# Advances in Permeation of Solutes into Hair: Influencing Factors and Theoretical Models
## 2 Hair Structure and Composition  
### Hair Structure  
Hair consists of a hair follicle, hair root, and hair shaft. The hair follicle is a cystic tissue surrounding the hair root; the inner layer is an epithelial tissue hair follicle, which is connected with the epidermis; and the outer layer is a connective tissue hair follicle, which is connected with the dermis. The hair roots are hidden in the scalp, and their main function is biosynthesis and directional growth. The hair shaft is the part exposed outside of the skin, and it can be divided into three layers known as the cuticle, the cortex, and the medulla, from outside to inside. The cuticle cells, cortex cells, and the space between them are connected by the cell membrane complex (CMC) (Figure 1) [35; 36].  
The cuticle covers the outermost layer of the hair shaft, and it is covered with a stratum corneum layer. It is generally composed of lipid layers and 7 to 10 layers of flat cuticle cells that are staggered and overlapped into fish-scale or tile-like structures. Each cuticle cell has a thickness of 0.5 um, a length of approximately 5 to 10 um, and a total thickness of 5 um in the cuticle layer [37]. The cuticle structure can be divided into 3 parts [11]: the epidermis (cystine content of -12%), the exocutic (cystine content of -15%), and the endo-cutic (cystine content of -3%). The epidermis consists of a thin fat layer and a high-sulfur, protein-rich layer (a-layer, cystine content of -30%) [38]. At the molecular level,  
Figure 1: Structure of the human hair shaft.  
the cuticle is a protein-lipid structure that is severely crosslinked by cystine, and it is not highly organized. This structure provides a barrier to prevent hair damage.  
The cortex, which accounts for 70% to 90% of the hair's weight, is the main part of the hair shaft, and it is an important component in determining the physical properties of hair [11]. The cortex cells are spindle-shaped with thicknesses of approximately 1-6 \(\upmu\)m and lengths of approximately 50-100 \(\upmu\)m [39]. The cells are closely arranged along the direction parallel to the axial direction of the hair, and the edges of the cells are serrated [38]. The cortex cells comprise many macrofibrils, hard \(\alpha\)- keratin intermediate filaments, and coiled-coil ropes. They are arranged spirally to each other, embedded in a cell matrix composed of high-sulfur proteins, and form a very tight fiber bundle, giving hair a certain degree of elasticity, strength, and toughness [40].  
The medulla is located in the center of the hair shaft. Due to different hair types, the medulla may be continuous, intermittent, or absent [36]. Morphologically, the medulla is a loose structure composed of spongy keratin and amorphous substances, and it is generally considered to have no important physiological function.  
The CMC is the only continuous phase in the hair structure [36], and it acts as an adhesive between cuticle cells and cortex cells and has an important influence on the physical and chemical properties of hair. The main components of the CMC are low-sulfur protein and fat, which is the main structural lipid in hair and accounts for 5% to 7% of the hair mass [41]. The CMC lipids are similar to human epidermal keratin lipids, and their main components are ceramide, cholesterol sulfate, cholesterol, fatty alcohols, and free fatty acids [42].