Hair Structure and Morphology Revisited 2024-2025 PDF

Hair structure and morphology revisited Advanced Cosmetic Science G. Daniels 2024-2025  Lecture plan Hair chemistry and hair fibre morphology Hair cuticle and cortex structures Current scientific knowledge of different hair types Approache...

Hair structure and morphology revisited Advanced Cosmetic Science G. Daniels 2024-2025  Lecture plan Hair chemistry and hair fibre morphology Hair cuticle and cortex structures Current scientific knowledge of different hair types Approaches to product formulation for diverse consumer needs 2 Hair chemical composition http://www.livescience.com/15819-ancient- (Trevor and Randal, 2012, pp.2-3) egyptian-hair-product.html Hair is made of proteins; however their composition and structure make them different from the proteins in living tissue The presence of sulfur-containing proteins renders the hair resistant to mechanical damage and to chemical degradation, thus being an useful protection of our brains/eyes Lipids are present, in the intercellular spaces (will revisit in a few minutes) 3 Hair chemical composition: proteins True or False? Amino acid: All protein chains have the same backbone? Carboxyl Amino H group group Peptides and Peptide bond: Protein: 4-Dec-24 G Daniels 4 The amino acids (AAs) of hair Important properties of amino acids relevant to hair The amino acid which are the building blocks of the hair fibre are the same as in other (living tissue) but are organised in a specific way. Hair proteins: primary structure Primary structure: % presence of different amino acids (Aas) in the hair fibre Cystine (sulfur-binding): the highest content Polar acidic content is higher than other amino acids – this makes the hair absorb and hold water AA with additional acidic group of higher % than basic AAs, this gives wet hair overall negative charge Some grooming techniques cause intentionally or unintentionally chemical reactions in the hair which alter the amino acid residues (Trevor and Randal, 2012, pp.2-3) 6 Hair proteins: secondary structure As proteins (made up of 20 different amino acids) have so many peptide bonds and residues, they fold into 3D shapes. There two common shapes for hair proteins i) Alpha helix is stabilised via hydrogen bonds ii) Beta pleated sheets stabilised by a variety of bonds (mostly hydrogen) The presence of the side functional groups (called amino acid residues) provides the opportunities for inter and intra molecular bonding Many of the residues are hydrophilic groups hence they attract and bind water https://www.youtube.com/watch?v=cMhkt2H3kpE Water is essential for maintaining the proteins’ 3D structural configurations, hence some water is constantly bound to hair proteins Our hair is NEVER completely dry 4-Dec-24 G Daniels 7 Secondary structure bonds  Hydrogen bonds  Salt bridges (electrostatic bonds)  Chemical bonds (disulfide and isodipeptide)  Hydrophobic  Intermolecular  Intramolecular Disulfide bond Thiol group Thiol groups oxidise to form disulphide bridge This process occurs in the follicle The disulfide bonds play a key role in stabilising protein structures (more in a Disulphide bond few minutes) This also plays a role in hair shape formation Some grooming and UV radiation cause an irreversible breakdown of the disulfide bonds 9 Hair structure: crystalline regions A large proportion of the hair is composed of proteins that are twisted in “α-helix” shapes: approx. 3 and 2/3 amino acids per turn Hydrogen bonds play a major role for the stabilisation of the α-helixes A hydrogen bond links the carbonyl oxygen of each amino acid in the polypeptide chain, with the imido hydrogen in an amino acid three positions away from it. Hydrogen bonds are weak, but due to the Right-handed α-helix high number present, they impact (Swift, 1997, p23) substantially on the shape and elasticity of hair: https://www.youtube.com/watch?v=4l1Hjf6DBW0 10  Crystalline hair protein structure Two coils coil around each other forming a dimer. The coils are held together by intermolecular forces between the aliphatic groups of the non-polar amino acids and some electrostatic attraction. This renders the crystalline structure hydrophobic Electrostatic attraction between basic and acidic groups Van der Walls forces between (Swift, 1997, p.28) aliphatic side chains  Hair structure: crystalline regions - IFs Coiled coils are formed (dimers) Left handed twist These dimers are packed tightly into more complex groups of 32 strands which form small building (Swift, 1997, p.24) blocks called Intermediate Filaments (IFs): approximate diameter: 10nm This structure leads to the specific mechanical properties of hair: tensile strength is high, as well as ability of hair to stretch a lot before breaking 12  Hair structure: amorphous matrix (Keratin-associated proteins or KAPs) Intermediate filaments (IFs) IFs are embedded in a matrix of keratin-associated proteins (KAP) The KAPs are amorphous (globular), with β-sheet segments and kinks, stabilised by hydrogen bonds and S-S bonds The KAP structures have high level of sulfur containing amino acids (½ cystine) It is well recognised that the KAPs are chemically cross-linked with the IF proteins These bonds are formed by the sulfur-containing amino acids from the KAPs Matrix and adjacent IFs reacting with each other (KAPs) The KAP support the elasticity of the hair fibre, the ability to stretch to about 5-10 % and return to its initial length when the force is released 13 Hair fibre shaft: structural schematics Zhang Y, Alsop RJ, Soomro A, Yang F, Rheinstädter MC. 2015. Effect of shampoo, conditioner and permanent waving on the molecular 14 structure of human hair. PeerJ 3:e1296 https://doi.org/10.7717/peerj.1296  Lecture plan Hair cuticle and cortex structures 15 AFM imaging of hair structure  Hair structure Two types of cortical cells exist: Orto cortex cells (more matrix) are found on the outside of the hair crimp Paracortex cells (very little matrix) are found on the inner side of the hair crimp Overall, studies offer inconsistent conclusions on the bilateral distribution of cortex cells However, it is understood that the ratio and distribution of these cells play a role in (Evans, 2012, p.17) hair shape formation. Cortex The inner and the largest structural element of the fibre Consists of spindle-shaped longitudinally orientated cells: dimeter=3µm, length=5-100µm Each cortical cells contains rod-shaped units: macrofibrils SEM of a cuticle cell (Cu) and a The macrofibrils have a complex structure cortical cell (Co) Cortex cells vary (two major types) based on their (Swift, 1997, p.50) structure Melanin (melanosomes) are dispersed amongst the macrofibrils 18 Cuticle The outer structure of the hair shaft Composed of 5-10 layers of overlapping flat cuticle cells Each cell is approx. square with: side lengths = 50-75 µm aver. thickness = 0.5µm Inner ends (proximal) are covalently bonded to the cortex http://www.dralinsyed.com/blog/2015/4/ Outer (distal) ends point in the direction of 5/the-structure-of-hair-part-1 the tip and are not bonded 19 Cuticle cell structure Each cuticle cell has laminated structure: is made up of several compressed layers A-layer (thickness - approx. 110nm): high ½ cystine content ca. 37% (Swift, 1997) Exocuticle (thickness vary from 100to 300nm): high ½ cystine content ca 25% (Swift, 1997) A-layer and exocuticle combined contain approx. 30% of proteins cross linked by cystine – forming a tough and resilient barrier Endocuticle contains significantly lower levels of cysteine 3%. It is mechanically weak and swellable: (Swift, 1997, p.44) 20 Cuticle structure summary Cloete et al, (2019) The what, why and how of curly hair: a review, The Royal Society Publishing External tear and wear of fine straight fibre examples Cuticle shape: chipped; adhesion – reduced; cysteic acid increases These effects are attributed to: chemical oxidative treatments, photooxidation, regular washing and related 22 mechanical damage – for example wet rubbing, or tight pleating Studies have identified the different lipid content of the CMC within cortex, cuticle and between them. Proteins are also present in the CMC but in very small quantities.  Cell Membrane Complex – cont. Cell membrane complex also contains chemically bound fatty acids and some free fatty acids: C18, C18:2, C16 Cholesterol sulfate, cholesterol and ceramides These materials form the β-layers Sandwiched between two β-layers is the δ-layer made up of proteins The CMC is the “weakest link” of the fibre and internal cracks develop under mechanical stress If stress continues, cracks will extend to the CMC between the cuticle and the cortex and a catastrophic failure occurs: fibre breakage There are some differences in the content of the three CMC shown on the image 24 Robbins, 2012 p.77  Water Water binds to the hydrophilic amino acid residues of the hair proteins: this is considered tightly bound water. Such water will remain in hair after drying More water will bind to the tightly-bound water, via hydrogen boning, this water will be considered loosely bound or free water Water is found in the KAPs, whilst the IF are hydrophobic Water acts as a plasticiser - the hair is less rigid Relative ambient humidity and hair are in equilibrium At low humidity (up to 60%) the hair fibre Barba et al, (2009) contains approximately 10% water 25 Melanin Tyrosine (amino acid) Melanin biosynthesis = melanogenesis Key function of melanin: UV protection of hair. Melanogenesis = regulated by specific enzymes Dopaquinone + Cysteine Eumelanin (dark Pheomelanin brown/black colour) (yellow/orange colour) 26 Hair fibres cross sectional images: melanin distribution with each fibre 27 Current scientific knowledge of the different hair types 28  Some defined differences CI= confidence interval Cross sectional area differences Ellipticity differences Overall shape differences Mechanical properties, as defined by the above: different Evans and Wickett (2013), p.204 29  Cross sectional area CSA The cross-sectional area reflects how “thick” the fibre is and thus partially affects its strength; Asian hair has the highest CSA, followed by African followed by Caucasian The hair ellipticity = the ratio between the longer and shorter diameters. Highly elliptical hair tangles more and appears to suffer more surface and load damage Cross sectional area Caucasian 3857 μm2 ± 132 African 4274 μm2 ± 215 Images reproduced with the permission of Dr Roger McMullen, TRI Symposium on Franbourg et al., J. Am. Acad. Dermatol., 48, S115-S119 (2003). 30 textured hair (23 Sept 19) NJ  Cuticle differences The exact shape of the hair fibre is determined by the shape of the hair follicle in the zone of keratinisation Scan Electron Micrograph shows below: African hair = highly elliptical cross section, with multiple twists along the fibre axis Asian hair=round cross section, mostly straight Caucasian hair=elliptical, but not twisted Bushan (2010), p.4 31  Cuticle differences cont. Surface properties: 1. Shine: Asian hair is the shiniest. Higher percentage of specular light (reflection from the cuticles) vs low diffused light (the melanin in the cortex absorbs the light and reduces transmission and defused light) 2. Cuticle spacing; the highest is in African hair, the lowest in Asian 3. Overall number of cuticle layers: African hair has 1-2 layers less on the side of the minor axis 4. The protein composition of the hair is not different to an extend that influences products and treatments Evans and Wickett (2013), p.203 32 Hair assembly properties as based on: Hair density Hair colour Hair shape Hair growth rate Evans and Wickett, 2007 pp.196-197 33  Global hair shape classification The hair fibre shape is critical for the assembly properties Hair shape also determines hair manageability: combing forces and style retention On that basis, hair integrity and quality is dependent on the shape Loussouarn et al, 2007 34 Caucasian hair specific data Caucasian hair shows the greatest variability in shape, size and colour than the other types Hair fibre becomes finer with age too A different study of Caucasian hair suggest a “fine” hair cross sectional area average of 2000-2500 µm Bouabbache et al (2016) 35  SEM images of highly elliptical and curly hair Damage: cuticle removal Images reproduced with the permission of Dr Roger McMullen, TRI Symposium on textured hair (6 June 2023) NJ 36 Hair knots and breakage – the effect of shape Caucasian (straight) hair knotting (single fibre) Complex knotting in African (kinky) hair and corresponding surface and structural damage Aguh and Okuye, 2017 37 The tensile curve (tensile deformation of the hair fibre) Post-yield region =breakage of disulphide bonds, allowing for cracks within the cortex Yield region: breakage of H-bonds in the alpha helixes and unfolding = changes in IF structure Elastic region 5% strain: hydrogen and ionic bonds, some disulfide bonds breakage = IF disconnect with the KAPs Graph adaptation from the proceedings of the 7th International Haircare Conference TRI Princeton Hair manageability When hair is straight and circular in shape, its manageability is higher: fibre alignment is better; interfibre friction is lower; mechanical stress is more evenly distributes across its cross-sectional area; volume and shape are difficult to achieve and maintain Wet combing require more force than dry combing When hair is curved and highly elliptical: contact points between non- adjacent fibres increase the chance for knotting Inter fibre friction increases; mechanical stress in not distributed evenly over the cross-sectional area; volume and shape are difficult to achieve and maintain. Wet combing requires less force than dry combing 39 Approaches to product formulation for diverse consumer needs 40 Diverse consumer needs How different in human hair? The difference in geometric shape/cross sectional size and the degree of curl have the most significant impact on how the hair assembly appearance The next level of differentiation is created via chosen hair length and style Temporary and permanent changes are achievable via different cosmetic products and techniques Daniels, G., A., Fraser, Westgate G. (2022) How different is human hair? A critical appraisal of the reported differences in global hair fibre characteristics and properties towards defining a more relevant framework for hair type classification. International Journal of Cosmetic Science. doi: 10.1111/ics.12819 Revision material Bouillon and Wilkinson (2005) The Science of Hair Care, Chapter 1 Lecture slides and notes 43