Development and Characterization of Bigels for the Topical Delivery of Curcumin

Abstract

The topical application of curcumin can act directly on the tissue, but there are problems related to solubility and permeation. Bigels combine hydrogels and organogels to enhance the release and transport of bioactives through the skin. The aim of this study was to develop bigels for the topical delivery of curcumin. Employing a rheology test, it was found that all bigels showed a solid-like behavior structure (G′ > G″) with stiffness increasing with higher organogel content. The principle of time–temperature superposition (TTS) was used to generate master curves. Microscopy revealed a morphological structure that depended on the organogel/hydrogel ratio. The bigels exhibited a pH compatible with that of human skin, and the curcumin content met the standards for uniform dosage. Thermal characterization showed the presence of three peaks in coconut oil bigels and two peaks in castor oil bigels. Bigels with a 45% castor oil organogel/55% hydrogel ratio exhibited a longer controlled release of curcumin, while bigels with coconut oil showed a faster release. The release data were fitted to mathematical models indicating non-Fickian release. The permeability of curcumin through Strat-M membranes was investigated, and greater permeation was observed with increasing organogel content. The developed bigels could be a promising option for the topical delivery of curcumin.

1. Introduction

Curcumin is a compound isolated from turmeric, a plant known for its medicinal uses. There is growing evidence that curcumin may be effective in the treatment of various conditions, such as treating chronic pain, digestive problems, and dermatitis; promoting wound healing; treating skin infections and other problems in cosmetics as an anti-pigmentation agent [1]. Although turmeric has been used in alternative medicine for thousands of years and its benefits are widely known, its therapeutic potential remains limited due to its poor oral bioavailability [2]. Several efforts to increase the solubility of curcumin have been described in the literature, including the use of nanoparticles, nanosuspensions, or complexes with cyclodextrins [3]; however, topical application of curcumin would allow it to act directly on the affected tissue, making it useful for the treatment of skin conditions. However, there are still limitations, such as poor solubility and poor permeation. These points are related to inefficient absorption by epithelial cells [4]. Also, there are reports about instability at a certain pH [3]. At physiological pH, curcumin degrades to bicyclopentadione, vanillin, and ferulic acid [5]. The curcumin that is not chemically degraded undergoes biotransformation by metabolic enzymes to form reduction products and glucuronide in the liver and several organs of the gastrointestinal tract. Thus, there is a lot of work on increasing the use of curcumin by oral delivery [6]; however, the use of alternative routes to oral delivery could be useful.

Pandey and Dalvi [7] demonstrate the effect of the temperature on curcumin stability; they found that the curcumin shows three polymorphs (two orthorhombic and one monoclinic), and the most thermally stable form was monoclinic.

The continuous search for innovative and effective therapeutic strategies in the field of dermatology has led to the development of topical delivery systems that allow controlled administration of bioactive substances. Structured gel-like systems have excellent properties for the topical application of curcumin. Gels are biocompatible systems that are easy to prepare, as they do not require toxic reagents or catalysts and do not generate by-products, making them suitable and advantageous as drug delivery vehicles. Gels are viscoelastic materials that exist in both solid and liquid consistencies due to their cross-linking properties. In general, gel-based formulations are classified into two main categories based on the polarity of the liquid phase, with water being the liquid phase in hydrogels and oil being the liquid phase in organogels (also known as oleogels) [8], from which other, more complex systems such as emulsion organogels, emulgels, and bigels are derived [9].

Bigels are novel biphasic systems prepared by mixing hydrogels with organogels at high shear rates, resulting in a stable, doubly structured system [10]. They can be formulated as emulsified or bicontinuous systems and, therefore, have the advantages of hydrogels, organogels, and even emulsions [11]. They can contain lipophilic and hydrophilic active ingredients and administer them simultaneously. The gelation of the water and oil phases makes it difficult for the droplets to escape from the three-dimensional gel network so that they do not agglomerate or flocculate. Some other interesting properties of bigels are their refreshing and moisturizing effect; they are washable after application, easy to spread, and improve the absorption of drugs by the skin [12]. Research has focused on their use for cosmetic and pharmaceutical applications and, more recently, on the development of functional foods [8]. Also, several authors have found good thermomechanical and photostability of bigels in comparison with oleo- and hydrogels and emulsions [13]. Several research studies have found that the rate of oleogel/hydrogel has an important effect on the mechanical stability of the bigels [14]; these authors reported an increase in the thermomechanical stability of bigels with a higher proportion of oleogel.

The current applications of bigels are described by Francavilla et al. [15], including cosmetic, ingredient replacement or substitution, novel matrices, preventive care, therapeutic care, and controlled release and protection.

There is an increase in work about bigels as a transdermal controlled release; however, there are limitations, including stability assessments and permeation experiments [16].

The oily fraction used in bigels (oleogel) has been probed by several vegetable oils. In the field of cosmetics and pharmaceutics, castor oil is an interesting alternative. Reports about their use in bigels have been performed by several authors [17,18,19] with improved thermomechanical properties in comparison with other vegetable oils. Also, the use of coconut oil in bagels has been reported with higher oil-binding capacity (>99%), according to [20].

Castor oil is derived from the seeds of the Ricinus communis plant, and it is predominantly composed of triglycerides of ricinoleic acid. Ricinoleic acid is distinctive among other fatty acids found in vegetable oils due to the presence of a hydroxyl group in its structure. This characteristic renders it highly appealing for a broad range of applications, particularly within the pharmaceutical and cosmetic industries [17].

Conversely, utilization of coconut oil in organogels comprising various compounds, including piroxicam [21], has been demonstrated to enhance permeability. Kumar et al. [22] demonstrated that the incorporation of coconut oil into organogels enhances the biological activity of luliconazole, a pharmaceutical agent, facilitating more precise regulation of its release.

To the best of our knowledge, the utilization of bigels as a topical delivery system for curcumin has not yet been documented. A bigel is a system that can be tailored to a specific texture and rheological properties by varying the formulation and process parameters, depending on the desired application. This behavior is driven by several interactions between different components of the bigel, which modify its rheological properties. Rheology is, therefore, a useful tool for characterizing bigels [23].

The three most significant factors that alter the properties of bigel systems are the type of solvent, the organogel/hydrogel ratio, and the type and structure of the gelators [24]. Several reports about the rheological characterization of bigels are found in the literature, for example, emulsion gel [25], and Lupi et al. [26] developed a semi-empirical model relating rheological properties of bigels to single-phase properties and fractions. Mata-Mota et al. [27] used several models in order to establish a correlation between bigel complex modulus and the rheological properties of both the dispersed and continuous phase, as well as their volumetric ratio. It was determined that these models were not applicable to bigels. Consequently, an empirical modification was proposed with the objective of enhancing the degree of fit. Nakano et al. [28] demonstrated the applicability of the empirical rule of Cox–Merz on bigels. However, the utilization of time–temperature superposition is highly desirable, as it would facilitate the prediction of relaxation properties over both extended and shorter time intervals, as well as at lower and higher frequencies [29].

The time–temperature principle can be employed to elucidate the structural change/deformation of food and biopolymers at selected temperatures. The novelty of the principle is associated with the fact that the frequency and time domain of the viscoelastic properties can be extended to higher ranges. The generation of master curves under specified stresses is a fundamental aspect of the model, with the corresponding temperature shift factor exhibiting a dependence on the stress level at which time shifting is employed [30]. To the best of our knowledge, there have been no reports of the utilization of time–temperature master curves in bigels. The main objective of the present study was to evaluate the use of time–temperature master curve to evaluate thermomechanical properties of bigels made with castor oil and coconut oil for topical delivery of curcumin, including in vitro permeation and release mechanism.

2. Materials and Methods

2.1. Materials

The coconut oil was purchased at a local supermarket, manufactured by Vaserco S de RL de CV (Zapopan, Jal, Mexico), the main producer of coconut oil in Mexico, and the fatty acid composition declared by the producer was caproic acid 0–1.2%, linoleic acid 1–2.5%, stearic acid 1–4%, caprylic acid 5.5–9.5%, capric acid 4.5–5–5%, oleic acid 3.5–11%, palmitic acid 7.5–12%, myristic acid 13–19%, and lauric acid 41–56%. The castor oil was purchased at Comercializadora BR PROQUIM (Atizapán de Zaragoza, State of Mexico, Mexico) the main distributor of castor oil in Mexico. The fatty acid composition declared by the producer was palmitic acid 2%, stearic acid 2.5%, oleic acid 2.5–6%, linoleic acid 2.5–7%, linolenic acid 1%, eicosanoic acid 1%, ricinoleic acid 85–92%, and others fatty acids 1%. The myverol 18–04 PK (49% glycerol monostearate, 48% glycerol monopalmitate and 3% calcium silicate) used as an organogelating agent was provided by Kerry, SW FOOD TECNOLOGÍA, S.A de C.V (Monterrey, Nuevo León, Mexico). The xanthan gum was purchased from Ingen Manufacturing S.A. de C.V. (Tepalcingo, Morelos, Mexico). Curcumin (CAS Number 458-37-7) was declared to have a purity of ≥65% by HPLC, and there was a minor presence of curcuminoids in the form of demethoxycurcumin and bisdemethoxycurcumin, from Sigma-Aldrich (St. Louis, MO, USA), without additional purification procedures used. The cosmetic grade preservative Geogard Ultra (70–80% gluconolactone, 22–28% sodium benzoate, and 1% calcium gluconate) was purchased from ABREIKO S DE RL DE CV (Zapopan, Jal, México). Regenerated cellulose membranes with a pore size of 12–14 kDa were purchased from Fisher Scientific and Strat-M^® membranes from Merck Millipore.

2.2. Bigel Formulation

Organogels were prepared using castor oil (CaOG) and coconut oil (CnOG) with 6 wt% myverol. Hydrogels (HG) were prepared with a constant concentration of 1.5 wt% of Geogard Ultra preservative and 2% xanthan gum as a structuring agent (see

Table 1. Organogels and hydrogel composition (% wt/wt).

	Codes
Materials	CaOG	CnOG	HG
Myverol	6	6	-
Castor oil	94	-	-
Coconut oil	-	94	-
Xanthan Gum	-	-	2
Water	-	-	96.5
Geogard Ultra			1.5

CaOG is the organogel made with castor oil; CnOG is the organogel made with coconut oil; HG is the hydrogel.

). The preparation of these organogels and hydrogels followed the methodology described by [17].

The obtention of the bigels was initiated by preparing the hydrogel and organogels at 60 °C, followed by a 5-min mixing process at 1000 rpm (IKA Eurostar ST-P-DV) with a rotor-stator-type disperser. The organogel was incorporated to the hydrogel in the ratios 15/85, 30/70, and 45/55 of Ca OG/HG and Cn OG/HG, respectively (see

Table 2. Bigels composition (% wt/wt). HG is the hydrogel, CaOG is the organogel made with castor oil, CnOG the is organogel made with coconut oil.

	Code	CaOG15 (%)	CaOG30 (%)	CaOG45 (%)	CnOG15 (%)	CnOG30 (%)	CnOG45 (%)
Materials
HG		85	70	55	85	70	55
CaOG		15	30	45	-	-	-
CnOG		-	-	-	15	30	45

). Subsequently, the samples were cooled to 25 °C and stored in darkness for at least 15 days prior to analysis.

CaOG15 indicates the use of 15% of organogel made with castor oil and 85% of hydrogel, CaOG30 indicates the use of 30% of organogel made with castor oil and 70% of hydrogel, CaOG45 indicates the use of 45% of organogel made with castor oil and 55% of hydrogel, CnOG15 indicates the use of 15% of organogel made with coconut oil and 85% of hydrogel, CnOG30 indicates the use of 30% of organogel made with coconut oil and 70% of hydrogel, CnOG45 indicates the use of 45% of organogel made with coconut oil and 55% of hydrogel.

2.3. Rheological Characterization

2.3.1. Small Amplitude Oscillatory Shear Flow (SAOS) Methodology

The oscillatory shear tests were conducted within a frequency range of 1–300 rad/s. Previously, the linear viscoelastic zone was identified in instances where the amplitude of the oscillations was small. The linear zone was determined based on the evolution of the moduli as a function of the percentage deformation at different fixed frequencies (1, 10 and 100 Hz). For the purpose of this study, a 5% strain was utilized in the fabrication of all bigels. The measurements were conducted at 25, 37, and 45 °C (room temperature, human body temperature and stress temperature condition, respectively). The later condition was selected to apply high temperature without visual damage to the bigel, i.e., no physical change was observed upon visual observation. A stress-controlled rheometer with rough-surface parallel-plate fixture of 40 mm diameter (DHR3, TA Instruments, New Castle, DE, USA) and Peltier system with recirculating bath enable temperature control (Cole Parmer Polystat and Peltier AR–G2). All experiments were conducted in triplicate (n = 3).

2.3.2. Application of the Principle of Time-Temperature Superposition (TTS)

Rheological evaluation of the viscoelastic behavior is constrained to a finite range of the viscoelastic spectrum. However, rheological properties may exhibit significant variation across several orders of magnitude in response to changes in temperature. A methodology employed to address this issue is the time–temperature superposition (TTS) principle, which involves the lateral shift in isothermal oscillatory frequency data using a shift factor to generate a single master curve [31].

The oscillation test at 37 °C was taken as a reference point, and it was necessary to multiply the time values of each curve by a different unique scalar number, $a_T$, for each curve to shift all remaining curves horizontally in the direction of the reference curve. The curves, which initially exhibited a horizontal shift in direction toward the reference curve, now demonstrated a vertical trajectory, intersecting at $b_T$, to form a unified, superimposed curve. This collective curve is designated as the master curve. This superimposition of the curves was performed manually, with the changes factor values being arbitrarily selected, until the connection of all curves was visually confirmed. After the generation of the master curve, the experimentally determined change factors $a_T$ and $b_T$ were fitted to Arrhenius’ equation [32]:

$$a_T=exp\left[{\displaystyle \frac{E_a}{R}}\left({\displaystyle \frac{1}{T}}-{\displaystyle \frac{1}{T_o}}\right)\right]$$

(1)

$$b_T=exp{\displaystyle \frac{1}{\left[\frac{E_b}{R}\left(\frac{1}{T}-\frac{1}{T_o}\right)\right]}}$$

(2)

where $a_T$ and $b_T$ are the change factors, $E_a$ is the flow activation energy, $E_b$ is the relaxation activation energy, $T_o$ is the reference temperature in °K, and $R$ is the ideal gas constant.

In the literature, the vertical displacement factor $b_T$ = 1 is generally given or is negligible in some cases [33]. In this work, however, it had to be considered for the correct connection of the curves Cn45 and Ca45. Linear regression was employed to derive the a_T enabling the construction of the master curve (TTS) of G′(a_ωT) and G″(a_ωT). The time–temperature superposition principle posits that the structural integrity of the sample remains constant and unaltered across the entire temperature range of the experimental tests.

2.4. Microscopy Methodology

Approximately 15 mg of the sample was placed on a glass slide for observation at room temperature (≈25 °C). An optical microscope with a polarizing filter (ZEISS, Axio-Lab A.1, Jena, Germany) equipped with an Axio-Cam ERc 5S digital camera and ZEN 2.3 lite software (blue version, Zeiss, Jena, Germany) was used. The photomicrographs were obtained in bright field and using a polarization filter.

2.5. Determination of the pH Value Methodology

For the pH measurements, a 1% solution of Bigels in distilled water was prepared. The pH was measured using a digital pH meter [34]. All determinations were performed in triplicate (n = 3).

2.6. Curcumin Content

Curcumin content was analyzed following the method of [35]. Briefly, 7 mg of each of the curcumin-loaded bigels were dissolved in 2 mL of 96% ethanol, vortexed for curcumin extraction and then centrifuged at 10,000 rpm for 20 min to obtain a clean solution. The bigels were loaded with 423 μg of curcumin/g of bigel, according to Perez-Salas et al., [17]. Briefly, curcumin was solubilized in the oily phase at the indicated concentration. Then, the obtained organogel was added to the hydrogel as indicated by [36]. Quantification of curcumin was performed by spectrophotometry at 425 nm. A calibration curve was prepared in 96% ethanol from a concentration of 0.125 to 6 µg/mL. All determinations were made in triplicate (n = 3).

Differential Scanning Calorimetry Methodology

The thermal properties of the bigels were analyzed using a DSC Q-2000 Differential Scanning Calorimeter (TA Instruments, New Castle, DE, USA). The DSC device was calibrated with indium. Approximately 6 to 9 mg of each Bigel sample was hermetically sealed in aluminum capsules, and an empty aluminum dish was used as a reference. Nitrogen flow was supplied at a rate of 40 mL/min. The thermal profile was determined in the heating temperature range from 20 to 200 °C at a rate of 3 °C/min. The temperatures and enthalpies corresponding to the endotherms were determined using Universal Analysis 2000 Ver. 4. Experimental determinations were performed in triplicate (n = 3).

2.7. Curcumin Release Mechanisms

The release profiles were determined using Franz cell device (PermeGear, Hellertown, PA, USA) with an effective diffusion area of 7.07 cm². A cellulose membrane with a pore size of 12 to 14 kDa was placed between the donor and receptor compartments. The receiving compartment was filled with 105 mL of a 1:1 ethanol/PBS solution with a pH of 7.4 to establish sink conditions [23]. The receiving solution was subjected to continuous stirring using a constant magnetic stirring (600 rpm) and maintained at 32 °C. 0.85 g of each of the bigels was added to the donor compartment. At time intervals of 1, 2, 3, 6, 9, 12, and 24 h, 2 mL of the sample was removed from the receptor and immediately replaced with an equal volume of the fresh receptor solution. The samples were then subjected to analysis using a spectrophotometer set to a calibration curve ranging from 0.125 to 8 µg/mL. The limit of detection (LOD) was obtained (0.15 µg/mL), and the limit of quantification (LOQ) was determinate (7.317 µg/mL). The correlation coefficient was 0.9992. The analysis was conducted using a S2000 spectrometer, equipped with an A DT1000 deuteruim light source, SAD500 serial port interface (Optics, Inc. Brunswick, OH, USA) and a 10 nm path length quartz cuvette (Prolab). All the experiments were performed in triplicate (n = 3).

In order to understand the mechanism of drug release, the release kinetics were fitted to different kinetic models using DDSolver software (version 1.0) [37].

$$\mathrm{Zero} \mathrm{order}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}F=k_0\times t$$

(3)

$$\mathrm{First} \mathrm{order}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}F=100\times \left[1-Exp\left({-k}_1\times t\right)\right]$$

(4)

$$\mathrm{Higuchi}\hspace{1em}\hspace{1em} \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}F=K_H\times t^{0.5}$$

(5)

$$\mathrm{Korsmeyer}-\mathrm{Peppas} \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}F=k_{KP}\times t^n$$

(6)

where $F$ is the fraction (%) of the drug released at time $t$, $k_0$ is the zero order release constant, $k_1$ is the first order release constant, $K_H$ is the Higuchi release constant, $k_{KP}$ is the release constant that incorporates structural and geometric characteristics of the pharmaceutical form of the drug, and $n$ is the diffusion exponent that indicates the drug release mechanism.

2.8. In Vitro Permeation Study

A Franz diffusion cell, with an effective diffusion area of 7.07 cm², was utilized in this study. The permeation studies were performed using a 47 mm-thick synthetic Stat-M membrane, which has structural and chemical properties like those of human skin. It is noteworthy that this membrane does not necessitate hydration or any form of pretreatment. The Strat-M^® membrane was positioned with its shiny side oriented towards the donor compartment. A total of 0.85 g of each of the bigels was added to the donor compartment along with curcumin. The uptake compartment was filled with 105 mL of a 1:1 ethanol:PBS solution with a pH of 7.4 [35]. The receiving solution was continuously stirred using a magnetic bar at 600 rpm and kept at 32 °C. At time intervals of 1, 2, 3, 6, 9, 12 and 24 h, 2 mL of the sample was removed from the receptor and promptly replaced with an equivalent volume of the freshly prepared receptor solution. Furthermore, the curcumin retained in the synthetic membrane was subjected to analysis. At the end of each experiment, the membrane was cleaned of excess sample with distilled water, cut into small pieces, and placed in 50 mL of 96% ethanol.

To extract the retained curcumin, the membrane was stirred with a magnetic bar at 600 rpm for 5 h. The samples were analyzed using a spectrophotometer set to a calibration curve between 0.125 and 8 µg/mL. The amount of drug accumulated per square centimeter is plotted against time for each sample. The lag time and flow were determined from the intercept and slope of the x axis, respectively. All experiments were conducted in triplicate (n = 3).

2.9. Statistical Analysis

The data obtained were analyzed using two-way ANOVA (p < 0.05 indicates statistical differences) and a mean comparison test (Tukey’s test, p < 0.05, indicates statistical differences) with Statistica Software 12 (StatSoft, Tulsa, OK, USA). Power law model parameters were obtained using Trios V5.1.1 software, and values of R² ≥ 99 were obtained in all cases. All experiments were performed, at least, in triplicate (n = 3).

3. Results and Discussion

3.1. Bigels Obtention

The bigels were formulated through the use of organogels, which were prepared with castor or coconut oil, using as gelator, Myverol, at a constant concentration of 6% (w/v). The use of these oils has been reported in previous studies [17]. The use of xanthan gum as a gelator in the hydrogels to use in bigels has also been reported [38]. The next step was the obtention of the bigels, using the next nomenclature: Ca for castor oil organogel, Cn for coconut oil organogel, and the number 15, 30, and 45, for the proportion in percentage of organogel used in the bigel formulation. The bigel samples showed no flow in a 1 h rapid test in an inverted tube at room temperature. This finding suggest that the concentration of gelling agents employed to form each phase, and the organogel/hydrogel ratios selected were appropriate for loading curcumin.

3.2. Formulation of the Curcumin-Loaded Bigels

The bigel samples showed no flow in a 1 h rapid test in an inverted tube at room temperature, indicating that the concentration of gelling agents used to form each phase and the organogel/hydrogel ratios chosen were suitable for the formulation of self-supporting systems.

3.3. Small Amplitude Oscillatory Shear Flow (SAOS)

In viscoelastic materials, such as gels, the solid behavior is characterized by the storage modulus or modulus of elasticity G′, which measures the ability of a gel to stretch and return to its original shape. Conversely, the liquid phase’s behavior is characterized by the viscous component, denotated as the loss modulus G″, which measures the resistance of the gel to flow [39]. Results about the effect of the proportion and type of organogel used in rheological behavior of bigels are showed in Figure 1. All bigels exhibit G′ greater than G″ throughout the frequency range (G′ > G″) indicating the formation of a gel-like structure [40]. The modules G′ and G″ do not intercept at the range of frequencies that were utilized, which may suggest that the bigel does not show gel-to-sol transformation [41]. This behavior can indicate the presence of stronger internal forces in the bigel, which manifest as a solid-like behavior [42].

As the organogel content in the bigels increased, the modulus of elasticity (G′) was higher. Thus, when the organogel increase in the bigel, it improves particle arrangements, increasing the G′ response of these materials [30]. However, several differences were observed related with the type of oil used. The magnitude of the difference in the higher and lower G′ value of bigels made with castor oil was higher than the observed for bigels made with coconut oil. Martins et al., [43] indicate that the use of coconut oil in organogels created a denser triglycerides crystal network, enhancing the rheological properties of the resulting bigels. On the other hand, the use of castor oil in bigels increase the presence of the OH groups, reducing the interaction between the castor oil and the gelator, yielding a more significative changes in the viscoelastic functions [44]. The magnitude of the highest G′ module was observed for Cn45 (around 1000 Pa), a similar value to that reported by [41] for bigels made with coconut oil.

Figure 2 shows the dependence of G′ and G″ as frequency changes in the Ca45 bigel at 25, 37, and 45 °C. The temperature does not affect the mechanical module behavior of the material in the range of 25–45 °C, and only changes in the magnitude were observed [45]. The formation of the α-gel around 20 °C has been evidenced in bigels, resulting in a decline in the G′ values of bigels with increasing temperature until the temperature transition gel-sol. Viscoelastic materials with high elastic modulus are typically more structurally stable, indicating that the bigels forms thermo-rheological-stable systems. The bigels can be interpreted as evidence that the temperature changes accelerate or decelerate the dominant viscoelastic process(es). Thus, in turn, this allows us to apply the principle of time–temperature superposition for the construction of master curves. The net effect is a compound curve over a much broader range of time (frequency) than that accessible in the original test [46]. This behavioral pattern was consistent across all experimental conditions, with Ca45 serving merely as a representative example.

Figure 3a,b displays the master curves reduced to a reference temperature of 37 °C for a fixed concentration. As illustrated in Figure 3a,b, the master curve (TTS) is the result of shifting the curves in Figure 3a,b along the horizontal axis. The net effect is a compound curve over a much broader range of time (frequency) than that accessible in the original test [47]. Furthermore, it facilitates the prediction of viscoelastic behavior in conditions that fall outside the conventional range of relevant rheological measurements. This method increases the span of the relevant rheological properties as functions of frequency allowing the assessment of the structure changes in complex systems in thermo-reversible (thermo-rheologically simple) systems within a wide region of temperature.

3.4. Master Curves Arising from Time-Temperature Superposition (TTS)

The principle of time (frequency)–temperature superposition assumes that the material exhibits thermorheologically simple behavior. By selecting a reference temperature and applying a horizontal change factor, $aT$, and a vertical, $bT$, the isothermal curves can be shifted until they merge and form a single curve, the so-called master curve. As an example, a sample (Cn15) was randomly selected to show the general behavior. Figure 3a,b displays the master curve reduced to a reference temperature of 37 °C for the Cn15 bigel. First, the experimental viscoelastic data of the bigels must be determined at different temperatures, specifically 25, 37, and 45 °C (Figure 3a) as environmental temperature [48], human body temperature [49], and thermal stress condition [50], respectively. Subsequently, a reference temperature is defined, and the other curves are shifted to this temperature as required when calculating the change factors $aT$ and $bT$. This superimposition of the curves was performed manually, using arbitrary values for the shift factor, until visual confirmation of the merging of all curves was available (Figure 3b). The independence of the superposition method TTS, with the reference temperature (37 °C) indicates a relationship between the relaxation process and the free volume. According to [30], good superposibility of TTS suggests that the material is thermorheologically simple, where relaxation times for all mechanisms change identically with temperature. All samples showed the same behavior.

Finally, the change factors were modeled using the Arrhenius equation for calculating the activation energy of retardation and relaxation (Figure 3c), which is shown in

Table 3. Retardation activation energy and relaxation activation energy from bigels.

Sample	Retardation Activation Energy $\mathbf{E}\mathbf{a}$ kcal/mol	Relaxation Activation Energy $\mathbf{E}\mathbf{b}$ kcal/mol
Ca45	142.69	6.56
Ca30	98.73	0
Ca15	23.10	0
Cn45	132.01	4
Cn30	66.85	0
Cn15	47.51	0

Ea is the retardation activation energy; Eb is the relaxation activation energy. Ca indicates bigels made with organogel castor oil and hydrogel, and Cn indicates bigels made with coconut oil organogel and hydrogel. The numbers 15, 30, and 45 indicate the proportion of organogel used in the bigels expressed in %.

. Considering that the activation energy reflects the need to overcome an energy barrier for local rearrangements from one state to another [51]. Higher activation energy values with increasing organogel fractions, independent of the oil used (castor oil or coconut oil), indicate a stronger cross-linking network. To the best of our knowledge, the utilization of TTS in bigels has not been previously documented. However, an increased activation energy has been observed, which is indicative of the large configurational adjustments that polymers undergo to contribute to the reduction in free volume [52]. Where the free volume is defined as the amount of unoccupied space within the material, it is influenced by both temperature and polymer mobility [53]. Ferry, [54] attributed this rise with declining temperature to difficulties for transverse vibrations to occur over polymer chains and reflects the energy needed to overcome these difficulties.

Ca refers to organogel made with castor oil, and Cn refers to the organogel made with coconut oil, and 15, 30, and 45 are the proportion (in percent) of organogel used in bigels.

The results obtained for the bigels (Figure 3a–c) indicated they are thermo-rheological simple materials. This categorization facilitates the construction of a master curve by time–temperature superposition principle. According to [30], good superposition of TTS suggests that the material is thermorheologically simple, meaning that the relaxation times for all mechanisms change identically with temperature. The Arrhenius equation fits the data adequately to obtain the factor aT.

The TTS method allows us to extend the frequency range of experimental data beyond six decades, thereby enabling the determination of gel strength. This capacity represent a useful tool for industrial applications of gelling systems.

3.5. Microscopy

The microstructure of the bigels was examined using light field and polarized light microscopy (bright and polarized field, respectively). Microscopic images of bigels prepared from Ca OG formed emulsions with defined organogel droplets, confirmed by polarized light microscopy (Figure 4a,b). A Previous observation indicated that the birefringence of the monoglyceride structures was found to be greater in comparison to that observed in hydrogels with xanthan gum [17]. On the other hand, the bigels formed from Cn OG in the lower part of Figure 4a,b demonstrate that emulsions with well-defined organogel droplets are formed at low proportions of Cn OG. As the percentage increases to 30%, there are still many droplets, but they start to form aggregates due to their packing. Finally, a high fraction (45%) leads to the formation of a more complex bicontinuous system in which the oil and water phases are entrapped within each other.

Kasapis and Sworn [55] investigated the properties of bigels prepared by mixing a cosmetic emulsion (hydrogel emulsion) with the organogel phase. The authors also observed the transition from an oil-in-water-like morphology to a complex bicontinuous morphology as well as the variation in the consistency of the system depending on the organogel/hydrogel ratio. These structural morphological differences depending on the organogel/hydrogel ratio have also been recently observed by other authors [56].

Several authors are reported that thermomechanical properties of bigels are influenced by the chemical nature of the fatty acids in the organogel phase. Machado et al., [20] demonstrate that bigels made with medium chain fatty acid showed strong internal structure, on the other hand the presence of olive oil (rich in monounsaturated fatty acids) affects the crystalline structure of oleogels and then of bigels [44]. However, further experimentation is necessary to study this effect.

3.6. Determination of the pH Value

The pH range of 4.58 to 4.85 determined in the bigels (

Table 4. Curcumin content and pH value of bigels.

Sample	Curcumin Content (%)	pH
Ca15	101.48 ± 4.30 a	4.85 ± 0.19 abc
Ca30	98.08 ± 1.93 a	4.61 ± 0.03 ac
Ca45	102.09 ± 4.25 a	4.58 ± 0.08 ac
Cn15	101.77 ± 2.4 a	4.64 ± 0.03 ac
Cn30	96.87 ± 0.54 a	4.73 ± 0.02 b
Cn45	102.38 ± 1.64 a	4.81 ± 0.21 abc

Different superscript letter in the same column for Ca or Cn indicates a significant difference. Tukey’s α ˂ 0.05, n = 3. Ca refers to the organogel made with castor oil, and Cn refers to the organogel made with coconut oil. 15, 30, and 45 are the proportion (in percent) used of the organogel in bigels.

) is compatible with the normal pH value of the skin [57]. The pH values below 5 are beneficial for the barrier function, hydration, and exfoliation. Microorganisms inhabiting superficial skin layers are known as “skin microbiota” and include bacteria, viruses, archaea, and fungi [58]. An acidic pH such as 4.0–4.5 inhibited the presence of pathogens and keeps the resident microbiota in the skin [59]. There is sufficient evidence for the importance of the acid skin surface conditions and the influence of the topical products on the pH of the skin surface. Consequently, there is a high level of agreement that topical products should have pH values in the range of 4 to 6 [10].

3.7. Determination of the Curcumin Content

The uniform distribution of drugs in any pharmaceutical dosage form can be confirmed by the percentage of active ingredient content. According to the Pharmacopeia of the United Mexican States, dose uniformity is accepted if the amount of active ingredient in not less than 9 out of 10 dosage units is within the range of 85.0 to 115.0%, and no amount is outside the range of 75.0 to 125.0% of the declared amount [60]. Furthermore, it is imperative to ensure that the coefficient of variation should not be greater than 6.0%. The content was found to be within the acceptable range (Table 4).

3.8. Differential Scanning Calorimetry

Characterization of the thermal properties of the bigels was performed using differential scanning calorimetry (DSC), which allowed us to understand the changes in the microstructure and physical transitions as a function of temperature. In the thermograms, for the bigels prepared with castor oil, two endothermic peaks were observed, where the first of them was small, (Figure 5a); the first peak, at ≈45 °C, was assigned to the melting of the crystals of monoglycerides used as a gelator, which can be considered as a loss of the structure of the organogel [61]. No effect of the concentration of the organogel (p < 0.05) in the bigels was observed. For the second endothermic peak at ≈107, which was assigned to the evaporation of water from the bigel, no differences regarding the organogel concentration were observed (p < 0.05).

On the other hand, bigels prepared with coconut oil (Figure 5b) showed three endothermic peaks, with the first peak (≈27 °C) assigned to the melting point of coconut oil. Several authors reported a range of melting point between 22 and 27 °C [62].

The thermal behavior of coconut oil is complex in function of their composition. It consists mainly of medium-chain saturated fatty acids (C12:0 and C14:0) and certain amounts (≈10%) of long-chain fatty acids (C18:0 and C18:1). Several authors claim that coconut oil does not have a well-defined melting point [63]. Also, in the present experiment, a macroscopic phase separation was observed during preparation of the gels, indicating that the complete entrapment of the oil did not occur, and there was no formation of a continuous network, similar to that reported by [63]. The second peak was observed at ≈54 °C. Zampouni et al., [44] reported a melting point of organogels of coconut oil and monoglycerides (10%) at 56 °C ± 1 °C, through the use of the softening drop method; however, the method is not comparable to the DSC. Reference [64] reported that the melting point of the monoglycerides is around 50 °C, and it depends of the length of the acyl chain. Thus, this peak could be assigned to the crystal of the monoglycerides; however, it is at a higher temperature than the melting point of the castor organogel, and the melting temperature of the binary or ternary systems is reduced, as expected by solubility laws [26]. This effect was more evident in the coconut oil, and it broadens the melting transition (i.e., less symmetrical than the pure crystalline state) due to solubility effects upon raising the temperature [65]; however, this effect was not evident for castor oil. Also, the presence of the third peak, which corresponds to the evaporation of water ≈105 °C, was observed, and no differences related with the concentration of the organogel were observed (p < 0.05). For better visualization, peaks are included in

Table 5. Melting point and enthalpy of fusion of bigels.

		Tmax °C			Enthalpy J/g
	Peak	1	2	3	1	2	3
Sample
Ca15		-	44.98 ± 0.66 a	106.68 ± 1.41 a	-	0.17 ± 0.03 a	1475.50 ± 85.56 c
Ca30		-	45.00 ± 0.18 a	107.68 ± 1.38 a	-	0.35 ± 0.04 b	1324.00 ± 82.02 b
Ca45		-	45.16 ± 0.28 a	109.49 ± 3.71 a	-	0.60 ± 0.03 c	988.90 ± 76.51 a
Cn15		27.27 ± 0.12 a	54.23 ± 0.35 b	104.89 ± 2.33 a	3.43 ± 0.08 a	0.20 ± 0.02 a	1654.50 ± 41.72 d
Cn30		27.65 ± 0.04 a	54.37 ± 0.48 b	109.84 ± 0.97 a	4.33 ± 0.17 b	0.28 ± 0.04 b	1367.50 ± 43.14 b
Cn45		27.34 ± 0.21 a	54.67 ± 0.24 b	104.95 ± 3.66 a	9.78 ± 0.05 c	0.55 ± 0.03 c	962.55 ± 11.39 a

A different superscript letter in the same column as Ca or Cn indicates a significant difference. Tukey α ˂ 0.05, n = 3. Ca is referred to organogel made with castor oil and Cn is referred to the organogel made with coconut oil, 15, 30, and 45 are the proportion (in percent) used of organogel in bigels.

.

Enthalpy data are shown in Table 5. Differences were observed by the organogel concentration; when the organogel content in the bigels increases, enthalpy increases, while the opposite occurs when the hydrogel content decreases. The enthalpy data can be related to the degree of monoglyceride crystallization in the samples; at lower enthalpy of fusion, lower mass of monoglycerides crystallized in the matrix [66]. Zhang et al., [64] indicated that higher crystallinity indicates higher thermal stability and, thus, bigels with high concentration of organogels (Ca45 and Cn45) showed better thermal stability independently of the oil used. However, in the bigels system, there are two components, organogel and hydrogel, and several reports indicate that the proportion of organogel into the bigel influences the thermal stability. References [43,67] demonstrate that the use of a high concentration of organogel in bigels increases the thermal stability of the system, like that observed in the present work.

3.9. Mechanisms of Curcumin Release

Figure 6 showed distinct release patterns of curcumin depending on the organogel/hydrogel ratio and the type of organogel used in the bigel formulation. Formulations containing 45% organogel (Ca45 and Cn45) display a longer and more controlled release over the entire 24 h period. These bigels manage to maintain a constant release rate, indicating a greater ability to retain and gradually distribute curcumin. In contrast, formulations containing 30% (Ca30 and Cn30) exhibit a sustained release profile, albeit with faster release rates than those containing 45%.

Finally, formulations containing 15% (Ca15 and Cn15) exhibit a faster and immediate release, achieving significant release percentages within the initial hours. This could be related to the degree of structuring of the system. Thus, higher activation energy levels result in lower curcumin release rate. This relationship is characterized by an inverse proportionality between the percentage of curcumin release and the activation energy.

Regarding the type of oil used, bigels with organogels made with coconut oil consistently show faster release compared to castor oil at the given proportions and test conditions, which could be because coconut oil tends to be less viscous than castor oil. The lower viscosity of coconut oil facilitates its fluidity and, thus, its ability to move faster through the bigel matrix, resulting in faster release of curcumin. In addition, the intermolecular dipole–dipole interactions between coconut oil and curcumin may favor faster release than the specific interactions that castor oil can form with curcumin due to the hydroxyl group at carbon 12, which allows a stronger interaction with the polar groups of curcumin due to hydrogen bonds [68].

The release kinetics were modeled using various mathematical models (

Table 6. Curcumin release from bigels. Models, parameters, and data adjust.

	${\mathit{k}}_0$	${\mathit{R}}^2$	${\mathit{k}}_1$	${\mathit{R}}^2$	${\mathit{k}}_{\mathit{H}}$	${\mathit{R}}^2$	${\mathit{k}}_{\mathit{K}\mathit{P}}$	$\mathit{n}$	${\mathit{R}}^2$
	Zero Order		First Order		Higuchi		Korsmeyer-Peppas
Ca15	4.12 ± 0.04	0.83	0.07 ± 0.00	0.99	15.61 ± 0.04	0.89	9.50 ± 0.53	0.70 ± 0.02	0.95
Ca30	2.57 ± 0.05	0.92	0.04 ± 0.00	0.99	9.60 ± 0.22	0.89	5.02 ± 0.29	0.76 ± 0.01	0.98
Ca45	2.15 ± 0.04	0.88	0.03 ± 0.00	0.96	8.11 ± 0.16	0.88	4.64 ± 0.14	0.73 ± 0.01	0.96
Cn15	9.53 ± 0.26	0.87	0.19 ± 0.01	0.97	26.73 ± 0.65	0.90	17.17 ± 0.53	0.73 ± 0.02	0.96
Cn30	5.08 ± 0.07	0.22	0.013 ± 0.01	0.97	20.28 ± 0.61	0.87	12.23 ± 0.63	0.80 ± 0.01	0.96
Cn45	4.20 ± 0.09	0.52	0.08 ± 0.00	0.98	16.88 ± 0.13	0.94	10.01 ± 1.20	0.74 ± 0.04	0.98

Data shown the mean ± standard deviation (n = 3). Ca refers to the organogel made with castor oil, and Cn is referring to the organogel made with coconut oil. 15, 30, and 45 are the proportion (in percent) used of organogel in bigels. k₀ is the release rate constant of the zero-order model, k₁ is the release rate constant of the first order model, k_H is the Higuchi dissolution constant, and k_kp is the release rate constant. R² is the coefficient of determination.

). The models with high degree of fit included first order kinetics and Korsmeyer–Peppas (R² ≥ 0.95). The first order model shows a concentration-dependent drug release; thus, the bioactive release is only a function of the remaining drug concentration [69]. The use of Korsmeyer–Peppas has been used successful in system rich in lipids as liposomes [70], nanoparticles [17,71], indicating a non-linear behavior. On the other hand, the Higuchi model implies that the amount of drug liberated is a function of the square root of the time [69]. In this model, the drug is released in proportion to the amount of drug remaining in the dosage form used due to the dissolution phenomenon. Although the bigel formulations show good agreement with first order kinetics, analysis with the Korsmeyer–Peppas model indicates a non-Fickian release mechanism with n values between 0.5 and 1.0 (Table 6). This indicates that the release of curcumin is not only controlled by diffusion, but there are also other phenomena that modify the release process [72]; these authors indicate that sink conditions in several circumstances are not satisfied, and due the saturation, drug release is slowed. Also, the carrier could prevent the release by modifying the conditions of release. These release profiles would allow the manufacture of tailor-made delivery systems, depending on the release period sought and curcumin doses for each situation.

3.10. Permeability

The permeability of curcumin through the Strat-M membranes was evaluated using Franz cell device. Figure 7 showed the permeation kinetics of curcumin within 24 h. The formulations with the lowest curcumin permeation are those containing small amounts of organogel Ca15 and Cn15, followed by Ca30 and the remaining formulations with a potentially very similar permeation of ≈19 µg/cm². This behavior is due to the increase in the lipophilic fraction of the formulations that used oils as penetration enhancers due to their “safety profile”. These oils contain fatty acids that could promote skin permeability by liquefying lipids in the stratum corneum and, thus, effectively improve transdermal drug delivery [73]. However, the release results are relevant as the Ca45 and Ca30 formulation had a greater affinity for curcumin and was released more slowly than its counterpart Cn45 and Cn30. Although Cn45 contains a higher proportion of the organogel, its permeation did not differ from that of Cn30, possibly due to the complexity of the system observed during microscopy. The accumulated curcumin permeation agrees with the results of other researchers [35].

The values obtained for the steady-state fluxes, the lag time (T lag), the accumulated permeation of curcumin in 24 h (Q₂₄) per unit area cm², and the amount of curcumin retained in the membrane (Q_ret) per unit area cm² of the bigels with curcumin are shown in

Table 7. Parameters of the in vitro permeability study. Steady-state flux J_ss, the delay time T lag, the cumulative permeation of curcumin in 24 h Q₂₄ per unit area cm², and the amount of curcumin retained in the membrane Q_ret per unit area cm².

Bigel	Jss (µg cm−2 h−1)	T Lag (h)	Q24 (µg/cm2)	Qret
Ca 15	0.32 ± 0.03 a	3.41 ± 0.06 bc	6.73 ± 0.64 a	23.16 ± 0.44 c
Ca 30	0.75 ± 0.01 c	2.71 ± 0.11 ab	15.99 ± 0.49 c	15.93 ± 1.03 a
Ca 45	0.85 ± 0.02 d	2.44 ± 0.22 ab	18.84 ± 0.25 d	18.91 ± 0.49 a
Cn 15	0.52 ± 0.04 b	3.64 ± 0.47 c	10.54 ± 0.49 b	34.79 ± 0.85 d
Cn 30	0.88 ± 0.01 d	2.05 ± 0.22 a	19.72 ± 0.13 d	20.60 ± 0.71 b
Cn 45	0.86 ± 0.02 cd	2.35 ± 0.25 a	19.34 ± 0.26 d	20.37 ± 0.89 b

Different superscript letter in the same column for Ca or Cn indicates a significant difference. Tukey’s test α ˂ 0.05, n = 3. Ca refers to the organogel made with castor oil, and Cn refers to the organogel made with coconut oil. 15, 30, and 45 are the proportion (in percent) used of organogel in the bigels. J_ss is the steady-state flux, T lag is the delay time, Q₂₄ is the cumulative permeation in 24 h (curcumin), and Q_ret is the amount of curcumin retained in the membrane at the end of the experiment.

.

The J_ss flow describes the accumulated amount of drug per exposed area penetrating the vessel per unit time. Increasing the organogel content allowed a higher permeation flux. The formulations with the higher permeation flux were Cn30, Cn45, and Ca45. In studies, a J_ss steady flux value of 0.61 μg cm⁻² h⁻¹ was determined for the penetration of curcumin nanoparticles using porcine skin, which is comparable to our results [74].

The latency time (T lag) is defined as the time at which the steady state begins in which the permeation flux remains constant. Formulations with a low organogel content take longer to reach this point, but the times are comparable to experiments by other authors [75].

The amount of curcumin retained (Q_ret) in the membrane at the end of the experiment was greater in the Cn15 formulation, which was not very good for permeation. This is because in the release profiles, the curcumin left this formulation in a short time. Except for Ca15 and Cn15, the amounts of curcumin that permeate the membrane and the amounts that are in the superficial layers of the skin (trapped in the membrane) are very similar.

4. Conclusions

Curcumin was encapsulated; this study shows that bigels are promising as topical delivery systems for curcumin. The rheological results indicate that they are thermorheologically simple systems in which the principle of time–temperature superposition (TTS) is correctly applied. Under small amplitude oscillatory shear flow, samples were viscoelastic with a predominant elastic behavior (G′ > G″). The Arrhenius equation fits the data adequately to obtain the factor a_T. The TTS method allows to extend the frequency span of experimental data to more than six decades, allowing to the viscoelastic behavior for different temperatures outside the usual range of rheometric measurements. The degree of structuring, represented by the flow activation energy for the different formulations, is inversely proportional to the percentage release of curcumin. Formulations containing 45% organogel showed a more controlled and prolonged release, while coconut oil favored a faster release compared to castor oil. The release kinetics were fitted to mathematical models, and non-Fickian diffusion was found to be predominant. Different release profiles were obtained under various conditions, which allows for the possibility of preparing tailor-made encapsulated systems, depending on the preferred release period and adequate doses for specific applications, such as antioxidant systems, functional pharmaceuticals, organic pigments, etc. The permeation of curcumin was influenced by the organogel fraction and was higher in formulations with higher percentages. However, the versatility of these formulations provides a valuable tool to customize treatments to the specific needs of skin lesions. Finally, formulations such as Ca45, Cn30, and Cn45 showed a high permeation capacity. On the other hand, Ca15, Ca30, and Cn15 showed a faster and more superficial release.

Future direction: The use of bigels showed a great potential for innovative applications including their use in 3D printing and bioactive delivery as described in [76].