Applying pressure-sensitive adhesive (PSA) specifically to the peripheral area rather than the entire surface serves to eliminate physical barriers between the drug reservoir and the skin. By limiting the adhesive to the edges, you ensure the drug moves directly from the rate-controlling membrane into the skin, avoiding the increased resistance and potential chemical interactions caused by a full adhesive coating.
Core Insight: In reservoir-type patches, the adhesive layer itself can inadvertently act as a secondary, uncontrolled rate-limiting membrane. Peripheral coating decouples the mechanical function (adhesion) from the therapeutic function (delivery), allowing for maximum drug flux without compromising patch retention.
The Mechanism of Release
Avoiding the "Double Barrier" Effect
When a patch is fully coated with adhesive, that adhesive layer sits directly between the rate-controlling membrane and the stratum corneum (the outer layer of skin).
This creates an additional physical barrier that drug molecules must traverse. Peripheral application removes this obstacle, allowing the rate-controlling membrane to function as the sole governor of drug release.
Preventing Chemical Interference
Coating the center of the patch brings the adhesive into direct contact with the drug gel or reservoir.
This can lead to interactions between the adhesive polymer and the gel formulation. These interactions often increase penetration resistance, unpredictably slowing the delivery rate.
Optimizing Performance
Maximizing Transdermal Delivery Rate
The primary advantage of the peripheral design is efficiency.
By ensuring the drug pathway is unobstructed, the system achieves a higher and more consistent delivery rate. The drug penetrates the skin directly from the control membrane, adhering to the theoretical release profile more accurately.
Balancing Fixation and Function
Peripheral coating maintains the necessary mechanical properties without sacrificing therapeutic efficacy.
The adhesive ring provides strong skin adhesion to keep the patch in place. Meanwhile, the active center remains "clean," dedicated entirely to drug transport.
Understanding the Trade-offs
The Contact Challenge
While peripheral coating improves drug kinetics, it introduces a mechanical challenge regarding skin contact.
Supplementary data suggests that a seamless adhesive interface (full coating) eliminates gaps between the system and the skin. By removing adhesive from the center, you must ensure the peripheral ring is strong enough to keep the central membrane flush against the skin to prevent air pockets, which could interrupt delivery.
Stability vs. Breathability
Full adhesive layers can trap moisture, potentially leading to maceration.
While peripheral coating reduces the amount of adhesive on the skin, the stability of the patch relies heavily on the backing material. Using porous elastic tape in conjunction with peripheral adhesive can help maintain a stable physiological environment by allowing the skin to breathe.
Making the Right Choice for Your Goal
- If your primary focus is maximizing drug bioavailability: Choose peripheral coating to eliminate the resistance and chemical interactions caused by a full adhesive layer.
- If your primary focus is preventing skin maceration: Leverage the reduced adhesive surface area of a peripheral design to allow better air circulation over the central drug reservoir.
- If your primary focus is ensuring absolute physical contact: Be aware that peripheral coating requires a highly flexible backing or porous tape to prevent the non-adhesive center from lifting off the skin.
Design the adhesive profile to support the drug's kinetics, not impede them.
Summary Table:
| Feature | Peripheral Adhesive Coating | Full Surface Adhesive Coating |
|---|---|---|
| Drug Flux | Maximum; no secondary barrier | Reduced; adhesive acts as a barrier |
| Chemical Stability | High; no contact with drug gel | Lower; potential for polymer interaction |
| Skin Breathability | Better; reduced adhesive coverage | Lower; higher risk of skin maceration |
| Contact Consistency | Requires flexible backing for flush fit | Naturally seamless interface |
| Best Use Case | Maximizing bioavailability | Ensuring absolute physical contact |
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References
- Zhen Yang, Huimin Hou. Enhancement of skin permeation of bufalin by limonene via reservoir type transdermal patch: Formulation design and biopharmaceutical evaluation. DOI: 10.1016/j.ijpharm.2013.02.048
This article is also based on technical information from Enokon Knowledge Base .
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