Scanning Electron Microscope (SEM) image of human hair

WLI image of human hair with height heat map

Digitally flattened WLI image of human hair with height heat map

The structure of hair is intimately linked to its chemistry, which imparts particular physical properties and characteristics. Hair is an outgrowth of the epidermal region of the skin, comprising the hair follicle and the hair shaft. New cells are continuously produced in the lower portion of the hair bulb and push the previously formed cells upwards. As cells reach the upper portion of the bulb, they begin to change into six cylindrical layers. Melanocytes in the hair bulb produce the melanin pigment which gives hair its colouration. The structure and chemistry of these hair features can be explored through extraction, drying, and surface analysis using SEM (Scanning Electron Microscopy) and mass spectrometry, specifically ToFSIMS (Time-of-Flight Secondary Ion Mass Spectrometry).

In the mid-follicle region, the growing cells die, harden, and form the hair, which is a mixture of hair proteins such as keratin. These proteins and protective fatty acid esters on the hair surface can be characterised analytically to monitor the effects of hair treatments and damage. The most abundant surface lipid on human hair, 18-methyl eicosanoic acid (18-MEA) can be readily detected using ToFSIMS, from which its relative surface levels can be directly related to the surface treatment(s) applied (1). 18-MEA is the only covalently bound surface lipid, the level of which is related to the gloss or sheen of hair, and its removal during, say, bleaching processes can be duly measured (2).

ToFSIMS image of human hair with false colour overlay of alkyl sulphate (pink) on keratin (green)

The hair shaft comprises dead cells and 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.

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 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 (3). These dimensions vary with age and ethnicity (4). 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 or hair straighteners, which compress the scale heights and again produce a glossy hair finish.

3D SEM (3D scanning electron microscopy)

Available software provides the capability to convert pairs of stereoscopic digital SEM images into 3D representations of the area sampled. The resultant calibrated image contains Z (height) information, allowing metrology of macro- and micro-areas. Through this, the metrology of materials which are difficult to determine by other methods (such as rough surfaces, angular metals, cutting tools, fibres) can be easily resolved. Furthermore, compared to WLI methods, the lateral resolution in X and Y is significantly superior (5). The use of 3D SEM allows for quantification of subtle structural changes on hair. This data can be leveraged to support R&D decisions, optimise active ingredient delivery, and provide evidence for scientifically robust claims.

White Light Interferometry (WLI)

WLI is an optical metrology technique that provides measurement of the physical characteristics of a material, including micro-topography, form and texture surface topography, roughness, dimensional metrology, and layer thickness measurement. The resolution of the measurements is <1 μm in the x and y axes and <1 nm in the z-axis, allowing for micro-features and topographic variations to be monitored in detail.

Processing of the raw data allows for the generation of a range of data formats including pseudo-colour height maps, profilometry, contour map plots (axonometric plots), 3D modelling, and film thickness for sufficiently transparent layers.

WLI is ideal for characterising the effects of hair applications, such as conditioners, on the surface structure modifications achieved from their use. This data can then be correlated with qualifiable product performance metrics such as texture and reduced friction, allowing manufacturers to make rationalised and evidence-based decisions when refining product formulations.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS)

In ToF-SIMS the sample surface is bombarded with a pulsed beam of primary ions. Secondary ions are sputtered from the sample surface and then mass analysed to provide detailed surface chemical information on elements, chemical groups, molecules, and polymer fragments. ToF-SIMS is analytically highly sensitive (<ppm detection levels) and although it is not directly quantitative it can provide some semi-quantitative information.

ToF-SIMS analysis provides mass spectra and chemical species spatial distribution images. The retrospective data processing capability of the instrument is particularly useful when a selected area of the sample is scanned in spectrum-per-point mode. In this mode a mass spectrum is acquired at each (μm scale) pixel point for the field of view and the resulting two-dimensional array of spectra is used to construct area specific species maps.

The versatile chemical mapping of molecular species available from the ToF-SIMS technique has been applied to the study of residual substances on the surface of human hair. These include both natural (e.g. sebum) and synthetic (e.g. hair products) materials. Human hair has a diameter of around 60 - 70 μm. The sub-micron capability of the SIMS technique generates spectacular high-resolution images showing the spatial distribution of residues with ppm sensitivity, as can be seen in figure 5 where brightness of colour indicates abundance.

ToFSIMS image of shampoo treated / rinsed cross sectioned human hair

Product residues can be readily identified and their spatial distribution followed using this technique. This is important in hair product development, not only from an efficacy standpoint but also in terms of ensuring potential adverse effects are addressed effectively.

In one recent case, work was undertaken for a multinational manufacturer of personal care products to investigate the distribution of key surfactant ingredients following treatment of human hair with a range of formulations. Through ToF-SIMS analysis in mass spectral and imaging modes, various formulation components on the hair were identified:

• Protein, detected as CN- and CNO-, from keratin
• Silicon oil (polydimethyl-siloxane - PDMS) and quaternary ammonium compounds from conditioning agents
• Alkyl sulfates from surfactant additives
• Lipids, detected as palmitate, oleate, stearate and 18-methyleicosanoate species, from the hair structure

The ToF-SIMS analysis showed the distributions of these key species, allowing for formulation modification that optimised coverage of the surfactant and other additive species on the hair surface following washing treatment.

Surface chemical mapping and topographical profiling have become increasingly valuable tools in the development of hair care products, supporting areas such as formulation research, failure analysis, manufacturing investigations, and claim substantiation. Over the past few decades these techniques have enabled a deeper understanding of how treatments interact with hair at the microscopic level.

This information is often vital in matching the product formulation to the stringent requirements of regulatory authorities in major market areas such as the USA.