34 TESTING
keratins along with binding material and small amounts of water. Terminal hairs on the scalp are lubricated by sebum from the sebaceous glands of the follicle which can lead to greasy hair. The uptake of product ingredients from greasy hair treatments can be monitored using surface analysis techniques such as ToFSIMS and XPS (X-ray Photoelectron Spectroscopy). The keratin strands in the hair cortex lie
longitudinally through the length of the hair and low-sulphur keratin fibres are compressed into bundles held together by a mass of sulphur- rich keratins in a fibre-matrix. The fibre-matrix is strong, giving hair its particular physical properties. The outermost layer of the hair (the cuticle) is made up of between six and ten overlapping layers of long cells or scales. Each of these scales is around 0.3 μm thick, 100 μm in length, and 10 μm in width. These dimensions vary with age and ethnicity. The scales lie along the surface of the hair like tiles on a roof, with their free edges directed towards the tip. Microscopy techniques such as SEM allow
scale height measurements to be made in relation to the use of conditioning treatments, where a glossy appearance may be indicative of scale coverage, or to the use of curling tongs/ hair straighteners, which compress the scale heights, again producing a glossy hair finish.
Physical properties of hair The physical properties of hair depend on its geometry. Physical properties such as elasticity, porosity and texture can vary widely from hair-type to hair-type, and the surface physical condition arising from hair care products can vary. These variations allow a range of treatments to be exploited to create different effects and finishes (dye treatments, perms, shampoos, conditioners).
Elasticity Elasticity enables human hair to resist forces that can change shape, length, and volume, allowing it to recover shape without damage. Wet hair can stretch by up to 30% and return to its original length when dry. Over-stretching hair leads to permanent damage and shaft breakage. Hair elasticity is dependent on the keratin
fibres in the cortex. Chemical treatments of hair, such as bleaching, affect the cortex and such damage changes the hair’s elasticity. Hair with poor elasticity stretches less, will not curl, breaks easily on grooming and is difficult to perm. The elasticity of hair is a key parameter to determine when developing hair treatments.
‘Static’ ‘Fly away’ hair is caused by static electricity. When dry hair is rubbed, static electricity builds up on the hair shafts with like charges repelling each other to give the fly-away effect. High levels of conditioning agents in shampoos and/ or conditioners reduce this phenomenon. The distribution and location of components
from shampoos and conditioners can be monitored using imaging ToFSIMS to produce chemical maps. Strategies to counteract the effect include the use of positively charged
PERSONAL CARE November 2025 Figure 3: Digitally flattened white light interferometry (WLI) image of human hair with height heat map
polymers to neutralise the negative charges built up through combing.
Texture Hair texture is related to the diameter of individual hairs (larger hair diameter gives ‘rougher’ hair) weathering exposure and the use of hair treatments. Hair texture can be monitored using white light interferometry which allows the surface roughness and scale heights to be quantified with nanometre resolution.
Porosity The porous nature of hair shafts is exploited by semi-permanent changes such as dyeing and
perming. Treatments open the scales on the cuticle for penetration, enabling reaction with the keratin to produce the effect. Over time, the scales gradually close up again to protect the hair shaft. Damage to the cuticle and scales makes
the fibres prone to split ends. White light interferometry and 3D SEM can be used to monitor surface damage, while mass spectrometry, and specifically ToFSIMS, can be used to monitor the distribution and location of perming and dyeing ingredients. An understanding of the physical and
chemical properties of human hair has been instrumental in the growth of that part of the consumer healthcare industry. The surface
www.personalcaremagazine.com Figure 2: White light interferometry (WLI) image of human hair with height heat map
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