In the homogeneous matrix model, the APIs are molecularly dispersed in the lipid core or positioned as amorphous clusters. The obtaining of SLNs Type I is a result of optimal ratios of APIs and lipids due to HPH techniques (Balamurugan, and Chintamani 2018). According to their structure, SLNs Type I can possess controlled release properties suitable for incorporating different APIs, e.g., prednisolone, which can show release from 1 day up to weeks (Pandey et al. 2022).

Distinctive about this model is the low API concentration in the melted lipid. When using the hot HPH technique, during the cooling of the homogenized nanoemulsion, initially, the lipid phase precipitates, leading to gradually increasing concentration of API in the remaining lipid melt with a raised fraction of solidified lipid. Thus is formed API-free lipid core. When API’s solubility reaches its saturation level in the remaining melt, an outer shell encompassing both API and lipid begins to harden around this core (Sumera et al. 2017; Ezzati et al. 2018). The formed core contains only a small amount of API, therefore the described model is not suitable to achieve a modified (prolonged) drug release. It suits to obtain an immediate release of API, and also, the occlusive properties of the lipid core can be used (Ganesan and Narayanasamy 2017).

In this model, API molecules are solubilized in the lipid melt up to its saturation solubility. The cooling of the lipid emulsion causes the supersaturation of API in the lipid melt. Under these conditions, first crystallizes the used API. The continuous cooling process causes lipid recrystallization too, and this circumstance contributes to the formation of the membrane around the already crystallized API-comprised core. This model is suited for APIs, which meet the criteria for prolonging their release (Uner and Yener 2007; Wu et al. 2021).

The essence of this type is in the construction of a matrix with many free spaces, which can be filled by API molecules. Тhis architecture is achieved by mixing solid lipids and a sufficient amount of liquid lipids (oils). The inclusion of components with different chain lengths such as fatty acids and a mixture of mono-, di- and triacylglycerols, does not allow the matrix to form NLC with a highly ordered structure, thus creating free spaces (structural imperfections) (Salunkhe et al. 2015; Xu et al. 2022).

This structure type is obtained by mixing some specific lipids (dibutyl adipate, hydroxy octacosanyl hydroxyl stearate, isopropyl palmitate), which do not recrystallize after homogenization and cooling of the nanoemulsion. These lipids form amorphous matrices and minimize the expulsion of API during storage (Salvi and Pawar 2019; Xu et al. 2022).

The solubility of the lipophilic API in liquid lipids (oils) is higher than in solid lipids. This staging is used to develop the “multiple” type NLCs. In this approach, a higher amount of oil is blended in solid lipids. At low saturation, oil molecules are efficiently dispersed into the lipid matrix. The additional amount of oil leads to phase separation and the creation of oily nano compartments surrounded by the solid lipid sheath. This method allows the modeling of controlled API release. Furthermore, the lipid matrix prevents API expulsion. Lipophilic APIs can be solubilized in oils, and “multiple” types of NLCs could be formed during the cooling process after HHM (Borges et al. 2020; Akbari et al. 2022).

High-energy-based approaches include the use of equipment causing deformation under pressure, creating significant shear forces and severe mechanical forces, as well as applying other mechanisms to reduce particle size (Che Marzuki et al. 2019). In summary, high-energy processes can be identified in two main stages: (i) transformation of macrodroplets into smaller ones; (ii) the adsorption of a surfactant on their surface for the purposes of stabilization (Håkansson and Rayner 2018).

HHM is one of the most commonly exploited methods for the preparation of LNs, tightly bound to HPH (Huang et al. 2017). HPH is a suitable, reliable, and well-known method for producing different types of LNs. Besides, it becomes clear that this powerful method does not have scaling-up problems (Shirodkar et al. 2019). HPH has been used for a long time to produce nanoemulsions for parenteral nutrition (Azmi et al. 2019). It has currently been applied for SLNs and NLCs production and represents the primary method adopted for these LNs (Yadav and Kale 2019). During the process, homogenizers push a liquid with high pressure (100–2000 bar) through a narrow size space (few µm). The liquid obtains high velocity (more than 1000 km/h) by enforcing high pressure, resulting in a break down the accelerated particles to submicron size, caused by generated shear stress and cavitation forces (Khosa et al. 2018). The used lipid content in this procedure is most often in the 5–10% range and does not cause technological problems. Data exist that even mixtures with higher lipid content (up to 40%) can be homogenized and brought to a nanodispersion state (Musielak et al. 2022).

The HPH technique was used for preparation of NLCs loaded with coenzyme Q10 at a throughput of 25 kg/h (Hu et al. 2016).

HPH method was reported to prepare efavirenz-loaded SLNs, characterized by improved drug bioavailability (vs. oral admintration) and brain targeting feasibility (Gupta et al. 2017). Enhanced bioavailability, along with lower dosing and side effects was reported also by Gorle et al. (2023), which applied the HPH method to incorporate the tyrosine kinase inhibitor axitinib in NLCs.

The process of HHPH takes place at temperatures above the melting temperature of the used lipid. The lipids and APIs are melted and combined with an aqueous surfactant at the same temperature. A hot pre-emulsion is formed by using the high-shear device (Jain and Thareja 2020). A representative for the process is the production of smaller particles due to decreasing viscosity of the internal phase due to high operating temperatures (Singh 2019). It is essential to be kept in mind that HHPH increases the temperature of the sample (approximately 108 °C for 500 bar) (Chandana et al. 2021). Finally, the obtained nanoemulsion is cooled to room temperature resulting in the lipid recrystallization and formation of nanoparticles (Qushawy and Nasr 2020).

Makoni et al. developed efavirenz-loaded SLNs and NLCs by the HHPH method and investigated the physical stability of the dispersions for two months at different conditions (25 °C/60% RH and 40 °C/75% RH). The optimal LNs formulation exhibited high entrapment efficiency (90%), sustained drug release and excellent physical stability at ambient temperature (Makoni et al. 2019).

HHPH method was applied to produce atorvastatin-loaded SLNs as potential self-administrable eye drop formulation with increased bioavailability in aqueous and vitreous humor and improved stability (Yadav et al. 2020). Also, celecoxib-loaded NLCs with PS of < 200 nm and sustained release were prepared for potential treatment of leukemia and breast cancer (Üner et al. 2019).

Recently, the hot homogenization process followed by ultrasonication was applied to achieve high drug loading (over 90%), entrapment efficiency (of 5%), and enhanced oral bioavailability of raloxifene with NLCs (3.19 – fold as compared to drug-free suspension in female Wistar rats) (Murthy et al. 2020).

CHPH has been developed in order to eliminate the problems appearing when applying an HHPH technique (Duan et al. 2020), to wit:

1. API degradation due to the high operating temperature;
2. API dispersal into the aqueous phase during homogenization;
3. Expressed variability after the nanoemulsion crystallization step.

CHPH, to some extent is similar to HHPH. API molecules are incorporated into the matrix by dissolving or dispersing in the heated and molten solid lipid. The API-containing lipid melt is rapidly solidified by cooling with dry ice or liquid nitrogen. The rapid cooling leads to homogeneous stowage of API inside the lipid material. Afterwards, grinding of the solid bases and obtaining of a fine powder of microparticles is achieved, which is subsequently dispersed in a cold aqueous surfactant solution. The resulting dispersion is subjected to HPH in order for SLNs to be generated (Salvi and Pawar 2019; Lammari et al. 2021).

Typical for CHPH is a process of solid lipids homogenization as opposed to a lipid melt in HHPH. Attainment to solid lipids’ dispersion state requires high-energy input and strict homogenization conditions (Kovačević et al. 2020; Sastri et al. 2020).

Makhmalzade et al. (2021) prepared deferoxamine-loaded SLNs applying the CHPH method and a full factorial design to evaluate the impact of formulation constituents on the physicochemical properties of the nanoparticles. The optimal formulation was composed of Compritol/ oleic acid as solid/liquid lipid, Tween 80 and lecithin (surfactants), and exhibited narrow size distribution, high loading and sustained drug release. According to the authors, the selected method of preparation (CHPH) contributed to the observed high EE values (60%), as it restricts hydrophilic drugs’ partition between lipid and aqueous phases.

In another study Duong et al. (2019a) investigated the effect of formulation parameters on ondansetron hydrochloride encapsulation in NLCs. As optimal CHPH conditions were estbalished: pressure (500 bars); cycles (6), leading to the formation of NLCs with high entrapment efficiency and prolonged ondansetron release (up to 96 h).

HSH /US are actually dispersing techniques. LNs dispersions are obtained by dispersing the melted lipid in a warm aqueous phase containing surfactants by a high shear homogenization followed by ultrasonication (Cheaburu-Yilmaz et al. 2019).

The technique primarily involves heating a solid lipid to approximately 5–10 °C above its melting point. The lipid melt is dispersed in an aqueous surfactant solution at the same temperature under high-speed stirring and an emulsion is formed. The following sonication reduces the droplet size of the emulsion. Gradual cooling of the warm emulsion below the lipid crystallization temperature yields LNs dispersion (Souto et al. 2019). HSH/US are successfully used to obtain dispersions of SLNs and NLCs without organic solvents with a tight particle distribution (Bazzaz et al. 2018; Jannin et al. 2018; Zardini et al. 2018).

HSH/US method was applied for insulin-loaded SLNs with improved stability (Ansari et al. 2017).

Recently, the technique of HSH followed by ultrasonication was used to prepare SLNs with improved solubility of total flavonoid extract from Dracocephalum moldavica L. with myocardial protective function (Tan et al. 2017). In another study, Ajiboye et al. (2021) investigated the effect of HSH/US on the properties of olanzapine loaded NLCs.

Shah et al. (2020) applied HSH (15,000 rpm for 10 min) followed by US (~3 min) to prepare optimized linagliptin-loaded SLNs that increased drug permeability and oral bioavailability by 3-fold compared to administered solution of linagliptin in rats.

The main advantages, disadvantages, and limitations of the high-energy approaches for LNs preparation are shown in Table 1.

Low-energy-based methods do not consume a substantial amount of energy to obtain LNPs and sometimes are even spontaneous. Тhe type of these techniques determines as thermal or isothermal. The thermal processes are characterized by emulsion formation due to temperature-dependent changes in surfactant properties. Specific to isothermal methods is the formation of an emulsion due to continuous temperature changes. Low-energy-based approaches are easy to use and cost-effective as they have the advantage to generate tiny droplets without specialized equipment (Chircov and Grumezescu 2019; Li et al. 2020).

The formation of microemulsion as a phase of the production of SLNs and NLCs is reported in the early 90s (Gasco 1993). Distinctive for this method is the spontaneously formed microemulsion due to the high surfactants/lipid ratio. А few simple steps build this method. At first, it proceeds to melt the lipids and mix the melt with a hot surfactant solution thru a constant cautious stirring until the formation of a microemulsion. Then, the hot microemulsion is dispersed in a high volume of cold water (2–3 °C) under continuous moderate stirring. This operation leads to the solidification of the liquid droplets (Masiiwa and Gadaga 2018). The obtained SLNs and NLCs are spherical with a narrow size distribution. One of the drawbacks of the microemulsion technique is the high dilution level of the final dispersion (ranging from 1:25 to 1:50), which may require additional concentration. Another drawback is using a high concentration of surfactants/co-surfactants (Thepwatee et al. 2019).

Joshi et al. (2019) described the preparation of NLC-based gels from the microemulsion technique for drug delivery through the skin, particularly concerning the method of obtaining and the characterization of the final nanobased carriers.

Some cases for applying the microemulsion technique in preparing SLNs and NLCs formulations were reported such as methotrexate-loaded SLNs, characterized by small particles size (238 nm), excellent physical stability and controlled drug release (Krishnasailaja and Gazi 2023), as well as phytostanol ester-loaded SLNs serving as a promising DDS in the therapy of hypercholesterolemia (Shrestha et al. 2021).

In another study Ali et al. (2022) developed allopurinol-loaded NLC subsequently incorporated into hydrogels for potential treatment of gout. The formulated LNs served as feasible transdermal drug delivery platform, showing approx. 28-fold increase in skin penetration and superior anti-gout effect vs. free drug (Ali et al. 2022).

The technical setting for this method is the use of a cylindrical membrane module. The aqueous phase containing a surfactant flows from the inside of the membrane and the molten lipid is compressed through the pores of the membrane from outside to inside to pass into the aqueous phase (Fig. 1). This allows the formation of small droplets which are differentiated in the aqueous phase. The water is maintained at the lipid melting temperature. Then SLNs and NLCs are formed thru cooling at room temperature. The method is scalable and the particle size can be adjusted by using membranes with different pore sizes (Lombardo and Kiselev 2022).

Furthermore, the MC technique was applied to obtain SLNs with high vitamin E LC (Chaves et al. 2023).

Figure 1. 

Schematic drawing of the process of preparation of LNs using a membrane contactor.

In practice, the transformation of an emulsion from O/W type to a W/O type is termed “phase inversion.” It can be induced by changing the operating temperature. The temperature at which the inversion occurs is referred to as PIT (Jintapattanakit 2018). In essence, this technique depends on the change in the properties of polyoxyethylated surfactants at different temperatures. The Griffin’s hydrophilic-lipophilic balance (HLB) value of surfactants is valid at 25 °C. At this temperature, the hydrophilic parts of the surface-active compounds are hydrated to a certain level. Аt elevated temperatures are observed dehydration of the ethoxy groups, which leads to the rising of the lipophilicity of the molecules of the surface-active compounds and HLB value decreases. The affinity of surface-active compounds to the aqueous and lipid phase is equal when the PIT has been reached. This particulate state is characterized by very low surface tension and intricate structures in the system. Further increasing the temperature rises enough the affinity of the surface-active compounds to the lipid phase, which allows stabilization of W/O type emulsions. If rapid cooling is applied, stable particles with desirable size and polydispersity can be obtained (Gomes et al. 2019).

Weerapol et al. (2022) developed quercetin-loaded SLNs characterized by small sizes, excellent physical stability, high entrapment efficiency values and sustained drug release. The selected PIT technique was reported as feasible approach to incorporate poorly aqueous soluble compounds into LNs, as confirmed also by Simão et al. 2020. The authors elaborated hesperetin-loaded NLCs with high entrapment efficiency (72.7%), narrow size distribution (PDI < 0.2), controlled drug release (up to 72 h) and excellent in vitro cytotoxic potential (Simão et al. 2020).

In another study, furosemide-loaded SLNs were synthesized by the PIT technique using 3² factorial design. The parameter sensitivity analysis demonstrated a pronounced effect of particle size and reference solubility on the AUC_0–∞, C_max, and t_max. The results showed that the optimized formulation could provide а controlled release and improve the formulation’s physicochemical stability for furosemide oral delivery (Ali and Singh 2018).

LNs can be produced by acidification of a micellar solution of fatty acid alkaline salts. At first, a stock solution of the polymeric stabilizer must be prepared by heating in hot water. The alkaline salt of the fatty acid is homogeneously dispersing in the polymeric stabilizer stock solution, heated to a precisely defined degree with constant stirring until a clear solution is obtained. Then addition of the API (commonly solubilized in ethanol) to the clear solution under permanent stirring is performed until establishing a single-phase state. The gradual input of a coacervating solution to this mixture formed a suspension. In the next step, the suspension is cooled in a water bath under permanent agitation until the launch of well dispersed and API-loaded NPs (Clemente et al. 2018).

The coacervation technique was applied to prepare quercetin-loaded SLNs with high drug loading, controlled release and preserved antioxidant activity of the encapsulated phytochemical (Talarico et al. (2021).

In another study Muntoni et al. (2019) used a fatty acid coacervation method to entrap peptides (insulin and an insulin analog) into NLCs for oral administration. The authors observed an optimized ex vivo and in vivo intestinal uptake of glargine insulin (the insulin analog) expressed by significantly higher permeation (till 30% dose/mL) than free peptide and approximately 6% absolute bioavailability.

The preparation of SLNs and NLCs through a double emulsion technique is suitable for hydrophilic APIs and peptides. In this method, an APIs aqueous solution is emulsified in a melted lipid blend to form a primary W/O emulsion, stabilized with suitable excipients. Second, the primary W/O emulsion is dispersed in an aqueous solution of hydrophilic emulsifier to form a double W/O/W emulsion. Finally, the double emulsion is stirred and isolated by filtration (Mazur et al. 2019).

Subroto et al. (2022) used the DE technique to prepare SLNs as promising DDS for ferrous sulfate, capable to overcome its low stability and unpleasant flavor. The prepared nanoparticles exhibited a spherical shape and high drug entrapment efficiency (Subroto et al. 2022). Doxycycline-loaded SLNs, prepared by DE technique, significantly decreased the Brucella melitensis loading within macrophages (3.5 Log) in comparison with the free doxycycline (Hosseini et al. 2019).

John and Jay (2006) patented a micro emulsion-based method for the preparation of SLNs. This method essentially presents the preparation of o/w microemulsion, where the emulsifying wax is melted at 37–55 °C. At the next step, water heated to the same temperature is added under gentle stirring. As a result, a homogeneous milky suspension is formed. Then a suitable polymeric surfactant is added into water until a stable and clear o/w microemulsion in the form of a liquid matrix is produced. SLNs are molded by precipitation from already obtained and cooled o/w microemulsion at room temperature or 4 °C. As a benefit, this method is reproducible, simple, and easy to scale up. No organic solvents are used, and all components are biodegradable.

Kuo and Lee (2016) used the MEC technique to prepare etoposide-loaded SLNs grafted with 83–14 monoclonal antibody (8314MAb) and anti-epithelial growth factor receptor (AEGFR). The authors reported that incorporating both 8314MAb and AEGFR increased the particle size but decreased the zeta potential, etoposide release rate, and viability of HBMECs and HAs. In this regard, the conjugation of 8314MAb and AEGFR on etoposide-loaded SLNs can be a promising strategy to deliver antitumor etoposide to the brain and restrain the propagation of glioblastoma multiforme.

In their study Cirri et al. (2018) investigated the effect of methods of preparation (homogenization-ultrasonication and microemulsion cooling technique) on the main physicochemical characteristics of hydrochlorthiazide-loaded NLCs. The prepared LNs via the microemulsion method with subsequent cooling exhibited smaller particles size (within 300 nm to 400 nm diapason) higher EE values and prolonged drug release vs. the carriers prepared by the homogenization-ultrasonication method.

The discussed above low-energy approaches have distinct advantages and suffer from some disadvantages and limitations, which are briefly summarized in Table 2.

In general, this technique involves the following steps of preparation:

1. Preparation of organic phase: the lipophilic material is dissolved in an organic solvent (water-immiscible) by magnetic stirring.
2. Pre-emulsification step: the lipid-containing organic phase is dispersed in aqueous solution using a high-speed homogenizer.
3. Nano emulsification step: the coarse pre-emulsion in the fastest way was passed through a high-pressure homogenizer to obtain nanodispersion.

Formed nanodispersion is kept on the magnetic stirrer for a relatively long time (commonly overnight), sometimes in a fume hood, to drive the organic solvent. Upon solvent evaporation, nanodispersion is formed by the precipitation of lipid material in the aqueous medium. Solidified nanodispersion is filtered through a glass filter in order to remove lipid and API agglomerates (Chaudhary et al. 2021).

The emulsification solvent-evaporation method was used to prepare SLNs for the improvement of oral bioavailability of ramipril (Vakhariya et al. 2019). Also, SLNs prepared by the same technique with glyceryl monostearate and poloxamer 407 have enhanced bioavailability with sustained release of irbesartan being an attractive approach for oral administration of the drug (Soma et al. 2017).

A certain amount of solvents, partially soluble in water, such as butyl lactate, isobutyric acid, benzyl alcohol, tetrahydrofuran, and isovaleric acid are used to solubilize the solid lipids in the preparation of LNs. Transient O/W emulsion is passed into water under prolonged stirring, which leads to solidification of the dispersed phase and forming LNs due to diffusion of the organic solvent. Typical O/W ratios are 1/5 or 1/10 (Chaudhary et al. 2021).

The ESD technique was used to prepare SLNs for topical delivery of tretinoin (Patravale and Mirani 2019).

In another study, tenofovir disoproxil fumarate-loaded NLCs, obtained via the same method were reported, characterized by sustained drug release, and suitable physicochemical characteristics to achieve nose to brain delivery (Sarma and Das 2019).

The principle of the SI method is very close to that of the solvent-diffusion method. When the solvent injection technique is applied, lipids are mixed with a water-miscible solvent or mixture of solvents (e.g., acetone, isopropanol or methanol) and after that is immediately injected into an aqueous solution of surfactants through an injection needle (Schubert et al. 2003).

Two parallel and simultaneous circumstances lead to the effective formation of SLNs and NLCs: Firstly, diffusion of the solvent out of lipid-solvent droplets into the water causes a reduction of their size and increases lipid concentration. Secondly, the diffusion of pure solvent from the lipid-solvent droplet reduces the size of droplets. Obtaining SLNs and NLCs can be influenced by the control of technological parameters such as lipid concentration, lipid concentration in the solvent phase, injected solvent, injected volume of solvent, and viscosity of the aqueous phase (Lakshmi and Joni 2021).

Elvitegravir-loaded SLNs with improved drug aqueous solubility (800–1030-fold vs. free drug) were successfully prepared via the SI technique (Kommavarapu et al. 2015).

In another study Duong et al. (2019b) used the SI technique to encapsulate the hydrophilic ondasetron hydrochloride into NLCs. The prepared LNs were characterized by high entrapment efficiency (more than 90%), narrow size distribution and sustained drug release.

The production of LNs from emulsions using SCF technology is referred to as “supercritical fluid extraction of emulsions” (SFEE). The organic solution is prepared by solubilizing the lipid and the API in an organic solvent (e.g., chloroform) with a suitable surfactant followed up by dispersing this organic solution into an aqueous solution (with or without co-surfactant). The mixture is passed through a high-pressure homogenizer in order to be formed an O/W emulsion. The obtained O/W emulsion is fed from the one end of the extraction column (usually the top) at a constant flow rate. The SCF, maintained at constant temperature and pressure, is fed counter-currently at a constant flow rate. The LN dispersions are formulated by the continuous extraction of solvent from the O/W emulsions (Akbari et al. 2020).

Couto et al. (2017) applied the SCF technique for preparing vitamin B2-loaded SLNs, characterized by controlled API release, high EE, bioactive load, and proper API protection. In their study Andrade et al. (2019) developed praziquantel-loaded SLNs, characterized by decreased lipid (cetyl palmitate) crystallinity and high (83%) entrapment efficiency as result of the applied SCF technique for LNs preparation.

PGSS method consists of the melting of the material, which dissolves the SCF under pressure. It is performed incorporation of CO₂ in melted or liquid suspended substance(s), leading to a gas-saturated solution/suspension that is further expanded through a nozzle with the formation of SPs or droplets (Melgosa et al. 2019). The substances do not need to be soluble in CO₂. Some of the most crucial PGSS expedients applied to lipid NPs are the so-called Gas Assisted Melting Atomization (GAMA). Lipids are placed in a thermostated mixing chamber (MC), where they are melted and kept in contact with SCF CO₂ at selected conditions. Then, the saturated lipid fluid is forced through a nozzle by opening the valve at the bottom of the MC to produce NPs. NPs are gathered by a collection device and dispersed in water by vortexing and sonicating to obtain suspensions. Polyethylene glycol (PEG) can be added to the formulation to increase the dispersion rate in water (Chakravarty et al. 2019).

Pedro et al. (2016) developed curcumin-loaded SLNs via PGSS method characterized by homogenous size distribution, high loading and preserved drug’s chemical stability.

The discussed technological methods that use organic solvents with their remarkable advantages, the main disadvantages, and limitations are summarized in Table 3.