# Applications of Capacitive Imaging in Human Skin Texture and Hair Analysis
## 1 Introduction  
The electrical properties of skin and hair alongside their texture and anatomy provide information about a person's health, efficiency of drug delivery and effects from application of cosmetic products. For these reasons, scientists from a variety of fields use non-invasive instruments to extract such information and achieve experimental results that strengthen their research. As part of this introduction, selected publications are illustrating the above points right after a summary of human skin structure and hair anatomy. Then, the use of capacitive imaging in various research fields is demonstrated by summarizing selected research in the literature. The skin is the largest human organ in terms of surface area and its thickness varies from 5 \(\upmu\)m to 1 mm (or more) in different areas of the body [1]. It protects internal organs from environmental influences, and it regulates the water body loss. As illustrated in Figure 1 (left), the skin is separated into three main layers: epidermis, dermis and subcuits. Among other functions, the epidermis provides chemical and diffusion protection, the dermis protects from external mechanical forces and the subcuits connects the skin to the underlying tissue [1]. The outermost sublayer, target of non-invasive instruments, is called stratum coreum and consists mainly of dead keratinocytes. The textural information of this layer, or skin microrelief, is affected by the internal body health, age and living habits as well as by environmental influences [2]. The visible part of hair (Figure 1 right) is shaped as a three-layered shaft of dead protein filament: the cuticle, cortex and medulla [3]. The cuticle is a thin surrounding layer of roof tiles-like structures with about five degrees inclination from the hairshaft core. The cortex is the thickest layer of the hair, it consists of tightly packed keratin cells and it is responsible for the hair color as well as most of the water holding capabilities. Last, the medulla is a soft unstructured keratin that forms the core in the thicker human hair (e.g., scalp hair) [4]. In the literature, a variety of methods have been used to extract information from the human skin surface. [PERSON] et al. [7] used a capacitive imaging system to measure solvent concentration and penetration in human skin to demonstrate how such a system can support skin clinical trial studies. In the same work, skin damage by intense washing, tape stripping and SLS irritation was characterized. In the area of skin aging analysis, [PERSON] et al. [8] studied the skin furrow response during arm extension based on image analysis of negative skin replicas. They provided evidence that younger people can buffer skin strain between primary and secondary line orientations while elderly subjects tend to have furrows only in one orientation that rotate during extension. In more recent years, [PERSON] et al. [9] achieved better classification between primary and secondary skin lines by measuring their depth using three-dimensional confocal microscopy. Experimental results on 120 Caucasian women confirmed that secondary lines fade with age while the depth of primary lines increases. A different approach of evaluating skin aging was employed by [10; 11], where the surface area of polygons shaped between skin furrow was measured and it was found to associate well with subjects' chronological age. As illustrated for the area of skin research, hair samples are also analyzed by scientists to detect health and cosmetic conditions. [PERSON] et al. [12] reviewed 39 studies that associate hair cortisol concentrations to stress psychiatric symptoms and disorders. This approach was found to be more accurate in detecting long-term stress because cortisol concentration measurements from blood, urine and saliva samples are influenced by living habits and environment conditions. Furthermore, [PERSON] et al. [13] used hair samples from 266 women to determine that hair dyeing and frequent washing does not affect cortisol measurements.