Ascorbic acid (AA), also known as vitamin C (IUPAC name, (5R)-5-[(1S)-1,2-Dihydroxyethyl]-3,4-dihydroxyfuran-2(5H)-one; molecular formula, C₆H₈O₆), is derived from a six-carbon sugar called L-threo-hex-2-enono-1,4-lactone, as shown in Fig. 1 [1-3]. Structurally similar to glucose, it comprises two interconvertible compounds: AA (a strong reducing agent) and its oxidized derivative L-dehydroascorbic acid (DHA) [4]. AA is an essential water-soluble nutrient that enables the maintenance of optimal physiological processes in the human body. Its activities include the promotion of collagen biosynthesis, reduction of melanin production, photoprotection, free-radical scavenging, and immunity enhancement, particularly against viral infections. Moreover, it helps prevent scurvy, aids in iron absorption to prevent anemia, and contributes to the treatment of common cold, cataracts, age-related muscular degeneration, cardiovascular diseases, and certain types of cancer [5,6]. Notably, AA consumption is vital for humans, primates, and other mammals that are unable to synthesize it owing to mutations in the gene that encodes the final enzyme in the AA-biosynthesis pathway (i.e., L-gulono-1,4-lactone oxidase) [1].

Figure 1. Molecular structure of ascorbic acid (IUPAC name, (5R)-5-[(1S)-1,2-Dihydroxyethyl]-3,4-dihydroxyfuran-2(5H)-one; molecular formula, C6H8O6) and its oxidation scheme (drawn by the authors).

AA deficiency is associated with the disease scurvy, which exhibits symptoms such as bleeding gums, slow wound healing, dry skin, muscle pain, and fatigue. Scurvy occurs when the synthesis of collagen, the primary protein in animal tissues, is impeded by low AA availability [3]. Currently, the recommended dietary allowance for AA is 90 and 75 mg/day for male and female, respectively. The daily consumption of an adequate quantity of fruits and vegetables leads to a steady-state concentration of AA (~80 µmol/L) in plasma; the oral intake of 1.25 g of AA can increase the AA concentration in plasma to 134.8 ± 20.6 µmol/L [7]. To maintain an optimal AA concentration within the body, the consumption of dietary supplements or AA-fortified foods along with naturally occurring fruits and vegetables is being increasingly favored by consumers [7].

Notably, on exposure to oxygen, moisture, light, alkaline pH, high temperature, and heavy-metal ions, AA undergoes a color change and degrades into biologically inactive compounds (such as oxalic acid) [8-10]. Owing to its antioxidant potency, AA undergoes rapid oxidation to DHA, irreversibly hydrolyzing into 2,3-L-diketogulonate under alkaline conditions (at high pH), which lacks the activity of AA [10,11]. Moreover, AA is prone to oxidation and decomposition in acidic environments (for example, in the stomach). Therefore, the delivery of intact AA (protected from gastric fluid) into the intestine can significantly enhance the therapeutic effects of dietary AA [12]. Loading AA into carriers can protect the active compound from degradation due to external conditions, enhancing the stability and shelf life of vitamin-based products [9,11].

Furthermore, AA plays a crucial role in various physiological functions, and stability enhancements could enable its application in different fields. This review discusses recent advances (reported over the past five years) in the stabilization of AA by carrier loading, highlighting the medical applications of these stabilized moieties. It could serve as a useful repository of information and facilitate future research on AA-loaded carriers.

Reactive oxygen species (ROS), produced as a result of normal physiological processes, are vital for cell signaling and tissue homeostasis [13]. However, these highly reactive species oxidize biological materials, exerting diverse effects on organisms. In animals, ROS are associated with cancer, aging, cataracts, Alzheimer’s disease, and blood-circulation disorders [14]. AA exhibits potent antioxidant properties, facilitating ROS detoxification [15]; its free-radical scavenging ability is attributed to the fully oxidized dehydroascorbate species generated from AA [16]. While AA primarily undergoes cycling between its fully reduced form and the monodehydroascorbate radical (A^•–), complete oxidation process leading to DHA can indeed take place. A significant pathway involves the disproportionation of the A^•–, wherein two A^•– react to yield one molecule of ascorbate and one molecule of DHA. The rapid and essential disproportionation of the A^•– plays a crucial role in the antioxidant properties exhibited by AA. This process effectively maintains the concentration of the A^•– at an extremely low level, thus facilitating the oxidation of AA [16,17]. Notably, AA exhibits antioxidant properties that inhibit scar formation and skin damage, improving the functionality and moisture content of the skin barrier [18]. During inflammation, the immune system generates ROS to eliminate microorganisms. However, excessive ROS levels can adversely affect (and even kill) healthy host cells at the defect site (e.g., fibroblasts). Antioxidants minimize the negative impacts of ROS on healthy cells by activating cell-signaling pathways [19]. Furthermore, AA reduces the risk of cardiovascular disease by preventing the oxidation of low-density lipoproteins, a key factor in the development of atherosclerosis [20]. AA is also used as an antioxidant in the food industry [21]. The biological activity of AA in food (which includes the protection of oxidizable compounds, inhibition of enzymatic browning, oxygen scavenging, and prevention of nitrosamine formation) makes it a useful food additive [22].

Notably, AA exhibits both prooxidant and antioxidant properties [23]. It reduces transition-metal ions (such as Fe³⁺ and Cu³⁺) to generate highly reactive free radicals by the Fenton reaction, which cause DNA damage (by interacting with the phosphodiester backbone of DNA and modifying the DNA bases). The prooxidant activity of AA, which induces cytotoxicity, has been extensively explored for the prevention and treatment of cancer. Its efficacy is dose-dependent [24-26]. Low concentrations of AA (administered orally) exhibit antioxidant properties. Contrarily, high concentrations of AA (gram doses; administered intravenously) exhibit prooxidant properties, and can enhance the efficacy of certain cytostatic drugs (such as paclitaxel, carboplatin, and arsenic trioxide) and radiotherapy [27].

AA is a cofactor for enzymes and several bioactive processes associated with cellular functions (including neuromodulation, hormone regulation, immune-system support, and collagen synthesis) that enable the formation of connective tissue, bone matrix, and scar tissue during wound healing. As an enzyme cofactor, AA enables the regulation of hydroxylation in multiple enzymatic reactions [28,29]. AA is a cofactor for proline and lysine hydroxylases, which stabilize the tertiary structure of collagen molecules and promote collagen gene expression [30-32]. Moreover, AA contributes to all phases of the wound-healing process, accelerating healing in superficial and deep wounds across various body tissues (including the skin and bones). It facilitates neutrophil clearance during the inflammatory phase and supports collagen synthesis and maturation during the proliferative phase [31,32]. Notably, AA simultaneously increases the expression of genes related to osteogenesis (such as bone morphogenetic protein-2, osteocalcin, and RUNX2) and reduces the expression of those related to osteoclast differentiation (such as tartrate-resistant acid phosphatase, cathepsin K, receptor activator of nuclear factor kappa-B, and receptor activator of nuclear factor kappa-B ligand) [33]. Additionally, it upregulates endogenous retroviruses in lymphoma cells, inducing the viral defense pathway to activate the immune system and recognize tumor cells, thereby triggering interferon response [34]. Moreover, AA epigenetically modulates T-cells, promoting both innate and adaptive immune responses [35,36]. The activity of AA is schematically represented in Fig. 2.

Figure 2. Schematic representation of the activity of ascorbic acid (drawn by the authors).

Although AA is vital for several biochemical and metabolic processes, it is highly sensitive to environmental changes. Therefore, AA has been loaded into different types of carriers to protect it from external factors that adversely affect its stability [37-39].

AA-GCh-PCNC nanocapsules, comprising AA-encapsulated glycidyltrimethylammonium chloride (GTMAC)–chitosan (GCh) complexes (stabilized through electrostatic interactions) cross-linked with phosphorylated-cellulose nanocrystals (PCNC), exhibit a high encapsulation efficiency of 90.3% ± 0.42%. The synthesis of these nanocapsules is shown in Fig. 3A. AA-GCh-PCNCs exhibit the sustained and gradual release of AA, with an approximate cumulative release of 18% over 14 days. The release profiles of AA-GCh-PCNC under simulated human-digestive-system conditions (with pH 7.4 as the control) are shown in Figs. 3B, C; a significantly high cumulative release of AA over 8 hours is observed under these conditions. At 8 hours, ~27% of AA release occurs under digestive-system conditions, with less than 10% of AA release under control conditions (at neutral pH). The enhanced release of AA under digestive-system conditions can be attributed to the mucoadhesive properties of chitosan and cellulose nanocrystals, which facilitate the attachment of AA to the mucous membrane, enabling the controlled release of AA over an extended duration [11].

Figure 3. Modified cellulose nanocrystal/chitosan nanocapsules for the controlled release of ascorbic acid (AA). (A) Schematic illustration of the formation of AA-GCh-TPP and AA-GCh-PCNC nanocapsules from AA, chitosan (GCh), sodium tripolyphosphate (TPP), and phosphorylated-cellulose nanocrystals (PCNC). (B) Diagram of the human digestive system. (C) In-vitro simulation of digestive-system conditions to analyze the activity of AA-GCh-PCNC. Reused from the article of Baek et al. (Curr Res Food Sci 2021;4:215-23) [11] with original copyright holder’s permission.

Microencapsulation is widely used to maintain the stability of sensitive compounds under different conditions [29,40]. Kalaycioglu and Aydogan [41] reported microcapsule development by the layer-by-layer method for the dual drug delivery of AA and ibuprofen; the reported microcapsules comprise gold nanoparticles and solid lipid nanoparticles alternately coating the drug crystals, and chitosan was used as the final building block. Additionally, AA encapsulated by microcapsules comprising gelatin and gum arabic exhibit high stability. Thermogravimetric data indicates that pure AA exhibits two mass losses of 31.8% and 12.4% within the temperature ranges of 193°C-250°C and 275°C-311°C, respectively, whereas microencapsulated AA exhibits mass losses of 4% and ~85% in the temperature ranges of 168°C-218°C and 377°C-456°C. These results confirm that encapsulation significantly enhances the thermal stability of AA. Furthermore, after 15 days of storage, the concentration of free AA decreases to 15%, whereas the AA microcapsules exhibit a high retention rate of 90.3% [42].

Liposomes are structurally similar to cell membranes; moreover, they exhibit high biocompatibility, low toxicity, and low immunogenicity. Therefore, they effectively enhance the stability of bioactive compounds through encapsulation [10,43]. Jacob et al. [21] reported the development of stabilized fiber-reinforced-phospholipid-based powdered formulations of AA (Zeal-AA) using nanofiber (NF)-weaving (Zeal) technology that do not undergo rapid clearance from the bloodstream (unlike conventional liposomal formulations). The synthesis is shown in Fig. 4. Zeal-AA exhibits an encapsulation efficiency of 83.58% ± 2.18%. Moreover, the oral administration of Zeal-AA causes a 5.9-fold higher bioavailability of AA than that of pure AA. Several studies have evaluated the storage stability of AA-loaded liposomes [44,45]. According to Maione-Silva et al. [44], AA-loaded liposomes are stable for at least 30 days; after 30 days, samples stored at 4°C and 25°C retain ~95% and >90% of their initial AA content, respectively.

Figure 4. Schematic representation of the fabrication of phospholipid vesicles reinforced with turmeric fibers loaded with ascorbic acid. Reused from the article of Jacob et al. (ACS Omega 2021;6:5560-8) [21] with original copyright holder’s permission.

Electrospinning has emerged as a promising method for encapsulating bioactive molecules in the pharmaceutical field [46]. This technique enables the production of NFs with unique characteristics (such as a large surface-area-to-volume ratio, nanoscale structure, high porosity, and low molecular weight) [47,48]. Electrospinning has been used to incorporate a diverse range of drugs into NF webs/meshes, facilitating their application in pharmaceuticals. Khan et al. [49] reported the fabrication of AA-loaded NFs comprising hydroxypropyl-β-cyclodextrin (HP-β-CD). Thermogravimetric analysis indicates that the main degradation step of pure AA occurs within the temperature range of 190°C-220°C, whereas that of AA loaded in HP-β-CD-NF occurs within a wider temperature range (240°C-310°C). Moreover, the latter undergoes lower degradation than the former. Therefore, HP-β-CD improves the thermal stability of AA by protecting it from degradation. Avizheh et al. [50] reported the development of electrospun NFs incorporating AA and caffeine. The storage stability of AA in these NFs is greater than that of pure AA (the control). After 45 days of storage at room temperature, the control samples of AA exhibit a degradation rate of ~40%; contrarily, the NF formulations exhibit an AA content of ~80% after 45 days of storage at both room temperature and under refrigeration.

Furthermore, several studies have attempted to enhance the stability of AA by incorporating it into different types of carriers (such as scaffolds [32,51,52], hydrogels [5,26], NF molecular capsules [6], lipid nanocarriers [53], biopolymers [54], thin films/membranes [18], and water-in-oil-in-water emulsions [8]). Table 1 summarizes the stability of several AA-loaded carriers reported to date.

Table 1 . Stabilization of ascorbic acid on loading in different carriers

System	Material	Stability improvements	Reference
Nanoparticles	Glycidyl trimethylammonium chloride, chitosan and phosphorylated-cellulose nanocrystals	Shows sustained release of ascorbic acid over 14 days	[11]
	Tripolyphosphate-β-cyclodextrin and chitosan	After 3 weeks of storage at 4°C, the retention of ascorbic acid is ~98.3% Protection of ascorbic acid in acidic conditions, with sustained release in weakly alkaline conditions	[28]
	Layered double hydroxide and tetraethyl orthosilicate	Drug elimination range increases at high temperatures Delays the release of ascorbic acid	[38]
Microparticles	Sodium alginate and gum arabic	Maintains the stability of ascorbic acid up to 188°C	[29]
	Fully hydrogenated palm oil and palm oil	Gradually releases ascorbic acid	[37]
	Corn starch and gum arabic	Improves the storage stability of ascorbic acid	[39]
	Gelatin, sodium caseinate and genipin	Shows controlled release of ascorbic acid	[40]
	Gold nanoparticles, solid lipid nanoparticles and chitosan	Shows controlled release of ascorbic acid	[41]
	Gelatin and gum arabic	Significantly improves the thermal stability of ascorbic acid After 30 days of storage at 20°C, the retention of ascorbic acid is ~80% Shows controlled release of ascorbic acid	[42]
Liposomes	Phospholipids, cholesterol and phytosterols	Enhances the stability and controlled release of ascorbic acid	[10]
	Phospholipid and turmeric fibers	Exhibits 5.9 times more bioavailability than normal ascorbic acid A single oral dose is capable of increasing the ascorbic acid levels in the body (relative to the control) for up to 24 hours	[21]
	Yolk lecithin and cholesterol	After 30 days of storage at 4°C and 25°C, the retention of ascorbic acid is ~80%	[43]
	Cholesterol, soybean phosphatidylcholine, 1,2-dioleoyl-3-trimethylammoniopropane and 1,2-distearoyl-sn-glycero-3-phospho-(1’-rac-glycerol)	After 30 days of storage at 4°C and 25°C, the retention of ascorbic acid is >90%	[44]
	Cholesterol, Xanthan gum and Tween-80	The average diameter changes negligibly on storage for 5 weeks at 4°C and 25°C	[45]
Nanofibers	Hydroxypropyl-β-cyclodextrin	Drug elimination range increases at high temperatures	[49]
	Alginate and polyethylene oxide	Shows controlled release of ascorbic acid	[46]
	PVA	After 45 days of storage at room temperature and under refrigeration, the retention of ascorbic acid is ~80%	[50]
Scaffolds	Polycaprolactone and β-tricalcium phosphate	Shows prolonged and sustained release of ascorbic acid over 60 days	[51]
	Polylactic acid, polycaprolactone, and gelatin	Shows continuous release of ascorbic acid for 256 hours	[32]
	Poly(urethane-urea) and layered double hydroxides	Shows sustained release of ascorbic acid for 5 days	[52]
Hydrogels	Salecan and chitosan	Protects ascorbic acid from simulated gastric fluid Maintains high levels of ascorbic acid in blood for 6 hours	[5]
	Alkyl chain	Shows sustainable release of ascorbic acid in the presence of esterase	[26]
Nanofiber-molecular capsules	PVA and β-CD	Loading ascorbic acid into β-CD enables it to withstand heating up to 350°C Exhibits programmable release profiles of ascorbic acid from the zeroth-order constant-release rate to the nonlinear burst, biphasic, and triphasic release profiles	[6]
Lipid nanocarrier	Tween® 20, 40, 60, and 80, stearic acid, and oleic acid	Exhibits controlled release of ascorbic acid	[53]
Biopolymer	Genipin and gelatin	Shows controlled release of ascorbic acid	[54]
Thin film/membrane	Chitosan and agarose	Shows controlled release of ascorbic acid	[18]
W/O/W emulsions	Gelatin, ethylenediaminetetraacetic acid disodium salt, polyglycerol polyricinoleate, hydrophobic fumed silica particles, β-CD, and Tween 80	After 6 weeks of storage at 25°C, the retention of ascorbic acid is >80% Shows controlled release of ascorbic acid	[8]

PVA, poly vinyl alcohol; β-CD, β-cyclodextrin.

Loading AA into carrier systems reduces its inherent instability, facilitating a diverse range of applications. This section discusses recent advances (over the past five years) in the medical application of stabilized AA, highlighting their significance.

AA is extensively used in wound healing owing to its capacity to promote fibroblast proliferation and stimulate collagen deposition during the re-epithelialization phase of wound repair [19]. Demir et al. [55] reported the development and characterization of novel regenerative, temporary, flexible wound dressings that can be easily applied to the skin and replaced after three days without causing damage to the wound. Xanthan/gelatin and keratin/xanthan/gelatin hydrogels (KXGHs) have been used in these wound dressings because of their high absorption capacity and ability to locally deliver AA. AA is continuously released from these hydrogels over 100 hours. Fibroblast viability analyses indicate that these AA-containing KXGHs exhibit cytocompatibility. Sirius red staining experiments indicate greater collagen synthesis in hydrogel-treated fibroblasts than in those of the control group.

Rezvani et al. [56] reported the development of vesicles loaded with AA and tocopherol (Eudragit^® L100, Nutriose^® FM06, and Lipoid S75). These vesicles protect the loaded drugs from intestinal oxidative-stress damage on oral administration, facilitating excellent intestinal-wound healing. The co-loaded vesicles show high biocompatibility, with >100% cell viability at different tested concentrations. The nutriosomes exhibit antioxidant properties, protect cells against oxidative stress, and promote cell proliferation with 110% viability. In-vitro studies indicate that neither the pure drug solutions nor liposomes effectively stimulate cell migration and proliferation for wound closure, whereas the nutriosomes and Eudragit^® L100-nutriosomes accelerate the healing process. The nutriosomes enable complete wound closure within 96 hours, possibly owing to a synergistic effect of the antioxidants and Nutriose^®.

Patients with diabetes often experience slow wound healing, which can lead to severe outcomes (such as limb amputation or death). Sun et al. [57] reported the development of novel epigallocatechin gallate (EGCG), AA, gelatin, and chitosan nanoparticles (EV NPS) that accelerate wound healing. Over a period of 2-8 days, diabetic mice administered EV NPS (comprising the EV NPS group) exhibit a significantly higher wound-healing rate than the model, no-load, and EGCG groups (all p < 0.001). Moreover, during the same period, the wound-healing rate of the model group is significantly different from that of the normal control group (with p < 0.05 and p < 0.001, respectively). These results indicate that EV NPS effectively promotes wound healing in diabetic mice, possibly because of increased collagen accumulation, which promotes angiogenesis and reduces inflammatory cell infiltration into the diabetic wounds.

In addition to the aforementioned treatments, diverse forms of AA have been extensively investigated for potential skin-related applications. Intact chitosan-AA-lactic acid composite membranes, enhanced by the addition of glycerol and polyethylene glycol (PEG), exhibit high potential for skin-tissue-engineering applications [31]. Skin patches comprising composite poly vinyl alcohol (PVA) NF-based molecular capsules exhibit the controlled and linear release of AA upon contact with skin moisture [6]. An adapalene-loaded transfersome gel incorporating AA has been proposed for the combination therapy of acne vulgaris [58]. AA-incorporated sodium alginate/polyethylene oxide species (fabricated in the core-shell and blended configurations using two distinct electrospinning setups) exhibit high potential as drug-delivery systems for the treatment of pigmented purpuric dermatosis [46].

Corneal stromal regeneration is challenging because of its complexity; moreover, there is a shortage of suitable donor corneas for transplantation. To resolve these issues, several studies have attempted to develop functional corneal stromal substrates. Moghanizadeh-Ashkezari et al. [52] reported a polymeric substrate comprising AA-loaded poly(urethane-urea)/ZnAl-layered double hydroxide-aligned scaffolds (PUU-AA-LDH), which mimics the extracellular matrix of the native corneal stroma tissue, facilitating tissue regeneration (Fig. 5A). According to in-vivo experimentation, the synthesized PUU shows biocompatibility in the rat model, with gradual degradation over time; it forms connective-tissue capsules and vessels around the sample with no evidence of complications (Fig. 5B). Histological analysis (with hematoxylin and eosin staining) 30 and 60 days after implantation indicates the formation of new blood vessels in the polymeric film. Moreover, the PUU-AA-LDH scaffold exhibits higher cell viability and keratocyte proliferation than other scaffolds, possibly owing to the presence of AA and zinc (Figs. 5C, D). Transcriptional and protein investigations indicate that the expression of vimentin, a major structural protein in normal keratocytes, is up-regulated in human keratocytes cultivated on PUU-AA-LDH, which exhibit a lower content of α-SMA than the control, indicating the differentiation of fewer cells into the myofibroblast phenotype (Figs. 5E, F). Therefore, the PUU-AA-LDH scaffold exhibits high potential as a substrate for the regeneration of corneal stromal tissue.

Figure 5. (A) Schematic representation of the synthesis of ascorbic acid-loaded poly(urethane-urea)/ZnAl-LDH aligned scaffolds. (B) Images of rats and PUU films stained with hematoxylin and eosin, 30 and 60 days after implantation. Black arrows indicate vessel formation in samples during the implantation period. (C) Cell viability results of PUU, PUU-LDH, and PUU-AA-LDH scaffolds according to the AlamarBlue assay after 24 hours and 1 week (*p < 0.05). (D) SEM images of keratocyte cell adhesion on the scaffolds after 24 hours and 1 week. (E) Immunocytochemistry of human corneal keratocytes for vimentin after 1 week of cultivation on PUU, PUU-LDH, and PUU-AA-LDH. Immune reactivity (green) of cells in all the groups to FITC-conjugated vimentin antibodies. (F) Percentage of human keratocytes that test positive for ALDH and α-SMA proteins, 24 hours and 1 week after cultivation in PUU, PUU-LDH, and PUU-AA-LDH. The graphs indicate a significantly higher percentage of ALDH protein expression by keratocytes cultivated for 24 hours and 1 week on PUU-AA-LDH than on PUU (p = 0.0001) or PUU-LDH (p = 0.0001). Notably, keratocytes cultivated on PUU-AA-LDH for 24 hours and 1 week exhibit a significantly lower percentage of α-SMA protein expression than those cultivated on PUU (p = 0.0001) or PUU-LDH (p = 0.041 after 24 hours and p = 0.003 after 1 week). Reused from the article of Moghanizadeh-Ashkezari et al. (ACS Appl Mater Interfaces 2019;11:35525-39) [52] with original copyright holder’s permission.

AA significantly influences bone formation by promoting osteoblast differentiation and inhibiting osteoclast differentiation; therefore, its incorporation into scaffolds could facilitate bone regeneration [32,51]. Hashemi et al. [32] reported the development of a highly porous three-dimensional scaffold comprising polylactic acid/polycaprolactone (PCL)/gelatin and AA for bone regeneration. In-vivo analysis indicates strong cell attachment to the pore walls of the scaffold; moreover, it confirms that AA-containing scaffolds exhibits better bone-healing properties than AA-free scaffolds. Bose et al. [51] reported enhanced bone regeneration using PCL-coated β-tricalcium phosphate (β-TCP) species containing AA. Microstructural analysis indicates a greater formation of multilayered osteoblast cells on the AA-incorporated sample surface compared with that on the control, consistent with 3-(4,5-di-methylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assays. Moreover, the osteoblast cell viability increases two-fold in the presence of these AA-loaded species, regardless of the time. Additionally, the alkaline phosphatase (ALP) density increases on day 11, confirming the influence of AA on osteoblast differentiation. In-vitro osteosarcoma cell cultures show a four-fold decrease in cell proliferation on day 3 with the AA-loaded species compared with the control. Therefore, the sustained release of AA from PCL-coated TCP scaffolds effectively enhances osteoblast proliferation and differentiation, making it a promising drug-delivery system for bone-tissue engineering with minimal to no side effects.

Cano et al. [59] reported the synthesis of dual-loaded nanoparticles incorporating AA (with antioxidant and anti-inflammatory properties) and EGCG, which effectively reduce cognitive impairment in APPswe/PS1dE9 mice, a model of familial Alzheimer’s disease.

Research indicates that pharmacological concentrations of AA sensitize cancer cells to chemotherapeutic agents, enhancing their anticancer effects [60]. Synergistic interactions between AA and chemotherapeutics have been used for the treatment of several types of cancer (including breast cancer) [61,62]. Fahmy et al. [63] reported the development of a pH-responsive dual-loaded nanosystem (AA-OX/PEG-CS NPs) that uses PEGylated chitosan nanoparticles (PEG-CS NPs) to deliver both AA and oxaliplatin (OX) for breast-cancer treatment. This nanosystem exhibits significantly higher release rates of AA and OX in the acidic microenvironment of cancer cells (pH 5.5) than in the neutral microenvironment of healthy cells (pH 7.4) (Figs. 6A, B). After 72 hours at 37°C and pH 7.4, AA-OX/PEG-CS NPs release ~32.43% and 25.07% of AA and OX, respectively, whereas they release ~83.2% and 90% of AA and OX, respectively, at pH 5.5 (keeping all other conditions constant). MTT assays analyzing the effectiveness of AA and OX in reducing cell viability indicate that the IC₅₀ value of AA alone is 150.8 ± 26.5 µg/ml (Fig. 6C), whereas that of AA encapsulated in CS NPs is IC₅₀ 44.87 ± 11.49 µg/ml (Fig. 6D). Additionally, systems comprising AA and OX individually encapsulated in PEG-CS NPs exhibit excellent reductions in cellular viability, with IC₅₀ values of 23.3 ± 3.73 and 17.98 ± 3.99 µg/ml, respectively (Fig. 6E). Testing the combination of AA and OX in both CS and CS-PEG NPs on MCF-7 cells indicates that AA-OX/CS NPs exhibit an IC₅₀ of 18.69 ± 2.22 µg/ml, while AA-OX/CS-PEG NPs show the lowest IC₅₀ value (7.50 ± 0.69 µg/ml) (Figs. 6F, G). Therefore, PEGylated CS NPs loaded with AA and OX (individually and in combination) exhibit high potential for breast-cancer treatment. The medical applications of AA-loaded carriers are summarized in Table 2.

Table 2 . Medical applications of ascorbic-acid-loaded carriers

Application	System	Material	Drug	Test	Reference
Wound healing	Thin film/membrane	Chitosan and agarose	Ascorbic acid	In vitro	[18]
	Membranes	Bacterial cellulose and pullulan	Ascorbic acid and vitamin E	In vitro	[19]
	Nanofibers	Poly vinyl alcohol	Ascorbic acid and caffeine	In vitro and in vivo	[50]
	Hydrogels	Keratin, xanthan, and gelatin	Ascorbic acid	In vitro	[55]
Intestinal wound healing	Liposomes	S75 phospholipid, Nutriose® FM06, and Eudragit® L100	Ascorbic acid and tocopherol	In vitro	[56]
Diabetic wound healing	Nanoparticles	Gelatin and chitosan	Ascorbic acid and epigallocatechin gallate	In vivo	[57]
Skin tissue engineering	Membranes	Chitosan, lactic acid, glycerol and polyethylene glycol	Ascorbic acid	In vitro	[31]
Skin patch	Nanofiber-molecular capsules	Poly vinyl alcohol and β-cyclodextrin	Ascorbic acid	In vitro	[6]
Acne Vulgaris treatment	Membranes	Transfersome	Ascorbic acid and adapalene	In vitro and in vivo	[58]
Pigmented purpuric dermatosis treatment	Nanofibers	Alginate and polyethylene oxide	Ascorbic acid	In vitro	[46]
Corneal stromal tissue regeneration	Scaffolds	Poly(urethane-urea) and layered double hydroxide	Ascorbic acid and zinc	In vitro and in vivo	[52]
Bone regeneration	Scaffolds	Polylactic acid, polycaprolactone, and gelatin	Ascorbic acid	In vitro and in vivo	[32]
	Scaffolds	Polycaprolactone and β-tricalcium phosphate	Ascorbic acid	In vitro	[51]
Alzheimer’s disease	Nanoparticles	Poly(lactic-co-glycolic acid) and polyethylene glycol	Ascorbic acid and epigallocatechin-3-gallate	In vitro and in vivo	[59]
Breast cancer treatment	Nanoparticles	Polyethylene glycol and chitosan	Ascorbic acid and oxaliplatin	In vitro	[63]
Cancer immunotherapy	Hydrogels	Alkyl chain	Ascorbic acid and interferon genes (STING) agonist-4 (SA)	In vitro, ex vivo, and in vivo	[26]

Figure 6. Time-dependent release profiles of ascorbic acid (AA) from (A) AA/PEG-CS NPs and (B) AA-OX/PEG-CS NPs at 37℃ and pH 7.4 and 5.5. Concentration-dependent effects of the tested compounds on the cellular viability of MCF-7 cells. MCF-7 cells treated with increasing concentrations (0.625, 1.25, 2.5, 5, 10, 20, 40, 80, 160, and 320 μg/ml) of (C) AA, (D) AA/CS NPs, (E) AA/PEG-CS NPs, (F) AA-OX/CS NPs, and (G) AA-OX/PEG-CS NPs for 48 hours. Their cellular viability is measured using the MTT assay (in triplicates). Relative IC50 values are commonly reported in pharmacological studies. Moreover, the curve-fitting approach considers the maximal and minimal responses (not simply the values between two adjacent concentrations). Reused from the article of Fahmy et al. (Pharmaceutics 2022;14:407) [63] with original copyright holder’s permission.

AA, a powerful antioxidant and essential water-soluble nutrient, enables the maintenance of optimal physiological processes in the human body. It exhibits several useful activities in the body; for example, it promotes collagen biosynthesis, scavenges free radicals, and boosts immunity against viral infections. However, AA is chemically unstable and prone to degradation, particularly on exposure to oxygen, moisture, light, high temperatures, and alkaline/acidic environments. To increase the stability of AA and facilitate its widespread application, several studies have investigated the loading of AA into carriers.

This review highlights recent advances in AA stabilization through carrier loading. Various carrier systems (including nanocapsules, microcapsules, liposomes, and NFs) have been used for the effective protection and delivery of AA. In summary, the loading of AA into carriers could open new frontiers in the medical utilization of AA. Carrier systems simultaneously protect AA from degradation, enhance its effectiveness, and promote its controlled delivery to specific target sites. AA-loaded carriers exhibit high potential for advanced wound healing, skin-related therapies, bone regeneration, and cancer treatment. Further research in this field could enable the development of highly efficient medical treatments and therapies that utilize the full potential of AA.

None.

None.

Conceptualization: GC. Data curation: all authors. Formal analysis: all authors. Funding acquisition: GC. Investigation: GC. Methodology: all authors. Project administration: GC. Visualization: all authors. Writing–original draft: all authors. Writing–review & editing: all authors.

No potential conflict of interest relevant to this article was reported.

This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2022R1F1A1076459) and the research fund of Dankook University Research and Business Development Foundation in 2023.

None.