1.  Field of the Invention
The present invention relates to a device and a method for delivering or withdrawing a substance through the skin of an animal, including humans, and in particular to a method and device for withdrawing or delivering a substance such as a drug, protein or vaccine to a subject.  The invention also relates to a device for enhancing the penetration of one or more dermal-access members. 
2.  Related Art
The skin is made up of several layers with the upper composite layer being the epithelial layer.  The outermost layer of the skin is the stratum corneum that has well known barrier properties to prevent molecules and various substances from entering the body and analytes from exiting the body.  The stratum corneum is a complex structure of compacted keratinized cell remnants having a thickness of about 10–30 microns.  The stratum corneum forms a waterproof membrane to protect the body from invasion by various substances and the outward migration of various compounds. 
The natural impermeability of the stratum corneum prevents the administration of most pharmaceutical agents and other substances through the skin.  Numerous methods and devices have been proposed to enhance the permeability of the skin and to increase the diffusion of various substances through the skin in order to be utilized by the body.  According to some methods and devices, the delivery of substances through the skin is enhanced by either increasing the permeability of the skin or increasing the force or energy used to direct the substance through the skin. 
Other methods of sampling and delivering various substances through the skin include forming micropores or cuts through the stratum corneum.  Numerous substances can be effectively administered by piercing the stratum corneum and delivering a substance in or below the stratum corneum.  In a similar manner, some substances can be extracted from the body through cuts or pores formed in the stratum corneum.  The devices for piercing the stratum corneum generally include a plurality of microneedles or blades having a length to pierce the stratum corneum.  Examples of these devices are disclosed in U.S. Pat.  No.  5,879,326 to Godshall et al.;  U.S. Pat.  No.  6,494,865 to Alchas; U.S. Pat.  No.  5,997,501 to Gross et al.;  U.S. Pat.  No.  4,886,499 to Cirelli et al.;  U.S. Pat.  No.  6,183,434 to Eppstein; U.S. Pat.  No.  5,250,023 to Lee et al.;  International publication WO 97/48440; U.S. Pat.  No.  5,527,288 to Gross et al.;  and U.S. Pat.  No.  3,595,231 to Pistor.  Each of the foregoing documents is incorporated herein by reference in their entirety. 
Some of the above-noted devices include micron-sized needles or blades and can be effective in delivering or sampling substances.  However, many of these needles and blades have a length of a few microns to a few hundred microns and typically do not penetrate the skin to a uniform depth.  The natural elasticity and resilience of the skin often result in the skin being deformed by the needles rather than pierced.  Therefore, when a microneedle array is pressed against the skin, the outermost needles penetrate the skin while the innermost needles do not penetrate the skin or only penetrate to a depth less than the outermost needles. 
Moreover, conventional devices have problems with overall height and ease of use.  As a result, the prior methods and devices for the sampling and administering of substances have exhibited limited success.  Accordingly, a continuing need exists in the industry for an improved device for the sampling and administering of various substances to the body.