Winner of the Young Investigator Award of the Society for Biomaterials at the 10th World Biomaterials Congress, May 17-22, 2016, Montreal QC, Canada: Microribbon-based hydrogels accelerate stem cell-based bone regeneration in a mouse critical-size cranial defect model

Injectable microcapillary network hydrogels engineered by liquid-liquid phase separation for stem cell transplantation

Injectable hydrogels are promising carriers for cell delivery in regenerative medicine. However, injectable hydrogels composed of crosslinked polymer networks are often non-microporous and prevent biological communication with host tissues through signals, nutrients, oxygen, and cells, thereby limiting graft survival and tissue integration. Here we report injectable hydrogels with liquid-liquid phase separation-induced microcapillary networks (μCN) as stem cell-delivering scaffolds. The molecular modification of gelatin with hydrogen bonding moieties induced liquid-liquid phase separation when mixed with unmodified gelatin to form μCN structures in the hydrogels. Through spatiotemporally controlled covalent crosslinking and dissolution processes, porous μCN structures were formed in the hydrogels, which can enhance mass transport and cellular activity. The encapsulation of cells with injectable μCN hydrogels improved cellular spreading, migration, and proliferation. Transplantation of mesenchymal stem cells with injectable μCN hydrogels enhanced graft survival and recovered hindlimb ischemia by enhancing material-tissue communication with biological signals and cells through μCN. This facile approach may serve as an advanced scaffold for improving stem cell transplantation therapies in regenerative medicine.

In situ forming macroporous biohybrid hydrogel for nucleus pulposus cell delivery

Degenerative intervertebral disc disease is a common source of chronic pain and reduced quality of life in people over the age of 40. While degeneration occurs throughout the disc, it most often initiates in the nucleus pulposus (NP). Minimally invasive delivery of NP cells within hydrogels that can restore and maintain the disc height while regenerating the damaged NP tissue is a promising treatment strategy for this condition. Towards this goal, a biohybrid ABA dimethacrylate triblock copolymer was synthesized, possessing a lower critical solution temperature below 37 °C and which contained as its central block an MMP-degradable peptide flanked by poly(trimethylene carbonate) blocks bearing pendant oligoethylene glycol groups. This triblock prepolymer was used to form macroporous NP cell-laden hydrogels via redox initiated (ammonium persulfate/sodium bisulfite) crosslinking, with or without the inclusion of thiolated chondroitin sulfate. The resulting macroporous hydrogels had water and mechanical properties similar to those of human NP tissue and were mechanically resilient. The hydrogels supported NP cell attachment and growth over 28 days in hypoxic culture. In hydrogels prepared with the triblock copolymer but without the chondroitin sulfate the NP cells were distributed homogeneously throughout in clusters and deposited collagen type II and sulfated glycosaminoglycans but not collagen type I. This hydrogel formulation warrants further investigation as a cell delivery vehicle to regenerate degenerated NP tissue. STATEMENT OF SIGNIFICANCE: The intervertebral disc between the vertebral bones of the spine consists of three regions: a gel-like central nucleus pulposus (NP) within the annulus fibrosis, and bony endplates. Degeneration of the intervertebral disc is a source of chronic pain in the elderly and most commonly initiates in the NP. Replacement of degenerated NP tissue with a NP cell-laden hydrogel is a promising treatment strategy. Herein we demonstrate that a crosslinkable polymer with a lower critical solution temperature below 37 °C can be used to form macroporous hydrogels for this purpose. The hydrogels are capable of supporting NP cells, which deposit collagen II and sulfated glycosaminoglycans, while also possessing mechanical properties matching those of human NP tissue.

Aspirin synergizes with mineral particle-coated macroporous scaffolds for bone regeneration through immunomodulation

Rationale: Mineral particles have been widely used in bone tissue engineering scaffolds due to their osteoconductive and osteoinductive properties. Despite their benefits, mineral particles can induce undesirable inflammation and subsequent bone resorption. Aspirin (Asp) is an inexpensive and widely used anti-inflammatory drug. The goal of this study is to assess the synergistic effect of Asp and optimized mineral particle coating in macroporous scaffolds to accelerate endogenous bone regeneration and reduce bone resorption in a critical-sized bone defect model. Methods: Four commonly used mineral particles with varying composition (hydroxyapatite v.s. tricalcium phosphate) and size (nano v.s. micro) were used. Mineral particles were coated onto gelatin microribbon (µRB) scaffolds. Macrophages (Mφ) were cultured on gelatin µRB scaffolds containing various particles, and Mφ polarization was assessed using PCR and ELISA. The effect of conditioned medium from Mφ on mesenchymal stem cell (MSC) osteogenesis was also evaluated in vitro. Scaffolds containing optimized mineral particles were then combined with varying dosages of Asp to assess the effect in inducing endogenous bone regeneration using a critical-sized cranial bone defect model. In vivo characterization and in vitro cell studies were performed to elucidate the effect of tuning Asp dosage on Mφ polarization, osteoclast (OC) activity, and MSC osteogenesis. Results: Micro-sized tricalcium phosphate (mTCP) particles were identified as optimal in promoting M2 Mφ polarization and rescuing MSC-based bone formation in the presence of conditioned medium from Mφ. When implanted in vivo, incorporating Asp with mTCP-coated µRB scaffolds significantly accelerated endogenous bone formation in a dose-dependent manner. Impressively, mTCP-coated µRB scaffolds containing 20 µg Asp led to almost complete bone healing of a critical-sized cranial bone defect as early as week 2 with no subsequent bone resorption. Asp enhanced M2 Mφ polarization, decreased OC activity, and promoted MSC osteogenesis in a dosage-dependent manner in vivo. These results were further validated using in vitro cell studies. Conclusions: Here, we demonstrate Asp and mineral particle-coated microribbon scaffold provides a promising therapy for repairing critical-sized cranial bone defects via immunomodulation. The leading formulation supports rapid endogenous bone regeneration without the need for exogenous cells or growth factors, making it attractive for translation. Our results also highlight the importance of optimizing mineral particles and Asp dosage to achieve robust bone healing while avoiding bone resorption by targeting Mφ and OCs.

Stem Cell Membrane-Coated Microribbon Scaffolds Induce Regenerative Innate and Adaptive Immune Responses in a Critical-Size Cranial Bone Defect Model

Naturally-derived cell membranes have shown great promise in functionalizing nanoparticles to enhance biointerfacing functions for drug delivery applications. However, its potential for functionalizing macroporous scaffolds to enhance tissue regeneration in vivo remains unexplored. Engineering scaffolds with immunomodulatory functions represents an exciting strategy for tissue regeneration but is largely limited to soft tissues. Critical-sized bone defects cannot heal on their own, and the role of adaptive immune cells in scaffold-mediated healing of cranial bone defects remains largely unknown. Here, mensenchymal stem cell membrane (MSCM)-coated microribbon (µRB) scaffolds for treating critical size cranial bone defects via targeting immunomodulation are reported. Confocal imaging and proteomic analyses are used to confirm successful coating and characterize the compositions of cell membrane coating. It is demonstrated that MSCM coating promotes macrophage (Mφ) polarization toward regenerative phenotype, induces CD8+ T cell apoptosis, and enhances regulatory T cell differentiation in vitro and in vivo. When combined with a low dosage of BMP-2, MSCM coating further accelerates bone regeneration and suppresses inflammation. These results establish cell membrane-coated microribbon scaffolds as a promising strategy for treating critical size bone defects via immunomodulation. The platform may be broadly used with different cell membranes and scaffolds to enhance regeneration of multiple tissue types.

Differential dynamics of bone graft transplantation and mesenchymal stem cell therapy during bone defect healing in a murine critical size defect

Background

A critical size bone defect is a clinical scenario in which bone is lost or excised due to trauma, infection, tumor, or other causes, and cannot completely heal spontaneously. The most common treatment for this condition is autologous bone grafting to the defect site. However, autologous bone graft is often insufficient in quantity or quality for transplantation to these large defects. Recently, tissue engineering methods using mesenchymal stem cells (MSCs) have been proposed as an alternative treatment. However, the underlying biological principles and optimal techniques for tissue regeneration of bone using stem cell therapy have not been completely elucidated.

Methods

In this study, we compare the early cellular dynamics of healing between bone graft transplantation and MSC therapy in a murine chronic femoral critical-size bone defect. We employ high-dimensional mass cytometry to provide a comprehensive view of the differences in cell composition, stem cell functionality, and immunomodulatory activity between these two treatment methods one week after transplantation.

Results

We reveal distinct cell compositions among tissues from bone defect sites compared with original bone graft, show active recruitment of MSCs to the bone defect sites, and demonstrate the phenotypic diversity of macrophages and T cells in each group that may affect the clinical outcome.

Conclusion

Our results provide critical data and future directions on the use of MSCs for treating critical size defects to regenerate bone.Translational Potential of this article: This study showed systematic comparisons of the cellular and immunomodulatory profiles among different interventions to improve the healing of the critical-size bone defect. The results provided potential strategies for designing robust therapeutic interventions for the unmet clinical need of treating critical-size bone defects.

A 3D Osteosarcoma Model with Bone-Mimicking Cues Reveals a Critical Role of Bone Mineral and Informs Drug Discovery

Osteosarcoma (OS) is an aggressive bone cancer for which survival has not improved over three decades. While biomaterials have been widely used to engineer 3D soft-tissue tumor models, the potential of engineering 3D biomaterials-based OS models for comprehensive interrogation of OS pathology and drug discovery remains untapped. Bone is characterized by high mineral content, yet the role of bone mineral in OS progression and drug response remains unknown. Here, a microribbon-based OS model with bone-mimicking compositions is developed to elucidate the role of 3D culture and hydroxyapatite in OS signaling and drug response. The results reveal that hydroxyapatite in 3D is critical to support retention of OS signaling and drug resistance similar to patient tissues and mouse orthotopic tumors. The physiological relevance of this 3D model is validated using four established OS cell lines, seven patient-derived xenograft (PDX) cell lines and two animal models. Integrating 3D OS PDX models with RNA-sequencing identified 3D-specific druggable target, which predicts drug response in mouse orthotopic model. These results establish microribbon-based 3D OS models as a novel experimental tool to enable discovery of novel therapeutics that would be otherwise missed with 2D model and may serve as platforms to study patient-specific OS heterogeneity and drug resistance mechanisms.

Hydroxyapatite-coated gelatin microribbon scaffolds induce rapid endogenous cranial bone regeneration in vivo

Hydroxyapatite (HA) has a composition similar to mineral bone and has been used for coating macroporous scaffolds to enhance bone formation. However, previous macroporous scaffolds did not support minimally invasive delivery. Our lab has reported on gelatin-based microribbon (μRB) shaped hydrogels, which combine injectability with macroporosity and support cranial bone formation in an immunocompromised mouse model. However, gelatin alone was not sufficient to support cranial bone formation in immunocompetent animals. To overcome this challenge, here we evaluated two methods to incorporate HA into gelatin μRB scaffolds using either modified simulated body fluid (mSBF) or commercially available HA nanoparticles (HAnp). HA incorporation and distribution were characterized using scanning electron microscopy and energy-dispersive X-ray spectroscopy. While both methods enhanced MSC osteogenesis and mineralization, the mSBF method led to undesirable reduction in mechanical properties. HAnp-coated μRB scaffolds were further evaluated in an immunocompetent mouse cranial defect model. Acellular HAnp-coated gelatin μRB scaffolds induced rapid and robust endogenous cranial bone regeneration as shown by MicroCT imaging and histology. Co-delivery with exogenous MSCs led to later bone resorption accompanied by increased osteoclast activity. In summary, our results demonstrate the promise of gelatin μRBs with HAnps as a promising therapy for cranial bone regeneration without the need for exogenous cells or growth factors.

Ageing attenuates bone healing by mesenchymal stem cells in a microribbon hydrogel with a murine long bone critical-size defect model

BACKGROUND

Despite the high incidence of fractures and pseudoarthrosis in the aged population, a potential role for the use of mesenchymal stem cells (MSCs) in the treatment of bone defects in elderly patients has not been elucidated. Inflammation and the innate immune system, including macrophages, play crucial roles in the differentiation and activation of MSCs. We have developed lentivirus-transduced interleukin 4 (IL4) over-expressing MSCs (IL4-MSCs) to polarize macrophages to an M2 phenotype to promote bone healing in an established young murine critical size bone defect model. In the current study, we explore the potential of IL4-MSCs in aged mice.

METHODS

A 2 mm femoral diaphyseal bone defect was created and fixed with an external fixation device in 15- to 17-month-old male and female BALB/c mice. Microribbon (µRB) scaffolds (Sc) with or without encapsulation of MSCs were implanted in the defect sites. Accordingly, the mice were divided into three treatment groups: Sc-only, Sc + MSCs, and Sc + IL4-MSCs. Mice were euthanized six weeks after the surgery; subsequently, MicroCT (µCT), histochemical and immunohistochemical analyses were performed.

RESULTS

µCT analysis revealed that bone formation was markedly enhanced in the IL4-MSC group. Compared with the Sc-only, the amount of new bone increased in the Sc + MSCs and Sc + IL4-MSC groups. However, no bridging of bone was observed in all groups. H&E staining showed fibrous tissue within the defect in all groups. Alkaline phosphatase (ALP) staining was increased in the Sc + IL4-MSC group. The Sc + IL4-MSCs group showed a decrease in the number of M1 macrophages and an increase in the number of M2 macrophages, with a significant increase in the M2/M1 ratio.

DISCUSSION

IL4 promotes macrophage polarization to an M2 phenotype, facilitating osteogenesis and vasculogenesis. The addition of IL4-MSCs in the µRB scaffold polarized macrophages to an M2 phenotype and increased bone formation; however, complete bone bridging was not observed in any specimens. These results suggest that IL4-MSCs are insufficient to heal a critical size bone defect in aged mice, as opposed to younger animals. Additional therapeutic strategies are needed in this challenging clinical scenario.

Sex Differences in Mesenchymal Stem Cell Therapy With Gelatin-Based Microribbon Hydrogels in a Murine Long Bone Critical-Size Defect Model

Mesenchymal stem cell (MSC)-based therapy and novel biomaterials are promising strategies for healing of long bone critical size defects. Interleukin-4 (IL-4) over-expressing MSCs within a gelatin microribbon (µRB) scaffold was previously shown to enhance the bridging of bone within a critical size femoral bone defect in male Balb/c mice. Whether sex differences affect the healing of this bone defect in conjunction with different treatments is unknown. In this study, we generated 2-mm critical-sized femoral diaphyseal bone defects in 10–12-week-old female and male Balb/c mice. Scaffolds without cells and with unmodified MSCs were implanted immediately after the primary surgery that created the bone defect; scaffolds with IL-4 over-expressing MSCs were implanted 3 days after the primary surgery, to avoid the adverse effects of IL-4 on the initial inflammatory phase of fracture healing. Mice were euthanized 6 weeks after the primary surgery and femurs were collected. MicroCT (µCT), histochemical and immunohistochemical analyses were subsequently performed of the defect site. µRB scaffolds with IL-4 over-expressing MSCs enhanced bone healing in both female and male mice. Male mice showed higher measures of bone bridging and increased alkaline phosphatase (ALP) positive areas, total macrophages and M2 macrophages compared with female mice after receiving scaffolds with IL-4 over-expressing MSCs. Female mice showed higher Tartrate-Resistant Acid Phosphatase (TRAP) positive osteoclast numbers compared with male mice. These results demonstrated that sex differences should be considered during the application of MSC-based studies of bone healing.

Translational Applications of Hydrogels

Advances in hydrogel technology have unlocked unique and valuable capabilities that are being applied to a diverse set of translational applications. Hydrogels perform functions relevant to a range of biomedical purposes-they can deliver drugs or cells, regenerate hard and soft tissues, adhere to wet tissues, prevent bleeding, provide contrast during imaging, protect tissues or organs during radiotherapy, and improve the biocompatibility of medical implants. These capabilities make hydrogels useful for many distinct and pressing diseases and medical conditions and even for less conventional areas such as environmental engineering. In this review, we cover the major capabilities of hydrogels, with a focus on the novel benefits of injectable hydrogels, and how they relate to translational applications in medicine and the environment. We pay close attention to how the development of contemporary hydrogels requires extensive interdisciplinary collaboration to accomplish highly specific and complex biological tasks that range from cancer immunotherapy to tissue engineering to vaccination. We complement our discussion of preclinical and clinical development of hydrogels with mechanical design considerations needed for scaling injectable hydrogel technologies for clinical application. We anticipate that readers will gain a more complete picture of the expansive possibilities for hydrogels to make practical and impactful differences across numerous fields and biomedical applications.

Injectable hydrogels for vascular embolization and cell delivery: The potential for advances in cerebral aneurysm treatment

Cerebral aneurysms are vascular lesions caused by the biomechanical failure of the vessel wall due to hemodynamic stress and inflammation. Aneurysmal rupture results in subarachnoid hemorrhage often leading to death or disability. Current treatment options include open surgery and minimally invasive endovascular options aimed at secluding the aneurysm from the circulation. Cerebral aneurysm embolization with appropriate materials is a therapeutic approach to prevent rupture and the resultant clinical sequelae. Metallic platinum coils are a typical, practical option to embolize cerebral aneurysms. However, the development of an alternative treatment modality is of interest because of poor occlusion permanence, coil migration, and coil compaction. Moreover, minimizing the implanted foreign materials during therapy is of importance not just to patients, but also to clinicians in the event an open surgical approach has to be pursued in the future. Polymeric injectable hydrogels have been investigated for transcatheter embolization and cell therapy with the potential for permanent aneurysm repair. This review focuses on how the combination of injectable embolic biomaterials and cell therapy may achieve minimally invasive remodeling of a degenerated cerebral artery with promise for superior outcomes in treatment of this devastating disease.

Alginate-magnetic short nanofibers 3D composite hydrogel enhances the encapsulated human olfactory mucosa stem cells bioactivity for potential nerve regeneration application

The design of 3D hydrogel constructs to elicit highly controlled cell response is a major field of interest in developing tissue engineering. The bioactivity of encapsulated cells inside pure alginate hydrogel is limited by its relatively inertness. Combining short nanofibers within a hydrogel serves as a promising method to develop a cell friendly environment mimicking the extracellular matrix. In this paper, we fabricated alginate hydrogels incorporating different magnetic short nanofibers (M.SNFs) content for olfactory ecto-mesenchymal stem cells (OE-MSCs) encapsulation. Wet-electrospun gelatin and superparamagnetic iron oxide nanoparticles (SPIONs) nanocomposite nanofibers were chopped using sonication under optimized conditions and subsequently embedded in alginate hydrogels. The storage modulus of hydrogel without M.SNFs as well as with 1 and 5 mg/mL of M.SNFs were in the range of nerve tissue. For cell encapsulation, OE-MSCs were used as a new hope for neuronal regeneration due to their neural crest origin. Resazurin analyses and LIVE/DEAD staining confirmed that the composite hydrogels containing M.SNFs can preserve the cell viability after 7 days. Moreover, the proliferation rate was enhanced in M.SNF/hydrogels compared to alginate hydrogel. The presence of SPIONs in the short nanofibers can accelerate neural-like differentiation of OE-MSCs rather than the sample without SPIONs.

Injectable and Crosslinkable PLGA-Based Microribbons as 3D Macroporous Stem Cell Niche

Poly(lactide-co-glycolide) (PLGA) has been widely used as a tissue engineering scaffold. However, conventional PLGA scaffolds are not injectable, and do not support direct cell encapsulation, leading to poor cell distribution in 3D. Here, a method for fabricating injectable and intercrosslinkable PLGA microribbon-based macroporous scaffolds as 3D stem cell niche is reported. PLGA is first fabricated into microribbon-shape building blocks with tunable width using microcontact printing, then coated with fibrinogen to enhance solubility and injectability using aqueous solution. Upon mixing with thrombin, firbornogen-coated PLGA microribbons can intercrosslink into 3D scaffolds. When subject to cyclic compression, PLGA microribbon scaffolds exhibit great shock-absorbing capacity and return to their original shape, while conventional PLGA scaffolds exhibit permanent deformation after one cycle. Using human mesenchymal stem cells (hMSCs) as a model cell type, it is demonstrated that PLGA μRB scaffolds support homogeneous cell encapsulation, and robust cell spreading and proliferation in 3D. After 28 days of culture in osteogenic medium, hMSC-seeded PLGA μRB scaffolds exhibit an increase in compressive modulus and robust bone formation as shown by staining of alkaline phosphatase, mineralization, and collagen. Together, the results validate PLGA μRBs as a promising injectable, macroporous, non-hydrogel-based scaffold for cell delivery and tissue regeneration applications.

Gelatin-Based Microribbon Hydrogels Support Robust MSC Osteogenesis across a Broad Range of Stiffness

Scaffold macroporosity has been shown to be critical for promoting bone regeneration. Although injectable materials are preferred for minimally invasive delivery, conventional macroporous scaffolds were not injectable and do not support homogeneous cell encapsulation. We recently reported a gelatin-based microribbon (μRB) scaffold that offers macroporosity while also supporting homogeneous cell encapsulation. Compared to conventional gelatin hydrogels, macroporous gelatin μRB scaffolds demonstrated great advantage in enhancing mesenchymal stem cell (MSC)-based cartilage formation. However, whether gelatin-based μRBs support MSC osteogenesis and bone formation remains unknown. The goal of this study is to assess the potential of gelatin-based μRBs for supporting MSC-based osteogenesis and bone formation in vitro. Given recent evidence from the literature that osteogenesis is sensitive to substrate stiffness, we further investigate how varying μRB stiffness modulates MSC osteogenesis. We first determine the maximal stiffness range of gelatin μRBs that can be fabricated (13-57 kPa), which supports both retention of μRB shape and macroporosity within scaffolds after inter-cross-linking. Interestingly, varying μRB stiffness across a broad range of stiffness did not significantly impact osteogenesis, with all groups supporting upregulation of bone markers and extensive collagen deposition. All gelatin μRBs also supported a comparable level of cell spreading and upregulation of mechanosensing markers. However, soft μRB (13 kPa) scaffolds did not maintain structural integrity and condensed into a pellet over time. Both intermediate and stiff gelatin μRB-based scaffolds maintained their integrity and supported robust bone formation, leading to a more than 10-fold increase in the compressive moduli of engineered bone after 5 weeks of culture in osteogenic media. Incorporating hydroxyapatite (HA) nanoparticle coating onto the gelatin μRB surface further accelerated the maturation of MSCs into osteoblasts and mineralization. Together, these results validate that gelatin μRBs can support MSC osteogenesis across a broad range of stiffness and offers an injectable macroporous scaffold for enhancing stem-cell-based bone regeneration.

Injectable and in situ crosslinkable gelatin microribbon hydrogels for stem cell delivery and bone regeneration in vivo

Rationale: Injectable matrices are highly desirable for stem cell delivery. Previous research has highlighted the benefit of scaffold macroporosity in enhancing stem cell survival and bone regeneration in vivo. However, there remains a lack of injectable and in situ crosslinkable macroporous matrices for stem cell delivery to achieve fast bone regeneration in immunocompetent animal model. The goal of this study is to develop an injectable gelatin-based μRB hydrogel supporting direct cell encapsulation that is available in clinics as macroporous matrices to enhance adipose-derived stromal cell (ASC) survival, engraftment and accelerate bone formation in craniofacial defect mouse. Methods: Injectable and in situ crosslinkable gelatin microribbon (μRB)-based macroporous hydrogels were developed by wet-spinning. Injectability was optimized by varying concentration of glutaraldehyde for intracrosslinking of μRB shape, and fibrinogen coating. The efficacy of injectable μRBs to support ASCs delivery and bone regeneration were further assessed in vivo using an immunocompetent mouse cranial defect model. ASCs survival was evaluated by bioluminescent imaging and bone regeneration was assessed by micro-CT. The degradation and biocompatibility were determined by histological analysis. Results: We first optimized injectability by varying concentration of glutaraldehyde used to fix gelatin μRBs. The injectable μRB formulation were subsequently coated with fibrinogen, which allows in situ crosslinking by thrombin. Fluorescence imaging and histology showed majority of μRBs degraded by the end of 3 weeks. Injectable μRBs supported comparable level of ASC proliferation and bone regeneration as implantable prefabricated μRB controls. Adding low dosage of BMP2 (100 ng per scaffold) with ASCs substantially accelerated the speed of mineralized bone regeneration, with 90% of the bone defect refilled by week 8. Immunostaining showed M1 (pro-inflammatory) macrophages were recruited to the defect at day 3, and was replaced by M2 (anti-inflammatory) macrophages by week 2. Adding μRBs or BMP2 did not alter macrophage response. Injectable µRBs supported vascularization, and BMP-2 further enhanced vascularization. Conclusions: Our results demonstrated that µRB-based scaffolds enhanced ASC survival and accelerated bone regeneration after injection into critical sized cranial defect mouse. Such injectable µRB-based scaffold can provide a versatile biomaterial for delivering various stem cell types and enhancing tissue regeneration.

Interleukin-4 overexpressing mesenchymal stem cells within gelatin-based microribbon hydrogels enhance bone healing in a murine long bone critical-size defect model

Mesenchymal stem cell (MSC)-based therapy is a promising strategy for bone repair. Furthermore, the innate immune system, and specifically macrophages, plays a crucial role in the differentiation and activation of MSCs. The anti-inflammatory cytokine Interleukin-4 (IL-4) converts pro-inflammatory M1 macrophages into a tissue regenerative M2 phenotype, which enhances MSC differentiation and function. We developed lentivirus-transduced IL-4 overexpressing MSCs (IL-4 MSCs) that continuously produce IL-4 and polarize macrophages toward an M2 phenotype. In the current study, we investigated the potential of IL-4 MSCs delivered using a macroporous gelatin-based microribbon (μRB) scaffold for healing of critical-size long bone defects in Mice. IL-4 MSCs within μRBs enhanced M2 marker expression without inhibiting M1 marker expression in the early phase, and increased macrophage migration into the scaffold. Six weeks after establishing the bone defect, IL-4 MSCs within μRBs enhanced bone formation and helped bridge the long bone defect. IL-4 MSCs delivered using macroporous μRB scaffold is potentially a valuable strategy for the treatment of critical-size long bone defects.

Nanoparticle-Mediated TGF-β Release from Microribbon-Based Hydrogels Accelerates Stem Cell-Based Cartilage Formation In Vivo

Conventional nanoporous hydrogels often lead to slow cartilage deposition by MSCs in 3D due to physical constraints and requirement for degradation. Our group has recently reported macroporous gelatin microribbon (μRB) hydrogels, which substantially accelerate MSC-based cartilage formation in vitro compared to conventional gelatin hydrogels. To facilitate translating the use of μRB-based scaffolds for supporting stem cell-based cartilage regeneration in vivo, there remains a need to develop a customize-designed drug delivery system that can be incorporated into μRB-based scaffolds. Towards this goal, here we report polydopamine-coated mesoporous silica nanoparticles (MSNs) that can be stably incorporated within the macroporous μRB scaffolds, and allow tunable release of transforming growth factor (TGF)-β3. We hypothesize that increasing concentration of polydopamine coating on MSNs will slow down TGF- β3 release, and TGF-β3 release from polydopamine-coated MSNs can enhance MSC-based cartilage formation in vitro and in vivo. We demonstrate that TGF-β3 released from MSNs enhance MSC-based cartilage regeneration in vitro to levels comparable to freshly added TGF-β3 in the medium, as shown by biochemical assays, mechanical testing, and histology. Furthermore, when implanted in vivo in a mouse subcutaneous model, only the group containing MSN-mediated TGF-β3 release supported continuous cartilage formation, whereas control group without MSN showed loss of cartilage matrix and undesirable endochondral ossification. The modular design of MSN-mediated drug delivery can be customized for delivering multiple drugs with individually optimized release kinetics, and may be applicable to enhance regeneration of other tissue types.

PDGFBB-modified stem cells from apical papilla and thermosensitive hydrogel scaffolds induced bone regeneration

Bone defects caused by cancer surgery or trauma have a strong negative impact on human health. Treatment with cell and material-based complexes provides an alternative strategy for the regeneration of damaged bone tissue. The good physical properties and suitable biodegradability of a thermosensitive hydrogel has been shown to act as a valuable scaffold. Platelet derived growth factor BB (PDGFBB) is mainly secreted by platelets and promotes the migration and angiogenesis of mesenchymal stem cells (MSCs). Although PDGFBB is known to indirectly enhance bone repair in vivo, the effects of PDGFBB on stem cells from apical papilla (SCAPs) require further investigation. In our study, the proliferation of cells was investigated by the cell counting kit-8 and live/dead staining methods. The results indicated that PDGFBB promoted the proliferation of SCAPs. Real-time polymerase chain reaction and Western blot experiments were used to detect osteogenic genes and proteins. Moreover, calvarial defects were created in Sprague-Dawley rats and different complexes implanted. The results shown by micro-CT and hematoxylin and eosin analysis demonstrated that the hydrogel combined with lentiviral supernatant-green fluorescent protein-PDGFBB significantly improved new bone formation and mineralization compared with the other three groups. In summary, our research showed that a thermosensitive hydrogel can be used as a scaffold for 3D cell culture, and PDGFBB gene-modified SCAPs can improve bone formation in calvarial defects.

Gelatin-Based Microribbon Hydrogels Accelerate Cartilage Formation by Mesenchymal Stem Cells in Three Dimensions

Hydrogels (HGs) are attractive matrices for cell-based cartilage tissue regeneration given their injectability and ability to fill defects with irregular shapes. However, most HGs developed to date often lack cell scale macroporosity, which restrains the encapsulated cells, leading to delayed new extracellular matrix deposition restricted to pericellular regions. Furthermore, tissue-engineered cartilage using conventional HGs generally suffers from poor mechanical property and fails to restore the load-bearing property of articular cartilage. The goal of this study was to evaluate the potential of macroporous gelatin-based microribbon (μRB) HGs as novel 3D matrices for accelerating chondrogenesis and new cartilage formation by human mesenchymal stem cells (MSCs) in 3D with improved mechanical properties. Unlike conventional HGs, these μRB HGs are inherently macroporous and exhibit cartilage-mimicking shock-absorbing mechanical property. After 21 days of culture, MSC-seeded μRB scaffolds exhibit a 20-fold increase in compressive modulus to 225 kPa, a range that is approaching the level of native cartilage. In contrast, HGs only resulted in a modest increase in compressive modulus of 65 kPa. Compared with conventional HGs, macroporous μRB scaffolds significantly increased the total amount of neocartilage produced by MSCs in 3D, with improved interconnectivity and mechanical strength. Altogether, these results validate gelatin-based μRBs as promising scaffolds for enhancing and accelerating MSC-based cartilage regeneration and may be used to enhance cartilage regeneration using other cell types as well.

It's All in the Delivery: Designing Hydrogels for Cell and Non-viral Gene Therapies

Hydrogels provide a regenerative medicine platform with their ability to create an environment that supports transplanted or endogenous infiltrating cells and enables these cells to restore or replace the function of tissues lost to disease or trauma. Furthermore, these systems have been employed as delivery vehicles for therapeutic genes, which can direct and/or enhance the function of the transplanted or endogenous cells. Herein, we review recent advances in the development of hydrogels for cell and non-viral gene delivery through understanding the design parameters, including both physical and biological components, on promoting transgene expression, cell engraftment, and ultimately cell function. Furthermore, this review identifies emerging opportunities for combining cell and gene delivery approaches to overcome challenges to the field.

Three-Dimensional (3D) Printed Microneedles for Microencapsulated Cell Extrusion

Cell-hydrogel based therapies offer great promise for wound healing. The specific aim of this study was to assess the viability of human hepatocellular carcinoma (HepG2) cells immobilized in atomized alginate capsules (3.5% (w/v) alginate, d = 225 µm ± 24.5 µm) post-extrusion through a three-dimensional (3D) printed methacrylate-based custom hollow microneedle assembly (circular array of 13 conical frusta) fabricated using stereolithography. With a jetting reliability of 80%, the solvent-sterilized device with a root mean square roughness of 158 nm at the extrusion nozzle tip (d = 325 μm) was operated at a flowrate of 12 mL/min. There was no significant difference between the viability of the sheared and control samples for extrusion times of 2 h (p = 0.14, α = 0.05) and 24 h (p = 0.5, α = 0.05) post-atomization. Factoring the increase in extrusion yield from 21.2% to 56.4% attributed to hydrogel bioerosion quantifiable by a loss in resilience from 5470 (J/m³) to 3250 (J/m³), there was no significant difference in percentage relative payload (p = 0.2628, α = 0.05) when extrusion occurred 24 h (12.2 ± 4.9%) when compared to 2 h (9.9 ± 2.8%) post-atomization. Results from this paper highlight the feasibility of encapsulated cell extrusion, specifically protection from shear, through a hollow microneedle assembly reported for the first time in literature.

Wet electrospun alginate/gelatin hydrogel nanofibers for 3D cell culture

Convergence of biological and biofabrication approaches is necessary to progress new biomaterials promoting three-dimensional (3D) cell growth and maturation towards tissue regeneration and integration. Here, we have developed a novel approach to fabricate 3D macroporous, alginate/gelatin hydrogel nanofibers (Alg/GelF-MA), which provide superior cell adhesion, motility, proliferation and maturation. The electrospinning process greatly depends on the ionic strength and viscoelastic behavior of the solution. The polyelectrolyte nature of alginate favors intramolecular bundles over intermolecular entanglement, which hinders its electrospinnability. Electrospinning of alginate was achieved by the aid of a supporting polymer, polyethylene oxide and a surfactant, Pluronic®F127. Furthermore, the Ca²⁺-mediated coagulation process of alginate was realized in situ during wet electrospinning, where the rapid physical crosslink-ability of alginate was applied in conjunction with the jet entrance into the wet electrospinning collector, a coagulation bath. Consequently, the rapid formation of Ca²⁺-alginate complex stabilized the nanofiber morphology. The low surface tension of the non-solvent ethanol used in the bath prevented fibers from dense packing, thus allowing the generation of 3D macroporous structure favoring cell motility. The subsequent UV-mediated chemical crosslinking further stabilized the gelatin content in the Alg/GelF-MA hydrogel nanofibers. It is demonstrated that the Alg/GelF-MA nanofibers with low cytotoxicity (below 10%) supported an over 8-fold proliferation of mesenchymal stem cells over 5 weeks and supported the maturation of human iPSC-derived ventricular cardiomyocytes, which significantly outperform the cell encapsulated bulk GelF-MA hydrogel. The work provides an insight for rational design and development of 3D cell culture matrix for advancement of stem cell therapy and tissue regeneration.

Recent Progress in Developing Injectable Matrices for Enhancing Cell Delivery and Tissue Regeneration

Biomaterials are key factors in regenerative medicine. Matrices used for cell delivery are especially important, as they provide support to transplanted cells that is essential for promoting cell survival, retention, and desirable phenotypes. Injectable matrices have become promising and attractive due to their minimum invasiveness and ease of use. Conventional injectable matrices mostly use hydrogel precursor solutions that form solid, cell-laden hydrogel scaffolds in situ. However, these materials are associated with challenges in biocompatibility, shear-induced cell death, lack of control over cellular phenotype, lack of macroporosity and remodeling, and relatively weak mechanical strength. This Progress Report provides a brief overview of recent progress in developing injectable matrices to overcome the limitations of conventional in situ hydrogels. Biocompatible chemistry and shear-thinning hydrogels have been introduced to promote cell survival and retention. Emerging investigations of the effects of matrix properties on cellular function in 3D provide important guidelines for promoting desirable cellular phenotypes. Moreover, several novel approaches are combining injectability with macroporosity to achieve macroporous, injectable matrices for cell delivery.

Stem Cells in Bone Regeneration

Bone has the capacity to regenerate and repair itself. However, this capacity may be impaired or lost depending on the size of the defect or the presence of certain disease states. In this review, we discuss the key principles underlying bone healing, efforts to characterize bone stem and progenitor cell populations, and the current status of translational and clinical studies in cell-based bone tissue engineering. Though barriers to clinical implementation still exist, the application of stem and progenitor cell populations to bone engineering strategies has the potential to profoundly impact regenerative medicine.

Modeling Physiological Events in 2D vs. 3D Cell Culture

Cell culture has become an indispensable tool to help uncover fundamental biophysical and biomolecular mechanisms by which cells assemble into tissues and organs, how these tissues function, and how that function becomes disrupted in disease. Cell culture is now widely used in biomedical research, tissue engineering, regenerative medicine, and industrial practices. Although flat, two-dimensional (2D) cell culture has predominated, recent research has shifted toward culture using three-dimensional (3D) structures, and more realistic biochemical and biomechanical microenvironments. Nevertheless, in 3D cell culture, many challenges remain, including the tissue-tissue interface, the mechanical microenvironment, and the spatiotemporal distributions of oxygen, nutrients, and metabolic wastes. Here, we review 2D and 3D cell culture methods, discuss advantages and limitations of these techniques in modeling physiologically and pathologically relevant processes, and suggest directions for future research.