Transdermal devices can be found in multiple versions. There are patches that are placed on the skin to allow medication to be absorbed through the skin into the bloodstream. Implants are also available that create a port for medicine to be delivered. Essentially, transdermal delivery is any drug administration that involves active ingredients being delivered across the skin for systemic distribution.
Perhaps the most promising devices being introduced today are those involving microneedles, which are divided into four types:
Transdermal devices using microneedles solve a long-standing medical problem: The skin’s anatomical peculiarities make it difficult to cross. The skin’s major barrier consists of the stratum corneum, the outermost layer. However, the layer underneath, the viable epidermis, also plays a protective role. According to research published in Pharmaceutics, only compounds that are able to get through the stratum corneum and diffuse through both layers of the epidermis have the potential to reach circulation and achieve systemic effects.
Ideal drug properties for transdermal delivery
Keep in mind that transdermal administration is not appropriate for all types of drugs. The optimal physicochemical properties of the drug and its biological properties must be considered, along with the pharmacokinetic and pharmacodynamic properties of the drug. The most important requirement is the need for controlled delivery, such as short half-life, adverse effect associated with another route, or a complex oral or IV dose regime.
The parameters for ideal candidates can be divided into physicochemical properties, biological characteristics, and polymer variables.
Advances in transdermal drug delivery technology have been rapid because of sophisticated polymer science that allows incorporation of polymers in transdermal systems in adequate quantity. The release rate from transdermal systems can be tailored by varying polymer composition. Selection of a polymeric membrane is important in designing a variety of membrane-permeation controlled transdermal systems.
Silicone elastomer blend networks, sugar siloxanes, amphiphilic resin linear polymers, and silicone hybrid pressure sensitive adhesives are showing promise for potential performance advantages and improved drug delivery efficacy.
Early on, transdermal delivery systems were used mainly for delivery of small, lipophilic, low-dose drugs. More recently, delivery systems began using chemical enhancers, non-cavitational ultrasound, and iontophoresis to enhance the efficacy of transdermal patches. Today, the ability of iontophoresis to control delivery rates in real time is providing added functionality in a number of instances.
At the same time, microneedles combined with thermal ablation are progressing through clinical trials for delivery of macromolecules and vaccines, including insulin, parathyroid hormone, and influenza. With these enhancement strategies, transdermal delivery is poised to significantly impact drug delivery choices.
Both chemical enhancers and the newest physical enhancers (ultrasound, thermal ablation, and microneedles) have begun expanding transdermal delivery of macromolecules and vaccines. These scientific and technological advances enable targeted disruption of the stratum corneum while protecting deeper tissues, positioning all types of transdermal drug delivery to have a widespread impact on medicine.