Emerging Applications of Pickering Emulsions in Pharmaceutical Formulations: A Comprehensive Review (2025)

Abbreviations: PLGA-PVA, poly(lactic-co-glycolic acid)- poly(vinyl alcohol); CGPLAP, cashew tree gum grafted with polylactide; SiO₂, silicon dioxide; CaCO₃, talcum carbonate; Fe₃O₄, ferric oxide.

Natural Polymer Nanoparticles

In emulsions, it is important to prevent droplets from coalescence to maintain stability. Some polysaccharides, which are natural polymers, are insoluble in both water and oil. For instance, the most representative polysaccharides such as gum Arabic, modified cellulose, and starch can be utilized to stabilize emulsions.^Citation38 In recent years, polysaccharide-based nanoparticles have shown tremendous capacity for applications in the fields of biotechnology, pharmaceuticals, and food industries. The information detailed is succinctly presented in .

Cellulose, a widely found biopolymer, is an ideal material option due to its sustainability, biodegradability, and non-toxic nature. Cellulose nanocrystals (CNCs) and cellulose nanofibrils are the primary forms of cellulose nanoparticles.^Citation39 CNCs, prepared by acid hydrolysis, possess a highly crystalline structure and have rod-like nanosizes. They can form a stable interfacial film at the oil-water interface, thereby maintaining the stability of the emulsion.^Citation63 Dong et al successfully synthesized spherical cellulose nanocrystals (S-CNCs) with diameters ranging from 30 nm to 60 nm using mixed acid hydrolysis of mercerized microcrystalline cellulose combined with ultrasonic treatment.^Citation39 The PEs’ droplets stabilized by S-CNCs showed remarkable stability across a wide range of pH, ionic strength, and temperature variations. Consequently, the utilization of S-CNCs as novel stabilizers for PEs presents promising opportunities in the fields of biomedical, cosmetic, and food applications. Furthermore, bacterial cellulose nanocrystals (BCNs) proved to be another promising emulsifier for the preparation of stable oil-in-water PEs.^Citation40 Bacterial cellulose (BC) can be further purified with sodium hydroxide to yield a form of BC with an endotoxin value below 20 endotoxin units, meeting the acceptable range for implant applications as defined by the US Food and Drug Administration. This suggests that BCNs may be suitable for intravenous applications.^Citation64 However, the wider application of nanocellulose-based PEs is hindered by safety concerns, and there is a lack of established protocols for evaluating the immunotoxicity, genotoxicity, and nutrient absorption associated with these emulsions. Additionally, the lack of standardized nanocellulose products, the limited number of manufacturers, and high production costs pose significant challenges.^Citation65

Starch, the second most abundant natural polymer after cellulose, finds extensive usage in the food and pharmaceutical industries.^Citation66 The inherent hydrophilic structure of natural starch granules restricts their ability to effectively stabilize the oil-water interface. Consequently, chemical modifications were performed to enhance their hydrophobic properties, thereby facilitating improved adsorption at the oil-water interface.^Citation67 One commonly used modification method is the application of octenyl succinic anhydride (OSA). For instance, Wang et al conducted a study in which enzymatically branched glutinous maize and potato starch were recrystallized to form starch globule crystals.^Citation42 These crystals were then modified with OSA to increase their hydrophobicity, ultimately serving as emulsifiers for the preparation of PEs. The researchers discovered that the resulting octenyl succinate starch spherocrystals played a role in protecting the stability of encapsulants and slowing down lipid oxidation. This finding contributes to our current understanding of emulsifiers for edible particles. To address the inherent limitations of OSA modification, Ma et al successfully synthesized butyric acid (BA)-modified porous starch (PS) with varying degrees of substitution.^Citation43 Through an esterification reaction, BA-PS with high degree of substitution can effectively stabilize the encapsulation of PTX in PEs, significantly enhancing emulsification and PTX retardation capabilities. This suggests that BA-PS has the capacity to be utilized as an emulsifier for slow-release drug delivery. Furthermore, there is still ample room for exploration in the application and development of environmentally friendly methods for producing starch granules to stabilize PEs.^Citation68

Chitosan is derived from chitin through deacetylation in the presence of hot alkali, and it possesses remarkable properties such as biocompatibility, biodegradability, mucous adhesion, and low toxicity.^Citation69 In comparison to traditional emulsions, PEs incorporating chitosan enhance the bioavailability of various compounds. The stability of the PEs can be achieved through the use of self-aggregated chitosan particles,^Citation70 chitosan nanocrystals (also known as chitosan whiskers, nanowhiskers, or rod-shaped chitosan), or complexes of chitosan with proteins^{Citation71,Citation72} such as corn protein or gelatin.^Citation73 As an example, Alehosseini et al tried to explore the PEs based on chitosan nanoparticles (CSNPs) as food-grade delivery systems for d-limonene, which is a bioactive compound.^Citation44 If they are able to provide data related to the in vivo safety studies of the obtained emulsion, the stable PEs of CSNPs holds promise for application in the delivery of active drug nanoparticles.

Moreover, both lignin and agarose (agarose microgels)^Citation74 are natural polymers. Lignin, a phenolic polymer, creates a three-dimensional network structure in the cell wall.^Citation75 Lignin has extensive applications in packaging materials, hydrogels, and even as a carrier for bioactive substances like enzymes and drugs.^Citation76 Nevertheless, the limited solubility and dispersibility of structurally diverse lignin pose challenges for its advancements in the realm of green energy and materials. Consequently, diverse approaches must be employed to improve its properties. For instance, Shomali et al utilized carboxylated lignosulfonate with different grafted alkyl side chains to synthesize lignosulfonate nanoparticles (LNPs).^Citation45 By adjusting the length of the alkyl chains, the resulting LNPs exhibited a higher affinity to the oil-water interface. As a result, a green, environmentally friendly, and simple water-based acid precipitation process was employed to obtain emulsions with smaller and more stable droplet sizes. In addition, Zhang et al have reported a green and simple method for the production of LNPs using beet alkaloid and lactic acid deep eutectic solvent through water droplet-induced self-assembly, which effectively addresses the issue of environmentally unfriendly organic solvents.^Citation76 Moreover, Dai et al have developed a novel multi-functional PEs stabilized by LNPs to enhance the stability and thermal-controlled release of trans-resveratrol.^Citation46 The research findings suggest that LNPs-based PEs hold great capacity in the storage and thermal-controlled release of light-unstable and poorly water-soluble drugs.

While certain natural substances such as proteins or polysaccharides are utilized in the formulation of these emulsions, the resultant particles typically exhibit polydispersity, instability, or excessive hydrophilicity, rendering them inadequate for emulsion stabilization. Indeed, the challenges associated with quality control of natural materials, influenced by factors like production locale and processing parameters, further complicate matters.^Citation77 Consequently, research pertaining to PEs in pharmaceutical applications warrants further exploration.

Synthetic Polymer Nanoparticles

Biodegradable polymer particles have garnered attention specially in the field of drug delivery due to their reduced toxicity, ability to control drug release kinetics, and enhanced biocompatibility.^Citation78 Biodegradable polymers comprise synthetic polymers like polylactic acid (PLA), and polyglycolic acid, as well as their copolymers, such as PLGA, and polycaprolactone.^Citation17 For instance, to enhance Amphotericin B (AmB) oral bioavailability and control drug release, Richter et al first reported the utilization of self-assembled cashew gum grafted with PLA nanoparticles, obtained through nano-precipitation, to obtain stable AmB-loaded PEs through spontaneous emulsification.^Citation47 Biodegradable particles made of PLGA offer the advantages of biocompatibility and rapid clearance from the body, which can help prevent inflammatory reactions due to accumulation. In a separate study, Deschamps et al employed a simple emulsification technique using two connected syringes to incorporate PLGA nanoparticles into the aqueous phase, resulting in the formation of an oil-in-water lipiodol emulsion with an average droplet size of approximately 40μm, which is comparable to the diameter of blood vessels near tumors.^Citation48 They loaded doxorubicin (DOX) into the internal phase and found that complete release of DOX from the stable lipiodol emulsion required up to 10 days, demonstrating the capacity of these solid particle-stabilized emulsions for efficient encapsulation and sustained release of chemotherapy drugs for the treatment of tumors. In line with prior research, in vitro experiments have revealed that PLGA nanoparticles exhibit no discernible toxicity until reaching relatively high concentrations of 3 mg/mL.^Citation79 However, the toxicity of PLGA up to 3 mg/mL needs further exploration, because stabilization of PEs requires larger concentrations of solid particles than this. Notably, in a significantly dynamic physiological environment, the interaction between nanoparticles and cellular components would occur at a reduced concentration level necessitating continuous fluid replenishment, thereby mitigating the risk of cytotoxic effects.^Citation48 As a result, more in-depth studies on toxicity tests including genotoxicity assay are needed, which hold significance for use in the injectable administration of chemotherapeutic drug.^Citation80

Inorganic Nanoparticles

Inorganic solid particles commonly employed for the preparation of PEs comprise SiO₂, TiO₂, ZnO, CaCO₃, Fe₃O_4,^Citation81 carbon nanotubes, kaolin, quantum dots (QD), and various modified inorganic solid particles. Extensive research has demonstrated that modified solid particles can enhance the stability of PEs and regulate their demulsification.^Citation82 Silica nanoparticles, also known as nano-silica, are extensively utilized owing to their cost-effectiveness, favorable biocompatibility, low in vivo toxicity, and antibacterial properties. Therefore, SiO₂ is a frequently utilized material in the synthesis of PEs.^Citation83 For example, Heidari et al employed the Taguchi approach to modify SiO₂ nanoparticles using cetyltrimethylammonium bromide to stabilize PEs, making them bioactive compounds delivery system.^Citation49 In the study by Tang et al, natural kaolinite was modified to prepare phosphatidylcholine-kaolinite and used to stabilize PEs encapsulating curcumin (Cur).^Citation50 The resulting PEs were effectively absorbed by cells, non-toxic, and biocompatible, and they could completely release Cur in the intestines to improve the bioavailability of Cur. These findings may contribute to the effective encapsulation of lipophilic food or drugs in PEs to enhance their bioavailability. Some inorganic particles also exhibit characteristics of responsiveness to external stimuli. For example, Huang et al developed QD-based nanoreactors by creating amphiphilic PbS QD-stabilised PEs through the formation of Janus ligand shells. Their research offers valuable insights for enhancing the effectiveness of nanoreactors and microemulsions in facilitating multiphase photocatalytic reactions.^Citation51 In addition, Xie et al proposed a photomagnetic dual-responsive PEs micro-reactor, in which photosensitive TiO₂ and Fe₃O₄ nanoparticles are co-adsorbed at the oil-water interface of the emulsion droplets.^Citation52 By approaching each other under an external magnetic field, the reactants came into contact and triggered a chemical reaction under ultraviolet light irradiation. Their research has opened up broad prospects for the construction of multifunctional PEs. Further research could investigate the utilization of visible light instead of ultraviolet light in photomagnetically driven PEs microreactors, offering significant potential for advancements in the field of life sciences. However, there is relatively limited research on the in vivo safety of these inorganic nanoparticles, which hinders their application as drug carriers.^Citation84

Protein Nanoparticles

Protein nanoparticles possess several advantages that make them well-suited for the preparation and stabilization of traditional PEs. These advantages include their natural origin, non-toxicity, biocompatibility, biodegradability, potential health benefits, good surface activity, and emulsion stability.^Citation85 In the context of PEs, protein nanoparticles can take on various shapes, including non-deformable solid spherical nanoparticles,^Citation53 nanofibers, nanotubes, nanogels, nanocages,^Citation86 and plate-like nanoparticles. Protein-based PEs have greater applications in the food and pharmaceutical industries compared to traditional inorganic particle-stabilized PEs. These applications include substituting partially hydrogenated fats in food,^Citation87 encapsulating, stabilizing, and releasing active ingredients,^Citation53 and inhibiting lipid oxidation.^Citation88 For instance, in the work of Sun et al, the recombinant oil bodies protein (ROP) was induced using guanidine hydrochloride.^Citation53 In this approach, the ROP first formed a core-shell structure to encapsulate Cur, and then the formed ROP-Cur colloidal particles were used to stabilize PEs for the preparation of a double-chamber microdroplet system to achieve co-encapsulation of Cur and vitamin D. This study provides an important avenue for developing nutrition delivery systems and improving the stability and bioactivity of hydrophobic nutrients. Furthermore, Xu et al utilized a GroEL-derived protein nanoring from Escherichia coli to stabilize PEs for encapsulating β-carotene.^Citation54 In the context of promoting biocompatibility and sustainability, this work offers a more environmentally friendly solution for emulsion-based applications. Moreover, Feng et al successfully prepared a nanocomplex of corn zein and sodium caseinate by surface adsorption, which enhanced the stability and dispersibility of corn zein colloidal particles in the aqueous phase.^Citation55 This approach was employed to explore a novel method of utilizing food-grade protein surface modification to stabilize PEs. The aforementioned studies indicate that proteins have the ability to stabilize emulsions. However, it is important to note that protein-stabilized PEs are highly susceptible to alterations in environmental factors such as pH, ionic strength, and temperature. If these conditions surpass certain thresholds, the PEs have a tendency to flocculate and expedite the destabilization process. Consequently, further adjustments are required in order to utilize protein particles effectively in the future for the production of PEs.^{Citation89,Citation90}

Nanocrystalline Drugs, Fat Crystals

Recently, a new delivery system called drug nanocrystal (NC) self-stabilized PEs (NSSPE) has been developed for poorly water-soluble drugs. This system is stabilized by insoluble drug nanocrystals, eliminating the need for additional stabilizers. NSSPE offers the combined benefits of NC and PEs while addressing the side effects associated with inorganic solid particles like silica. One of the key advantages of NSSPE is its ability to enhance the water solubility of insoluble drugs such as quercetin, geraniol,^Citation91 and silymarin flavonoids,^Citation92 leading to improved oral bioavailability.^Citation93 This delivery system, consisting of just three components (water, oil, and drug), significantly increases the drug loading capacity as the drug no longer needs to be dissolved in the oil phase.^Citation94 In the work by Sheng et al, Cur was adopted as a typical Biopharmaceutics Classification System (BCS) IV drug, and a stable PEs of Cur-NCs was prepared using either poorly digestible (isopropyl palmitate, IPP) or easily digestible (soybean oil, SO) oils.^Citation94 After optimization, they found that the concentration of Cur in the formulation far exceeded its solubility in IPP or SO. Their research provides a new strategy for improving the oral bioavailability of Cur and other BCS IV drugs. Additionally, Wang et al proposed the introduction of traditional Chinese medicine volatile oil - turmeric oil as the oil phase in the concept of “drug excipient combination” to prepare a drug called NSSPE, and Cur-NSSPE lyophilized powder was prepared using the freeze-drying method.^Citation56 This Cur-NSSPE lyophilized powder showed more significant advantages in its anti–inflammatory effects both in vitro and in vivo compared to Cur-API and Cur-NCs.

Solid lipid nanoparticles (SLNs), commonly referred to as fat crystals, have been employed as colloidal nanocarrier systems in drug delivery.^Citation95 Nevertheless, there is a lack of pharmaceutical applications that utilize fat crystal stabilized PEs. These fat crystals, however, have demonstrated their capability as suitable candidates for this purpose due to their exceptional biocompatibility, minimal toxicity, ability to safeguard thermally unstable molecules against degradation, and their specific targeting capabilities. Lipid crystals, such as triglycerides, contain small amounts of monoglycerides and diglycerides and can serve as solid particles with unique temperature-responsive properties.^Citation57 They provide stability to W/O emulsions and effectively build the emulsion droplet by forming an interfacial film and a crystalline network, creating the desired texture.^{Citation19,Citation96} In the work of Li et al, they developed SLNs based on medium- and long-chain diglycerides (MLCD) as interface stabilizers for W/O PEs.^Citation57 By using MLCD as the lipid core and controlling the crystallization curve, crystal form, contact angle, and interface adsorption curve through the selection and concentration of surfactants, the rigidity of SLNs can be adjusted. The high rigidity of SLNs makes water-in-oil PEs a viable option for reducing saturated fatty acid levels and serving as a carrier for water-soluble bioactive compounds. While researchers have demonstrated the safety of the SLNs, the inclusion of surfactants to enhance SLNs properties in the formulation of PEs drug delivery systems necessitates further research to ensure safety.^Citation97

Microbial Nanoparticles

Various microscopic particles can be found in nature, including viruses, spores, bacteria, and yeast, at both micro- and nano-scale levels. These particles exhibit uniform size distribution and can be tailored for pharmaceutical purposes.^Citation17 For example, virus-like nanoparticles lack genetic material and consist of protein shells. They are robust structures capable of encapsulating drug molecules at the nanoscale. Within nanomedicine, viruses have been engineered as intelligent carriers for targeted drug delivery.^Citation98 Nonetheless, concerns regarding the immunogenicity, cytotoxicity, inflammatory response, and other toxicities associated with mammalian viral vectors have prompted the exploration of plant viruses or plant virus nanoparticles as alternative nanocarriers.^Citation99 Extensive research has revealed the ability of these natural particles to stabilize PEs, enhancing biocompatibility, bioactivity, and bioavailability in body fluids while reducing toxicity. Notably, the Tobacco mosaic virus can self-assemble into nanorods with amphiphilic properties and inherent self-assembly capability, demonstrating significant potential for drug delivery and viral detection.^{Citation58,Citation99} Similarly, Ganoderma lucidum spores have been identified as effective stabilizers of PEs due to their unique organic composition and structure.^Citation59 Furthermore, in a study by Firoozmand et al, yeast and lactic acid bacteria were employed as micrometer-sized colloidal particles to stabilize oil-in-water emulsions, resulting in long-term stability of the dispersed oil droplets (lasting over one year) upon optimization.^Citation60 These microbial cells have demonstrated the ability to produce a monophasic emulsion with high oil volume fractions, characterized by closely packed oil droplets arranged in a polyhedral manner. This innovative method of emulsification stabilization holds great capacity for various food applications; however, the research in the realm of drug delivery remains limited, necessitating further exploration by the scientific community.^Citation100

Composite Particles

The utilization of emulsifiers such as starch and cellulose to stabilize emulsions may be limited by certain properties of the particles. To overcome this limitation, composite particles can be created through interactions between particles with different properties, resulting in more stable PEs. For instance, solid biopolymer nanoparticles with prolonged coalescence lifetimes were successfully produced by leveraging effective associative interactions between proteins and polysaccharides.^Citation101 These composite particles, exemplified by lactoferrin nanoparticle-inulin polysaccharide,^Citation102 were utilized to stabilize the oil-water interface and shield protein particles from pepsin, thus enhancing the kinetic stability of the emulsions under gastric conditions. Moreover, Zhang et al demonstrated a sustainable approach by modifying the surface of bacterial cellulose through the physical adsorption of soybean isolate proteins onto cellulose protofibrils. The resulting complexes effectively stabilized PEs with diameters ranging from 10μm to 40μm, and subsequent in vivo digestion studies revealed enhanced resistance to lipid hydrolysis in these emulsions.^Citation61 These findings underscore the capacity of composite particles in the stabilization of PEs. Composite particles are increasingly being designed as multifunctional systems, offering simultaneous stabilization and active functionalities such as antibacterial effects, drug delivery, or environmental responsiveness. This has driven the exploration of advanced emulsion types, including multiple emulsions and double PEs, expanding their application capacity in the pharmaceutical field. Additionally, innovations in nanocomposite particles provide precise control over emulsion properties and enhanced interfacial performance, further advancing their role in complex formulation systems.

Sustained or Controlled Drug Release

PEs have emerged as a reliable method for encapsulating and delivering active pharmaceutical ingredients, utilizing solid particles to create a protective barrier.^Citation7 Indeed, the solid particle film acts as a hindrance, slowing down the diffusion of therapeutic drugs from the emulsion to the target site. This controlled release mechanism offers numerous advantages, including prolonged drug persistence, protection against degradation, reduced administration frequency, and enhanced patient compliance.^Citation103 For instance, a recent study demonstrated that pea isolate protein (PPI) could be transformed into nanoparticles at pH 3.0,^Citation104 leading to the development of a stable gel-like emulsion with sustained β-carotene release properties through microfluidic emulsification.^Citation105 Moreover, to overcome the disadvantages of high toxicity of antimicrobial peptides, high hemolysis of human cells, short circulating plasma half-life and poor stability in vivo, researchers have stabilized PEs with Parasin I-interacted lecithin and chitosan nanoparticles.^Citation106 The results of the in vitro study indicated that the emulsion exhibited the ability to enhance membrane damage and coalescence area upon bacteria binding. Furthermore, the study revealed that the peptide-composite nanoparticles used to stabilize emulsions enabled the sustained release of Parasin I, ultimately boosting their antimicrobial efficacy. In addition, the sustained drug release properties of PEs are also beneficial to cancer treatment. For instance, oil-in-water lipid iodine emulsion remains the preferred choice for localized chemotherapy in hepatocellular carcinoma treatment.^Citation107 These findings underscore the benefits of utilizing PEs for the sustained or controlled release of active drugs, such as vitamins, antimicrobial peptides, and anticancer drugs.

Targeted Drug Delivery

Targeted delivery is a key benefit of drug delivery systems, offering the competence to minimize adverse effects and enhance therapeutic effectiveness by facilitating the accumulation of substances in specific tissues.^Citation108 PEs play a crucial role in enabling precise drug delivery through mechanisms such as particle adsorption on interfaces, bonding with target cells, and increasing cellular binding of modified target ligands by expanding the contact area.^Citation109 For instance, in the context of cancer vaccine administration, conventional liquid mixtures of immunostimulants, antigens, and target molecules often struggle with diffusion and degradation issues, hindering an optimal immune response.^Citation110 In contrast, PEs can be strategically formulated to encapsulate various components with diverse characteristics, such as size, hydrophobicity, or charges. To illustrate, Du et al utilized the recognition ability of mannose by antigen-presenting cell surface receptors to create mannose-based PEs for delivering complex vaccines aimed at synergistically combating tumors in conjunction with PD-1 antibodies.^Citation111 This targeted approach not only amplified the cellular immune response but also heightened the anti-tumor effects synergistically. Insights gained from these findings shed light on the ability of PEs-based targeted drug delivery strategies. Moreover, the effectiveness of drug delivery from PEs is closely tied to the properties of the particles involved. For example, researchers have successfully stabilized β-carotene emulsions with PPI nanoparticles at pH 3.0 for intestinal targeting, leveraging the isoelectric point nature of protein particles to suit different parts of the digestive system.^Citation105

Stimulus Responsive Drug Delivery Systems

In numerous application scenarios, the surface characteristics of solid particles can undergo various physical or chemical changes triggered by environmental factors like pH, temperature, specific reagents, light, and magnetic fields. These changes can impact the wettability, charge, or size of the particles and disrupt the arrangement of the emulsion’s interface^Citation112(). Consequently, PEs can serve as responsive delivery systems by striking a balance between droplet stability and controlled rupture, enabling researchers to achieve controlled or targeted drug release. For instance, in environments with weakly acidic conditions like inflammation sites, lysosomal compartments, or tumor tissues that necessitate in vivo treatment, researchers have harnessed the acid sensitivity and degradability of acetalized starch-based nanoparticles to enhance the efficacy of targeted drug delivery systems for anticancer medications, such as Cur.^Citation113 Moreover, studies have explored thermosensitive PEs, exclusively stabilized by SLNs, for drug encapsulation and delivery like ketoprofen. These SLNs exhibited controlled drug release upon experiencing specific temperature changes, such as partial melting at 37°C.^Citation114 The versatility of PEs, whether responsive to pH or temperature changes, introduces novel opportunities for precise drug delivery systems. Notably, nano delivery systems activated by external stimuli, like photothermal responses, offer distinct advantages over those reliant on internal triggers such as pH and heat, due to their ease of manipulation and precise treatment control. Furthermore, owing to its excellent tissue penetration capacity, near-infrared radiation (NIR)-light responsive drug delivery systems present a promising avenue in cancer treatment.^Citation115 Recent research has explored the use of polydopamine (PDA) bowl-shaped mesoporous nanoparticles as Pickering stabilizers, resulting in pH-responsive emulsions with notable photothermal responsiveness under NIR light due to PDA integration.^Citation116 These properties hold significant potential for diverse biomedical applications, including drug delivery. It is important to note that the release of drugs is influenced by factors like the rupture and deformation of PEs, which could affect their stability. Hence, future stimulus-responsive PEs are likely to focus on achieving a balance between stability and effective drug release mechanisms.

Multilayer Structure

In cancer treatment, combining various medications through cocktail therapy has proven to be a promising strategy for combatting tumor regression and multidrug resistance.^Citation119 However, the challenge lies in effectively delivering multiple drugs with diverse hydrophobicity and targets in a spatiotemporal manner.^Citation109 Hence, there is a demand for the development of multi-drug delivery systems to enhance the bioavailability and effectiveness of treatments. PEs, known for their intricate structure, serve as an efficient multi-drug delivery system. The multilayer configuration of W/O/W or O/W/O emulsions offers a better solution for loading different components with varying hydrophobicity or charge.^Citation120 For instance, a recent study by Tang et al introduced PEs loaded with betanin and Cur, where betanin and Cur were dissolved in the inner aqueous and oil phases, respectively, to create the W/O emulsion. The W/O emulsion was then combined with betanin-bovine serum albumin nanoparticles dispersed in the outer aqueous phase to form a composite W/O/W emulsion.^Citation121 The synergistic effect of betanin and curcumin in inhibiting A549 cell growth highlights the capability of this multilayer structure for co-delivering multiple drugs with diverse properties, suggesting its significance in future clinical applications. Furthermore, the multilayer structure of PEs can be further combined with active targeting ligands, stimuli-responsive designs, or sustained-release techniques to achieve precise and sequential drug delivery of multiple drugs.

Oral Administration

PEs are primarily used for the oral delivery of hydrophobic as well as poorly bioavailable drugs in the expectation of increasing the stability and improving the aqueous solubility of orally administered drugs. Poorly water-soluble drugs present challenges in formulating oral drug products due to their inconsistent bioavailability and limited dissolution in the gastrointestinal tract.^Citation123 Lipid-based formulations have been explored to enhance the oral bioavailability of such drugs, but the high surfactant content in these formulations may cause gastrointestinal irritation, leading to reduced drug effectiveness and bioavailability.^Citation124 PEs offer a novel drug delivery approach using solid particles instead of surfactants, which can minimize irritation. For instance, Eudragit^® RL100 nanoparticles have been utilized to stabilize PEs, extending the efficacy of drugs like ketoprofen and reducing the dosing frequency.^Citation125 Furthermore, recent advancements in PEs involve utilizing solid particles to create a protective barrier at the water-oil interface, safeguarding the drug core from degradation in particularly acidic environments or under various stress conditions. Li et al successfully enhanced the bioavailability of β-carotene by encapsulating it in PEs stabilized with chitosan hydrochloride-carboxymethyl starch nanogels^Citation117 (see ). The Pickering particles should be biocompatible and able to withstand or respond to the acidic environment of the stomach and the enzymatic conditions of the gastrointestinal tract. For instance, Hu et al hypothesized that media-milled black rice particle-stabilized PEs would be a good oral delivery vehicle for 5-demethylnobiletin (5-DMN), which has the potential to enhance 5-DMN absorption and metabolic conversion in the colon ().^Citation126 Black rice particles are high in dietary fiber and phenolic compounds, which could be selectively broken down by the colonic microbiota. They found that black rice-stabilized PEs encapsulated 5-DMN targeting the colon could alleviate dextran sulfate sodium-induced weight loss and diarrhea. However, comprehensive data on the pharmacokinetics and bioavailability of PEs remain limited, with in vitro lipolysis models commonly employed to assess their suitability for oral drug delivery. Moreover, utilizing refractory drug nanocrystals to stabilize emulsions for oral drug delivery systems can increase drug loading capacity without the need for surfactants or polymer stabilizers.^Citation92 Besides, the elucidation of the in vivo degradation of PEs, encompassing particle stabilization and emulsion digestion, holds paramount importance. The employment of a pioneering bioimaging methodology reliant on aggregation-induced burst fluorescent probes presents a promising avenue to address this intricate issue.^{Citation127,Citation128}

Transdermal Administration

PEs demonstrate promising capabilities in the topical administration of antibiotics for skin infections and in the transdermal delivery of lipophilic drugs, as evidenced by research findings. Topical drug delivery is a challenging field with many advantages, including extended administration duration, reduced frequency of dosing, and minimized systemic side effects.^Citation130 While conventional emulsions with surfactants may compromise the skin barrier, PEs offer a promising solution due to their low toxic, non-carcinogenic nature and excellent dermal compatibility. PEs could enhance the skin permeation of drugs by augmenting the moisture level within the stratum corneum, while certain solid particles may induce changes in the structure of the stratum corneum proteins.^Citation143 Recent research by Frelichowska et al demonstrated that PEs effectively enhance the delivery of lipophilic drugs like retinol into the skin, particularly within the stratum corneum, when compared to traditional emulsions.^Citation129 These nanoparticle-stabilized emulsions exhibit unique characteristics suitable for targeting the stratum corneum or enabling controlled drug release. This phenomenon could be attributed to the capacity of PEs for coalescence in contrast to surfactants-stabilized droplets, potentially stemming from differences in droplet flexibility. Moreover, in the context of skin barrier damage and associated bacterial infections, topical application of antibiotics via PEs shows capability for treating infections and preventing further complications.^Citation130 While PEs hold promise for sustained antibiotic delivery to prevent wound infections, further research is needed to investigate their skin penetration and safety profiles in vivo.^Citation8 Techniques like confocal laser scanning microscopy were employed to track particles and investigate the penetration capability of emulsion droplets through the stratum corneum into the deeper layers of the skin. This method was utilized to assess the potential entry of the topically administered drug into the bloodstream, enabling the evaluation of in vivo skin penetration and safety of PEs.^Citation131 However, the application of PEs to transdermal drug delivery has some limitations because of emulsions with strong mobility. Consequently, certain researchers have explored the application of PEs in conjunction with a gel matrix containing Cur to address skin wound treatment, thereby extending drug retention on the skin.^Citation132 In addition, the potential for irritation caused by particles and emulsions on the skin, mucous membranes, and damaged skin is a critical consideration for ensuring the safety of drug delivery systems, which is essential for the advancement of future research.

Subcutaneous Administration

Subcutaneous injections of PEs are predominantly utilized in vaccines. A significant obstacle in vaccine development involves producing highly effective and safe adjuvants to assist antigens to elicit strong humoral and cellular immune responses.^Citation144 Research on adjuvants has particularly concentrated on utilizing particulate carriers that imitate the characteristics of microorganisms or diseased cells.^Citation145 PEs, which utilize solid particles as stabilizers, have shown promise in maintaining the deformability and migration of presented antigens while offering high biosafety and antigen-loading capabilities.^Citation146 To improve vaccine delivery efficiency, Du et al have developed a multicomponent cancer vaccine delivery system by incorporating target molecules, immunostimulants, and antigens into these emulsions.^Citation111 In the B16-MUC1 tumor model, the large interfacial area of PEs enhanced interactions between antigens and antigen-presenting cells (APCs), thereby improving antigen uptake and promoting immune activation. Notably, no signs of inflammation or toxicity were observed in the treated mice, indicating a favorable biosafety profile and effective tumor suppression. Furthermore, adjuvants, in the form of solid particles, can stabilize PEs, as demonstrated by Peng and collaborators, who created an aluminum-stabilized PEs by adsorbing alum within squalene/water, based on the nature of the microgel of alum.^Citation133 This approach not only addresses the limitations of alum in eliciting T-cell responses but also enables effective co-loading of antigens and adjuvants to enhance immune reactions. Analogously, Guo et al designed PEs loaded with adeno-associated viruses (AAVs), stabilized by biomineralized manganese nanoparticles and aluminum hydroxide.^Citation147 This platform conferred AAVs with favorable in vivo distribution kinetics and improved the safety of AAVs vaccine, ultimately enhancing AAVs infection efficiency in APCs (). Furthermore, compared to the AAVs vaccination dose, the PEs with one-fifth AAVs induced effective immune responses. Stimuli-responsive PEs offer a promising approach for the controlled delivery of antigens and adjuvants tailored for cancer treatment requirements. For instance, researchers have successfully created PEs stabilized with Fe₃O₄ cellulose nanocomposites via ultrasound-assisted in situ co-precipitation.^Citation148 This magnetic emulsion shows promise as a smart nanotherapeutic carrier for biomedical and drug delivery applications. Coating this Fe₃O₄-stabilized emulsion with antigens allowed for lymph node retention of the vaccine via subcutaneous delivery and external stimulation, thereby achieving strong activation of APCs.

Intravascular Administration

Intravascular administration of PEs is mainly used as the form of nanodroplets, especially in the treatment of tumors. Recent researchers have demonstrated that nanodroplets can passively target tumor tissues through enhanced penetration and retention effects.^Citation6 For instance, utilizing the temperature-sensitive properties of poly(N-isopropylacrylamide), researchers have developed nanoscale droplets encapsulated with PTX,^Citation27 inducing higher accumulation of the drug at the tumor site and demonstrating the capability of PEs for cancer therapy in the realm of nanomedicine. However, the tightness of the arrangement of nanoparticles at the water-oil interface may have an effect on the stability of the nanoemulsion. Consequently, more stable nano-PEs loaded with active molecules could be created using deformable or soft building blocks as nanoparticle emulsifiers, which were able to form stronger interfacial barriers to resist droplet-droplet coalescence.^Citation27 Notably, once nanocarriers enter cells via endocytosis, they are typically captured by lysosomes, reducing the efficiency of drug delivery.^Citation149 To address this issue, various delivery vectors with different lysosome escape mechanisms have been designed.^{Citation150,Citation151} For instance, PEs stabilized by hybridized nanoparticles (HNPs) composed of cetyl trimethylamine bromide (CTAB) and linoleic acid (LA) possessed the lysosomal escape ability. In an acidic environment, HNPs disintegrated the emulsion upon protonation, with CTAB disrupting lysosomal membranes and LA acting on lysosomal and mitochondrial membranes through interactions involving lysosomal iron ions, leading to tumor cell death.^Citation134 In vivo studies in a CT26 melanoma tumor-bearing mouse model demonstrated that PEs stabilized by HNPs had no potential off-target toxicity in major organs, supporting the good biocompatibility of this formulation. If employed in the transportation of anticancer medications, these nanocarriers have the possibility to generate a more potent anti-tumor effect. While scholars have investigated the safety profile of the emulsion in healthy cells and animal models, establishing its remarkable tumor-targeting specificity with minimal off-target tissue accumulation, the cellular toxicity of CTAB warrants careful consideration. Further experimentation is imperative to delineate the safety parameters comprehensively.^Citation152

Furthermore, hepatic artery injection embolization serves as a temporary reservoir for chemotherapeutic agents, allowing a controlled drug release into hepatic tumor tissues, thus minimizing systemic exposure.^Citation153 Lipiodol, a radiopaque contrast agent, preferentially accumulates in liver tumor tissues and was adjuvanted with chemotherapeutic drugs to create stable PEs.^Citation135 Li et al developed temperature-sensitive nanogels with a shell-and-core structure using two gel materials, resulting in Pickering gel emulsions with a temperature-sensitive sol-gel phase transition in vivo, enhancing intravascular embolization properties and long-term angiography of lipiodol in orthotopic liver cancer mice.^Citation136 The Pickering gel emulsion has been shown to have 7.0 times higher mechanical strength than hairy p(NIPAM-acrylic acid) nanogels and better plugging properties through creep testing. In a recent study, calcium phosphate nanoparticles-stabilized lipiodol PEs were prepared for hepatocellular carcinoma, which encapsulated a clinically approved vascular disrupting agent. The emulsion responded to the tumor microenvironment and released combretastatin A4-phosphate to disrupt tumor nutrient delivery.^Citation154 Therefore, these studies hold great promise for potential clinical translation of vascular embolization lipiodol PEs.^{Citation12,Citation154}

Overall, when formulating PEs for intravascular drug delivery systems, it is imperative to prioritize factors such as sterility and non-pyrogenicity to ensure both the safety and regulatory compliance of the system, thereby optimizing its therapeutic efficacy.^Citation155

Intratumoral Administration

PEs are recognized for their suitability in delivering drugs within tumors as they exhibit excellent stability and the capability to regulate drug release effectively. Numerous research studies have revealed that administering low-selective chemotherapeutic agents systemically may lead to severe side effects due to dosage constraints. Over the past few decades, there have been significant advancements in the technology of directly injecting long-acting liquid formulations into tumors, effectively achieving high concentrations within the targeted tumor area.^Citation156 Local administration of immunotherapy emerges as a promising strategy to mitigate the autoimmune and inflammatory toxicity associated with systemic delivery, thereby enhancing the therapeutic effectiveness of such treatments. Consequently, researchers developed a radiopaque PEs by dispersing an anti-CTLA-4 antibody in lipiodol with PLGA nanoparticles as stabilizers. The results demonstrated that the anti-CTLA-4 antibody’s structure remained intact and retained its anti-tumor efficacy in vivo when released from this PEs.^Citation137 In addition, Dai and his team designed a W/O PEs gel loaded with PD-1 antibody in the internal phase and iron death inducer (1S, 3R)-RSL-3 (RSL-3) in the continuous phase, which promoted iron death of the tumor cells and activated the body’s immune response upon intratumoral administration^Citation118 (). Furthermore, numerous studies have explored the incorporation of small molecule inhibitors and anti-tumor drugs into PEs, which, when administered intratumorally, collaborate to enhance anti-tumor efficacy.^{Citation6,Citation36,Citation83} These findings underscore the ability of intratumoral injections to effectively target the entire tumor and its microenvironment, showcasing significant promise in the field of oncology. PEs, with their unique slow-release property, deformability, and capacity to transport various medications, present a promising approach for intratumoral administration and controlled release of a diverse array of antineoplastic agents, showing great capability for therapeutic use in oncology.

Other Drug Delivery Systems Based on PEs Preparation Methods

It is challenging to achieve the encapsulation of two or more drugs in the same carrier using traditional nanocarriers. PEs, with their unique structural features, can fulfill this requirement. Researchers have utilized PEs’ droplets as templates to develop multi-drug co-delivery systems, especially for tumor treatment. The fundamental concept of this method involves utilizing solid particles to stabilize oil-in-water emulsions, then fixing the solid particles through cross-linking or volatile oil phases to create hollow structures resembling microspheres.^Citation139 These structures hold great ability as carriers for drug delivery. For instance, Hu et al employed galactose-functionalized hydroxyethyl starch-polycaprolactone nanoparticles as templates to create carbon nanotubes encapsulating DOX/indocyanine green.^Citation140 These nanotubes demonstrated strong tumor-targeting capabilities through intravenous injection, enhanced tumor penetration, and facilitated photothermal effects. The researchers also identified these nanocolloids as a beneficial drug delivery system for actively targeting tumors in imaging-guided combination therapies for conditions like hepatocellular carcinoma.^Citation140 Nanocapsules present an appealing approach to achieving highly efficient anti-cancer drug delivery. Traditional techniques like emulsion-diffusion and nanoprecipitation, which involve small-molecule surfactants, often result in high toxicity issues. To address this concern, researchers have utilized the Pickering technique to produce polymer nanogels for stabilizing nanocapsules, incorporating a cyclic peptide for enhanced tumor targeting and disulfide bond for stimulus-response. The capsules containing these are proposed as a great nanocarrier for precise drug release and targeted delivery.^Citation141 Furthermore, hydrogel-based drug delivery systems offer significant advantages in safeguarding bioactive compounds from degradation and improving their bioavailability. When integrated with PEs, a novel stimuli-responsive drug delivery system is created, characterized by improved stability and controlled drug release kinetics. For example, a hydrogel composed of carboxymethyl cellulose, sodium alginate, and knotted cold gel was utilized as a carrier for loading PEs containing hydrophobically active drugs, representing a promising immobilization and pH-responsive controlled drug release approach.^Citation142