Int J Thermophys (2013) 34:1489–1498 DOI 10.1007/s10765-013-1523-y Genetic Algorithm for Opto-thermal Skin Hydration Depth Profiling Measurements Y. Cui · Perry Xiao · R. E. Imhof Received: 11 January 2012 / Accepted: 27 September 2013 / Published online: 15 October 2013 © Springer Science+Business Media New York 2013 Abstract Stratum corneum is the outermost skin layer, and the water content in stra- tum corneum plays a key role in skin cosmetic properties as well as skin barrier func- tions. However, to measure the water content, especially the water concentration depth profile, within stratum corneum is very difficult. Opto-thermal emission radiometry, or OTTER, is a promising technique that can be used for such measurements. In this paper, a study on stratum corneum hydration depth profiling by using a genetic algo- rithm (GA) is presented. The pros and cons of a GA compared against other inverse algorithms such as neural networks, maximum entropy, conjugate gradient, and sin- gular value decomposition will be discussed first. Then, it will be shown how to use existing knowledge to optimize a GA for analyzing the opto-thermal signals. Finally, these latest GA results on hydration depth profiling of stratum corneum under different conditions, as well as on the penetration profiles of externally applied solvents, will be shown. Keywords Depth profiling · Genetic algorithm · Hydration · Inverse problem · Opto-thermal radiometry · Stratum corneum 1 Introduction The water in stratum corneum plays a key role in many aspects of skin functions, such as the cosmetic properties and the barrier functions. However, to measure the water Y. Cui · P. Xiao (B ) Faculty of ESBE, London South Bank University, 103 Borough Road, London SE1 0AA, UK e-mail: xiaop@lsbu.ac.uk P. Xiao · R. E. Imhof Biox Systems Ltd, 90 London Road, London SE1 6LN, UK 123