Drug delivery to skin

A recent study used hollow microneedles with the following dimensions 20-100 pm in diameter and 100-150 pm in length to deliver insulin through skin [22]. In vivo tests in diabetic animals, however, were unable to demonstrate any functional delivery of insulin through the hollow microneedles. It is therefore essential that the engineering processes evolve to ensure that both microneedle length and tip sharpness are optimized for systemic drug delivery to be seen in vivo. 

Roberts, M.S. (1997). Targeted drug delivery to the skin and deeper tissues role of physiology, solute stmeture and disease. Clin. Exp. Pharmacol. Physiol. 24 874-9. 

Because of the large surface area of the skin and its bypass of the liver as a first pass step in metabolism, many drug delivery systems have been developed that control the rate of drug delivery to the skin for subsequent absorption. Effective transdermal drug delivery systems of this type deliver uniform quantities of drug to the skin over a period of time. Technically, transdermal drug delivery systems may be classified into monolithic and membrane-controlled systems (9). 

Drugs chosen for delivery via a transdermal drug-delivery system must adequately penetrate the skin in such a way that the system determines the delivery rate that should be fairly constant. In addition, the drug must not irritate or sensitize the skin. It is hoped that in the future more drugs will be developed for transdermal delivery. This could become an alternative route for drug delivery to children who have difficulty with oral administration. 

Because of the very extensive barrier properties of the stratum corneum, it is often necessary to increase the intrinsic rate of dermal or transdermal drug delivery to achieve the required therapeutic drug levels. In these instances, skin penetration and permeation enhancement... 

There is a paucity of published data on in vitro studies of dermal penetration in food animals. In one study comparing the effects of freezing on human and cow skin (27), it is alluded to that the follicular route of penetration predominates in the cow. These authors have also written an excellent review on topical drug delivery to cattle and sheep which should be consulted for further details (2S). In this work, studies on the dermal application of twenty four different substances are reviewed. Some interesting data on differential effects of dermal enhancers in different species underlines the problems inherent to interspecies extrapolations. One conclusion of this study was that "the barrier properties of sheep and cattle skin are not well understood." This situation has not improved in the intervening years. 

M. Sun, A. Fan, Z. Wang, Y. Zhao, Dendrimer-mediated drug delivery to the skin. Soft Matter 8 (2012) 4301-4305. 

Nanocapsules appear a promising approach as a drug system for topical application. The transport of chlorhexidine-loaded poly(e-caprolactone) nanocapsules through full-thickness and stripped hairless rat skin was investigated [48]. The flexibility of nanocapsules assured a satisfying bioadhesion to the skin, whereas the rigidity of the carrier limited the molecular spill into the skin and controlled the drug delivery to the skin. 

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