Here is a simple research paper I wrote on using capacitance as a medium to measure skin hydration:

Introduction
Previous studies show that capacitance based Fingerprint card sensors, originally designed for fingerprint imaging, can be used for skin hydration imaging, surface analysis, and skin micro relief measurements[1-3]. In this paper, we present our latest work on stratum corneum (SC) dynamic water concentration measurements by using Fingerprint card sensors.

To further validate the measurement results, we will also compare the Fingerprint card sensors results with SC surface hydration results measured by using opto-thermal transient emission radiometry (OTTER) [4,5] and TEWL (trans-epidermal water loss) results measured by using condenser-chamber TEWL method [6,7].

Apparatus
A hand-held probe, based on the FingerPrint Cards area sensor development kit (FPC-AMD FPC6410), has been developed and
used in this research, see Figure 1. It contains a FPC AMD array area sensor chip, a processor board, and a USB connecting cable for connecting to a PC. A dedicated VC++ software has also been developed, which can capture the images, process the images, perform grayscale value calculation, and display the images.

The area sensor chip unit has an array of 152 by 200 capacitive sensors, which generates a 30400 pixels black and white skin
capacitance image with a 50x50um special resolution. In the images, each pixel is represented by an 8 bit grayscale value, 0~255, with 0 represent white (low capacitance) and 255 represent black (high capacitance). The image data are saved in standard RAW image format files.

The average grayscale value and its standard deviation of an image are calculated by averaging all the pixel values which are above a certain threshold.

Neat isn't it, how dielectric field can be used to measure water!