Additive-Mediated Stability Enhancement of Bakuchiol-Loaded Alginate Core–Shell Microcapsules Fabricated by Coaxial Electrospray

Abstract

Bakuchiol is a bioactive compound with promising cosmetic and pharmaceutical applications. However, its poor stability toward light, oxygen, and environmental conditions limits its practical use. In this study, bakuchiol-loaded alginate microcapsules were fabricated via coaxial electrospray, and the effects of polymer additives on the stability and structural properties of the microcapsules were systematically investigated. Optimized electrospray conditions produced spherical microcapsules with uniform particle size and morphological characteristics consistent with the proposed core–shell configuration. The optimized formulation achieved an encapsulation efficiency of 91.0 ± 1.5%. Polyvinylpyrrolidone (PVP) and carboxymethyl chitosan (CMCS) were incorporated as functional additives to modulate the alginate polymer network. Rheological, FTIR, and mechanical analyses indicated that additive-mediated intermolecular interactions significantly influenced shell rigidity, swelling behavior, and storage stability. PVP-containing formulations exhibited improved dimensional stability and retained more than 90% of the encapsulated bakuchiol after 12 weeks of storage, whereas excessive CMCS incorporation promoted swelling and accelerated bakuchiol leakage, particularly under alkaline conditions. The enhanced stability of the PVP-containing formulations was attributed to hydrogen-bonding interactions that reinforced the polymer network, demonstrating an effective strategy for improving the long-term stability of oxidation-sensitive bioactive compounds in cosmetic and pharmaceutical encapsulation systems.

1. Introduction

Microencapsulation has emerged as an effective strategy for protecting bioactive compounds from degradation induced by environmental factors such as heat, light, oxygen, and moisture. , In addition to improving the physicochemical stability and shelf life, microencapsulation can also regulate the release and retention behavior of encapsulated compounds under specific environmental conditions. Biopolymer-based encapsulation systems have attracted considerable attention because of their biodegradability, biocompatibility, low toxicity, and broad applicability in pharmaceutical, cosmetic, and food-related applications.

Sodium alginate (SA) is one of the most widely used biopolymers for microcapsule fabrication because of its low cost, nontoxicity, and excellent gel-forming capability through ionic crosslinking with divalent cations such as calcium ions (Ca²⁺), following the well-known egg-box model. , However, alginate-based microcapsules still exhibit several limitations, including high porosity, poor mechanical integrity, and excessive swelling behavior, which may lead to the leakage of encapsulated compounds under aqueous or high-humidity conditions. , These limitations significantly restrict the long-term stability and practical applicability of alginate-based encapsulation systems.

To improve the structural stability of alginate matrices, various functional additives have been incorporated to modulate intermolecular interactions and polymer-network density. Such interactions, including hydrogen bonding, electrostatic attractions, and chain entanglement, can strongly influence the swelling behavior, permeability, mechanical stability, and retention performance. − For example, chitosan has been reported to form dense polyelectrolyte complexes with alginate, thereby improving the structural stability and reducing the permeability. Functional additives may therefore provide an effective strategy for tailoring the physicochemical properties of alginate-based encapsulation systems. However, the influence of additive-mediated polymer interactions on the stability and retention behavior of alginate-based core–shell structures fabricated via coaxial electrospray remains insufficiently understood.

Among the available fabrication techniques, electrospray technology has attracted increasing interest as an effective approach for producing micro- and nanoscale encapsulation systems. , In particular, coaxial electrospray enables the fabrication of well-defined core–shell microstructures in a single processing step. This technique operates under mild conditions, including ambient temperature and atmospheric pressure, making it suitable for encapsulating thermally sensitive and chemically unstable compounds. , In addition, important processing parameters such as polymer concentration, flow rate, and applied voltage can be precisely adjusted to control the particle morphology, shell thickness, and encapsulation performance. The resulting core–shell configuration also enhances protection efficiency by physically separating sensitive compounds from the external environment through a tunable polymeric shell layer. −

Bakuchiol, a natural meroterpene isolated from Psoralea corylifolia, has recently gained considerable interest in cosmetic and cosmeceutical applications because of its antioxidant, antiaging, and retinol-like activities with low irritation potential. − However, bakuchiol is highly susceptible to degradation upon exposure to light, oxygen, and heat, which significantly limits its physicochemical stability and practical applicability in commercial formulations. ,, Therefore, the development of effective encapsulation systems capable of improving bakuchiol’s stability is of considerable importance. ,

In this study, bakuchiol-loaded alginate core–shell microcapsules were fabricated via coaxial electrospray to improve the storage stability of bakuchiol. Polyvinylpyrrolidone (PVP) and carboxymethyl chitosan (CMCS) were incorporated as functional additives to modulate alginate network structures through hydrogen bonding and electrostatic interactions, respectively. − Their effects on intermolecular interactions, shell integrity, swelling behavior, − and bakuchiol retention under different environmental conditions were systematically investigated. Unlike controlled-release systems, these microcapsules were designed to maintain structural stability during storage and release of bakuchiol upon mechanical rupture during product application. This work provides insight into additive-mediated stabilization strategies for oxidation-sensitive bioactive compounds through the modulation of polymer networks.

2. Methods

2.1. Materials and Chemicals

Bakuchiol (purity ≥ 99%, cosmetic grade) and carboxymethyl chitosan (CMCS; MW = 240 kDa, degree of substitution of 90%, cosmetic grade) were supplied from Chanjao Longevity Co., Ltd. (Bangkok, Thailand). Coconut oil (food grade) was purchased from Krungthepchemi Co., Ltd. (Bangkok, Thailand). Sodium alginate (viscosity grade 270–330 mPa·s, MW = 250–350 kDa, food grade), calcium chloride (purity ≥ 75%, food grade), and polyvinylpyrrolidone (PVP; K30, food grade) were purchased from Krungthepchemi Co., Ltd. (Bangkok, Thailand). Hydrochloric acid (purity 37%, AR grade) and sodium hydroxide (purity ≥ 98%, AR grade) were obtained from Merck (Darmstadt, Germany). Methanol (purity ≥ 99%, AR grade) was purchased from Lab Valley (Bangkok, Thailand). All chemicals were used as received without further purification. Ultrapure water (18.2 MΩ·cm) was produced using an Arium Pro water purification system (Sartorius Lab Instruments GmbH & Co. KG, Göttingen, Germany).

2.2. Preparation of Solutions for Microcapsule Fabrication

A bakuchiol core solution (1.0%, w/v) was prepared by dissolving bakuchiol in coconut oil under continuous magnetic stirring until a homogeneous solution was obtained. Shell solutions with a total concentration of 1.0% (w/v) were prepared using sodium alginate (SA) as the primary shell-forming material. Polyvinylpyrrolidone (PVP) and carboxymethyl chitosan (CMCS) were incorporated according to the formulations listed in Table . The polymer mixtures were dissolved in ultrapure water and stirred at 50 °C for approximately 45 min until homogeneous solutions were obtained. Calcium chloride (CaCl₂) solutions (1.0–3.0%, w/v) were prepared in ultrapure water and used as ionic crosslinking baths during microcapsule fabrication.

1. Composition of Shell Formulations Used for Fabrication of Alginate-Based Core–Shell Microcapsules .

	Polymer Composition (% w/v)
Sample	SA	PVP	CMCS
SA	1.00	0	0
SA/PVP0.25	0.75	0.25	0
SA/PVP0.50	0.50	0.50	0
SA/PVP0.75	0.25	0.75	0
SA/CMCS0.25	0.75	0	0.25
SA/CMCS0.50	0.50	0	0.50
SA/CMCS0.75	0.25	0	0.75

^aThe total polymer concentration of all shell formulations was maintained at 1.0% (w/v).

2.3. Fabrication of Core–Shell Alginate Microcapsules via Coaxial Electrospray

Bakuchiol-loaded alginate microcapsules were fabricated by using a coaxial electrospray technique. As illustrated schematically in Figure , the core and shell solutions were independently delivered through the concentric channels of a coaxial nozzle at controlled flow rates, enabling simultaneous dispensing of both phases during electrospraying. The resulting droplets were collected directly in a calcium chloride solution, where instantaneous ionic gelation of the alginate shell produced the hydrogel microcapsules. The coaxial nozzle configuration was designed to produce microcapsules with a proposed core–shell architecture. The fabrication parameters investigated in this study, including sodium alginate concentration, core-to-shell flow rate, calcium chloride concentration, and applied voltage, were selected on the basis of preliminary optimization experiments and previously reported electrospray conditions using comparable coaxial electrospray systems. , The operating conditions employed during the electrospray process are summarized in Table .

1.

Schematic illustration of the fabrication process of bakuchiol-loaded alginate -based microcapsules via coaxial electrospray, created by Apisittipong Srinusud.

2. Experimental Conditions Investigated for Fabrication of Alginate-Based Core–Shell Microcapsules via Coaxial Electrospray.

Parameter	Condition	Unit
Bakuchiol concentration (core phase)	1.0	% (w/v)
Sodium alginate concentration (shell phase)	1.0, 1.5, and 2.0	% (w/v)
Calcium chloride (crosslinking bath)	1.0, 2.0, and 3.0	% (w/v)
Core-to-shell flow rate	15:35, 15:40, 20:35, 20:40, and 30:50	mL h–1
Applied voltage	0 and 1000	V
Operating frequency	400	Hz
Inner nozzle diameter	450	μm
Outer nozzle diameter	700	μm
Nozzle-to-collector distance	15	cm

2.4. Viscosity Measurement

The viscosity of the shell solutions was measured at 25 ± 1 °C using a Brookfield viscometer (Model RVT, Brookfield Engineering Laboratories, Middleboro, MA, USA). Measurements were performed using spindle No.4 at a rotational speed of 100 rpm. Prior to measurements, the samples were equilibrated at room temperature. All measurements were conducted in triplicate, and the results are reported as mean ± standard deviation (SD).

2.5. Optical Microscopy

The morphology and particle size of freshly prepared microcapsules were examined using an optical microscope (Olympus Corporation, Tokyo, Japan) at 5× magnification. Images of the microcapsules and a calibrated vernier scale (Mitutoyo Corporation, Kanagawa, Japan) were acquired under identical magnification conditions. Particle diameters were determined using ImageJ software. At least 100 microcapsules were analyzed for each formulation, and the average diameter was reported as mean ± SD.

2.6. Determination of Encapsulation Efficiency

The encapsulation efficiency (EE) of the optimized SA/PVP0.25 formulation was determined by quantifying the amount of bakuchiol retained within the microcapsules using UV–vis spectrophotometry. Five freshly prepared microcapsules were dissolved in methanol with gentle stirring until complete disruption of the polymer matrix. The resulting solution was filtered, and the bakuchiol concentration was determined at 262 nm using a previously established calibration curve. Encapsulation efficiency was calculated according to eq :

$$\mathrm{EE}(\%)=({\mathrm{M}}_{\mathrm{e}}{/\mathrm{M}}_{\mathrm{t}})\times 100$$ 1

where M_e is the amount of bakuchiol encapsulated in the microcapsules and M_t is the initial amount of bakuchiol used during microcapsule preparation. All measurements were performed in triplicate.

2.7. FT-IR Spectroscopy

The chemical structures of the fabricated microcapsules were characterized using Fourier transform infrared (FT-IR) spectroscopy (Spectrum 100, PerkinElmer, Inc., MA, USA). Dried microcapsules were finely ground with potassium bromide and compressed into transparent pellets using a hydraulic press. Spectra were recorded over the wavenumber range of 4000–400 cm^–1 with 64 scans.

2.8. Mechanical Properties

The mechanical properties of the fabricated microcapsules were evaluated using a TA.XTplus Texture Analyzer (Stable Micro Systems Ltd., Surrey, UK) following the method reported by Li et al. with minor modifications. Individual microcapsules were compressed using a cylindrical aluminum probe (10 mm diameter) at a test speed of 0.5 mm s^–1 with a 5 g load cell. Compression was continued until 50% deformation was achieved. Force–distance curves were recorded, and the maximum compression force was used to determine microcapsule hardness.

2.9. Determination of Encapsulated Bakuchiol Content

The encapsulated bakuchiol content was quantified using a SPECORD 210 Plus UV–visible spectrophotometer (Analytik Jena GmbH + Co. KG, Jena, Germany). The maximum absorption wavelength (λ_max) of bakuchiol was determined at 262 nm.

For quantitative analysis, the microcapsules were dissolved in methanol to release the encapsulated bakuchiol. Absorbance was measured at 262 nm, and the bakuchiol concentration was calculated by using a calibration curve prepared from bakuchiol standard solutions. The average bakuchiol content was subsequently determined from the calibration equation. The calibration curve exhibited good linearity over the investigated concentration range (R² > 0.99).

2.10. Stability Study of Microcapsules

2.10.1. Stability Study under Different Storage Temperatures

Microcapsule stability was evaluated under different storage conditions by monitoring changes in particle size and bakuchiol retention. Samples were immersed in ultrapure water and stored at 4 °C and room temperature under either light or dark conditions. At predetermined intervals (0, 14, 30, and 60 days), particle size and bakuchiol retention were determined. Three independent samples were analyzed for each formulation and storage condition. The percentage retention of bakuchiol was calculated according to eq :

$$\mathrm{Bakuchiol\; retention}(\mathrm{\%})=({\mathrm{M}}_{\mathrm{t}}{/\mathrm{M}}_0)\times 100$$ 2

where M_t represents the bakuchiol content at a given storage time and M₀ represents the initial bakuchiol content.

2.10.2. Stability Study under Different pH Conditions

The pH stability of the fabricated microcapsules was investigated by using buffer solutions at pH 1, 3, 5, 7, and 9. Samples were incubated in the respective buffer solutions and analyzed after 0, 14, and 30 days. Changes in particle size and bakuchiol retention were monitored to evaluate the influence of the environmental pH on microcapsule stability. Three independent samples were analyzed for each formulation under each pH condition.

2.11. Statistical Analysis

All experiments were performed in triplicate unless otherwise stated, and the results are presented as the mean ± standard deviation (SD). Statistical analyses were performed using SPSS software (version 28.0, IBM Corp., Armonk, NY, USA). Differences among groups were evaluated by one-way analysis of variance (ANOVA) followed by Tukey’s posthoc test. Differences were considered statistically significant at p < 0.05.

3. Results and Discussion

3.1. Fabrication of Alginate Microcapsules

3.1.1. Effect of SA Concentration and Core-to-Shell Flow Rate

The effects of SA concentration and core-to-shell flow rate on the formation of bakuchiol-loaded alginate microcapsules were systematically evaluated using coaxial electrospray. SA concentrations of 1.0, 1.5, and 2.0% (w/v) were investigated in combination with core-to-shell flow rates of 15:35, 15:40, 20:35, 20:40, and 30:50 mL h^–1. The bakuchiol concentration was fixed at 1.0% (v/v), the CaCl₂ concentration in the crosslinking bath at 2.0% (w/v), the operating frequency at 400 Hz, and the applied voltage at 1000 V, respectively.

The morphology, size uniformity, and core–shell integrity of the fabricated microcapsules were used as the primary criteria for identifying suitable processing conditions (Figure A). At 1.0% (w/v) SA, well-defined core–shell microcapsules were obtained, with the most favorable morphology observed at a core-to-shell flow rate of 20:40 mL h^–1. Under this condition, the microcapsules exhibited good sphericity and a relatively uniform size distribution with morphological features consistent with the proposed core–shell structure. In contrast, lower flow rate combinations produced microcapsules with smaller core regions, thicker shell layers, broader size distributions, and occasional multicore structures, suggesting an insufficient balance between core and shell delivery during droplet formation.

2.

Optical micrographs showing the effects of sodium alginate concentration and core-to-shell flow rate (A), calcium chloride concentration (B), and applied voltage (C) on alginate microcapsule formation.

Increasing the SA concentration to 1.5% (w/v) altered the droplet formation behavior. The most acceptable morphology at this concentration was obtained at a higher core-to-shell flow rate of 30:50 mL h^–1; however, the resulting microcapsules were larger and less uniform than those prepared using 1.0% (w/v) SA. This behavior was likely associated with the increased viscosity of the shell phase, which enhanced resistance to droplet breakup and promoted the formation of larger droplets during electrospray processing. − At 2.0% (w/v) SA, stable core–shell microcapsules could not be obtained under the investigated conditions. The high viscosity of the shell solution likely hindered stable jet formation and droplet detachment, resulting in irregular morphology, coalescence, and poor core–shell formation. −

These results demonstrate that both the sodium alginate (SA) concentration and the core-to-shell flow rate play critical roles in regulating droplet formation, particle morphology, and the proposed core–shell structure during coaxial electrospray. Among the conditions investigated, an SA concentration of 1.0% (w/v) and a core-to-shell flow rate of 20:40 mL h^–1 provided the best balance between solution viscosity, shell-forming ability, and droplet breakup behavior, producing uniform spherical microcapsules. Accordingly, this formulation was selected as the optimized baseline for subsequent additive incorporation studies and is highlighted by the red boxes in Figure .

3.1.2. Effect of Calcium Chloride Concentration

The effect of CaCl₂ concentration in the crosslinking bath on microcapsule fabrication was investigated at 1.0, 2.0, and 3.0% (w/v) under the optimized electrospray condition of 1.0% (w/v) SA and core-to-shell flow rate of 20:40 mL h^–1. During this experiment, the operating frequency and applied voltage were maintained at 400 Hz and 1000 V, respectively. The resulting morphologies and particle sizes are presented in Figure B and Figure B.

3.

Average diameter of bakuchiol-loaded alginate microcapsules fabricated using CaCl₂ crosslinking baths at concentrations of 1.0, 2.0, and 3.0% (w/v). Data are presented as mean ± SD.

The average microcapsule diameter showed a slight decreasing trend with an increasing CaCl₂ concentration (Figure ). Although the overall size difference was modest, noticeable changes in the microcapsule morphology were observed. At 1.0% (w/v) CaCl₂, the microcapsules exhibited irregular shapes, rough surfaces, and fragile shell structures; this behavior was attributed to insufficient ionic crosslinking between alginate chains and Ca²⁺ ions, which limited the formation of mechanically stable calcium alginate networks. −

Increasing the CaCl₂ concentration to 2.0% (w/v) produced spherical microcapsules with smoother surfaces, more uniform size distribution, and improved shell integrity. This result indicates that 2.0% (w/v) CaCl₂ provided sufficient Ca²⁺ availability for effective ionic gelation, while allowing adequate microcapsules prepared using 3.0% (w/v) CaCl₂ to exhibit more irregular morphology. This behavior is likely due to rapid surface gelation induced by the high Ca²⁺ concentration, which may have limited further Ca²⁺ diffusion into the alginate matrix and produced heterogeneous crosslinking. −

These results demonstrate that the CaCl₂ concentration plays a critical role in controlling the balance between the gelation rate and Ca²⁺ diffusion during calcium alginate microcapsule formation. Among the investigated conditions, 2.0% (w/v) CaCl₂ was selected as the optimal crosslinking bath concentration because it produced spherical microcapsules with smooth surfaces, uniform morphology, and adequate shell integrity.

3.1.3. Effect of Applied Voltage

The effect of applied voltage on the formation of bakuchiol-loaded alginate microcapsules was investigated at 0 and 1000 V using 1.0% (w/v) SA as the shell phase and 2.0% (w/v) CaCl₂ as the crosslinking bath. The core-to-shell flow rate and operating frequency were maintained at 20:40 mL h^–1 and 400 Hz, respectively. The resulting microcapsule morphologies are shown in Figure C.

In the absence of an applied voltage (0 V), the droplets were mainly generated by gravitational and hydrodynamic forces. Under these conditions, the fabricated microcapsules exhibited an irregular morphology, a broad size distribution, and multicore structures, indicating unstable droplet detachment and insufficient control over core–shell droplet formation. In contrast, applying a voltage of 1000 V markedly improved the microcapsule morphology. The resulting microcapsules exhibited improved sphericity, a more uniform particle size distribution, and morphological characteristics consistent with those expected for the coaxial electrospray process.

The improvement observed at 1000 V can be attributed to the applied electric field, which promoted stable Taylor cone formation and facilitated controlled droplet breakup at the coaxial nozzle tip. , The electrostatic force helped overcome surface tension, allowing more uniform droplet generation and improved alignment between the core and shell streams. , Without the electric field, droplet formation was dominated by gravitational dripping, which resulted in unstable breakup and nonuniform encapsulation structures.

These results demonstrate that the applied voltage is essential for stable coaxial electrospray processing and the formation of uniform alginate microcapsules. Therefore, 1000 V was selected as the operating voltage for the subsequent fabrication experiments.

3.2. Effect of Additives on Microcapsule Formation

The effects of PVP and CMCS incorporation on the formation of bakuchiol-loaded alginate core–shell microcapsules were investigated using the optimized fabrication conditions identified in the previous section. The microcapsules were fabricated using 2.0% (w/v) CaCl₂ as the crosslinking bath, an applied voltage of 1000 V, an operating frequency of 400 Hz, and a core-to-shell flow rate of 20:40 mL h^–1. The shell formulations containing different proportions of SA, PVP, and CMCS are summarized in Table .

As shown in Figure A–F, all shell formulations successfully produced spherical microcapsules with morphological features consistent with the proposed core–shell configuration, indicating that the incorporation of PVP or CMCS did not prevent alginate gelation or core–shell droplet formation during coaxial electrospray. The successful formation of microcapsules was mainly attributed to the ability of SA to undergo ionic crosslinking with Ca²⁺ ions through the egg-box gelation mechanism.

4.

Optical micrographs of bakuchiol-loaded alginate core–shell microcapsules fabricated using different shell formulations: (A) SA/PVP_0.25, (B) SA/PVP_0.50, (C) SA/PVP_0.75, (D) SA/CMCS_0.25, (E) SA/CMCS_0.50, and (F) SA/CMCS_0.75.

Although all formulations generated microcapsules, differences in morphology were observed depending on the type and concentration of the additive. PVP-containing formulations (Figure A–C) produced relatively uniform spherical microcapsules with clear core–shell boundaries. In contrast, CMCS-containing formulations (Figure D–F), particularly SA/CMCS_0.50 and SA/CMCS_0.75, appeared larger and showed more opaque shell regions. This behavior may be associated with increased polymer chain interactions and higher shell solution viscosity, which can influence droplet breakup and shell formation during electrospray processing.

These observations indicate that both PVP and CMCS were compatible with alginate-based core–shell microcapsule fabrication. However, the additive type and concentration influenced the physical appearance and structural characteristics of the resulting microcapsules, suggesting that additive-mediated polymer interactions play an important role in determining the shell morphology and microcapsule formation.

3.3. Particle Size of Microcapsules

The particle size distribution of bakuchiol-loaded alginate microcapsules containing different shell additives was evaluated by optical microscopy and ImageJ analysis (Figure ). All formulations exhibited relatively narrow size distributions, indicating that the optimized coaxial electrospray conditions provide stable droplet generation for both PVP- and CMCS-containing shell formulations. However, the additive type and concentration clearly influenced the average microcapsule diameters.

5.

Particle size distribution of bakuchiol-loaded alginate microcapsules containing different concentrations of PVP and CMCS: (A) SA/PVP_0.25, (B) SA/PVP_0.50, (C) SA/PVP_0.75, (D) SA/CMCS_0.25, (E) SA/CMCS_0.50, and (F) SA/CMCS_0.75, as determined by optical microscopy and ImageJ analysis.

For PVP-containing formulations (Figure A–C), the average diameter increased slightly from approximately 0.85 mm to 0.99 mm as the PVP concentration increased from 0.25 to 0.75% (w/v). In contrast, CMCS-containing formulations (Figure D–F) produced larger microcapsules, with average diameters ranging from approximately 0.99 mm to 1.06 mm. The greatest particle size observed in the CMCS-containing formulations was consistent with the optical micrographs in Figure , where these microcapsules appeared larger and exhibited more opaque shell regions.

The increased particle size was likely associated with changes in the shell formulation properties after polymer-additive incorporation. In coaxial electrospray, higher shell viscosity can increase resistance to droplet deformation and delay droplet breakup, leading to the formation of larger droplets before gelation. − In particular, CMCS may promote stronger intermolecular interactions with alginate through hydrogen bonding and electrostatic interactions, resulting in higher resistance to droplet breakup and larger microcapsule formation. By comparison, PVP-containing formulations caused only a moderate increase in the particle size, suggesting a less pronounced effect on droplet breakup under the investigated conditions.

These results indicate that additive-mediated changes in shell formulation properties play an important role in controlling the microcapsule size during coaxial electrospray. The larger particle size and more opaque appearance of CMCS-containing microcapsules further suggest differences in shell matrix organization compared with PVP-containing formulations.

3.4. Apparent Viscosity, FT-IR Analysis, and Mechanical Properties of Microcapsules

The apparent viscosity of the shell formulations was strongly influenced by additive type and concentration (Figure A). The pure SA formulation exhibited the lowest viscosity of 172.50 ± 0.61 cP. Incorporation of PVP gradually increased viscosity to 185.90 ± 0.50, 202.80 ± 0.65, and 219.20 ± 0.76 cP for SA/PVP_0.25, SA/PVP_0.50, and SA/PVP_0.75, respectively. A more pronounced increase was observed for CMCS-containing formulations, with viscosities of 229.20 ± 2.15, 266.90 ± 1.47, and 304.60 ± 1.66 cP for SA/CMCS_0.25, SA/CMCS_0.50, and SA/CMCS_0.75, respectively.

6.

Apparent viscosity of shell formulations (A) and hardness of bakuchiol-loaded alginate microcapsules (B) containing different concentrations of PVP and CMCS. Data are presented as mean ± SD (n = 3). Different lowercase letters indicate significant differences (p < 0.05), while groups sharing the same letter are not significantly different (one-way ANOVA with Tukey’s posthoc test).

The increase in apparent viscosity was attributed to enhanced intermolecular interactions and polymer chain entanglement with the shell formulations. In PVP-containing systems, hydrogen bonding may occur between the hydroxyl groups of SA and the carbonyl groups of PVP, leading to increased resistance to molecular movement. , In CMCS-containing systems, the stronger viscosity enhancement was likely associated with multiple interactions involving hydroxyl (−OH), amino (−NH₂), and carboxylate (−COO^–) groups interacting with alginate chains. , These interactions can increase polymer association and restrict chain mobility, resulting in a more viscous shell phase.

It should also be noted that increasing additive concentration was accompanied by proportional reduction in SA content to maintain constant subsequent microcapsule properties, likely resulting from the combined effects of additive incorporation and changes in the alginate-to-additive ratio. The increase in apparent viscosity was also consistent with the particle size trends discussed in Section , where CMCS-containing formulations produced larger microcapsules than PVP-containing formulations. This relationship suggests that additive-induced changes in shell formulation viscosity influenced droplet breakup and microcapsule formation during the coaxial electrospray.

FT-IR spectroscopy was used to evaluate the chemical structures and possible intermolecular interactions within the fabricated microcapsules (Figure ). Bakuchiol exhibited characteristic absorption bands at 3359 cm^–1, corresponding to O–H stretching, 2966 cm^–1, corresponding to aliphatic C–H stretching, 1632 cm^–1, corresponding to CC stretching. SA showed characteristic bands at 3446 cm^–1 for O–H stretching, 1615 cm^–1 for asymmetric carboxylate (COO^–) stretching, and 1032 cm^–1 for C–O stretching. , Coconut oil displayed absorption bands at 2854 cm^–1, 1747 cm^–1, and 1160 cm^–1, corresponding to aliphatic C–H, ester CO, and C–O stretching vibrations, respectively.

7.

FT-IR spectra of bakuchiol, sodium alginate, coconut oil (A), and bakuchiol-loaded alginate microcapsules fabricated using different shell formulations containing PVP (B) and CMCS (C).

The FT-IR spectra of the fabricated microcapsules retained characteristic absorption bands associated with bakuchiol, coconut oil, and alginate, supporting the successful incorporation of the bakuchiol-containing oil core within the alginate-based shell matrix. In addition, shifts and broadening of the O–H stretching region were observed after the incorporation of PVP and CMCS, indicating the presence of intermolecular interactions within the polymer matrix. For PVP-containing formulations, the O–H stretching band shifted to lower wavenumbers, together with slight changes in the carbonyl stretching region, suggesting hydrogen bonding between SA and PVP. , For CMCS-containing formulations, more pronounced broadening and red shifts of the O–H stretching band were observed, , which may be attributed to stronger hydrogen bonding and electrostatic interactions between SA and CMCS. The proposed interactions between SA and the additives are illustrated in Figure .

8.

Schematic illustration of the proposed intermolecular interactions within alginate-based shell matrices: (top) hydrogen bonding between SA and PVP and (bottom) hydrogen bonding and electrostatic interactions between SA and CMCS.

The mechanical properties of the fabricated microcapsules further supported the role of additive-mediated polymer interactions (Figure B). Pure SA microcapsules exhibited the lowest hardness, indicating that the shell matrix was mainly stabilized by Ca²⁺-mediated ionic crosslinking. Incorporation of PVP gradually increased the microcapsule hardness, which was attributed to hydrogen bonding between SA and PVP that enhanced polymer network compactness and resistance to compressive deformation. CMCS-containing formulations showed substantially higher hardness values than both pure SA- and PVP-containing formulations, with SA/CMCS_0.75 exhibiting the highest hardness. The enhancement was likely due to the combined contribution of hydrogen bonding, electrostatic interactions, and Ca²⁺-mediated ionic cross-linking, which together produced a more rigid shell matrix.

Overall, the apparent viscosity, FT-IR, and mechanical analyses collectively suggested that additive incorporation modified the alginate shell matrix through intermolecular interactions. PVP moderately increased viscosity and hardness while maintaining a relatively uniform microcapsule morphology, whereas CMCS produced stronger viscosity enhancement, larger microcapsules, and higher mechanical rigidity. These structural differences are expected to influence swelling behavior and bakuchiol retention stability, as discussed in the following section.

3.5. Storage Stability Study

3.5.1. Stability Study at 4 °C and Room Temperature

The storage stability of bakuchiol-loaded alginate microcapsules was evaluated by monitoring changes in particle diameter and bakuchiol retention during storage in ultrapure water under different temperatures. As shown in Figure , the average microcapsule diameter increased gradually during storage at both 4 °C and room temperature, indicating progressive swelling of the alginate-based shell matrix. The swelling behavior was comparable at both storage temperatures, suggesting that the relatively small temperature difference had only a limited influence on water uptake. Instead, the extent of swelling appeared to be governed primarily by the polymer composition and crosslinked network structure of the microcapsules.

9.

Percentage change in diameter of bakuchiol-loaded alginate microcapsules containing different concentrations of PVP and CMCS during storage for 2, 4, and 8 weeks under (A) 4 °C and (B) room-temperature (RT) conditions. Data are presented as mean ± SD (n = 3). Different lowercase letters indicate significant differences (p < 0.05), while groups sharing the same letter are not significantly different (one-way ANOVA with Tukey’s posthoc test).

The swelling behavior strongly depended on the additive type and concentration. CMCS-containing formulations exhibited greater diameter changes than the PVP-containing formulation, particularly at higher CMCS contents. This behavior was attributed to the hydrophilic hydroxyl, amino, and carboxylate groups of CMCS, which can enhance water uptake and promote expansion of the shell matrix. In addition, the increased CMCS fraction reduced the relative alginate content available for Ca²⁺-mediated ionic cross-linking, which may have contributed to a less compact network and swelling tendency. In contrast, PVP-containing formulations, especially SA/PVP_0.25 and SA/PVP_0.50, showed comparatively smaller diameter changes (<6%), indicating improved bonding between SA and PVP, which reinforced the polymer network and restricted excessive water penetration.

The encapsulation efficiency (EE) of the optimized SA/PVP0.25 formulation was determined to evaluate the initial encapsulation performance. The formulation exhibited an EE of 91.0 ± 1.5%, indicating efficient incorporation of bakuchiol within the alginate microcapsules. This high initial encapsulation efficiency provided a basis for the subsequent evaluation of bakuchiol retention during storage.

Bakuchiol retention during storage at 4 °C, room temperature in the dark, and room temperature under light exposure is shown in Figure . Retention decreased gradually with storage time for all formulations, which may be associated with swelling-induced leakage of bakuchiol from the microcapsules and possible degradation of the encapsulated compound during storage. , Although the increased swelling of the polymer matrix is expected to facilitate diffusion of bakuchiol from the microcapsules, bakuchiol is also known to be susceptible to oxidative degradation during storage. Therefore, the observed reduction in bakuchiol retention is likely the combined result of physical leakage and chemical degradation. Since the analytical method employed in this study quantified the remaining bakuchiol without distinguishing between these two mechanisms, their individual contributions could not be determined. Storage at 4 °C generally provided better bakuchiol retention than under room-temperature conditions. Although swelling behavior was comparable between the two temperatures, lower storage temperature may have reduced the rate of bakuchiol degradation, thereby contributing to improved retention. Additive incorporation markedly influenced bakuchiol retention. PVP-containing formulations, particularly SA/PVP_0.25 and SA/PVP_0.50, maintained more than 90% bakuchiol retention after 12 weeks under the investigated storage conditions. This enhanced retention was attributed to the formation of a more compact alginate-PVP network, which reduced water penetration and limited diffusion of the encapsulated oil phase from the shell matrix. In contrast, CMCS-containing formulations showed lower retention, especially at higher CMCS concentrations. The reduced retention was consistent with their greater swelling behavior, which likely weakened shell integrity and facilitated bakuchiol leakage during storage. Although CMCS increased viscosity and mechanical rigidity, excessive CMCS incorporation did not improve long-term retention because the resulting shell matrix was more susceptible to hydration and swelling.

10.

Bakuchiol retention in alginate-based core–shell microcapsules containing different concentrations of PVP and CMCS during storage for 2, 4, 8, and 12 weeks under (A) 4 °C, (B) room temperature in the dark, and (C) room temperature under light exposure. Data are expressed as mean ± SD (n = 3). Different lowercase letters indicate significant differences (p < 0.05), while groups sharing the same letter are not significantly different (one-way ANOVA with Tukey’s posthoc test).

Overall, these results demonstrated that storage temperature and additive-mediated polymer interactions strongly affected the dimensional stability and bakuchiol retention of alginate-based core–shell microcapsules. Among the tested formulations, SA/PVP_0.25 and SA/PVP_0.50 provided the best balance between shell stability and retention performance, making them the most suitable formulations for the long-term stabilization of bakuchiol-loaded microcapsules.

3.5.2. Stability Study at Different pH Conditions

The pH stability of bakuchiol-loaded alginate microcapsules was evaluated by monitoring changes in the particle diameter and bakuchiol retention after storage in buffer solutions at pH 1, 3, 5, 7, and 9 for 4 weeks. As shown in Figure A, all formulations exhibited pH-dependent swelling behavior with greater diameter changes observed under neutral to alkaline conditions than under acidic conditions. This behavior is commonly associated with the ionization of carboxylic acid groups in alginate into negatively charged carboxylate groups, which increases electrostatic repulsion between polymer chains, promotes matrix relaxation, and enhances water uptake.

11.

Effect of pH conditions on the stability of bakuchiol-loaded alginate microcapsules after 4 weeks of storage: (A) percentage change in average particle diameter and (B) bakuchiol retention in buffer solutions at pH 1, 3, 5, 7, and 9. Data are presented as mean ± SD. Different lowercase letters indicate significant differences (p < 0.05), while groups sharing the same letter are not significantly different (one-way ANOVA with Tukey’s posthoc test).

Under acidic conditions, particularly at pH values of 1 and 3, the microcapsules showed relatively limited swelling. Protonation of carboxylate groups can reduce electrostatic repulsion and promote a more compact alginate network, thereby preserving the shell integrity. In contrast, alkaline conditions, especially at pH 9, induced pronounced swelling and structural deformation due to enhanced ionization of alginate carboxyl groups and pH-induced matrix relaxation. ,, The morphological images in Figure further confirmed that several formulations exhibited enlarged, distorted, or partially ruptured microcapsules after exposure to an alkaline medium, indicating reduced shell stability under these conditions.

12.

Morphological appearance of bakuchiol-loaded alginate microcapsules containing PVP and CMCS additives after storage in buffer solutions at different pH conditions for 4 weeks.

The extent of pH-induced swelling depended strongly on the incorporated additive. PVP-containing formulations, especially SA/PVP_0.25 and SA/PVP_0.50, showed comparatively lower diameter changes across the tested pH range. This result suggests that hydrogen bonding between SA and PVP improved shell compactness and restricted excessive water penetration. In contrast, CMCS-containing formulations exhibited greater swelling, particularly at higher CMCS contents. The pronounced swelling of SA/CMCS_0.75 was likely related to the hydrophilic nature of CMCS and the presence of amino and carboxylate groups, which can enhance water uptake and alter polymer–polymer interactions under pH variation.

Bakuchiol retention also decreased with increasing pH (Figure B). The lowest retention was observed under alkaline conditions, consistent with the greater swelling and structural deformation of the microcapsules. This reduction in retention was mainly attributed to pH-induced shell relaxation and increased diffusion pathways, which facilitated the leakage of the encapsulated bakuchiol-containing oil phase from the alginate matrix. Possible degradation of bakuchiol under alkaline or aqueous storage conditions may also contribute to the decrease in the measured retention; however, this study primarily evaluated total retained bakuchiol rather than degradation products.

Among the tested formulations, SA/PVP_0.25 and SA/PVP_0.50 provided the highest retention (>80%) across the pH range, maintaining more stable microcapsule structures than the other formulations. In contrast, CMCS-containing formulations showed reduced retention, particularly at pH 9, where excessive swelling and partial structural disruption were observed. These findings indicate that environmental pH strongly affects both the dimensional stability and the bakuchiol retention of alginate-based core–shell microcapsules. Overall, moderate PVP incorporation was more effective than CMCS incorporation in maintaining the shell integrity and retention stability under varying pH conditions.

4. Conclusions

Bakuchiol-loaded alginate-based microcapsules were successfully fabricated using the coaxial electrospray technique. The optimal fabrication conditions, including 1.0% (w/v) sodium alginate, a core-to-shell flow rate of 20:40 mL h^–1, 2.0% (w/v) CaCl₂, and an applied voltage of 1000 V, produced spherical microcapsules with uniform morphology. The proposed core–shell architecture was inferred from the coaxial electrospray configuration and was further supported by the observed physicochemical characteristics, although direct structural confirmation was beyond the scope of this study.

The incorporation of polymer additives markedly influenced the structural integrity and storage stability of microcapsules. FT-IR, apparent viscosity, and mechanical analyses demonstrated that additive-mediated intermolecular interactions played a crucial role in regulating the polymer network compactness, shell rigidity, and swelling behavior. CMCS incorporation increased the viscosity, particle size, and mechanical strength, likely because of stronger hydrogen bonding and electrostatic interactions with alginate. However, excessive CMCS promoted water uptake and swelling, resulting in reduced bakuchiol retention during storage. In contrast, PVP-containing formulations, particularly SA/PVP_0.25 and SA/PVP_0.50, exhibited improved dimensional stability and retention performance because hydrogen bonding between SA and PVP produced a more compact shell matrix.

Storage temperature and environmental pH strongly affected microcapsule stability. Storage at 4 °C and acidic conditions minimized swelling and improved bakuchiol retention, whereas alkaline conditions promoted matrix relaxation, shell deformation, and bakuchiol leakage. Among the investigated formulations, SA/PVP0.25 and SA/PVP0.50 showed the best overall stability, maintaining high bakuchiol retention during long-term storage. These findings demonstrate that additive-mediated modulation of the alginate polymer network is an effective strategy for controlling microcapsule performance. Notably, increased mechanical strength alone did not guarantee improved retention stability, as CMCS-containing formulations exhibited greater swelling and lower bakuchiol retention despite their higher hardness. The present study focused on the stability of alginate-based microcapsules under aqueous environments relevant to cosmetic storage and application. The behavior of these microcapsules in organic solvents was not investigated and remains an important subject for future research, as solvent polarity may influence polymer hydration, dimensional stability, and bakuchiol retention. The present work therefore establishes a structure–property–stability relationship among polymer additive selection, shell matrix organization, swelling behavior, and retention performance, providing useful design guidelines for the development of alginate-based encapsulation systems for oxidation-sensitive bioactive compounds.

A.S.: Data curation, Investigation, Methodology, Formal analysis, Visualization, and Writingoriginal draft. S.T., S.S., L.S., and I.S.: Validation and Writingreview and editing. N.T.: Conceptualization, Funding acquisition, Project administration, Supervision, and Writingreview and editing.

The authors declare no competing financial interest.