Fabrication and evaluation of polymeric microneedles containing a chitosan-coated curcumin nanoemulsion: structural characterization and transdermal performance

Abstract

Curcumin is a natural compound with promising anticancer potential, but its clinical application is limited by its poor aqueous solubility and low skin permeability. To address these challenges, this study investigates the transdermal delivery of curcumin using dissolving microneedles (DMNs) fabricated using three different polymers: poly(vinyl alcohol) (PVA), hyaluronic acid (HA), and carboxymethyl cellulose (CMC). A chitosan-coated curcumin nanoemulsion (NE-Cur-CS) was prepared and characterized for its physicochemical properties, including particle size (26.37 ± 0.42 nm) and zeta potential (−21.90 ± 0.98 mV). NE-Cur-CS was incorporated into 10 × 10 pyramidal microneedle arrays, fabricated using the micromolding method. Structural and mechanical characterization showed that the PVA- and HA-based microneedles (639 × 238 µm and 566 × 299 µm, respectively) maintained their integrity under a 32 N compression force and successfully penetrated the stratum corneum, as confirmed by H&E staining. Permeation studies confirmed their ability to penetrate the stratum corneum layer. Permeation studies at physiological pH (7.4) showed curcumin release rates of 59.74% for HA-based microneedles and 17.71% for those made with PVA after 7 hours. These results indicate that the type of polymer matrix plays a crucial role in controlling the transdermal delivery efficiency of the curcumin nanoemulsion.

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Introduction

Curcumin is a natural polyphenol compound extracted from the turmeric rhizome (Curcuma longa L.). It has exhibited several pharmacological activities, including anti-inflammatory, antioxidant, anticancer, antiviral, and antibacterial effects. Among these, its anticancer activity has shown the most promising results. Notably, the US National Cancer Institute has recognized curcumin as a chemopreventive agent and classified it as a third-generation chemotherapeutic.^1–5 However, despite its potential therapeutic effects, curcumin suffers from poor water solubility, rapid metabolism, and low oral absorption, which significantly limit its clinical application.^2,3,6,7 To overcome these challenges, an advanced drug delivery system is required to enhance curcumin's solubility, stability, and targeted release.

Transdermal drug delivery systems (TTDS) provide a non-invasive route of administration that circumvents the gastrointestinal tract and hepatic first-pass metabolism, thereby enhancing drug bioavailability and minimizing systemic side effects. This approach also improves patient compliance, particularly in long-term therapies, by enabling sustained and controlled drug release.^8,9 However, the primary limitation of conventional TDDS lies in the low permeability of the skin, predominantly due to the barrier function of the stratum corneum, which restricts the diffusion of most therapeutic agents, especially those with high molecular weight or hydrophilic properties.⁹ To address these challenges, advanced transdermal strategies such as microneedles and nanocarrier-assisted systems have been developed to facilitate drug transport across the skin and expand the range of deliverable compounds.¹⁰ These innovations offer promising avenues to enhance the efficacy and applicability of transdermal therapies.

Microneedles (MNs) are tiny structures, typically less than 1000 µm in length, with conical, pyramidal, or multifaceted shapes, offering significant advantages for transdermal drug delivery. When applied, MNs create temporary microchannels in the stratum corneum, allowing for the delivery of therapeutic compounds that would otherwise be unable to penetrate the skin barrier.¹¹ The short length of MNs allows them to penetrate the stratum corneum without reaching deeper nerve endings, making their application painless. This minimally invasive approach improves patient compliance, particularly among individuals with trypanophobia.¹²

Among the various types of microneedles, dissolving microneedles (DMNs) have gained significant attention in both academic research and pharmaceutical applications due to their biodegradable nature, biocompatibility, cost-effectiveness, and ease of fabrication.^13–15 DMNs are fabricated from water-soluble polymers, which dissolve upon application, releasing their encapsulated drug cargo into the skin.¹⁶ DMNs have been widely explored for the transdermal delivery of poorly permeable drugs, including antimicrobial and anticancer agents. A recent study demonstrated the successful development of Amphotericin B-loaded DMN patches for the treatment of cutaneous leishmaniasis (CL) using polyvinylpyrrolidone (PVP) and carboxymethyl cellulose (CMC) as biodegradable polymer matrices. The MNs exhibited strong mechanical properties, effective penetration into the rat skin, and rapid dissolution for drug release, resulting in potent in vitro Leishmanicidal activity (up to 86% parasite death).¹⁷ This study highlights the potential of DMNs in delivering therapeutic agents transdermally while maintaining high efficacy and minimal toxicity.

Various biocompatible polymers, such as hyaluronic acid (HA),¹⁸ polyvinyl alcohol (PVA),^6,19 and carboxymethyl cellulose (CMC),²⁰ have been widely employed in DMN formulations for curcumin delivery due to their favourable physicochemical and biomedical properties. These polymers represent diverse structural and functional characteristics that influence the performance of microneedles in terms of mechanical strength, dissolution behaviour, and drug delivery efficiency. PVA is a synthetic polymer with high crystallinity and mechanical robustness, suitable for fabricating microneedles that maintain their integrity and offer sustained release.²¹ HA, a naturally occurring polysaccharide, dissolves rapidly and exhibits excellent skin compatibility, promoting efficient drug release upon insertion. In contrast, CMC provides high water absorption and film-forming capability.²² This diversity offers a rational basis for systematically investigating how different polymer matrices affect the physical characteristics and transdermal performance of microneedle systems.

However, native curcumin exhibits extremely low aqueous solubility and poor miscibility with hydrophilic polymers such as PVA and HA, which hinder its direct incorporation into microneedle formulations. These limitations often result in phase separation and non-uniform drug distribution, ultimately affecting the structural integrity and delivery performance of DMNs. To overcome these challenges, curcumin was formulated as a chitosan-coated nanoemulsion, improving its solubility, stability, and compatibility with the polymer matrices. This approach was expected to facilitate uniform dispersion of curcumin within the polymer network, thereby enhancing the reproducibility and efficiency of transdermal delivery.

The combination of microneedles with nanoemulsion formulations enables a dual-functional system, where the nanoemulsion enhances drug solubility and skin penetration. In contrast, the microneedle array facilitates efficient transdermal delivery by physically disrupting the stratum corneum. Nanoemulsions (NE) have emerged as a promising lipid-based drug delivery system due to their high solubilization capacity, thermodynamic stability, ease of large-scale production, and ability to improve the bioavailability of hydrophobic drugs.^23,24 Encapsulating curcumin in a nanoemulsion (Cur-NE) significantly enhances its aqueous solubility and physicochemical stability, thereby facilitating more efficient transdermal penetration and enabling controlled drug release.²⁵ To further improve the structural and functional performance of Cur-NE within the microneedle system, chitosan (CS) has been employed as a coating agent. Due to its positive surface charge, chitosan forms a protective electrostatic barrier around the nanoemulsion droplets, preventing coalescence and aggregation during the solvent evaporation stage of microneedle fabrication. This stabilising effect ensures a uniform dispersion of the active compound within the polymer matrix, supporting the formation of well-defined and intact microneedle arrays. In addition to its stabilizing properties, chitosan exhibits excellent film-forming ability and mechanical reinforcement, resulting in improved structural integrity and water uptake capacity, both of which are critical for microneedle formation.^26,27 Moreover, chitosan has demonstrated the capacity to enhance skin permeation by reversibly interacting with epidermal lipids and proteins, reinforcing its dual function as both a formulation stabiliser and a transdermal permeation enhancer.²⁸

Although previous studies have demonstrated the potential of PVA-based microneedles loaded with curcumin nanosuspension (Cur-NS) in enhancing transdermal delivery due to their excellent mechanical strength and rapid dissolution,⁶ challenges such as poor solubility, low bioavailability, and limited skin permeability of curcumin remain major obstacles. The integration of Cur-NE-CS into microneedle systems remains largely unexplored. Previous research has generally focused on either improving curcumin's physicochemical properties through nanoformulation or enhancing skin penetration using microneedles as physical enhancers, but not on combining both strategies into a unified and synergistic delivery system. Studies have shown that lipid-based nanocarriers, including nanoemulsions, can enhance the solubility and stability of hydrophobic drugs, while microneedle arrays facilitate direct transdermal transport by breaching the stratum corneum barrier.²⁹ While both chitosan-coated nanoemulsions and polymeric microneedles have been individually investigated for transdermal drug delivery, most existing research has focused either on improving curcumin's physicochemical properties via nanoformulation or enhancing skin penetration using microneedles, without combining these strategies into a unified platform. Furthermore, limited comparative data exist on how different polymeric matrices, such as PVA, HA, and CMC, influence the morphology, mechanical behaviour, and drug delivery performance of microneedles loaded with Cur-NE-CS. Addressing these gaps is essential for guiding the rational design of microneedle-based transdermal systems with optimized performance.

Therefore, this study aims to develop and characterize dissolving microneedles fabricated using three distinct polymers—PVA, HA, and CMC—each loaded with Cur-NE-CS. The objective is to systematically evaluate how the choice of polymer affects the microneedles’ structural integrity, mechanical strength, and transdermal delivery efficiency of curcumin. This work offers a novel approach by integrating chitosan-coated nanoemulsions into dissolving microneedles and providing a comparative analysis across multiple polymer platforms. The findings are expected to offer new insights into the structure–function relationships of microneedle matrices, thereby contributing to the advancement of transdermal drug delivery technologies.

Results and discussion

Droplet size, polydispersity index, and zeta potential of Cur-SNEDDS-CS

Particle size measurements of the Cur-loaded SNEDDS-CS were conducted using DLS analysis (Table 1). The average particle size of the nanoemulsions was determined to be 26.37 ± 0.42 nm, and the polydispersity index (PDI) stands at 0.501 ± 0.005, suggesting that the nanoemulsions are almost monodisperse. Chitosan is renowned for its ability to interact with the components of SNEDDS, including oil, surfactant, co-surfactant, and curcumin, through electrostatic interactions and hydrogen bonding.

Table 1 Particle size, polydispersity index, and zeta potential of Cur-NE-CS

Particle size (nm)	PDI	Zeta potential (mV)
26.37 ± 0.42	0.501 ± 0.005	−21.90 ± 0.98

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These interactions can help form a stable and compact structure within SNEDDS, typically resulting in particles with a nanometer size. Additionally, chitosan is known to act as a stabilizing agent in nanoemulsions, further contributing to droplet size reduction.^30,31

The nanoscale droplet size (∼26 nm) suggests high stability and a large interfacial area, which enhances drug solubilization and dispersion within the microneedle matrix. Although nanoparticles below 100 nm may potentially undergo lymphatic uptake, this possibility is minimal in the present system due to localized release within the viable skin layers via dissolving microneedles, which favor capillary absorption and systemic delivery rather than lymphatic sequestration.³²

The polydispersity index (PDI) is a critical indicator of physical stability, representing the breadth of the particle size distribution. Systems that exhibit good distribution homogeneity typically have PDI values less than 0.5.³³ The PDI value of Cur-SNEDDS-CS, at 0.501 ± 0.005, indicates that the addition of chitosan maintains a homogeneous system. Similarly, the zeta potential of Cur-SNEDDS-CS, at −21.90 ± 0.98 mV, can be attributed to the electrostatic interaction between the amine group on chitosan and the hydroxyl group on the PEG co-surfactant.

Scanning electron microscopy of DMNs

The successful formation of DMN arrays composed of HA, CMC, and PVA incorporating Cur-NE was confirmed, as illustrated in Fig. 1. Digital microscope images revealed that the microneedle tips exhibited a well-defined, two-layered pyramidal structure, indicating the proper fabrication of the polymer-based MNS. The morphology and dimensions of the fabricated microneedles were analyzed using SEM, as shown in Fig. 2. The SEM images revealed that the PVA-based microneedles exhibited an average height of 639 µm and a width of 238 µm; the hyaluronic acid-based microneedles measured 528 µm in height and 401 µm in width. The carboxymethyl cellulose-based microneedles had a height of 566 µm and a width of 299 µm. These results confirm the successful fabrication of well-defined microneedles, with dimensions falling within the expected micrometer range for effective transdermal application.

Fig. 1 Digital microscopy images of Cur-NE-CS integrated DMNs of HA (a), CMC (b), and PVA (c).

Fig. 2 SEM images of DMNs prepared using PVA, HA, and CMC polymers containing Cur-NE-CS: overall morphology of the microneedle arrays (a) and cross-sectional images showing the internal structure (b).

These differences in needle height are attributed to the intrinsic physicochemical properties of each polymer, particularly the viscosity, drying rate, and mechanical strength during the molding process. Polymers with higher viscosity or slower drying kinetics, such as hyaluronic acid, may not completely fill the micromold cavities or may experience more shrinkage upon drying, leading to shorter needle structures. Meanwhile, polymers like PVA, known for better film-forming and mechanical properties, tend to maintain structural integrity and form taller microneedles under the same fabrication conditions. These variations have also been previously reported in microneedle fabrication using different hydrogel-forming polymers, where the molecular weight, crosslinking ability, and swelling capacity greatly influenced the microneedle geometry and mechanical behaviour.^34,35

FTIR analysis

Fourier transform infrared (FTIR) spectroscopy assessed the molecular interactions among PVA, HA, CMC, and curcumin nanoemulsion (Cur-NE) in DMNs. The notable FTIR spectral alterations reflect intermolecular interactions. In the PVA-based microneedles, a distinct –OH stretching peak was observed at 2917 cm⁻¹, while absorption bands at 1647 cm⁻¹ and 1419 cm⁻¹ corresponded to C=O and C–O stretching vibrations, respectively.³⁶ Following the addition of curcumin, a significant shift in the hydroxyl peak to 2945 cm⁻¹ and modifications in the C=O stretching region (1651 cm⁻¹) suggested potential hydrogen bonding between PVA and curcumin, resulting in improved structural stability (Fig. 3a).

Fig. 3 FTIR spectrum of blank DMNs and Cur-NE-loaded DMNs: PVA (a), HA (b) and CMC (c).

In the HA-based microneedles, the blank formulation exhibited a broad –OH stretching band at 3302 cm⁻¹, alongside characteristic amide and carboxylate absorption peaks at 1604 cm⁻¹, 1407 cm⁻¹, and 1038 cm⁻¹.³⁷ Upon curcumin loading, a shift in the –OH peak to 3286 cm⁻¹ and a new peak at 1230 cm⁻¹ suggest molecular interactions between HA and curcumin, potentially hydrogen bonding or electrostatic interactions. A shift in the C=O stretching region to 1651 cm⁻¹ further supports these interactions (Fig. 3b). These changes indicate that curcumin was successfully incorporated into the microneedle matrix and suggest modification in the polymeric network that may influence drug release behaviour.

In the blank CMC DMNs, a broad absorption band at 3290 cm⁻¹ was observed, corresponding to the hydroxyl (–OH) stretching vibration, which is characteristic of CMC's hydrophilic nature. Additionally, peaks at 1588 cm⁻¹ and 1410 cm⁻¹ were attributed to the asymmetric and symmetric stretching of carboxylate (–COO–) groups, respectively, while the band at 1022 cm⁻¹ was assigned to the C–O–C stretching of the polysaccharide backbone (Fig. 3c). Upon curcumin loading, a shift in the –OH peak and reduction in intensity were observed, suggesting hydrogen bonding between curcumin and CMC, which could influence the microneedle profile.

Mechanical properties and X-ray diffraction of DMNs

The resulting microneedles must possess strong mechanical properties, as they should not break or become damaged when applied to the skin. If the microneedles break or are compromised, the active substance will not penetrate the stratum corneum and therefore will not reach the bloodstream. The mechanical properties of the microneedles made from PVA, hyaluronic acid (HA), and carboxymethyl cellulose (CMC) polymers are displayed in Fig. 4a and b. Before being subjected to a force of 32 N, the microneedles made from the three polymers exhibited a regular and rigid structure with sharp tips, except for those made using CMC, which had less sharp tips than those of the PVA and HA microneedles. After applying the force, the microneedles made using PVA and HA maintained their shape, with a reduction in height of 12.3% for PVA and 4.7% for HA, respectively.

Fig. 4 SEM images of the DMNs before and after applying a force of 32N (a), the height of the DMNs before and after using a force of 32N (n = 3) (b), and the XRD spectrum (c). Asterisks (*) denote statistically significant differences (p < 0.05), whereas “n.s” indicates the absence of significant differences (p > 0.05), as determined by a paired sample t-test (n = 3).

Microneedles fabricated using CMC exhibited significantly weaker mechanical properties than those made using PVA and HA, as evidenced by substantial deformation when subjected to a 32 N compression force. Among the tested polymers, CMC-based microneedles experienced the most significant height reduction (28.7%), indicating lower structural integrity.

This mechanical weakness can be attributed to carboxymethyl functional groups, which impart high hydrophilicity to the polymer, making it prone to rapid moisture absorption and softening upon exposure to water. Furthermore, CMC possesses the lowest degree of crystallinity among the three polymers, resulting in a more amorphous structure that lacks the strong intermolecular interactions necessary to maintain mechanical rigidity.^38,39 In contrast, PVA and HA exhibit higher crystallinity and stronger hydrogen bonding, which enhance their structural stability and resistance to deformation.

This is consistent with the XRD results shown in Fig. 4c, where PVA (blue) exhibits sharp peaks at 2θ = 19°–22°, indicating its semi-crystalline nature and high crystallinity. This contributes to superior mechanical properties such as hardness and stiffness. In contrast, HA (red) and CMC (black) show broad, diffuse patterns, characteristic of amorphous structure with lower crystallinity. These materials are more flexible but mechanically weaker, with CMC exhibiting the lowest crystallinity and mechanical strength among the three.

Hygroscopicity of DMNs

The hygroscopic properties of the MNs were assessed by subjecting them to high-humidity conditions. The water absorption of PVA, HA, and CMC DMNs was approximately 5.3%, 5.5%, and 20.1%, respectively (Fig. 5). This indicates that CMC exhibits high hygroscopicity, which is attributed to its abundant carboxymethyl (–CH₂–COO–) functional groups, allowing for strong interactions with water molecules through hydrogen bonding and electrostatic attraction. This finding is consistent with the mechanical test results, which indicate that CMC has the lowest mechanical strength among the three polymers used.

Fig. 5 Hygroscopic analysis of DMNs under 80% RH conditions, showing the weight of the DMNs (a) and the water absorption rate of the DMNs (b) over time. Asterisks (*) indicate statistically significant differences (p < 0.05), while “n.s” denotes no significant difference (p > 0.05) based on a paired sample t-test (n = 3).

Ex vivo DMNs’ dissolution study

Solubility is a critical factor in the design of MNs, as it directly affects drug release, skin penetration efficiency, and therapeutic applications. In this study, the solubility of MNs fabricated with PVA, HA, and CMC was analysed, and the results are presented in Fig. 6. Solubility tests revealed notable differences in dissolution behaviour among the polymers, which were closely related to their structural characteristics. After 120 seconds, PVA remained partially undissolved, whereas HA had completely dissolved by 120 seconds, and CMC achieved complete dissolution within 90 seconds. The extended dissolution time of PVA can be attributed to its semi-crystalline structure, which provides high resistance to water penetration.⁴⁰ In contrast, MNs made with HA dissolved faster than those with PVA but slower than those with CMC. HA's amorphous structure and strong hydrophilicity facilitate the formation of a gel matrix upon contact with liquid, enabling quicker dissolution compared to PVA.^18,41,42

Fig. 6 The solubility of PVA, HA, and CMC DMNs.

Meanwhile, the rapid dissolution of CMC underscores its utility for applications that require immediate drug release. These findings emphasize the significance of polymer selection in designing MNs with tailored dissolution profiles to address specific therapeutic needs.

Insertion properties of DMNs, H&E staining, and skin recovery

Skin sections from the DMN insertion site were histologically stained to evaluate the insertion depth using Haematoxylin and Eosin (H&E). As shown in Fig. 7, the DMNs formed cavities in the skin with an average depth of 364.33 ± 23.46 µm for PVA-based DMNs and 305.33 ± 14.19 µm for HA-based DMNs, indicating that they successfully penetrated the stratum corneum (SC) and reached the deeper skin layers. A study conducted by Zhao et al. reported an insertion depth of approximately 218 µm for microneedles with a total height of 907 µm, further supporting the observation that actual penetration tends to be lower than the designed microneedle length due to skin deformation and mechanical resistance.⁴³

Fig. 7 Surface of rat skin before and after DMN insertion (a), images of skin recovery after peeling DMNs (b), and normal skin and skin treated with PVA and HA DMNs (c).

These results suggest that the MNs used in this study, with a height of 639 µm (PVA) and 528 µm (HA), effectively penetrated the skin without structural failure, ensuring the successful delivery of Cur into the dermal layer. H&E staining was not performed for CMC-based DMNs due to their weak mechanical properties, which could hinder effective skin penetration.

HPLC analysis

The chromatographic method was developed to analyse Cur under different pH conditions, with a retention time (RT) of 10.81 minutes at pH 7.4. Minor peaks corresponding to desmethoxycurcumin (RT of 10.010 minutes) and bisdemethoxycurcumin (RT of 9.279 minutes) were observed, though present in negligible amounts, consistent with previous studies.⁴⁴Fig. 8a and b show the chromatograms and calibration curve of Cur. System suitability tests confirmed the method's reliability, demonstrating compliance under the defined conditions and ensuring accurate detection and quantification of Cur. The prepared calibration curve demonstrated linearity within the 0.5–8 mg L⁻¹ range, with a regression coefficient (R²) of 0.9995, as summarized in Table 2.

Fig. 8 Chromatogram indicating the peaks (a) and the calibration curve of curcumin (b).

Table 2 HPLC system suitability of Cur

Retention time (min)	Area (cm^2)	Tailing factor	Theoretical plates	Height equivalent to theoretical plates	LOD (µg mL^−1)	LOQ (µg mL^−1)
10.81	2413.57	1.2	39 612.84	0.006	0.24	0.72

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Drug content of DMNs

Drug content is the amount of active pharmaceutical ingredient (API) encapsulated or incorporated within a delivery system. Accurate determination of drug loading content is crucial, as it directly influences the therapeutic efficacy, bioavailability, and consistency of formulation. The obtained drug content was 24.93 ± 0.28, 23.92 ± 1.90, and 24.96 ± 0.29 µg for PVA, HA, and CMC DMNs, respectively.

Ex vivo transdermal permeation

The permeation test results for DMNs are presented in Fig. 9. After 7 hours, HA-based DMNs demonstrated the highest cumulative curcumin permeation, reaching 14.44 µg (59.74%). In contrast, PVA-based DMNs exhibited markedly lower permeation, with 3.80 µg (17.71%) of the total. This disparity can be attributed to the intrinsic properties of the polymers, including molecular weight, crystallinity, and hydration dynamics. HA, a low-molecular-weight, amorphous polysaccharide, rapidly absorbs water and swells upon contact with the buffer, forming a hydrated gel matrix that facilitates faster curcumin diffusion through widened aqueous channels. In contrast, PVA, with semi-crystalline domains and a higher molecular weight, exhibits slower hydration and limited swelling, maintaining a denser network that restricts diffusion and results in a more controlled and sustained release profile.

Fig. 9 Cumulative amount of curcumin permeated for HA and PVA DMNs at pH 7.4.

These findings are consistent with previous reports that highlight the influence of polymer characteristics on drug permeation. Prabhu et al. (2022) noted that HA-based MNs promote rapid drug release due to their hydrophilicity and amorphous structure. In contrast, PVA matrices prolong release by resisting water ingress and maintaining structural rigidity.¹⁹

Similarly, Abdelghany et al. (2019) demonstrated that PVA-based MNs, while effective in enhancing curcumin delivery compared to topical application, release the drug more gradually due to their crystalline structure.⁶ Collectively, these results underscore that polymer selection is a critical determinant of permeation performance, with HA enabling rapid and extensive release of curcumin. At the same time, PVA supports prolonged, controlled delivery, which is suited for sustained transdermal applications.

The ex vivo permeation study of the present work demonstrated that HA-based Cur-NE-CS microneedles achieved a cumulative drug permeation of 59.74% within 7 hours, indicating a rapid and efficient transdermal delivery performance. In comparison, Prabhu et al. (2022) reported a 69.31 ± 0.67% drug release after 12 hours using curcumin-loaded solid lipid nanoparticle (SLN) microneedles.¹⁹ The markedly faster permeation observed in our HA-based system suggests that the combination of chitosan-coated curcumin nanoemulsions (Cur-NE-CS) with HA microneedles facilitates enhanced diffusion of curcumin through the microchannels formed in the skin.

This improvement can be attributed to the high hydrophilicity and swelling ability of HA, which promote faster microneedle dissolution and efficient drug release, as well as the nano-sized droplets (∼26 nm) of the Cur-NE-CS system that improve skin permeation. Collectively, these factors enable our formulation to achieve nearly comparable permeation in almost half the time reported by Prabhu et al., underscoring the superior efficiency of the Cur-NE-CS-loaded HA microneedle system for transdermal curcumin delivery.

Experimental

Materials

Curcumin was purchased from the Tokyo Chemical Industry. The chitosan used, with a molecular weight (M_w) of ∼100 000 g mol⁻¹, was purchased from AbMole Science. Sigma Aldrich, Germany, supplied cinnamon oil, the surfactant Cremophor RH 40, and the co-surfactant polyethylene glycol 200 (PEG 200). Poly(vinyl alcohol) (PVA), M_w ∼ 74 900 g mol⁻¹, carboxymethyl cellulose (CMC) sodium salt (M_w ∼ 81 900 g mol⁻¹), and hyaluronic acid (HA), M_w ∼ 83 070 g mol⁻¹, were purchased from Tokyo Chemical Industry.

Animals

All animal experiments in this study were conducted following approval from the Ethics Committee of Universitas Bakti Tunas Husada (Approval Code: 042-01/E.01/KEPK-BTH/III/2025). Three male Sprague-Dawley rats, aged 7–8 weeks and weighing approximately 200–250 g, were utilised. The rats were housed individually in separate enclosures under a 12-hour light/dark cycle, under controlled environmental conditions at 25 °C and 50–60% humidity. Food and water were supplied ad libitum.

Preparation of Cur-NE-CS

To prepare Cur-loaded NE (Cur-NE), approximately 250 mg of Cur was added to the preconcentrate of Cremophor RH 40 30%, PEG 200 30%, and cinnamon oil 40%. The Cur-NE formulation was then supplemented with 2 mg mL⁻¹ of chitosan and stirred for 30 minutes to obtain Cur-NE-CS. This formulation was selected based on the optimization study by Wulandari et al. (2025), which identified it as the most effective composition in terms of particle size, stability, and curcumin entrapment efficiency.⁴⁵

Preparation of dissolving microneedles

A volume of 100 µL of the previously prepared Cur-NE-CS was diluted with 1 mL of distilled water. From this diluted solution, 500 µL was mixed with 1000 mg of either PVA, HA, or CMC, each at a final polymer concentration of 10% (w/v), to obtain a homogeneous formulation. The mixtures were sonicated for 10 minutes to ensure uniform dispersion. The detailed composition of each DMN formulation is presented in Table 3.

Table 3 Composition of dissolving microneedles (DMNs) of Cur-NE-CS

Formulation Code	Polymer	Polymer concentration (%w/v)	Polymer amount (mg)	Volume of Cur-NE-CS solution (µL)
DMNs-P	PVA	10	1000	500
DMNs-H	HA	10	1000	500
DMNs-C	CMC	10	1000	500

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The resulting homogeneous solution was then poured into silicone microneedle molds (ST-08; Micropoint Technologies, Singapore), which were made of PDMS and featured a 10 × 10 pyramidal needle array (800 µm height, 200 µm base diameter, and 500 µm inter-needle spacing) on an 8 mm × 8 mm patch. The filled molds were subjected to vacuum degassing at 0.15 mmHg for 3 hours using a desiccator to eliminate trapped air and facilitate complete filling of the microneedle cavities by pressure differential. This method aligns with the fabrication approaches described by Arshad et al. (2019) and Hao Feng et al. (2020), which demonstrate effective mold filling without the use of external positive pressure.^46,47

After degassing, an additional layer of the same polymer solution was added to ensure a consistent backing layer. The molds were then left to dry at room temperature for 24 hours. The solidified microneedle patches were carefully demolded and stored in a desiccator until further characterization. The fabrication setup is illustrated in Fig. 10.

Fig. 10 Preparation of dissolving microneedles (DMNs) of Cur-NE-CS.

DMN characterization

Scanning electron microscopy. The morphology of the prepared microneedle arrays was analysed using a Hitachi SU3500 scanning electron microscope (SEM). The samples were mounted on a coverslip attached to a pin stub and sputter-coated with a thin layer of gold under vacuum to enhance surface conductivity. The coated samples were examined at an accelerating voltage of 5 kV, with appropriate magnifications to assess the overall shape, tip sharpness, and surface integrity of the microneedles.

FTIR analysis. The FTIR spectra were recorded using an Agilent Cary-630 spectrometer equipped with an Attenuated Total Reflectance (ATR) accessory featuring a ZnS crystal. Measurements were performed at ambient temperature. The FTIR spectra were collected for blank DMNs loaded with a curcumin nanoemulsion, utilizing Origin Lab software across the 4000–650 cm⁻¹ spectral range.

Mechanical properties of DMNs. The microneedle array was tested for its mechanical properties using a texture analyzer with a flat aluminium block as the rigid surface to stimulate the skin-like resistance. A 32 N force was applied for 30 seconds at a compression rate of 0.5 mm s⁻¹, with pre-test and post-test speeds set at 1 mm s⁻⁶. After the test, the morphology of the microneedles was observed using scanning electron microscopy (SEM) to evaluate any structural changes. The height of the microneedles before and after the test was measured using ImageJ software to quantify any deformation.

X-ray diffraction analysis. The X-ray diffraction analysis was conducted using a powder X-ray diffractometer (Rigaku, Tokyo, Japan) equipped with Cu-Kα radiation (λ = 1.54184 Å), operating at 45 kV and 200 mA. Measurements were performed over a 2θ range of 5° to 40°, at room temperature, at a scan rate of 2° per minute. Before analysis, the sample was finely pulverised and mounted on a thin polyester film (Mylar, USA).

Hygroscopicity of the DMNs. The DMNs’ hygroscopicity was evaluated at 80% relative humidity (RH) and 25 °C for 24 hours using a saturated ammonium sulphate solution. Their water absorption capacity was determined by measuring their weight at specific intervals.

Ex vivo DMN dissolution study. For the dissolution assessment of the DMN patches, a Sprague-Dawley rat was anesthetized via intraperitoneal injection of xylazine hydrochloride (5 mg kg⁻¹, Germany) and ketamine hydrochloride (45 mg kg⁻¹, Germany), followed by euthanasia. The rat's back was shaved using an electric shaver and further treated with a hair removal cream. The excised skin was then collected and preserved at −80 °C until further analysis. Before experimentation, the skin was rehydrated by immersing it in a normal saline solution. The DMN patches were then applied to the skin using gentle thumb pressure and secured with medical adhesive tape for predetermined time intervals. Following the exposure period, the patches were carefully removed and examined under an optical microscope to evaluate their dissolution behaviour.¹⁷

Insertion properties of DMNs, H&E staining, and skin recovery. To assess the skin penetration ability of DMNs, rat skin samples were pretreated as previously described. For penetration testing, Cur-NE-CS-loaded DMNs were manually inserted into excised rat skin by applying gentle thumb pressure for 3 minutes and subsequently removed. The treated skin samples were immediately fixed in 10% neutral buffered formalin for 24 hours, then dehydrated in a graded ethanol series, and finally embedded in paraffin. Thin sections of 7 µm thickness were obtained using a Dakewe MT1 Microtome. The sections were stained with hematoxylin and eosin (H&E) and examined under an Olympus Biological Microscope CX23 equipped with a calibrated scale bar. The vertical distance from the skin surface (stratum corneum) to the deepest visible microchannel was measured using ImageJ software to estimate the penetration depth of the DMNs.

For skin recovery assessment, DMNs were applied to the dorsal skin of anesthetized rats for 3 minutes. Afterward, the treated sites were monitored at predetermined time intervals (0, 2, 4, 8, and 24 hours) using the same optical microscope. The progression of skin closure and healing of the microchannels was visually documented until complete recovery was observed.

HPLC system. Cur was analysed using an HPLC system (Agilent 1100, Santa Clara, CA, USA) with detection at 426 nm. Separation was carried out on a Zorbax SB-C18 column (4.6 × 250 mm, 5 µm), using a mobile phase composed of acetonitrile, methanol, and 0.3% (v/v) phosphoric acid solution in a ratio of 40 : 20 : 40 (v : v : v). The flow rate was maintained at 1 mL min⁻¹.

Detection of drug loading in DMNs. To quantify the amount of Cur encapsulated in DMNs, the Cur-NE-CS-loaded microneedles were wholly dissolved in methanol and acidic buffer (pH 5.4), followed by vortexing to ensure complete drug release.

Ex vivo transdermal permeation. A Franz diffusion cell was employed to evaluate the transdermal permeation of curcumin. The donor compartment was fitted with previously hydrated and isolated rat abdominal skin, positioned between the donor and receptor chambers with the stratum corneum facing upward. Cur-NE-CS-loaded microneedle arrays were applied onto the skin surface and secured within the donor chamber. The receptor compartment was filled with phosphate buffer (pH 7.4) and maintained at 37 °C under continuous stirring at 250 rpm to simulate physiological conditions. The permeation study was conducted over a 7-hour period, during which 500 µL aliquots were withdrawn from the receptor medium at hourly intervals (i.e., at 1, 2, 3, 4, 5, 6, and 7 hours). After each sampling, the same volume of fresh buffer was added to maintain sink conditions. The collected samples were appropriately diluted and analysed using HPLC to determine the amount of curcumin diffused through the skin.¹⁹

Conclusions

This study demonstrated the successful development of dissolving microneedles (DMNs) incorporating a chitosan-coated curcumin nanoemulsion using three different polymers: PVA, HA and CMC, as matrix materials. The findings underscore the importance of polymer selection in determining microneedle performance for transdermal delivery applications. Overall, the use of biocompatible polymers in combination with nanoemulsion technology offers a promising platform for non-invasive, controlled drug administration. Future research should focus on optimizing polymer formulations to improve drug loading capacity, stability, and tunable release characteristics. In addition, in vivo evaluations and clinical translation studies are crucial for confirming the therapeutic efficacy and safety of these systems for broader biomedical applications.

Author contributions

Winda Trisna Wulandari: conceptualisation, investigation, visualisation, and writing – original draft. Mia Ledyastuti: conceptualisation, methodology, validation, writing – review and editing. Marselina Irasonia Tan: conceptualisation, methodology, validation, and writing – review and editing. I Made Arcana: conceptualisation, methodology, supervision, validation, and writing – review and editing.

Conflicts of interest

The authors declare that they have no conflicts of interest.

Data availability

The data supporting the findings of this study are not publicly available as they have not been deposited in a public repository. However, the data may be made available from the corresponding author upon reasonable request.

Acknowledgements

This work was supported by the Ministry of Education, Culture, Research, and Technology of Indonesia through a doctoral grant program and by the Research Grant of the Research, Community Service, and Innovation Program of Institut Teknologi Bandung (ITB).

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