Transdermal drug delivery systems are a method of administering medication through the skin to achieve a systemic effect. These systems, commonly known as patches, are designed to release a specific dose of a drug at a controlled rate into the bloodstream. This approach offers an alternative to other delivery methods, like oral medications or injections. A familiar example is the nicotine patch, which helps individuals quit smoking by providing a steady supply of nicotine. This method of delivery is valued for its convenience and non-invasive nature.
The Mechanism of Transdermal Delivery
The effectiveness of transdermal delivery hinges on a drug’s ability to cross the skin’s primary protective barrier, the stratum corneum. This thin but tough outermost layer serves as a rate-limiting barrier controlling what passes through. To be effective, a drug must navigate this layer and the epidermis to reach the dermis, which contains blood capillaries for absorption into the general circulation.
The process driving a drug from a patch into the body is passive diffusion, where substances move from an area of higher concentration to one of lower concentration. The patch contains a high concentration of medication, while the skin initially has none. This concentration gradient powers the drug’s journey through the skin’s layers until it is absorbed.
A benefit of this route is that it bypasses the digestive system and liver, avoiding the “first-pass effect.” When drugs are taken orally, the liver metabolizes them, which can reduce the amount of active drug entering circulation. Delivering medication through the skin avoids this initial breakdown, improving the drug’s bioavailability and effectiveness.
The rate of drug absorption is influenced by several factors, including the drug’s characteristics and the skin’s condition. Smaller, fat-soluble molecules pass through the stratum corneum more easily. The skin’s thickness, hydration, and the presence of hair follicles also affect how well a drug is absorbed.
Types of Transdermal Systems
Transdermal systems come in several designs, with patches being the most prevalent. Patches are categorized into two main types: reservoir and matrix systems. Each design uses a different mechanism to store and release medication, which influences how the drug is delivered over time.
Reservoir patches contain medication in a liquid or gel compartment. This reservoir is separated from the skin by a rate-controlling membrane. This design allows for a consistent release rate, meaning the drug is delivered at a constant speed as long as the reservoir’s concentration is higher than the skin’s.
Matrix systems have the drug dispersed directly within a solid or semi-solid polymer matrix. In some designs, this drug-infused matrix also serves as the adhesive. The rate of delivery is controlled by the drug’s diffusion through the polymer. Because the drug is evenly distributed, these patches may offer a lower risk of accidental overdose if damaged.
Beyond patches, other forms of transdermal delivery include medicated gels and creams intended for systemic absorption. Emerging technologies are also expanding the field, such as microneedle patches. These feature microscopic needles that painlessly penetrate the stratum corneum. This creates tiny channels that allow larger drug molecules, which normally cannot pass through the skin, to be delivered.
Anatomy of a Transdermal Patch
A transdermal patch is a multi-layered product, with each layer serving a specific function to deliver medication safely and effectively. While designs vary, most share a fundamental structure consisting of four primary components.
The outermost backing layer is impermeable, protecting the patch’s contents from the environment and providing structural support. It is made of flexible material, allowing the patch to conform to the body’s contours.
Beneath the backing is the drug reservoir or matrix, which contains the active medication. The formulation of this layer is designed to ensure the drug remains stable and is released at the intended rate.
The adhesive layer secures the patch to the skin. It must be strong enough to hold for the prescribed duration but gentle enough to avoid significant skin irritation upon removal. In some designs, the medication is incorporated directly into this adhesive layer.
Finally, the release liner is a protective layer that is peeled off just before application. It covers the adhesive, keeping it clean and preserving the patch’s integrity during storage.
Common Medications Delivered Through the Skin
The transdermal route is beneficial for medications requiring a steady, controlled release over a prolonged period, making it well-suited for chronic conditions. The first patch, approved in 1979, delivered scopolamine to prevent motion sickness over three days.
A common application is for pain management. Fentanyl patches provide continuous relief for severe, chronic pain, avoiding the concentration peaks and valleys of oral medication for more stable control. Lidocaine patches are also used for localized neuropathic pain.
Hormones are frequently administered this way for contraception and hormone replacement therapy. Estradiol, testosterone, and combination birth control patches deliver hormones directly into the bloodstream. This maintains stable levels and avoids the need for daily pills or injections.
Other medications are also delivered via patches for various conditions:
- Cardiovascular drugs, such as nitroglycerin for angina and clonidine for high blood pressure.
- Nicotine for smoking cessation, which helps manage withdrawal symptoms and cravings.
- Rivastigmine for Alzheimer’s disease.
- Rotigotine for Parkinson’s disease.