Advances in Herbal Nanoemulgels for Acne Treatment: Formulation, Evaluation, and Future Prospects

Abstract

Acne vulgaris is a multifactorial chronic inflammatory disorder of the pilosebaceous unit affecting a significant proportion of adolescents and adults worldwide. Conventional therapies, including retinoid, benzoyl peroxide, and systemic or topical antibiotics, are associated with adverse effects, skin irritation, relapse, and increasing antimicrobial resistance. Herbal bioactive possessing antimicrobial, anti-inflammatory, antioxidant, and sebum-regulating properties have emerged as promising alternatives. However, their therapeutic application is often limited by poor aqueous solubility, chemical instability, low dermal penetration, and rapid degradation. Nanoemulgels, hybrid delivery systems combining nanoemulsions with gel matrices, have gained considerable attention for enhancing solubility, stability, controlled release, and follicular targeting of herbal actives. This review critically discusses the pathogenesis of acne, the therapeutic role of plant-derived compounds, formulation strategies of herbal nanoemulgels, physicochemical and biological evaluation parameters, recent technological advances, regulatory challenges, and future translational prospects. Herbal nanoemulgels represent a promising bridge between phytotherapy and advanced nanotechnology-driven dermatological treatment.

Keywords

Introduction

Figure 1: Fabrication of Nanoemulgel for topical delivery of EO.

2. PATHOGENESIS OF ACNE VULGARIS

Successful therapeutic strategies require a deep understanding of acne pathophysiology, which is driven by interconnected mechanisms:

2.1 Follicular Hyperkeratinization

Hyperproliferation and impaired desquamation of follicular epithelial cells lead to microcomedo formation — the earliest lesion in acne. The accumulation of keratinous material obstructs sebum flow, creating an anaerobic niche favourable to microbial growth [2].

2.2 Sebum Overproduction

Sebaceous glands, under androgenic stimulation, produce excessive sebum rich in triglycerides and wax esters. Sebum itself acts as a nutrient source for C. acnes, further exacerbating inflammation [8].

2.3 Microbial Colonization and Biofilm Formation

Cutibacterium acnes, a commensal anaerobic bacterium, proliferates in the lipid-rich follicle. It releases lipases that hydrolyze sebum triglycerides into pro-inflammatory free fatty acids, triggering innate immune responses [9].

2.4 Inflammatory Mediators

Activation of pattern recognition receptors triggers release of pro-inflammatory cytokines (IL-1β, IL-8, TNF-α) and recruitment of immune cells, leading to pustule and nodule formation. Oxidative stress further perpetuates inflammation and tissue damage [10]

3. HERBAL BIOACTIVES IN ACNE MANAGEMENT

Herbal phytochemicals present multiple mechanisms that can intercept acne pathogenic pathways:

3.1 Antimicrobial Activity

Many plant extracts exhibit bacteriostatic and bactericidal activity against C. acnes and Staphylococcus epidermidis. For example:

• Tea tree oil — rich in terpinen-4-ol — disrupts bacterial cell membranes and inhibits growth [11].
• Neem (Azadirachta indica) extract inhibits acne pathogens through multiple phytochemicals including nimbidin and azadirachtin [12].

3.2 Anti-Inflammatory Mechanisms

Inflammation is central to acne progression. Herbal actives modulate inflammatory pathways:

• Curcumin (from Curcuma longa) inhibits NF-κB signaling and reduces pro-inflammatory cytokine release [13].
• Green tea polyphenols (EGCG) decrease inflammatory marker expression and sebum secretion [14].

3.3 Sebostatic and Antioxidant Effects

• Camellia sinensis and Rubia cordifolia contain polyphenolic compounds that reduce sebum output and neutralize free radicals, preventing lipid peroxidation in skin tissues [15].

3.4 Limitations of Herbal Phytochemicals

Despite promising preclinical results, obstacles include:

• Instability to oxidation and light
• Low partitioning into stratum corneum
• Rapid degradation and metabolic transformation upon topical application

These limitations underscore the need for advanced delivery systems like nanoemulgels to enhance therapeutic utility.

These phytoconstituents inhibit C. acnes, reduce inflammatory mediators, regulate sebum production, and promote skin repair. However, challenges such as volatility, oxidative degradation, low aqueous solubility, and limited dermal permeation restrict their clinical effectiveness.

4. NANOEMULGEL DRUG DELIVERY SYSTEM

Nanoemulgels combine the advantages of nanoemulsions and hydrophilic gel matrices to improve topical drug delivery.

4.1 Nanoemulsions: Core Benefits

Nanoemulsions are submicron (20–200 nm) oil-in-water (O/W) or water-in-oil (W/O) dispersions stabilized by surfactants and co-surfactants that:

• Solubilize lipophilic agents
• Increase surface area for absorption
• Facilitate enhanced permeation across the stratum corneum
• Avoid creaming and sedimentation due to nanometer droplet size [16].

4.2 Limitation of Nanoemulsions in Topical Use

While permeation is improved, nanoemulsions often suffer from:

• Low viscosity
• Short contact time on skin
• Poor retention at the application site

4.3 Nanoemulgel: Structure and Function

Incorporation of nanoemulsions into a gel matrix (e.g., Carbopol®, HPMC) increases:

• Viscosity and bioadhesive force
• Sustained release profiles
• Mechanical stability during application

The gel matrix enhances residence time, reduces run-off, and improves user acceptability [17].

5. FORMULATION STRATEGIES

5.1 Oil Phase Considerations

Selection of oil influences solubility, penetration, and therapeutic effect:

• Oleic acid enhances skin penetration via disruption of intercellular lipids [18].
• Essential oils like lavender and eucalyptus contribute intrinsic antimicrobial activity.

5.2 Surfactant and Co-surfactant Selection

Non-ionic surfactants (e.g., Tween 80, Span 80) are preferred due to:

• Reduced irritancy
• Better biocompatibility
• Improved thermodynamic stability

Co-surfactants (e.g., PEG 400, Transcutol P) increase flexibility of the interfacial film, enabling smaller droplet formation [19].

5.3 Preparation Methods

High-Energy Techniques

• Ultrasonication: Provides intense shear forces to reduce droplet size
• High-pressure homogenization: Yields uniform nanoemulsions

Low-Energy Techniques

• Phase inversion temperature (PIT): Utilizes temperature changes to induce nanoemulsion formation
• Spontaneous emulsification: Results from the diffusion of surfactants across phase boundaries

Each method has advantages and limitations in scalability, energy requirement, and droplet uniformity.

5.4 Gel Matrix Development

Hydrophilic polymers such as Carbopol and HPMC increase gel strength and control drug diffusion. pH adjustment near skin pH (4.5–6.5) minimizes irritation and enhances tolerance [20].

6. EVALUATION PARAMETERS OF HERBAL NANOEMULGELS

Comprehensive evaluation of herbal nanoemulgels is essential to ensure formulation stability, safety, efficacy, and reproducibility. Evaluation parameters are broadly categorized into physicochemical, stability, and biological performance studies.

6.1 Physicochemical Characterization

6.1.1 Droplet Size and Polydispersity Index (PDI)

Droplet size is a critical determinant of skin permeation, stability, and follicular targeting. Nanoemulgels intended for topical delivery typically exhibit droplet sizes in the range of 20–200 nm, which allows enhanced penetration through the stratum corneum and accumulation within hair follicles.

The polydispersity index (PDI) reflects size distribution uniformity. A PDI value below 0.3 indicates a homogeneous system with reduced risk of phase separation. Smaller droplets provide increased surface area, improving drug dissolution and release kinetics.

6.1.2 Zeta Potential

Zeta potential measures the surface charge of nanoemulsion droplets and predicts colloidal stability. High absolute zeta potential values (≥ ±30 mV) generate electrostatic repulsion between droplets, preventing aggregation and coalescence.

In herbal nanoemulgels, both electrostatic and steric stabilization mechanisms (from non-ionic surfactants) contribute to long-term stability.

6.1.3 pH Determination

Skin-compatible pH is essential to prevent irritation and maintain barrier integrity. Herbal nanoemulgels are typically adjusted to a pH range of 4.5–6.5, aligning with natural skin pH. Maintaining this range ensures user safety and improves formulation tolerability during prolonged application.

6.1.4 Viscosity and Rheological Behavior

Rheological analysis determines flow behavior and mechanical stability. Ideal nanoemulgels exhibit pseudoplastic (shear-thinning) behavior, where viscosity decreases upon shear stress during application and recovers afterward.

Thixotropic behavior enhances:

• Ease of application
• Uniform spreading
• Retention at the application site

Brookfield viscometers or rotational rheometers are commonly used for rheological analysis.

6.1.5 Spreadability

Spreadability is a measure of formulation ease of application and uniform drug distribution over the skin surface. Higher spreadability improves patient compliance and dosing consistency. Nanoemulgels with optimized polymer concentration demonstrate superior spreadability compared to conventional gels.

6.2 Stability Studies

Stability testing ensures formulation integrity during storage and transportation.

6.2.1 Physical Stability

Evaluated by observing:

• Phase separation
• Creaming
• Cracking
• Changes in droplet size

Accelerated stability studies are commonly conducted at 40°C ± 2°C / 75% RH ± 5%.

6.2.2 Freeze–Thaw Cycles

Repeated cycles between low (4°C) and high (40°C) temperatures assess resistance to temperature-induced stress. Stable nanoemulgels maintain droplet size, pH, and viscosity throughout these cycles.

6.2.3 Chemical Stability

Herbal actives are prone to oxidation and degradation. Stability studies evaluate:

• Drug content uniformity
• Changes in pH
• Loss of therapeutic activity

Nanoemulgels protect phytoconstituents by encapsulation within oil droplets, reducing exposure to environmental stressors.

6.3 Performance Evaluation

6.3.1 In vitro Drug Release Studies

In vitro release studies are commonly performed using Franz diffusion cell with synthetic or dialysis membranes. Release kinetics help predict drug availability at the skin surface and are often analysed using mathematical models such as Higuchi or Korsmeyer–Peppas equations.

Nanoemulgels typically show controlled and sustained release compared to conventional gels.

6.3.2 Ex vivo Skin Permeation Studies

Ex vivo permeation studies using excised animal or human skin evaluate drug penetration depth and flux. Nanoemulgels enhance dermal retention while minimizing systemic absorption — a critical requirement for acne therapy.

6.3.3 Antimicrobial Activity

Antimicrobial efficacy is assessed against acne-causing bacteria such as Cutibacterium acnes and Staphylococcus epidermidis using:

• Agar well diffusion
• Disc diffusion
• Minimum inhibitory concentration (MIC) assays

Nanoemulgels often demonstrate superior antibacterial activity due to enhanced penetration and sustained release of herbal actives.

7. RECENT ADVANCES IN HERBAL NANOEMULGELS FOR ACNE TREATMENT

Recent research has focused on improving therapeutic efficiency, safety, and patient compliance.

7.1 Follicular-Targeted Nanoemulgels

Nano-sized droplets preferentially accumulate in hair follicles, enabling targeted delivery to sebaceous glands — the primary site of acne pathology. This reduces off-target effects and enhances therapeutic efficacy.

7.2 Polyherbal Nanoemulgels

Combining multiple herbal extracts provides synergistic antimicrobial and anti-inflammatory effects while reducing the dose of individual actives, minimizing irritation.

7.3 Controlled and Sustained Release Systems

Advanced nanoemulgels are engineered to release actives gradually, maintaining therapeutic concentrations over extended periods and reducing dosing frequency.

7.4 Green and Sustainable Formulation Approaches

Eco-friendly surfactants, biodegradable polymers, and green emulsification techniques are being explored to minimize environmental impact and toxicity.

7.5 Clinical Translation Efforts

Emerging clinical studies indicate improved efficacy and reduced irritation compared to conventional topical formulations, though large-scale trials remain limited.

8. CHALLENGES AND LIMITATIONS

Despite encouraging outcomes, several challenges hinder widespread adoption:

• Variability in herbal raw materials
• Lack of phytochemical standardization
• Batch-to-batch inconsistency
• Scale-up difficulties
• Potential surfactant-induced irritation
• Limited long-term safety data
• Regulatory uncertainty for nano-phytopharmaceuticals

Addressing these issues is essential for clinical translation.

9. REGULATORY AND COMMERCIAL PERSPECTIVES

Regulatory classification of herbal nanoemulgels varies across regions:

• Cosmetic products
• Over-the-counter (OTC) drugs
• Phytopharmaceuticals
• Nanomedicines

Challenges include:

• Absence of unified global guidelines
• Nanotoxicology assessment requirements
• GMP-compliant manufacturing
• Stability and shelf-life validation

Clear regulatory frameworks are crucial for commercialization and market acceptance.

10. FUTURE PROSPECTS

Future research directions include:

• Artificial intelligence-assisted formulation design
• Personalized acne treatment strategies
• Stimuli-responsive nanoemulgels
• Integration with microneedle systems
• Long-term clinical trials
• Regulatory harmonization

Collaborative efforts between academia, industry, and regulatory agencies will accelerate translation.

CONCLUSION

Herbal nanoemulgels represent a scientifically advanced and patient-friendly approach for acne management. By combining the therapeutic potential of phytochemicals with nanotechnology-based delivery systems, nanoemulgels overcome key limitations of conventional therapies, including poor solubility, instability, and limited dermal penetration. While formulation strategies and preclinical outcomes are promising, large-scale clinical validation, standardization, and regulatory clarity remain essential. Continued innovation and translational research will establish herbal nanoemulgels as a next-generation topical therapy for acne vulgaris.

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