Liposomes can be defined as artificial spherical nanostructures that consist of one or more phospholipid bilayer membranes ^[1][2][3][1,2,3].

They are the most commercialized active substance delivery systems with a prime encapsulation capacity and therapeutic index, biocompatibility, biodegradability, flexibility, and safety due to their structural similarity to cell membranes (lipid bilayer) [4]. The liposome structures consisting of phospholipids come from various sources of lecithin (soy, egg yolk, sunflower, rice beans, and rape seed). The main phospholipids used are phosphatidylcholines, phosphatidylethanolamines, phosphatidylserines, and phosphatidylglycerols. These lipids are amphiphilic because they have a group of polar heads and a lipophilic tail, making them suitable for use in obtaining liposomes ^[5][6][5,6].

The aqueous core of liposomes is enclosed by the phospholipid bilayer. The core encapsulates the water-soluble drugs, and the hydrophobic domain entraps insoluble compounds. Due to their structure, their stability, bioactivity, and bioavailability in the human body are increased ^[7][8][7,8]. Liposomal nanostructures can have one or more layer membranes.

The number of layers influences the liposome size from 20 nm to 1000 nm [9]. Based on size and lamellarity, liposomes are classified into several categories: small unilamellar vesicles (20–100 nm) [10], large unilamellar vesicles (>100 nm) ^[11][12][11,12] giant unilamellar vesicles (>1000 nm) ^[13][14][13,14], oligolamellar vesicles (100–1000 nm) ^[15][16][15,16], multilamellar large vesicles (>500 nm) ^[17][18][17,18] and multivesicular vesicles (>1000 nm) [19].

The methods used to obtain liposomes can be grouped into conventional and modern methods. The conventional methods used are thin-film hydration, ethanol/ether injection, reverse phase evaporation (detergent depletion, microfluidic channel, heating, membrane extrusion, high shear homogenization, and sonication), and the proliposome method. The modern methods used are freeze drying, dual asymmetric centrifugation, and supercritical fluid methods (supercritical anti-solvent, supercritical CO₂ reverse phase evaporation process, the rapid expansion of a supercritical solution, depressurization of an expanded liquid organic solution suspension, and super-critical assisted liposome formation) [20].

It is recommended that the encapsulation of the active ingredients in liposomes be conducted by appropriate methods to protect them both from the food processing conditions and the action of the gastrointestinal tract (GT) juices to ensure targeted, controlled release. Different categories of active substances can be encapsulated in liposomes, with applications in the pharmaceutical, cosmetic, and food industries, such as vitamins, enzymes, antimicrobial polypeptides, essential oils, phenolic compounds, minerals, antioxidants, food additives, flavors, fatty acids, drugs, toxins, proteins (peptides), antigens, and nucleotides ^{[21][22][23][24]}[21,22,23,24].

The use of the liposome-based delivery system in the pharmaceutical field is predominant because the therapeutic effect is achieved by reducing the dose and frequency of administration of the encapsulated compounds [25]. Currently, there are various liposomal products on the market with applications in the pharmaceutical and nutraceutical fields, such as Abhope, Liposomal Vegan D3 K2 Magnesium, VENTUS liposomal Omega 3, LVC5, and others In cosmetics, there are many commercial nano-liposomal products.

Liposomes and niosomes are particularly interesting due to the phospholipid component that has a high-esterified essential fatty acid content. Phosphatidylcholine is preferred as an essential component in obtaining liposome membranes because of its conditioning and softening properties. Unilamellar liposomes are most often used in the cosmetic field, and they do not need an additional coating because they must have an easy and fast penetration into the skin. Studies have shown that liposomes are suitable for delivering active compounds due to their compatibility with the skin and their extended and slow dermal release.

2. Coating Materials for Liposomes with Applications in Drug Delivery

Liposomes used as nanocarriers with amphiphilic character can deliver different drugs by their encapsulation. In the bloodstream, they are treated as foreign objects and are destroyed by immune system cells by phagocytosis ^[26][62]. Protecting liposomes from the rapid reaction of the immune system and increasing their stability against low pHs in the stomach represent challenges. To improve and achieve a good biological interaction (with blood and tissues) while using liposomes as carrier drugs, studies have been conducted to improve liposomes’ surface. Liposomes were coated with film-forming compounds to improve the stability of their membranes. The most common materials used for coating and the change in the surface of liposomes are found in the following classes of compounds: saccharides and their derivatives, polymers, and proteins. The general coating materials for liposomes encapsulating drugs and their properties are illustrated in Table 13. Saccharides and their derivatives tend to be more physically and chemically stable, resulting in site-vesicle structures specific to their biological environments. In the coating process of liposomes using saccharides, the greatest influence on the permeability, fluidity, and integrity of the membrane is given by the mechanisms (adsorption, coagulation, and bridging) and by the method of binding the saccharide to the lipid bilayer of the surface ^[89][123].

3. Coating Materials for Liposomes with Applications in the Food Industry

In the food industry, the application of liposomes mainly focuses on texture alteration and water retention improvement. In recent years, studies have been conducted on the encapsulation of food components using various technologies (LipoCellTech, Kerkplein, The Netherlands, stealth liposomes, or non-PEGylated liposome technology). Active ingredients must be formulated in such a way as to protect them against production technology and environmental conditions so that they can be safely delivered to the targeted organs and cells. The results of various studies have shown that the coating of liposomes with different types of compounds by creating a layer on the surface of the membrane and providing electrostatic repulsion has led to increased physical stability, resistance to mechanical stress, and a low release speed of charged compounds ^[90][153]. Different types of materials have been used in the coating of liposome carriers for food-active compounds. Two groups of compounds have been used most frequently due to their food-grade qualities: saccharides and their derivatives and proteins. These groups of compounds have been chosen for their biocompatibility, low or nontoxicity, and neutral organoleptic properties ^[91][154]. The general coating materials for liposomes with applications in the food industry and their properties are illustrated in Table 24. Saccharides and their derivatives studied as coatings for food ingredients entrapped in liposomes have been shown to improve the thermal, physical and chemical, functional, and structural stability of liposomes during storage, with better release during in vitro digestion ^[47][97][98][82,155,156]. Some researchers ^[73][108] synthesized vitamin C and introduced it in mandarin juice. The multi-layered liposomes were the result of depositing positive chitosan and negative sodium alginate on the surface of the anionic nanoliposomes. The coated structure of nanoliposomes modified the surface characteristics of these. After 90 days, the vitamin C was still protected, and no significant organoleptic changes were observed in the fortified samples. Low methoxyl pectin (LMP) can be used as a macromolecular material to modify the surface of liposomes. The liposomes formed not only have a good particle size and potential, but also have better stability during long-term storage. The double layer is protected from oxidation and maintains a good active compound release efficiency. LMP-coated liposomes added to orange juice create a bridge with metal ions and form a network-like gel to establish the stability of the liposomes. The addition of pectin with a different degree of esterification leads to an increase in the particle size of the liposomes. This is mainly due to the adsorption of pectin on the surface of the liposomes. Some environmental factors, such as pH, ionic strength, and temperature, have a significant effect on the appearance, particle size, and flow rate of liposomes, but LMP as a coating material can have a protective effect ^[103][104][161,162]. Inulin-coated liposomes showed better stability in the presence of surfactants and electrolytes. The use of cationic inulin helps to create a physical barrier to prevent the aggregation and fusion/coalescence phenomena, while using long-chain inulin has been shown to increase liposome stability during storage and improve gastric viability ^[105][163]. Coating liposomes with lactose or inulin prevents their rapid dissolution in alcohol, providing a protective effect. In addition, because lactose is a molecule present on the surface of blood cells, the affinity avoids macrophages. Depending on the microstructural order and the molecular weight of the saccharide, the structure of the liposome changes; for example, inulin as a coating material confers longer release properties due to its higher molecular weight. Thus, the thickness of the liposomes is closely related to the molecular weight; the higher the molecular weight, the thicker and more resistant the liposomes, which last longer in the gastrointestinal tract ^[91][105][154,163]. The use of proteins as a coating material has led to the enhancement of thermal and light stability with an improvement in the active compound stability under gastric conditions. Whey protein isolates used as a coating material for astaxanthin entrapped in liposomes improved the bio-accessibility and protected the liposome membranes against alteration during in vitro digestion ^[58][59][93,94]. The use of chitosan in combination with other compounds (alginate) to obtain a liposome coating material led to an improvement in the antimicrobial and prolonged antioxidant activity of the encapsulated compounds and the improvement of thermal and light storage stability ^{[73][78][100][101]}[108,113,158,159]. Challenges in using coating materials to improve the stability of liposomes when used in the food industry include the need to ensure that the coating material is safe for consumption, the cost and complexity of manufacturing, the consistency of liposomal properties during storage, and the difficulty in determining the optimal parameters for coating. In addition, some modifications to the liposome formulation may be required to ensure that the liposomes remain stable over a long period of time and can survive during storage, handling, and shipment.

4. The Influence of Polymer Coatings on the Absorption of Liposomes

The stages of digestion for liposomes as vehicles for drug or bio-compound delivery have been studied under in vitro conditions in simple mono-compartment to complex multi-compartment dynamic digestive systems. In addition, an artificial gastric digestive system with a 3D-printed shape was developed and validated to follow the food digestion mechanisms ^[106][164]. The oral administration of liposomes raises several challenges, namely susceptibility to physiological factors in the GT, poor permeability of liposomes across gastrointestinal epithelia (the main absorption barrier), and liposomal formulations (manufacturing). Under the action of physiological factors, liposomes composed of phospholipids and cholesterol lose their integrity (are unstable), and the active ingredients are released but not in the target cell or tissues ^[107][165]. The liposome delivery systems cross the GT and change until they reach the intestinal mucosa where absorption takes place. Only some of the ingested liposomes reach full form and are absorbed by the lymph pathway [4]. At the level of the stomach, acid-stable gastric lipase-initiated digestion takes place. This can slightly affect the structure of the liposomes because they have a lipid bilayer membrane and because cholesterol from the structure increases the rigidity of membranes. So, they decrease slightly in diameter due to the pressure difference between the inside and outside of the two sides of the liposomes ^{[108][109][110]}[34,166,167]. When the simulated digestion time was extended to 120 min, aggregation of the liposomes was observed due to the reduction in the electrostatic repulsion force between the liposomes under the conditions of a low pH. The encapsulated compounds, such as betacyanins, lutein, and β-carotene, could be degraded slower or faster without affecting the liposomes by crossing the liposomal membrane and exposure to gastric acid ^[111][168]. Most of the liposome digestion takes place in the small intestine under the action of pancreatic enzymes (colipase-dependent pancreatic lipase acts on unhydrolyzed triacylglycerols from the stomach, pancreatic lipase-related protein 2 acts as phospholipase and galactolipase, carboxyl ester hydrolase, bile salt-stimulated lipase, hydrolyses cholesterol esters, triacylglycerols, monoacylglycerols, vitamin (A, E) esters, phospholipids carotenoid esters, galactolipids, and polyethylene glycol mono- and di-esters, and pancreatic phospholipase A2, involved in the digestion of phospholipids, catalyzes the hydrolysis of the sn-2 fatty acyl ester bond of 3-sn-glycerophospholipids) ^[112][169]. Bile salts mediate digestion in several ways: (i) they weaken the interfacial stresses between molecules by facilitating the action of phospholipase A2 and lipase on the liposomal lipid phase; (ii) they weaken the structure of the phospholipid bilayers by the insertion of bile salt molecules and the formation of channels, which make the membranes more susceptible to the lipolysis process by fluidizing them; (iii) bile salts facilitate the hydrolysis of phospholipids and the release of fatty acids by the adsorption of lipase to phospholipid bilayers; (iv) they increase lipolysis by eliminating the accumulation of fatty acids and increasing the accessibility of lipase; and (v) they aid in the solubilization and absorption of the lipolysis products by forming mixed micelles ^[113][114][170,171]. When biopolymers are deposited on the surface of liposomes, their properties in different GT fluids may change the ability of enzymes to act on the surface of the lipids, which could improve the digestive stability of the liposomes. Their digestion in the GT involves a complex set of physical-chemical and biochemical reactions that affect the uptake of hydrophilic and hydrophobic-loaded molecules ^[115][172]. The role of chitosan in liposome digestion and absorption is still controversial. Some authors ^[115][116][172,173] report that the polycationic nature of chitosan prolongs the retention time through the intestinal mucosa. The explanation is that mucin, an anionic glycosylated protein negatively charged at the mouth pH, covers the chitosan-coated liposomes, offering further protection to the loaded active molecules during the other digestion phases. It takes place in the electrostatic interaction between the amine group (NH3+) of chitosan and the carboxylate (COO⁻) or sulfonate (SO3⁻) group of mucins. In addition, the adhesion of chitosan-coated liposomes to the mucosal membranes, negatively charged, enhances; so, the bioactive compounds are more available for absorption and the half-time of clearance increases. The mechanism responsible for the permeation is based on the positive charges of this polysaccharide, which structurally reorganizes (opens) the tight junctions of the mucosal cell membrane proteins, facilitating the paracellular transport of hydrophilic macromolecules. Chitosan’s molecular weight and degree of deacetylation influence the increase in membrane permeability. Thus, a high degree of deacetylation and a high molecular mass contribute to the increase in the chitosan charge density, which leads to the increase in epithelial permeability and implicitly to the increase in drug transportation ^[117][174]. Highly methoxylated pectin is a widely used liposomal coating because it increases the stability of liposomes during storage and adheres to the intestinal epithelium without influencing membrane permeability [4]. Coating liposomes with PEG increases their intravenous circulation time and increases the stability of liposomes at the intestinal level through a mechanism of adhesion to the mucus of intestinal epithelia. The adhesion mechanism of positively charged mucoadhesive polymers is based on the ionic interaction between them and negative compounds from the mucus layer ^[118][119][175,176]. The polymer coating is a promising way to modify the surface characteristics of the vesicle’s stability to improve its applicability ^[120][177]. The liposome content is delivered in the cell by four mechanisms ^[121][178]. The first mechanism is the adsorption of liposomes on cells which can be specific—through specific receptors on the cell membrane and liposomes—and nonspecific, realized through attractive forces. The second mechanism represents the exchange of lipids between the cell membrane and the liposomal membrane due to their similarity. The third mechanism is endocytosis (for large particles by phagocytosis and receptor-dependent internalization by pinocytosis). The fourth mechanism is the fusion between the plasma and liposomal membranes. The liposomal content is delivered directly into the cell ^[122][123][179,180]. The liposome membrane is broken, and the encapsulated active compounds are released; these can be internalized into the cell in three ways: simple diffusion, facilitated diffusion, and active transport ^[124][181]. The cell uptake of liposomal oral or injectable products can be influenced by the liposomes’ size and surface charge. Experiments with liposomal formulations with surface charge and varied lipid compositions have shown that anionic or neutral liposomes are efficiently absorbed by monocyte-derived DCs ^[111][125][168,182]. Depending on the size of the liposomes, they follow different pathways. Studies show that liposomes between 40.6 nm and 276.6 nm in diameter are up-taken by Caco-2 cells ^[126][183]. From a pharmacokinetic perspective, the main goals of liposome drug delivery systems are improved in vivo drug release profiles, including enhanced drug absorption, targeted drug delivery, a modified metabolic pattern, a prolonged residence time of the drug in the body (e.g., in the bloodstream), and delayed and/or reduced renal excretion of the drug. From the initial stages of liposome system design through to the final clinical evaluations, absorption, distribution, metabolism, and excretion (ADME) must be considered to accurately understand the pharmacokinetic properties of this drug delivery system. In terms of ADME affecting the pharmacokinetic behavior of the drugs, for liposome delivery systems, the lipid bilayers serve as barriers between aqueous compartments and distribution compartments ^[127][184]. A quantitative explanation of the in vivo conditions under which a drug dose leads to therapeutic or side effects is provided by pharmacokinetics. For this purpose, the drug concentrations in the biophase and/or toxic phase must be considered. The concentration–time curves of drugs serve as the basis for pharmacokinetic research, which in turn serves as the starting point for estimating pharmacokinetic parameters with the help of corresponding mathematical models ^[128][185]. These factors should provide a quantitative link between biological concentrations and drug effects. In this situation, liposomes as drug carriers can be used as “pharmacokinetic modifiers” to achieve predetermined spatial and/or temporal targeted drug delivery. Recently, coated liposomes have been developed for the targeted delivery of therapeutic drugs to increase oral drug bioavailability, solubilize drugs for intravascular delivery, maintain the effects of drugs or genes in target tissues, reduce the potential for toxic effects or adverse reactions, and/or improve the stability of therapeutic drugs against enzymatic hydrolysis or other particular nutrients, peptides, and nucleic acids ^[129][186]. Liposomes, due to their subcellular size, can penetrate the tissue through the capillary walls and cross epithelial tissues and are usually taken up by cells. A therapeutic concentration must be achieved in the target tissues by modulating the physicochemical properties of the liposomes, as unfavorable exposure of nontarget tissues to these drugs can potentially lead to adverse effects. Various characterization experiments are often performed during the development of these nanocarriers, mainly in vitro and in vivo tests, to optimize the drug delivery of liposome systems. Particle size, shape, chemical composition, surface hydrophilicity, polarity, drug release profile, and other physicochemical characteristics are used in in vitro studies to provide an indirect measurement of the drug delivery capabilities of different compounds ^[130][187]. On the other hand, successful in vitro tests are followed by in vivo studies to test the liposome drug carriers for efficacy in a living, intact organism or in specific organs or tissues. The produced drug nanocarriers are often subjected to two different types of in vivo experiments: pharmacodynamic tests on the pharmacological effects and pharmacokinetic studies for the expected effects of the particle and/or the drug associated with the particle. Coated and uncoated liposome drug carriers often consist of a large number of individual parts that interact as integrated systems in a special structure. Compared to the free drug, these different components, especially the therapeutic portion (drug), have different ADME properties (absorption, distribution, metabolism, and excretion). Therefore, the localization of the drug and coated liposome drug carriers in the biological system is a problem in ADME-related animal studies with coated liposome drug carriers. in vivo studies in which each component is independently tracked may not be sufficient to determine the therapeutic efficacy and toxicity of coated liposome carriers ^[131][132][188,189]. Coating liposomes with certain materials can alter the ADME of the drug they are designed to deliver. Advantages of coating liposomes include increased protection of the drug from degradation, improved permeation through cell membranes, and the ability to control the release of the drug at a specific rate. The main disadvantages of coating liposomes are the impairment of pharmacokinetics, bioavailability, and the biological half-life of the drug, the difficulty in determining the optimal parameters for coating, and the possible toxic effects of the coating material. Toxicity depends on the material used, but general potential toxic effects include inflammation and irritation of skin and tissues, an increase in antigenicity, an increased risk of sensitization and allergic reactions, and disruption of cell membrane barrier functions.