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One of the most important functions of our skin is its barrier function. The skin keeps the water inside and protects us against environmental noxins such as light, pollutions or germs. Therefore, it is not easy to transmit substances into the skin even when it is desired. The uppermost layer of the skin, the horny layer (stratum corneum), acts as a physical barrier. Only small moderate lipophilic drugs can pass this barrier to achieve a therapeutic effect. For this reason the pharmaceutical industry has invested a high amount of effort in designing formulations or delivery methods to overcome the skin barrier and to enhance the penetration of active substances across the stratum corneum. One important novel area of research is the development of vaccines, which can be applied directly onto the skin without any pain sensation. However, the active part of the vaccine, the antigen, is a highly molecular weight substance, which makes it difficult to deliver by the dermal route using the more conventional methods. One of the novel methods for the delivery of highly molecular weight substances through the skin barrier is the use of microneedles. Microneedles were introduced in the field of transdermal and dermal drug delivery more than a decade ago (Henry et al., 1998). Microneedles are shorter than hypodermal needles, but long enough to breach the stratum corneum barrier, the upper 20 μm of the skin. The application of 0.5 mm microneedles is described as painless (Kaushik et al., 2001). They have been developed as a replacement for the traditional needle and syringe and to facilitate the transport across the skin barrier without any pain sensation. When using solid microneedle arrays, small conduits are created in the skin, which are large enough to allow the penetration of high molecular

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Application of in vivo Laser Scanning Microscopy to Visualise the Penetration of a Fluorescent Dye in Solution and in Liposomes into the Skin after Pre-Treatment with Microneedles