Occlusion refers to covering the skin by any means of film or substance;1 this extends to wound dressings, which can be differentiated as being fully occlusive or semi-occlusive. Wounds are either covered or left exposed to air and while studies have shown that occlusion may speed the healing process of wounds faster than air exposure,2,3 completely occlusive dressings have some disadvantages, particularly when compared to semi-occlusive dressings.4 For instance, if bacteria are trapped under the wound dressing, the fully occlusive dressing provides a warm and moist environment for the bacteria to reproduce, potentially leading to infection.5
On the other hand, a semi-occlusive dressing presumably allows more water and oxygen transfer. Thus, bacteria multiply less. Some experiments have shown semi-occlusion to be preferable for wound healing.5,6 Therefore, water vapor may be a differentiating variable between these dressings and is also an indicator of epidermal lipid synthesis, a signal for recovery.7,8
The present study uses an evaporimeter to measure the degree of water loss from in vitro skin samples covered by occlusive and semi-occlusive wound dressings to serve as a model for determining the effectiveness of occlusive cosmetic formulations. The purpose of this work was to develop a model for determining the effectiveness of occlusive cosmetic formulations.
As an example, acrylates/octylacrylamide copolymer is a hydrophobic, high molecular weight carboxylated acrylic copolymer that is inherently moisture resistant and as such can be used in waterproof sunscreens and a variety of creams and lotions. Its film-forming properties help to maintain active ingredients on the site of application by imparting resistance to abrasion or rub-off. This material was compared with other occlusion films to determine its potential as a wound healing dressing.
Materials and Methods
Wound dressings: Laboratory filma, a putative occlusive membrane; adhesive bandagesb, presumably semi-permeable from their fabric covering; and acrylates/octylacrylamide copolymerc were purchased for the study.
Human skin: Human cadaver skin was obtainedd and dermatomed to a 500-micrometer thickness. Skin samples were stored in Eagle’s MEM with Earle’s balanced salt solution (BSS)e at 4ºC and used within 5 days to ensure cell viability.9-11 A total of 8 skin samples from 8 different donors were used after being examined for TEWL integrity.
For each trial, four glass vials were filled with saline (0.9% NaCl) to mimic live body conditions. The skin samples were placed over vial openings, secured with latex bands, and allowed to equilibriate for 30 min, after which the baseline TEWL values were measured by evaporimeterf. The first sample was covered and banded with the occlusive film, and the second with a semi-occlusive dressing. For the third sample, 100 uL of 5% acrylates/octylacrylamide copolymer solution was spread over the skin and allowed to aerate to form a thin, presumably semi-permeable membrane upon the skin. The final skin sample was left exposed to the surrounding air, serving as a blank (untreated) control. After 30 min, TEWL values again were recorded with the evaporimeter. One trial consisted of 4 treatments, including 3 topical agents and a blank control; this trial was repeated 7 additional times for a total of 8 trials.
TEWL measurements: TEWL was measured noninvasively by lightly placing a evaporimeter on the skin or dressing without added pressure, per the standard guidelines.12-14 TEWL values were expressed as g/m2/h. During the experiment, the relative humidity (RH) and room temperature were recorded as RH = 44–55% (50 ± 3%) and temperature = 19–21°C (20±1°C), respectively.
Statistical methods: The results were statistically calculatedg and the differences were analyzed utilizing the one way repeated measure ANOVA. Statistical significance was accepted at p < 0.05.
No significant results were observed between TEWL values among the wound dressings, except in the case of the purported occlusive membrane, which showed a statistically significant lower value (p < 0.05) when compared with the blank control (Figure 1).
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 shown7,8 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
As a reminder, the present study measuring water vapor was to determine the dressing’s permeability to water, not the actual healing process itself. These results supplement previous findings1-6, 15 and may prove valuable when considering future in vivo wound-related occlusion experiments. In this pilot study, the results were generated from a limited sample size; the authors suggest a validation study with larger samples.
The results of this study show the occlusive film yielded the least amount of water vapor while the semi-occlusive bandage allowed relatively more; the copolymer yielded about the same amount as the control. Thus, the experiment could be paired with the findings of other experiments on the effectiveness of various dressings to determine whether a particular dressing is desirable or not. Moreover, future experiments could elaborate on the unknown properties of the copolymer to possibly produce a more effective way to heal human skin wounds.
Further, in vivo studies would provide in depth details as to the correlation between the permeability of a dressing to water and possibly air. Recent review provides additional insight that wound dressings remain a standard treatment since its advantages outweigh its disadvantages.15 The authors believe that this new method may aid in the development and evaluation of more occlusive cosmetic formulations.
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2. GD Winter, Formation of the scab and the rate of epithelization of superficial wounds in the skin of the young domestic pig, Nature 193 293-294 (1962)
3. CD Hinman and HI Maibach, Effect of air exposure and occlusion on experimental human skin wounds, Nature 200 377-378 (1963)
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13. P Elsner, E Berardesca and HI Maibach, eds, Bioengineering of The Skin: Water and the Stratum Corneum, Boca Raton: CRC Press (1994)
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