Synergistic enhancement of ectopic bone formation by supplementation of freshly isolated marrow cells with purified MSC in collagen-chitosan hydrogel microbeads

Hydrogel Microparticles for Bone Regeneration

Hydrogel microparticles (HMPs) stand out as promising entities in the realm of bone tissue regeneration, primarily due to their versatile capabilities in delivering cells and bioactive molecules/drugs. Their significance is underscored by distinct attributes such as injectability, biodegradability, high porosity, and mechanical tunability. These characteristics play a pivotal role in fostering vasculature formation, facilitating mineral deposition, and contributing to the overall regeneration of bone tissue. Fabricated through diverse techniques (batch emulsion, microfluidics, lithography, and electrohydrodynamic spraying), HMPs exhibit multifunctionality, serving as vehicles for drug and cell delivery, providing structural scaffolding, and functioning as bioinks for advanced 3D-printing applications. Distinguishing themselves from other scaffolds like bulk hydrogels, cryogels, foams, meshes, and fibers, HMPs provide a higher surface-area-to-volume ratio, promoting improved interactions with the surrounding tissues and facilitating the efficient delivery of cells and bioactive molecules. Notably, their minimally invasive injectability and modular properties, offering various designs and configurations, contribute to their attractiveness for biomedical applications. This comprehensive review aims to delve into the progressive advancements in HMPs, specifically for bone regeneration. The exploration encompasses synthesis and functionalization techniques, providing an understanding of their diverse applications, as documented in the existing literature. The overarching goal is to shed light on the advantages and potential of HMPs within the field of engineering bone tissue.

Synthesis, Characterization and Biological Properties of Type I Collagen-Chitosan Mixed Hydrogels: A Review

Type I collagen and chitosan are two of the main biological macromolecules used to design scaffolds for tissue engineering. The former has the benefits of being biocompatible and provides biochemical cues for cell adhesion, proliferation and differentiation. However, collagen hydrogels usually exhibit poor mechanical properties and are difficult to functionalize. Chitosan is also often biocompatible, but is much more versatile in terms of structure and chemistry. Although it does have important biological properties, it is not a good substrate for mammalian cells. Combining of these two biomacromolecules is therefore a strategy of choice for the preparation of interesting biomaterials. The aim of this review is to describe the different protocols available to prepare Type I collagen-chitosan hydrogels for the purpose of presenting their physical and chemical properties and highlighting the benefits of mixed hydrogels over single-macromolecule ones. A critical discussion of the literature is provided to point out the poor understanding of chitosan-type I collagen interactions, in particular due to the lack of systematic studies addressing the effect of chitosan characteristics.

New Challenges and Prospective Applications of Three-Dimensional Bioactive Polymeric Hydrogels in Oral and Craniofacial Tissue Engineering: A Narrative Review

Regenerative medicine, and dentistry offers enormous potential for enhancing treatment results and has been fueled by bioengineering breakthroughs over the previous few decades. Bioengineered tissues and constructing functional structures capable of healing, maintaining, and regenerating damaged tissues and organs have had a broad influence on medicine and dentistry. Approaches for combining bioinspired materials, cells, and therapeutic chemicals are critical in stimulating tissue regeneration or as medicinal systems. Because of its capacity to maintain an unique 3D form, offer physical stability for the cells in produced tissues, and replicate the native tissues, hydrogels have been utilized as one of the most frequent tissue engineering scaffolds during the last twenty years. Hydrogels' high water content can provide an excellent conditions for cell viability as well as an architecture that mimics real tissues, bone, and cartilage. Hydrogels have been used to enable cell immobilization and growth factor application. This paper summarizes the features, structure, synthesis and production methods, uses, new challenges, and future prospects of bioactive polymeric hydrogels in dental and osseous tissue engineering of clinical, exploring, systematical and scientific applications.

Effects of pre-osteogenic differentiation on the bone regeneration potentiality of marrow mesenchymal stem cells/poly(ethylene glycol)-diacrylate hydrogel using a rat cranial defect model

Transplanting cell/hydrogel constructs into a bone defect site is an effective strategy to repair the damaged tissues. However, before transplantation, there are various methods to culture cell/hydrogel constructs. Especially, the preferred pre-osteogenic differentiation period to achieve satisfied bone regeneration should be determined. To this end, Bone marrow mesenchymal stem cells (BMSCs) were firstly photo-encapsulated into poly(ethylene glycol)-diacrylate (PEGDA) hydrogel. Then the constructs were implanted in rat calvarial defects after being osteogenically induced for 0, 7, 14, and 21 days. In vitro experiments demonstrated that the proliferation of BMSCs in the hydrogels deceased significantly from 0 day to 7 days. The activity and the gene expression of alkaline phosphatase, besides the gene expression of bone morphogenetic protein-2 peaked at day 14, whereas the gene expression of osteocalcin and the formation of calcium nodules increased with the prolongation of differentiation time. In vivo results showed that limited areas of newly formed bone were found in the day0 and day21 groups. In the day7 group, obvious new bone with bone marrow space was found, while the day14 group nearly achieved complete bone healing. Our data suggested that the period of in vitro pre-osteogenic differentiation played a crucial role for the osteogenesis of BMSCs/PEGDA hydrogels. Furthermore, we found that a pre-differentiation for 14 days is preferable for bone regeneration in the rat cranial defects.

Coupling Osteogenesis and Vasculogenesis in Engineered Orthopedic Tissues

Inadequate vascularization of engineered tissue constructs is a main challenge in developing a clinically impactful therapy for large, complex, and recalcitrant bone defects. It is well established that bone and blood vessels form concomitantly during development, as well as during repair after injury. Endothelial cells (ECs) and mesenchymal stromal cells (MSCs) are known to be key players in orthopedic tissue regeneration and vascularization, and these cell types have been used widely in tissue engineering strategies to create vascularized bone. Coculture studies have demonstrated that there is crosstalk between ECs and MSCs that can lead to synergistic effects on tissue regeneration. At the same time, the complexity in fabricating, culturing, and characterizing engineered tissue constructs containing multiple cell types presents a challenge in creating multifunctional tissues. In particular, the timing, spatial distribution, and cell phenotypes that are most conducive to promoting concurrent bone and vessel formation are not well understood. This review describes the processes of bone and vascular development, and how these have been harnessed in tissue engineering strategies to create vascularized bone. There is an emphasis on interactions between ECs and MSCs, and the culture systems that can be used to understand and control these interactions within a single engineered construct. Developmental engineering strategies to mimic endochondral ossification are discussed as a means of generating vascularized orthopedic tissues. The field of tissue engineering has made impressive progress in creating tissue replacements. However, the development of larger, more complex, and multifunctional engineered orthopedic tissues will require a better understanding of how osteogenesis and vasculogenesis are coupled in tissue regeneration. Impact statement Vascularization of large engineered tissue volumes remains a challenge in developing new and more biologically functional bone grafts. A better understanding of how blood vessels develop during bone formation and regeneration is needed. This knowledge can then be applied to develop new strategies for promoting both osteogenesis and vasculogenesis during the creation of engineered orthopedic tissues. This article summarizes the processes of bone and blood vessel development, with a focus on how endothelial cells and mesenchymal stromal cells interact to form vascularized bone both during development and growth, as well as tissue healing. It is meant as a resource for tissue engineers who are interested in creating vascularized tissue, and in particular to those developing cell-based therapies for large, complex, and recalcitrant bone defects.

Development of a modular, biocompatible thiolated gelatin microparticle platform for drug delivery and tissue engineering applications

The field of biomaterials has advanced significantly in the past decade. With the growing need for high-throughput manufacturing and screening, the need for modular materials that enable streamlined fabrication and analysis of tissue engineering and drug delivery schema has emerged. Microparticles are a powerful platform that have demonstrated promise in enabling these technologies without the need to modify a bulk scaffold. This building block paradigm of using microparticles within larger scaffolds to control cell ratios, growth factors and drug release holds promise. Gelatin microparticles (GMPs) are a well-established platform for cell, drug and growth factor delivery. One of the challenges in using GMPs though is the limited ability to modify the gelatin post-fabrication. In the present work, we hypothesized that by thiolating gelatin before microparticle formation, a versatile platform would be created that preserves the cytocompatibility of gelatin, while enabling post-fabrication modification. The thiols were not found to significantly impact the physicochemical properties of the microparticles. Moreover, the thiolated GMPs were demonstrated to be a biocompatible and robust platform for mesenchymal stem cell attachment. Additionally, the thiolated particles were able to be covalently modified with a maleimide-bearing fluorescent dye and a peptide, demonstrating their promise as a modular platform for tissue engineering and drug delivery applications.

Microfluidics generation of chitosan microgels containing glycerylphytate crosslinker for in situ human mesenchymal stem cells encapsulation

Human mesenchymal stem cells (hMSCs) are an attractive source for cell therapies because of their multiple beneficial properties, i.e. via immunomodulation and secretory factors. Microfluidics is particularly attractive for cell encapsulation since it provides a rapid and reproducible methodology for microgel generation of controlled size and simultaneous cell encapsulation. Here, we report the fabrication of hMSC-laden microcarriers based on in situ ionotropic gelation of water-soluble chitosan in a microfluidic device using a combination of an antioxidant glycerylphytate (G1Phy) compound and tripolyphosphate (TPP) as ionic crosslinkers (G1Phy:TPP-microgels). These microgels showed homogeneous size distributions providing an average diameter of 104 ± 12 μm, somewhat lower than that of control (127 ± 16 μm, TPP-microgels). The presence of G1Phy in microgels maintained cell viability over time and upregulated paracrine factor secretion under adverse conditions compared to control TPP-microgels. Encapsulated hMSCs in G1Phy:TPP-microgels were delivered to the subcutaneous space of immunocompromised mice via injection, and the delivery process was as simple as the injection of unencapsulated cells. Immediately post-injection, equivalent signal intensities were observed between luciferase-expressing microgel-encapsulated and unencapsulated hMSCs, demonstrating no adverse effects of the microcarrier on initial cell survival. Cell persistence, inferred by bioluminescence signal, decreased exponentially over time showing relatively higher half-life values for G1Phy:TPP-microgels compared to TPP-microgels and unencapsulated cells. In overall, results position the microfluidics generated G1Phy:TPP-microgels as a promising microcarrier for supporting hMSC survival and reparative activities.

Modular microcarrier technologies for cell-based bone regeneration

A variety of materials-based approaches to accelerate the regeneration of damaged bone have been developed to meet the important clinical need for improved bone fillers. This comprehensive review covers the materials and technologies used in modular microcarrier-based methods for delivery of progenitor cells in orthopaedic repair applications. It provides an overview of the field and the rationale for using microcarriers combined with osteoprogenitor cells for bone regeneration in particular. The general concepts and methods used in microcarrier-based cell culture and delivery are described, and methods for fabricating and characterizing microcarriers designed for specific indications are presented. A comprehensive review of the current literature on the use of microcarriers in bone regeneration is provided, with emphasis on key developments in the field and their impact. The studies reviewed are organized according to the broad classes of materials that are used for fabricating microcarriers, including polysaccharides, proteins and peptides, ceramics, and synthetic polymers. In addition, composite microcarriers that incorporate multiple material types or that are mineralized biomimetically are included. In each case, the fabrication, processing, characterization, and resulting function of the microcarriers is described, with an emphasis on their ability to support osteogenic differentiation of progenitor cells in vitro, and their effectiveness in healing bone defects in vivo. In addition, a summary of the current state of the field is provided, as are future perspectives on how microcarrier technologies may be enhanced to create improved cell-based therapies for bone regeneration.

Hydrogel microparticles for biomedical applications

Hydrogel microparticles (HMPs) are promising for biomedical applications, ranging from the therapeutic delivery of cells and drugs to the production of scaffolds for tissue repair and bioinks for 3D printing. Biologics (cells and drugs) can be encapsulated into HMPs of predefined shapes and sizes using a variety of fabrication techniques (batch emulsion, microfluidics, lithography, electrohydrodynamic (EHD) spraying and mechanical fragmentation). HMPs can be formulated in suspensions to deliver therapeutics, as aggregates of particles (granular hydrogels) to form microporous scaffolds that promote cell infiltration or embedded within a bulk hydrogel to obtain multiscale behaviours. HMP suspensions and granular hydrogels can be injected for minimally invasive delivery of biologics, and they exhibit modular properties when comprised of mixtures of distinct HMP populations. In this Review, we discuss the fabrication techniques that are available for fabricating HMPs, as well as the multiscale behaviours of HMP systems and their functional properties, highlighting their advantages over traditional bulk hydrogels. Furthermore, we discuss applications of HMPs in the fields of cell delivery, drug delivery, scaffold design and biofabrication.

Injectable osteogenic microtissues containing mesenchymal stromal cells conformally fill and repair critical-size defects

Repair of complex fractures with bone loss requires a potent, space-filling intervention to promote regeneration of bone. We present a biomaterials-based strategy combining mesenchymal stromal cells (MSC) with a chitosan-collagen matrix to form modular microtissues designed for delivery through a needle to conformally fill cavital defects. Implantation of microtissues into a calvarial defect in the mouse showed that osteogenically pre-differentiated MSC resulted in complete bridging of the cavity, while undifferentiated MSC produced mineralized tissue only in apposition to native bone. Decreasing the implant volume reduced bone regeneration, while increasing the MSC concentration also attenuated bone formation, suggesting that the cell-matrix ratio is important in achieving a robust response. Conformal filling of the defect with microtissues in a carrier gel resulted in complete healing. Taken together, these results show that modular microtissues can be used to augment the differentiated function of MSC and provide an extracellular environment that potentiates bone repair.

Two-step stem cell therapy improves bone regeneration compared to concentrated bone marrow therapy

Adult stem cells are a promising tool to positively influence bone regeneration. Concentrated bone marrow therapy entails isolating osteoprogenitor cells during surgery with, however, only low cells yield. Two step stem cell therapy requires an additional harvesting procedure but generates high numbers of progenitor cells that facilitate osteogenic pre-differentiation. To further improve bone regeneration, stem cell therapy can be combined with growth factors from platelet rich plasma (PRP) or its lysate (PL) to potentially fostering vascularization. The aim of this study was to investigate the effects of bone marrow concentrate (BMC), osteogenic pre-differentiation of mesenchymal stromal cells (MSCs), and PL on bone regeneration and vascularization. Bone marrow from four different healthy human donors was used for either generation of BMC or for isolation of MSCs. Seventy-two mice were randomized to six groups (Control, PL, BMC, BMC + PL, pre-differentiated MSCs, pre-differentiated MSCs + PL). The influence of PL, BMC, and pre-differentiated MSCs was investigated systematically in a 2 mm femoral bone defect model. After a 6-week follow-up, the pre-differentiated MSCs + PL group showed the highest bone volume, highest grade of histological defect healing and highest number of bridged defects with measurable biomechanical stiffness. Using expanded and osteogenically pre-differentiated MSCs for treatment of a critical-size bone defect was favorable with regards to bone regeneration compared to treatment with cells from BMC. The addition of PL alone had no significant influence; therefore the role of PL for bone regeneration remains unclear. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1318-1328, 2019.

Microgels: Modular, tunable constructs for tissue regeneration

Biopolymer microgels are emerging as a versatile tool for aiding in the regeneration of damaged tissues due to their biocompatible nature, tunable microporous structure, ability to encapsulate bioactive factors, and tailorable properties such as stiffness and composition. These properties of microgels, along with their injectability, have allowed for their utilization in a multitude of different tissue engineering applications. Controlled release of growth factors, antibodies, and other bioactive factors from microgels have demonstrated their capabilities as transporters for essential bioactive molecules necessary for guiding tissue reconstruction. Additionally, recent in vitro studies of cellular interaction and proliferation within microgel structures have laid the initial groundwork for regenerative tissue engineering using these materials. Microgels have even been crosslinked together in various ways or 3D printed to form three-dimensional scaffolds to support cell growth. In vivo studies of microgels have pioneered the clinical relevance of these novel and innovative materials for regenerative tissue engineering. This review will cover recent developments and research of microgels as they pertain to bioactive factor release, cellular interaction and proliferation in vitro, and tissue regeneration in vivo. STATEMENT OF SIGNIFICANCE: This review is focused on state-of-the-art microgel technology and innovations within the tissue engineering field, focusing on the use of microgels in bioactive factor delivery and as cell-interactive scaffolds, both in vitro and in vivo. Microgels are hydrogel microparticles that can be tuned based on the biopolymer from which they are derived, the crosslinking chemistry used, and the fabrication method. The emergence of microgels for tissue regeneration applications in recent years illuminates their versatility and applicability in clinical settings.

Bioactive hydrogels for bone regeneration

Bone self-healing is limited and generally requires external intervention to augment bone repair and regeneration. While traditional methods for repairing bone defects such as autografts, allografts, and xenografts have been widely used, they all have corresponding disadvantages, thus limiting their clinical use. Despite the development of a variety of biomaterials, including metal implants, calcium phosphate cements (CPC), hydroxyapatite, etc., the desired therapeutic effect is not fully achieved. Currently, polymeric scaffolds, particularly hydrogels, are of interest and their unique configurations and tunable physicochemical properties have been extensively studied. This review will focus on the applications of various cutting-edge bioactive hydrogels systems in bone regeneration, as well as their advantages and limitations. We will examine the composition and defects of the bone, discuss the current biomaterials for bone regeneration, and classify recently developed polymeric materials for hydrogel synthesis. We will also elaborate on the properties of desirable hydrogels as well as the fabrication techniques and different delivery strategies. Finally, the existing challenges, considerations, and the future prospective of hydrogels in bone regeneration will be outlined.

Biofabrication of injectable fibrin microtissues for minimally-invasive therapies: application of surfactants

Microtissues created from the protein fibrin and containing embedded cells can be used in modular tissue engineering approaches to create larger, hierarchical and complex tissue structures. In this paper we demonstrate an emulsification-based method for the production of such fibrin microtissues containing fibroblasts (FB) and endothelial cells (EC) and designed to promote tissue vascularization. Surfactants can be beneficial in the microtissue fabrication process to reduce aggregation and to facilitate recovery of microtissues from the emulsion, thereby increasing yield. The nonionic surfactants Pluronic L101^® and Tween 20^® both increased microtissue yield in a dose-dependent fashion. Cell viability of both human FB and human EC remained high after exposure to low surfactant concentrations but decreased with increasing surfactant concentration. L101 was markedly less cytotoxic than Tween, and therefore was the surfactant of choice in this application. The yield of cell-laden microtissues increased with increasing L101 concentration, though microtissues were slightly larger at low concentrations. The total metabolic activity of cells in retrieved microtissues was bimodal and was highest at an L101 concentration of 0.10% wt/vol. Network formation by EC in microtissues embedded in surrounding 3D fibrin hydrogels was also most extensive in microtissues made using an L101 concentration of 0.10% wt/vol. Minimally-invasive delivery of microtissue populations was demonstrated by injection through a standard 18 G needle, and the ability to form robust endothelial networks was maintained in injected microtissue populations. Taken together, these data demonstrate a facile emulsification-based method to create modular, cell-laden hydrogel microtissues that can be delivered by injection to promote tissue regeneration. Appropriate selection of the type and concentration of surfactant used in the process can be used to maximize viability and specialized function of the embedded cells. Such biomaterial-based microtissues may have broad applicability in cell-based therapies and tissue engineering.

Secretome released from hydrogel-embedded adipose mesenchymal stem cells protects against the Parkinson's disease related toxin 6-hydroxydopamine

Neurodegenerative diseases, as Parkinson’s disease (PD), involve irreversible neural cell damage and impairment. In PD, there is a selective degeneration of the dopaminergic neurons leading to motor symptoms. A common finding in PD neurodegeneration is the increase of reactive oxygen species (ROS), leading to oxidative stress. To date there are only interventions to relieve PD symptoms, however progress has been made in the development of therapies that target the immune system or use its components as therapeutic agents; among these, mesenchymal stem cells (MSCs), which are able to express neuroprotective factors as cytokines, chemokines and angiogenic molecules, collectively named secretome, that accumulate in MSC culture medium. However, lasting cell-free administration of secretome in vitro or in vivo is challenging. We used the conditioned media from rat adipose tissue-derived MSCs (RAA-MSCs) to check for neuroprotective activity towards pro-oxidizing agents such as hydrogen peroxide (H₂O₂) or the dopaminergic selective toxin 6-hydroxydopamine (6-OHDA) that is commonly used to model PD neurodegeneration. When neuroblastoma SH-SY5Y cells were pre-conditioned with 100% RAA-MSC media, then treated with H₂O₂ and 6-OHDA, mortality and ROS generation were reduced. We implemented the controlled release of RAA-MSC secretome from injectable biodegradable hydrogels that offer a possible in situ implant with mini-invasive techniques. The hydrogels were composed of type I bovine collagen (COLL) and low-molecular-weight hyaluronic acid (LMWHA) or COLL and polyethylene glycol (PEG). Hydrogels were suitable for RAA-MSC embedding up to 48 h and secretome from these RAA-MSCs was active and counteracted 6-OHDA toxicity, with upregulation of the antioxidant enzyme sirtuin 3 (SIRT3). These results support a biomaterials-based approach for controlled delivery of MSC-produced neuroprotective factors in a PD-relevant experimental context.

Advances and challenges in stem cell culture

Stem cells (SCs) hold great promise for cell therapy, tissue engineering, and regenerative medicine as well as pharmaceutical and biotechnological applications. They have the capacity to self-renew and the ability to differentiate into specialized cell types depending upon their source of isolation. However, use of SCs for clinical applications requires a high quality and quantity of cells. This necessitates large-scale expansion of SCs followed by efficient and homogeneous differentiation into functional derivatives. Traditional methods for maintenance and expansion of cells rely on two-dimensional (2-D) culturing techniques using plastic culture plates and xenogenic media. These methods provide limited expansion and cells tend to lose clonal and differentiation capacity upon long-term passaging. Recently, new approaches for the expansion of SCs have emphasized three-dimensional (3-D) cell growth to mimic the in vivo environment. This review provides a comprehensive compendium of recent advancements in culturing SCs using 2-D and 3-D techniques involving spheroids, biomaterials, and bioreactors. In addition, potential challenges to achieve billion-fold expansion of cells are discussed.

In-vivo evaluation of subcutaneously implanted cell-loaded apatite microcarriers for osteogenic potency

Cell-loaded apatite microcarriers present as potential scaffolds for direct in-vivo delivery of cells post-expansion to promote bone regeneration. The objective of this study was to evaluate the osteogenic potency of human foetal mesenchymal stem cells (hfMSC)-loaded apatite microcarriers when implanted subcutaneously in a mouse model. This was done by examining for ectopic bone formation at 2 weeks, 1 month and 2 months, which were intended to coincide with the inflammation, healing and remodelling phases, respectively. Three histological examinations including haematoxylin and eosin staining to examine general tissue morphology, Masson's trichrome staining to identify tissue type, and Von Kossa staining to examine extent of tissue mineralisation were performed. In addition, immunohistochemistry assay of osteopontin was conducted to confirm active bone formation by the seeded hfMSCs. Results showed a high level of tissue organisation and new bone formation, with active bone remodelling being observed at the end of 2 months, and an increase in tissue density, organisation, and mineralisation could also be observed for hfMSC-loaded apatite microcarriers. Various cell morphology resembling that of osteoblasts and osteoclasts could be seen on the surfaces of the hfMSC-loaded apatite microcarriers, with presence of woven bone tissue formation being observed at the intergranular space. These observations were consistent with evidence of ectopic bone formation, which were absent in group containing apatite microcarriers only. Overall, results suggested that hfMSC-loaded apatite microcarriers retained their osteogenic potency after implantation, and provided an effective platform for bone tissue regeneration.

The mechanism of a chitosan-collagen composite film used as biomaterial support for MC3T3-E1 cell differentiation

Natural composite biomaterials are good structural supports for bone cells to regenerate lost bone. Here, we report that a chitosan-collagen composite film accelerated osteoblast proliferation, differentiation and matrix mineralization in MC3T3-E1 cells. Intriguingly, we observed that the film enhanced the phosphorylation of Erk1/2. We showed that the chitosan-collagen composite film increased the transcriptional activity of Runx2, which is an important factor regulating osteoblast differentiation downstream of phosphorylated Erk1/2. Consistent with this observation, we found that the chitosan-collagen composite film increased the expression of osteoblastic marker genes, including Type I Collagen and Runx2 in MC3T3-E1 cells. We conclude that this film promoted osteoblast differentiation and matrix mineralization through an Erk1/2-activated Runx2 pathway. Our findings provide new evidence that chitosan-collagen composites are promising biomaterials for bone tissue engineering in bone defect-related diseases.