Assessment of Water Content in Biological Samples by Terahertz Time-Domain Spectroscopy

In recent years, different techniques using terahertz (THz) radiation have been applied in a variety of scopes due to unique properties of this type of emission. One of the most significant areas of interest is the diagnosis of different diseases or stresses through examination of biological tissues and objects. Sensitive and precise tool for noninvasive water concentration measurement would help in such areas as cancer diagnosis, products quality control or the plants' stress responses monitoring. THz radiation is characterized by the strong water absorption, which allows this frequency range to be used as a sensitive noninvasive hydration probe. The study is devoted to the development and testing of the methods for noninvasive measurement of water concentrations in biological tissues and samples by THz TDS. Leaves and fruits from different plants along with samples of pork meat were used as the objects of investigation. The measurement of water concentration in botanic tissues and objects is significant due to the steadily worsening ecological situation in the world. Giving that plants and trees are essential for humanity living, there is a need for noninvasive tools to measure and evaluate stress responses of different types of plants, including response for drought. The investigation of humidity of pork meat is a step towards development of the methods for diagnostics of human skin and organs pathologies. Generally, water content in plants can be monitored using such methods as gravimetric or pressure chamber, which lead to the destruction of the sample. Besides, there also are some non-destructive methods which are used for water content measurement (chlorophyll fluorescence, visual estimate), which are usually not very precise. Moreover, there have already been reported the methods for noninvasive water concentration measurement using THz TDS or continuous THz setups, based on correlation between water content level and attenuation of transmitted through the sample THz signal. However, these methods work in transmission mode, which significantly limits their application. Due to the strong water absorption, only thin tissues can be used for the investigation. In order to examine thicker biological objects (fruit skins and pork meat samples), the reflection geometry was applied. Previous studies demonstrated that water content of human or animal tissues of skin or organs differs in zones with pathologies and healthy zones. It should be noted, that the THz radiation is non-ionizing, therefore in general it is harmless for biological tissues. A water content sensor based on the reflection geometry would be a helpful tool for distinguishing healthy and injured tissues when visual assessment is difficult.

The method of water concentrations determination is based on the extended Landau-Looyenga-Lifshitz model. Equation (1) shows the relation between effective permittivity of a water-containing sample and the dielectric function of its components

$where \epsilon {ˆ}_{mix} , \epsilon {ˆ}_w , \epsilon {ˆ}_1 and \epsilon {ˆ}_2 are permittivities of the whole sample, water, the 1st and 2nd components, respectively; a_w , a_1 and a_2 are volume fractions of water, 1st and 2nd component in the sample.$