Oxygen-generating nanofiber cell scaffolds with antimicrobial properties

Fabrication of fiber-particle structures by electrospinning/electrospray combination as an intrinsic antioxidant and oxygen-releasing wound dressing

In this study, we employed a combination of electrospinning and electrospray techniques to fabricate wound dressings with a particle-fiber structure, providing dual characteristics of oxygen-releasing and intrinsic antioxidant properties, simultaneously. The electrospun part of the dressing was prepared from a blend of polycaprolactone/gallic acid-grafted-gelatin (GA-g-GE), enabling intrinsic ROS scavenging. To the best of our knowledge, this is the first time that PCL/GA-g-GE was fabricated by electrospinning. Furthermore, polyvinyl pyrrolidone (PVP) microparticles, containing calcium peroxide nanoparticles (CNPs), were considered as the oxygen production agent through the electrospray part. The CNP content was 1% and 3% w/w of PVP while biopolymer:PCL was 10% w/w. The fabricated structures were characterized in terms of fiber/particle morphology, elemental analysis, oxygen release behavior, ROS inhibition capacity, and water contact angle assessments. The covalent bonding of gallic acid to gelatin was confirmed by 1H-NMR, UV spectroscopy, and FTIR. According to the SEM results, the morphology of the prepared PCL/biopolymer fibers was bead-free and with a uniform average diameter. The analysis of released oxygen showed that by increasing the weight percentage of CNPs from 1 to 3 wt%, the amount of released oxygen increased from 120 mmHg to 195 mmHg in 24 h, which remained almost constant until 72 h. The obtained DPPH assay results revealed that the introduction of GA-g-GE into the fibrous structure could significantly improve the antioxidant properties of wound dressing compared to the control group without CNPs and modified gelatine. In vitro, the fabricated wound dressings were evaluated in terms of biocompatibility and the potential of the dressing to protect human dermal fibroblasts under oxidative stress and hypoxia conditions by an MTT assay. The presence of GA-g-GE led to remarkable protection of the cells against oxidative stress and hypoxia conditions. In vivo studies revealed that the incorporation of intrinsic ROS inhibition and oxygen-releasing properties could significantly accelerate the wound closure rate during the experimental period (7, 14, and 21 days). Additionally, histopathological investigations in terms of H&E and Masson's trichrome staining showed that the incorporation of the two mentioned capabilities remarkably facilitated the wound-healing process.

Microfluidic strategies for engineering oxygen-releasing biomaterials

In the field of tissue engineering, local hypoxia in large-cell structures (larger than 1 mm3) poses a significant challenge. Oxygen-releasing biomaterials supply an innovative solution through oxygen ⁠ delivery in a sustained and controlled manner. Compared to traditional methods such as emulsion, sonication, and agitation, microfluidic technology offers distinct benefits for oxygen-releasing material production, including controllability, flexibility, and applicability. It holds enormous potential in the production of smart oxygen-releasing materials. This review comprehensively covers the fabrication and application of microfluidic-enabled oxygen-releasing biomaterials. To begin with, the physical mechanism of various microfluidic technologies and their differences in oxygen carrier preparation are explained. Then, the distinctions among diverse oxygen-releasing components in regards for oxygen-releasing mechanism, oxygen-carrying capacity, and duration of oxygen release are presented. Finally, the present obstacles and anticipated development trends are examined together with the application outcomes of oxygen-releasing biomaterials based on microfluidic technology in the biomedical area. STATEMENT OF SIGNIFICANCE: Oxygen is essential for sustaining life, and hypoxia (a condition of low oxygen) is a significant challenge in various diseases. Microfluidic-based oxygen-releasing biomaterials offer precise control and outstanding performance, providing unique advantages over traditional approaches for tissue engineering. However, comprehensive reviews on this topic are currently lacking. In this review, we provide a comprehensive analysis of various microfluidic technologies and their applications for developing oxygen-releasing biomaterials. We compare the characteristics of organic and inorganic oxygen-releasing biomaterials and highlight the latest advancements in microfluidic-enabled oxygen-releasing biomaterials for tissue engineering, wound healing, and drug delivery. This review may hold the potential to make a significant contribution to the field, with a profound impact on the scientific community.

Printable oxygen-generating biodegradable scaffold for thicker tissue-engineered medical products

BACKGROUND

Due to the increasing demand to generate thick and vascularized tissue-engineered constructs, novel strategies are currently being developed. An effective example is the fabrication of a 3D scaffold containing oxygen-releasing biomaterials to solve the limitations of gas diffusion and transport within transplanted tissues or devices.

METHODS

In this study, we developed a biodegradable scaffold made of polycaprolactone (PCL) mixed with oxygen-generating calcium peroxide (CPO) to design new structures for regenerative tissue using a 3D printer capable of forming arbitrarily shapes.

RESULTS AND CONCLUSION

When osteoblast progenitor cells (MC3T3-E1 cells) were cultured under hypoxic conditions on scaffolds fabricated with this technique, it was shown that cell death was reduced by the new scaffolds. Therefore, the results suggest that 3D-printed scaffolds made from biodegradable oxygen-releasing materials may be useful for tissue engineering and regeneration.

Oxygen Delivery Biomaterials in Wound Healing Applications

Oxygen (O2 ) delivery biomaterials have attracted great interest in the treatment of chronic wounds due to their potential applications in local and continuous O2 generation and delivery, improving cell viability until vascularization occurs, promoting structural growth of new blood vessels, simulating collagen synthesis, killing bacteria and reducing hypoxia-induced tissue damage. Therefore, different types of O2 delivery biomaterials including thin polymer films, fibers, hydrogels, or nanocomposite hydrogels have been developed to provide controlled, sufficient and long-lasting O2 to prevent hypoxia and maintain cell viability until the engineered tissue is vascularized by the host system. These biomaterials are made by various approaches, such as encapsulating O2 releasing molecules into hydrogels, polymer microspheres and 3D printed hydrogel scaffolds and adsorbing O2 carrying reagents into polymer films of fibers. In this article, different O2 generating sources such as solid inorganic peroxides, liquid peroxides, and photosynthetic microalgae, and O2 carrying perfluorocarbons and hemoglobin are presented and the applications of O2 delivery biomaterials in promoting wound healing are discussed. Furthermore, challenges encountered and future perspectives are highlighted.

Oxygen generating biomaterials at the forefront of regenerative medicine: advances in bone regeneration

Globally, an annual count of more than two million bone transplants is conducted, with conventional treatments, including metallic implants and bone grafts, exhibiting certain limitations. In recent years, there have been significant advancements in the field of bone regeneration. Oxygen tension regulates cellular behavior, which in turn affects tissue regeneration through metabolic programming. Biomaterials with oxygen release capabilities enhance therapeutic effectiveness and reduce tissue damage from hypoxia. However, precise control over oxygen release is a significant technical challenge, despite its potential to support cellular viability and differentiation. The matrices often used to repair large-size bone defects do not supply enough oxygen to the stem cells being used in the regeneration process. Hypoxia-induced necrosis primarily occurs in the central regions of large matrices due to inadequate provision of oxygen and nutrients by the surrounding vasculature of the host tissues. Oxygen generating biomaterials (OGBs) are becoming increasingly significant in enhancing our capacity to facilitate the bone regeneration, thereby addressing the challenges posed by hypoxia or inadequate vascularization. Herein, we discussed the key role of oxygen in bone regeneration, various oxygen source materials and their mechanism of oxygen release, the fabrication techniques employed for oxygen-releasing matrices, and novel emerging approaches for oxygen delivery that hold promise for their potential application in the field of bone regeneration.

Scaffolds with high oxygen content support osteogenic cell survival under hypoxia

Regeneration of large bone defects is a significant clinical challenge with variable success, but tissue engineering strategies are promising for rapid and effective bone regeneration. Maintaining an adequate oxygen level within implanted scaffolds is a major obstacle in bone tissue engineering. We developed a new oxygen-generating scaffold by electrospinning polycaprolactone with calcium peroxide (CaO2) nanocuboids (CPNCs) and characterized the physical, chemical, and biological properties of the resulting composites. Our scaffolds are highly porous and composed of submicron fibers that include CPNC as confirmed with XRD and FTIR analyses. Scaffolds containing CPNC provided controlled oxygen release for 14-days and supported cell proliferation while protecting preosteoblasts from hypoxia-induced cell death. Oxygen-generating scaffolds also facilitated bone mimetic defect contraction in vitro. The results suggest that our approach can be used to develop tissue-engineered products which target bone defects.

Hydrogel-Impregnated Self-Oxygenating Electrospun Scaffolds for Bone Tissue Engineering

Bone defects resulting from trauma, disease, or aging present significant challenges in the clinic. Although biomaterial scaffolds for bone-tissue engineering have shown promising results, challenges remain, including the need for adequate mechanical strength and suitable bioactive agents within scaffolds to promote bone formation. Oxygen is a critical factor for successful bone formation, and low oxygen tension inhibits it. In this study, we developed gelatin methacryloyl (GelMA) hydrogel-impregnated electrospun polycaprolactone (PCL) scaffolds that can release oxygen over 3 weeks. We investigated the potential of composite scaffolds for cell survival in bone-tissue engineering. Our results showed that the addition of an increased amount of CaO2 nanoparticles to the PCL scaffolds significantly increased oxygen generation, which was modulated by GelMA impregnation. Moreover, the resulting scaffolds showed improved cytocompatibility, pre-osteoblast adhesion, and proliferation under hypoxic conditions. This finding is particularly relevant since hypoxia is a prevalent feature in various bone diseases. In addition to providing oxygen, CaO2 nanoparticles also act as reinforcing agents improving the mechanical property of the scaffolds, while the incorporation of GelMA enhances cell adhesion and proliferation properties. Overall, our newly developed self-oxygenating composite biomaterials are promising scaffolds for bone-tissue engineering applications.

Radially Electrospun Fibrous Membrane Incorporated with Copper Peroxide Nanodots Capable of Self-Catalyzed Chemodynamic Therapy for Angiogenesis and Healing Acceleration of Diabetic Wounds

Vascular dysfunction severely hinders the healing process of diabetic wounds. Therefore, a radially structured fibrous membrane was fabricated through electrospinning by using a polycaprolactone (PCL) and polyvinylpyrrolidone (PVP) mixed solution containing copper peroxide nanoparticles (CPs) as the chemodynamic therapy (CDT) agents, aiming to simultaneously accelerate tissue regeneration and angiogenesis. The fabricated membrane allowed for the in situ H₂O₂ generation activated by the acidic diabetic microenvironment and the subsequent Fenton-type reactions to realize 99.4% elimination against Staphylococcus aureus. Besides, the released Cu²⁺ ions significantly enhanced the expression of hypoxia-inducible factor-1α (HIF-1α) and vascular endothelial growth factor (VEGF) in human umbilical vein endothelial cells (HUVECs), and they showed enhanced in vitro angiogenesis. Interestingly, the CP-embedded membrane also guided cell spreading and orientated migration of L929 fibroblasts along the fiber distribution through the radially aligned topology. The in vivo implantation indicated that the raidally structured membrane modified by CPs not only dramatically accelerated wound healing of diabetic Sprague-Dawley (SD) rats in 14 days but also promoted angiogenesis in wound sites. The combination of the in situ CDT with the radially structured morphology of the functional membrane is highly promising in applications to promote diabetic wound healing through anti-infection and revascularization.

Oxygen-releasing biomaterials for chronic wounds breathing: From theoretical mechanism to application prospect

Chronic wounds have always been considered as "gordian knots" in medicine, in which hypoxia plays a key role in blocking healing. To address this challenge, although tissue reoxygenation therapy based on hyperbaric oxygen therapy (HBOT) has been performed clinically for several years, the bench to bedside still urges the evolution of oxygen-loading and -releasing strategies with explicit benefits and consistent outcome. The combination of various oxygen carriers with biomaterials has gained momentum as an emerging therapeutic strategy in this field, exhibiting considerable application potential. This review gives an overview of the essential relationship between hypoxia and delayed wound healing. Further, detailed characteristics, preparation methods and applications of various oxygen-releasing biomaterials (ORBMs) will be elaborated, including hemoglobin, perfluorocarbon, peroxide, and oxygen-generating microorganisms, those biomaterials are applied to load, release or generate a vast of oxygen to relieve the hypoxemia and bring the subsequent cascade effect. The pioneering papers regarding to the ORBMs practice are presented and trends toward hybrid and more precise manipulation are summarized.

Oxygen-Releasing Hyaluronic Acid-Based Dispersion with Controlled Oxygen Delivery for Enhanced Periodontal Tissue Engineering

Periodontitis is a chronic biofilm-associated inflammatory disease of the tooth-supporting tissues that causes tooth loss. It is strongly associated with anaerobic bacterial colonization and represents a substantial global health burden. Due to a local hypoxic environment, tissue regeneration is impaired. Oxygen therapy has shown promising results as a potential treatment of periodontitis, but so far, local oxygen delivery remains a key technical challenge. An oxygen (O₂)-releasing hyaluronic acid (HA)-based dispersion with a controlled oxygen delivery was developed. Cell viability of primary human fibroblasts, osteoblasts, and HUVECs was demonstrated, and biocompatibility was tested using a chorioallantoic membrane assay (CAM assay). Suppression of anaerobic growth of Porphyromonas gingivalis was shown using the broth microdilution assay. In vitro assays showed that the O₂-releasing HA was not cytotoxic towards human primary fibroblasts, osteoblasts, and HUVECs. In vivo, angiogenesis was enhanced in a CAM assay, although not to a statistically significant degree. Growth of P. gingivalis was inhibited by CaO₂ concentrations higher than 256 mg/L. Taken together, the results of this study demonstrate the biocompatibility and selective antimicrobial activity against P. gingivalis for the developed O₂-releasing HA-based dispersion and the potential of O₂-releasing biomaterials for periodontal tissue regeneration.

Oxygen-generating microparticles downregulate HIF-1α expression, increase cardiac contractility, and mitigate ischemic injury

Myocardial hypoxia is the low oxygen tension in the heart tissue implicated in many diseases, including ischemia, cardiac dysfunction, or after heart procurement for transplantation. Oxygen-generating microparticles have recently emerged as a potential strategy for supplying oxygen to sustain cell survival, growth, and tissue functionality in hypoxia. Here, we prepared oxygen-generating microparticles with poly D,L-lactic-co-glycolic acid, and calcium peroxide (CPO), which yielded a continuous morphology capable of sustained oxygen release for up to 24 h. We demonstrated that CPO microparticles increased primary rat cardiomyocyte metabolic activity while not affecting cell viability during hypoxia. Moreover, hypoxia-inducible factor (HIF)-1α, which is upregulated during hypoxia, can be downregulated by delivering oxygen using CPO microparticles. Single-cell traction force microscopy data demonstrated that the reduced energy generated by hypoxic cells could be restored using CPO microparticles. We engineered cardiac tissues that showed higher contractility in the presence of CPO microparticles compared to hypoxic cells. Finally, we observed reduced myocardial injuries in ex vivo rabbit hearts treated with CPO microparticles. In contrast, an acute early myocardial injury was observed for the hearts treated with control saline solution in hypoxia. In conclusion, CPO microparticles improved cell and tissue contractility and gene expression while reducing hypoxia-induced myocardial injuries in the heart. STATEMENT OF SIGNIFICANCE: Oxygen-releasing microparticles can reduce myocardial ischemia, allograft rejection, or irregular heartbeats after heart transplantation. Here we present biodegradable oxygen-releasing microparticles that are capable of sustained oxygen release for more than 24 hrs. We then studied the impact of sustained oxygen release from microparticles on gene expresseion and cardiac cell and tissue function. Previous studies have not measured cardiac tissue or cell mechanics during hypoxia, which is important for understanding proper cardiac function and beating. Using traction force microscopy and an engineered tissue-on-a-chip, we demonstrated that our oxygen-releasing microparticles improve cell and tissue contractility during hypoxia while downregulating the HIF-1α expression level. Finally, using the microparticles, we showed reduced myocardial injuries in rabbit heart tissue, confirming the potential of the particles to be used for organ transplantation or tissue engineering.

Oxygen-Generating Scaffolds: One Step Closer to the Clinical Translation of Tissue Engineered Products

The lack of oxygen supply in engineered constructs has been an ongoing challenge for tissue engineering and regenerative medicine. Upon implantation of an engineered tissue, spontaneous blood vessel formation does not happen rapidly, therefore, there is typically a limited availability of oxygen in engineered biomaterials. Providing oxygen in large tissue-engineered constructs is a major challenge that hinders the development of clinically relevant engineered tissues. Similarly, maintaining adequate oxygen levels in cell-laden tissue engineered products during transportation and storage is another hurdle. There is an unmet demand for functional scaffolds that could actively produce and deliver oxygen, attainable by incorporating oxygen-generating materials. Recent approaches include encapsulation of oxygen-generating agents such as solid peroxides, liquid peroxides, and fluorinated substances in the scaffolds. Recent approaches to mitigate the adverse effects, as well as achieving a sustained and controlled release of oxygen, are discussed. Importance of oxygen-generating materials in various tissue engineering approaches such as ex vivo tissue engineering, in situ tissue engineering, and bioprinting are highlighted in detail. In addition, the existing challenges, possible solutions, and future strategies that aim to design clinically relevant multifunctional oxygen-generating biomaterials are provided in this review paper.

Scaffolds in the microbial resistant era: Fabrication, materials, properties and tissue engineering applications

Due to microbial infections dramatically affect cell survival and increase the risk of implant failure, scaffolds produced with antimicrobial materials are now much more likely to be successful. Multidrug-resistant infections without suitable prevention strategies are increasing at an alarming rate. The ability of cells to organize, develop, differentiate, produce a functioning extracellular matrix (ECM) and create new functional tissue can all be controlled by careful control of the extracellular microenvironment. This review covers the present state of advanced strategies to develop scaffolds with antimicrobial properties for bone, oral tissue, skin, muscle, nerve, trachea, cardiac and other tissue engineering applications. The review focuses on the development of antimicrobial scaffolds against bacteria and fungi using a wide range of materials, including polymers, biopolymers, glass, ceramics and antimicrobials agents such as antibiotics, antiseptics, antimicrobial polymers, peptides, metals, carbon nanomaterials, combinatorial strategies, and includes discussions on the antimicrobial mechanisms involved in these antimicrobial approaches. The toxicological aspects of these advanced scaffolds are also analyzed to ensure future technological transfer to clinics. The main antimicrobial methods of characterizing scaffolds’ antimicrobial and antibiofilm properties are described. The production methods of these porous supports, such as electrospinning, phase separation, gas foaming, the porogen method, polymerization in solution, fiber mesh coating, self-assembly, membrane lamination, freeze drying, 3D printing and bioprinting, among others, are also included in this article. These important advances in antimicrobial materials-based scaffolds for regenerative medicine offer many new promising avenues to the material design and tissue-engineering communities.

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Antibacterial, antifungal and antibiofilm scaffolds.

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Antimicrobial scaffold fabrication techniques.

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Antimicrobial biomaterials for tissue engineering applications.

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Antimicrobial characterization methods of scaffolds.

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Bone, oral tissue, skin, muscle, nerve, trachea, cardiac, among other applications.

Enzyme-Mediated Alleviation of Peroxide Toxicity in Self-Oxygenating Biomaterials

Oxygen releasing biomaterials can facilitate the survival of living implants by creating environments with a viable oxygen level. Hydrophobic oxygen generating microparticles (HOGMPs) encapsulated calcium peroxide (CPO) have recently been used in tissue engineering to release physiologically relevant amounts of oxygen for several weeks. However, generating oxygen using CPO is mediated via the generation of toxic levels of hydrogen peroxide (H2 O2 ). The incorporation of antioxidants, such as catalases, can potentially reduce H2 O2 levels. However, the formulation in which catalases can most effectively scavenge H2 O2 within oxygen generating biomaterials has remained unexplored. In this study, three distinct catalase incorporation methods are compared based on their ability to decrease H2 O2 levels. Specifically, catalase is incorporated within HOGMPs, or absorbed onto HOGMPs, or freely laden into the hydrogel entrapping HOGMPs and compared with control without catalase. Supplementation of free catalase in an HOGMP-laden hydrogel significantly decreases H2 O2 levels reflecting a higher cellular viability and metabolic activity of all the groups. An HOGMP/catalase-laden hydrogel precursor solution containing cells is used as an oxygenating bioink allowing improved viability of printed constructs under severe hypoxic conditions. The combination of HOGMPs with a catalase-laden hydrogel has the potential to decrease peroxide toxicity of oxygen generating tissues.

Strontium Peroxide-Loaded Composite Scaffolds Capable of Generating Oxygen and Modulating Behaviors of Osteoblasts and Osteoclasts

The reconstruction of bone defects remains challenging. The utilization of bone autografts, although quite promising, is limited by several drawbacks, especially substantial donor site complications. Recently, strontium (Sr), a bioactive trace element with excellent osteoinductive, osteoconductive, and pro-angiogenic properties, has emerged as a potential therapeutic agent for bone repair. Herein, a strontium peroxide (SrO₂)-loaded poly(lactic-co-glycolic acid) (PLGA)-gelatin scaffold system was developed as an implantable bone substitute. Gelatin sponges serve as porous osteoconductive scaffolds, while PLGA not only reinforces the mechanical strength of the gelatin but also controls the rate of water infiltration. The encapsulated SrO₂ can release Sr²⁺ in a sustained manner upon exposure to water, thus effectively stimulating the proliferation of osteoblasts and suppressing the formation of osteoclasts. Moreover, SrO₂ can generate hydrogen peroxide and subsequent oxygen molecules to increase local oxygen tension, an essential niche factor for osteogenesis. Collectively, the developed SrO₂-loaded composite scaffold shows promise as a multifunctional bioactive bone graft for bone tissue engineering.

Oxygen-Carrying and Antibacterial Fluorinated Nano-Hydroxyapatite Incorporated Hydrogels for Enhanced Bone Regeneration

Insufficient oxygen availability in tissue engineering is one of the major factors for the failure of clinical transplantation. One potential strategy to conquer this limitation is the fabrication of spontaneous and continuous oxygen supplying scaffolds for in situ tissue regeneration. In this work, a versatile fluorine-incorporating hydrogel is designed which can not only timely and continuously supply oxygen for mesenchymal stem cells (MSCs) to overcome deficient oxygen before vascularization in scaffolds, but can present a higher antibacterial capability to avoid bacterial infections. The HAp@PDA-F nanoparticles are first prepared and then incorporated with the quaternized and methacrylated chitosan forming CS/HAp@PDA-F by photo-crosslinking. In vitro results indicate that CS/HAp@PDA-F hydrogel has outstanding mechanical performance, moreover, it also has the oxygen-carrying ability to prolong survival ability, enhance proliferation activity, and preserve osteogenic differentiation potency and promote osteogenic-related genes expression of rat bone mesenchymal stem cells (rBMSCs) under hypoxic environment. Furthermore, the CS/HAp@PDA-F hydrogel can inhibit the growth of Staphylococcus aureus and Escherichia coli, providing a good antibacterial activity. Additionally, in vivo experiments demonstrate higher bone volume and bone mineral density, and more new bone tissue generation in CS/HAp@PDA-F group than in CS/HAp@PDA group. These results indicate that the rational design of fluorinated hydrogel possesses a good clinical application prospect for bone regeneration.

Oxygenation therapies for improved wound healing: Current trends and technologies

Degree of oxygenation is one of the important parameters governing various processes, including cell proliferation, angiogenesis, extracellular matrix production, and even combating the microbial burden at the wound site, all...

Recent advances in in situ oxygen-generating and oxygen-replenishing strategies for hypoxic-enhanced photodynamic therapy

Cancer is a leading cause of death worldwide, accounting for an estimated 10 million deaths by 2020. Over the decades, various strategies for tumor therapy have been developed and evaluated. Photodynamic therapy (PDT) has attracted increasing attention due to its unique characteristics, including low systemic toxicity and minimally invasive nature. Despite the excellent clinical promise of PDT, hypoxia is still the Achilles' heel associated with its oxygen-dependent nature related to increased tumor proliferation, angiogenesis, and distant metastases. Moreover, PDT-mediated oxygen consumption further exacerbates the hypoxia condition, which will eventually lead to the poor effect of drug treatment and resistance and irreversible tumor metastasis, even limiting its effective application in the treatment of hypoxic tumors. Hypoxia, with increased oxygen consumption, may occur in acute and chronic hypoxia conditions in developing tumors. Tumor cells farther away from the capillaries have much lower oxygen levels than cells in adjacent areas. However, it is difficult to change the tumor's deep hypoxia state through different ways to reduce the tumor tissue's oxygen consumption. Therefore, it will become more difficult to cure malignant tumors completely. In recent years, numerous investigations have focused on improving PDT therapy's efficacy by providing molecular oxygen directly or indirectly to tumor tissues. In this review, different molecular oxygen supplementation methods are summarized to alleviate tumor hypoxia from the innovative perspective of using supplemental oxygen. Besides, the existing problems, future prospects and potential challenges of this strategy are also discussed.

Oxygen-Releasing Composites: A Promising Approach in the Management of Diabetic Foot Ulcers

In diabetes, lower extremity amputation (LEA) is an irreversible diabetic-related complication that easily occurs in patients with diabetic foot ulcers (DFUs). Because DFUs are a clinical outcome of different causes including peripheral hypoxia and diabetic foot infection (DFI), conventional wound dressing materials are often insufficient for supporting the normal wound healing potential in the ulcers. Advanced wound dressing development has recently focused on natural or biocompatible scaffolds or incorporating bioactive molecules. This review directs attention to the potential of oxygenation of diabetic wounds and highlights current fabrication techniques for oxygen-releasing composites and their medical applications. Based on different oxygen-releasable compounds such as liquid peroxides and solid peroxides, for example, a variety of oxygen-releasing composites have been fabricated and evaluated for medical applications. This review provides the challenges and limitations of utilizing current oxygen releasable compounds and provides perspectives on advancing oxygen releasing composites for diabetic-related wounds associated with DFUs.

Advances in Skin Wound and Scar Repair by Polymer Scaffolds

Scars, as the result of abnormal wound-healing response after skin injury, may lead to loss of aesthetics and physical dysfunction. Current clinical strategies, such as surgical excision, laser treatment, and drug application, provide late remedies for scarring, yet it is difficult to eliminate scars. In this review, the functions, roles of multiple polymer scaffolds in wound healing and scar inhibition are explored. Polysaccharide and protein scaffolds, an analog of extracellular matrix, act as templates for cell adhesion and migration, differentiation to facilitate wound reconstruction and limit scarring. Stem cell-seeded scaffolds and growth factors-loaded scaffolds offer significant bioactive substances to improve the wound healing process. Special emphasis is placed on scaffolds that continuously release oxygen, which greatly accelerates the vascularization process and ensures graft survival, providing convincing theoretical support and great promise for scarless healing.

Methods for fabricating oxygen releasing biomaterials

Sustained external supply of oxygen (O₂) to engineered tissue constructs is important for their survival in the body while angiogenesis is taking place. In the recent years, the trend towards the fabrication of various O₂-generating materials that can provide prolonged and controlled O₂ source to the large volume tissue constructs resulted in preventing necrosis associated with the lack of O₂ supply. In this review, we explain different methods employed in the fabrication of O₂-generating materials such as emulsion, microfluidics, solvent casting, freeze drying, electrospraying, gelation, microfluidic and three-dimensional (3D) bioprinting methods. After discussing pros and cons of each method, we review physical, chemical, and biological characterisation techniques used to analyse the resulting product. Finally, the challenges and future directions in the field are discussed.

Engineering a macroporous oxygen-generating scaffold for enhancing islet cell transplantation within an extrahepatic site

Insufficient oxygenation is a serious issue arising within cell-based implants, as the hypoxic period between implantation and vascularization of the graft is largely unavoidable. In situ oxygen supplementation at the implant site should significantly mitigate hypoxia-induced cell death and dysfunction, as well as improve transplant efficacy, particularly for highly metabolically active cells such as pancreatic islets. One promising approach is the use of an oxygen generating material created through the encapsulation of calcium peroxide within polydimethylsiloxane (PDMS), termed OxySite. In this study, OxySite microbeads were incorporated within a macroporous PDMS scaffold to create a single, streamlined, oxygen generating macroporous scaffold. The resulting OxySite scaffold generated sufficient local oxygenation for up to 20 days, with nontoxic levels of reaction intermediates or by-products. The benefit of local oxygen release on transplant efficacy was investigated in a diabetic Lewis rat syngeneic transplantation model using a clinically relevant islet dosage (10,000 IEQ/kg BW) with different isolation purities (80%, 90%, and 99%). Impure islet preparations containing pancreatic non-islet cells, which are common in the clinical setting, permit examination of the effect of increased overall oxygen demand. Our transplantation outcomes showed that elevating the oxygen demand of the graft with decreasing isolation purity resulted in decreased graft efficacy for control implants, while the integration of OxySite significantly mitigated this impact and resulted in improved graft outcomes. Results highlight the superior clinical translational potential of these off-the-shelf OxySite scaffolds, where islet purity and the overall oxygen demands of implants are increased and highly variable. The oxygen-generating porous scaffold further provides a broad platform for enhancing the survival and efficacy of cellular implants for numerous other applications. STATEMENT OF SIGNIFICANCE: Hypoxia is a serious issue within tissue engineered implants. To address this challenge, we developed a distinct macroporous scaffold platform containing oxygen-generating microbeads. This oxygen-generating scaffold showed the potential to support clinically relevant cell dosages for islet transplantation, leading to improved treatment efficacy. This platform can also be used to mitigate hypoxia for other biomedical applications.

Design Considerations for Macroencapsulation Devices for Stem Cell Derived Islets for the Treatment of Type 1 Diabetes

Stem cell derived insulin producing cells or islets have shown promise in reversing Type 1 Diabetes (T1D), yet successful transplantation currently necessitates long-term modulation with immunosuppressant drugs. An alternative approach to avoiding this immune response is to utilize an islet macroencapsulation device, where islets are incorporated into a selectively permeable membrane that can protect the transplanted cells from acute host response, whilst enabling delivery of insulin. These macroencapsulation systems have to meet a number of stringent and challenging design criteria in order to achieve the ultimate goal of reversing T1D. In this progress report, the design considerations and functional requirements of macroencapsulation systems are reviewed, specifically for stem-cell derived islets (SC-islets), highlighting distinct design parameters. Additionally, a perspective on the future for macroencapsulation systems is given, and how incorporating continuous sensing and closed-loop feedback can be transformative in advancing toward an autonomous biohybrid artificial pancreas.

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.

Survival and Proliferation under Severely Hypoxic Microenvironments Using Cell-Laden Oxygenating Hydrogels

Different strategies have been employed to provide adequate nutrients for engineered living tissues. These have mainly revolved around providing oxygen to alleviate the effects of chronic hypoxia or anoxia that result in necrosis or weak neovascularization, leading to failure of artificial tissue implants and hence poor clinical outcome. While different biomaterials have been used as oxygen generators for in vitro as well as in vivo applications, certain problems have hampered their wide application. Among these are the generation and the rate at which oxygen is produced together with the production of the reaction intermediates in the form of reactive oxygen species (ROS). Both these factors can be detrimental for cell survival and can severely affect the outcome of such studies. Here we present calcium peroxide (CPO) encapsulated in polycaprolactone as oxygen releasing microparticles (OMPs). While CPO releases oxygen upon hydrolysis, PCL encapsulation ensures that hydrolysis takes place slowly, thereby sustaining prolonged release of oxygen without the stress the bulk release can endow on the encapsulated cells. We used gelatin methacryloyl (GelMA) hydrogels containing these OMPs to stimulate survival and proliferation of encapsulated skeletal myoblasts and optimized the OMP concentration for sustained oxygen delivery over more than a week. The oxygen releasing and delivery platform described in this study opens up opportunities for cell-based therapeutic approaches to treat diseases resulting from ischemic conditions and enhance survival of implants under severe hypoxic conditions for successful clinical translation.

Fabrication of Photoactive Electrospun Cellulose Acetate Nanofibers for Antibacterial Applications

The aim of the study was to investigate the process of electrostatic fabrication of cellulose acetate (CA) nanofibers containing methylene blue (MB) as a photosensitizer. The electrical, physicochemical, and biocidal properties of the prepared material were given. CA nanofibers were prepared by electrospinning method using a solvent mixture of acetone and distilled water (9:1 vv−1) and different concentrations of CA (i.e., 10–21%). Additionally, methylene blue was implemented into the polymer solution with a CA concentration of 17% to obtain fibers with photo-bactericidal properties. Pure electrospun CA fibers were more uniform than fibers with MB (i.e., ribbon shape). Fiber diameters did not exceed 900 nm for the tested polymer solutions and flow rate below 1.0 mL h−1. The polymer properties (i.e., concentration, resistivity) and other parameters of the process (i.e., flow rate, an applied voltage) strongly influenced the size of the fibers. Plasma treatment of nanofibers resulted in reduced biofilm formation on their surface. The results of photo-bactericidal activity (i.e., up to 180 min) confirmed the high efficiency of inactivation of Staphylococcus aureus cells using fibers containing methylene blue (i.e., with and without plasma treatment). The most effective reduction in the number of biofilm cells was equal to 99.99 ± 0.3%.

Nanomaterials for Tumor Hypoxia Relief to Improve the Efficacy of ROS-Generated Cancer Therapy

Given the fact that excessive levels of reactive oxygen species (ROS) induce damage to proteins, lipids, and DNA, various ROS-generating agents and strategies have been explored to induce cell death and tumor destruction by generating ROS above toxic threshold. Unfortunately, hypoxia in tumor microenvironment (TME) not only promotes tumor metastasis but also enhances tumor resistance to the ROS-generated cancer therapies, thus leading to ineffective therapeutic outcomes. A variety of nanotechnology-based approaches that generate or release O2 continuously to overcome hypoxia in TME have showed promising results to improve the efficacy of ROS-generated cancer therapy. In this minireview, we present an overview of current nanomaterial-based strategies for advanced cancer therapy by modulating the hypoxia in the TME and promoting ROS generation. Particular emphasis is put on the O2 supply capability and mechanism of these nanoplatforms. Future challenges and opportunities of design consideration are also discussed. We believe that this review may provide some useful inspiration for the design and construction of other advanced nanomaterials with O2 supply ability for overcoming the tumor hypoxia-associated resistance of ROS-mediated cancer therapy and thus promoting ROS-generated cancer therapeutics.

Oxygen releasing materials: Towards addressing the hypoxia-related issues in tissue engineering

Manufacturing macroscale cell-laden architectures is one of the biggest challenges faced nowadays in the domain of tissue engineering. Such living constructs, in fact, pose strict requirements for nutrients and oxygen supply that can hardly be addressed through simple diffusion in vitro or without a functional vasculature in vivo. In this context, in the last two decades, a substantial amount of work has been carried out to develop smart materials that could actively provide oxygen-release to contrast local hypoxia in large-size constructs. This review provides an overview of the currently available oxygen-releasing materials and their synthesis and mechanism of action, highlighting their capacities under in vitro tissue cultures and in vivo contexts. Additionally, we also showcase an emerging concept, herein termed as "living materials as releasing systems", which relies on the combination of biomaterials with photosynthetic microorganisms, namely algae, in an "unconventional" attempt to supply the damaged or re-growing tissue with the necessary supply of oxygen. We envision that future advances focusing on tissue microenvironment regulated oxygen-supplying materials would unlock an untapped potential for generating a repertoire of anatomic scale, living constructs with improved cell survival, guided differentiation, and tissue-specific biofunctionality.

Polymer microcapsules and microbeads as cell carriers for in vivo biomedical applications

Polymer microcarriers are being extensively explored as cell delivery vehicles in cell-based therapies and hybrid tissue and organ engineering. Spherical microcarriers are of particular interest due to easy fabrication and injectability. They include microbeads, composed of a porous matrix, and microcapsules, where matrix core is additionally covered with a semipermeable membrane. Microcarriers provide cell containment at implantation site and protect the cells from host immunoresponse, degradation and shear stress. Immobilized cells may be genetically altered to release a specific therapeutic product directly at the target site, eliminating side effects of systemic therapies. Cell microcarriers need to fulfil a number of extremely high standards regarding their biocompatibility, cytocompatibility, immunoisolating capacity, transport, mechanical and chemical properties. To obtain cell microcarriers of specified parameters, a wide variety of polymers, both natural and synthetic, and immobilization methods can be applied. Yet so far, only a few approaches based on cell-laden microcarriers have reached clinical trials. The main issue that still impedes progress of these systems towards clinical application is limited cell survival in vivo. Herein, we review polymer biomaterials and methods used for fabrication of cell microcarriers for in vivo biomedical applications. We describe their key limitations and modifications aiming at improvement of microcarrier in vivo performance. We also present the main applications of polymer cell microcarriers in regenerative medicine, pancreatic islet and hepatocyte transplantation and in the treatment of cancer. Lastly, we outline the main challenges in cell microimmobilization for biomedical purposes, the strategies to overcome these issues and potential future improvements in this area.

Needle-injectable microcomposite cryogel scaffolds with antimicrobial properties

Porous three-dimensional hydrogel scaffolds have an exquisite ability to promote tissue repair. However, because of their high water content and invasive nature during surgical implantation, hydrogels are at an increased risk of bacterial infection. Recently, we have developed elastic biomimetic cryogels, an advanced type of polymeric hydrogel, that are syringe-deliverable through hypodermic needles. These needle-injectable cryogels have unique properties, including large and interconnected pores, mechanical robustness, and shape-memory. Like hydrogels, cryogels are also susceptible to colonization by microbial pathogens. To that end, our minimally invasive cryogels have been engineered to address this challenge. Specifically, we hybridized the cryogels with calcium peroxide microparticles to controllably produce bactericidal hydrogen peroxide. Our novel microcomposite cryogels exhibit antimicrobial properties and inhibit antibiotic-resistant bacteria (MRSA and Pseudomonas aeruginosa), the most common cause of biomaterial implant failure in modern medicine. Moreover, the cryogels showed negligible cytotoxicity toward murine fibroblasts and prevented activation of primary bone marrow-derived dendritic cells ex vivo. Finally, in vivo data suggested tissue integration, biodegradation, and minimal host inflammatory responses when the antimicrobial cryogels, even when purposely contaminated with bacteria, were subcutaneously injected in mice. Collectively, these needle-injectable microcomposite cryogels show great promise for biomedical applications, especially in tissue engineering and regenerative medicine.

An ultrasound activated oxygen generation nanosystem specifically alleviates myocardial hypoxemia and promotes cell survival following acute myocardial infarction

Hypoxemia after acute myocardial infarction (AMI) causes severe damage to cardiac cells and induces cardiac dysfunction. Protection of cardiac cells and reconstruction of cardiac functions by re-introducing oxygen into the infarcted myocardium represents an efficient approach for the treatment of AMI. However, the established methods for oxygen supplementation mainly focus on systemic oxygen delivery, which always results in inevitable oxidative stress on normal tissues. In this work, an ultrasound (US) activated oxygen generation nanosystem has been developed, which specifically releases oxygen in the infarcted myocardium and alleviates the hypoxemic myocardial microenvironment to protect cardiac cells after AMI. The nanosystem was constructed through the formation of calcium peroxide in the mesopores of biocompatible mesoporous silica nanoplatforms, followed by the assembly of the thermosensitive material heneicosane and polyethyleneglycol. The mild hyperthermia induced by US irradiation triggered the phase change of heneicosane, thus achieving US responsive diffusion of water and release of oxygen. The US-activated oxygen release significantly alleviated the hypoxia and facilitated the mitigation of oxidative stress after AMI. Consequently, the survival of cardiac cells under hypoxic conditions was substantially improved and the damage in the infarcted myocardial tissue was minimized. This US-activated oxygen generation nanosystem may provide an efficient modality for the treatment of AMI.

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 Antibacterial Nanofibrous Scaffolds for Tissue Engineering Applications

Lack of suitable auto/allografts has been delaying surgical interventions for the treatment of numerous disorders and has also caused a serious threat to public health. Tissue engineering could be one of the best alternatives to solve this issue. However, deficiency of oxygen supply in the wounded and implanted engineered tissues, caused by circulatory problems and insufficient angiogenesis, has been a rate-limiting step in translation of tissue-engineered grafts. To address this issue, we designed oxygen-releasing electrospun composite scaffolds, based on a previously developed hybrid polymeric matrix composed of poly(glycerol sebacate) (PGS) and poly(ε-caprolactone) (PCL). By performing ball-milling, we were able to embed a large percent of calcium peroxide (CP) nanoparticles into the PGS/PCL nanofibers able to generate oxygen. The composite scaffold exhibited a smooth fiber structure, while providing sustainable oxygen release for several days to a week, and significantly improved cell metabolic activity due to alleviation of hypoxic environment around primary bone-marrow-derived mesenchymal stem cells (BM-MSCs). Moreover, the composite scaffolds also showed good antibacterial performance. In conjunction to other improved features, such as degradation behavior, the developed scaffolds are promising biomaterials for various tissue-engineering and wound-healing applications.

Regenerative engineered vascularized bone mediated by calcium peroxide

One of the main challenges hindering the clinical translation of bone tissue engineering scaffolds is the lack of establishment of functional vasculature. Insufficient vascularization and poor oxygen supply limit cell survival within the constructs resulting in poor osseointegration with the host tissue and eventually leading to inadequate bone regeneration. Inspired by cues from developmental biology, we regenerative engineered a composite matrix by incorporating calcium peroxide (CaO₂ ) into poly(lactide-co-glycolide) (PLGA) microsphere-based matrices and sought to assess whether the delivery of the byproducts of CaO₂ decomposition, namely O₂ , Ca²⁺ , and H₂ O₂ could enhance the regeneration of vascularized bone tissue. The composite microspheres were successfully fabricated via the oil-in-water emulsion method. The presence and encapsulation of CaO₂ was confirmed using scanning electron microscopy, energy dispersive x-ray spectroscopy, thermogravimetric analysis, and X-ray powder diffraction. The microspheres were further heat sintered into three-dimensional porous scaffolds and characterized for their degradation and release of byproducts. The in vitro cytocompatibility of the matrices and their ability to support osteogenic differentiation was confirmed using human adipose-derived stem cells. Lastly, an in vivo study was performed in a mouse critical-sized calvarial defect model to evaluate the capacity of these matrices in supporting vascularized bone regeneration. Results demonstrated that the presence of CaO₂ increased cellularization and biological activity throughout the matrices. There was greater migration of host cells to the interior of the matrices and greater survival and persistence of donor cells after 8 weeks, which in synergy with the composite matrices led to enhanced vascularized bone regeneration.

Chemoreactive Nanotherapeutics by Metal Peroxide Based Nanomedicine

The advances of nanobiotechnology and nanomedicine enable the triggering of in situ chemical reactions in disease microenvironment for achieving disease-specific nanotherapeutics with both intriguing therapeutic efficacy and mitigated side effects. Metal peroxide based nanoparticles, as one of the important but generally ignored categories of metal-involved nanosystems, can function as the solid precursors to produce oxygen (O2) and hydrogen peroxide (H2O2) through simple chemical reactions, both of which are the important chemical species for enhancing the therapeutic outcome of versatile modalities, accompanied with the unique bioactivity of metal ion based components. This progress report summarizes and discusses the most representative paradigms of metal peroxides in chemoreactive nanomedicine, including copper peroxide (CuO2), calcium peroxide (CaO2), magnesium peroxide (MgO2), zinc peroxide (ZnO2), barium peroxide (BaO2), and titanium peroxide (TiOx) nanosystems. Their reactions and corresponding products have been broadly explored in versatile disease treatments, including catalytic nanotherapeutics, photodynamic therapy, radiation therapy, antibacterial infection, tissue regeneration, and some synergistically therapeutic applications. This progress report particularly focuses on the underlying reaction mechanisms on enhancing the therapeutic efficacy of these modalities, accompanied with the discussion on their biological effects and biosafety. The existing gap between fundamental research and clinical translation of these metal peroxide based nanotherapeutic technologies is finally discussed in depth.

Oxygenation strategies for encapsulated islet and beta cell transplants

Human allogeneic islet transplantation (ITx) is emerging as a promising treatment option for qualified patients with type 1 diabetes. However, widespread clinical application of allogeneic ITx is hindered by two critical barriers: the need for systemic immunosuppression and the limited supply of human islet tissue. Biocompatible, retrievable immunoisolation devices containing glucose-responsive insulin-secreting tissue may address both critical barriers by enabling the more effective and efficient use of allogeneic islets without immunosuppression in the near-term, and ultimately the use of a cell source with a virtually unlimited supply, such as human stem cell-derived β-cells or xenogeneic (porcine) islets with minimal or no immunosuppression. However, even though encapsulation methods have been developed and immunoprotection has been successfully tested in small and large animal models and to a limited extent in proof-of-concept clinical studies, the effective use of encapsulation approaches to convincingly and consistently treat diabetes in humans has yet to be demonstrated. There is increasing consensus that inadequate oxygen supply is a major factor limiting their clinical translation and routine implementation. Poor oxygenation negatively affects cell viability and β-cell function, and the problem is exacerbated with the high-density seeding required for reasonably-sized clinical encapsulation devices. Approaches for enhanced oxygen delivery to encapsulated tissues in implantable devices are therefore being actively developed and tested. This review summarizes fundamental aspects of islet microarchitecture and β-cell physiology as well as encapsulation approaches highlighting the need for adequate oxygenation; it also evaluates existing and emerging approaches for enhanced oxygen delivery to encapsulation devices, particularly with the advent of β-cell sources from stem cells that may enable the large-scale application of this approach.

Mathematical modeling of oxygen release from hyperbarically loaded polymers

Polymer-based scaffolds are used extensively in the field of regenerative medicine. These biomaterials may induce therapeutic responses through modulating a wound microenvironment with or without the addition of cells. It has long been known that oxygen is a crucial component of the microenvironment that influences cellular and physiological processes such as metabolism, proliferation, differentiation, matrix deposition, phagocytic killing, and wound healing. Consequently, several studies have investigated the potential for using oxygen-eluting biomaterials to regulate the oxygen tension within a wound microenvironment and to tune the regenerative response. We recently demonstrated that hyperbarically loaded polymers could be used as oxygen delivery devices for biomedical uses. To further develop this strategy, it is important to quantitatively characterize the spatiotemporal oxygen diffusion profile from scaffolds. Here, we use analytical and numerical solutions to describe the profiles of oxygen diffusion from hyperbarically loaded polymers as a function of different scaffold geometries, material compositions, and ambient temperatures. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 2018 © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 35: e2751, 2019.

Oxygen Regulation in Development: Lessons from Embryogenesis towards Tissue Engineering

Oxygen is a vital source of energy necessary to sustain and complete embryonic development. Not only is oxygen the driving force for many cellular functions and metabolism, but it is also involved in regulating stem cell fate, morphogenesis, and organogenesis. Low oxygen levels are the naturally preferred microenvironment for most processes during early development and mainly drive proliferation. Later on, more oxygen and also nutrients are needed for organogenesis and morphogenesis. Therefore, it is critical to maintain oxygen levels within a narrow range as required during development. Modulating oxygen tensions is performed via oxygen homeostasis mainly through the function of hypoxia-inducible factors. Through the function of these factors, oxygen levels are sensed and regulated in different tissues, starting from their embryonic state to adult development. To be able to mimic this process in a tissue engineering setting, it is important to understand the role and levels of oxygen in each developmental stage, from embryonic stem cell differentiation to organogenesis and morphogenesis. Taking lessons from native tissue microenvironments, researchers have explored approaches to control oxygen tensions such as hemoglobin-based, perfluorocarbon-based, and oxygen-generating biomaterials, within synthetic tissue engineering scaffolds and organoids, with the aim of overcoming insufficient or nonuniform oxygen levels and nutrient supply.

Oxygen-Releasing Antioxidant Cryogel Scaffolds with Sustained Oxygen Delivery for Tissue Engineering Applications

With the advancement in biomaterial sciences, tissue-engineered scaffolds are developing as a promising strategy for the regeneration of damaged tissues. However, only a few of these scaffolds have been translated into clinical applications. One of the primary drawbacks of the existing scaffolds is the lack of adequate oxygen supply within the scaffolds. Oxygen-producing biomaterials have been developed as an alternate strategy but are faced with two major concerns. One is the control of the rate of oxygen generation, and the other is the production of reactive oxygen species (ROS). To address these concerns, here, we report the development of an oxygen-releasing antioxidant polymeric cryogel scaffold (PUAO-CPO) for sustained oxygen delivery. PUAO-CPO scaffold was fabricated using the cryogelation technique by the incorporation of calcium peroxide (CPO) in the antioxidant polyurethane (PUAO) scaffolds. The PUAO-CPO cryogels attenuated the ROS and showed a sustained release of oxygen over a period of 10 days. An in vitro analysis of the PUAO-CPO cryogels showed their ability to sustain H9C2 cardiomyoblast cells under hypoxic conditions, with cell viability being significantly better than the normal polyurethane (PU) scaffolds. Furthermore, in vivo studies using an ischemic flap model showed the ability of the oxygen-releasing cryogel scaffolds to prevent tissue necrosis upto 9 days. Histological examination indicated the maintenance of tissue architecture and collagen content, whereas immunostaining for proliferating cell nuclear antigen confirmed the viability of the ischemic tissue with oxygen delivery. Our study demonstrated an advanced approach for the development of oxygen-releasing biomaterials with sustained oxygen delivery as well as attenuated production of residual ROS and free radicals because of ischemia or oxygen generation. Hence, the oxygen-releasing PUAO-CPO cryogel scaffolds may be used with cell-based therapeutic approaches for the regeneration of damaged tissue, particularly with ischemic conditions such as myocardial infarction and chronic wound healing.

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.

3D-printed biphasic calcium phosphate scaffolds coated with an oxygen generating system for enhancing engineered tissue survival

Tissue engineering scaffolds with oxygen generating elements have shown to be able to increase the level of oxygen and cell survivability in specific conditions. In this study, biphasic calcium phosphate (BCP) scaffolds with the composition of 60% hydroxyapatite (HA) and 40% beta-tricalcium phosphate (β-TCP), which have shown a great potential for bone tissue engineering applications, were fabricated by a direct-write assembly (robocasting) technique. Then, the three-dimensional (3D)-printed scaffolds were coated with different ratios of an oxygen releasing agent, calcium peroxide (CPO), which encapsulated within a polycaprolactone (PCL) matrix through dip-coating, and used for in situ production of oxygen in the implanted sites. The structure, composition and morphology of the prepared scaffolds were characterized by different techniques. The oxygen release kinetics and biological investigations of the scaffolds were also studied in vitro. The results showed that oxygen release behaviour was sustained and dependant on the concentration of CPO encapsulated in the PCL coating matrix. It was also demonstrated that the coated scaffolds, having 3% CPO in the coating system, could provide a great potential for promoting bone ingrowth with improving osteoblast cells viability and proliferation under hypoxic conditions. The findings indicated that the prepared scaffolds could play a significant role in engineering of large bone tissue implants with limitations in oxygen diffusion.

Regulation of Scaffold Cell Adhesion Using Artificial Membrane Binding Proteins

The rapid pace of development in biotechnology has placed great importance on controlling cell-material interactions. In practice, this involves attempting to decouple the contributions from adhesion molecules, cell membrane receptors, and scaffold surface chemistry and morphology, which is extremely challenging. Accordingly, a strategy is presented in which different chemical, biochemical, and morphological properties of 3D biomaterials are systematically varied to produce novel scaffolds with tuneable cell affinities. Specifically, cationized and surfactant-conjugated proteins, recently shown to have non-native membrane affinity, are covalently attached to 3D scaffolds of collagen or carboxymethyl-dextran, yielding surface-functionalized 3D architectures with predictable cell immobilization profiles. The artificial membrane-binding proteins enhance cellular adhesion of human mesenchymal stem cells (hMSCs) via electrostatic and hydrophobic binding mechanisms. Furthermore, functionalizing the 3D scaffolds with cationized or surfactant-conjugated myoglobin prevents a slowdown in proliferation of seeded hMSCs cultured for seven days under hypoxic conditions.