A Comprehensive Review on Fabrication Techniques, Transdermal Applications and Future Prospects of Solid Lipid Nanoparticles

Abstract

Transdermal drug delivery systems have become a viable substitute to traditional drug administration methods because they can avoid first-pass metabolism, offer regulated drug release, and increase patient compliance. However, drug penetration through the skin is severely restricted by the stratum corneum's barrier function. Delivery systems based on nanotechnology have drawn a lot of attention recently as a means of getting around these restrictions. Solid lipid nanoparticles (SLNs) have demonstrated significant promise as transdermal medication delivery vehicles. SLNs, which are submicron colloidal systems made of biocompatible and biodegradable lipids stabilised by surfactants, provide benefits like increased skin penetration, regulated drug release, high encapsulation efficiency, and improved drug stability. The composition, manufacturing processes, and drug loading and release mechanisms of SLNs are all covered in detail in this review. A number of preparation processes are covered, including solvent evaporation, solvent injection, microemulsion, high-pressure homogenisation, and ultrasonication. The article also emphasises the use of SLNs in transdermal drug administration, specifically for neuropsychiatric therapy, anti-inflammatory medications, dermatological treatments, and metabolic disorders. Additionally, the prospects for the future, difficulties in large-scale production, stability problems, and safety concerns of SLNs are discussed. In brief, SLNs are a promising nanocarrier method for boosting therapeutic efficacy and transdermal drug delivery in contemporary pharmaceutical research.

Keywords

Introduction

Fig.1 Structure of solid lipid nanoparticle

2. COMPOSITION

A solid lipid matrix containing drug molecules and a layer of surfactant that stabilizes SLN in the aqueous phase make up solid lipid nanoparticles, which have a spherical form. ⁽¹⁹⁾ The main distinction between SLN and NLC is found in their lipid components, even though both are colloidal particles with sizes between 10 and 1000 nm. ^(20). NLCs are a combination of liquid and solid lipids, whereas SLN are made from solid lipids^{. (21, 22).}  

The two fundamental parts of SLN are the surface stabilizers and the lipid matrix. It also contains other substances, such as charge modifiers, cryoprotectants, preservatives, and co-surfactants. ^{(23, 24)}

2.1 Lipids

The stability, release, encapsulation, and loading of APIs in SLNs depend heavily on lipids, which are the basic building blocks of SLNs. As a result, choosing the right lipids is essential to developing an effective formulation. The best lipids to use when creating SLN are those that are physiologic, biocompatible, and biodegradable, have a melting point of at least 40°C, and maintain their solid-state properties at both body temperature and room temperature. ⁽²⁵⁾ The ability to create submicron-sized particles, biodegradability, biocompatibility, carrying capacity to hold the medication within, and storage stability are the variables that determine the choice of lipids for the development of SLN formulations. ⁽²⁶⁾

The polarity of the drug, the viscosity of the lipid, the degree of crystallinity and polymorphism, and the angle of contact with the surfactant solution are the most crucial factors that must be taken into account when creating SLN. Since only lipophilic medications may be readily integrated and dissolved in lipid media, the drug's polarity is crucial. The size of SLN produced is influenced by viscosity and contact angle. ^(27). The internal resistance rises in tandem with the viscosity, preventing the lipid from fragmenting into droplets.

Furthermore, increasing viscosity has an impact on homogenization effectiveness since utilizing high temperatures to break down lipids may cause big, thick particles to precipitate out if the operating temperature is around the melting point range. The contact angle determines whether stable SLN can be produced. Smaller, more evenly distributed, and more stable SLNs can be obtained by reducing the contact angle. ^(28). The degree of crystallinity affects the properties of the release from SLNs. Different polymorphic forms of lipids exist, and they frequently go through polymorphic transitions that ultimately result in drug extrusion from nanoparticles. Long chain fatty acids improve drug stability and lower the risk of drug leakage ^(29).

For the formation of SLN, glycero-lipids such monoacylglycerol and diacylglycerol are the most often utilized lipids. Monoacylglycerols include lipids like glyceryl monostearate and glyceryl caprylate, while diacylglycerols include most frequently glyceryl dibehenate, sometimes referred to as Compritol 888 ATO. Emulcire, a polyglycerol ester of fatty acids, is one example of a non-ionic self-emulsifying wax that is employed. Precirol is a combination of stearic acid and palmitic acid mono, di, and tri glycerides. Stearic acids and other medium-chain fatty acids are also utilized in the production of SLN.

Cationic lipids, like stearyl amine, can improve drug penetration and encourage cellular internalization by creating ionic connections between negatively charged cells and positively charged molecular components. This prolongs the drug's surface residence duration and increases its bioavailability. ^{(30, 31, 32)} The most often used surface stabilizers include phospholipids, bile salts, soy lecithin, egg lecithin, phosphatidylcholine, poloxamers, and polysorbates. When appropriate surfactants are present, the solid-lipid core of SLNs dissolves hydrophobic molecules. ⁽³³⁾

2.2 Surfactants

Surfactants are generally important in the preparation of lipid nanoparticles because they help disperse the lipid melt in the aqueous phase and stabilize the lipid nanoparticles in dispersions after cooling ^[34]. The main factors to take into account when using surfactants in the preparation of SLN are their safety, compatibility with other excipients, ability to produce the desired size with the least amount consumed, and also providing sufficient stability to the SLNs, covering their surfaces ^[35]. The surfactants used to create these carriers can enhance epithelial permeability (e.g., disrupt the cell membrane) and, thus, overcome drug absorption limitations ^[36].

However, in addition to ensuring the particles' steric stability in aqueous dispersion, the surfactants that envelop them also produce particular surface chemical characteristics and have the power to alter the biopharmaceutical profile ^[37]. A number of factors need to be taken into account when choosing the appropriate surfactant, including particle size, hydrophilic-lipophilic balance (HLB) values, and its impact on lipid polymorphism. For stabilizing oil dispersions in water, HLB values range from 8 to 18. The possibility of creating particle aggregates that could jeopardize the stability of the distribution in vitro and its efficacy in vivo is reduced when the right surfactant is used.

Based on their electrical charge, surfactants can be divided into three classes: amphoteric, non-ionic, and ionic. Amphoteric surfactants include phospholipid-egg phosphatidylcholine, soybean phosphatidylcholine, hydrogenated egg phosphatidylcholine, hydrogenated soybean phosphatidylcholine, egg phospholipid, steroid-cholesterol, cholesteryl oleate etc. Because of their negatively and positively charged functional groups, amphoteric surfactants behave like cationic and anionic surfactants at low and high pH levels, respectively [38]. Ionic surfactants provide electrostatic stability, whereas non-ionic surfactants provide steric repulsion stability. these include anionic surfactants like sodium lauryl sulphate, bile salts like sodium dihydrofolate, sodium taurocholate, sodium cholate, sodium glycocholate, sodium Tauro deoxycholate and cationic surfactants like Dimethyldioctadecylammonium bromide, cetrimonium bromide, 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), chlorhexidine salts,DOTMA and co-surfactant alcohols such as butanol, ethanol, polyvinyl alcohol (PVA) and others such as Diethylene glycol monoethyl ether, low molecular weight PEG, propylene glycol, sorbitan monostearate, butyric acid, sodium dioctyl sulfosuccinate, sodium monooctyl phosphoric acid. Non ionic surfactants  include - Polyethylene glycol / polyoxypropylene copolymers-(eg. poloxamer 188, poloxamer 182, poloxamer 407, poloxamine 908) , Polyoxyethylene sorbitan copolymers (polysorbate 20, polysorbate 60, polysorbate 80, polysorbate 85, sorbitan monooleate Polyoxyethylene), alkyl/aryl ethers (tyloxapol, polyoxyethylene-20-cetyl ether, polyoxyethylene-20-isohexadecyl ether, polyoxyethylene-20-oleyl ether)and others like polyglyceryl-6 distearate, polyglyceryl-3 methyl glucose distearate, caprylic/capric triglycerides of PEG, macrogol [15]—hydroxy stearate, polyoxyethylene glyceryl monostearate A surfactant's toxicity is a crucial factor to take into account, and not all surfactants can be utilized to make every kind of SLN. ⁽³⁰⁾

2.3 Other ingredients

Lipid nanoparticle formulations may also include charge modifiers, the surface, and cryoprotectants utilized in SLN drying processes such lyophilization and spray drying, in addition to lipids and surfactants. Lipid nanoparticle absorption by the reticuloendothelial system (RES) can be decreased by altering their surface with surface modifiers like hydrophilic polymers. Coating with a biocompatible polymer, like poly (ethylene) glycol (PEG), can delay the fast absorption of SLNs and lengthen blood circulation ^[38].

3. PREPARATION METHODS FOR SLN

Solid lipid, emulsifier, and water/solvent make up SLNs. Triglycerides (tri-stearin), partial glycerides (Imwitor), fatty acids (stearic acid, palmitic acid), steroids (cholesterol), and waxes (cetyl palmitate) are examples of lipids that can be utilized. The lipid dispersion has been stabilized using a variety of emulsifiers and their combination (Pluronic F 68, F 127). Particle agglomeration may be more effectively prevented by the combination of emulsifiers. ⁽³⁹⁾ There are several ways to prepare SLN; the method chosen will rely on the drug's physicochemical characteristics regarding the lipid matrix, the administration route, and other factors. The techniques used to prepare SLN can be broadly divided into three groups: (i) high-energy techniques that require the precipitation of nanoparticles from homogeneous systems (like high-pressure homogenization), (ii) low-energy techniques that require the dispersion of the lipid phase (like microemulsions), and (iii) techniques based on organic solvents. ⁽²⁵⁾

3.1 High Energy Methods

3.1.1 High Pressure Homogenisation

Initially, solid lipid nano dispersions were produced using the high shear homogenization process. ⁽⁴⁰⁾ This method can be categorized as either hot homogenization or cold homogenization, depending on the temperature at which SLN is produced. This approach has the benefit of producing SLNs with high entrapment effectiveness and tiny particle sizes. In high-pressure homogenization, a molten lipid is rapidly forced through a small opening at a pressure of 500–5000 bar. Although up to 40% lipid content has been studied, 5–10% lipid content is often employed. Hot homogenization and cold homogenization are two common methods for High pressure Homogenisation.

SLN is made via melt emulsification using a high-speed homogenization process. Olbrich et al. looked into how the zeta potential and particle size were affected by several process variables, such as the cooling condition, stirring rate, and emulsification time. Trimyristin, tripalmitin, and a combination of mono, di, and triglycerides (Witepsol W35, Witepsol H35) were among the lipids used in this investigation. Glycerol behenate and poloxamer 188 were employed as steric stabilizers (0.5% w/w). ^(42). The optimum SLN quality for Witepsol W35 dispersions was achieved after 8 minutes of stirring at 20,000 rpm, 10 minutes of chilling, and 5000 rpm stirring at room temperature. On the other hand, emulsification at 25,000 rpm for 10 minutes and cooling at 5,000 rpm for 5 minutes in chilly water (≈16°) were the ideal conditions for Dynasan116 dispersions. Increased stirring speeds marginally increased the polydispersity index but had no discernible effect on particle size. ⁽⁴³⁾

3.1.1.1 Hot Homogenisation

The process is carried out at temperatures higher than the lipid's melting point when using the hot homogenization method. At the same temperature, the medication and lipid are fused and mixed with an aqueous surfactant. The high shear device is used to create a heated pre-emulsion. To create the SLNs, the hot colloidal emulsion droplets are recrystallized by cooling the emulsion to room temperature. ⁽⁴³⁾ Higher temperatures generally cause the internal phase's viscosity to drop, which leads to smaller particle sizes. High temperatures, however, may also accelerate the drug's and the vehicle's rate of degradation. Three to five homogenization cycles at 500–1500 bar are usually adequate. Particle size is frequently increased by increasing the number of cycles or the homogenization pressure, which causes coalescence because of its high kinetic energy. It is possible to obtain particles smaller than 500 nm^{. (44)}

3.1.1.2 Cold Homogenisation

This method involves cooling the drug-containing lipid melt. Lipid microparticles are created by grinding solid lipids. Pre-excitation is produced by dispersing these lipid microparticles in a cold surfactant solution. Gravity is strong enough to break lipid microparticles straight into SLNs once this hypothetical process is homogenized at room temperature or lower. This method typically produces particle sizes between 50 and 1000 nm. ⁽⁴⁵⁾

The following issues with the hot homogenization process have been addressed by cold homogenization: Accelerated drug payload degradation caused by temperature, drug partitioning and subsequent loss into the aqueous phase during homogenization, Uncertain lipid polymorphic transitions brought on by the intricacy of the nano emulsion’s crystallization phase, which can result in many alterations and/or supercooled melts. However, cold homogenized samples often have a wider size dispersion and bigger particle sizes than hot homogenization. Because the lipid/drug mixture melts in the first step, the cold homogenization approach reduces but does not completely eliminate the sample's exposure to heat. ⁽⁴⁶⁾

3.1.2 Ultrasonication

High-speed homogenization, sometimes referred to as ultrasonication, is another method for developing SLN. The foundation of this technique is the use of ultrasonic waves to create cavitation processes, including the formation, growth, and implosive collapse of microbubbles or cavities inside the medium; Extremely high temperatures of up to 5000 K and pressures of up to 1000 bar are involved. Using this approach, the emulsion duration, cooling temperature, and stirring speed dictate the particle size of the SLNs, which can achieve heights of less than 100 nm^{. (47)}

One of the main benefits is that the equipment utilized here is widely available in all labs. This method's wider particle size distribution, which reaches into the micrometre range, is its drawback. When stored, this causes physical instability, such as particle proliferation. Another major issue with this approach is the possibility of metal contamination via ultrasonication. The combination of high-speed stirring and ultrasonication, carried out at high temperature, has therefore been studied by a number of research groups in order to create a stable formulation. ⁽⁴⁸⁾

3.1.3 Electrospray method

To far, over 30 polymers have been successfully electro spun using the electrospray process. A spout connected to a high-voltage control supply and a fluid to be atomized make up the basic electrostatic atomization setup. Typically, a metal capillary linked to an electrode with a high-power supply is used to fill the syringe with the matrix solution. As a counter electrode, a foil-based collector is positioned across from the metal capillary. ⁽⁴⁹⁾

3.1.4 Supercritical fluid method

Compounds can dissolve more readily in a supercritical fluid (SCF), which is created when a fluid's temperature and pressure surpass critical values. One special feature of supercritical fluid technology is its ability to create microscopic, irregularly shaped solids. great diffusivity, low viscosity, and great compressibility are some of the special characteristics of supercritical fluids (SCF). Because it is non-toxic, non-flammable, and readily available, supercritical CO2 (SC-CO2) is the most often used SCF. Five primary SCF techniques can be used to construct SLNs: (i) supercritical fluid extraction of emulsions (SFEE), (ii) particles of gas-saturated solutions/suspensions (PGSS), and (iii) rapid expansion of supercritical solutions (RESS^).(50)

3.2 Low energy methods

Low energy techniques reduce particle size without using a lot of energy, and some even happen on their own. The system's characteristics and intricate interfacial hydrodynamic mechanics serve as the foundation for these subpar energy techniques. It is thought that the change in the spontaneous curvature of surfactant molecules from negative to positive (o/w) or from positive to negative (w/o) is what causes the chemical energy released during emulsification^.(51)

3.2.1 Microemulsion methods

According to the IUPAC, a microemulsion is an anisotropic, thermodynamically stable dispersion of water, oil, and surfactant (s) with a dispersed domain diameter that ranges from around 1 to 100 nm, usually 10 to 50 nm (52).SLN preparation methods based on microemulsion dilution were developed by Gasco and colleagues. A low melting fatty acid (stearic acid), an emulsifier (polysorbate 20, polysorbate 60, soy phosphatidylcholine, and sodium taurodeoxycholate), co-emulsifiers (sodium monooctylphosphate), and water are usually combined to make them by stirring an optically transparent mixture at 65–70°. The hot microemulsion diffuses in the cold water (2°C–3°C). The dilution process can be fixed based on the microemulsion combination. The primary requirements for the production of nanoparticles are that they can only be produced with specific solvents that quickly disperse into the aqueous phase , while more lipophilic solvents are used to obtain the large particle sizes. This method uses no additional energy to achieve the submicron size. This method's primary benefit is its adequate low mechanical energy input. ^(53,54)

3.2.2 Membrane contractor technique

Using this technique, the lipid phase is forced through the membrane's pores, causing microscopic droplets to develop at the pore outlets. These droplets are then removed by the water that is flowing through them. Once the preparation has cooled to room temperature, SLNs are produced .Particle sizes of 100–200 nm can be attained with this technique^{. (55)}

Lipid content, lipid phase pressure, and aqueous cross-flow velocity are examples of process parameters that can be controlled to control the particle size. The advantages of this new SLN process seemed to be its practicality for use, the capacity to easily scale up, and the ability to adjust particle size with appropriate process parameters. ⁽⁵⁶⁾

3.2.3 Coacervation technique

The precipitation of free fatty acids from their micelles in the presence of a surfactant is the basis of the coacervation technique. A fatty acid salt is evenly distributed throughout the stabilizer solution during this procedure. Until a clear solution was achieved, the mixture was heated to the fatty acid salt's Krafft point while being constantly agitated. To obtain a single phase, an ethanolic solution of the API is then gradually added while being constantly stirred. The suspension of nanoparticles is then obtained by adding an acidifying solution or a coacervation agent .The concentration of the micellar solution and the amount of polymer employed for stabilization determine the particle size of SLN, which can range from 260 to 500 nm^.(57)

3.2.4 Phase inversion temperature technique

The mechanical emulsification at the phase inversion temperature and the abrupt cooling to room temperature, when an emulsion with a large number of tiny droplets is found, are the key components of the phase inversion temperature method. The two primary components of this approach are an aqueous phase that contains NaCl and an oil phase that contains solid lipids and nonionic surfactant. At about 90°C, which is higher than the phase transition temperature, both phases are heated. W/O Emulsion is created by gradually adding an aqueous phase to the oily phase while maintaining a steady temperature and stirring. After that, the mixture is continuously stirred as it cools to room temperature. The turbid mixture clears at the phase inversion temperature, and a PIT O/W nano emulsion forms below.(⁵⁸⁾

3.2.5 Double emulsion technique

One of the most popular methods for creating nanoparticles encapsulated with hydrophilic active ingredients is the double emulsion technique, which uses stabilizers or surfactants. This method is also known as the multiple emulsion method, where it has three basic steps: (i) formation of the water-in-oil emulsion or reverse emulsion, (ii) addition of the W1/O emulsion in the aqueous surfactant solution to form a W1/O/W2 emulsion with continuous stirring (sonication or homogenization) and (iii) evaporation of the solvent or filtration of the multiple emulsion to form the nanoparticles  ⁽⁵⁹⁾

3.2.6 Melt dispersion technique

This method uses a low HLB surfactant to disperse the lipid phase in water after heating it above its melting point. Once more, a high HLB surfactant aqueous solution is added to the emulsion without causing it to form; this double w/o/w emulsion is then placed into cold water while being gently stirred to encourage the development of SLN^.(60)

3.3 Organic solvent method

3.3.1 Solvent evaporation emulsification method

There are three fundamental processes in the solvent evaporation emulsion (SEE) method for making nanoparticles. To create a transparent, uniform lipid solution, lipid material is added to a known volume of organic solvent (immiscible in water) and properly mixed in step (I). In step (II), a high-speed homogenizer is used to create a thick emulsion by adding the previously prepared solution to the appropriate volume of a heated aqueous solution that contains surfactant above the melting temperature of lipids. In step (III), a high-pressure homogenizer is used to create the nanoemulsion. The high pressure causes the coarse emulsion to change into a nanoemulsion.

Since the lipid material will precipitate in the water when the organic solvent evaporates, nanodispersion is created. The sintered disk filter funnel is used to filter the precipitated lipids in an aqueous media. This method produces nanoparticles with a high trapping efficiency that are nanosized and not flocculated (single entity). Particle sizes of 30–500 nm can be produced with this technique.(⁶¹⁾

3.3.2 Solvent emulsification diffusion technique

The process relies on the initial saturation and thermodynamic equilibrium of the organic phase with a stabilizer that contains an aqueous phase. The medicine is dissolved using a homogenizer in the resulting saturated solution, and then it is dispersed in an aqueous solution using an emulsifier to create an o/w emulsion. When additional water is added to the emulsion in a suitable ratio while significant magnetic stirring is present, the solvent diffuses into the water, causing nanoprecipitation and the creation of SLNs. ⁽⁶²⁾

3.3.3 Solvent injection method

The precipitation of dissolved lipid in a solution is a basic principle of this technique. After dissolving solid lipid in an organic solvent, the mixture is syringed into the surfactant-containing stirred aqueous phase. To get rid of any extra fat, the obtained dispersion is filtered. By lowering the surface tension between the water and lipid phases, the aqueous emulsifier aids in the production of lipid droplets at the injection site and also helps to stabilize the SLNs^.(63)

3.4 Additional production steps

3.4.1 Lyophilisation

One method that shows promise for improving chemical and physical stability over long periods of time is lyophilization. For a product comprising hydrolyzable medications or a product appropriate for per oral administration, lyophilization was necessary to achieve long-term stability. Hydrolytic processes and Oswald ripening would be avoided by transformation into the solid state. All of the lipid matrices utilized in the product's freeze-drying process result in larger solid lipid nanoparticles with a wider size distribution because of the aggregates that form between the nanoparticles. Aggregation among SLNs is encouraged by the circumstances of the freeze-drying procedure and the water removal. The aggregation of solid lipid nanoparticles during the freeze-drying process can be prevented with a sufficient quantity of cryoprotectant. ^(64,65,66)

3.4.2 Spray drying

It is a less expensive and alternative method than lyophilization. This suggests using lipids that have a melting point higher than 70°C. SLN concentrations of 1% in a trehalose in water solution or 20% in an ethanol-water mixture produced the best results. In spray drying, the low lipid content and addition of carbohydrates help to maintain the colloidal particle size. Using ethanol-water mixtures rather than pure water can reduce the amount of lipid that melts since cooling produces small, uneven crystals and lowers inlet temperatures^{. (67)}

4. DRUG LOADING AND RELEASE FROM SLN

Particle size and the kind of drug entrapment model of SLNs have an impact on drug release, according to the standard theory of drug release from any nanoparticle. Parameters such as the drug solution and its interaction with the lipid matrix might influence the release of medicines.. Temperature changes can alter the release profile of SLNs in response to both internal and external stimuli. Chen et al. examined the pH-sensitive release profile of cholesterol-PEG coated SLNs loaded with doxorubicin and discovered that the drug released more quickly at pH 4.7 than at pH 7.4^.(68)

In general, SLNs showed burst release. As particle size increases, the drug's burst release may decrease and a longer release will be accomplished. Using tetracaine, etomidate, and prednisolone as model drugs, zur Mühllen et al. found that tetracaine and etomidate SLNs had a burst drug release (100% release <1 minute) because of their huge surface area and drug enhancement in the outer shell. Contrary to this information, prednisolone-loaded SLNs were shown to have a 5-week extended release. Burst (83.8%) and regulated releases (37.1%) were attained as a result of the lipid matrix's distinct chemical activity, such as that of cholesterol and compritol.(⁶⁹⁾

According to Olbrich and Muller, a lipid interface is necessary for the activation of the enzymes that break down the lipid matrix. It is advised to modify the surface of SLNs using a hydrophilic carrier (such as PEG) to prevent lipase enzymes from readily recognizing them in order to alter their release and improve their stability. Appropriate steric stabilizers and other surfactants should also be adjusted^.(70)

Drugs having a high melting point (numerically not defined) and a Log P value of 2 are typically poor candidates for lipid systems, according to a recent review by Savla et al. For the extremely lipophilic medication (Log P>5) (BCS Class-II), lipid-based formulations are a great carrier. For lipid formulations, Chen et al. also suggested the following pharmacological profile: hydrophilicity (water solubility) <10 mcg/mL; Log P >5; solubility in oils and fats >25 mg/mL; relatively low melting point; and strong chemical stability^.(71)

5. APPLICATIONS IN TRANSDERMAL DELIVERY

5.1 Handling Metabolic Disorders

As specialized carriers for the transdermal distribution of Repaglinide (REP), solid lipid nanoparticles (SLNs) have shown great promise, especially in addressing the drawbacks of traditional oral therapy, such as considerable first-pass metabolism and frequent dosage requirements. Researchers successfully created REP-SLNs through ultrasonic hot-melt emulsification using a Box-Behnken Design (BBD) for optimization. They found that surfactant concentration significantly affects particle size and entrapment efficiency, whereas lipid content positively correlates with these parameters. By incorporating these enhanced nanocarriers into transdermal patches made of chitosan, a delivery system with controlled drug release and enhanced skin penetration has been produced. Comparative results show that this SLN-based transdermal method is a highly promising technological alternative for controlled anti-diabetic therapy, providing better patient compliance and more effective blood glucose management than conventional oral pills.^[72]

5.2 Non-steroidal anti-inflammatory drug delivery

5.2.1 Transdermal delivery of flurbiprofen

The use of solvent-emulsified flurbiprofen-loaded SLNs is an example of how lipid-based nanocarriers can be precisely surface-engineered to maximize transdermal delivery. Researchers were able to achieve a sustained-release profile by using stearic acid as the core lipid, which is a result of the drug's persistent encapsulation within the solid matrix. During the coating procedure, a crucial functional transition was noticed: whereas untreated SLNs retained a negative surface charge, the chitosan coating effectively reversed the charge to a positive one, further delaying drug release and stabilizing the nanoentities. The formulation's chemical compatibility was verified by analytical evaluations using ATR/FTIR, which revealed no negative interactions between the medication and the lipid-polymer matrix. Moreover, decreased surface tension and the natural lipophilic attraction between the solid lipids and the phospholipid bilayers of the skin were the main causes of the improved skin penetration seen in both coated and uncoated systems. These findings highlight how well SLNs work to get beyond the stratum corneum barrier and offer a highly physicochemically stable, optimal route for NSAID administration.^[73]

5.2.2 Transdermal delivery of Piroxicam

The incorporation of SLNs into transdermal patches for the systemic administration of Piroxicam further demonstrates their adaptability in improving the delivery of NSAIDs. Researchers overcame the problems of poor drug solubility and skin permeability by adding Piroxicam-loaded SLNs to a patch matrix. By creating a localized concentration gradient, the SLN framework greatly increases the flow of Piroxicam through the stratum corneum, outperforming the penetration of conventional topical formulations. In order to maintain stable therapeutic plasma levels and reduce the gastrointestinal side effects commonly associated with oral piroxicam, this application demonstrates the synergistic effect of combining nanotechnology with patch-based delivery systems to provide a controlled release environment. Thus, SLN-incorporated patches are a significant technological development for raising the therapeutic index and bioavailability of weakly anti-inflammatory substances that are permeable.^[73]

5.3 Photoprotection and Wound Healing in Dermatology

SLNs have proven to be quite useful in dermoprotective applications, particularly as carriers for photoprotective compounds like Caffeic Acid (CF), in addition to medicine delivery for systemic disorders. CF-loaded SLNs, which were created by hot homogenization and sonication, stabilized particle sizes below 200 nm and had good encapsulation efficiency. Low cytotoxicity toward L929 fibroblasts indicated the formulations' outstanding biocompatibility and sustained-release profile. Functional tests showed that the SLN matrix actively promoted wound healing processes in addition to offering efficient photoprotection against UV-induced cellular irradiation. These results establish CF-loaded SLNs as viable options for cutting-edge topical treatments in dermatological care and regenerative medicine by reducing UV-induced skin photodamage and photoaging at the cellular level.^[74]

5.4 Applications in Neuropsychiatry: Transdermal Administration of Antidepressants

In order to get around the drawbacks of oral administration, such as significant first-pass metabolism and varying plasma concentrations, SLN technology has also been used to deliver antidepressants like paroxetine. The creation of SLN-based patches loaded with paroxetine offers an advanced delivery system that guarantees a regulated and extended release of the medication. When compared to traditional formulations, ex-vivo permeation experiments have shown that the SLN matrix dramatically increases the flux of paroxetine across skin barriers. The occlusive action of the solid lipids, which encourages skin hydration and breaks up the densely packed lipid bilayers of the stratum corneum, is responsible for this enhanced permeability behaviour.^[75]

6. FUTURE PROSPECTIVES AND CONCLUSION

Compared to other colloidal systems, SLNs have many benefits, including improved stability, decreased cytotoxic effects, greater drug absorption, and drug protection. SLNs have been used in numerous studies to distribute different active substances because it has been discovered that these systems greatly enhance the delivery and effectiveness of these items. The main reasons for choosing SLNs is their scalability and ease of modification. The potential to alter the surface of SLNs with various compounds, which can improve therapeutic efficacy, is another encouraging feature.

These nanostructures are already found in items that are sold commercially, mostly in the cosmetics industry, while others are undergoing clinical studies. The toxicity of these nanoparticles, however, requires further investigation, particularly for formulations that do not solely employ biocompatible and biodegradable components. To appropriately assess exposure and potential dangers, a detailed understanding of the mechanisms by which SLNs interact with the body as well as their biodistribution, degradation, and excretion processes is essential. This would make it possible to reformulate current medications or get new ones onto the market with more dependable outcomes and greater success in clinical therapy.

Research on these nanostructures' potential medicinal applications is ongoing and progressing in a number of areas. The potential of SLNs to transport drugs, particularly to target certain tissues or organs and guarantee regulated pharmacological release, is being studied. They are used to transport a wide range of medicinal chemicals, such as genetic materials, anti-inflammatory medicines, anticancer treatments, and vaccinations. To assess their efficacy and safety for various medical applications, numerous clinical trials and investigations have been conducted. Bio-distribution, biocompatibility, and the ability to get beyond biological barriers like the blood–brain barrier are some of the topics covered in these studies.

For SLNs, no toxicity information is now available. These nanocarriers primarily consist of physiological lipids and well accepted safe excipients. Research indicates that these lipid nanoparticles, whether administered orally, parenterally, dermally, or ocularly, do not result in toxicity. However, it is important to understand how these nanocarriers are dispersed in tissues and interact with biological systems because they are intended to transport bioactive chemicals to the human body.

Issues with metabolism and excretion must also be examined because it is unclear if tissue buildup could have negative consequences down the road. While there is enough information on the effectiveness and quality of SLNs, little is known about the safety of these lipid nanoparticles, particularly with regard to surfactants, which have the potential to trigger the immune system. In addition to being used in combined therapies inside a single delivery system or for diagnostic technologies, the utilization of these structures in medicine holds promise for the development of personalized therapies that adapt treatments to the unique characteristics of each patient.

Delivering outcomes pertaining to these nanostructures' safety is essential for their large-scale manufacturing. Consistent manufacturing of nanoparticles is necessary for large-scale production since differences in manufacturing circumstances can impact the products' safety and effectiveness. Although it is difficult, quality control requires careful monitoring of medication distribution, particle size, and encapsulation effectiveness in huge quantities. PEGylation and other surface modifications of SLNs are challenging to carry out successfully on a wide scale. For large-scale production, nanostructure development must also be standardized through protocols and SOPs. Depending on the source and batch, the quality and properties of the lipids used in the production of these nanoparticles can differ. The final product's uniformity and quality may be impacted by this variability, necessitating strict quality control.

Lipid polymorphism can also be triggered by radiation exposure and high temperatures in sterilization-required formulations. To maintain the stability and effectiveness of SLNs throughout industrial production, choosing the right sterilizing technique is still essential. Additionally important is the stability of lipids throughout synthesis and storage. The integrity and effectiveness of the finished products may be jeopardized by oxidation or hydrolysis of lipids. Therapeutic efficacy may be jeopardized by lipid oxidation during storage, which can alter the particles' surface charge, stability, and drug release characteristics ^[71]. Precise process parameters including temperature, pressure, and agitation are necessary for the manufacturing of SLNs. It can be difficult to scale these conditions industrially for large production because even little changes can have a big impact on the stability.

The upgrading of lipid nanoparticles comes at a high expense. High-pressure homogenization and ultrasonication are popular methods for producing SLNs, although they need costly equipment. Furthermore, it takes a lot of time and money to comply with pharmaceutical industry rules for the manufacturing of lipid nanoparticles, including paperwork, process validation, and conformity testing ^[71]. SLNs have a bright future ahead of them, with the ability to provide fresh solutions to the market once these issues are resolved.^[69]

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