The bioink: A comprehensive review on bioprintable materials

This paper discusses "bioink", bioprintable materials used in three dimensional (3D) bioprinting processes, where cells and other biologics are deposited in a spatially controlled pattern to fabricate living tissues and organs. It presents the first comprehensive review of existing bioink types including hydrogels, cell aggregates, microcarriers and decellularized matrix components used in extrusion-, droplet- and laser-based bioprinting processes. A detailed comparison of these bioink materials is conducted in terms of supporting bioprinting modalities and bioprintability, cell viability and proliferation, biomimicry, resolution, affordability, scalability, practicality, mechanical and structural integrity, bioprinting and post-bioprinting maturation times, tissue fusion and formation post-implantation, degradation characteristics, commercial availability, immune-compatibility, and application areas. The paper then discusses current limitations of bioink materials and presents the future prospects to the reader.

Mechanical properties of cell sheets and spheroids: the link between single cells and complex tissues

Cell aggregates, including sheets and spheroids, represent a simple yet powerful model system to study both biochemical and biophysical intercellular interactions. However, it is becoming evident that, although the mechanical properties and behavior of multicellular structures share some similarities with individual cells, yet distinct differences are observed in some principal aspects. The description of mechanical phenomena at the level of multicellular model systems is a necessary step for understanding tissue mechanics and its fundamental principles in health and disease. Both cell sheets and spheroids are used in tissue engineering, and the modulation of mechanical properties of cell constructs is a promising tool for regenerative medicine. Here, we review the data on mechanical characterization of cell sheets and spheroids, focusing both on advances in the measurement techniques and current understanding of the subject. The reviewed material suggest that interplay between the ECM, intercellular junctions, and cellular contractility determines the behavior and mechanical properties of the cell aggregates.

Engineering cell aggregates through incorporated polymeric microparticles

Ex vivo cell aggregates must overcome significant limitations in the transport of nutrients, drugs, and signaling proteins compared to vascularized native tissue. Further, engineered extracellular environments often fail to sufficiently replicate tethered signaling cues and the complex architecture of native tissue. Co-cultures of cells with microparticles (MPs) is a growing field directed towards overcoming many of these challenges by providing local and controlled presentation of both soluble and tethered proteins and small molecules. Further, co-cultured MPs offer a mechanism to better control aggregate architecture and even to report key characteristics of the local microenvironment such as pH or oxygen levels. Herein, we provide a brief introduction to established and developing strategies for MP production including the choice of MP materials, fabrication techniques, and techniques for incorporating additional functionality. In all cases, we emphasize the specific utility of each approach to form MPs useful for applications in cell aggregate co-culture. We review established techniques to integrate cells and MPs. We highlight those strategies that promote targeted heterogeneity or homogeneity, and we describe approaches to engineer cell-particle and particle-particle interactions that enhance aggregate stability and biological response. Finally, we review advances in key application areas of MP aggregates and future areas of development.

STATEMENT OF SIGNIFICANT

Cell-scaled polymer microparticles (MPs) integrated into cellular aggregates have been shown to be a powerful tool to direct cell response. MPs have supported the development of healthy cartilage, islets, nerves, and vasculature by the maintenance of soluble gradients as well as by the local presentation of tethered cues and diffusing proteins and small molecules. MPs integrated with pluripotent stem cells have directed in vivo expansion and differentiation. Looking forward, MPs are expected to support both the characterization and development of in vitro tissue systems for applications such as drug testing platforms. However, useful co-cultures must be designed keeping in mind the limitations and attributes of each material strategy within the context of the overall tissue biology. The present review integrates prospectives from materials development, drug delivery, and tissue engineering to provide a toolbox for the development and application of MPs useful for long-term co-culture within cell aggregates.

Dual functional construct containing kartogenin releasing microtissues and curcumin for cartilage regeneration

BACKGROUND

Regeneration of articular cartilage poses a tremendous challenge due to its limited self-repair capability and inflammation at the damaged site. To generate the desired structures that mimic the structure of native tissue, microtissues with repeated functional units such as cell aggregates have been developed. Multicellular aggregates of mesenchymal stem cells (MSCs) can be used as microscale building blocks of cartilage due to their potential for cell-cell contact, cell proliferation, and differentiation.

METHODS

Chondrogenic microtissues were developed through incorporation of kartogenin-releasing poly (lactic-co-glycolic acid) (PLGA) microparticles (KGN-MP) within the MSC aggregates. The chondrogenic potential of KGN-MP treated MSC aggregates was proven in vitro by studying the chondrogenic markers at the RNA level and histological analysis. In order to address the inflammatory responses at the defect site, the microtissues were delivered in vivo via an injectable, anti-inflammatory hydrogel that contained gelatin methacryloyl (GelMA) loaded with curcumin (Cur).

RESULTS

The KGN-MPs were fabricated to support MSCs during cartilage differentiation. According to real-time RT-PCR analysis, the presence of KGN in the aggregates led to the expression of cartilage markers by the MSCs. Both toluidine blue (TB) and safranin O (SO) staining demonstrated homogeneous glycosaminoglycan production throughout the KGN-MP incorporated MSC aggregates. The curcumin treatment efficiently reduced the expressions of hypertrophy markers by MSCs in vitro. The in vivo results showed that implantation of chondrogenic microtissues (KGN-MP incorporated MSC aggregates) using the curcumin loaded GelMA hydrogel resulted in cartilage tissue regeneration that had characteristic features close to the natural hyaline cartilage according to observational and histological results.

CONCLUSIONS

The use of this novel construct that contained chondrogenic cell blocks and curcumin is highly desired for cartilage regeneration.

Inelastic mechanics: A unifying principle in biomechanics

Many soft materials are classified as viscoelastic. They behave mechanically neither quite fluid-like nor quite solid-like - rather a bit of both. Biomaterials are often said to fall into this class. Here, we argue that this misses a crucial aspect, and that biomechanics is essentially damage mechanics, at heart. When deforming an animal cell or tissue, one can hardly avoid inducing the unfolding of protein domains, the unbinding of cytoskeletal crosslinkers, the breaking of weak sacrificial bonds, and the disruption of transient adhesions. We classify these activated structural changes as inelastic. They are often to a large degree reversible and are therefore not plastic in the proper sense, but they dissipate substantial amounts of elastic energy by structural damping. We review recent experiments involving biological materials on all scales, from single biopolymers over cells to model tissues, to illustrate the unifying power of this paradigm. A deliberately minimalistic yet phenomenologically very rich mathematical modeling framework for inelastic biomechanics is proposed. It transcends the conventional viscoelastic paradigm and suggests itself as a promising candidate for a unified description and interpretation of a wide range of experimental data. This article is part of a Special Issue entitled: Mechanobiology.

Enhanced survival and insulin secretion of insulinoma cell aggregates by incorporating gelatin hydrogel microspheres

Introduction

The objective of this study is to evaluate the survival and glucose-induced insulin secretion of rat-derived insulinoma cells (INS-1) from their aggregates incorporating different size of gelatin hydrogel microspheres comparing with microspheres-free cell aggregates.

Methods

The gelatin hydrogel microspheres were prepared by the conventional w/o emulsion method. The INS-1 cells were cultured in a V-bottomed well, combining with or without the gelatin hydrogel microspheres to form their aggregates with or without microspheres.

Results

When the cell viability, the live cell number, the reductase activity, and the insulin secretion of cell aggregates were evaluated 7 or 14 days after incubation, the cell aggregates incorporating gelatin hydrogel microspheres showed higher cell viability, reductase activity and a larger number of live cells. The cell aggregates incorporating larger size and number of gelatin hydrogel microspheres secreted a larger amount of insulin, compared with those incorporating smaller size and number of microspheres or without microspheres.

Conclusion

It is conceivable that the incorporation of gelatin hydrogel microspheres in cell aggregates is promising to improve their survival and insulin secretion function.

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INS-1 cell aggregates incorporating gelatin hydrogel microspheres are prepared.

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Gelatin hydrogel microspheres incorporation improves cell viability and glucose-induced insulin secretion of cell aggregates.

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The size and number of gelatin hydrogel microspheres affected the cell condition and function.

Enhanced efficacy of human brain-derived neural stem cells by transplantation of cell aggregates in a rat model of Parkinson's disease

Objective

Neural tissue transplantation has been a promising strategy for the treatment of Parkinson's disease (PD). However, transplantation has the disadvantages of low-cell survival and/or development of dyskinesia. Transplantation of cell aggregates has the potential to overcome these problems, because the cells can extend their axons into the host brain and establish synaptic connections with host neurons. In this present study, aggregates of human brain-derived neural stem cells (HB-NSC) were transplanted into a PD animal model and compared to previous report on transplantation of single-cell suspensions.

Methods

Rats received an injection of 6-OHDA into the right medial forebrain bundle to generate the PD model and followed by injections of PBS only, or HB-NSC aggregates in PBS into the ipsilateral striatum. Behavioral tests, multitracer (2-deoxy-2-[¹⁸F]-fluoro-D-glucose ([¹⁸F]-FDG) and [¹⁸F]-N-(3-fluoropropyl)-2-carbomethoxy-3-(4-iodophenyl)nortropane ([¹⁸F]-FP-CIT) microPET scans, as well as immunohistochemical (IHC) and immunofluorescent (IF) staining were conducted to evaluate the results.

Results

The stepping test showed significant improvement of contralateral forelimb control in the HB-NSC group from 6-10 weeks compared to the control group (p<0.05). [¹⁸F]-FP-CIT microPET at 10 weeks posttransplantation demonstrated a significant increase in uptake in the HB-NSC group compared to pretransplantation (p<0.05). In IHC and IF staining, tyrosine hydroxylase and human β2 microglobulin (a human cell marker) positive cells were visualized at the transplant site.

Conclusion

These results suggest that the HB-NSC aggregates can survive in the striatum and exert therapeutic effects in a PD model by secreting dopamine.

Aggregate and Microcarrier Cultures of Human Pluripotent Stem Cells in Stirred-Suspension Systems

Pluripotent stem cells can differentiate to any cell type and contribute to damaged tissue repair and organ function reconstitution. The scalable culture of pluripotent stem cells is essential to furthering the use of stem cell products in a wide gamut of applications such as screening of candidate drugs and cell replacement therapies. Human stem cell cultivation in stirred-suspension vessels enables the expansion of stem cells and the generation of differentiated progeny in quantities suitable for use in animal models and clinical studies. We describe methods of culturing human pluripotent stem cells in spinner flasks either as aggregates or on microcarriers. Techniques for assessing the quality of the culture and characterizing the cells based on the presentation of pertinent markers are also presented. Spinner flask culture with its relatively low capital and operating costs is appealing to laboratories interested in scaling up their production of stem/progenitor cells.

Laser reflectance measurement for the online monitoring of Chlorella sorokiniana biomass concentration

Fast and reliable methods to determine biomass concentration are necessary to facilitate the large scale production of microalgae. A method for the rapid estimation of Chlorella sorokiniana biomass concentration was developed. The method translates the suspension particle size spectrum gathered though laser reflectance into biomass concentration by means of two machine learning modelling techniques. In each case, the model hyper-parameters were selected applying a simulated annealing algorithm. The results show that dry biomass concentration can be estimated with a very good accuracy (R²=0.87). The presented method seems to be suited to perform fast estimations of biomass concentration in suspensions of microalgae cultivated in moderately turbid media with tendency to aggregate.

Improved quorum sensing capacity by culturing Vibrio harveyi in microcapsules

Microcapsule entrapped low density cells with culture (ELDCwc), different from free cell culture, conferred stronger stress resistance and improved cell viability of microorganisms. In this paper, the quorum sensing (QS) system of Vibrio harveyi was used to investigate changes when cells were cultured in microcapsules. Cells in ELDCwc group grew into cell aggregates, which facilitated cell-cell communication and led to increased bioluminescence intensity. Moreover, the luxS-AI-2 system, a well-studied QS signal pathway, was detected as both luxS gene and the AI-2 signaling molecule, and the results were analyzed with respect to QS capacity of unit cell. The V. harveyi of ELDCwc also showed higher relative gene expression and stronger quorum sensing capacity when compared with free cells. In conclusion, the confined microcapsule space can promote the cell aggregates formation, reduce cell-cell communication distance and increase local concentration of signal molecule, which are beneficial to bacterial QS.

Preparation of epithelial cell aggregates incorporating matrigel microspheres to enhance proliferation and differentiation of epithelial cells

The objective of this study is to investigate the effect of matrigel microspheres (MM), gelatin hydrogel microspheres (GM), and matrigel-coated GM on the proliferated and biological functions of epithelial cells in cell aggregates incorporating the microspheres. The MM were prepared by a coacelvation method. When mammary epithelial EpH4 cells were cultured with the MM, GM, and matrigel-coated GM in round U-bottom wells of 96-multiwell culture plates which had been coated with poly (vinyl alcohol) (PVA) to suppress the cell adhesion, EpH4 cell aggregates with each microspheres homogeneously incorporated were formed. Higher EpH4 cells proliferation was observed for cell aggregates incorporating MM, GM, and matrigel-coated GM compared with the conventional 3-dimensional (3D) culture method. When examined to evaluate the epithelial differentiation of EpH4 cells, the β-casein expression was significantly higher for the cell aggregates incorporating MM than that of aggregates incorporating GM and matrigel-coated GM or the conventional 3D culture method. It is concluded that the proliferation and differentiation of mammary epithelial EpH4 cells were promoted by the incorporation of MM.

A Diffuse Interface Framework for Modeling the Evolution of Multi-cell Aggregates as a Soft Packing Problem Driven by the Growth and Division of Cells

We present a model for cell growth, division and packing under soft constraints that arise from the deformability of the cells as well as of a membrane that encloses them. Our treatment falls within the framework of diffuse interface methods, under which each cell is represented by a scalar phase field and the zero level set of the phase field represents the cell membrane. One crucial element in the treatment is the definition of a free energy density function that penalizes cell overlap, thus giving rise to a simple model of cell-cell contact. In order to properly represent cell packing and the associated free energy, we include a simplified representation of the anisotropic mechanical response of the underlying cytoskeleton and cell membrane through penalization of the cell shape change. Numerical examples demonstrate the evolution of multi-cell clusters and of the total free energy of the clusters as a consequence of growth, division and packing.

Droplet based vitrification for cell aggregates: Numerical analysis

Cell aggregates represent the main format of cells existing in vivo and have been widely used as tissue and disease models in vitro. Nevertheless, the preservation of cell aggregates while maintaining their functionalities for off-the-shelf applications is still challenging. Among various preservation methods, droplet-based vitrification exhibits superior advantages for the cryopreservation of cell aggregates; however, the physical mechanisms underlying droplet-based vitrification of cell aggregate using this method remain elusive. To address this issue, we proposed a voronoi model to construct two-dimensional geometric morphologies of cell aggregates and established a coupled physical model to describe the diffusion, heat transfer and crystallization processes during vitrification. Based on these models, we performed a numerical study on the variation and distribution of cryoprotectant (CPA) concentration, temperature and crystallization in cell aggregates during droplet-based vitrification. The results show that although cell membrane is not an obvious barrier in heat transfer, it affects the diffusion of CPA remarkably as a biologic film and thus the following crystallization in cell aggregates. The effective protection of CPA during vitrification occurs during the initial stage of CPA diffusion, thus a longer CPA loading time does not necessarily lead to significant decrease in crystallization, but rather may induce more toxicity to cells.

Compartmentalized microfluidic perfusion system to culture human induced pluripotent stem cell aggregates

Microfluidic perfusion systems enable small-volume cell cultures under precisely controlled microenvironments, and are typically developed for cell-based high-throughput screening. However, most such systems are designed to manipulate dissociated single cells, not cell aggregates, and are thus unsuitable to induce differentiation in human induced pluripotent stem cells (hiPSCs), which is conventionally achieved by using cell aggregates to increase cell-cell interactions. We have now developed a compartmentalized microfluidic perfusion system with large flow channels to load, culture, and observe cell aggregates. Homogeneously sized cell aggregates to be loaded into the device were prepared by shredding flat hiPSC colonies into squares. These aggregates were then seeded into microchambers coated with fibronectin and bovine serum albumin (BSA) to establish adherent and floating cultures, respectively, both of which are frequently used to differentiate hiPSCs. However, the number of aggregates loaded in fibronectin-coated microchambers was much lower than in BSA-coated microchambers, suggesting that fibronectin traps cell aggregates before they reach the chambers. Accordingly, hiPSCs that reached the microchambers subsequently adhered. In contrast, BSA-coated microchambers did not allow cell aggregates to adhere, but were sufficiently deep to prevent cell aggregates from flowing out during perfusion of media. Immunostaining for markers of undifferentiated cells showed that cultures on both fibronectin- and BSA-coated microchambers were successfully established. Notably, we found that floating aggregates eventually adhered to surfaces coated with BSA upon differentiation, and that differentiation depends on the initial size of aggregates. Collectively, these results suggest that the microfluidic system is suitable for manipulating hiPSC aggregates in compartmentalized microchambers.

Dextran sulfate prevents excess aggregation of human pluripotent stem cells in 3D culture by inhibiting ICAM1 expression coupled with down-regulating E-cadherin through activating the Wnt signaling pathway

Background

Human pluripotent stem cells (hPSCs) have great potential in applications for regenerative medicine and drug development. However, 3D suspension culture systems for clinical-grade hPSC large-scale production have been a major challenge. Accumulating evidence has demonstrated that the addition of dextran sulfate (DS) could prevent excessive adhesion of hPSCs from forming larger aggregates in 3D suspension culture. However, the signaling and molecular mechanisms underlying this phenomenon remain elusive.

Methods

By using a cell aggregate culture assay and separating big and small aggregates in suspension culture systems, the potential mechanism and downstream target genes of DS were investigated by mRNA sequence analysis, qRT-PCR validation, colony formation assay, and interference assay.

Results

Since cellular adhesion molecules (CAMs) play important roles in hPSC adhesion and aggregation, we assumed that DS might prevent excess adhesion through affecting the expression of CAMs in hPSCs. As expected, after DS treatment, we found that the expression of CAMs was significantly down-regulated, especially E-cadherin (E-cad) and intercellular adhesion molecule 1 (ICAM1), two highly expressed CAMs in hPSCs. The role of E-cad in the adhesion of hPSCs has been widely investigated, but the function of ICAM1 in hPSCs is hardly understood. In the present study, we demonstrated that ICAM1 exhibited the capacity to promote the adhesion in hPSCs, and this adhesion was suppressed by the treatment with DS. Furthermore, transcriptomic analysis of RNA-seq revealed that DS treatment up-regulated genes related to Wnt signaling resulting in the activation of Wnt signaling in which SLUG, TWIST, and MMP3/7 were highly expressed, and further inhibited the expression of E-cad.

Conclusion

Our results demonstrated that DS played an important role in controlling the size of hPSC aggregates in 3D suspension culture by inhibiting the expression of ICAM1 coupled with the down-regulation of E-cad through the activation of the Wnt signaling pathway. These results represent a significant step toward developing the expansion of hPSCs under 3D suspension condition in large-scale cultures.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-02890-4.

Discovering the Latest Scientific Pathways on Tissue Spheroids: Opportunities to Innovate

Tissue spheroids consist of a three-dimensional model of cells which is capable of imitating the complicated composition of healthy and unhealthy human tissue. Due to their unique properties, they can bring innovative solutions to tissue engineering and regenerative medicine, where they can be used as building blocks for the formation of organ and tissue models used in drug experimentation. Considering the rapid transformation of the health industry, it is crucial to assess the research dynamics of this field to support the development of innovative applications. In this research, a scientometric analysis was performed as part of a Competitive Technology Intelligence methodology, to determine the main applications of tissue spheroids. Papers from Scopus and Web of Science published between 2000 and 2019 were organized and analyzed. In total, 868 scientific publications were identified, and four main categories of application were determined. Main subject areas, countries, cities, authors, journals, and institutions were established. In addition, a cluster analysis was performed to determine networks of collaborations between institutions and authors. This article provides insights into the applications of cell aggregates and the research dynamics of this field, which can help in the decision-making process to incorporate emerging and innovative technologies in the health industry.

Elaboration of a finite element model of pancreatic islet dielectric response to gap junction expression and insulin release

Dielectric spectroscopy could potentially be a powerful tool to monitor isolated human pancreatic islets for applications in diabetes therapy and research. Isolated intact human islets provide the most relevant means to understand the cellular and molecular mechanisms associated with diabetes. The advantages of dielectric spectroscopy for continuous islet monitoring are that it is a non-invasive, inexpensive and real-time technique. We have previously assessed the dielectric response of human islet samples during stimulation and differentiation. Because of the complex geometry of islets, analytical solutions are not sufficiently representative to provide a pertinent model of islet dielectric response. Here, we present a finite element dielectric model of a single intact islet that takes into account the tight packing of islet cells and intercellular junctions. The simulation yielded dielectric spectra characteristic of cell aggregates, similar to those produced with islets. In addition, the simulation showed that both exocytosis, such as what occurs during insulin secretion, and differential gap junction expression have significant effects on islet dielectric response. Since the progression of diabetes has some connections with dysfunctional islet gap junctions and insulin secretion, the ability to monitor these islet features with dielectric spectroscopy would benefit diabetes research.

Definitive Endoderm from EpiSC Aggregates in Matrigel

This chapter describes the protocol to derive definitive endoderm cells from epiblast stem cells (EpiSCs) via a process analogous to gastrulation in embryos. The basis of this protocol mimicking the in vivo gastrulation process makes a contrast with those using sequential administration of pharmacological molecules and recombinant signaling proteins even at nonphysiological levels. In the experimental setup, EpiSCs are first freed from the dish-adherent condition to form free-floating aggregates, where endoderm precursor pools are produced. Embedding the EpiSC aggregates in the Matrigel allows the endoderm precursors to interact with the Matrigel mimicking the laminin-rich basement membrane underlying the egg cylinder epiblast in embryos, and let the precursors migrate into the Matrigel-filled external zone and develop into endodermal epithelial tissues.

Evaluation of the Capability of the Wharton's Jelly Mesenchymal Stem Cell Aggregates to Express the Markers of Three Germ Cell Lineages

BACKGROUND

The ability of stem cells to differentiate into different cell types makes them a key component of healing damage in regenerative medicine. As human umbilical cord Wharton's jelly (HUCWJ) is available non-invasively, HUCWJ does not raise any ethical issues with higher differentiation potential compared to adult stem cells. With the ability to express embryonic stem cell markers, HUCWJ can be considered as a good candidate in regenerative medicine applications. The objective of this study was to find if these cells form cell aggregates with the same features as that formed by embryonic stem cells (embryoid body) and could form three germ layers.

METHODS

Eighteen umbilical cords were of healthy infants with parent permission. The umbilical cords were cut into small pieces and the explants were cultured. At the third passage, 1000, 5000 and 10000 cells/ 20 µL were cultured in hanging drops for 3 days. Then, they were incubated for additional 3 days in non-adhesive dishes. As the center of cell aggregates formed from 5000 and 10000 cells were darker than those formed from 1000 cells, this study focused on the aggregates formed by 1000 cells for further assessments. The immunocytochemistry and flowcytometry were performed using 3 color antibodies to detect the markers for three germ cell lineages.

RESULTS

The immunohistochemistry data showed that the embryoid-body-like aggregates expressed a low amount of ectodermal and endodermal markers and most of the cells expressed mesodermal markers. The flowcytometry percentage of the cells in each aggregate that expressed ectodermal marker Otx2 was17.1% and endodermal marker, Sox 17 was 5.49%. The frequency of cells expressing mesodermal marker Brachyury was high (75.0%). Flowcytometry also showed the percentages by mathematical evaluation and we did this three times for our result accuracy.

CONCLUSION

These aggregates mainly kept their mesenchymal state and showed a poor differentiation potential toward ectoderm and endoderm identity.

High-throughput 3D Spheroid Formation and Effective Cardiomyocyte Differentiation from Human iPS Cells Using the Microfabric Vessels EZSPHERETM

High-throughput 3D spheroid formation from human induced pluripotent stem cells (hiPSCs) can be easily performed using the unique microfabric vessels EZSPHERE, resulting in effective and large scale generation of differentiated cells such as cardiomyocytes or neurons. Such hiPSC-derived cardiomyocytes (hiPSC-CMs) or neurons are very useful in the fields of regenerative medicine or cell-based drug safety tests. Previous studies indicated that 3D spheroids arising from hiPSCs are effectively differentiated into high quality hiPSC-CMs by controlling Wnt signals through utilization of the microfabric vessels EZSPHERE. Here, we describe a simple and highly efficient protocol for generating a large number of uniformly sized hiPSC spheroids and inducing them for cardiac differentiation using the EZSPHERE. This method comprises the collection and dissociation of the spheroids from cardiac differentiation medium, in the middle stage of the whole cardiac differentiation process, and re-seeding the obtained single cells into the EZSPHERE to re-aggregate them into uniform hiPSC-CM spheroids with controlled size. This re-aggregation process promotes non-canonical Wnt signal-related cardiac development and improves the purity and maturity of the hiPSC-CMs generated. Graphic abstract: Overview of cardiac differentiation from iPSCs by spheroid formation and reaggregation using EZSPHERE.

The maximum size of cell-aggregates is determined by the competition between the strain energy and the binding energy of cells

The development of tissues and organs is affected by how cells interact with each other to form aggregates. Such an interaction is in turn determined by several different factors, such as inter-cellular attractive forces, cell motility, and the strain energy of cells. Here, we have used mathematical modelling and numerical simulations to explore how the interplay between these factors can influence the formation and stability of 2D cell aggregates. Cell aggregates were created by incrementally accumulating cells over an initial seed. The binding energy density of these aggregates was determined using the harmonic approximation and was integrated into a probabilistic model to estimate the maximum cluster size, beyond which the aggregate becomes unstable and breaks into smaller fragments. Our simulations reveal that the ratio of strain energy to internal adhesive energy (U s / U b ) critically impacts cell aggregation; smaller ratios allow for larger cluster sizes. These findings have significant implications for tissue engineering, in-vitro modeling, the study of neurodegenerative diseases, and tissue regeneration, providing insights into how physical and biological characteristics of cells influence their aggregation and stability.

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.

Regenerative endodontic therapy: From laboratory bench to clinical practice

BACKGROUND

Maintaining the vitality and functionality of dental pulp is paramount for tooth integrity, longevity, and homeostasis. Aiming to treat irreversible pulpitis and necrosis, there has been a paradigm shift from conventional root canal treatment towards regenerative endodontic therapy.

AIM OF REVIEW

This extensive and multipart review presents crucial laboratory and practical issues related to pulp-dentin complex regeneration aimed towards advancing clinical translation of regenerative endodontic therapy and enhancing human life quality.

KEY SCIENTIFIC CONCEPTS OF REVIEW

In this multipart review paper, we first present a panorama of emerging potential tissue engineering strategies for pulp-dentin complex regeneration from cell transplantation and cell homing perspectives, emphasizing the critical regenerative components of stem cells, biomaterials, and conducive microenvironments. Then, this review provides details about current clinically practiced pulp regenerative/reparative approaches, including direct pulp capping and root revascularization, with a specific focus on the remaining hurdles and bright prospects in developing such therapies. Next, special attention was devoted to discussing the innovative biomimetic perspectives opened in establishing functional tissues by employing exosomes and cell aggregates, which will benefit the clinical translation of dental pulp engineering protocols. Finally, we summarize careful consideration that should be given to basic research and clinical applications of regenerative endodontics. In particular, this review article highlights significant challenges associated with residual infection and inflammation and identifies future insightful directions in creating antibacterial and immunomodulatory microenvironments so that clinicians and researchers can comprehensively understand crucial clinical aspects of regenerative endodontic procedures.