Hyaluronic Acid-Based Dissolving Microneedles: An Innovative Transdermal Approach for Osteoarthritis Management

Abstract

Osteoarthritis (OA), a leading cause of disability worldwide, is characterized by the progressive degeneration of articular cartilage and inflammation of synovial tissues. Current therapies—including NSAIDs and intra-articular injections—are palliative and often associated with systemic side effects and invasiveness. Transdermal drug delivery systems (TDDS) present a non-invasive alternative, yet are hindered by the stratum corneum barrier. Microneedle (MN) technology has emerged as a minimally invasive strategy capable of bypassing this barrier for efficient drug delivery. This review explores the integration of hyaluronic acid (HA), a key component of synovial fluid, into dissolving MNs for localized OA treatment. HA provides intrinsic therapeutic benefits including joint lubrication, anti-inflammatory activity, and chondroprotection. Fabricated using polymers such as polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP), HA-based dissolving MNs offer dual functionality as both drug and delivery matrix. We detail the preformulation, fabrication, and evaluation of HA MNs, highlighting morphological, physicochemical, and mechanical assessments. The review underscores the therapeutic potential and translational promise of HA-MNs in OA management and discusses future research directions.

Keywords

Introduction

Osteoarthritis: Burden and Challenges

Osteoarthritis (OA) is a chronic degenerative disorder affecting synovial joints, marked by cartilage erosion, osteophyte formation, subchondral bone remodelling, and synovial inflammation. These pathological events culminate in joint stiffness, chronic pain, reduced mobility, and functional limitations. OA predominantly affects elderly populations and constitutes a substantial public health burden. Existing pharmacological treatments provide only symptomatic relief and are often associated with adverse effects, necessitating alternative approaches that target the disease more effectively [1-5].

1.2 Transdermal Drug Delivery Systems (TDDS)

TDDS offer advantages including avoidance of hepatic first-pass metabolism, sustained drug plasma concentrations, improved adherence, and reduced systemic toxicity. However, the stratum corneum remains a formidable barrier to the permeation of macromolecules and hydrophilic drugs. Conventional enhancers such as surfactants or iontophoresis present limitations, highlighting the need for novel transdermal strategies [6,7].

1.3 Microneedle Technology

Microneedles (MNs) provide a minimally invasive transdermal platform by puncturing the stratum corneum without reaching pain receptors or blood vessels. Classification includes solid, coated, hollow, dissolving, and hydrogel-forming MNs. Among them, dissolving MNs fabricated from biocompatible and biodegradable polymers present unique advantages in safety, precision, and patient compliance.

1.4 Hyaluronic Acid in OA Therapy

Hyaluronic acid (HA) is a high molecular weight polysaccharide naturally present in synovial fluid. It contributes to joint lubrication, mechanical shock absorption, and cellular signaling. In OA, HA concentration and molecular weight decline, impairing joint function. HA injections are FDA-approved for knee OA but are invasive and limited by local side effects. HA’s film-forming, hydrophilic, and viscoelastic properties make it an ideal candidate for dissolving MN matrix systems [8-13].

1.5 Research Gap and Novelty

Despite increasing research in MN-based delivery systems, limited studies explore the use of HA-loaded dissolving MNs for OA treatment. This novel approach merges HA’s therapeutic functions with a minimally invasive delivery method, potentially overcoming the drawbacks of systemic administration and intra-articular injections.

2. MATERIALS AND METHODS

2.1 MATERIALS

The formulation of dissolving microneedles commonly involves biocompatible polymers and safe solvents. Hyaluronic acid (HA), obtained from Yarrow Chem, is widely used for its biodegradability and skin compatibility. Polyvinyl alcohol (PVA) and polyvinylpyrrolidone K90 (PVP K90), both sourced from Sigma-Aldrich, contribute to mechanical strength and rapid dissolution, respectively. Fluorescein isothiocyanate (FITC) from Yarrow Chem serves as a fluorescent marker in permeability studies. Purified water and anhydrous ethanol are used as solvents and cleaning agents during the fabrication process.

2.2 Preformulation Studies

2.2.1 Solubility Studies

The solubility of HA, PVP, and PVA was determined qualitatively in both purified water and ethanol. An excess amount of each polymer was added to 10 mL of solvent in a beaker, stirred at room temperature for 24 hours, and filtered. Solubility was recorded as soluble, slightly soluble, or insoluble.

2.2.2 pH Measurement of Polymer Solutions

The pH of individual polymer solutions (1% w/v) was measured using a calibrated digital pH meter at 25?±?1?°C. Each solution was prepared freshly, and the average of three readings was recorded.

2.2.3 Viscosity Determination

The viscosity of polymeric gels and solutions was measured using a Brookfield viscometer (Model DV-E) at 25?±?1?°C, employing spindle No. 4 at 50 rpm. Each measurement was conducted in triplicate.

2.2.4 Compatibility Studies

FTIR spectroscopy was performed to assess possible chemical interactions between the polymers and FITC. Individual spectra of HA, PVA, PVP, and FITC, along with their physical mixtures, were recorded in the range of 4000–400 cm?¹ using KBr pellets. Visual assessment for color change, precipitation, or turbidity was also conducted.

2.3 Fabrication of Dissolving Microneedles

2.3.1 Micromold and Casting Procedure

Microneedles were fabricated using polydimethylsiloxane (PDMS) molds with 97 conical cavities (0.45 mm diameter, 1.2 mm height). A two-layer casting process was adopted:

• Tip Layer Formation: A viscous blend of HA and PVP containing FITC was poured into the mold cavities and centrifuged at 3000 rpm for 5 minutes to ensure cavity filling.
• Backing Layer Application: A 15% w/v PVA solution was poured over the tip layer, followed by centrifugation to form the base.

2.3.2 Drying and Demolding

The filled molds were oven-dried at 37–40?°C for 24 hours. After drying, microneedles were carefully demolded and stored in a desiccator until use.

2.4 Morphological and Physicochemical Characterization

2.4.1 Visual and Morphological Inspection

Microneedles were visually inspected for uniformity, transparency, surface smoothness, and the presence of air bubbles. Surface integrity and shape were also noted.

2.4.2 Scanning Electron Microscopy (SEM)

SEM imaging was performed to evaluate microneedle tip sharpness, surface topography, and structural precision. Dried samples were sputter-coated with gold and observed under SEM at varying magnifications.

2.4.3 Confocal Laser Scanning Microscopy (CLSM)

FITC-loaded microneedles were evaluated using CLSM to assess drug localization and distribution. Z-stack imaging was performed to determine depth-wise fluorescence penetration and uniformity.

2.4.4 Thickness Measurement

The overall thickness of microneedle patches was measured using a digital Vernier caliper at five random locations, and the mean ± SD was calculated.

2.4.5 Weight Uniformity

Individual microneedle patches were weighed using an analytical balance. Ten patches were selected randomly, and the mean weight and standard deviation were determined.

2.4.6 Folding Endurance

To evaluate mechanical integrity, a single microneedle patch was folded repeatedly at the same point until visible cracks or breakage appeared. The total number of folds before failure was recorded.

2.4.7 Moisture Content Determination

Moisture content was measured gravimetrically. Weighed samples were dried in a hot air oven at 60?°C until constant weight. The percentage of moisture content was calculated by:

Where:

• WiW_i = Initial weight
• WfW_f = Final weight after drying

2.4.8 Moisture Uptake Study

Samples were stored in a desiccator at 75% RH (using saturated sodium chloride solution) for 72 hours. Weight gain was noted, and the moisture uptake was calculated as:

2.4.9 Surface pH Measurement

Surface pH was measured by moistening the microneedle patch with 1 mL of distilled water and allowing it to equilibrate for 1 hour. A flat-surface pH electrode was gently placed on the patch surface to record the pH.

2.4.10 Swelling Index

The swelling index was assessed by immersing microneedle patches in phosphate buffer (pH 6.8) at 37?°C for 6 hours. Swollen patches were weighed after blotting excess surface water, and swelling index (%) was calculated as:

Where:

• WdW_d = Initial dry weight
• WsW_s = Swollen weight after 6 hours

3. RESULTS

This section presents a comprehensive evaluation of the morphological and physicochemical parameters of the formulated hyaluronic acid-based microneedle patches. Each test was performed in triplicate unless otherwise stated.

3.1 Morphological Assessment

3.1.1 Visual Inspection

All prepared microneedle arrays were visually inspected for defects such as broken tips, irregular shapes, or incomplete needle formation. The patches exhibited consistent transparency and flexibility, with smooth surfaces and uniformly arranged needles. No discoloration, phase separation, or particulate aggregation was observed, indicating proper formulation and compatibility of excipients.

3.1.2 Scanning Electron Microscopy (SEM)

SEM analysis revealed that the microneedles had sharply defined conical structures with an average height of approximately 1.2 mm and a base diameter of 0.45 mm. The tips appeared symmetric and free of structural deformities, which are critical for skin penetration efficiency. SEM images are shown in Figure 1.

3.1.3 Confocal Laser Scanning Microscopy (CLSM)

CLSM was used to assess the distribution of fluorescein isothiocyanate (FITC) within the microneedle matrix. Fluorescence imaging confirmed a homogeneous distribution of FITC throughout the tip region, indicating uniform drug loading. CLSM images are depicted in Figure 2.

3.2 Physicochemical Parameters

3.2.1 Thickness

The microneedle patches showed a mean thickness of 0.45 ± 0.01 mm, measured using a calibrated digital micrometer. The low variation indicates consistency in fabrication and drying across batches.

3.2.2 Weight Uniformity

Patches weighed on an analytical balance displayed weight variations within ±6.2% from the mean, satisfying pharmacopeial standards.

Table 1: Weight Uniformity of Microneedle Patches

3.2.3 Folding Endurance

The patches were folded repeatedly at the same spot until a visible crack or break occurred. All patches withstood more than 300 folding cycles, confirming excellent mechanical durability suitable for practical use and handling.

3.2.4 Moisture Content

Moisture content was measured gravimetrically. Samples were dried in a hot air oven at 60?°C until constant weight. The percentage of moisture content was calculated using the formula:

Where:

• WiWi = Initial weight
• WfW_f = Final weight after drying

The average moisture content was ~3.4%, indicating adequate drying and stability during storage. Minimal residual moisture reduces microbial risk while maintaining flexibility.

3.2.5 Moisture Uptake

Patches were stored in a 75% RH chamber for 24 hours. Moisture uptake was recorded and calculated as follows:

Table 2: Moisture Uptake Under 75% RH

The results suggest moderate hygroscopicity and suitability for transdermal application without rapid disintegration.

3.2.6 Surface pH

Surface pH was measured by placing moistened patches on pH paper and also using a flat-surface pH probe. Values ranged from 5.6 to 6.4, which aligns well with normal human skin pH, minimizing the risk of irritation.

3.2.7 Swelling Index

Swelling behavior was evaluated by immersing patches in phosphate buffer (pH 6.8) at 37°C. The percentage increase in weight indicated water uptake capacity and gel-forming behavior.

Table 3: Swelling Index of Microneedle Patches

These results collectively indicate that the developed microneedle patches possess desirable physicochemical and mechanical characteristics suitable for transdermal delivery applications.

4. CONCLUSION

The development of polymeric microneedle patches using hyaluronic acid (HA), polyvinylpyrrolidone (PVP), and polyvinyl alcohol (PVA) represents a promising strategy for transdermal drug delivery, offering several key advantages over traditional methods. The results of this study underscore the potential of microneedle systems as a minimally invasive, patient-friendly, and efficient platform for drug administration, particularly for macromolecular and hydrophilic compounds that typically exhibit poor skin permeability. Comprehensive preformulation studies confirmed the aqueous solubility and physicochemical compatibility of the selected polymers. The pH measurements indicated that the polymeric solutions are within a physiologically acceptable range for skin application, while viscosity analysis ensured optimal consistency and processability during casting. FTIR spectroscopy validated the absence of any significant chemical incompatibility between the polymers, a critical factor in maintaining microneedle integrity and drug stability. Fabrication through a two-step layer casting method successfully produced well-defined microneedle arrays. The combination of a drug-loaded tip layer composed of HA and PVP with a structurally supportive backing layer of PVA enabled the fabrication of sturdy, dissolvable microneedles. The optimization of drying conditions further contributed to their mechanical stability and usability. The use of PDMS molds allowed for consistent geometrical formation, critical for reproducible performance. Morphological assessment via SEM revealed conically shaped, uniform microneedles without deformities, while CLSM imaging confirmed homogeneous FITC distribution within the needle tips, demonstrating successful and uniform drug loading. The microneedle patches exhibited a mean thickness of 0.45 mm and maintained weight uniformity within acceptable pharmacopeial limits, highlighting the reproducibility of the fabrication technique. Mechanical property evaluations, such as folding endurance, showed that patches could withstand more than 300 repeated folds without cracking, indicating excellent flexibility and mechanical integrity. This parameter is especially vital for user comfort and ensuring the patches do not break during handling or application. Additionally, surface pH testing confirmed that the final patches-maintained skin-compatible pH levels between 5.6 and 6.4, minimizing the risk of irritation. The moisture content and uptake tests demonstrated the patches' stability and response to ambient humidity, crucial for packaging and storage considerations. The swelling index results illustrated that the patches could absorb moisture in a controlled manner, which plays a vital role in patch dissolution and drug release once applied to the skin. Overall, this study confirms that HA-PVP-PVA-based dissolvable microneedle patches can be successfully fabricated with reproducible physical characteristics, mechanical strength, and drug-loading capabilities. These findings strongly support further in vitro drug release and ex vivo skin permeation studies to confirm bioavailability, pharmacokinetics, and therapeutic efficacy. In conclusion, polymeric microneedle patches offer a forward-looking platform for painless, controlled, and effective transdermal delivery of active pharmaceutical ingredients. With appropriate optimization and scalability, such delivery systems hold promise for future clinical application, particularly in areas requiring self-administration, improved compliance, and targeted therapy.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the support and insights provided by colleagues and mentors in the field of pharmaceutics and pain management.

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