Challenges and strategies: Scalable and efficient production of mesenchymal stem cells-derived exosomes for cell-free therapy

Exosomes are small membrane vesicles secreted by most cell types, and widely exist in cell supernatants and various body fluids. They can transmit numerous bioactive elements, such as proteins, nucleic acids, and lipids, to affect the gene expression and function of recipient cells. Mesenchymal stem cells (MSCs) have been confirmed to be a potentially promising therapy for tissue repair and regeneration. Accumulating studies demonstrated that the predominant regenerative paradigm of MSCs transplantation was the paracrine effect but not the differentiation effect. Exosomes secreted by MSCs also showed similar therapeutic effects as their parent cells and were considered to be used for cell-free regenerative medicine. However, the inefficient and limited production has hampered their development for clinical translation. In this review, we summarize potential methods to efficiently promote the yield of exosomes. We mainly focus on engineering the process of exosome biogenesis and secretion, altering the cell culture conditions, cell expansion through 3D dynamic culture and the isolation of exosomes. In addition, we also discuss the application of MSCs-derived exosomes as therapeutics in disease treatment.

Hierarchical and heterogeneous hydrogel system as a promising strategy for diversified interfacial tissue regeneration

A state-of-the-art review on the design and preparation of hierarchical and heterogeneous hydrogel systems for interfacial tissue regeneration.

Fabrication of protease XIV-loaded microspheres for cell spreading in silk fibroin hydrogels

Due to their excellent mechanical strength and biocompatibility, silk fibroin(SF) hydrogels can serve as ideal scaffolds. However, their slow rate of natural degradation limits the space available for cell proliferation, which hinders their application. In this study, litchi-like calcium carbonate@hydroxyapatite (CaCO₃@HA) porous microspheres loaded with proteases from Streptomyces griseus (XIV) were used as drug carriers to regulate the biodegradation rate of SF hydrogels. The results showed that litchi-like CaCO₃@HA microspheres with different phase compositions could be prepared by changing the hydrothermal reaction time. The CaCO₃@HA microspheres controlled the release of Ca ions, which was beneficial for the osteogenic differentiation of mesenchymal stem cells (MSCs). The adsorption and release of protease XIV from the CaCO₃@HA microcarriers indicated that the loading and release amount can be controlled with the initial drug concentration. The weight loss test and SEM observation showed that the degradation of the fibroin hydrogel could be controlled by altering the amount of protease XIV-loaded CaCO₃@HA microspheres. A three-dimensional (3D) cell encapsulation experiment proved that incorporation of the SF hydrogel with protease XIV-loaded microspheres promoted cell dispersal and spreading, suggesting that the controlled release of protease XIV can regulate hydrogel degradation. SF hydrogels incorporated with protease XIV-loaded microspheres are suitable for cell growth and proliferation and are expected to serve as excellent bone tissue engineering scaffolds.

Assessment of Cytocompatibility and Anti-Inflammatory (Inter)Actions of Genipin-Crosslinked Chitosan Powders

Chitosan is a polysaccharide commonly used, together with its derivatives, in the preparation of hydrogel formulations, scaffolds and films for tissue engineering applications. Chitosan can be used as such, but it is commonly stabilized by means of chemical crosslinkers. Genipin is one of the crosslinkers that has been considered that is a crystalline powder extracted from the fruit of Gardenia jasminoides and processed to obtain an aglycon compound. Genipin is gaining interest in biological applications because of its natural origin and anti-inflammatory actions. In this paper, the ability of chitosan-based materials crosslinked with genipin to exert anti-inflammation properties in applications such as bone regeneration was studied. Powders obtained from chitosan–genipin scaffolds have been tested in order to mimic the natural degradation processes occurring during biomaterials implantation in vivo. The results from osteoblast-like cells showed that specific combinations of chitosan and genipin stimulate high permissiveness towards cells, with higher performance than the pure chitosan. In parallel, evidences from monocyte-like cells showed that the crosslinker, genipin, seems to promote slowing of the monocyte-macrophage transition at morphological level. This suggests a sort of modularity of pro-inflammatory versus anti-inflammatory behavior of our chitosan-based biomaterials. Being both the cell types exposed to microscale powders, as an added value our results bring information on the cell–material interactions in the degradative dynamics of chitosan scaffold structures during the physiological resorption processes.

Mechanics-Controlled Dynamic Cell Niches Guided Osteogenic Differentiation of Stem Cells via Preserved Cellular Mechanical Memory

Stem cells sense and respond to their local dynamic mechanical niches, which further regulate the cellular behaviors. While in naturally, instead of instantly responding to real-time mechanical changes of their surrounding niches, stem cells often present a delayed cellular response over a time scale, namely cellular mechanical memory, which may finally influence their lineage choice. Here, we aim to build a dynamic mechanical niche model with alginate-based hydrogel, therein the dynamic mechanical switching can be easily realized via the introduce or removal of Ca²⁺. The results show that stiffening hydrogel (from soft to stiff) suppresses osteogenic differentiation of human mesenchymal stem cells (hMSCs) early on, though it finally promoted osteogenic differentiation over a long time period. Instead, softening hydrogel (from stiff to soft) still retains the strong osteogenic differentiation in the early days, though it finally showed a lower level of osteogenic differentiation compared with stiff hydrogel. Further, microRNA miR-21 has been found as a long-term mechanical memory sensor of the osteogenic program in hMSCs, as its level remains to match early mechanics of substrate over a period of time. Regulation of miR-21 level is efficient to erase the past mechanical memory and resensitize hMSCs to subsequent substrate mechanics. Our findings highlight cellular mechanical memory effect as a key factor of cell and cellular microenvironment interactions, which has been largely neglected before, and as a crucial design element of biomaterials for cell culture.

Microparticles in Contact with Cells: From Carriers to Multifunctional Tissue Modulators

For several decades microparticles have been exclusively and extensively explored as spherical drug delivery vehicles and large-scale cell expansion carriers. More recently, microparticulate structures gained interest in broader bioengineering fields, integrating myriad strategies that include bottom-up tissue engineering, 3D bioprinting, and the development of tissue/disease models. The concept of bulk spherical micrometric particles as adequate supports for cell cultivation has been challenged, and systems with finely tuned geometric designs and (bio)chemical/physical features are current key players in impacting technologies. Herein, we critically review the state of the art and future trends of biomaterial microparticles in contact with cells and tissues, excluding internalization studies, and with emphasis on innovative particle design and applications.

Porous tissue strands: avascular building blocks for scalable tissue fabrication

The scalability of cell aggregates such as spheroids, strands, and rings has been restricted by diffusion of nutrient and oxygen into their core. In this study, we introduce a novel concept in generating tissue building blocks with micropores, which represents an alternative solution for vascularization. Sodium alginate porogens were mixed with human adipose-derived stem cells, and loaded into tubular alginate capsules, followed by de-crosslinking of the capsules. The resultant cellular structure exhibited a porous morphology and formed cell aggregates in the form of strands, called 'porous tissue strands (pTSs).' Three-dimensional reconstructions show that pTSs were able to maintain ∼25% porosity with a high pore interconnectivity (∼85%) for 3 weeks. Owing to the porous structure, pTSs showed up-regulated cell viability and proliferation rate as compared to solid counterparts throughout the culture period. pTSs also demonstrated self-assembly capability through tissue fusion yielding larger-scale patches. In this paper, chondrogenesis and osteogenesis of pTSs were also demonstrated, where the porous microstructure up-regulated both chondrogenic and osteogenic functionalities indicated by cartilage- and bone-specific immunostaining, quantitative biochemical assessment and gene expression. These findings indicated the functionality of pTSs, which possessed controllable porosity and self-assembly capability, and had great potential to be utilized as tissue building blocks in distinct applications such as cartilage and bone regeneration.

Collagen structure regulates MSCs behavior by MMPs involved cell–matrix interactions

Various scaffolds have been studied in the formation of cell niches and regulation of mesenchymal stem cells (MSCs) behaviors.

Degradation regulated bioactive hydrogel as the bioink with desirable moldability for microfluidic biofabrication

Bioink development is vital in biofabriacation for generating three-dimensional (3D) tissue-like constructs. As potential candidates of bioinks, hydrogels need to meet the requirements of good moldability, initially strong mechanical properties and prominent bioactivity to guarantee cell vitality and further assembly. Enzyme-induced dynamic degradation is an efficient and biocompatible approach to improve the bioactivity of hydrogels through releasing space continuously for cell proliferation and promoting the functional establishing of engineered tissue. Here a novel bioink was designed by introducing alginate lyase into composite Alginate-GelMA hydrogels. Results showed that bioink with proper lyase content exhibited desirable modability and cytocompatibility. Then cell-laden osteon-like microfibers were engineered with the microfluidic device and diverse complex 3D constructs were also successfully assembled. This degradation-regulated bioink showed great promise in a variety of applications in tissue engineering and biomedical investigation.

Macroporous Hydrogel Scaffolds for Three-Dimensional Cell Culture and Tissue Engineering

Hydrogels have been promising candidate scaffolds for cell delivery and tissue engineering due to their tissue-like physical properties and capability for homogeneous cell loading. However, the encapsulated cells are generally entrapped and constrained in the submicron- or nanosized gel networks, seriously limiting cell growth and tissue formation. Meanwhile, the spatially confined settlement inhibits attachment and spreading of anchorage-dependent cells, leading to their apoptosis. In recent years, macroporous hydrogels have attracted increasing attention in use as cell delivery vehicles and tissue engineering scaffolds. The introduction of macropores within gel scaffolds not only improves their permeability for better nutrient transport but also creates space/interface for cell adhesion, proliferation, and extracellular matrix deposition. Herein, we will first review the development of macroporous gel scaffolds and outline the impact of macropores on cell behaviors. In the first part, the advantages and challenges of hydrogels as three-dimensional (3D) cell culture scaffolds will be described. In the second part, the fabrication of various macroporous hydrogels will be presented. Third, the enhancement of cell activities within macroporous gel scaffolds will be discussed. Finally, several crucial factors that are envisaged to propel the improvement of macroporous gel scaffolds are proposed for 3D cell culture and tissue engineering.

Vascularization in Engineered Tissue Construct by Assembly of Cellular Patterned Micromodules and Degradable Microspheres

Tissue engineering aims to generate functional tissue constructs in which proper extracellular matrix (ECM) for cell survival and establishment of a vascular network are necessary. A modular approach via the assembly of modules mimicking the complex tissues' microarchitectural features and establishing a vascular network represents a promising strategy for fabricating larger and more complex tissue constructs. Herein, as a model for this modular tissue engineering, engineered bone-like constructs were developed by self-assembly of osteon-like modules and fast degradable gelatin microspheres. The collagen microspheres acting as osteon-like modules were developed by seeding human umbilical vein endothelial cells (HUVECs) onto collagen microspheres laden with human osteoblast-like cells (MG63) and collagenase. Both HUVECs and MG63 cells were well spatially patterned in the modules, and collagen as ECM well supported cell adhesion, spreading, and functional expression due to its native RGD domains and enzymatic degradation activity. The patterned modules facilitated both the cellular function expression of osteogenic MG63 cells and vasculogenic HUVECs; that is, the osteon-like units were successfully achieved. The assembly of the osteon-like modules and fast degradable gelatin microspheres promoted the vascularization, thus facilitating the osteogenic function expression. The study provides a highly efficient approach to engineering complex 3D tissues with micropatterned cell types and interconnected channels.