The Synergy of Scaffold-Based and Scaffold-Free Tissue Engineering Strategies. Trends Biotechnol. 2018;36:348-357. [PMID: 29475621 DOI: 10.1016/j.tibtech.2018.01.005]

Research Progress in Hydrogels for Cartilage Organoids

The repair and regeneration of cartilage has always been a hot topic in medical research. Cartilage organoids (CORGs) are special cartilage tissue created using tissue engineering techniques outside the body. These engineered organoids tissues provide models that simulate the complex biological functions of cartilage, opening new possibilities for cartilage regenerative medicine and treatment strategies. However, it is crucial to establish suitable matrix scaffolds for the cultivation of CORGs. In recent years, utilizing hydrogel to culture stem cells and induce their differentiation into chondrocytes has emerged as a promising method for the in vitro construction of CORGs. In this review, the methods for establishing CORGs are summarized and an overview of the advantages and limitations of using matrigel in the cultivation of such organoids is provided. Furthermore, the importance of cartilage tissue ECM and alternative hydrogel substitutes for Matrigel, such as alginate, peptides, silk fibroin, and DNA derivatives is discussed, and the pros and cons of using these hydrogels for the cultivation of CORGs are outlined. Finally, the challenges and future directions in hydrogel research for CORGs are discussed. It is hoped that this article provides valuable references for the design and development of hydrogels for CORGs.

Vascular tissues bioprinted with smooth muscle cell-only bioinks in support baths mimic features of native coronary arteries

This study explores the bioprinting of a smooth muscle cell-only bioink into ionically crosslinked oxidized methacrylated alginate (OMA) microgel baths to create self-supporting vascular tissues. The impact of OMA microgel support bath methacrylation degree and cell-only bioink dispensing parameters on tissue formation, remodeling, structure and strength was investigated. We hypothesized that reducing dispensing tip diameter from 27 G (210μm) to 30 G (159μm) for cell-only bioink dispensing would reduce tissue wall thickness and improve the consistency of tissue dimensions while maintaining cell viability. Printing with 30 G tips resulted in decreased mean wall thickness (318.6μm) without compromising mean cell viability (94.8%). Histological analysis of cell-only smooth muscle tissues cultured for 14 d in OMA support baths exhibited decreased wall thickness using 30 G dispensing tips, which correlated with increased collagen deposition and alignment. In addition, a TUNEL assay indicated a decrease in cell death in tissues printed with thinner (30 G) dispensing tips. Mechanical testing demonstrated that tissues printed with a 30 G dispensing tip exhibit an increase in ultimate tensile strength compared to those printed with a 27 G dispensing tip. Overall, these findings highlight the importance of precise control over bioprinting parameters to generate mechanically robust tissues when using cell-only bioinks dispensed and cultured within hydrogel support baths. The ability to control print dimensions using cell-only bioinks may enable bioprinting of more complex soft tissue geometries to generatein vitrotissue models.

Formation and culture of cell spheroids by using magnetic nanostructures resembling a crown of thorns

In contrast to traditional two-dimensional cell-culture conditions, three-dimensional (3D) cell-culture models closely mimic complexin vivoconditions. However, constructing 3D cell culture models still faces challenges. In this paper, by using micro/nano fabrication method, including lithography, deposition, etching, and lift-off, we designed magnetic nanostructures resembling a crown of thorns. This magnetic crown of thorns (MCT) nanostructure enables the isolation of cells that have endocytosed magnetic particles. To assess the utility of this nanostructure, we used high-flux acquisition of Jurkat cells, an acute-leukemia cell line exhibiting the native phenotype, as an example. The novel structure enabled Jurkat cells to form spheroids within just 30 min by leveraging mild magnetic forces to bring together endocytosed magnetic particles. The size, volume, and arrangement of these spheroids were precisely regulated by the dimensions of the MCT nanostructure and the array configuration. The resulting magnetic cell clusters were uniform in size and reached saturation after 1400 s. Notably, these cell clusters could be easily separated from the MCT nanostructure through enzymatic digestion while maintaining their integrity. These clusters displayed a strong proliferation rate and survival capabilities, lasting for an impressive 96 h. Compared with existing 3D cell-culture models, the approach presented in this study offers the advantage of rapid formation of uniform spheroids that can mimicin vivomicroenvironments. These findings underscore the high potential of the MCT in cell-culture models and magnetic tissue enginerring.

Bioactive polymers: A comprehensive review on bone grafting biomaterials

The application of bone grafting materials in bone tissue engineering is paramount for treating severe bone defects. In this comprehensive review, we explore the significance and novelty of utilizing bioactive polymers as grafts for successful bone repair. Unlike metals and ceramics, polymers offer inherent biodegradability and biocompatibility, mimicking the native extracellular matrix of bone. While these polymeric micro-nano materials may face challenges such as mechanical strength, various fabrication techniques are available to overcome these shortcomings. Our study not only investigates diverse biopolymeric materials but also illuminates innovative fabrication methods, highlighting their importance in advancing bone tissue engineering.

Proanthocyanidin surface preconditioning of dental pulp stem cell spheroids enhances dimensional stability and biomineralization in vitro

AIM

Lack of adequate mechanical strength and progressive shrinkage over time remain challenges in scaffold-free microtissue-based dental pulp regeneration. Surface collagen cross-linking holds the promise to enhance the mechanical stability of microtissue constructs and trigger biological regulations. In this study, we proposed a novel strategy for surface preconditioning microtissues using a natural collagen cross-linker, proanthocyanidin (PA). We evaluated its effects on cell viability, tissue integrity, and biomineralization of dental pulp stem cell (DPSCs)-derived 3D cell spheroids.

METHODOLOGY

Microtissue and macrotissue spheroids were fabricated from DPSCs and incubated with PA solution for surface collagen cross-linking. Microtissue viability was examined by live/dead staining and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, with transverse dimension change monitored. Microtissue surface stiffness was measured by an atomic force microscope (AFM). PA-preconditioned microtissues and macrotissues were cultured under basal or osteogenic conditions. Immunofluorescence staining of PA-preconditioned microtissues was performed to detect dentin sialophosphoprotein (DSPP) and F-actin expressions. PA-preconditioned macrotissues were subjected to histological analysis, including haematoxylin-eosin (HE), alizarin red, and Masson trichrome staining. Immunohistochemistry staining was used to detect alkaline phosphatase (ALP) and dentin matrix acidic phosphoprotein 1 (DMP-1) expressions.

RESULTS

PA preconditioning had no adverse effects on microtissue spheroid viability and increased surface stiffness. It reduced dimensional shrinkage for over 7 days in microtissues and induced a larger transverse-section area in the macrotissue. PA preconditioning enhanced collagen formation, mineralized nodule formation, and elevated ALP and DMP-1 expressions in macrotissues. Additionally, PA preconditioning induced higher F-actin and DSPP expression in microtissues, while inhibition of F-actin activity by cytochalasin B attenuated PA-induced dimensional change and DSPP upregulation.

CONCLUSION

PA surface preconditioning of DPSCs spheroids demonstrates excellent biocompatibility while effectively enhancing tissue structure stability and promoting biomineralization. This strategy strengthens tissue integrity in DPSC-derived spheroids and amplifies osteogenic differentiation potential, advancing scaffold-free tissue engineering applications in regenerative dentistry.

Challenges and Opportunities in Developing Tracheal Substitutes for the Recovery of Long-Segment Defects

Tracheal resection and reconstruction procedures are necessary when stenosis, tracheomalacia, tumors, vascular lesions, or tracheal injury cause a tracheal blockage. Replacement with a tracheal substitute is often recommended when the trauma exceeds 50% of the total length of the trachea in adults and 30% in children. Recently, tissue engineering and other advanced techniques have shown promise in fabricating biocompatible tracheal substitutes with physical, morphological, biomechanical, and biological characteristics similar to native trachea. Different polymers and biometals are explored. Even with limited success with tissue-engineered grafts in clinical settings, complete healing of tracheal defects remains a substantial challenge due to low mechanical strength and durability of the graft materials, inadequate re-epithelialization and vascularization, and restenosis. This review has covered a range of reconstructive and regenerative techniques, design criteria, the use of bioprostheses and synthetic grafts for the recovery of tracheal defects, as well as the traditional and cutting-edge methods of their fabrication, surface modification for increased immuno- or biocompatibility, and associated challenges.

Advancing bone regeneration: Unveiling the potential of 3D cell models in the evaluation of bone regenerative materials

Bone, a rigid yet regenerative tissue, has garnered extensive attention for its impressive healing abilities. Despite advancements in understanding bone repair and creating treatments for bone injuries, handling nonunions and large defects remains a major challenge in orthopedics. The rise of bone regenerative materials is transforming the approach to bone repair, offering innovative solutions for nonunions and significant defects, and thus reshaping orthopedic care. Evaluating these materials effectively is key to advancing bone tissue regeneration, especially in difficult healing scenarios, making it a critical research area. Traditional evaluation methods, including two-dimensional cell models and animal models, have limitations in predicting accurately. This has led to exploring alternative methods, like 3D cell models, which provide fresh perspectives for assessing bone materials' regenerative potential. This paper discusses various techniques for constructing 3D cell models, their pros and cons, and crucial factors to consider when using these models to evaluate bone regenerative materials. We also highlight the significance of 3D cell models in the in vitro assessments of these materials, discuss their current drawbacks and limitations, and suggest future research directions. STATEMENT OF SIGNIFICANCE: This work addresses the challenge of evaluating bone regenerative materials (BRMs) crucial for bone tissue engineering. It explores the emerging role of 3D cell models as superior alternatives to traditional methods for assessing these materials. By dissecting the construction, key factors of evaluating, advantages, limitations, and practical considerations of 3D cell models, the paper elucidates their significance in overcoming current evaluation method shortcomings. It highlights how these models offer a more physiologically relevant and ethically preferable platform for the precise assessment of BRMs. This contribution is particularly significant for "Acta Biomaterialia" readership, as it not only synthesizes current knowledge but also propels the discourse forward in the search for advanced solutions in bone tissue engineering and regeneration.

In vitro vascularized liver tumor model based on a microfluidic inverse opal scaffold for immune cell recruitment investigation

Liver cancer, characterized as a kind of malignant tumor within the digestive system, poses great health harm, and immune escape stands out as an important reason for its occurrence and development. Chemokines, pivotal in guiding immune cells' migration, is necessary to initiate and deliver an effective anti-tumor immune response. Therefore, understanding the chemotactic environment and identifying chemokines that regulate recruitment of immune cells to the tumor microenvironment (TME) are critical to improve current immunotherapy interventions. Herein, we report a well-defined inverse opal scaffold generated with a microfluidic emulsion template for the construction of a vascularized liver tumor model, offering insights into immune cells' recruitment. Due to the excellent 3D porous morphology of the inverse opal scaffold, human hepatocellular carcinoma cells can aggregate in the pores of the scaffold to form uniform multicellular tumor spheroids. More attractively, the vascularized liver tumor model can be achieved by constructing a 3D co-culture system involving endothelial cells and hepatocellular carcinoma cells. The results demonstrate that the 3D co-cultured tumor cells increase the neutrophil chemokines remarkably and recruit neutrophils to tumor tissues, then promote tumor progression. This approach opens a feasible avenue for realizing a vascularized liver tumor model with a reliable immune microenvironment close to that of a solid tumor of liver cancer.

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.

Biomimetic Approaches in Scaffold-Based Blood Vessel Tissue Engineering

Cardiovascular diseases remain a leading cause of mortality globally, with atherosclerosis representing a significant pathological means, often leading to myocardial infarction. Coronary artery bypass surgery, a common procedure used to treat coronary artery disease, presents challenges due to the limited autologous tissue availability or the shortcomings of synthetic grafts. Consequently, there is a growing interest in tissue engineering approaches to develop vascular substitutes. This review offers an updated picture of the state of the art in vascular tissue engineering, emphasising the design of scaffolds and dynamic culture conditions following a biomimetic approach. By emulating native vessel properties and, in particular, by mimicking the three-layer structure of the vascular wall, tissue-engineered grafts can improve long-term patency and clinical outcomes. Furthermore, ongoing research focuses on enhancing biomimicry through innovative scaffold materials, surface functionalisation strategies, and the use of bioreactors mimicking the physiological microenvironment. Through a multidisciplinary lens, this review provides insight into the latest advancements and future directions of vascular tissue engineering, with particular reference to employing biomimicry to create systems capable of reproducing the structure-function relationships present in the arterial wall. Despite the existence of a gap between benchtop innovation and clinical translation, it appears that the biomimetic technologies developed to date demonstrate promising results in preventing vascular occlusion due to blood clotting under laboratory conditions and in preclinical studies. Therefore, a multifaceted biomimetic approach could represent a winning strategy to ensure the translation of vascular tissue engineering into clinical practice.

Manufacturing of 3D-Printed Hybrid Scaffolds with Polyelectrolyte Multilayer Coating in Static and Dynamic Culture Conditions

Three-dimensional printing (3DP) has emerged as a promising method for creating intricate scaffold designs. This study assessed three 3DP scaffold designs fabricated using biodegradable poly(lactic) acid (PLA) through fused deposition modelling (FDM): mesh, two channels (2C), and four channels (4C). To address the limitations of PLA, such as hydrophobic properties and poor cell attachment, a post-fabrication modification technique employing Polyelectrolyte Multilayers (PEMs) coating was implemented. The scaffolds underwent aminolysis followed by coating with SiCHA nanopowders dispersed in hyaluronic acid and collagen type I, and finally crosslinked the outermost coated layers with EDC/NHS solution to complete the hybrid scaffold production. The study employed rotating wall vessels (RWVs) to investigate how simulating microgravity affects cell proliferation and differentiation. Human mesenchymal stem cells (hMSCs) cultured on these scaffolds using proliferation medium (PM) and osteogenic media (OM), subjected to static (TCP) and dynamic (RWVs) conditions for 21 days, revealed superior performance of 4C hybrid scaffolds, particularly in OM. Compared to commercial hydroxyapatite scaffolds, these hybrid scaffolds demonstrated enhanced cell activity and survival. The pre-vascularisation concept on 4C hybrid scaffolds showed the proliferation of both HUVECs and hMSCs throughout the scaffolds, with a positive expression of osteogenic and angiogenic markers at the early stages.

Enhanced osteochondral regeneration with a 3D-Printed biomimetic scaffold featuring a calcified interfacial layer

The integrative regeneration of both articular cartilage and subchondral bone remains an unmet clinical need due to the difficulties of mimicking spatial complexity in native osteochondral tissues for artificial implants. Layer-by-layer fabrication strategies, such as 3D printing, have emerged as a promising technology replicating the stratified zonal architecture and varying microstructures and mechanical properties. However, the dynamic and circulating physiological environments, such as mass transportation or cell migration, usually distort the pre-confined biological properties in the layered implants, leading to undistinguished spatial variations and subsequently inefficient regenerations. This study introduced a biomimetic calcified interfacial layer into the scaffold as a compact barrier between a cartilage layer and a subchondral bone layer to facilitate osteogenic-chondrogenic repair. The calcified interfacial layer consisting of compact polycaprolactone (PCL), nano-hydroxyapatite, and tasquinimod (TA) can physically and biologically separate the cartilage layer (TA-mixed, chondrocytes-load gelatin methacrylate) from the subchondral bond layer (porous PCL). This introduction preserved the as-designed independent biological environment in each layer for both cartilage and bone regeneration, successfully inhibiting vascular invasion into the cartilage layer and preventing hyaluronic cartilage calcification owing to devascularization of TA. The improved integrative regeneration of cartilage and subchondral bone was validated through gross examination, micro-computed tomography (micro-CT), and histological and immunohistochemical analyses based on an in vivo rat model. Moreover, gene and protein expression studies identified a key role of Caveolin (CAV-1) in promoting angiogenesis through the Wnt/β-catenin pathway and indicated that TA in the calcified layer blocked angiogenesis by inhibiting CAV-1.

Induced pluripotent stem cells in cartilage tissue engineering: a literature review

Osteoarthritis (OA) is a long-term, persistent joint disorder characterized by bone and cartilage degradation, resulting in tightness, pain, and restricted movement. Current attempts in cartilage regeneration are cell-based therapies using stem cells. Multipotent stem cells, such as mesenchymal stem cells (MSCs), and pluripotent stem cells, such as embryonic stem cells (ESCs), have been used to regenerate cartilage. However, since the discovery of human-induced pluripotent stem cells (hiPSCs) in 2007, it was seen as a potential source for regenerative chondrogenic therapy as it overcomes the ethical issues surrounding the use of ESCs and the immunological and differentiation limitations of MSCs. This literature review focuses on chondrogenic differentiation and 3D bioprinting technologies using hiPSCS, suggesting them as a viable source for successful tissue engineering.

METHODS

A literature search was conducted using scientific search engines, PubMed, MEDLINE, and Google Scholar databases with the terms 'Cartilage tissue engineering' and 'stem cells' to retrieve published literature on chondrogenic differentiation and tissue engineering using MSCs, ESCs, and hiPSCs.

RESULTS

hiPSCs may provide an effective and autologous treatment for focal chondral lesions, though further research is needed to explore the potential of such technologies.

CONCLUSIONS

This review has provided a comprehensive overview of these technologies and the potential applications for hiPSCs in regenerative medicine.

Polyethylenimine Inclusion to Develop Aqueous Alginate-Based Core-Shell Capsules for Biomedical Applications

Aqueous core-shell structures can serve as an efficient approach that allows cells to generate 3D spheroids with in vivo-like cell-to-cell contacts. Here, a novel strategy for fabricating liquid-core-shell capsules is proposed by inverse gelation of alginate (ALG) and layer-by-layer (LbL) coating. We hypothesized that the unique properties of polyethylenimine (PEI) could be utilized to overcome the low structural stability and the limited cell recognition motifs of ALG. In the next step, alginate dialdehyde (ADA) enabled the Schiff-base reaction with free amine groups of PEI to reduce its possible toxic effects. Scanning electron microscopy and light microscopy images proved the formation of spherical hollow capsules with outer diameters of 3.0 ± 0.1 mm for ALG, 3.2 ± 0.1 mm for ALG/PEI, and 4.0 ± 0.2 mm for ALG/PEI/ADA capsules. The effective modulus increased by 3-fold and 5-fold when comparing ALG/PEI/ADA and ALG/PEI to ALG capsules, respectively. Moreover, PEI-coated capsules showed potential antibacterial properties against both Staphylococcus aureus and Escherichia coli, with an apparent inhibition zone. The cell viability results showed that all capsules were cytocompatible (above 75.5%). Cells could proliferate and form spheroids when encapsulated within the ALG/PEI/ADA capsules. Monitoring the spheroid thickness over 5 days of incubation indicated an increasing trend from 39.50 μm after 1 day to 66.86 μm after 5 days. The proposed encapsulation protocol represents a new in vitro platform for developing 3D cell cultivation and can be adapted to fulfill the requirements of various biomedical applications.

Bioprinting of human dermal microtissues precursors as building blocks for endogenousin vitroconnective tissue manufacturing

The advent of 3D bioprinting technologies in tissue engineering has unlocked the potential to fabricatein vitrotissue models, overcoming the constraints associated with the shape limitations of preformed scaffolds. However, achieving an accurate mimicry of complex tissue microenvironments, encompassing cellular and biochemical components, and orchestrating their supramolecular assembly to form hierarchical structures while maintaining control over tissue formation, is crucial for gaining deeper insights into tissue repair and regeneration. Building upon our expertise in developing competent three-dimensional tissue equivalents (e.g. skin, gut, cervix), we established a two-step bottom-up approach involving the dynamic assembly of microtissue precursors (μTPs) to generate macroscopic functional tissue composed of cell-secreted extracellular matrix (ECM). To enhance precision and scalability, we integrated extrusion-based bioprinting technology into our established paradigm to automate, control and guide the coherent assembly ofμTPs into predefined shapes. Compared to cell-aggregated bioink, ourμTPs represent a functional unit where cells are embedded in their specific ECM.μTPs were derived from human dermal fibroblasts dynamically seeded onto gelatin-based microbeads. After 9 days,μTPs were suspended (50% v/v) in Pluronic-F127 (30% w/v) (µTP:P30), and the obtained formulation was loaded as bioink into the syringe of the Dr.INVIVO-4D6 extrusion based bioprinter.µTP:P30 bioink showed shear-thinning behavior and temperature-dependent viscosity (gel atT> 30 °C), ensuringµTPs homogenous dispersion within the gel and optimal printability. The bioprinting involved extruding several geometries (line, circle, and square) into Pluronic-F127 (40% w/v) (P40) support bath, leveraging its shear-recovery property. P40 effectively held the bioink throughout and after the bioprinting procedure, untilµTPs fused into a continuous connective tissue.µTPs fusion dynamics was studied over 8 days of culture, while the resulting endogenous construct underwent 28 days culture. Histological, immunofluorescence analysis, and second harmonic generation reconstruction revealed an increase in endogenous collagen and fibronectin production within the bioprinted construct, closely resembling the composition of the native connective tissues.

Engineering of a decellularized bovine skin coated with antibiotics-loaded electrospun fibers with synergistic antibacterial activity for the treatment of infectious wounds

An ideal antibacterial wound dressing with strong antibacterial behavior versus highly drug-resistant bacteria and great wound-healing capacity is still being developed. There is a clinical requirement to progress the current clinical cares that fail to fully restore the skin structure due to post-wound infections. Here, we aim to introduce a novel two-layer wound dressing using decellularized bovine skin (DBS) tissue and antibacterial nanofibers to design a bioactive scaffold with bio-mimicking the native extracellular matrix of both dermis and epidermis. For this purpose, polyvinyl alcohol (PVA)/chitosan (CS) solution was loaded with antibiotics (colistin and meropenem) and electrospun on the surface of the DBS scaffold to fabricate a two-layer antibacterial wound dressing (DBS-PVA/CS/Abs). In detail, the characterization of the fabricated scaffold was conducted using biomechanical, biological, and antibacterial assays. Based on the results, the fabricated scaffold revealed a homogenous three-dimensional microstructure with a connected pore network, a high porosity and swelling ratio, and favorable mechanical properties. In addition, according to the cell culture result, our fabricated two-layer scaffold surface had a good interaction with fibroblast cells and provided an excellent substrate for cell proliferation and attachment. The antibacterial assay revealed a strong antibacterial activity of DBS-PVA/CS/Abs against both standard strain and multidrug-resistant clinical isolates of Acinetobacter baumannii, Pseudomonas aeruginosa, and Escherichia coli. Our bilayer antibacterial wound dressing is strongly suggested as an admirable wound dressing for the management of infectious skin injuries and now promises to advance with preclinical and clinical research.

Strategies for Constructing Tissue-Engineered Fat for Soft Tissue Regeneration

BACKGROUND

Repairing soft tissue defects caused by inflammation, tumors, and trauma remains a major challenge for surgeons. Adipose tissue engineering (ATE) provides a promising way to solve this problem.

METHODS

This review summarizes the current ATE strategies for soft tissue reconstruction, and introduces potential construction methods for ATE.

RESULTS

Scaffold-based and scaffold-free strategies are the two main approaches in ATE. Although several of these methods have been effective clinically, both scaffold-based and scaffold-free strategies have limitations. The third strategy is a synergistic tissue engineering strategy and combines the advantages of scaffold-based and scaffold-free strategies.

CONCLUSION

Personalized construction, stable survival of reconstructed tissues and functional recovery of organs are future goals of building tissue-engineered fat for ATE.

A micro-fragmented collagen gel as a cell-assembling platform for critical limb ischemia repair

Critical limb ischemia (CLI) is a devastating disease characterized by the progressive blockage of blood vessels. Although the paracrine effect of growth factors in stem cell therapy made it a promising angiogenic therapy for CLI, poor cell survival in the harsh ischemic microenvironment limited its efficacy. Thus, an imperative need exists for a stem-cell delivery method that enhances cell survival. Here, a collagen microgel (CMG) cell-delivery scaffold (40 × 20 μm) was fabricated via micro-fragmentation from collagen-hyaluronic acid polyionic complex to improve transplantation efficiency. Culturing human adipose-derived stem cells (hASCs) with CMG enabled integrin receptors to interact with CMG to form injectable 3-dimensional constructs (CMG-hASCs) with a microporous microarchitecture and enhanced mass transfer. CMG-hASCs exhibited higher cell survival (p < 0.0001) and angiogenic potential in tube formation and aortic ring angiogenesis assays than cell aggregates. Injection of CMG-hASCs intramuscularly into CLI mice increased blood perfusion and limb salvage ratios by 40 % and 60 %, respectively, compared to cell aggregate-treated mice. Further immunofluorescent analysis revealed that transplanted CMG-hASCs have greater muscle regenerative and angiogenic potential, with enhanced cell survival than cell aggregates (p < 0.05). Collectively, we propose CMG as a cell-assembling platform and CMG-hASCs as promising therapeutics to treat CLI.

Enhanced articular cartilage regeneration using costal chondrocyte-derived scaffold-free tissue engineered constructs with ascorbic acid treatment

Background

Cartilage tissue engineering faces challenges related to the use of scaffolds and limited seed cells. This study aims to propose a cost-effective and straightforward approach using costal chondrocytes (CCs) as an alternative cell source to overcome these challenges, eliminating the need for special culture equipment or scaffolds.

Methods

CCs were cultured at a high cell density with and without ascorbic acid treatment, serving as the experimental and control groups, respectively. Viability and tissue-engineered constructs (TEC) formation were evaluated until day 14. Slices of TEC samples were used for histological staining to evaluate the secretion of glycosaminoglycans and different types of collagen proteins within the extracellular matrix. mRNA sequencing and qPCR were performed to examine gene expression related to cartilage matrix secretion in the chondrocytes. In vivo experiments were conducted by implanting TECs from different groups into the defect site, followed by sample collection after 12 weeks for histological staining and scoring to evaluate the extent of cartilage regeneration. Hematoxylin-eosin (HE), Safranin-O-Fast Green, and Masson's trichrome stainings were used to examine the content of cartilage-related matrix components in the in vivo repair tissue. Immunohistochemical staining for type I and type II collagen, as well as aggrecan, was performed to assess the presence and distribution of these specific markers. Additionally, immunohistochemical staining for type X collagen was used to observe any hypertrophic changes in the repaired tissue.

Results

Viability of the chondrocytes remained high throughout the culture period, and the TECs displayed an enriched extracellular matrix suitable for surgical procedures. In vitro study revealed glycosaminoglycan and type II collagen production in both groups of TEC, while the TEC matrix treated with ascorbic acid displayed greater abundance. The results of mRNA sequencing and qPCR showed that genes related to cartilage matrix secretion such as Sox9, Col2, and Acan were upregulated by ascorbic acid in costal chondrocytes. Although the addition of Asc-2P led to an increase in COL10 expression according to qPCR and RNA-seq results, the immunofluorescence staining results of the two groups of TECs exhibited similar distribution and fluorescence intensity. In vivo experiments showed that both groups of TEC could adhere to the defect sites and kept hyaline cartilage morphology until 12 weeks. TEC treated with ascorbic acid showed superior cartilage regeneration as evidenced by significantly higher ICRS and O'Driscoll scores and stronger Safranin-O and collagen staining mimicking native cartilage when compared to other groups. In addition, the immunohistochemical staining results of Collgan X indicated that, after 12 weeks, the ascorbic acid-treated TEC did not exhibit further hypertrophy upon transplantation into the defect site, but maintained an expression profile similar to untreated TECs, while slightly higher than the sham-operated group.

Conclusion

These results suggest that CC-derived scaffold-free TEC presents a promising method for articular cartilage regeneration. Ascorbic acid treatment enhances outcomes by promoting cartilage matrix production. This study provides valuable insights and potential advancements in the field of cartilage tissue engineering.

The translational potential of this article

Cartilage tissue engineering is an area of research with immense clinical potential. The approach presented in this article offers a cost-effective and straightforward solution, which can minimize the complexity of cell culture and scaffold fabrication. This simplification could offer several translational advantages, such as ease of use, rapid scalability, lower costs, and the potential for patient-specific clinical translation. The use of costal chondrocytes, which are easily obtainable, and the scaffold-free approach, which does not require specialized equipment or membranes, could be particularly advantageous in clinical settings, allowing for in situ regeneration of cartilage.

A review of the therapeutic potential of dental stem cells as scaffold-free models for tissue engineering application

In the realm of regenerative medicine, tissue engineering has introduced innovative approaches to facilitate tissue regeneration. Specifically, in pulp tissue engineering, both scaffold-based and scaffold-free techniques have been applied. Relevant articles were meticulously chosen from PubMed, Scopus, and Google Scholar databases through a comprehensive search spanning from October 2022 to December 2022. Despite the inherent limitations of scaffolding, including inadequate mechanical strength for hard tissues, insufficient vents for vessel penetration, immunogenicity, and suboptimal reproducibility-especially with natural polymeric scaffolds-scaffold-free tissue engineering has garnered significant attention. This methodology employs three-dimensional (3D) cell aggregates such as spheroids and cell sheets with extracellular matrix, facilitating precise regeneration of target tissues. The choice of technique aside, stem cells play a pivotal role in tissue engineering, with dental stem cells emerging as particularly promising resources. Their pluripotent nature, non-invasive extraction process, and unique properties render them highly suitable for scaffold-free tissue engineering. This study delves into the latest advancements in leveraging dental stem cells and scaffold-free techniques for the regeneration of various tissues. This paper offers a comprehensive summary of recent developments in the utilization of dental stem cells and scaffold-free methods for tissue generation. It explores the potential of these approaches to advance tissue engineering and their effectiveness in therapies aimed at tissue regeneration.

Beyond Vision: An Overview of Regenerative Medicine and Its Current Applications in Ophthalmological Care

Regenerative medicine (RM) has emerged as a promising and revolutionary solution to address a range of unmet needs in healthcare, including ophthalmology. Moreover, RM takes advantage of the body's innate ability to repair and replace pathologically affected tissues. On the other hand, despite its immense promise, RM faces challenges such as ethical concerns, host-related immune responses, and the need for additional scientific validation, among others. The primary aim of this review is to present a high-level overview of current strategies in the domain of RM (cell therapy, exosomes, scaffolds, in vivo reprogramming, organoids, and interspecies chimerism), centering around the field of ophthalmology. A search conducted on clinicaltrials.gov unveiled a total of at least 209 interventional trials related to RM within the ophthalmological field. Among these trials, there were numerous early-phase studies, including phase I, I/II, II, II/III, and III trials. Many of these studies demonstrate potential in addressing previously challenging and degenerative eye conditions, spanning from posterior segment pathologies like Age-related Macular Degeneration and Retinitis Pigmentosa to anterior structure diseases such as Dry Eye Disease and Limbal Stem Cell Deficiency. Notably, these therapeutic approaches offer tailored solutions specific to the underlying causes of each pathology, thus allowing for the hopeful possibility of bringing forth a treatment for ocular diseases that previously seemed incurable and significantly enhancing patients' quality of life. As advancements in research and technology continue to unfold, future objectives should focus on ensuring the safety and prolonged viability of transplanted cells, devising efficient delivery techniques, etc.

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.

Biofabrication methods for reconstructing extracellular matrix mimetics

In the human body, almost all cells interact with extracellular matrices (ECMs), which have tissue and organ-specific compositions and architectures. These ECMs not only function as cellular scaffolds, providing structural support, but also play a crucial role in dynamically regulating various cellular functions. This comprehensive review delves into the examination of biofabrication strategies used to develop bioactive materials that accurately mimic one or more biophysical and biochemical properties of ECMs. We discuss the potential integration of these ECM-mimics into a range of physiological and pathological in vitro models, enhancing our understanding of cellular behavior and tissue organization. Lastly, we propose future research directions for ECM-mimics in the context of tissue engineering and organ-on-a-chip applications, offering potential advancements in therapeutic approaches and improved patient outcomes.

From cells to organs: progress and potential in cartilaginous organoids research

While cartilage tissue engineering has significantly improved the speed and quality of cartilage regeneration, the underlying metabolic mechanisms are complex, making research in this area lengthy and challenging. In the past decade, organoids have evolved rapidly as valuable research tools. Methods to create these advanced human cell models range from simple tissue culture techniques to complex bioengineering approaches. Cartilaginous organoids in part mimic the microphysiology of human cartilage and fill a gap in high-fidelity cartilage disease models to a certain extent. They hold great promise to elucidate the pathogenic mechanism of a diversity of cartilage diseases and prove crucial in the development of new drugs. This review will focus on the research progress of cartilaginous organoids and propose strategies for cartilaginous organoid construction, study directions, and future perspectives.

Effect of GelMA Hydrogel Properties on Long-Term Encapsulation and Myogenic Differentiation of C2C12 Spheroids

Skeletal muscle regeneration and engineering hold great promise for the treatment of various muscle-related pathologies and injuries. This research explores the use of gelatin methacrylate (GelMA) hydrogels as a critical component for encapsulating cellular spheroids in the context of muscle tissue engineering and regenerative applications. The preparation of GelMA hydrogels at various concentrations, ranging from 5% to 15%, was characterized and correlated with their mechanical stiffness. The storage modulus was quantified and correlated with GelMA concentration: 6.01 ± 1.02 Pa (5% GelMA), 75.78 ± 6.67 Pa (10% GelMA), and 134.69 ± 7.93 Pa (15% GelMA). In particular, the mechanical properties and swelling capacity of GelMA hydrogels were identified as key determinants affecting cell sprouting and migration from C2C12 spheroids. The controlled balance between these factors was found to significantly enhance the differentiation and functionality of the encapsulated spheroids. Our results highlight the critical role of GelMA hydrogels in orchestrating cellular dynamics and processes within a 3D microenvironment. The study demonstrates that these hydrogels provide a promising scaffold for the long-term encapsulation of spheroids while maintaining high biocompatibility. This research provides valuable insights into the design and use of GelMA hydrogels for improved muscle tissue engineering and regenerative applications, paving the way for innovative approaches to muscle tissue repair and regeneration.

A Synergic Strategy: Adipose-Derived Stem Cell Spheroids Seeded on 3D-Printed PLA/CHA Scaffolds Implanted in a Bone Critical-Size Defect Model

Bone critical-size defects and non-union fractures have no intrinsic capacity for self-healing. In this context, the emergence of bone engineering has allowed the development of functional alternatives. The aim of this study was to evaluate the capacity of ASC spheroids in bone regeneration using a synergic strategy with 3D-printed scaffolds made from poly (lactic acid) (PLA) and nanostructured hydroxyapatite doped with carbonate ions (CHA) in a rat model of cranial critical-size defect. In summary, a set of results suggests that ASC spheroidal constructs promoted bone regeneration. In vitro results showed that ASC spheroids were able to spread and interact with the 3D-printed scaffold, synthesizing crucial growth factors and cytokines for bone regeneration, such as VEGF. Histological results after 3 and 6 months of implantation showed the formation of new bone tissue in the PLA/CHA scaffolds that were seeded with ASC spheroids. In conclusion, the presence of ASC spheroids in the PLA/CHA 3D-printed scaffolds seems to successfully promote bone formation, which can be crucial for a significant clinical improvement in critical bone defect regeneration.

Recent advances in nano-scaffolds for tissue engineering applications: Toward natural therapeutics

Among the promising methods for repairing or replacing tissue defects in the human body and the hottest research topics in medical science today are regenerative medicine and tissue engineering. On the other hand, nanotechnology has been expanded into different areas of regenerative medicine and tissue engineering due to its essential benefits in improving performance in various fields. Nanotechnology, a helpful strategy in tissue engineering, offers new solutions to unsolved problems. Especially considering the excellent physicochemical properties of nanoscale structures, their application in regenerative medicine has been gradually developed, and a lot of research has been conducted in this field. In this regard, various nanoscale structures, including nanofibers, nanosheets, nanofilms, nano-clays, hollow spheres, and different nanoparticles, have been developed to advance nanotechnology strategies with tissue repair goals. Here, we comprehensively review the application of the mentioned nanostructures in constructing nanocomposite scaffolds for regenerative medicine and tissue engineering. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Implantable Materials and Surgical Technologies > Nanotechnology in Tissue Repair and Replacement Diagnostic Tools > Biosensing.

Selective Extracellular Matrix Guided Mesenchymal Stem Cell Self-Aggregate Engineering for Replication of Meniscal Zonal Tissue Gradient in a Porcine Meniscectomy Model

Degenerative meniscus tears (DMTs) are prevalent findings in osteoarthritic knees, yet current treatment is mostly limited to arthroscopic partial meniscectomy rather than regeneration, which further exacerbates arthritic changes. Translational research regarding meniscus regeneration is hindered by the complex, composite nature of the meniscus which exhibit a gradient from inner cartilage-like tissue to outer fibrous tissue, as well as engineering hurdles often requiring growth factors and cross-linking agents. Here, a meniscus zonal tissue gradient is proposed using zone-specific decellularized meniscus extracellular matrix (DMECM) and autologous synovial mesenchymal stem cells (SMSC) via self-aggregation without the use of growth factors or cross-linking agents. Combination with zone-specific DMECM during self-aggregation of MSCs forms zone-specific meniscus tissue that reflects the respective DMECM harvest site. The implantation of these constructs leads to the regeneration of meniscus tissue resembling the native meniscus, demonstrating inner cartilaginous and outer fibrous characteristics as well as recovery of native meniscal microarchitecture in a porcine partial meniscectomy model at 6 months. In all, the findings offer a potential regenerative therapy for DMTs that may improve current partial meniscectomy-based patient care.

Sound-based assembly of three-dimensional cellularized and acellularized constructs

Herein we show an accessible technique based on Faraday waves that assist the rapid assembly of osteoinductive β-Tricalcium phosphate (β-TCP) particles as well as human osteoblast pre-assembled in spheroids. The hydrodynamic forces originating at 'seabed' of the assembly chamber can be used to tightly aggregate inorganic and biological entities at packing densities that resemble those of native tissues. Additionally, following a layer-by-layer assembly procedure, centimeter scaled osteoinductive three-dimensional and cellularized constructs have been fabricated. We showed that the intimate connection between biological building blocks is essential in engineering living system able of localized mineral deposition. Our results demonstrate, for the first time, the possibility to obtain three-dimensional cellularized and acellularized anisotropic constructs using Faraday waves.

The Three-Dimensional In Vitro Cell Culture Models in the Study of Oral Cancer Immune Microenvironment

The onset and progression of oral cancer are accompanied by a dynamic interaction with the host immune system, and the immune cells within the tumor microenvironment play a pivotal role in the development of the tumor. By exploring the cellular immunity of oral cancer, we can gain insight into the contribution of both tumor cells and immune cells to tumorigenesis. This understanding is crucial for developing effective immunotherapeutic strategies to combat oral cancer. Studies of cancer immunology present unique challenges in terms of modeling due to the extraordinary complexity of the immune system. With its multitude of cellular components, each with distinct subtypes and various activation states, the immune system interacts with cancer cells and other components of the tumor, ultimately shaping the course of the disease. Conventional two-dimensional (2D) culture methods fall short of capturing these intricate cellular interactions. Mouse models enable us to learn about tumor biology in complicated and dynamic physiological systems but have limitations as the murine immune system differs significantly from that of humans. In light of these challenges, three-dimensional (3D) culture systems offer an alternative approach to studying cancer immunology and filling the existing gaps in available models. These 3D culture models provide a means to investigate complex cellular interactions that are difficult to replicate in 2D cultures. The direct study of the interaction between immune cells and cancer cells of human origin offers a more relevant and representative platform compared to mouse models, enabling advancements in our understanding of cancer immunology. This review explores commonly used 3D culture models and highlights their significant contributions to expanding our knowledge of cancer immunology. By harnessing the power of 3D culture systems, we can unlock new insights that pave the way for improved strategies in the battle against oral cancer.

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.

Three-dimensional (3D) cell culture studies: a review of the field of toxicology

Traditional two-dimensional (2D) cell culture employed for centuries is extensively used in toxicological studies. There is no doubt that 2D cell culture has made significant contributions to toxicology. However, in today's world, it is necessary to develop more physiologically relevant models. Three-dimensional (3D) cell culture, which can recapitulate the cell's microenvironment, is, therefore, a more realistic model compared to traditional cell culture. In toxicology, 3D cell culture models are a powerful tool for studying different tissues and organs in similar environments and behave as if they are in in vivo conditions. In this review, we aimed to present 3D cell culture models that have been used in different organ toxicity studies. We reported the results and interpretations obtained from these studies. We aimed to highlight 3D models as the future of cell culture by reviewing 3D models used in different organ toxicity studies.

Wetting of Cell Aggregates on Microdisk Topography Structures Achieved by Maskless Optical Projection Lithography

Cell aggregates as a 3D culture model can effectively mimic the physiological processes such as embryonic development, immune response, and tissue renewal in vivo. Researches show that the topography of biomaterials plays an important role in regulating cell proliferation, adhesion, and differentiation. It is of great significance to understand how cell aggregates respond to surface topography. Herein, microdisk array structures with the optimized size are used to investigate the wetting of cell aggregates. Cell aggregates exhibit complete wetting with distinct wetting velocities on the microdisk array structures of different diameters. The wetting velocity of cell aggregates reaches a maximum of 293 µm h^-1 on microdisk structures with a diameter of 2 µm and is a minimum of 247 µm h^-1 on microdisk structures of 20 µm diameter, which suggests that the cell-substrates adhesion energy on the latter is smaller. Actin stress fibers, focal adhesions (FAs), and cell morphology are analyzed to reveal the mechanisms of variation of wetting velocity. Furthermore, it is demonstrated that cell aggregates adopt climb and detour wetting modes on small and large-sized microdisk structures, respectively. This work reveals the response of cell aggregates to micro-scale topography, providing guidance for better understanding of tissue infiltration.

Using Odontoblasts Derived from Dog Endometrial Stem Cells Encapsulated in Fibrin Gel Associated with BMP-2 in a Rat Pulp-Capping Model

This study aimed to treat dental injuries by utilizing one of the most advanced tissue engineering techniques. In this study, an in vitro model was employed to investigate the proliferation and odontogenic differentiation of canine endometrial stem cells (C-EnSCs). Furthermore, the dentin regeneration potential of odontoblast like-cells (OD) derived from C-EnSCs was assessed in rats. The C-EnSCs were isolated by the enzymatic method and identified by flow cytometry. The C-EnSCs were encapsulated in fibrin gel associated with signaling factors to create the proper conditions for cell growth and differentiation. Then, the OD cells were associated with bone morphologic protein-2 (BMP-2) to promote dentin formation in vivo. The animal model used to evaluate the regenerative effect of cells and biomaterials included the preparation of the left maxillary first molar of rats for direct pulp capping operation. Animals were divided into four groups: group 1, a control group without any treatment, group 2, which received fibrin, group 3, which received fibrin with ODs (fibrin/ODs), and group 4, which received fibrin with ODs and BMP-2 (fibrin/ODs/BMP-2). The morphological observations showed the differentiation of C-EnSCs into adipose, bone, neural cells, and ODs. Furthermore, the histomorphometric data of the treated teeth showed how fibrin gel and BMP2 at a concentration of 100 ng/mL provided an optimal microenvironment for regenerating dentin tissue in rats, which was increased significantly with the presence of OD cells within eight weeks. Our study showed that using OD cells derived from C-EnSCs encapsulated in fibrin gel associated with BMP2 can potentially be an appropriate candidate for direct pulp-capping and dentin regeneration.

3D co-culture of macrophages and fibroblasts in a sessile drop array for unveiling the role of macrophages in skin wound-healing

Three-dimensional (3D) heterotypic multicellular spheroid models play important roles in researches of the proliferation and remodeling phases in wound healing. This study aimed to develop a sessile drop array to cultivate 3D spheroids and simulate wound healing stage in vitro using NIH-3T3 fibroblasts and M2-type macrophages. By the aid of the offset of surface tension and gravity, the sessile drop array is able to transfer cell suspensions to spheroids via the superhydrophobic surface of each microwell. Meanwhile, each microwell has a cylinder hole at its bottom that provides adequate oxygen to the spheroid. It demonstrated that the NIH-3T3 fibroblast spheroid and the 3T3 fibroblast/M2-type macrophage heterotypic multicellular spheroid can form and maintain physiological activities within nine days. In order to further investigate the structure without destroying the entire spheroid, we reconstructed its 3D morphology using transparent processing technology and the Z-stack function of confocal microscopy. Additionally, a nano antibody-based 3D immunostaining assay was used to analyze the proliferation and differentiation characteristics of these cells. It found that M2-type macrophages were capable of promoting the differentiation of 3T3 fibroblast spheroid. In this study, a novel, inexpensive platform was constructed for developing spheroids, as well as a 3D immunofluorescence method for investigating the macrophage-associated wound healing microenvironment.

Bioengineering and Clinical Translation of Human Lung and its Components

Clinical lung transplantation has rapidly established itself as the gold standard of treatment for end-stage lung diseases in a restricted group of patients since the first successful lung transplant occurred. Although significant progress has been made in lung transplantation, there are still numerous obstacles on the path to clinical success. The development of bioartificial lung grafts using patient-derived cells may serve as an alternative treatment modality; however, challenges include developing appropriate scaffold materials, advanced culture strategies for lung-specific multiple cell populations, and fully matured constructs to ensure increased transplant lifetime following implantation. This review highlights the development of tissue-engineered tracheal and lung equivalents over the past two decades, key problems in lung transplantation in a clinical environment, the advancements made in scaffolds, bioprinting technologies, bioreactors, organoids, and organ-on-a-chip technologies. The review aims to fill the lacuna in existing literature toward a holistic bioartificial lung tissue, including trachea, capillaries, airways, bifurcating bronchioles, lung disease models, and their clinical translation. Herein, the efforts are on bridging the application of lung tissue engineering methods in a clinical environment as it is thought that tissue engineering holds enormous promise for overcoming the challenges associated with the clinical translation of bioengineered human lung and its components.

Biomaterials for Testicular Bioengineering: How far have we come and where do we have to go?

Traditional therapeutic interventions aim to restore male fertile potential or preserve sperm viability in severe cases, such as semen cryopreservation, testicular tissue, germ cell transplantation and testicular graft. However, these techniques demonstrate several methodological, clinical, and biological limitations, that impact in their results. In this scenario, reproductive medicine has sought biotechnological alternatives applied for infertility treatment, or to improve gamete preservation and thus increase reproductive rates in vitro and in vivo. One of the main approaches employed is the biomimetic testicular tissue reconstruction, which uses tissue-engineering principles and methodologies. This strategy pursues to mimic the testicular microenvironment, simulating physiological conditions. Such approach allows male gametes maintenance in culture or produce viable grafts that can be transplanted and restore reproductive functions. In this context, the application of several biomaterials have been proposed to be used in artificial biological systems. From synthetic polymers to decellularized matrixes, each biomaterial has advantages and disadvantages regarding its application in cell culture and tissue reconstruction. Therefore, the present review aims to list the progress that has been made and the continued challenges facing testicular regenerative medicine and the preservation of male reproductive capacity, based on the development of tissue bioengineering approaches for testicular tissue microenvironment reconstruction.

Cell Encapsulation and 3D Bioprinting for Therapeutic Cell Transplantation

The promise of cell therapy has been augmented by introducing biomaterials, where intricate scaffold shapes are fabricated to accommodate the cells within. In this review, we first discuss cell encapsulation and the promising potential of biomaterials to overcome challenges associated with cell therapy, particularly cellular function and longevity. More specifically, cell therapies in the context of autoimmune disorders, neurodegenerative diseases, and cancer are reviewed from the perspectives of preclinical findings as well as available clinical data. Next, techniques to fabricate cell-biomaterials constructs, focusing on emerging 3D bioprinting technologies, will be reviewed. 3D bioprinting is an advancing field that enables fabricating complex, interconnected, and consistent cell-based constructs capable of scaling up highly reproducible cell-biomaterials platforms with high precision. It is expected that 3D bioprinting devices will expand and become more precise, scalable, and appropriate for clinical manufacturing. Rather than one printer fits all, seeing more application-specific printer types, such as a bioprinter for bone tissue fabrication, which would be different from a bioprinter for skin tissue fabrication, is anticipated in the future.

Scaffold-Free Tracheal Engineering via a Modular Strategy Based on Cartilage and Epithelium Sheets

Tracheal defects lead to devastating problems, and practical clinical substitutes that have complex functional structures and can avoid adverse influences from exogenous bioscaffolds are lacking. Herein, a modular strategy for scaffold-free tracheal engineering is developed. A cartilage sheet (Cart-S) prepared by high-density culture is laminated and reshaped to construct a cartilage tube as the main load-bearing structure in which the chondrocytes exhibit a stable phenotype and secreted considerable cartilage-specific matrix, presenting a native-like grid arrangement. To further build a tracheal epithelial barrier, a temperature-sensitive technique is used to construct the monolayer epithelium sheet (Epi-S), in which the airway epithelial cells present integrated tight junctions, good transepithelial electrical resistance, and favorable ciliary differentiation capability. Epi-S can be integrally transferred to inner wall of cartilage tube, forming a scaffold-free complex tracheal substitute (SC-trachea). Interestingly, when Epi-S is attached to the cartilage surface, epithelium-specific gene expression is significantly enhanced. SC-trachea establishes abundant blood supply via heterotopic vascularization and then is pedicle transplanted for tracheal reconstruction, achieving 83.3% survival outcomes in rabbit models. Notably, the scaffold-free engineered trachea simultaneously satisfies sufficient mechanical properties and barrier function due to its matrix-rich cartilage structure and well-differentiated ciliated epithelium, demonstrating great clinical potential for long-segmental tracheal reconstruction.

Transfer-Tattoo-Like Cell-Sheet Delivery Induced by Interfacial Cell Migration

A direct transfer of a cell sheet from a culture surface to a target tissue is introduced. Commercially available, flexible parylene is used as the culture surface, and it is proposed that the UV-treated parylene offers adequate and intermediate levels of cell adhesiveness for both the stable cell attachment during culture and for the efficient cell transfer to a target surface. The versatility of this cell-transfer process is demonstrated with various cell types, including MRC-5, HDFn, HULEC-5a, MC3T3-E1, A549, C2C12 cells, and MDCK-II cells. The novel cell-sheet engineering is based on a mechanism of interfacial cell migration between two surfaces with different adhesion preferences. Monitoring of cytoskeletal dynamics and drug treatments during the cell-transfer process reveals that the interfacial cell migration occurs by utilizing the existing transmembrane proteins on the cell surface to bind to the targeted surface. The re-establishment and reversal of cell polarity after the transfer process are also identified. Its unique capabilities of 3D multilayer stacking, freeform design, and curved surface application are demonstrated. Finally, the therapeutic potential of the cell-sheet delivery system is demonstrated by applying it to cutaneous wound healing and skin-tissue regeneration in mice models.

Embedded 3D printing of dilute particle suspensions into dense complex tissue fibers using shear thinning xanthan baths

In order to fabricate functional organoids and microtissues, a high cell density is generally required. As such, the placement of cell suspensions in molds or microwells to allow for cell concentration by sedimentation is the current standard for the production of organoids and microtissues. Even though molds offer some level of control over the shape of the resulting microtissue, this control is limited as microtissues tend to compact towards a sphere after sedimentation of the cells. 3D bioprinting on the other hand offers complete control over the shape of the resulting structure. Even though the printing of dense cell suspensions in the ink has been reported, extruding dense cellular suspensions is challenging and generally results in high shear stresses on the cells and a poor shape fidelity of the print. As such, additional materials such as hydrogels are added in the bioink to limit shear stresses, and to improve shape fidelity and resolution. The maximum cell concentration that can be incorporated in a hydrogel-based ink before the ink's rheological properties are compromised, is significantly lower than the concentration in a tissue equivalent. Additionally, the hydrogel components often interfere with cellular self-assembly processes. To circumvent these limitations, we report a simple and inexpensive xanthan bath based embedded printing method to 3D print dense functional linear tissues using dilute particle suspensions consisting of cells, spheroids, hydrogel beads, or combinations thereof. Using this method, we demonstrated the self-organization of functional cardiac tissue fibers with a layer of epicardial cells surrounding a body of cardiomyocytes.

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.

Expanding Quality by Design Principles to Support 3D Printed Medical Device Development Following the Renewed Regulatory Framework in Europe

The vast scope of 3D printing has ignited the production of tailored medical device (MD) development and catalyzed a paradigm shift in the health-care industry, particularly following the COVID pandemic. This review aims to provide an update on the current progress and emerging opportunities for additive manufacturing following the introduction of the new medical device regulation (MDR) within the EU. The advent of early-phase implementation of the Quality by Design (QbD) quality management framework in MD development is a focal point. The application of a regulatory supported QbD concept will ensure successful MD development, as well as pointing out the current challenges of 3D bioprinting. Utilizing a QbD scientific and risk-management approach ensures the acceleration of MD development in a more targeted way by building in all stakeholders' expectations, namely those of the patients, the biomedical industry, and regulatory bodies.

Advanced Robotics to Address the Translational Gap in Tendon Engineering

Tendon disease is a significant and growing burden to healthcare systems. One strategy to address this challenge is tissue engineering. A widely held view in this field is that mechanical stimulation provided to constructs should replicate the mechanical environment of native tissue as closely as possible. We review recent tendon tissue engineering studies in this article and highlight limitations of conventional uniaxial tensile bioreactors used in current literature. Advanced robotic platforms such as musculoskeletal humanoid robots and soft robotic actuators are promising technologies which may help address translational gaps in tendon tissue engineering. We suggest the proposed benefits of these technologies and identify recent studies which have worked to implement these technologies in tissue engineering. Lastly, key challenges to address in adapting these robotic technologies and proposed future research directions for tendon tissue engineering are discussed.

Tissue-Engineered Models of the Human Brain: State-of-the-Art Analysis and Challenges

Neurological disorders affect billions of people across the world, making the discovery of effective treatments an important challenge. The evaluation of drug efficacy is further complicated because of the lack of in vitro models able to reproduce the complexity of the human brain structure and functions. Some limitations of 2D preclinical models of the human brain have been overcome by the use of 3D cultures such as cell spheroids, organoids and organs-on-chip. However, one of the most promising approaches for mimicking not only cell structure, but also brain architecture, is currently represented by tissue-engineered brain models. Both conventional (particularly electrospinning and salt leaching) and unconventional (particularly bioprinting) techniques have been exploited, making use of natural polymers or combinations between natural and synthetic polymers. Moreover, the use of induced pluripotent stem cells (iPSCs) has allowed the co-culture of different human brain cells (neurons, astrocytes, oligodendrocytes, microglia), helping towards approaching the central nervous system complexity. In this review article, we explain the importance of in vitro brain modeling, and present the main in vitro brain models developed to date, with a special focus on the most recent advancements in tissue-engineered brain models making use of iPSCs. Finally, we critically discuss achievements, main challenges and future perspectives.

Highly Bioactive Cell-laden Hydrogel Constructs Bioprinted Using an Emulsion Bioink for Tissue Engineering Applications

The insufficient pore structure of cell-laden hydrogel scaffolds has limited their application in various tissue regeneration applications owing to low cell-to-cell/matrix interactions and low transfer of nutrients and metabolic wastes. Herein, we designed a highly porous cell-laden hydrogel scaffold fabricated using an emulsion bioink consisting of methacrylated collagen (CMA), mineral oil (MO), and human adipose stem cells (hASCs) to induce efficient cell infiltration and cellular activities. By selecting the most appropriate concentration of CMA and MO, the emulsion bioink can be successfully formulated with proper yield stress and printability. The cell-laden scaffold exhibited significantly greater cell growth and cytoskeletal reorganization than the normally printed cell-laden CMA scaffold. Furthermore, two bioactive components (kartogenin and bone morphogenetic protein 2) were physically encapsulated in the oil droplets of the cell construct, and the molecules in the cell constructs enhanced chondrogenic or osteogenic differentiation of hASCs in the printed structure. Based on these results, the cell-printed structure using an emulsion bioink can not only provide a good cellular microenvironment but also be a new potential method to accelerate stem cell differentiation by combining bioactive molecules and cell-laden scaffolds.