This chapter describes the fundamentals and advances of controlling tissue optical properties. As a major technology, the optical immersion method using exogenous optical clearing agents (OCAs) is discussed. Water transport in a tissue, and tissue swelling and hydration upon its interaction with an OCA, are considered. Optical clearing properties of fibrous and cell-structured tissues are analyzed by using spectrophotometry, frequency domain, fluorescence, IR vibrational, Raman, terahertz, and polarization measurements, confocal microscopy and OCT, as well as nonlinear spectroscopy techniques, such as two-photon fluorescence, SHG, and CARS. In vitro, ex vivo, and in vivo studies of a variety of human and animal tissues, such as eye sclera, skin, muscle, fat, cerebral membrane (dura mater), digestive tract tissue, tendon, blood vessels, and blood, are presented. OCA delivery, tissue permeation, and skin reservoir function are discussed. Imaging of cells and cell flows in optical clearing is also discussed. Some important applications of the tissue immersion technique are described, such as glucose sensing and precision tissue laser photodisruption, as well as other techniques of controlling tissue optical properties, such as tissue compression and stretching, noncoagulating and coagulating temperature action, and tissue whitening.
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