to produce cell-laden microgels can enhance cell-cell and cell-matrix interactions and boost nutrient and metabolite exchange. Moreover, these studies have demonstrated gene expression profiles and matrix regeneration that are superior compared to conventional injectable bulk hydrogels. As cell-laden microgels and their application in cartilage repair are moving closer to clinical translation, this review aims to present an overview of the recent developments in this field. Here we focus on the currently used biomaterials and crosslinking strategies, the innovative fabrication techniques being used for the production of microgels, the cell sources used, the signals used for induction of chondrogenic differentiation and the resultant biological responses, and the ability to create three-dimensional, functional cartilage tissues. In addition, this review also covers the current clinical approaches for repairing cartilage as well as specific challenges faced when attempting the regeneration of damaged cartilage tissue. New findings related to the macroporous nature of the structures formed by the assembled microgel building blocks and the novel use of microgels in 3D printing for cartilage tissue engineering are also highlighted. Finally, we outline the challenges and future opportunities for employing cell-laden microgels in clinical applications.

Probing Multicellular Tissue Fusion of Cocultured Spheroids-A 3D-Bioassembly Model

While decades of research have enriched the knowledge of how to grow cells into mature tissues, little is yet known about the next phase: fusing of these engineered tissues into larger functional structures. The specific effect of multicellular interfaces on tissue fusion remains largely unexplored. Here, a facile 3D-bioassembly platform is introduced to primarily study fusion of cartilage-cartilage interfaces using spheroids formed from human mesenchymal stromal cells (hMSCs) and articular chondrocytes (hACs). 3D-bioassembly of two adjacent hMSCs spheroids displays coordinated migration and noteworthy matrix deposition while the interface between two hAC tissues lacks both cells and type-II collagen. Cocultures contribute to increased phenotypic stability in the fusion region while close initial contact between hMSCs and hACs (mixed) yields superior hyaline differentiation over more distant, indirect cocultures. This reduced ability of potent hMSCs to fuse with mature hAC tissue further underlines the major clinical challenge that is integration. Together, this data offer the first proof of an in vitro 3D-model to reliably study lateral fusion mechanisms between multicellular spheroids and mature cartilage tissues. Ultimately, this high-throughput 3D-bioassembly model provides a bridge between understanding cellular differentiation and tissue fusion and offers the potential to probe fundamental biological mechanisms that underpin organogenesis.

Overcoming the Dependence on Animal Models for Osteoarthritis Therapeutics - The Promises and Prospects of In Vitro Models

Osteoarthritis (OA) is a musculoskeletal disease characterized by progressive degeneration of osteochondral tissues. Current treatment is restricted to the reduction of pain and loss of function of the joint. To better comprehend the OA pathophysiological conditions, several models are employed, however; there is no consensus on a suitable model. In this review, different in vitro models being developed for possible therapeutic intervention of OA are outlined. Herein, various in vitro OA models starting from 2D model, co-culture model, 3D models, dynamic culture model to advanced technologies-based models such as 3D bioprinting, bioassembly, organoids, and organ-on-chip-based models are discussed with their advantages and disadvantages. Besides, different growth factors, cytokines, and chemicals being utilized for induction of OA condition are reviewed in detail. Furthermore, there is focus on scrutinizing different molecular and possible therapeutic targets for better understanding the mechanisms and OA therapeutics. Finally, the underlying challenges associated with in vitro models are discussed followed by future prospective. Taken together, a comprehensive overview of in vitro OA models, factors to induce OA-like conditions, and intricate molecular targets with the potential to develop personalized osteoarthritis therapeutics in the future with clinical translation is provided.