Optical clearing of in vivo human skin: Implications for light-based diagnostic imaging and therapeutics

Lasers in Surgery and Medicine 34:83–85 (2004) Letter to the Editor Optical Clearing of In Vivo Human Skin: Implications for Light-Based Diagnostic Imaging and Therapeutics Misbah H. Khan, MD , Bernard Choi, PhD , Samuel Chess, BS , Kristen M. Kelly, MD , Jerry McCullough, and J. Stuart Nelson, MD , PhD * Beckman Laser Institute and Medical Clinic, University of California-Irvine, Irvine, CA 92612 The complex morphological nature of human skin provides a highly scattering medium for visible and near- infrared wavelengths due to variations in the indices of refraction of different components therein. Scattering diminishes the depth and clarity of images in light-based diagnostic imaging and attenuates the effective light dose that reaches targeted chromophores in laser therapeutics [1]. Light-based diagnostic techniques and therapeutics would likely be improved if scattering could be reduced, thereby enhancing light penetration into human skin. One method to enhance light penetration into skin is the use of hyperosmotic and biocompatible agents that induce an optical clearing effect [2]. Hyperosmotic agents result in refractive index matching between cells and ground substance in the dermis, which is believed to cause a reduction in optical scattering. Studies done by Vargas et al. [3] and Wang et al. [4], demonstrated that the injection of hyperosmotic agents into rat dermis could significantly reduce light scattering and thereby enhance the imaging depth of non-invasive techniques such as optical coherence tomography (OCT). If delivered by injection into the dermis, these hyperosmotic agents tend to dehydrate the skin and reduce the index mismatch between inter/in- tracellular components [5]. Although glycerol and polyethylene glycol (PEG) reduce optical scattering in human skin, their clinical utility has been very limited. Penetration of glycerol and PEG through intact skin is very minimal and extremely slow, because these agents are hydrophilic and penetrate the lipophilic stratum corneum poorly. In order to enhance skin penetra- tion, these agents need to be either injected into the dermis or the stratum corneum has to be removed, mechanically (e.g., tape stripping) or thermally (e.g., erbium: YAG laser ablation). Clinical utility would be improved if the clearing agent could be applied topically onto intact skin and there- after migrate across the stratum corneum and epidermis into the dermis. In this letter, we present our evaluation of the use of topically applied optical clearing agents in vitro as well as in vivo. Food and Drug Administration (FDA) approved lipophilic polypropylene glycol-based polymers (PPG) and hydrophilic PEG-based polymers, both with indices of refraction that closely match that of dermal collagen s 2004 Wiley-Liss, Inc. PhD , (1.47) were studied alone and in a combined pre-polymer mixture. To measure the optical properties (diffuse transmission and surface reflectance) of freshly excised in vitro human skin, an integrating sphere was used [6] with an inverse adding doubling algorithm [7] to estimate the reduced scattering coefficient ( s ’) of the samples. The optical clearing potential (OCP) was defined as the ratio of m s ’ immediately before and 24 hours after, agent application. Optical clearing agents including, glycerol and PPG- and PEG-based polymers compounds were applied alone, and in a PPG- and PEG-based combined pre-polymer mixture, topically on the epidermal side of the skin samples which were left undisturbed in an incubator at 378C for 24 hours. The OCP was found to be significantly higher for the lower molecular weight combined PPG- and PEG-based pre-polymer mixture as compared to glycerol or either pre-polymer alone. The combined PPG- and PEG-based pre-polymer mixture was also evaluated for its OCP in vivo. The volar forearm of a healthy volunteer was pre-cleaned, shaved and, divided into two areas, i.e., untreated control and where the combined pre-polymer mixture was topically applied onto the skin surface in the form of a thick layer under occlusion. After 2 hours, the mixture was removed and cross-polarized images of the hair shafts were obtained. Visualization of the intradermal portion of the hair shaft (approximately 1 mm deep) was enhanced on the area where the combined pre- polymer mixture was applied (Fig. 1). The combined PPG- and PEG-based pre-polymer mix- ture was topically applied to a 2 2 cm area on the leg of a healthy volunteer (that contained vascular telangiecta- sias). Cross-polarized images of the vessels before, and 2 hours after application under occlusion were obtained. Visibility of the telangiectasias was enhanced (approxi- mately 500 mm deep) and smaller telangiectatic branches *Correspondence to: J. Stuart Nelson, MD, PhD, Beckman Laser Institute, University of California, 1002 Health Sciences Road East Irvine, CA 92612-1475. E-mail: snelson@laser.bli.uci.edu Accepted 1 December 2003 Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/lsm.20014