Transdermal drug delivery system (TDDS) also known as “patches” (non-invasive delivery) is dosage form designed to deliver a medicine across a patient skin. Skin is that the largest and most accessible organ of physical body with the assistance of skin layers drug reaches into the blood stream given as sustained release, controlled release, or extended-release formulation. Transdermal delivery systems are specifically designed to obtain systemic blood levels and are utilized in the US since the 1950's. the primary transdermal system, transdermal SCOP was approved by FDA in 1979 for the prevention of nausea and vomiting related to travel. Most transdermal patches are designed to release the active ingredient from several hours to days following application to the skin. This is especially advantageous for prophylactic therapy in chronic conditions. The evidence of percutaneous drug absorption may be found through measurable blood levels of the drug, detectable excretion of the drug, and its metabolites within the urine and thru the clinical response of the patient to the administered drug therapy.

Transdermal permeation or percutaneous absorption are often defined because the passage of a substance, like a drug, from the surface of the skin through its various layers into the blood stream.

Certainly, each dosage form has its unique place in medicine, but some attributes of the transdermal delivery system provide distinct advantages over traditional methods. In the development of the transdermal delivery system, a series of interrelated elements must be taken into consideration. These elements are often classified into five basic areas;

The transport of medicine through the skin is complex since many factors influence their permeation to simplify the situation somewhat, one should consider the subsequent

• Skin structure and its properties.
• The penetrating molecule.
• Physical-chemical relationship to the skin.
• The delivery platform.
• The platform or delivery system carrying the penetrant.
• the mixture of skin.
• Penetrant.

Advantages include; Avoidance of first-pass metabolism Avoidance of gastrointestinal compatibility, minimizing undesirable side effects, Avoiding the fluctuation in drug levels, maintain plasma concentration of potent drugs and the limitations include; Transdermal patches cannot deliver ionic drugs, Transdermal patches cannot achieve high drug levels in blood/plasma, Transdermal patches cannot deliver drugs during a pulsatile fashion, It cannot develop for drugs of huge molecular size.

Some of the components used for skin patch are;

• Liner: It's removed before using it gives protection to patch during storage. for instance, polyester film.
• Drug: it's directly in touch with release layer. for instance, nicotine, methotrexate, and estrogen Adhesive: It helps in adhering the components of patch with one another and helps in adhering the patch to skin patch. For example, acrylates, polyisobutylene, silicones, etc.• Membrane: It helps in release of drug from reservoir and multilayer patches.
• Backing: It gives protection to the patch from the atmosphere. for instance, cellulose derivatives and polyvinyl alcohol.

Transdermal patches were categorized into four main types; Matrix type, Reservoir type, Membrane matrix hybrid, micro-reservoir type, Drug in adhesive and it is of Single layer drug in adhesive and Multilayer drug in adhesive, Miscellaneous which includes Hybrid matrix type, vapor patch, etc.