Advances in Gas Therapeutics for Wound Healing: Mechanisms, Delivery Materials, and Prospects

Wound repair, particularly chronic nonhealing wound repair, is a global health issue that affects millions of people worldwide. As an effective and safe therapeutic reagent, gas molecules in tissue repair have attracted considerable attention. Recent studies have shown that gas therapy (GT) plays an essential role in all phases of wound repair, including anti‐inflammatory and antimicrobial modulation, cell proliferation and migration, proangiogenesis, and extracellular matrix remodeling. This review aims to summarize recent progress in developing GT for wound repair. The characteristics of gas molecules with therapeutic functions, such as oxygen, nitric oxide, carbon monoxide, hydrogen sulfide, and others (such as sulfur dioxide, hydrogen, carbon dioxide, and plasma) in wound healing are introduced. GT has transitioned from pure gases to inorganic/organic materials as gas‐releasing materials, including gas‐producing and gas‐loaded materials, and controlled/long‐lasting stable‐releasing biomaterials. Finally, the limitations and prospects in the field of GT are analyzed and summarized.

therapeutic material platforms were introduced into tissue engineering in the last decade, offering a tremendous opportunity for significantly improving the efficacy of GT.Nanoparticles can achieve controlled release of gas through external stimulation such as magnetic/electric fields, X-rays, and UV/near-infrared (NIR) light. [48]The combination of inorganic/organic functional material structures with gas-loaded or synthetic gas donors synergistically promotes the efficient healing of skin wounds through the inherent biological efficacy of the gas, as well as the photodynamic (PDT), photothermal (PTT), or antibiotic therapy [49] of the material (Figure 1).This review systematically presents a series of GTs for wound healing based on different gas molecules and discusses their mechanisms for skin healing and recent advances in gas-releasing engineered materials.The controllability and long-term stability of gas-releasing materials are emphasized, and the therapeutic potential of combined treatment strategies is demonstrated.Additionally, this review highlights the limitations and prospects of GT applications and provides new insights into the future use of GT for wound repair.

Mechanism of Gas in the Phases of Wound Healing
Wound healing is an orderly process that consists of three overlapping phases: inflammatory, proliferation, and remodeling.Different gases are proven to play vital roles in all stages of wound healing (Figure 2):

Inflammatory Phase
The inflammatory phase of wound healing is characterized by heightened inducible NO synthase (iNOS) activity in macrophages and neutrophils.As an initial line of defense against bacteria, neutrophils are quickly recruited to the clot.Within 48-96 h of damage, macrophages are recruited and converted into tissueactivated macrophages at the wound site. [1]uring the inflammatory phase, neutrophils are activated, generating reactive oxygen species (ROS) to phagocytose and combat bacterial infection.However, neutrophils lose their ability to kill bacteria when partial pressure of oxygen (pO 2 ) levels decrease below 40 mmHg. [50]O 2 is essential for the production of cytokines and growth factors, such as nicotinamide adenine dinucleotide oxidase, [37] superoxide dismutase (SOD), and other detoxifying agents. [51]Thus, in the hypoxic microenvironment of chronic trauma, bacteria cannot be effectively eliminated, the detoxification process is hindered, and uncontrolled ROS can further exacerbate the infection and inflammatory response to damage host cells.Moreover, O 2 plays a crucial role in regulating macrophage polarization, facilitating the transition from the inflammatory phase to the proliferative phase during wound healing.
In addition, iNOS expression is upregulated in macrophages within 6-24 h after an injury.This leads to the production of NO, which is involved in the signaling pathway of anti-inflammatory cytokine secretion (e.g., transforming growth factor-β1 (TGF-β1) and interleukin-8 (IL-8)).Consequently, inflammation and the immune response to infection are regulated. [23]Moreover, NO exhibits potent antibacterial and antibiofilm properties against a range of pathogens, including Staphylococcus aureus (S. aureus), Escherichia coli (E.coli), methicillin-resistant Staphylococcus aureus (MRSA), Pseudomonas aeruginosa (P.aeruginosa), and Candida albicans (C.albicans) infections. [5]Specifically, NO reacts with oxygen and superoxide to produce a reactive intermediate, which elicits significant lipid peroxidation, DNA alterations, and protein/enzyme destruction in bacterial cells. [6]NO prevents biofilm formation and disperses established biofilms through cyclic guanosine 3,5-monophosphate (cGMP) (an intracellular second messenger), [7] which has the benefit of preventing the development of drug-resistant strains.H 2 S exerts its unique cytoprotective effects by inhibiting p38-and c-Jun N-terminal kinasedependent apoptosis and nuclear factor-κB (NF-κB)-dependent inflammatory pathways. [24]H 2 S can prevent inflammatory responses and upregulate endogenous antioxidant systems. [24] 2 S interacts with a variety of oxidants, such as oxygen radicals [25] and peroxynitrite, [26] and scavenges ROS, minimizing cellular oxidative stress and hastening the healing of wounds.H 2 S can significantly inhibit the growth of bacteria, such as S. aureus and E. coli, [8] and fungi, such as Aspergillus niger and Penicillium italicum; [9] however, it has not been applied to the treatment of infected wounds.Both CO gases and CO-releasing molecules (CORMs) exhibit potent anti-inflammatory effects because of their ability to reduce the production of inflammatory mediators.CO selectively inhibits the expression of the proinflammatory cytokines TNF-α, IL-1β, and macrophage inflammatory protein-1β, and increases the production of the anti-inflammatory cytokine IL-10. [27]CO increases the expression of genes and molecules, such as signal transducer and activator of tran-scription6, peroxisome proliferator-activated receptor, Ym1, Fizz1, arginine-1, and IL-10, which are involved in the M2 polarization and M2 phenotypic of macrophages.Massive CO generation by active macrophages promotes M2 proliferative, polarizing, and differentiating processes.Apoptotic cell removal, inflammation relief, and tissue remodeling may all be facilitated by CO. [52] As CORMs, complexes CORM-2 (tricarbonyl dichlororuthenium(II) dimer) and CORM-3 (tricarbonyl chloro (glycine)ruthenium(III)) exhibit broad-spectrum antibacterial activity.They are effective against bacteria, such as E. coli, [10] extended-spectrum β-lactamase E. coli, [10,11] S. aureus, [12] Helicobacter pylori, [13] and P. aeurginosa. [14]The mechanism of antibacterial activity of CO and CORM involves targeting the inhibition of terminal oxidase respiration, [15,16] enhancing the production of intracellular ROS, leading to DNA damage and death, [11,53] increasing membrane permeability, [16] and disrupting the biofilm structure. [14]H 2 is sufficiently mild to scavenge intracellular ROS without any toxic effects, even at high concentrations. [29]Macrophages utilize an endogenous SO 2 /aspartate aminotransferase (AAT) pathway, and SO 2 produced by macrophages possesses anti-inflammatory properties. [54]SO 2 also upregulates the cyclic adenosine monophosphate pathway, serving as an endogenous mast cell stabilizer [28] The ability of plasma to kill microorganisms is primarily mediated by ROS and reactive nitrogen particles.These particles are collectively referred to as reactive oxygen and nitrogen species and include powerful and stable antimicrobial compounds such as nitric oxide, hydrogen peroxide (H 2 O 2 ), hypochlorous acid (HOCl), ammonium chloride, and other species, which form in plasma from surrounding gases. [20,21]These oxidizing substances attack bacterial proteins, lipids, DNA, and RNA, and their concentrations increase proportionately with increases in applied voltage. [20,21]Three mechanisms are possible: electroporation and oxidation-induced cell wall/membrane dysfunction, which leads to leakage of cellular components; intracellular oxidation and nitrification, which cause protein damage and gene expression disorder; and direct DNA damage, such as those causing breaking of the double strand. [22]Cold plasma treatment could generate additional reaction products, including Cl 2À , ClO À , and HOCl, which result from peroxide formation by the plasma.The oxidation of bacterial macromolecules by HOCl can increase the uptake and processing of bacterial neoantigens by antigen-presenting cells. [20]

Proliferative Phase
Angiogenesis occurs after the inflammatory phase has ended.Angiogenesis is the process through which endothelial cells proliferate, migrate, and branch to produce new blood vessels. [1]uring the formation of new blood arteries, local fibroblasts proliferate and penetrate the clot to generate contractile granulation tissue.Some fibroblasts develop into myofibroblasts in this location, pulling the wound borders together. [1]uring the proliferative phase, O 2 plays a crucial role in regulation, neovascularization, cell proliferation, migration, and ECM formation.Under normoxic conditions, mitochondrial respiration consumes over 90% of O 2 , leaving 10% to degrade hypoxia-inducible factor-1α (HIF-1α).Meanwhile, acute hypoxia leads to the upregulation of HIF-1α, which stimulates neovascularization and tissue regeneration, vascular endothelial growth factor (VEGF), sirtuin, and mitochondrial metabolism. [55,56]owever, these vessels are immature with minimal surrounding stroma, which show frequent areas of hemorrhage. [37,57]Further, prolonged decrease in O 2 can lead to metabolic acidosis, which inhibits fibroblast proliferation and collagen synthesis. [34,35]38] The proliferation of keratinocytes at the wound margin is dependent on iNOS.This enzyme stimulates the migration and proliferation of endothelial cells.When activated, the angiogenic signaling regulator eNOS-derived NO participates in angiogenesis by regulating VEGF expression through the NO/cGMP/adenosine monophosphate-activated protein kinase (AMPK) cascade, which increases circulating GMP in smooth muscle cells to dilate blood vessels. [23,60]NO can stimulate vasodilation through permeable membranes in healthy skin through a chemical reaction between sodium nitrite and ascorbic acid. [39] 2 S improves the environment for wound healing, encourages angiogenesis, and regulates the amounts of numerous growth factors. [40]O dilates blood vessels in a manner similar to NO and significantly attenuates vasoconstriction in vitro and in vivo due to the HO-1-induced increase in endogenous CO in rats by increasing vascular cGMP levels.[41] CO directly enhances the activity of large-conductance calcium-activated potassium channels (bKCa) in rat vascular smooth muscle cells through a cGMP-nondependent mechanism.When bKCa channels open, the membrane becomes hyperpolarized, which causes voltagedependent calcium channels to close, resting calcium concentrations to drop, and vascular tissue to relax.[61] Exogenous CO has been demonstrated to prevent apoptosis in endothelial cells in vitro through a mechanism that is dependent on p38 mitogen-activated protein kinase but independent of cyclic guanosine monophosphate and inducible NO synthase.[62] H 2 acts as an anti-inflammatory, anti-allergic, and antiapoptotic molecule, while also stimulating energy metabolism.[29,30] CO 2 is effective in improving microcirculation and blood circulation, and its efficacy originates from the Bohr effect.[42] Cold atmospheric microwave plasma-treated cells indicate the downregulation of E-cadherin and upregulation of vimentin, Snail, and Slug at transcription and translation levels, thereby promoting cell migration.[63] These cells promote the cytoskeletal transformation of keratinocytes and migration due to changes in the expression of integrin-dependent focal adhesion molecules and MMPs.[64]

Remodeling Phase
This phase involves ECM remodeling, in which type III collagen is replaced by the more stable type I collagen.O 2 and NO contribute in ECM formation and remodeling. [1]oth collagen synthesis and deposition are O 2 -dependent processes.A pO 2 of 30-40 mmHg is necessary for type I collagen synthesis and is proportional to O 2 tension. [34]The enzymes involved in the posttranslational steps of collagen synthesis (prolyl hydroxylase, lysyl hydroxylase, and lysyl oxidase) require O 2 as a cofactor [37,43] to ensure tissue strength. [43,44]Hypoxia disrupts the balance between matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs), thereby enhancing MMP-2 expression and reducing TIMP-1 expression. [45]Thus, increasing wound oxygenation optimizes collagen deposition and tensile strength. [37,57]ound re-epithelialization is also dependent on NO, stimulating ECM synthesis. [39]The collagen content of the experimental wounds increased after exogenous NO treatment.It increases collagen synthesis, which enhances granulation tissue quality and may strengthen the wound. [39]old plasma production of H 2 O 2 , NO, and ONOO À can affect the side chains of amino acids, changing the fundamental makeup of collagen.In particular, proline/hydroxyproline (common in collagen) carbonyl groups can be converted by ONOO À to a nitroso to create the more basic N-nitrosopyrrolidine amino acid.Variations in ONOO À synthesis can result in various alterations to the side chains of amino acids, allowing for either positive or negative interactions with the cell matrix.By modulating collagen deposition, ECM disintegration, and the wound healing stage-dependent regulation of proteinases (such as MMPs), gas plasma-derived ROS support the physical integrity of repaired skin. [65]

Oxygen (O 2 ) Therapy
A key limiting factor for organ regeneration and wound healing is hypoxic stress, which is caused by damaged circulatory oxygenation and a hypermetabolic state at the cellular level.Chronic wounds are often accompanied by vascular pathologies, periwound fibrosis, tissue edema, and decreased perfusion, [66] resulting in a hypoxic microenvironment that leads to delayed wound healing. [55]At the molecular level, O 2 is essential for the cellular synthesis of adenosine triphosphate (ATP) by mitochondrial oxidative phosphorylation.Tissue hypoxia impairs mitochondrial oxidative phosphorylation and ATP production and may lead to a loss of tissue function and cell death.The process of wound healing is divided into four main overlapping phases: hemostasis, inflammatory, proliferative, and tissue remodeling phases. [1] 2 is essential to all processes, and all chronic hypoxia-induced diseases causing abnormal trauma healing processes include epithelial and stromal cell depletion, chronic inflammation, angiogenesis inhibition, ECM component imbalance, and impaired keratin-forming cell function. [31]Although acute hypoxia transiently promotes cell proliferation and migration, chronic hypoxia ultimately leads to poor trauma-healing outcomes. [31]

Roles of Oxygen in Wound Healing
Wound oxygenation levels are usually measured as pO 2 .Arterial blood is %100 mmHg of O 2 , [50] while normal skin usually has a pO 2 of 30-50 mmHg.However, nonhealing chronic wounds may have pO 2 levels as low as 5-20 mmHg (with diabetic wounds having pO 2 levels below 5 mmHg). [67,68]Because of disrupted blood vessels, circulating O 2 to reach the chronic wound is difficult, and atmospheric O 2 can only penetrate %0.3 mm of tissue.Consequently, exposed chronic wounds are usually covered by hypoxic tissue with necrosis, persistent inflammation, and anaerobic infection. [67]Important enzymes for wound healing require pO 2 levels between 25 and 100 mmHg to exert their catalytic activity. [37]Neutrophils are unable to kill bacteria when the partial pressure of O 2 is below 40 mmHg. [50]Fibroblasts can only proliferate actively at O 2 partial pressures greater than 15 mmHg, and collagen formation also requires 30-40 mmHg pO 2 levels. [34]herefore, treatments to improve hemodynamics and increase wound oxygenation have great significance for chronic wound repair.The significance of O 2 in all stages of wound healing is illustrated in Figure 2.

Gaseous Oxygen Therapy
Current therapies that utilize direct O 2 include hyperbaric oxygen and topical oxygen therapies (HBOT and TOT, respectively).
HBOT has been first introduced by Henshaw in 1662 [69] and has since become a common treatment for chronic wounds of various origins, such as diabetic foot ulcers (DFUs), [70,71] refractory venous ulcers (RVU), [72,73] complex wounds, [74] terminal radiation injury, [75] damaged skin flaps, and ischemic reperfusion disorders. [76]HBOT involves delivering systemic O 2 to the entire body at a pressure equal to or exceeding 1.4 atm, typically 2-3 atm.The mechanisms of hyperbaric O 2 therapy include 1) antimicrobial adjuvants, whereby HBOT upregulates the expression of NOS and virucidal peptides (defensins and histones, such as histidine-associated antimicrobial peptides), generates ROS and reactive nitrogen species (RNS), and promotes microorganism phagocytosis by neutrophils and monocytes. [77]) HBOT can improve blood circulation, reduce inflammatory reactions, promote cell proliferation and angiogenesis, and improve wound remodeling. [75,78]owever, the use of HBOT as a supplementary treatment for chronic wounds is a topic of debate in the medical community.This treatment may result in various complications, including central nervous system and pulmonary O 2 toxicity, ocular side effects, [75] air embolism, severe anemia, and idiopathic sensory hearing loss. [79]Notably, HBOT cannot improve local ischemia in the wound or tissue O 2 diffusion and cannot achieve sustained oxygenation.Once patients discontinue HBOT, oxygenation levels will return to baseline within minutes. [80]he use of TOT as an alternative O 2 therapy to HBOT offers several advantages, including low cost and no systemic O 2 toxicity. [74]Micro-/nanobubbles (MNB) are microbubbles in fluids, with sizes ranging from 100 μm to <1 μm, that provide a new and relatively stable mode of O 2 release for wounds as TOT. [36,81]MNBs have several advantages, including high stability in water, slow increase, and high affinity for O 2 , which significantly increase the solubility of O 2 in their production fluid (up to 800 mmHg of O 2 pressure) compared to that in HBOT.A shear generation system is used to produce O 2 -loaded MNBs in saline. [82]O 2 nanobubble water (O 2 NBW) used in TOT significantly improves collagen tissue, hair regeneration, and re-epithelialization in ischemic wounds. [83]However, both HBOT and TOT cannot achieve significant and effective wound healing because of their low O 2 solubility, poor tissue permeability, and difficulty in maintaining effective O 2 concentrations. [46]

Oxygen-Releasing Biomaterials
To address the limitations of direct O 2 therapy, the creation of O 2 delivery materials that offer control, long-lasting, and intelligently released O 2 is of great significance for wound repair and has been a frontier trend in recent years.The ideal state involved in dynamic O 2 generation systems for nanomaterials includes 1) an adequate supply of O 2 generation, 2) an appropriate rate of O 2 production, and 3) nanomaterial safety. [84,85]The O 2 release potential of diverse materials has been extensively explored, ranging from O 2 -carrying carriers to the chemical decomposition of H 2 O 2 to photosynthesis by microalgae. [80,86]he controlled release and effective delivery of gas are achieved by physicochemically sensitive materials, such as light response, [31,[87][88][89] pH-response, [90,91] and radial extracorporeal shockwave (rESW)-response materials. [92]These advancements offer potential for further clinical translation of GT.
O 2 -releasing materials are broadly divided into two groups.The first group is O 2 -generating materials, which include inorganic peroxide materials, in situ catalytic H 2 O 2 enzymes/ nanomaterials with enzymatic activity, and biological O 2 production systems.The second group is O 2 -carrying materials, which include two major classes of perfluorocarbon (PFC)-based O 2carrying scaffolds and hemoglobin O 2 -carrying materials.We will provide a detailed classification of material donors, properties, representative examples of O 2 -releasing biomaterials, and recent advances in three subsections.

Oxygen-Generating Biomaterials
The hypoxic microenvironment of chronic wounds can lead to excessive accumulation of ROS, resulting in oxidative stress and inflammation.Therefore, an in situ O 2 generation approach is highly desirable.One commonly employed strategy is to catalyze endogenous H 2 O 2 by enzymes or enzyme-mimicking active nanomaterials simultaneously. [93]This strategy includes the use of 1) inorganic peroxide-based materials, 2) biological O 2 production systems, and 3) catalase (CAT)-based O 2 generation scaffolds. [84]aterials for Inorganic Peroxides: Inorganic peroxide-based materials include sodium percarbonate (SPC), [94] sodium carbonate (SPO), [95] calcium peroxide (CPO), [95] and magnesium peroxide (MgO 2 ).Among these materials, CPO has a long history of application as an O 2 -generating compound, which can directly react with water and quickly release O 2 according to the following chemical equation: 2CaO However, the hydration of solid peroxides produces oxygen too rapidly, causing excessively high O 2 conditions that produce H 2 O 2 as a reaction intermediate and increase side reaction sensitivity.[98] CPO-based O 2 -generating particles have been incorporated into 3D scaffolds of poly (d,l-lactide-co-glycolide) (PLGA) to improve cell viability. [99]High-pressure O 2 -generation hydrogels comprising thiolated gelatin (Gel) can form a hydrogel network with O 2 generation in a calcium peroxide-mediated oxidative crosslinking reaction. [78]This network enhances the cell proliferation activity of human fibroblasts and endothelial cells and promotes wound healing. [78]Films composed of chitosan (CS) and gel polymers with CPO reached maximum O 2 release values on the first day and gradually approached constant values over 10 days, thereby enhancing antibacterial activity against Escherichia coli (E.coli) and promoting fibroblast growth. [97]he O 2 -generating antioxidant scaffolds antioxidant polyurethane (PUAO)-CPO, which is composed of PUAO cryogels and CPO, has the potential to increase the survival of perfused tissue in an ischemia flap model. [98]Collagen/CS scaffolds coated with CPO and loaded with ciprofloxacin demonstrated durable, enhanced, and sustained O 2 release over 10 days to promote wound healing. [96]Antimicrobial nanofibers loaded with calcium peroxide in poly(ε-caprolactone) (PCL) were used for antimicrobial therapy. [100]To improve O 2 release time, calcium peroxide was encapsulated in polydimethylsiloxane (PDMS)-CPO. [60]Owing to the high hydrophobicity of PDMS, the reactivity of CPO with water was significantly reduced, thereby preventing O 2 burst release and consequently regulating O 2 release for more than 40 days. [60]SPC is a mild water-soluble salt that decomposes to release H 2 O 2 and eventually O 2 . [101]PC-doped PLGA membranes reduced tissue necrosis and apoptosis in a mouse ischemia model. [101]In a porcine full-thickness skin trauma model, SPO/CPO dressings were found to accelerate wound healing, although these dressings only provided O 2 release for three days and needed frequent replacement. [95]lectrospun wound dressings based on PCL and SPC have the potential to promote full-thickness wound repair with a more vascularized and compact ECM for up to 10 days. [94]iological O 2 Production Systems: Biological O 2 production systems, such as microalgae, serve as O 2 -producing materials for light-triggered photosynthesis.In comparison to materials based on other compounds, these materials show excellent compatibility and efficient O 2 production.Additionally, algae are naturally occurring organisms, and their preparation process is relatively simple and cost-effective.[80] In regard to skin entry, microalgae gel patches have superior performance and is more than 100 times more effective than topical gaseous O 2 .[80] In one study, O 2 -producing patches generated from live Synechococcus elongatus (S. elongatus) PCC7942, a single-celled cyanobacterial seaweed gel pellet, were utilized to enhance chronic wound healing in diabetic rats by promoting cell proliferation and migration (Figure 3).[80] The microalgae with PCC7942 are fully active and capable of both respiration and photosynthesis.A diagram of the fabrication of an algae gel patch (AGP) using polyurethane (PU) film and a polytetrafluoroethylene membrane to conduct dissolved O 2 release in reaction to light is shown in Figure 3a.With sufficient light, the dissolved O 2 concentration of the PCC7942 solution grew gradually from 0 μM to over 600 μM in 30 min; in the dark, the concentration decreased from 600 μM to %0 μM in 30 min (Figure 3b).The top limit of dissolved O 2 generation in PCC7942 solutions at 0, 125, 250, and 500 μM Na 2 CO 3 steadily increased from %480 to 1400 μM, indicating that the concentration of inorganic carbon can improve PCC7942 performance (Figure 3b).The application of an AGP resulted in the recovery of the wound and skin flaps (Figure 3c).In a burn wound model, a novel topical hyaluronic acid (HA) biologic gel containing elongated S. elongatus, which produces O 2 through photosynthesis, enhanced tissue oxygenation levels by approximately seven times that of the control and accelerated wound healing.[86] No cyanobacteria were found in the wound beds of the animals, which could be attributed to the self-grooming behavior of the rodents and the ingestion of the gel.[86] However, given their nature as external organisms, algae invariably carry immunogenicity issues.Long-term exposure can also result in host colonization, which raises biosafety concerns.Sunlight exposure also has a negative impact on the hormonal and immune systems of animals.[86] To create a bioactive hydrogel ((berberine (BBR)@SP gel), the researcher used carboxymethyl CS and sodium alginate (SA) loaded with BBR, a quorum-sensing (QS) inhibitor and antibacterial drug, and the naturally occurring microalga Spirulina platensis (SP).[102] The BBR@SP gel suppresses and destroys MRSA biofilm formation and accelerates MRSA-infected diabetic wound healing by promoting angiogenesis and suppressing inflammatory response.[102] Because microalgae are living organisms that require respiration to consume O 2 and light to trigger O 2 production, biological O 2 -generating materials have a limited capacity to generate O 2 . Somemicroalgae could damage the trauma surface, and their colonization of the trauma surface could result in further biosafety issues.
CAT-Based O 2 Production Materials: CAT and its mimetic active materials, such as manganese dioxide (MnO 2 ), [87,88,103,104] nickel sulfide (NiS), [105] carbon dots, [106] molybdenum disulfide (MoS 2 ), [107,108] Fe 3 O 4 nanoparticles, [91] and cerium dioxide nanoparticles, [109] exhibit CAT-like and peroxidase (POD) activities and can be employed in cascade catalytic reactions.These nanoplatforms function by decomposing exogenous/endogenous H 2 O 2 to generate O 2 , with the main difference depending on the type of nanocarriers and multiple trauma models.Natural CAT, a biosafe natural strategy for producing O 2 , can hydrolyze exogenous or in situ H 2 O 2 to produce O 2 and provide sufficient O 2 for wound healing.A study designed an O 2 release system that consisted of PLGA, composite microspheres with H 2 O 2 and poly(2-vinlypyrridione), CAT, and an injectable thermosensitive hydrogel.This system was able to sustain O 2 release for at least two weeks, which enhanced cardiac stem cell therapy. [110]owever, the activity and stability of natural enzymes are significantly influenced by pH and temperature, which limit their ability to achieve a long-lasting catalytic effect.For more efficient and stable O 2 release, metal compounds with catalytic properties were manufactured.
The in situ production of O 2 from H 2 O 2 can be catalyzed by MnO 2 .For instance, to build porous Dex-SA-AEMA/MnO 2 / polydopamine (PDA) hydrogels by an amidation reaction, PDA and MnO 2 were combined with carboxylated dextran (Dex-SA) and 2-aminoethyl methacrylate hydrochloride (AEMA). [103]At the wound surface, MnO 2 nanosheets and a dual-net hydrogel made of natural biomaterials, such as silk fibroin (SF) and carboxymethyl CS (CMC), catalyze the conversion of excess ROS to O 2 , achieving higher O 2 levels (17-fold) and faster wound healing compared to that with a commercial 3M transparent pressurized film dressing. [103]To treat multidrugresistant (MDR) bacterial-infected diabetic wounds with accelerated healing in vivo, ε-polylysine-coated MnO 2 nanosheets were prepared with insulin-loaded aldehyde Pluronic F127 to form FEMI hydrogels with antibacterial and hyperglycemic characteristics, as well as reduced ROS and O 2 production. [104]Bioinspired MnO 2 hybrid (BMH) hydrogels were prepared by noncovalent self-assembly of catechol-functionalized CS and covalent oxidative polymerization of the catechol-functionalized CS mediated by MnO 2 nanosheets. [88]Excellent shear-thinning, injectable, redox/light sensitive, and contact-active antibacterial properties were demonstrated by the BMH hydrogel.In vivo studies have shown that these hydrogels significantly promote wound healing in MDR bacterial infections. [88]Other studies have designed metal-organic framework (MOF)-derived nanozymes, which consist of HA and MOF-derived POD-mimetic nanozymes (ε-polylysine-coated MnCoO nanozymes), to promote diabetic wound healing. [111]These nanozymes capture endogenously elevated ROS into O 2 , promoting macrophage anti-inflammatory (M2)-type differentiation. [111]Nickel sulfide possesses a special yolk-like structure and optimizes light utilization [105] ; moreover, it exhibits POD activity, perfect photothermal conversion efficiency, and excellent biocompatibility.The combination of Ag@NiS 2Àx with polyacrylamide and CS results in a composite antibacterial hydrogel that promotes wound healing by providing O 2 to newly formed granulation tissue. [105]dditionally, the utilization of H 2 O 2 -based glucose oxidase (GO x )/CAT (nanozyme) cascade-reaction materials shows potential for addressing the issues of insufficient endogenous H 2 O 2 and limited enzymatic activity of individual nanozymes.These materials are applied to diabetic wounds through the consumption of glucose by GO x to produce H 2 O 2 .Subsequently, CAT further catalyzes the cascade reaction of H 2 O 2 to O 2 , thus eliminating the hyperglycemic state of diabetic wounds and providing continuous O 2 . [46]For example, GO x , CAT, and CS (GCNC) hydrogel complexes supply O 2 continuously to aid in the healing of wounds. [46]Gold nanoparticles (Au-NPs) also were used to mimic natural GO x and catalyze glucose oxidation.2D materials such as molybdenum disulfide nanosheets (MoS 2 NSs) exhibit POD-like activity in acidic environments and SOD and CAT-like activity in neutral environments. [107]When defect-rich MoS 2 NSs loaded with bovine serum albumin-modified Au-NPs (MoS 2 @Au@BSA-NS) and oxidized dextran are crosslinked with glycol CS Schiff bases to form injectable hydrogels, they catalyze  [80] Copyright 2020, The Authors, some rights reserved; exclusive licensee AAAS.Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).
O 2 self-supplied glucose-powered cascade reactions that deplete endogenous and exogenous H 2 O 2 for O 2 -supplied antibacterial activity.GO x -like Au catalyzes the oxidation of glucose into gluconic acid and H 2 O, which is transformed into a hydroxyl radical (•OH) catalyzed by POD-like MoS 2 @Au@BSA to eradicate bacteria.MoS 2 @Au@BSA mimics SOD to transform superoxide anions into O 2 and H 2 O 2 , and decomposes endogenous and exogenous H 2 O 2 into O 2 via CAT-like mechanisms, thereby reducing oxidative stress, alleviating hypoxia, and facilitating glucose oxidation when the pH reaches an alkaline condition.This alleviates hypoxia and promotes epithelialization, collagen deposition, and angiogenesis for diabetic wound healing (Figure 4a). [107]MoS 2 NSs with triple enzyme-like activities (POD, CAT, and SOD) were loaded onto carbon nanotubes (CNTs) and combined with dynamic crosslinked hydrogels constructed with polyvinyl alcohol (PVA), SA, and borax. [108]The notable antibacterial effectiveness of the resulting hydrogel is ascribed to its POD-like activity, which under acidic conditions catalyzes H 2 O 2 into hydroxyl free radicals (•OH), results in GSH loss, and achieves photothermal treatment.The hydrogel is crosslinked by dynamic boron ester linkages.The adhesiveness, selfhealing, and shape-adaptivity of the multifunctional hydrogel allow it to fill the cavity of irregular wounds and encourage the nanozyme to perform its function as effectively as possible.
The nanosheets accelerate collagen deposition, downregulate the expression of inflammatory factors and ROS content, and upregulate the levels of angiogenic factors to promote skin regeneration (Figure 4b). [108]The US Food and Drug Administration (FDA) has granted permission for the use of Fe 3 O 4 NPs, which exhibit CAT and POD-like activities and catalyze pH-switchable glucose-initiated GO x /POD and GO x /CAT cascade reactions in acidic and neutral environments, respectively.Glucose oxidase (Fe 3 O 4 GO x ) shell-coated nanoparticles exhibit pH-responsive hypoglycemic, antibacterial, and oxygenation properties. [91]his nanoparticle targets and eliminates acidic biofilms (pH 5.5) to reduce inflammation and accelerate the natural healing process for wounds.The nanoparticle specifically targets neutral wound tissue, reducing hyperglycemia, hypoxia, and excessive oxidative stress.Furthermore, the GO x /CAT cascade reaction producing consecutive fluxes of oxygen spatially targets the neutral wound tissue and accelerates the proliferation and remodeling phases of wound repair (Figure 4c). [91]Alendronic acid and 2-methylimidazole formed dual-ligand molecules that were included in Ce-driven coassembly to establish a nanoparticle CHA and embed GO x to form nanozymes CHA@GO x with multiple enzyme activities. [109]The nanozymes can catalyze the extra H 2 O 2 resulting from the glucose oxidation reaction to produce O 2 , regulate the O 2 level in the wound, and lessen the toxic effects Figure 4. a) Synthesis of MoS 2 @Au@BSA nanosheets to reconstruct infected diabetic skin with glucose-powered cascade reaction for oxygen supply.Reproduced with permission. [107]Copyright 2022, Wiley-VCH GmbH.b) Schematic diagram of the triple nanozyme activities of CNT enhanced by NIR.Reproduced with permission. [108]Copyright 2021, WileysVCH GmbH.c) Diagram of the Fe 3 O 4 NPs designed to directly recover wound healing through spatial-temporal regulation of the wound microenvironment.Reproduced with permission under the terms of the Creative Commons CC BY license. [91]opyright 2022, the Authors.Published by Springer Nature.d) Schematic of the synthesis processes for CHA@GO x and nanozyme for diabetic wound healing.Reproduced with permission. [109]Copyright 2022, Elsevier Ltd.
of GO x .The chemical reaction of CHA@GO x is shown in Figure 4d.This enzyme promotes angiogenesis, collagen deposition, and re-epithelialization during wound healing in diabetic mice (Figure 4d). [109]Additionally, metal-free carbon dots, which display SOD, CAT-like, and POD-like activities, lessen liver inflammation. [106]ontinuous catalytic activity and long-term robustness are essential for nanocatalysts.However, the effectiveness of continuous ROS scavenging and O 2 production is diminished in metalor metal oxide-based enzyme nanoparticles because of their quick pH-responsive degradation. [111]Additionally, these materials display unpredictable biotoxicity and produce a worrisome range of byproducts.To improve material biosafety and reduce byproducts, building reliable and effective enzyme systems is necessary, and future researchers should concentrate on achieving superior enzyme stability and cascade efficiency in such systems. [46]

Oxygen-Carrying Biomaterials
Topical O 2 therapy requires the development of carriers that can release therapeutically significant amounts of total O 2 in an efficient and time-dependent manner.Hemoglobin-based O 2 carriers and PFC-based carriers are the two main types of O 2 -carrying carriers.The US FDA has licensed PFCs for use in select surgical procedures to increase blood oxygenation. [112]hese artificial O 2 carriers exhibit distinct properties in terms of O 2 uptake, release, and delivery.
Hemoglobin (Hb): Hemoglobin (Hb) is a metalloprotein that serves as physiological transport of O 2 in virtually all vertebrate erythrocytes.Hb is composed of four globular polypeptide subunits (α1, β1, α2, and β2), and its ability to transport O 2 is attributed to the heme group Fe 2þ to carry an O 2 molecule. [113]urrently, the unique O 2 transport capacity of Hb, which has been carried out as microneedles (MNs) [114] and hydrogels, [31] has generated significant interest in its application for wound treatment.
A study found that thermally responsive MNs can be produced with a backing layer of soluble PVA and tips composed of GelMA loaded with black phosphorus quantum dots (BP QDs) and Hb for wound healing. [114]Utilizing the quick dissolvability of PVA, the backing layer quickly dissolves after the MNs are applied to the skin, leaving the noncytotoxic, biocompatible GelMA tips in the skin.When exposed to NIR, BP QDs could quickly transform the light energy into heat to raise the local temperature, which sequentially decreased the capacity of Hb to bind O 2 and resulted in the controlled release of O 2 .MNs showed excellent wound healing capacity in treating the full-thickness wounds of a type I diabetes rat model (Figure 5a). [114]Hydrogels composed of hyaluronic acid-grafted dopamine (HA-DA) and PDA-coated Ti 3 C 2 Mxene nanosheet (Ti 3 C 2 @PDA or Mxene@PDANSs) were also generated by the oxidative coupling of catechol moieties catalyzed by the oxyhemoglobin (HbO 2 )/ H 2 O 2 system and combined with NIR light stimulation for the controlled release of HbO 2 . [31]This hydrogel allows for oxygenation of the wound even after the light source was switched off by adsorbing O 2 from the atmosphere, resulting in reproducible and effective oxygenation.The hydrogel also promoted M2 macrophage polarization, making it a promising treatment for diabetic wounds (Figure 5b). [31]Hb can be coupled to carboxylic acids on GelMa inverse opal particles via 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide chemistry. [115]he resulting microcarriers could carry O 2 extensively and facilitate tissue repair physically and biologically due to their distinctive porosity structure, interconnected nanochannels, and remarkable biocompatibility.In addition, due to the usual photothermal effect of 2D materials and the 2D QDs that are formed from them, the inverse opal particles combined with MoS 2 QDs are endowed with photoresponsive capability, enabling them to release O 2 photocontrollably for wound repair (Figure 5c). [115]FCs: PFCs are widely used for gas transport owing to their inertness, high O 2 affinity, and small size.[116] PFCs exhibit a positive correlation between pO 2 and O 2 concentration.The O 2 solubility of PFT is 35 mM, which is 20 times higher than that of water.[117] Compared to Hb, PFCs typically carry a greater proportion of O 2 that can be released into tissues, following an O 2 dissociation profile.[116] 2H,3H-decafluoropentane (DFP), perfluorooctyl bromide (PFOB), and other PFCs are frequently employed in combination with polymers and administered as aqueous emulsion systems for trauma treatment.
PFCs such as perfluorotributylamine (PFTBA) are used to enhance O 2 availability in hydrogels, thereby improving cell survival. [117]PFCs have been attached to methacrylamide CS to form photocrosslinked fluorinated methacrylamide CS (MACF), which could be reloaded with O 2 for regeneration.Even without oxygenation, O 2 delivery can still be facilitated, thus promoting wound healing in vivo. [112]Topical MACF application resulted in a pO 2 of 233.8 AE 9.9 mmHg after 2 h, which was %17 mmHg higher than that of the control group.MACF also significantly enhanced fibroblast and keratin-forming cell ATP levels. [118]egulating the balance of the macrophage phenotype and promoting elevated macrophage levels [96] promote wound healing by enhancing epithelialization and collagen synthesis. [93]everal studies have prepared erythrocyte-derived membranecoated PFC nanoemulsions of natural synthetic O 2 delivery carriers for long-term storage.These nanoemulsions exhibit high O 2 delivery capacity to help reduce hypoxia in vitro, with proven effects in animal models of hemorrhagic shock [111] to increase biocompatibility and decrease immunogenicity. [116] 2 -loaded nanodroplets (OLNs) composed of 2H, 3H-decafluoropentane (DFP), ascorbic acid, fluorinated carbon, and dextran or CS continuously released O 2 into the hypoxic environment.[45] This reversed the imbalance between MMP-2 and TIMP-1/2 caused by hypoxia, thereby enhancing migration and ECM invasion.[45] The hypoxic impact was reversed by DFP CS-loaded OLNs.[119] Excellent treatment of diabetic wounds was achieved with phosphatidylcholine (OXY-PFOB-NE) encapsulated by 1-bromoperfluorooctane (PFOB) and quercetin (QCN)-loaded nanoemulsion (QCN-NE).[120] In an acute trauma model, diabetic chronic trauma model, and skin flap graft model, perfluorododecane-encapsulated albumin nanoparticles (FDC@HSA) in combination with HA gel reduced cellular hypoxia, accelerated healing, and promoted graft survival.[67] The targeted release of O 2 into the wound by O 2 -carrying nano-PFCs was activated by the nanoradial PFCs in vitro with rESW-sensitive O 2 release characteristics, and wound healing was significantly accelerated in DFUs (Figure 5d). Hb also has several drawbacks, such as high O 2 concentration, poor stability, short circulation time, and easy catabolism. Although O 2 -carrying donors have a high O 2 release capability, the amount of O 2 they create must be regulated considering that excessive O 2 can cause cellular damage. [121]Therefore, the main purpose of designing O 2 donor materials in the future is to guarantee that the materials exhibit a controlled, long-term, stable, and low level of biotoxicity.
Growing evidence has shown that O 2 is essential for proper wound healing.However, different stages of wound healing have dynamic O 2 requirements.Evidently, the mission of researchers and clinicians is to provide the right O 2 transport system to the right patient at the right time.The risk tolerance of the patient and the expectations for treatment should be considered when using O 2 therapy, as with any advanced wound therapy.The development of a novel, highly effective O 2 transport system to improve therapeutic effect is urgently needed.

Nitric Oxide (NO) Therapy
NO, named as "molecule of the year" in 1992, is an endogenous messenger molecule that is synthesized by NOS catalyzing the oxidation of L-arginine (L-Arg). [122]NOS has three different isoforms: neuronal (nNOS), endothelial (eNOS), and inducible (iNOS) (Figure 11). [123]nNOS and eNOS can produce low levels of NO to regulate physiological processes, such as angiogenesis, vasodilation, control of blood pressure, and nerve conduction. [23]y contrast, iNOS can produce higher levels of NO in macrophages and neutrophils in response to foreign substances and pathogens, contributing to the pathophysiology of inflammatory diseases and infectious shock. [23,124]NO has been used alone or  [114] Copyright 2020, American Chemical Society.b) Schematic of the preparation and mechanism of HA-DA/Mxene@PDA hydrogel.Reproduced with permission. [31]Copyright 2022, Wiley-VCH.c) The light-responsive MoS 2 QDs deliver O 2 and heal wounds.Reproduced with permission. [115]Copyright 2019, American Chemical Society.d) Schematic of the fabrication of and rESW-responsive O 2 release from nano-PFC.Analyzing the blood circulation of the animal model after therapy.Quantitative evaluation of blood flow in a rat forepaw as measured by laser Doppler imaging.Reproduced with permission under the terms of the Creative Commons CC BY license. [92]Copyright 2019, the Authors.Published by Hindawi.
in combination with other materials as an emerging strategy for treating oncological, cardiovascular, and traumatic diseases.Due to its unique role in modulating inflammatory response, [23] antimicrobial activity, [6,124] and pro-angiogenic [23,60] and exogenous collagen synthesis abilities, [39] NO has also been investigated for use in an effective strategy for wound repair, especially in infected chronic wounds.

Roles of NO in Wound Healing
NO is closely related to wound healing, playing a crucial role in regulating inflammatory response, immune response, angiogenesis, cellular metabolism, ECM formation, and remodeling. [125]he effectiveness of NO therapy depends on its effective concentration in the wound.Low concentrations of NO (1 μM to 1 mM) act as signaling molecules and bind the soluble guanylate cyclase (sGC), which converts guanosine 5-triphosphate (GTP) to cGMP to regulate the PKG pathway.When activated, the angiogenic signaling regulator eNOS-derived NO participates in angiogenesis by regulating VEGF expression through the NO/cGMP/ adenosine monophosphate-activated protein kinase (AMPK) cascade, which increases the circulating GMP in smooth muscle cells to dilate blood vessels. [23,60]This promotes the growth and activity of immune cells and accelerates wound healing while causing minimal or no adverse effects.In contrast, high concentrations of NO (>1 mM) during the respiratory burst of neutrophils result in mitochondria dysfunction, enzyme inactivation, and lipid peroxidation to kill pathogens. [123,126]NO production at the diabetic wound site is diminished, thereby impeding the normal healing of wounds.The mechanism of NO at various phases of wound healing is shown in Figure 2.

NO-Releasing Biomaterials
Reduced NO production is a characteristic topic in chronic wounds.Therefore, enhancing the local delivery of exogenous NO and increasing endogenous NO production in chronic wounds have emerged as the primary NO therapeutic strategies.However, due to the extremely short biological duration of gaseous NO, its exogenous delivery is frequently hindered in trauma repair.Moreover, NO has several deficiencies such as a short half-life, chemical instability, uncertain complications and long-term effects, and the risk of carcinogenesis or cytotoxicity. [127]The ideal NO release platform should have the following qualities: 1) the ability to maintain stable NO concentrations in the trabeculae; 2) controlled release properties; and 3) excellent biocompatibility and nontoxicity of the carrier itself and byproducts.Various NO-carrying platforms (containing NO or NO donor systems) and NO-indirect treatment strategies (addition of "increments" such as Arg, which enhances NOS expression) have been developed.NO-releasing materials, NO storage, or the loading of NO donors can be combined with different biocompatible materials through ligand interactions, covalent bonding, doping, or other methods.These materials with the controlled triggered release of NO through responses to enzymes, [128,129] light, [130][131][132][133][134] temperature, [135] wound exudate, [136] and wound microacidic environment [137] were designed to efficiently and precisely deliver and release the NO platform for infected wound repair therapy. [5]In addition to NO GT alone, combination therapies including medicines, PDT, PTT, [132,133] or other gases (such as oxygen [32] ) can result in adaptable and powerful prorepair antibacterial therapy.

NO-Carrying Delivery Materials
Several low-molecular-weight compounds known as NO donors, which produce NO by hydrolysis, commonly include nitrosamines (such as N,N 0 -di-sec-butyl-N,N 0 -dinitroso-1,4-phenylenediamine (BNN6) [131][132][133] ), S-nitrosothiols (RSNO) [7,125,136,[138][139][140][141][142][143] (such as S-nitroso-N-acetyl-D-penicillamine (SNAP), [6,[144][145][146][147] Snitrosoglutathione (GSNO) [7,136,[138][139][140][141][142] ), diazeniumdiolate (NONOate), [129,[148][149][150] and metal-NO complexes (such as sodium nitroprusside [134] ).These compounds are widely applied in trauma repair as common materials to provide exogenous NO.They release and transport nitroso ions and their associated signaling molecules, such as RSNO. [147]irect NO Delivery Material: Asiaticoside NO gel has been found to modulate the Wnt/β-catenin signaling pathway to promote healing in rats with diabetic skin ulcers. [151]The fabrication of enzyme-responsive caged NO hydrogels encasing gaseous NO has been investigated.Intermolecular hydrogels have been prepared by combining Nap-FFGGG short peptide with caged NO molecules that are responsive to β-galactosidase.The inclusion of β-galactosidase removes the sugar-capped end groups of the NO molecules, thus achieving controlled NO release.This molecular hydrogel can continuously release NO at a controlled speed by regulating the amount of β-galactosidase in the system. [128]hen applied to fibroblasts, PVA NO-carrying hydrogels increase the synthesis of ECM, granulation tissue thickness in a diabetic mouse model of impaired wound healing, and scar tissue thickness following wound closure. [39]The potential of mesenchymal stem cells (MSCs) to stimulate angiogenesis is enhanced by the application of a gel hydrogel that releases NO following crosslinking via the transglutaminase reaction. [152] cold Poloxamer-407 (P 407 ) forms hydrogels (PNO) encapsulating NO MBs upon temperature alteration and regulates DFU inflammation accompanied by a decrease in proinflammatory cytokines (IL-1β, IL-6, and tumor necrosis factor α (TNF-α)), an increase in anti-inflammatory cytokines (IL-10, IL-22, and IL-13) levels, and proangiogenesis. [135]etal-Based NO Donors: Nitrite is a widely employed NO donor.The novel bactericidal tandem known as AB569 is composed of Na 2 -EDTA and NaNO 2 (A-NO 2 À ). [153]A-NO 2 À mediated NO production has shown excellent antimicrobial efficacy against P. aeurginosa in the treatment of a murine model of burn wound infection. [153]Surface-active NO donors include Ru nitrosyl and Mn nitrosyl (NO complexes).Owing to their swelling ability, oxygen permeability, and biocompatibility, hydrogels composed of poly(2-hydroxyethyl methacrylate) (pHEMA) have been extensively researched for biomedical applications.A HEMA-based polymer hydrogel has been modified to include a light-activated NO donor,[Mn(PaPy 3 ) (NO)]ClO 4 .A hybrid material with a pHEMA core, a PU covering, and a light-activated NO donor was fabricated as controlled-release wound dressings since nitrosyl only releases NO when exposed to light. [130]The dressing can therefore occasionally be illuminated to supply NO just to the wound area and maintain antiseptic conditions. [130]By transforming mononuclear {Fe(NO) 2 } 9 and dinuclear {Fe(NO) 2 } 9 -{Fe(NO) 2 } 9 dinitrosyl iron complexes (DNICs) in the low-molecular-weight and protein-bound forms, synthetic DNICs were created (Figure 6a). [154]The[Fe(NO) 2 ] À / NO delivery synthetic DNICs scaffold is capable of regulating NO in space, time, and dose and transferring the physiological activity of NO into biomedical applications. [154]odium nitroprusside (SNP) is another NO donor.MOF encapsulates SNP, Au acts as a photosensitizer on the MOF surface, and HOOC-PEG 5000 -Mal is attached to the outermost layer to form SNP@MOF@Au-Mal. [134] This nanogenerator releases NO through photothermal activity and targets Gram-negative bacterial-type IV pili (T4P) via maleimide to achieve precise antibacterial effects.The benefit of this strategy is its production of a high concentration of NO, which encourages the transfer of additional NO and its derivatives (N 2 O 3 , ONOO À ) to bacteria, thereby considerably enhancing the antibacterial impact.In vivo tests demonstrated a 97.7% reduction in the bacterial burden on the wound using this approach.The nanogenerator has also been demonstrated to encourage the release of growth factors, which are crucial for controlling inflammation and activating angiogenesis (Figure 6d). [134]Nitrosamine: BNN6 is one of the bis-N-nitroso compounds that are commonly used as NO donors with good thermal stability.The donor release of NO can be triggered by NIR and produce additional NO at low biotoxicity. [131]The combination of photothermal therapy and GT is expected to enhance the effectiveness of antibiotics and decrease drug resistance.NIR can induce BNN6 donors to release NO, increasing their antibacterial and prorepair properties. [131]Poly-dopamine nanosheets (PDA NS) were synthesized via a one-pot method by DNA templatemediated PDA polymerization. [132]BNN6 does not respond to NIR (808 nm), and transforming BNN6 into a NIR-responsive photothermal agent through π-π stacking interactions of PDA triggers NO release.As an amphiphilic triblock copolymer, F127-CHO possibly functions as a crosslinker and self-assembles into micelles in water.BNN6 2D polydopamine nanosheets (PDA NSs) were loaded with hydrazide-modified γ-polyglutamic acid (γ -PGA-ADH) and aldehyde-terminated Pluronic F127 through in situ dynamic Schiff bond and Pluronic F127 micellization, which could yield hydrogels with optimal temperature responsiveness and self-healing (Figure 6b).A synergistic antibacterial effect is produced when NO therapy is used in conjunction with PDA NS photothermal activity (Figure 6b). [132]NN6 was loaded into a NIR-responsive photothermal agent polydopamine (MPDA) via π-π stacking interactions form BNN6-loaded polydopamine nanoparticles (MPDA-BNN6 NPs).[133] Soluble fibrinogen is converted into insoluble fibrin gel after fibrinopeptides are resected from fibrinogen under the action of thrombin.This work combined MPDA-BNN6 NPs with a fibronectin-based hydrogel to produce (MPDA-BNN6@Gel) hydrogels.[133] Under NIR MPDA-BNN6@gel-treated MRSA infection, the MPDA-BNN6@gel effectively triggered the release of NO and caused hyperthermia, which disrupted the energy metabolism and damaged the cell membrane and genetic material, further inactivating the bacteria.In the absence of light, the release of a small amounts of NO gas promoted wound healing (Figure 6c).[133] A novel N-nitrosoaniline NO donor was designed based on 7-amino-4-methyl coumarin and prepared via the homogeneous breakage of the N─NO bond to achieve phototriggered NO release. [155]Phototriggered NO donors offer advantages such as easy synthesis, good stability, and controllable release compared to the other NO donors such as NONOate and RSNO compounds.The donors can accelerate the migration and proliferation of human umbilical vein vascular endothelial cells, thus assisting in angiogenesis.Moreover, the NO donor with levofloxacin showed synergistic effects to eradicate MRSA biofilms as a treatment for bacteria-infected wounds in vivo.[155] RSNOs: The intracellular signaling of NO involves the formation of RSNOs as an intermediate step.Several naturally existing S-thiols are present in tissues and blood, including GSNO, S-nitrosoalbumin, and S-nitrosohemoglobin, which are all safe as endogenous substances.Among these thiols, GSNO is the most commonly used as a NO carrier.
GSNO is an endogenous NO carrier found in mammals.It releases NO slowly according to the following dimerization reaction: 2 GSNO !GS-SG þ 2 NO. [136]GSNO directly applied to flap animal models has a higher rate of wound contraction and epithelialization, a lower amount of inflammatory cells, and increased collagen fiber density and histochemistry. [125]In a rat ischemia model, a membrane made of PVA and GSNO and incorporated into a hydrogel composed of Pluronic F127 was found to gradually release NO, resulting in increased collagen fibril maturation, accelerated wound contraction and re-epithelialization, and reduced inflammation. [138]In MRSAinfected diabetic wounds, CS membranes containing GSNO (CS/NO membranes) reduced wound size while increasing epithelialization rate and collagen deposition. [7]The integration of GSNO within supramolecular hydrogels can trigger NO release from wound exudate, thereby promoting angiogenesis and organizing collagen fibrils. [136]The excellent compressibility of GSNO-loaded hydrogels prepared by 3D printing containing cellulose nanocrystals (CNCs) and polyacrylic acid (PAA)/F127 should be exploited in the field of trauma repair. [139]raphene oxide (GO) and PVA are combined to generate novel hydrogel-forming MNs (HFMNs) called GO-GNSO-HFMNs when frozen.The photothermal effect of GO in NIR enhances the release of NO from GNSO, which not only inhibits MRSA biofilm formation but also boosts tissue regeneration by disrupting the biofilm structure using MN structures. [141]The CMC/ sodium alginate (Alg) porous structure (composite loaded with GSNO (CMC Alg GSNO) promoted diabetic wound healing in a rat model. [140]In another study, a single-jet electrospinning process was used to create SF-alcoholic protein (ZN)-GSNO composite nanofibers, which exhibited high antibacterial capabilities and biodegradability. [142]-nitrosated keratin (KSNO), an NO donor, is generated by keratin nitrosation.In addition to showing antibacterial capabilities, PU/KSNO biocomposites can continuously release NO for 72 h and enhance tissue regeneration, collagen synthesis, cell migration, epithelialization, and angiogenesis to accelerate wound healing.[143] S-nitroso-N-acetyl-D-penicillamine (SNAP) is a biocompatible NO donor that can release low doses of NO to promote endothelial cell growth.SNAP couples with a peptide-natural ECM scaffold that contains numerous primary amine groups via a carboxylic acid reaction and significantly affects the proliferation, morphology, and adhesion of MSCs on ECM scaffolds.[126] SNAP encapsulated in CS-PVA hydrogels is proven to promote angiogenesis.[144] SNAP bound to highly porous GelMA forms GelMA/SNAP hydrogel patches that promote cell proliferation and migration and diabetic wound healing.[145] Interpenetrating network hydrogel scaffolds containing SNAP based on SF and GelMA nonporous structures exhibit excellent antimicrobial properties and contribute to fibroblast proliferation.[6] Bone marrow MSCs coupled with SNAP-loaded CS and PVA hydrogels enhanced the viability and cytoprotective properties of bone marrow MSCs against H 2 O 2 -damaged cells and promoted vascular regeneration and collagen synthesis in a diabetic rabbit model.[146] CS and PVA hydrogels encapsulating SNAP resulted in threefold and fourfold increase in cell proliferation and cell migration, respectively.[147] NONOate: The most commonly used NO donor in research is NONOate, which is created by reacting secondary amines with superpressurized NO gas.NONOate formation needs the assistance of additional basic residues, such as unreacted amine substrates or metal alkoxide bases, to deprotonate the amine and thus help its nucleophilic attack by NO.7] This compound spontaneously decomposes in solution at physiological pH and temperature to produce two moles of NO molecules without needing specific metabolites or redox mechanisms.[150] To create a PCL/CS-NO dressing with an electrospun PCL pad and NO-carrying polymer (CS-NO) surface coating that is catalyzed by β-galactosidase to control the shift for sustained NO release, NONOates are linked to the side chains of natural CS.[129] The dressing promotes full-thickness skin wounds in mice, reduces swelling, promotes angiogenesis, and boosts collagen synthesis, all of which considerably accelerate the healing process.[129] Alg hydrogel (Alg-DETA/NO), a wound dressing containing diethylenetriamine (DETA)/ NONOate as an NO donor, has been demonstrated to reduce the possibility of DETA toxicity and encourage angiogenesis, re-epithelialization, and collagen deposition.[149] However, NONOate is susceptible to the potential toxicity of its parent compound and metabolic byproducts, particularly the carcinogenic secondary nitrosamines.[148] Using gaseous NO, propylene oxide (PO)-modified polyethylenimine (PEI) and a novel diazeniumdiolate NO donor were prepared.Within the polyethylene glycol (PEG) mixture system, the PEI-PO-NONOate polymer sustained a controlled release profile for more than 30 h and accelerated cutaneous healing and closure with improved granulation tissue development, collagen deposition, and angiogenesis.[158] Owing to risky reaction circumstances (high N 2 O concentrations, flammable solvent), which might result in an explosion, and its constrained scope, the synthesis of sodium NONOates has proven to be challenging.Synthesizing calcium NONOates may be the safer and more scalable alternative.[157] The release of NO in tissues by NO donors is dependent on NO synthase-catalyzed reactions.However, chronic wounds are often characterized by a deficiency in NOS, [135] which impedes the release process.Additionally, harmful byproducts, such as NONOate metabolic side products and poor biocompatibility, limit the availability of NO donors with unrestrained leakage. Thereore, the development of NO-carrying materials with minimal byproducts and excellent biocompatibility is now promising for biological applications.

NO-Indirect Treatment Strategy
Strategies for the Indirect Delivery of NO: Strategies for the indirect delivery of NO are mainly applied in trauma repair to increase exogenous L-Arg or enhance NOS activity.Biomaterials that indirectly deliver NO have the benefits of low toxicity and excellent biocompatibility.Although electrochemical devices have been available for NO production, they have not been employed in wound repair therapy.
L-Arg is an exogenous NO-producing natural resource that can continuously supply NO for healing trauma.For instance, to produce Arg-Lig-NF gels, electrostatic interactions attach arginine molecules to the surface of lignin nanofibers (Lig-NF). [159]ompared to other materials, these gels result in a more rapid boost in NO levels; they promoted collagen synthesis, angiogenesis, and re-epithelialization in a rat full-thickness wound model. [159]Cationic biodegradable Arg-UPEA/glycidyl methacrylate (GMA) CS hybrid hydrogels provide in situ Arg-rich environments that enhance arginase activity in macrophages, resulting in the production of TNF and NO. [160]A positively charged CS-grafted-polyarginine (CS-N-Parg) as the macromolecular NO donor, a negatively charged acetalated starch (AcSt-O-Pasp) as a glucose donor, glucoamylase (GA), and GO x were absorbed and assembled in gel sponges to build smart antimicrobial dressings (CS/St þ GO x /GA) through electrostatic interactions. [137]Glucose, H 2 O 2 , and NO release mechanisms under acidic conditions are shown in Figure 7a.Wound infection by E. coli in an acidic environment stimulates the disruption of CS/St þ GO x /GA acetal bonds, leading to the release of starch by decomposition and a cascade of reactions catalyzed by GO x oxidizing glucose to H 2 O 2 and L-Arg to NO.The antibacterial effects of this system can be achieved by successively inducing the synthesis of glucose, H 2 O 2 , and NO (Figure 7a). [137]OS: Chronic trauma frequently results in the downregulation of NOS, a crucial enzyme for NO production.Therefore, exogenous NOS delivery or upregulation of NOS expression is crucial for trauma repair.In a rabbit ear ulcer model, a fibrin scaffold was used to improve eNOS expression through the delivery of adenovirus encoding endothelial NOS and enhance epithelialization and angiogenesis.[161] Certain medicines, such as statins, decrease vascular oxidative stress, stimulate the phosphatidylinositol 3-kinase/protein kinase B (Akt) pathway to increase eNOS activity, inhibit vascular NADPH oxidase, and prevent eNOS uncoupling through GTP regulation to restore eNOS function.[23] A multifunctional hydrogel with antibacterial, ROS-scavenging, and O 2 -and NO-releasing properties is intended to control the excessive inflammation in rats with diabetic wounds that are infected with MRSA in vivo (Figure 7b).[32] Hydrophilic poly(PEGMA-co-GMA-co-Aam) polymers formed from the radical polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA), GMA, and acrylamide (Aam) were crosslinked with hyperbranched poly-L-lysine-modified MnO 2 nanozymes to produce a multifunctional hydrogel.Pravastatin sodium was further loaded to obtain the HMP hydrogel to  [137] Copyright 2022, Elsevier B.V. b) Production of a hydrogel dressing that can successfully kill MRSA, consume several types of ROS, and produce O 2 and NO.Reproduced with permission.[32] Copyright 2022, Elsevier Ltd. participate in the synthesis of NO. at 10 9 CFU mL À1 , the hydrogels scavenged different types of ROS, generating O 2 , and killed broad-spectrum bacteria up to 94.1-99.5% of P. aeurginosa, E.coli, and MRSA.The compound exhibits antibacterial properties, reduces the inflammatory response, and promotes the M2 polarization of macrophages.Dependence on O 2 and NO production promotes higher levels of angiogenesis and collagen deposition.[32] NO-induced smooth-muscle relaxation is mediated by cGMP, which is degraded via phosphodiesterases. The posphodiesterase type 5 (PDE-5) enzyme inhibitor sildenafil citrate (SC) controls the intracellular and intravascular L-Arg-NO-cGMP pathway by blocking PDE-5 and promoting NO production by NOS.[162] Different SC concentrations with phospholipid (PL) phosphatidylcholine (1:1) solution and triethanolaminesynthesized hydrogels for the treatment of rat skin wounds promoted skin tissue re-epithelialization, collagen synthesis, deposition, and regeneration.[162] Nebivolol (NB) is an antihypertensive medication that reduces diabetic neuropathy and restores neurological function in diabetic wounds by producing vasodilatory effects via the NO route.The ability of NO to restore endothelial function is among the potential strategies for reducing neuropathy and ischemia.The preparation of oil-in-oil emulsification solvent diffusion methods to develop slow-release gels loaded with NB for promoting wound healing in diabetic rats is being studied.
The materials and drugs discussed above indirectly promote NO production and exhibit superior biosafety, a longer NO halflife, and long-term NO storage compared to materials used in direct NO strategies. [127]Thus, they have good biological application prospects.

Hydrogen Sulfide (H 2 S) Therapy
Similarly to NO, H 2 S has been recently identified as an endogenous gaseous signaling molecule that controls intracellular and extracellular functions.Four enzymes, cystathionine β-synthase (CBS), CAT, cystathionine γ-lyase (CSE), and the tandem enzyme 3-mercaptopyruvate sulfurtransferase (3-MST), are considered to be primarily responsible for the endogenous production of H 2 S (Figure 11). [163]As a typical biological gas transport molecule, H 2 S provides indispensable effects in promoting wound healing, such as anti-inflammatory, proproliferative, and proangiogenic. [40]Thus, H 2 S has great therapeutic potential in biomedical research.

Roles of H 2 S in Wound Treatment
H 2 S exhibits a biphasic effect in burns: an initial proinflammatory effect and a subsequent anti-inflammatory effect. [40]H 2 S improves the environment for wound healing, encourages angiogenesis, and regulates the amounts of numerous growth factors. [40]H 2 S can significantly inhibit the growth of bacteria and scavenge ROS.Moreover, it may interact with RSNOs to form thionitrites, which are RSNOs with minimal metabolites, such as NO þ , NO, and NO À . [164]Additionally, H 2 S exerts its unique cytoprotective effects by inhibiting p38-and c-Jun N-terminal kinase-dependent apoptosis and NF-κB-dependent inflammatory pathways. [24]

H 2 S-Releasing Materials
H 2 S GT alone is limited by the short half-life of H 2 S. For example, volatilization causes an abrupt decrease in the concentration of H 2 S in aqueous solutions. [164]Thus, several H 2 S donors have been designed for use in wound healing treatments.In general, three types of H 2 S donors exist: sulfide salts, polysulfides, and synthetic H 2 S donors.Among them, the most commonly utilized are sulfide salts, such as sodium and sodium hydrogen sulfides, which quickly increase H 2 S concentration. [164]However, they are released in bursts, thus leading to overdose and local toxicity and making the mimicking of endogenous H 2 S production difficult. [164,165]Polysulfides, such as diallyl disulfide, diallyl trisulfide, (5-(4-hydroxyphenyl)-3H-1,2-dithiole-3-thione), and thionamide, are natural polysulfide chemicals.Finally, synthetic H 2 S donors (such as Lawesson's reagent derivative GYY4137) have also been developed.Biological thiols such as cysteine and GSH are primarily responsible for activating the polysulfides and synthetic H 2 S donors. [166,167]Therefore, the design of donor materials whose concentration and dosage can be regulated poses a significant challenge in H 2 S wound repair treatment.Most H 2 S donors show uncontrollable H 2 S release; however, the latest studies have designed controllable biomaterials with endogenous biothiol-triggered, [166,167] light-triggered, [168] and pH-responsive [33,164,169] release.

Natural Polysulfide Compounds
Polysulfide compounds have shown excellent H 2 S generation properties.Pry-Ps@ CP-PEG nanomodulators are constructed by encapsulating hydrophobic polysulfide-based H 2 S donors (Pry-Ps (2,2 0 -dipyridyl tetrasulfide) @ CP-PEG (where CP = conjugated polymer) in amphiphilic conjugated polymers (CP-PEG). [167]Endogenous GSH subsequently triggers the release of H 2 S from Pry Ps donors to achieve anti-inflammatory capacity and accelerate wound healing and re-epithelialization. [167] N-(Benzoylthio)benzamide (NSHD1) was incorporated as an H 2 S donor into PCL to prepare fibrous scaffolds, which were capable of triggering the controlled release of H 2 S with biothiols, such as cysteine and GSH, and promoting the expression of genes related to wound healing. [166]However, biothiols involve uncertainties, which may lead to unpredictable and unstable H 2 S release. [164,166]

Synthetic H 2 S Donor Materials
Carbonyl sulfide (COS)-releasing compounds act as a new type of H 2 S donor and are rapidly converted to H 2 S by ubiquitous carbonic anhydrase. [165]The active P─S bonded GYY4137 serves as a slow-release and water-soluble H 2 S donor, and the core structure of GYY4137-phosphorothioate is a valuable template for designing useful H 2 S donors. [164]Phenylphosphonothioic dichloride 4 has been used to create JK-1-JK-5, and their H 2 S release profiles under various pH are shown in Figure 8a.Phosphorothioates control the rate of H 2 S release by pH-regulated intramolecular cyclization reactions in novel cyclization reactions to control the rate of H 2 S release. [164]Specifically, protonation (at neutral or slightly acidic pH) of phosphoramidothiophosphates forms the corresponding phosphothiols.This process should facilitate the release of H 2 S if a nucleophilic carboxylate is present at the appropriate location.The materials exhibit a highly pH-dependent hydrogen sulfide release behavior.In the early stages of inflammation, the acidic pH inhibits bacterial growth, reduces proteolytic activity, and promotes fibroblast growth.The low pH of the early wound triggers gas release from the H 2 S donor JK.This pH-responsive release property is particularly suitable for wound healing treatment because the pH of the acute wound microenvironment is altered. [170]The drawback of small-molecule donors is their abrupt release, which can induce overdose and local toxicity.In contrast, large-molecule donors can prolong H 2 S release, decrease cytotoxicity, and enhance solubility.pH-dependent H 2 S-release PCL-JK1 fiber materials were used as novel wound healing scaffolds to enhance wound regeneration in a mouse fullthickness skin trauma model. [169]The fibrous matrix of PCL-JK1 was found to extend H 2 S release, as evidenced by lower releasing rates than those of JK1 in solutions.Furthermore, in vitro experiments revealed that PCL-JK1 had high cytocompatibility similar to that of PCL fibers. [169]However, the fibers exhibited low oxygen permeability; thus, a hydrogel system was created for gas delivery.To create a novel HA-JK1 hybrid system, an in situ forming biomimetic HA hydrogel was employed as a matrix to dope the pH-controllable H 2 S donor, JK1. [33]This HA-JK1 hydrogel was created to be the optimal JK1 delivery scaffold with a pH-dependent extended H 2 S releasing characteristics.The hydrogel was capable of significantly speeding up the regenerative process in vitro and functioning as a promising wound dressing by upregulating the M2 macrophage phenotype and enhancing epithelialization, collagen deposition, angiogenesis, and cell proliferation. [33]Recently, a photoactivable H 2 S donor (PRO-F) was designed. [168]Without the use of endogenous chemicals, activation of PRO-F was achieved using light, and the fluorescent signal accompanying the activation enables the real-time monitoring of released H 2 S. The onitrobenzyl group was selected as the donor molecule trigger because its high photolysis efficiency and low necessary photolysis energy concurrently ensure release efficiency and the least amount of concomitant photodamage.The thiocarbonate core functions as the sulfur source and connects the self-immolating trigger group with the dicyanoisophorone-based NIR dye.The self-elimination process begins when a certain wavelength of light is encountered, and the extruded COS is quickly converted into H 2 S by carbonic.Using diabetic models, the ability of PRO-F to improve chronic wound healing is confirmed. [168]y covalently connecting small-molecule donors to macromolecules, a controlled release material was synthesized.Here, a novel H 2 S donor of keratin-TA conjugate (KTC) was first prepared and then doped with PU and gel to afford nanofibrous PU/Gel/KTC mats (Figure 8b). [171]Nanofiber PU/gel/KTC mats were obtained via electrostatic spinning preparation by adding PU and gel.Both the KTC donor and biocomposite mats were capable of releasing H 2 S triggered by GSH under physiological conditions.Cell assays in vitro demonstrated that these mats promoted cell proliferation and migration.In a full-thickness defect model, the deposition of collagen was enhanced, and angiogenesis was encouraged (Figure 8b). [171] 2 S is a signaling molecule that affects different systems in the human body in a dose-dependent, time-dependent, and tissue-dependent manner.Therefore, different systems must be monitored using a validated measurement method that can conduct measurement for more than days.Controlling H 2 S levels in different systems in the body is also necessary to ensure that the therapy is not harmful to other systems.

Carbon Monoxide (CO) Therapy
Contrary to the common belief that CO is a substance that causes harm to organisms that depend on oxygen transport and  -5).H 2 S release profiles of JK-1, JK-2, and GYY4137 under various pH.Reproduced with permission. [164]Copyright 2016, American Chemical Society.b) Diagram of H 2 S released from the KTC triggered by thiols.Representative pictures of wounds treated with 3M Tegaderm HP films and PU/Gel/KTC mats.Reproduced with permission. [171]Copyright 2016, American Chemical Society.mitochondrial respiration, the physiological role played endogenously generated CO and the beneficial effects of the low-dose application of CO gas are evident. [61]CO is produced by the degradation of heme oxygenase (HO) (Figure 11).HO-1 is a stress-response enzyme that converts heme into CO, biliverdin, and iron. [62]HO-1 is highly induced in monocytes and macrophages after stimulation. [52]In contrast to NO and H 2 S, CO is a relatively stable small gas molecule [61] that exhibits anti-inflammatory and antibacterial effects.It is capable of penetrating cell membranes, [11] primarily reacting with transition metals in a specific redox state. [172]CO possesses anti-inflammatory, immunomodulatory, anti-infective, and vasodilatory properties. [173]1.Roles of CO in Wound Healing Soluble sGC, heme-containing potassium channels, NOS, and surface NADPH oxidase are the proximal targets of CO. [61] Low concentrations of CO may impact a variety of signaling pathways, including those that regulate sGC and/or activate the signaling pathways of p38 mitogen-activated protein kinase (MAPK), which is known to have strong anti-inflammatory effects.[62,174] 6.2.Gaseous CO Treatment CO release occurs through the following pathways: gaseous CO and CORMs.The administration of gaseous CO is risky and severely limited due to the high affinity of CO for Hb, which causes tissue hypoxia.[175] CO is not easily consumed in pathological situations, is generally stable, and does not interact with intracellular metabolites. Therore, CO lacks target selection and controllability.[176] Whether COHb is a reliable marker of CO intoxication and whether CO levels are proportional to the corresponding COHb levels are serious issues.[61] Therefore, the safety and feasibility of using CO as an inhalation gas remain a concern.
Gaseous CO therapy, such as rectal administration of COsaturated solutions, protects the intestinal mucosa from inflammation and accelerates colon ulcer healing by enhancing epithelial cell recovery. [177]The thickening agent PAA allows for the continuous release of CO.By considerably accelerating wound healing and increasing VEGF expression in wound granuloma tissue, the application of an aqueous PAA solution containing CO promoted the healing of skin ulcers.This activity was associated with an increase in neovascularization in wound granulation tissues. [178]

CORM
The development of CORM as a drug delivery system provides a desirable and secure alternative to gaseous CO administration.CORM is a transition metal carbonyl complex that includes a variety of organometallic complexes (Ru, Fe, Mn, V, Co, Ir, Cr, Mo, and W) and many major group compounds (a-dialkyl aldehydes, oxalates, boron carboxylates, and silyl carboxylates). [172]Ruthenium-based carbonyl compounds (CORM-2 and CORM-3) are the most commonly used CORMs that release CO in an efficient and controlled manner.CORMs are beneficial because they do not affect oxygen transport by Hb. [172] The ideal CORM should meet the following criteria: effective therapeutic action and low biotoxicity, suitable solubility and stability, controlled release behavior, and long-term sustained release. [172]Therefore, platforms were developed for integrating CORMs into various nanomaterials for the controlled release of CO after activation by triggers.The following two categories of triggers typically result in the release of CO: 1) endogenous biochemical stimuli (pH, [179] GSH, enzymes) [180] and 2) exogenous physical stimuli (heat and light). [176,181]In general, intramolecular cycloaddition reactions between dienones and alkynes are utilized to form a dienylheptanone intermediate that spontaneously undergoes chelation reactions to extrude CO. [49] 6.3.1.Endogenous Biochemical-Triggered CORM (ET CORM) Enzyme-triggered CO-releasing molecules (ET CORM) [175,182] are based on enzyme-sensitive and photoactivated CO-releasing molecules.O-Acetyl group-protected CORM (CORM-Ac) is a simple and viable unimolecular theranostic probe for the realtime detection of bacterial infection and subsequent treatment.Using exogenous bacterial lipases as targets, CORM-Ac may undergo enzymatic cleavage of the O-acetyl group and transform into CORM, thus easily triggering the progression of excitedstate intramolecular proton transfer and providing an early warning of infection via visualized fluorescence signal.Moreover, CORM-Ac turns on fluorescence for an infection early warning.The photoinduced release of CO is effective in the bactericidal treatment of MRSA infections.This "sense-and-treat" molecular formulation with considerably fewer production techniques has enormous potential for sensitive bacterial infection warning and therapy (Figure 9a). [180]

Phototriggered CORM (Photo CORM)
For application in antimicrobial prorepair therapy, a variety of CORMs, beginning with initial UV-triggered and visibletriggered releases to NIR-triggered releases, have been developed.
A sense-of-logic photoCORM detects thiols to determine the cellular environment before releasing CO when triggered by visible light (VIS) and O 2 . [183]Tryptophan CORM, also known as "VIS -activated CORM," is based on a tryptophan-containing manganese(I) tricarbonyl acetonitrile core.The tryptophanderived manganese-containing compound (TryptoCORM) is synthesized, subsequently releasing 1.4 and 2 moles of CO at 465 and 400 nm, respectively.When a light-induced breakdown occurs, tryptophan is produced, inhibiting E. coli. [184]UV activation offers poor tissue penetration and cytotoxicity.To release CO through exposure to red light (650 nm), nonmetallic COreleasing micelles were synthesized through the photo-oxidation mechanism of 3-hydroxyflavone (3-HF). [185]This ideal technique is beneficial for determining the antibacterial capacity of CO because, compared to that with metal carbonyls, the interference of transition metal ions with antimicrobial activity is eliminated.After exposure to 650 nm light, the excited tetraphenylporphyrin converted 3 O 2 into 1 O 2 , which then spontaneously oxidized 3-HF derivatives and released CO.In contrast to metal carbonyls that nonspecifically internalize into both Gram-positive and Gram-negative bacteria, the nonmetallic micelles have a selective bactericidal effect because they are selectively taken up by S. aureus rather than E. coli.This approach can effectively eradicate MRSA pathogens, treat wounds infected with MRSA, eliminate bacteria, and accelerate wound repair (Figure 9b). [185]FeCO is a commonly applied photothermal effect-mediated thermosensitive CO-releasing compound.Mesoporous polydopamine NPs (MPDA NPs), the surface of which is covalently anchored with deoxyribonuclease I (Dnase I), destroy extracellular DNA in biofilms and prevent biofilm densification by encapsulating the thermosensitive CO gas-releasing donor (FeCO).Dnase-CO@MPDA NP showed notable photothermal capacity when exposed to NIR and resulted in the on-demand supply of antibacterial CO gas, which completely penetrated the injured biofilm.In MRSA biofilm-induced infected wounds, MRSA was destroyed, and the infection-related inflammatory response was reduced (Figure 9c). [176]ome limitations exist in the use of CORMs, including their short duration of action, difficulty in maintaining stability under biological circumstances, and uncontrollable spontaneous release.Thus, future preparation of CORMs should consider increasing their stability and controllability, such as increasing the stability and delaying the release duration through the biogel combination.

H 2 Therapy
H 2 is beneficial in the selective scavenging of free radicals, such as the conversion of hydroxyl radicals to water (H 2 O).Produced by microorganisms containing hydrogenases with other compounds (Figure 11), H 2 reacts with intracellular hydroxyl radicals (-OH) and peroxynitrite (ONOO À ) to diminish cytotoxic ROS produced in inflammatory response. [186]Since the first report on the clinical application of H 2 , the gas has been demonstrated to have anti-inflammatory, antiapoptotic, and antitumor functions, making it a promising biotherapeutic gas.

Roles of H 2 in Wound Healing
Several therapeutic advantages of H 2 are as follows: 1) even at high concentrations, H 2 is sufficiently mild to scavenge intracellular ROS without any toxic effects; [29] 2) H 2 is the only antioxidant that can penetrate the blood-brain and blood-eye barriers. [186]H 2 has a well-distributed characteristic and the physical ability to penetrate biological membranes and diffuse into the cytoplasm. [29]3) In addition to its antioxidant effects, H 2 acts  [180] Copyright 2021, Wiley-VCH.b) PhotoCORMs can release CO under red light irradiation through a photo-oxygenation mechanism.Reproduced with permission. [185]Copyright 2021, Wiley-VCH.c) Diagram of the removal of MRSA biofilms based on Dnase I involvement and CO-potentiated PTT.Fluorescence staining images of MRSA treated by different NPs with a DCFH-DA probe and quantitative measurement of bacteria count in wounds by standard spread-plate assay.Reproduced with permission. [176]Copyright 2021, Wiley-VCH.

H 2 Therapy
Several ways to ingest H 2 include H 2 gas, H 2 -rich water (HRW), and H 2 -rich saline (HRS). [29]Consuming H 2 water assists in preventing inflammatory response by reducing the gene expression of proinflammatory cytokines. [29]HRW substantially scavenged peroxyl radicals (ROO À ) derived from 2,2 0 -azobis(2-amidinopropane) dihydrochloride in a dose-dependent manner under cellfree circumstances.The extract from HRW-treated human gingival fibroblasts (HGFs) more effectively scavenged ROO À than the extract from DDW-treated cells, suggesting that HRW can improve intracellular antioxidative capability and shield cells and tissue from oxidative harm. [187]Alkali burns can cause blindness; however, a new treatment for reducing corneal angiogenesis and effectively curing the disease involves immediate antioxidant treatment with an H 2 -rich rinse solution. [188]The injured cornea heals by recovering transparency because H 2 therapy reduces oxidative stress in the cornea. [188]RS treatment of burn wounds reduced malondialdehyde (MDA) while considerably increasing the activity of natural antioxidant enzymes.Additionally, HRS therapy reduced postburn apoptosis and autophagy elevation in wounds.In addition, HRS increased IL-10 while decreasing myeloperoxidase levels and IL-1 and IL-6 expression in the stasis zone.The increased expression levels of NF-κB p65 and Akt phosphorylation postburn were downregulated by HRS management.Additionally, the Akt/NF-κB signaling pathway may regulate the release of inflammatory cytokines. [30]Prophylactic instillation of HRW reduced inflammation caused by alkali burns possibly by increasing the expression of antioxidants, such as SOD-1 and PGC-1.In conclusion, the consumption of HRW showed a variety of positive effects by stimulating the Nrf2/antioxidant defense pathway. [189]By increasing SOD activity and decreasing the level of MDA in wound tissue, as well as the severity of oxidative damage in cutaneous tissue, HRW lessened the skin damage caused by radiation. [190]In rats exposed to acute radiation damage, HRW displayed a healing effect that was influenced by H 2 concentration. [190]HRS reduces oxidative stress through activation of the Nrf-2/HO-1 pathway and promotes healing through its antioxidant, anti-inflammatory, and antiapoptotic effects. [191]

H 2 -Releasing Materials
The explosive behavior of H 2 GT prevents it from being used effectively for wound healing, while the low solubility of HRW and HRS therapies prevents full penetration into the tissue. [192]onsequently, H 2 -releasing materials have gained attention in recent years as a safer H 2 therapy for healing wounds.The Bacillus-Chlorella (Bac-Chl) gel patch was filled with gel beads containing live Chlorella vulgaris (Chlorella) and Bacillus licheniformis (Bacillus). [192]Chlorella, a H 2 -producing algae, may produce H 2 more effectively by consuming extra oxygen when bacterial chaperones are present.Live Bacillus respires when exposed to light, reducing the oxygen content of the calcium-Alg hydrogel beads.As a result, hydrogen is quickly produced from Chlorella through photosynthesis.When exposed to light, the Bac-Chl gel patch provides a continuous H 2 transport environment for the skin, reducing toxic free radicals and secreting IL-10, which directly reduce inflammation.The patch promotes angiogenesis and tissue repair and effectively alleviates chronic inflammation in DFUs (Figure 10a). [192]However, the Bac-Chl gel patch had a limited lifespan (60 h).Moreover, Chlorella has potential biosecurity problems as an organism.The full-solution incorporation method was used to create H 2 -incorporated titanium oxide nanorods (HTONs) with a rutile single-crystal structure.In a VIS-photocatalytic high-glucose microenvironment, regulated and sustainable glucose depletion and H 2 production are achieved by H 2 incorporation-endowed HTON, and the data of VIS-photocatalytic hydrogen generation and glucose consumption are shown on Figure 10b. [193]HTON, a VIS-responsive photocatalyst, has the proper energy band structure to use glucose as a sacrificial agent in the high-glucose microenvironment for effective VIS-photocatalytic hydrogen generation.Photocatalytic glucose depletion and hydrogen molecule generation inhibit the synthesis of advanced glycation end products (AGEs) and the expression of their receptors (RAGE) in the diabetic wound microenvironment, respectively.By suppressing the RAGE gene, H 2 therapy greatly reduces the activity of the advanced glycation end-products and the AGEs-RAGE pathway in diabetic wounds.Together, these factors lessened the proapoptotic effects of elevated glucose and encouraged cell migration and proliferation to aid in the healing of diabetic wounds. [193]ne study proposed a multicomponent nanoreactor (NR) inspired by natural photosynthesis. [194]The photosensitizer expands the absorption spectrum while reducing the controllable sensitivity of H 2 release.The H 2 generation catalyst (Au-NPs), electron donors (L-ascorbic acid; AA), and photosensitizers (chlorophyll a; Chla) are all encapsulated in a liposome (Lip) system to form light-driven NRs.NRs produce H 2 gas in situ upon light absorption, which reduces inflammatory responses.The structure of NR and the mechanisms of H 2 gas photosynthesis are shown in Figure 10c.This Lip NR system offers the ideal possible environment for reactions, promoting the quick activation of H 2 gas photosynthesis and locally supplying a high therapeutic concentration of the gas.The phototriggered NR system reduces the degrees of overproduction of ROS and proinflammatory cytokines in vitro in RAW264.7 cells.In mice with lipopolysaccharide (LPS)-induced paw inflammation, the photodriven NR system lowers the levels of excessive ROS and proinflammatory cytokines (Figure 10c). [194]By integrating H 2 into Pd nanocubes, a biocompatible H 2 -releasing PdH nanohydride is produced, demonstrating on-demand regulated active H 2 release under NIR laser irradiation. [48]The force of the Pd-binding H is destroyed, and active H 2 is released after exposure to NIR laser.The power density and irradiation time of the NIR laser exert a significant impact on the active H 2 release rate.The created PdH nanohydride combines the benefits of both H 2 and the photothermal effect of Pd; consequently, it performs exceptionally well in treating wounds with significant bacterial infections in rats.In particular, the PdH nanohydride shows highly effective antibacterial, antibiofilm, and wound-healing activities.According to a comprehensive analysis of antibacterial mechanisms, two putative pathways are also implicated in the synergistic H 2 -photothermal antibacterial activity.One process involves genes that are essential for bacterial metabolisms, such as dmpI, narJ, and nark, which then increase the expression of oxidative metabolic enzymes to produce significant ROS and induce DNA damage.A second method involves severely damaging bacterial membranes to release intracellular materials, such as DNA. [48]lthough H 2 therapy has been studied extensively in the recent decades, many challenges to its preparation, delivery method, and clinical efficacy persist.The complex mechanisms of H 2 in wound-related cells remain to be further elucidated.The biocompatibility and stability of H 2 therapy and H 2 generators should be considered.Moreover, the combination of H 2 therapy with existing therapeutic approaches may provide synergistic effects.

Sulfur Dioxide (SO 2 ) Therapy
Environmentally hazardous SO 2 is now recognized as an endogenous gas-signaling molecule, along with NO, CO, and H 2 S, that regulates cellular activities, particularly those of the cardiovascular system.SO 2 acts synergistically with NO and has a vasodilatory activity similar to that of NO. [17] SO 2 is utilized frequently as an antibiotic, preservative, and antioxidant and is available in hydrated forms, such as bisulfite, HSO 3 À , and sulfite, SO 3 2À . [195]Cysteine dioxygenase (CDO) can oxidize L-cysteine to produce SO 2 and, consequently, cysteine sulfite, which can further react to produce SO 2 and pyruvate (Figure 11).Additionally, SO 2 is a byproduct of the metabolism of H 2 S. Endogenously formed H 2 S can be enzymatically oxidized to form thiosulfate (S 2 O 3 2À ), which then reacts with GSH in the presence of thiosulfate reductase to form SO 2 and oxidized GSH (GSSG).GSSH can also be oxidized to SO 2 by sulfur dioxygenase. [195,196]

Roles of SO 2 in Wound Healing
Macrophages utilize an endogenous SO 2 /AAT pathway, and SO 2 produced by macrophages possesses anti-inflammatory properties. [54]SO 2 also upregulates the cyclic adenosine monophosphate pathway, serving as an endogenous mast cell stabilizer. [28]SO 2 is toxic at high concentrations, [18] with autoxidation generating many hydrated forms of SO 2 , sulfite, and bisulfite free radicals that induce oxidative damage to biological macromolecules, such as proteins, lipids, and DNA.Therefore, the introduction of such reactive sulfur species into the cell may lead to irreversible changes in the intracellular redox balance. [18]Given the difficulty for the corresponding bacteria to overcome oxidative stress, the use of SO 2 as an antimicrobial agent may have great potential in antimicrobial applications.

SO 2 -Releasing Materials
The poor bioavailability of gaseous SO 2 hinders its use for therapeutic purposes.Sodium bisulfite, which can be applied as an Figure 10.a) The Bac-Chl hydrogel patch can supply H 2 to scavenge hydroxyl radicals and neutralize chemokines.Reproduced with permission. [192]opyright 2022, American Chemical Society.b) Depiction of the HTON-based photocatalytic treatment for diabetic wounds that is initiated by VIS.Data on VIS-photocatalytic hydrogen generation and glucose consumption.Reproduced with permission under the terms of the Creative Commons CC BY license. [193]Copyright 2022, the Authors.Published by Springer Nature.c) The in situ photosynthesis of H 2 gas by photodriven NR and its structural components.Reproduced with permission. [194]Copyright 2017, American Chemical Society.
SO 2 donor for the study SO 2 biology, shows poor controllability of the rate and amount of SO 2 production.2,4-Dinitrobenzenesulfonamide has been used as a thiol probe and is widely employed as a thiol-activated SO 2 donor for controlled SO 2 release. [195]However, given the inherent drawbacks of thiol-triggered materials, such as complex mechanisms and biotoxicity, other controllable scaffolds, such as phototriggered, [19] pH-responsive, [196] and enzyme-responsive [197] scaffolds, have been developed.SO 2 -releasing materials have been explored for the treatment of bacterial infectivities, such as Mycobacterium tuberculosis (Mtb), [17] MRSA, [18] and Enterobacter cloacae cells (MTCC 509). [19]n contrast, the complex inorganic sulfite mixtures typically utilized for SO 2 production in biological systems show poor controllability of the rate and amount of SO 2 production.The tunable-release organic SO 2 donor 2,4-dinitrobenzenesulfonamide (a thiol-activated SO 2 donor) is synthesized to combat Mtb, [17] and the mechanism of its inhibition of Mtb may involve the depletion of thiols during the activation stage, as well as SO 2 -induced oxidative stress and damage to biomolecules such as lipids, proteins, and DNA. [195]One proposed mechanism involved the thiol attacking the aromatic ring to form a Jackson-Meisenheimer complex, which then broke to produce SO 2 , benzylamine, and 2,4-dinitrophenylthioether.2,4-dinitrobenzenesulfonamide has also been demonstrated to inhibit MRSA. [18]This substance is cell permeable, and the results of treatments of MRSA cells with depleted intracellular thiols and enhanced oxidative species are both consistent with a mechanism involving thiol activation to generate SO 2 . [18]The utilization of thiols as triggers may complicate mechanistic research because the targets of SO 2 include biologically relevant sulfides and sulfur. [198]Damage can also be caused by its byproducts.Single-and two-photon activation of SO 2 donors based on the 4,5-dimethoxy-2-nitrobenzyl (DMNB) phototrigger initiates the generation of SO 2 and hydroxyl compounds through the photocleavage of C─S bonds in sulfonates.As a hydroxy-based drug, ferulic acid ethyl ester (FAEE) has unique features, such as broad antibacterial activity and fluorescent nature.FAEEs exhibit a fluorescent property that facilitates self-monitoring of the cage release process by increasing fluorescence intensity and enhancing antibacterial activity against MTCC 509. [19]Cell-permeable esterase-sensitive sulfonates are a novel category of esterasesensitive SO 2 donors that self-incinerate to form SO 2 .However, these donors have not yet been applied to trauma repair. [197]Under physiological and nonenzymatic conditions, 1-phenylbenzosulfonic acid can be utilized as a compound for controlled and effective SO 2 generation. [198]Benzothiazolyl sulfite (BTS) is a potential pH-dependent and water-soluble SO 2 donor.We discovered that BTS has the capacity to release SO 2 slowly but steadily and continuously at physiological pH. [196]hese therapies for trauma are highly anticipated in the future.
However, whether SO 2 possesses a biological target remains unclear, and the mechanisms of action of SO 2 and its role in redox homeostasis are still unknown. [195]Another limitation to the application of SO 2 donors is that they are significantly more effective against Gram-positive bacteria than Gram-negative bacteria, which is likely due to the low outer membrane permeability of the donors to Gram-negative bacteria. [195]

Other Gases Therapy
Produced by the tricarboxylic acid cycle in aerobic cellular respiration (Figure 11), carbon dioxide (CO 2 ) was first utilized in 1932 to treat patients with vascular ailments.CO 2 shows efficacy in improving microcirculation and improving blood circulation, which originates from the Bohr effect.The Bohr effect (pH/CO 2 on Hb O 2 affinity) [42] is a physiological phenomenon in which oxygen is released from Hb and oxygenation levels in tissues are increased owing to a decreased pH of the blood.CO 2 is capable of penetrating not only the complete cutaneous layer but also the granulation tissue of the wound.It improves tissue perfusion through the dilatation of small precapillary arteries, thereby improving pO 2 .Both chronic and acute wounds showed improved granulation formation and reduced excretion and odor within a week after CO 2 treatment. [199]CO 2 -rich water baths have a vasodilatory effect.Increased blood perfusion after CO 2 treatment in the ischemic limb of mice with unilateral hindlimb ischemia induces VEGF synthesis, leading to NO-dependent neocapillary formation associated with endothelial progenitor cell mobilization. [200]CuS NPs with photothermal conversion capabilities ligate CuS NPs driven by NIR to produce CO 2 . [201]Hollow porous CuS NPs decompose to CO 2 at 42 °C.The release of CO 2 leads to a Bohr effect, resulting in faster closure of mouse wounds. [201]However, the potential risk of overdose due to CO 2 toxicity (such as hypoxia or asphyxia) should also be considered accordingly. [201]omposition, method of action, and bacterial reaction when high-energy electrons collide with air molecules at temperatures that are bearable for human tissue produce cold atmospheric plasma, an ionized gas. [20,202]A decade ago, cold physical plasma was approved for the treatment of nonhealing and infected wounds in Europe. [203]Cold plasma may destroy biofilm matrices, kill bacteria, change the characteristics of the ECM, and control cell signaling to encourage particular cellular behavior. [20]he roles of cold physical plasma in wound healing are shown in Figure 2.
After plasma treatment, significant wound contraction occurred compared to untreated diabetic wounds, and plasma increased the wound-healing rate in non-diabetic rats. [202]A 2 min cold atmospheric-pressure plasma (CAP) treatment has been demonstrated to be effective against a variety of bacteria, including significant skin and wound pathogens such as E.coli, S. aureus, and MRSA; these results suggest that CAP has beneficial effects on wound healing.CAP treatment improves wound healing in diabetic mice by suppressing inflammation, reducing oxidative stress, and enhancing angiogenesis, involving the signaling of several proteins. [204]n the study, helium (He)-or a gas mixture of He and CAP effectively induced keratinocyte proliferation and migration mediated by the activation of epithelial-to-mesenchymal transition and cell cycle progression.Rat wound healing studies revealed that He/Ar-CAP therapy enhanced the creation of granulation tissue and reduced inflammation in cutaneous tissue, leading to an expedited healing process. [205]A novel type of low-temperature plasma with the advantages of a free gas source, long working distance, high gas flow rate, and production of abundant active substances (NOγ, OH, N 2 , and O) has been developed.In Type I diabetic rats, cellular and molecular analysis showed that plasma treatment significantly reduced inflammation and improved re-epithelialization, fibroblast proliferation, collagen deposition, neovascularization, and expression of TGF-β, SOD, GSH peroxidase, and catalase. [206].Limitations and Challenges The role of GT in wound healing is receiving increasing attention.The development of biosafe delivery platforms from simple gaseous GT to the use of inorganic or organic reagents and nanomaterials to construct gas-loaded or gas-generating biosafe delivery platforms with controlled, sustained release properties in response to various chemical and physical environments is of great significance for the translational clinical application of GT in trauma repair.However, current detailed problems must be further addressed, and only by improving these deficiencies can we achieve a safer and more efficient delivery system (Figure 12). 1) Biosafety.First, for gas molecules with chemical reactivity, such as NO and H 2 S, strict control of safe therapeutic dosages is crucial to prevent cytotoxicity because both antiinflammatory (low dose) and antibacterial (high dose) properties require such control.Overdose may result in irreversible damage, such as reduced oxygen-carrying capacity or respiratory depression due to high doses of as well as cell death that may result from high concentrations of NO and H 2 S. Therefore, establishing sophisticated platforms or instruments for gas content measurement is urgently needed to accurately assess the gas levels in the release platform.Continuous monitoring of the gas content at the tissue cell level for a period longer than days is possible.Second, when applied to the human body, all types of nanomaterials and inorganic or organic reagents must be considered for their toxicity or that of their byproducts.Avoiding possible extraneous products as much as possible is essential to reduce the biological toxicity of the material and maximize its therapeutic effect.For instance, natural biosafe amino acids or plant extracts with curative effects can be used.Inorganic materials have excellent stability and multifunctionality, while organic nanoparticles have inherent high biocompatibility and easy degradation properties.The development of organicinorganic hybrid systems can improve biosafety by adjusting the balance between stability and biodegradability.Investigation of in vivo biodegradation, biodistribution, excretion, hemocompatibility, histocompatibility, and even a comprehensive assessment of particular toxicity to various organs, tissues, and cells to confirm the likelihood of clinical translation is crucial.2) Molecular mechanisms.The molecular mechanisms of gas-only trauma treatment have been described in reviews.However, the underlying pathway mechanisms and interactions of each gas remain uncertain.Unraveling the interrelated networks among all gases, such as NO, H 2 S, and CO, is of great significance for the underlying mechanisms of treating traumatic wounds.Whether the inclusion of gas carrier-related gas therapies makes GT more complex and affects its therapeutic mechanisms remains unknown.Therefore, the future should exclude the therapeutic effect of gas release carriers to explore the role of gases in the overall nanomaterials.Defining the deeper molecular mechanisms of gas in the overall treatment of material platforms will allow the use of improved GT for future optimization.3) Combination of multiple strategies.At present, the application of GT to wounds has shown exhibited potential for efficiently alleviating the challenges of difficult healing and easy infection of wounds.Therefore, the development of multifunctional detection and treatment materials that combine prediction, diagnosis, and therapy is necessary.In the future, materials should be developed to precisely detect the trauma microenvironment and exhibit early warning capabilities, such as enabling bacterial lipase as a detection target and early control of the fluorescent switch to warn of trauma infection.Subsequently, the trauma can be treated with early intervention and controlled release of gas materials.The future development of more all-in-one mode materials helps real-time monitoring, diagnosis, and cure and has the potential for becoming commercialized products for clinical translation.4) Cost and patient compliance.The cost of GT can be high, which may limit its use for wound repair.Patients may find complying with GT difficult, especially if the treatment requires them to spend long periods of time in a hyperbaric chamber.Therefore, for the design of topical, portable gas delivery materials would be beneficial in improving patient compliance.Materials with long-term release properties and reusability characteristics can reduce the frequency of dressing changes for patients and reduce costs to a certain extent.

Conclusion
Early repair of acute and chronic wounds is a major clinical challenge, and various biosafe wound dressings, such as hydrogels, scaffolds, and fibers, have been developed to mimic ECM structures and to promote the proliferation and migration of cells.Gaseous molecules, including O 2 , NO, CO, H 2 , H 2 S, SO 2 , CO 2 , and plasma, exhibit unique physiological functions for wound repair, such as resistance to bacteria, relief of inflammation, proangiogenesis, and epithelialization.However, the inherent low solubility and dose toxicity of these gases limit their application in wound treatment.Therefore, the development of biomaterial platforms with gas-loaded or gas-generating properties provides a strategy for precise delivery, controlled release, and stable and sustained treatment of traumatic surfaces.With the rapid development of nanomaterials and nanotechnology, GT strategies incorporating biomaterials have become a popular research frontier.However, the clinical translation of gas-based therapeutic materials is still in its primary stage.Indepth researches on the overall biosafety and deep molecular mechanisms of the materials, multidisciplinary cooperation, and optimization of therapeutic strategies are still necessary for their early clinical application, as shown in Table 1.
Table 1.Abbreviations of gas signal molecules and their donors.

Figure 2 .
Figure 2. Mechanism of gas in the phases of wound healing.

Figure 3 .
Figure 3. a) Schematic of AGP fabrication using polyurethane film and a polytetrafluoroethylene membrane to conduct dissolved O 2 release in reaction to light.b) Comparison of the dissolved O 2 release capability.c) Illustrations of the wound area and skin flaps after various treatments.Reproduced from.[80]Copyright 2020, The Authors, some rights reserved; exclusive licensee AAAS.Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

Figure 5 .
Figure 5. a) BP QDs and Hb are contained within NIR-responsive MNs in wound healing.Reproduced with permission.[114]Copyright 2020, American Chemical Society.b) Schematic of the preparation and mechanism of HA-DA/Mxene@PDA hydrogel.Reproduced with permission.[31]Copyright 2022, Wiley-VCH.c) The light-responsive MoS 2 QDs deliver O 2 and heal wounds.Reproduced with permission.[115]Copyright 2019, American Chemical Society.d) Schematic of the fabrication of and rESW-responsive O 2 release from nano-PFC.Analyzing the blood circulation of the animal model after therapy.Quantitative evaluation of blood flow in a rat forepaw as measured by laser Doppler imaging.Reproduced with permission under the terms of the Creative Commons CC BY license.[92]Copyright 2019, the Authors.Published by Hindawi.

Figure 7 .
Figure 7. a) Steps in the synthesis of CS/St þ GO x /GA dressings for healing wounds with bacteria.Mechanisms of the glucose, H 2 O 2 , and NO release mechanisms in an acidic environment.Reproduced with permission.[137]Copyright 2022, Elsevier B.V. b) Production of a hydrogel dressing that can successfully kill MRSA, consume several types of ROS, and produce O 2 and NO.Reproduced with permission.[32]Copyright 2022, Elsevier Ltd.

Figure 8 .
Figure 8. a) Phenylphosphonothioic dichloride 4 was used to create a series of donors (JK-1 to JK-5).H 2 S release profiles of JK-1, JK-2, and GYY4137 under various pH.Reproduced with permission.[164]Copyright 2016, American Chemical Society.b) Diagram of H 2 S released from the KTC triggered by thiols.Representative pictures of wounds treated with 3M Tegaderm HP films and PU/Gel/KTC mats.Reproduced with permission.[171]Copyright 2016, American Chemical Society.

Figure 9 .
Figure 9. a) Synthetic process of CORM-Ac and diagram of lipase-and light-response CORM-Ac release of CO.Reproduced with permission.[180]Copyright 2021, Wiley-VCH.b) PhotoCORMs can release CO under red light irradiation through a photo-oxygenation mechanism.Reproduced with permission.[185]Copyright 2021, Wiley-VCH.c) Diagram of the removal of MRSA biofilms based on Dnase I involvement and CO-potentiated PTT.Fluorescence staining images of MRSA treated by different NPs with a DCFH-DA probe and quantitative measurement of bacteria count in wounds by standard spread-plate assay.Reproduced with permission.[176]Copyright 2021, Wiley-VCH.

Figure 11 .
Figure 11.The main biological production processes of endogenous gas. 1) O 2 is absorbed from the outside air by lung exchange and is mostly transported by Hb. 2) NO is produced as L-Arg breaks down in response to the activity of NOS.When skin is exposed to UV light, nitrite and RSNOs are photolyzed, resulting in the formation of NO. 3) Hb breakdown, which is carried out by HO enzymes, is necessary for the production of CO (HO-1 and HO-2).4) Both single-enzyme processes and nonenzymatic mechanisms can produce H 2 S. L-cysteine, L-cystathionine, L-homocysteine, and β-mercaptopyruvate, which are catalyzed by CSE, CBS, and 3-MST, are critical parts of the enzymatic route.In the nonenzymatic technique, cysteine is applied to produce H 2 S. 5) SO 2 is produced mainly through the metabolism of L-cysteine catalyzed by CDO and AAT.H 2 S catalyzed by NADPH oxidase can produce sulfite, the hydrated form of SO 2 .6) H 2 is mainly produced by micro-organisms containing hydrogenases.7) CO 2 is produced by the tricarboxylic acid cycle in aerobic cellular respiration.The common pathway between O 2 , H 2 S, and CO is MAPK, and the common target of NO and CO is sGC.NOS: nitric oxide synthase; HO: heme oxygenase; RSNOs: S-nitrosothiols; CBS: cystathionine β-synthase; CSE: cystathionine γ-lyase; 3-MST: 3-mercaptopyruvate sulfurtransferase; CDO: cysteine dioxygenase; AAT: aspartate aminotransferase; NADPH: reduced nicotinamide adenine dinucleotide phosphate; MAPK: p38 mitogen-activated protein kinase; sGC: soluble guanylate cyclase.
Phototriggered CO-releasing molecule photo CORM H 2 -rich water HRW H 2 -rich saline HRS