Enhanced Stability in High-Solvent and Oil-Rich Topical Formulations

Designing effective drug delivery systems remains a key challenge in the dermo pharmaceutical sector, often requiring elevated levels of polar solvents. These solvents play a critical role in enabling solubilization and supporting targeted skin delivery of Active Pharmaceutical Ingredients (APIs), but they can negatively impact the physical properties and emulsion stability of the final formulation. Conventional topical thickeners, including standard carbomers, frequently struggle to preserve structural integrity in such complex systems that contain high amounts of solvents and oils.

This article examines a study carried out to assess the performance of inverse latex polymers as reliable stabilizing agents in these challenging formulations. The main goal was to identify the functional limits of these rheology modifiers when exposed to high concentrations of polar solvents and a wide range of oil phases.

How does solvent concentration affect the stability of topical formulations?

The stability of topical formulations is heavily influenced by the solvent concentration, which affects the dielectric constant of the vehicle. High loads of polar solvents (e.g., Propylene Glycol, Transcutol® P) trigger the collapse of traditional carbomer networks by shielding electrostatic repulsions. Furthermore, these solvents can increase the solubility of the oil phase in the continuous medium, promoting droplet growth, coalescence, and eventual phase separation.

To maintain integrity, this study utilizes inverse latex polymers, which employ a steric stabilization mechanism coupled to electrostatic repulsion, delivering a more robust system for difficult to stabilize formulations. These polymers form a robust 3D micro-gel network that physically entraps oil and solvent phases, remaining less impacted by the chemical shifts that often destabilize conventional rheology modifiers.

Advanced polymeric stabilization

Inverse latex polymers offer a highly versatile approach for creating tailored vehicles for complex topical drug formulations. In contrast to conventional thickeners that depend on neutralization or extended hydration periods, these polymers are pre neutralized and immediately ready for use. Their functionality is based on a micro gel network that maintains stability over a broad pH range from 3 to 12.

In addition to their chemical robustness, these polymers provide significant advantages in process engineering. This technology enables formulators to achieve improved manufacturing efficiency:

• Energy efficiency: Unlike standard thickeners that require high temperature steps to melt waxes or properly disperse ingredients, these polymers support cold process manufacturing. This leads to reduced production time and lower energy consumption, contributing to a more sustainable process.
• Shear stability: These polymers maintain strong performance under a wide range of mixing conditions. Whether processed with high shear rotor stators or low shear anchor stirrers, the micro gel network preserves the desired viscosity without breakdown.
• API stability: The option to formulate at room temperature is essential for protecting thermo sensitive APIs, minimizing the risk of degradation that can occur during the heating phases required by traditional emulsifier systems.

Study parameters

The study concentrated on two specific polymers:

• SEPINEO™ P 600: A multifunctional liquid polymer based on Acrylamide and Sodium Acryloyldimethyl Taurate Copolymer.
• SEPINEO™ D.E.R.M: A highly concentrated powder polymer composed of Hydroxyethyl Acrylate and Sodium Acryloyldimethyl Taurate Copolymer.

To evaluate their performance and stability, the research incorporated high levels of different formulation components, including:

• Solvents: Propylene glycol, PEG 400, Transcutol® P, and glycerin.
• Oils: Mineral oil, caprylic capric triglycerides, and coco caprylate caprate.

Performance validation across dosage forms

Technical evaluations confirmed that inverse latex polymers provide superior emulsion stability across three distinct manufacturing processes compared to traditional polymers.

1.Cream-gels

These biphasic dosage forms achieve stability without additional surfactants by physically trapping the oil phase within the polymeric network.

• Process conditions: Formulations were successful at both room temperature and via hot process (80°C) across various shear levels
• Performance: Successful stabilization of up to 30% oil and 25% Transcutol® P.
• Target viscosity:  Achieved levels >100,000 mPa.s
• Benchmarking: Identical formulations using Carbomer 980 as a benchmark were found to be unstable, resulting in immediate phase separation.

2. High-solvent emulsion gels

These formulations serve as excellent vehicles for extremely high concentrations (>40%) of polar solvents.

• Process conditions: Manufactured at both room temperature and 80°C and employing a two-step process using a serrated disc for initial gel formation (medium-high shear) followed by an anchor stirrer for the emulsification phase (low shear).
• Target viscosity: Maintained a robust range between 80,000 to 180,000 mPa.s
• Versatility: The study validated stability with 50% PEG 400 and 40% levels of Glycerin or Propylene glycol.
• Patient Compliance: The unique texture helps offset the potential greasiness of high solvent levels, leading to a “cushion” effect that improves skin feel and patient adherence.

3. O/W Emulsions

In standard Oil-in-Water emulsions, these polymers acted as powerful co-stabilizers.

• Process conditions: Standard hot process (80°C) involving phase-stable emulsification, followed by slow cooling employing a high-shear using rotor-stator.
• Solvent capacity: Formulations remained stable even with a 60% load of Propylene glycol.
• Consistency: They demonstrated reliable stability regardless of the specific chemistry of the oil phase used.

Technical validation & stability metrics

The long-term performance and reliability of these formulations were confirmed through rigorous testing protocols:

• Durability: Multiple formulations maintained emulsion stability for up to one year at room temperature and three months at 45°C.
• Viscosity maintenance: These systems preserved high functional viscosity (typically 80,000 to 180,000 mPa.s), ensuring the product remains easy to apply yet structurally sound.
• Manufacturing versatility: Stability was maintained across various equipment types (anchor stirrers, rotor-stators) and temperature conditions, including both cold and hot processes (80°C).

The results clearly confirmSEPINEO™ P 600and SEPINEO™ D.E.R.Mas effective stabilizing polymers for flexible topical drug delivery systems. By delivering strong emulsion stability and supporting efficient API solubilization in formulations with high levels of solvents and oils, these inverse latex polymers provide a dependable foundation for developing advanced topical products.

This formulation versatility is illustrated in our High Oil and Solvent Content Cream Gel Formula. By enabling simultaneous thickening and stabilization of both solvent and oil phases without the need for additional emulsifiers, it highlights the distinctive network forming capabilities of SEPINEO™ P 600.

Sources

1. Gavinet B,Bulcourt C.  Flexible formulations to combine high amount of solvents and oils. Technical Study. 2022.
2. Torrado JJ, Anaya BJ, Kara A, et al. Unraveling the Impact of the Oil Phase on the Physicochemical Stability and Skin Permeability of Melatonin Gel Formulations. Pharmaceutics. 2021.
3. Bonacucina P, Cespi M, Misici-Falzi M, Palmieri GF. Characterization and Stability of Emulsion Gels Based on Acrylamide/Sodium Acryloyldimethyl Taurate Copolymer. AAPS PharmSciTech. 2009.