are called disulfide bonds (Toscani & Fino, 2024; Milady, 2023). Disulfide bonds are chemical bonds responsible for chemical changes, supporting a secure bond connection between other sulfur molecules. Disulfide bonds can permanently reform other cysteine amino acids, chemically altering the hair’s composition. Examples of this chemical change are noticeable among the visible part of a curly hair strand permanently altered into straight hair and vice versa (Toscani & Fino, 2024; Milady, 2023). Disulfide bonds are the only bonds facilitating the action of chemical services. Manipulation of the disulfide bonds alters the curl formation of the hair strands. The hair shaft The hair shaft is an elongated strand of fibrous protein continuously growing and protruding out of the skin’s epidermis. Keratin is responsible for the hair’s characteristics and growing ability. Keratin is a protein facilitating the hardening and maturing of all horny growths such as claws, hoofs, nails, and hair. Within the production of hair, keratin fills the developing hair strand within the derma papilla; this process is called keratinization. Keratinization involves the hardening and maturing of the new hair as it journeys upward in the hair follicle (Milady, 2023). The hair shaft has three layers: the outer (cuticle), the inner (cortex), and the innermost (medulla). The cuticle layer is a hard-coated cell, a protective sheath shielding the internal parts of the strand. It is formed by transparent scale-like cells overlapping each other on the outer part of the hair shaft, resembling shingles on a roof (Milady, 2023). Lynam (2024) described the cuticle layer as an essential part of the hair because it supports the health of the hair shaft. The cuticle layer consists of protein-based scales. The scales can open and close through activations of heat and cold environmental conditions and acidic and alkaline product ingredients. Milady (2023) noted that the rising and closing of the cuticle layer establishes different porosity levels of the hair. Porosity levels determine how much moisture is absorbed through the cuticle layer. Lynam (2024) noted that many cosmetologists refer to the cuticle layer as the “style layer” because it determines the hair’s ability to absorb moisture, how the hair feels, and how the style holds. When the cuticle scales rise, ingredients and chemicals can pass through the cuticle layer into the middle layers of the hair’s structure. The cuticle layer will only rise from heat and products high in pH, such as alkaline products, or strong alkaline-based chemicals, such as relaxers and permanent wave solutions. The inner layer of the hair strand is called the cortex (Milady, 2023). The cortex is a fibrous protein core of amino acids, peptide bonds, and cross-bonds. This unique protein structure provides elasticity, strength, and natural color to the hair shaft as well as 90% of the hair’s weight. Melanin cells (melanocytes) deposit into the hair cortex from inside the dermal papilla. This process distributes melanin particles into the cortex, which determines the natural color of the hair strand. Chemical changes in the hair take place in the cortex during chemical service applications (Milady, 2023). The innermost layer of the hair shaft is called the medulla. A medulla is not present in all hair strands, and it does not exist in very fine or naturally blonde hair. The medulla seems to support the larger diameters of hair textures, accommodating the extra space required for coarse and medium hair textures. It is composed of round cells, referred to as the marrow or pith of the hair shaft (Milady, 2023).
Salt bonds are affected by pH changes in the hair. Hair has a normal pH of 4 to 4.5. Substances with a pH of 5.5 or higher cause changes in the salt bonds’ original bonding grip. Salt bonds temporarily connect any two amino acids. When the hair returns to its normal pH, the salt bonds reform and stabilize their initial connections. Salt bonds support one- third of hairs’ strength (Toscani & Fino, 2024). The remaining cross-bond is the sulfur bond, which is much different from the hydrogen and salt bonds. The sulfur cross-bond is the strongest of the three, a chemical element consisting of cysteine amino acids that allow attachments to other sulfur-containing molecules. Sulfur cross bonds permanently connect two sulfur amino acids, such as the sulfur atoms of the COHNS elements; for this reason, they The structure of hair Hair is an adornment and representation of individuality. Yet, most people do not know or take note of the scientific factors of hair. Falde (2023) described hair as a defining characteristic among the mammal species. Many mammals share similar hair strands that do not exist in all mammals. As in humans, bodies partially covered with short, fine hair are a significant aspect of hairless mammals, including whales, dolphins, rhinoceroses, elephants, and hippopotamuses. According to Falde (2023), hair is an intricate part of the skin, more complicated than its appearance. Hair is much more than a retention of heat and protection of the skin. It also serves as a representative of gender identity and sexual dimorphism, a tool of attraction for potential mates, a reflector and absorber of sunlight, a protective barrier from foreign particles, and an assistant in transmitting sensory information. The hair’s many attributes all contribute to its structure. Hair is the product of the functions of the skin and, specifically, the skin’s two-part structure, consisting of the hair root (beneath the skin surface) and the hair strand (protruding above the skin surface) (Milady, 2023). Both parts of the skin, the epidermis and dermis, play essential roles in developing hair. The epidermis shows off the results of the production of hair by the dermis and supports hair’s healthfulness, manageability, and endurance. The dermis (true skin) is responsible for the hair’s development, growth, healthfulness, and sensory capability (Fernandes et al., 2023; Milady, 2023). Hair anatomy Figure 5: Hair Anatomy: The Structure of the Hair
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