Drugs are commonly administered in solid form through pills or capsules that can be orally taken. However, many biological drugs cannot be administered this way because of degradation in the gastrointestinal tract and quick elimination by the liver. Another common technique for administration of drugs in liquid form is through injection using a metal hypodermic needle that can cause significant pain and discomfort to patients. For example, U.S. Pat. No. 5,279,585 discloses an injection device is provided for injecting fluids such as insulin within body tissue. The device includes a housing, a piston rod movably mounted within the housing, and a dose setting mechanism for controlling the movement of the piston rod with respect to a fluid-containing cartridge removably positioned within the housing.
Similarly, a number of physical and chemical techniques including electroporation, laser ablation, ultrasound, thermal, iontophoresis, and chemical enhancers have been explored to develop painless transdermal drug delivery techniques. It was found that its very difficult for the molecules with a molecular weight higher than 500 or diameter larger than 1 nm to penetrate normal human skin. Further studies showed that the key barrier for transdermal delivery of substances is the stratum corneum layer, the outer layer of skin, that is about 4-30 micron thick. Invasive methods to overcome this skin barrier have been used in practice, such as intradermal (ID), intramuscular (IM), or subcutaneous (SC) injection using standard hypodermic needles and syringes. These methods cause pain and require a skilled professional. In addition, they may cause needle injuries. Similarly, current methods of extracting biologic fluids such as blood from patients suffer from the same disadvantages.
In order to improve the skin permeability of the therapeutic agents and other active ingredients, microneedles have been recently developed to disrupt the stratum corneum and facilitate the delivery of the active agents and ingredients to the epidermis. These active substances can then diffuse through the rest of the epidermis to the dermis and be absorbed by blood vessels and lymphatics there. The substance absorbed can then get into the circulation system. Thus, both topical and systemic-level delivery of drugs is possible. Since there are no nerves and blood vessels in the stratum corneum and epidermis, this is a minimally invasive, painless and blood-free method of drug delivery. An additional advantage of this method, when engineered for topical delivery of vaccines, can lead to an enhanced inoculation effect because the epidermis is rich in antigen presenting cells and is a desired target for vaccine delivery.