Oxygen-generating scaffolds: a new strategy for bone tissue engineering

In Situ Oxygen Generation Enhances the SCAP Survival in Hydrogel Constructs

Prolonged and severe hypoxia is the main cause of death of transplanted cells prior to the establishment of functional circulation. In situ generation of oxygen by oxygen-producing scaffolds-a unique solution that could produce and deliver oxygen to the adjacent cells independently of blood perfusion-has attracted considerable attention to enhance the survivability of the transplanted cells. However, the application of oxygen-generating scaffolds for facilitating cell survival in pulp-like tissue regeneration is yet to be explored. In this study, gelatin methacryloyl (GelMA)-a biocompatible scaffolding material that closely mimics the native extracellular matrix and is conducive to cell proliferation and differentiation-was used to fabricate oxygen-generating scaffolds by loading various concentrations of CaO₂. The CaO₂ distribution, topography, swelling, and pore size of CaO₂-GelMA hydrogels were characterized in detail. The release of O₂ by the scaffold and the viability, spreading, and proliferation of stem cells from apical papilla (SCAPs) encapsulated in the GelMA hydrogels with various concentrations of CaO₂ under hypoxia were evaluated. In addition, cellular constructs were engineered into root canals, and cell viability within the apical, middle, and coronal portions was assessed. Our findings showed that 0.5% CaO₂-GelMA was sufficient to supply in situ oxygen for maintaining the embedded SCAP viability for 1 wk. Furthermore, the 0.5% CaO₂-GelMA hydrogels improved the survivability of SCAPs within the coronal portion of the engineered cellular constructs within the root canals. This work demonstrated that 0.5% CaO₂-GelMA hydrogels offer a potential promising scaffold that enhances survival of the embedded SCAPs in endodontic regeneration.

Optimizing an Injectable Composite Oxygen-Generating System for Relieving Tissue Hypoxia

Oxygen deficiency resulting from bone fracture-induced vascular disruption leads to massive cell death and delayed osteoblast differentiation, ultimately impairing new bone formation and fracture healing. Enhancing local tissue oxygenation can help promote bone regeneration. In this work, an injectable composite oxygen-generating system consisting of calcium peroxide (CaO₂)/manganese dioxide (MnO₂)-encapsulated poly lactic-co-glycolic acid (PLGA) microparticles (CaO₂ + MnO₂@PLGA MPs) is proposed for the local delivery of oxygen. By utilizing a series of methodologies, the impacts of each component used for MP fabrication on the oxygen release behavior and cytotoxicity of the CaO₂ + MnO₂@ PLGA MPs are thoroughly investigated. Our analytical data obtained from in vitro studies indicate that the optimized CaO₂ + MnO₂@PLGA MPs developed in this study can effectively relieve the hypoxia of preosteoblast MC3T3-E1 cells that are grown under low oxygen tension and promote their osteogenic differentiation, thus holding great promise in enhancing fractural healing by increasing tissue oxygenation.

Oxygen-releasing nanofibers for breathable bone tissue engineering application

Oxygen is a vital molecule for cell and tissue processes. Electrospun fibers have been extensively used as drug loading carriers due to possibility of well control over drug release with modulating fiber properties. However, they have not been used as depots for oxygen release. In the present study, an oxygen-releasing nanofibrous scaffold has been developed by electrospinning of polylactic acid/nano-calcium peroxide suspension with different polylactic acid concentrations (6.5 and 13% w/v). The electrospun fibers with calcium peroxide cargo provided oxygen content of 30–94 mmHg in a period of 14 days which lies well within the oxygen level of osseous tissue. The release profile of 13% polylactic acid fibers was different with that of 6.5% fibers with respects to the initial content of released oxygen and the release rate. Not only did 13% fibers supply oxygen with a slower rate, but also they resulted in a lower burst release of oxygen. Cell culture studies in hypoxia corroborated that 13% polylactic acid fibers better preserve cell viability comparing 6.5% counterparts as perceived by MTT assay. Moreover, they endowed more favored milieu for adherence, arrangement and migration of mesenchymal stem cells as confirmed by microscopy images. The oxygen-releasing fibers equally affected alkaline phosphatase, osteocalcin, and calcium deposition by mesenchymal stem cells most likely due to interplay between topographical and metabolic cues offered by 6.5 and 13% formulations.

Hypoxia reduces the osteogenic differentiation of peripheral blood mesenchymal stem cells by upregulating Notch-1 expression

Purpose: Mesenchymal stem cells (MSCs) seeded on biocompatible scaffolds have therapeutic potential for bone defect repair. However, MSCs can be affected by hypoxia and nutritional deficiency due to a lack of blood vessels in the scaffolds. Here, we explored the effects of hypoxia on MSC differentiation to clarify these mechanisms. Methods: Peripheral blood mesenchymal stem cells (PBMSCs) were cultured in small individual chambers with oxygen concentrations of 1%, 9%, and 21%. Cell proliferation was evaluated by Cell Counting Kit 8 assays, and cell survival was determined using live/dead assays. Scratch assays were performed to evaluate cell migration. Ca²⁺ deposition/mineralization experiments, reverse transcription quantitative real-time polymerase chain reaction, and Western blotting were performed to assess the osteogenic differentiation of cells. Notch1 expression was downregulated by lentivirus-transfected PBMSCs to observe the effects of Notch1 knockdown on osteogenic gene and protein expression. Results: PBMSCs exposed to hypoxia (1% O₂) demonstrated accelerated proliferation, increased migration, and reduced survival in the absence of serum. Although 9% oxygen promoted osteogenic differentiation, the osteogenic differentiation of PBMSCs was significantly reduced by 1% O₂, and this effect was associated with increased Notch1 expression. Reducing Notch1 expression using small interfering RNA significantly restored the osteogenic differentiation of PBMSCs. Conclusions: Hypoxia accelerated proliferation, increased migration, and reduced PBMSC differentiation into osteoblasts by increasing Notch1 expression. These findings may contribute to the development of appropriate cell culture or in vivo transplantation conditions to maintain the full osteogenic potential of PBMSCs.

Controlled chemical synthesis of CaO2 particles coated with polyethylene glycol: characterization of crystallite size and oxygen release kinetics

The oxygen release pattern of CaO₂ nanoparticles was biphasic: an initial burst phase for the first 24 hours followed by a descending release rate for three days. The oxygen release rate in the second phase mainly depended on the shrinking-core model for CaO₂ hydrolysis.

Stimulating Fracture Healing in Ischemic Environments: Does Oxygen Direct Stem Cell Fate during Fracture Healing?

Bone fractures represent an enormous societal and economic burden as one of the most prevalent causes of disability worldwide. Each year, nearly 15 million people are affected by fractures in the United States alone. Data indicate that the blood supply is critical for fracture healing; as data indicate that concomitant bone and vascular injury are major risk factors for non-union. However, the various role(s) that the vasculature plays remains speculative. Fracture stabilization dictates stem cell fate choices during repair. In stabilized fractures stem cells differentiate directly into osteoblasts and heal the injury by intramembranous ossification. In contrast, in non-stable fractures stem cells differentiate into chondrocytes and the bone heals through endochondral ossification, where a cartilage template transforms into bone as the chondrocytes transform into osteoblasts. One suggested role of the vasculature has been to participate in the stem cell fate decisions due to delivery of oxygen. In stable fractures, the blood vessels are thought to remain intact and promote osteogenesis, while in non-stable fractures, continual disruption of the vasculature creates hypoxia that favors formation of cartilage, which is avascular. However, recent data suggests that non-stable fractures are more vascularized than stable fractures, that oxygen does not appear associated with differentiation of stem cells into chondrocytes and osteoblasts, that cartilage is not hypoxic, and that oxygen, not sustained hypoxia, is required for angiogenesis. These unexpected results, which contrast other published studies, are indicative of the need to better understand the complex, spatio-temporal regulation of vascularization and oxygenation in fracture healing. This work has also revealed that oxygen, along with the promotion of angiogenesis, may be novel adjuvants that can stimulate healing in select patient populations.

Delayed release singlet oxygen sensitizers based on pyridone-appended porphyrins

A new type of porphyrin photosensitizer capable of generating singlet oxygen upon irradiation, storing it through binding to pyridone subunits, followed by slow release at 20–40 °C, is reported.