Chess, Samuel

Choi, Bernard

Khan, Misbah H

M  Kelly, Kristen

McCullough, Jerry

Nelson, J Stuart

Kelly, Kristen M

eScholarship - University of California

UC IrvineUC Irvine Previously Published WorksTitleOptical clearing of in vivo human skin: Implications for light‐based diagnostic imaging and therapeuticsPermalinkhttps://escholarship.org/uc/item/8rq9n9wkJournalLasers in Surgery and Medicine, 34(2)ISSN0196-8092AuthorsKhan, Misbah HChoi, BernardChess, Samuelet al.Publication Date2004-02-01DOI10.1002/lsm.20014Copyright InformationThis work is made available under the terms of a Creative Commons Attribution License, availalbe at https://creativecommons.org/licenses/by/4.0/ Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaLasers in Surgery and Medicine 34:83–85 (2004)Letter to the EditorOptical Clearing of In Vivo Human Skin: Implications forLight-Based Diagnostic Imaging and TherapeuticsMisbah H. Khan, MD, Bernard Choi, PhD, Samuel Chess, BS, Kristen M. Kelly, MD, Jerry McCullough, PhD,and J. Stuart Nelson, MD, PhD*Beckman Laser Institute and Medical Clinic, University of California-Irvine, Irvine, CA 92612The complex morphological nature of human skinprovides a highly scattering medium for visible and near-infrared wavelengths due to variations in the indices ofrefraction of different components therein. Scatteringdiminishes the depth and clarity of images in light-baseddiagnostic imaging and attenuates the effective light dosethat reaches targeted chromophores in laser therapeutics[1]. Light-based diagnostic techniques and therapeuticswould likely be improved if scattering could be reduced,thereby enhancing light penetration into human skin.One method to enhance light penetration into skin is theuse of hyperosmotic and biocompatible agents that inducean optical clearing effect [2]. Hyperosmotic agents result inrefractive index matching between cells and groundsubstance in the dermis, which is believed to cause areduction in optical scattering. Studies done by Vargas et al.[3] and Wang et al. [4], demonstrated that the injection ofhyperosmotic agents into rat dermis could significantlyreduce light scattering and thereby enhance the imagingdepth of non-invasive techniques such as optical coherencetomography (OCT). If delivered by injection into thedermis, these hyperosmotic agents tend to dehydrate theskin and reduce the index mismatch between inter/in-tracellular components [5].Although glycerol and polyethylene glycol (PEG) reduceoptical scattering in human skin, their clinical utility hasbeen very limited. Penetration of glycerol and PEG throughintact skin is very minimal and extremely slow, becausethese agents are hydrophilic and penetrate the lipophilicstratum corneum poorly. In order to enhance skin penetra-tion, these agents need to be either injected into the dermisor the stratum corneum has to be removed, mechanically(e.g., tape stripping) or thermally (e.g., erbium: YAG laserablation). Clinical utility would be improved if the clearingagent could be applied topically onto intact skin and there-after migrate across the stratum corneum and epidermisinto the dermis.In this letter, we present our evaluation of the use oftopically applied optical clearing agents in vitro as well as invivo. Food and Drug Administration (FDA) approvedlipophilic polypropylene glycol-based polymers (PPG) andhydrophilic PEG-based polymers, both with indices ofrefraction that closely match that of dermal collagen(1.47) were studied alone and in a combined pre-polymermixture.To measure the optical properties (diffuse transmissionand surface reflectance) of freshly excised in vitro humanskin, an integrating sphere was used [6] with an inverseadding doubling algorithm [7] to estimate the reducedscattering coefficient (s’) of the samples. The opticalclearing potential (OCP) was defined as the ratio of ms’immediately before and 24 hours after, agent application.Optical clearing agents including, glycerol and PPG- andPEG-based polymers compounds were applied alone, and ina PPG- and PEG-based combined pre-polymer mixture,topically on the epidermal side of the skin sampleswhich were left undisturbed in an incubator at 378C for24 hours. The OCP was found to be significantly higher forthe lower molecular weight combined PPG- and PEG-basedpre-polymer mixture as compared to glycerol or eitherpre-polymer alone.The combined PPG- and PEG-based pre-polymer mixturewas also evaluated for its OCP in vivo. The volar forearm of ahealthy volunteer was pre-cleaned, shaved and, divided intotwo areas, i.e., untreated control and where the combinedpre-polymer mixture was topically applied onto the skinsurface in the form of a thick layer under occlusion. After 2hours, the mixture was removed and cross-polarized imagesof the hair shafts were obtained. Visualization of theintradermal portion of the hair shaft (approximately 1 mmdeep) 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 ahealthy volunteer (that contained vascular telangiecta-sias). Cross-polarized images of the vessels before, and2 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, BeckmanLaser Institute, University of California, 1002 Health SciencesRoad East Irvine, CA 92612-1475.E-mail: snelson@laser.bli.uci.eduAccepted 1 December 2003Published online in Wiley InterScience(www.interscience.wiley.com).DOI 10.1002/lsm.20014 2004 Wiley-Liss, Inc.previously obscured, were seen clearly after the combinedpre-polymer mixture was applied (Fig. 2).The ability of the combined PPG- and PEG-based pre-polymer mixture to enhance the contrast among skincomponents was also studied using OCT. The lateral fieldof view was adjusted to 5 mm and the maximum imagingdepth in control skin was 1.5 mm. The volar forearm of ahealthy volunteer was divided into three, 2 2 cm areas.After baseline images of each area were obtained by OCT,the following were topically applied to the different testareas: combined PPG- and PEG-based polymer mixture,glycerol, or immersion oil. All test areas were covered by ategaderm dressing and subsequently imaged 5, 10, 30, 90,and 120 minutes after occlusion.Surface reflection from the stratum corneum wassignificantly reduced by all three agents after 5 minutesof occlusion. Enhanced contrast among various compo-nents in the stratum corneum, epidermis, and dermis wasseen 30 minutes after occlusion of the combined PPG-and PEG-based pre-polymer mixture. Images obtainedFig. 1. Cross-polarized images of in vivo human skin before (A) and 2 hours after (B), topicalapplication of the combined PPG- and PEG-based pre-polymer mixture. Note enhancedvisibility of the intradermal portion of the hair shaft (arrows, scale bar¼ 1 mm).Fig. 2. Cross-polarized images of telangiectasias before (A) and 2 hours after (B), topicalapplication of the combined PPG- and PEG-based pre-polymer mixture. Noteworthy is thefinding that smaller branches of the main vessel, previously invisible, have become moreobvious (arrows, scale bar¼ 3 mm).84 KHAN ET AL.90–120 minutes after occlusion showed significant im-provement in contrast enhancement (Fig. 3) and anincrease in imaging depth (from 1.5 mm to approximately1.75 mm). In particular, imaging of dermal vasculature andhair follicles was substantially improved. OCT imaging ofthe areas where glycerol and immersion oil were applied,did not show any enhancement in contrast or imaging depthover 120 minutes.These intriguing findings merit further discussion.Although glycerol is an effective optical clearing agentwhen injected into the dermis, it does not penetrate intacthuman skin. Our preliminary experiments show that thecombined PPG- and PEG-based polymer mixture canpenetrate intact skin and reduce dermal scattering andsignificantly enhance OCP. The mechanism of opticalclearing by these topical agents remains incompletelyunderstood. It is, however, speculated that the PPG-based polymer acts as a permeation enhancer because ofits highly lipophilic nature, which allows better penetrationof the hyperosmotic PEG-based polymers, which arehydrophilic.The concept of reduced tissue optical scattering usingtopically applied clearing agents can benefit a number oflight-based diagnostic imaging techniques and therapeuticapplications. Although the combined PPG- and PEG-basedpre-polymer mixture is hyperosmotic with a high refractiveindex (1.47), it was demonstrated by OCT to enhancecontrast among dermal components and to reduce scatter-ing in the stratum corneum, epidermis and dermis, therebyincreasing imaging depth [8]. Optical clearing of skin,in vivo, might improve non-invasive diagnostic imagingtechniques by providing higher resolution of cellularmicrostructure over a greater imaging depth.Reduction in skin surface and dermal scattering mightalso improve light-based (e.g., laser) therapeutics. Thehighly turbid nature of human skin reduces the incidentlight that reaches targeted chromophores. A significantreduction in dermal scattering induced by use of topicallyapplied clearing agents will allow more photons to reach thetarget. This, in turn, should reduce the amount of energy,from lasers and other light sources, required to achieve thedesired clinical effect such as the removal of port winestains and other hypervascular skin lesions, hair, benignpigmented skin lesions, and tattoos.Our results clearly demonstrate that a combined PPG-and PEG-based polymer mixture causes optical clearingin vitro and in vivo when applied topically onto intacthuman skin. This pre-polymer mixture is FDA-approved,safe, effective and well tolerated for optical clearing of skinin vivo. Prospective, comparative, and controlled clinicalstudies on a multi-center basis against accepted treatmentregimens are required so that the role of topically appliedclearing agents to assist the treatment of various derma-toses amenable to laser therapy can be fully defined.REFERENCES1. Jacques SL. Skin optics. Oregon Medical Laser Center News1998.2. Tuchin VV. Tissue optics, TT38. Bellingham, WA: SPIE Press;2000.3. Vargas G, Chan EK, Barton JK, Grady HG III, Welch AJ. Use ofan agent to reduce scattering in the skin. Lasers Surg Med1999;24:133–141.4. Wang RK, Tuchin VV, Xu X, Elder JB. Concurrent enhance-ment of imaging depth and contrast for optical coherencetomography by hyperosmotic agents. J Opt Soc Am B 2001;18:948–953.5. Tuchin VV, Maksimova IL, Zimnyakov DA, Kon IL, MavlutovAK, Mishin AA. Light propagation in tissues with controlledoptical properties. J Biomed Opt 1997;2:401–417.6. Pickering JW, Prahl SA, van Wieringen N, Beek JF,Sterenborg HJCM, van Gemert MJC. Double-integratingsphere system for measuring the optical properties of tissue.Appl Opt 1993;32:399–410.7. Prahl SA, van Gemert MJC, Welch AJ. Determining the opticalproperties of turbid media by using the adding-doublingmethod. Appl Optics 1993;32:559–568.8. Vargas G, Chan EK, Thomsen SL, Welch AJ. Use of osmoticallyactive agents to alter optical properties of tissue: Effects on thedetected fluorescence signal measured through skin. LasersSurg Med 2001;29:213–220.Fig. 3. OCT images of in vivo human skin before (A) and 90–120 minutes after (B), the topicalapplication of the combined PPG- and PEG-based pre-polymer mixture. Noteworthy is theabsence of surface scattering and enhancement of dermal vasculature and hair follicles(arrows, scale bar¼ 200 mm). E¼ epidermis, D¼dermis, HF¼hair follicle, BV¼ blood vessels.OPTICAL CLEARING OF IN VIVO HUMAN SKIN 85

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

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

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

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