Evaluating Water Permeability and Occlusion in Wound Dressings and Topical Cosmetics

By: Hongbo Zhai, MD, and Howard I. Maibach, MD, University of California

Posted: June 30, 2009, from the July 2009 issue of Cosmetics & Toiletries.

page 3 of 4

Discussion
An abundance of experimental data has been published on the effects of moist wound healing under occlusive or semi-occlusive dressings.^{1-6, 15} Furthermore, even more information exists on the clinical use of these types of dressings. It is clear there is a clinical role for dressings that maintain hydration of wound tissues, and therefore, viability.

In vivo studies on the occlusion of both animal and human skin have been conducted.^2-8 These studies have used invasive methods such as skin biopsy as well as noninvasive methods such as the evaporimeter to determine the progress of wound healing. While such experiments are both complicated and time-consuming, the present in vitro model is simple and economical. The study described here compared the TEWL values from skin covered with a fully occlusive dressing, a semi-occlusive dressing, and a 5% copolymer solution.

Based on the results of this study, baseline TEWL measurements did not differ significantly among the 4 groups. Measurements taken 30 min post-treatment showed the putative occlusive film to be relatively impermeable with a significant difference (p < 0.05) as compared with the blank control. In addition, the semi-occlusive bandage was relatively more permeable than laboratory film and blank control but not as vapor-permeable as the film-forming copolymer.

Manufacturers of the tested copolymer claim that it can be applied in various waterproof products, such as sunscreen and lotion. In this study, only a thin layer of the copolymer was used as a potential wound dressing or model of a cosmetic occlusive formulation; it did not show occlusion character. Perhaps a higher concentrated solution of the copolymer would form an occlusive barrier. Further experiments at different concentrations and time points would assist in determining its occlusion properties.

Occlusion prevents water vapor from escaping, leading to rapid epithelization. Epithelization was enhanced by occlusion because rather than forming a scab on the wound, epithelium filled and spread around it, resulting in a smoother, more attractive scar.^2,3 Moreover, as recent studies have shown^7,8 semi-occlusive dressings allow not only vapor but also air to pass through. As mentioned, such findings also indicate that water vapor is a signal for epidermal recovery.^7,8