Dry Skin Testing 101: Measuring Skin Hydration Using Spectroscopic Techniques

Dr Samuel Gourion-Arsiquaud

Director, Skin & BioSubstrates

Dr Philippa Cranwell

Technical Content Creator

Flaky, itchy, tight….all adjectives associated with the dry feeling of skin, or ‘xerosis’ as it is known medically. This short blogpost will look at the causes of dry skin, how spectroscopy can be used to assess the efficacy of products, and provide case studies from TRI, allowing you to work towards restoring your customers’ radiant glow. 

Understanding dry skin: causes and symptoms

Xerosis, or dry, flaky skin, is characterized by disturbances in normal skin exfoliation (defined as the loss of dead skin cells from the skin surface). Instead of an ordered, gradual loss of dead skin cells from the surface of the stratum corneum, dry skin is characterised by skin inflammation, abnormally high proliferation and differentiation of epidermal skin cells, and by chunks of corneocytes being removed from the surface as scales. As result dry skin is dull, rough, flaky, tight, itchy, irritated and has reduced elasticity.

There are many potential causes of dry skin, some coming from inside the body and some coming from external insults to the skin barrier. Dry skin can be associated with nutritional deficiencies, genetic factors, and age. Some people, for example, can have a genetic abnormality that means they are deficient in filaggrin, one of the key ingredients in skin natural moisturising factor (NMF), and are thus more prone to dry, scaly skin. In addition, dry skin can arise from environmental insults. Cold winter air, and associated low humidity, can lead to skin dehydration and disturbances in skin barrier function, starting the cascade of events that lead to skin inflammation, further barrier impairment and scaling. Finally, the use of solvents and aggressive cleaning agents can also disturb barrier function and give rise to skin dryness.

Dry skin is particularly common on the lower leg, waist, back, arms and abdomen, with dryness sometimes severe enough to cause bleeding and fissures.

For a more detailed overview of dry skin (xerosis) try watching The Science of Dry Skin and Moisturisation Technologies by Prof K. P. Anathapadmanabhan (University of Cincinatti) in the TRI Library.

Restoring a rosy glow: common treatments for dry skin

There are four main approaches for developing products to treat xerosis:

1. Use of a humectant within a formulation, for example glycerol, urea or natural moisturizing factors (NMFs);

2. Protection of the natural barrier and prevention of lipid-stripping, for example by using mild cleansers and soap formulations;

3. Use of bio-sympathetic products that support and enhance the natural biological processes, for example exfoliation;

4. Enhancement or re-building the lipid barrier by replenishing lipids in the stratum corneum, and/or using materials that prevent lipid loss, for example mineral oils.

Measuring skin dryness: past, present and future

Traditionally, evaluation of skin hydration, or the impact of specific product upon skin dryness, was undertaken either using skin surface conductance or capacitance measurements, and, while thoroughly established and a mainstay of many laboratories, these techniques have several drawbacks. For example, conductance and capacitance results can be affected by the presence of electrolytes in test formulations or sweat and are not correlated directly to the water levels in the sample (conductance and capacitance are both indirect measures of water content). 

The use of vibrational spectroscopy to measure hydration levels is relatively new, but provides an alternative, direct technique that can give quantitative, reliable data, and provide insights into a product’s performance across the layers of the epidermis: the stratum basale, the stratum spinosum, the stratum granulosum, stratum lucidum, and the stratum corneum. The two most common spectroscopic techniques for measuring hydration levels are infrared (IR) and Raman, both of which are available at TRI Princeton. 

Measuring skin dryness: vibrational spectroscopy

Both IR and Raman spectroscopy are techniques in which chemical bonds are excited and use specific wavelengths of light to interact with a sample, in this case a biological sample: the skin. Importantly, these methods are non-destructive, meaning that both can be used in-vivo for clinical evaluation and for ex-vivo experiments. In addition, skin samples can be re-used after spectroscopic evaluation for additional analysis; there is no need for dyes, labels or stains, and key structural information in relation to proteins or lipids can be collected. When used in tandem, these spectroscopies are extremely powerful and can give mutually reinforcing data outputs. In addition, both techniques provide semi-quantitative data allowing for comparison between samples, giving an added dimension. 

Infrared (IR) or Raman: do I need to choose?

Although both techniques can be used to successfully observe a range of chemical bonds, IR is better when monitoring compounds containing chemical bonds with a strong dipole, for example O–H, N–H and C=O, whereas Raman is useful for studying bonds that are non-polar but have a polarizable electron cloud, for example C–H, C–O and C–N. This means that when using both techniques in tandem, a range of different molecules can be monitored, including water content, lipids and active ingredients e.g. caffeine. If possible, both IR and Raman data should be collected, as both can help build a more complete picture. For example, if a sample contained caffeine and a client wanted to know how far the caffeine penetrated into the skin, IR and Raman could be used to monitor for the presence different bonds across a transect of sample giving richer data, Figure 1. 

Figure 1: Molecule of caffeine. Bonds highlighted in blue are most suited to IR spectroscopy as they have a strong dipole. Bonds highlighted in green (C–H) are most suited to Raman spectroscopy as they are easily polarised. The C=N bond, highlighted in yellow, can be easily monitored by both techniques.

Case study: using confocal Raman spectroscopy to accurately probe skin hydration

Confocal Raman spectroscopy (CRS) enables the monitoring of particular chemical bonds either at a specific wavelength or over a range of wavelengths, allowing measurement of spectra in either two or three dimensions and, importantly, in vivo. When considering skin hydration, CRS can be used to measure the presence of water, fats or lipids and natural moisturizing factor (NMF) across the five layers of epidermis. During assessment of moisture levels, CRS allows for accurate investigation of depth, which can enable monitoring of active ingress through the skin or change in moisture-content. For example, Figure 2 shows the use of CRS to compare the hydration profile of skin up to a depth of 20 μm. In this example Product A only showed slightly elevated levels of hydration when compared with the control, whereas Product B led to significant hydration of the stratum corneum, as well as penetrating further into the epidermis. 

Figure 2: Use of Confocal Raman spectroscopy (CRS) for monitoring skin hydration and comparing a control with products A and B. Areas of lower water content are blue, and higher water content are red. The external face of the stratum corneum is indicated by a red line, and the beginning of the epidermis by a green line. The arrows show relative increase in levels of hydration when compared with the control; Product A gives slightly more hydration and Product B significantly more hydration.

Case study: using IR spectroscopy to compare scalp hydration when using shampoo

At TRI we use specific type of IR instrument, an FT-IR, which gives excellent spectroscopic quality and highly reproducible data.  In particular, this instrument allows for monitoring of specific bonds and measurement of spectra both in vivo and ex vivo. However, it is generally used to investigate the surface of the skin rather than penetrating the skin (which CRS does), therefore direct measurement of a sample, or tape-stripping, can be used. If penetration studies are required, sections of sample need to be prepared (ex vivo). 

Beside regular FT-IR instruments, which provide average molecular information of the sample without spatial information, advanced FT-IR imaging spectroscopy is also available at TRI Princeton. These systems can generate hyperspectral images, where the spectroscopic parameters can be spatially correlated inside samples allowing for investigation and visualization of water content, or a specific ingredient, inside skin samples.

As an example, researchers at TRI Princeton have used FT-IR spectroscopy to measure differences in the hydration levels of scalp when using a hydrating shampoo. Data were collected directly from the participants, Figure 3(a), and through comparison of the O–H peak areas – for water content the O–H peak intensity will increase with more hydration – and normalization of the N–H peak areas – due to amino acids present on the skin surface therefore the quantities present is unlikely to change – the relative water content could be determined, Figure 3(b). These data clearly show that the hydrating shampoo leads to more moisturization in the scalp. In addition, the long-lasting hydration benefits can be also investigated using this method.

Figure 3: Use of FT-IR spectroscopy for monitoring scalp hydration. (a) Data were collected directly on the scalp from participants; (b) comparison of water content between the untreated group and treated group with hydrating shampoo.

How can TRI Princeton help me?

TRI Princeton provide a suite of analytical services that can measure hydration levels in both hair and skin. Contact us to arrange a call with one of our experts to discuss use of spectroscopy within a project, or to gain further information about our services.