Abstract
With the growing research in the field of optical clearing and the various applications of this technique that have been recently developed, more than 1000 agents have been tested in various tissues in the past two decades to evaluate their clearing potential. To optimize the clearing treatments, knowledge on the dispersions and absorption spectra of the agents is necessary. We have gathered experimental and literature data to show that the absorption bands of typical clearing agents are located in the deep ultraviolet range, where the refractive index is significantly high. The desired characteristics for the clearing agents are presented, and their classification in three major groups is indicated. Solutions containing mixtures of optical clearing agents (OCAs) and diluted solutions are also important for certain applications, such as the enhancement of agent delivery or the evaluation of agent diffusion properties. Such applications are referred, and some examples are presented. A simple method to prepare diluted solutions of clearing agents is also described. © 2019, The Author(s), under exclusive license to Springer Nature Switzerland AG.

Abstract
In this chapter, we will describe methods based on simple measurements that allow one to acquire information of diverse nature. Regarding the characterization of optical clearing treatments and evaluation of their efficiency, we discuss a method to obtain the refractive index kinetics of the interstitial ground medium and another method to obtain the kinetics of the scattering properties of a tissue under study. The evaluation of the diffusion properties for the optical clearing agents and water involved in the fluxes between the tissue and the treating solution is also important. To obtain these properties, we describe in Sect. 6.4 a simple ex vivo method, which from collimated transmittance and thickness measurements allows one to estimate the diffusion time and the diffusion coefficient of these fluids. Such method can be used as a complementary diagnostic tool, since it allows also for discrimination between normal and pathological tissues. Also with the objective of obtaining physiological or pathological information from tissues, we describe in Sect. 6.5 the discovery of two new optical clearing windows in the ultraviolet range, which may turn possible the development of new diagnostic or treatment methodologies. © 2019, The Author(s), under exclusive license to Springer Nature Switzerland AG.

Abstract
In this chapter, other areas of application for optical clearing agents (OCAs) are presented. The osmotic properties of agents are highly important in dermatology, cosmetics, and pharmacology, if topical application to the skin is desired. After addressing this application in Sect. 8.2, tissue poisoning and discussing the osmotic properties of certain poisons or toxic compounds will be done in Sect. 8.3. The importance of evaluating the diffusion properties of those substances in the skin, eye, and other inner tissue is indicated as a tool for optimizing treatment or decontamination dosage and procedures. Section 8.4 is used to discuss the application of agents in food industry. The dehydration capabilities of certain agents, such as sodium chloride or glycerol, are presented, and the advantages of treating fruit, meat, or fish with sugars to improve their organoleptic properties during preservation are also presented. Finally, the application of OCAs for tissue or organ preservation is presented in Sect. 8.5, where some cases for preservation of eye tissues at room temperature made with glycerol will be discussed. The use of OCAs as cryoprotectants at low temperatures is also explained. In all these applications, we refer the applicability of the method described in Sect. 6.4 to evaluate the diffusion properties of water, poisons, or drugs for ex vivo tissue samples. © 2019, The Author(s), under exclusive license to Springer Nature Switzerland AG.

Abstract
To overcome the high light-scattering problem that occurs in biological tissues, we present in this chapter the different clearing methods known today. Most of these methods have benefits and downsides, depending on the application for which they are used. The optical immersion method is introduced as a better, reliable, and reversible way to turn tissues clear. The major benefits and advantages of this method such as its reversibility, the lack of side effects, and application in large wavelength range will be presented. A description of the molecular diffusion of optical clearing agents is given to explain the reduction in the refractive index mismatch that natural tissues have. © 2019, The Author(s), under exclusive license to Springer Nature Switzerland AG.

Abstract
Laser diagnostics and treatment procedures are commonly performed for visible and near-IR wavelengths. The knowledge of the wavelength dependences for the optical properties of various biological tissues in this spectral range is useful for clinical applications. Since the optical properties of human liver have been previously known only for near-IR wavelengths, the aim is to estimate their wavelength dependences between 400 and 1000 nm. Using spectral measurements from liver samples in this range, we determine their optical properties with the inverse adding-doubling method. The obtained results indicate the presence of bile, oxyhaemoglobin and deoxyhaemoglobin in human liver. The combination of these biological components results in strong absorption for wavelengths between 400 and 600 nm, with peaks at unusual wavelengths. For wavelengths above 600 nm, the wavelength dependences for all optical properties present the typical behavior, but strong and shifted absorption observed for wavelengths below 600 nm has been previously unknown and can be useful for clinical procedures with lasers working in this range.

Abstract
This book describes the Optical Immersion Clearing method and its application to acquire information with importance for clinical practice and various fields of biomedical engineering. The method has proved to be a reliable means of increasing tissue transparency, allowing the investigator or surgeon to reach deeper tissue layers for improved imaging and laser surgery. This result is obtained by partial replacement of tissue water with an active optical clearing agent (OCA) that has a higher refractive index and is a better match for the refractive index of other tissue components. Natural tissue scattering is thereby reduced. An exponential increase in research using this method has occurred in recent years, and new applications have emerged, both in clinical practice and in some areas of biomedical engineering. Recent research has revealed that treating ex vivo tissues with solutions containing active OCAs in different concentrations produces experimental data to characterize drug delivery or to discriminate between normal and pathological tissues. The obtained drug diffusion properties are of interest for the pharmaceutical and organ preservation industry. Similar data can be estimated with particular interest for food preservation. The free water content evaluation is also of great interest since it facilitates the characterization of tissues to discriminate pathologies. An interesting new application that is presented in the book regards the creation of two optical windows in the ultraviolet spectral range through the application of the immersion method. These induced transparency windows open the possibility to diagnose and treat pathologies with ultraviolet light. This book presents photographs from the tissues we have studied and figures that represent the experimental setups used. Graphs and tables are also included to show the numerical results obtained in the sequential calculations performed.