Polyethylene (PE) and polyurethane (PU) are the standard choices for backing layers because they offer an optimal balance of structural integrity and impermeability. These polymers act as a robust physical shield, preventing the volatile active ingredients in herbal formulas from evaporating while ensuring the medication is released exclusively toward the skin, rather than leaking into the environment.
The Core Insight The backing layer is not merely a cover; it is a functional component that enforces unidirectional drug delivery. By sealing the formulation against the external environment, these materials create a controlled "closed system" that forces active ingredients to migrate downward into the skin rather than dissipating into the air.
The Mechanism of Barrier Protection
Preventing Volatilization
Herbal formulations often contain volatile essential oils or active compounds that evaporate easily. Polyethylene and polyurethane provide a high-density barrier that locks these ingredients inside the patch matrix. Without this impermeable layer, a significant portion of the dosage would be lost to the atmosphere before entering the body.
Enforcing Directional Release
The primary reference highlights that these materials ensure medication is released directionally. By blocking the "exit" on the outer surface, the backing layer dictates that the only path for diffusion is through the adhesive and into the stratum corneum (the skin barrier). This maximizes the bioavailability of the herbal ingredients.
Shielding from Contamination
These polymers are chemically inert and water-resistant. They protect the drug reservoir from external threats such as moisture, bacteria, light, and physical friction. This ensures the chemical stability of the herbal formula remains intact throughout the wear period.
Structural Integrity and User Comfort
Flexibility and Conformity
A rigid patch will detach as soon as the user moves. PE and PU are selected for their inherent flexibility and tensile strength. They allow the patch to stretch and twist with the skin without delaminating (peeling off).
The Advantage of PE Foam
Supplementary data indicates that Polyethylene is often used in a foam format. This adds softness and physical elasticity, allowing the patch to mold closely to complex anatomical curves, such as joints or the abdomen. This close contact is critical for maintaining consistent drug permeation.
Mechanical Support
The backing layer serves as the "skeleton" of the patch. It provides the necessary physical support to maintain the patch's shape during manufacturing, storage, and application, preventing the adhesive layer from collapsing or distorting.
Enhancing Absorption via Occlusion
Increasing Skin Hydration
These backing materials typically possess a low water vapor transmission rate. By trapping moisture that naturally evaporates from the skin, they hydrate the underlying tissue.
The Permeability Effect
Hydrated skin acts like a sponge, becoming significantly more permeable than dry skin. This occlusive effect expands the pores and intercellular spaces, allowing large or complex herbal molecules to penetrate the skin barrier more effectively.
Understanding the Trade-offs
Occlusion vs. Irritation
While the sealing property of PE and PU is excellent for drug delivery, it creates a trade-off regarding skin health. Because these materials block air and moisture transfer, wearing the patch for extended periods can trap sweat and bacteria against the skin. This can potentially lead to maceration (whitening/softening of the skin) or irritation, particularly in sensitive individuals.
Balancing Adhesion and Flexibility
Highly flexible materials like PU are excellent for comfort but can be more expensive to manufacture than standard PE. Conversely, standard PE is cost-effective and chemically inert but may lack the superior elasticity required for high-mobility areas (like knees or elbows) compared to PU or PE foam variants.
Making the Right Choice for Your Goal
When selecting between specific grades of polyethylene or polyurethane, consider the specific nature of your herbal formulation:
- If your primary focus is Volatile Ingredients (Essential Oils): Prioritize high-density Polyethylene (PE) for its superior impermeability to prevent evaporation loss.
- If your primary focus is High Mobility Areas (Joints/Neck): Select Polyurethane (PU) or Polyethylene Foam for maximum elasticity and anatomical conformity.
- If your primary focus is Drug Absorption Efficiency: Rely on materials with low water vapor transmission rates to utilize the occlusion effect for deeper skin penetration.
Summary: The selection of PE or PU is a strategic decision to create a unidirectional, contamination-free environment that maximizes the stability and penetration of herbal ingredients.
Summary Table:
| Feature | Polyethylene (PE) / PE Foam | Polyurethane (PU) |
|---|---|---|
| Primary Strength | High impermeability & cost-effective | Exceptional elasticity & breathability |
| Mechanism | Best for preventing volatilization | Superior anatomical conformity |
| Best Use Case | Volatile herbal oils & essential oils | High-mobility areas like joints & neck |
| Effect on Skin | High occlusion for deeper penetration | Soft touch with reduced skin maceration |
| Durability | Excellent chemical & moisture barrier | High tensile strength & tear resistance |
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As a trusted manufacturer and wholesale partner, Enokon specializes in high-quality transdermal drug delivery systems. Whether you need custom R&D or bulk production, we provide expert solutions for Lidocaine, Menthol, Capsicum, Herbal, and Far Infrared pain relief patches, as well as specialized Eye Protection and Medical Cooling Gel products.
Our value to your business:
- Custom R&D: Expert selection of PE/PU backing layers tailored to your formula (excluding microneedle technology).
- Wholesale Manufacturing: Scalable production with strict quality control.
- Optimized Efficacy: We leverage the latest in material science to ensure your patches offer superior stability and absorption.
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References
- V. K. Singla. Exploring Transdermal Delivery of Traditional Herbal Medicine for Central Nervous System Disorders. DOI: 10.56025/ijaresm.2023.1201241700
This article is also based on technical information from Enokon Knowledge Base .
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