A number of papers at the 2006 IFSCC Congress (16-19 October) introduced novel and innovative testing methodologies for assessing the condition of skin and the behaviour of skin care products. SPC reports
It is not always easy for beauty advisors to assess the condition of a client’s skin below the surface, so to help provide a truer picture Pola has developed a non-invasive method for the assessment of interior skin damage caused by ageing and photoageing. Based on near-infrared spectroscopy (NIR), it enables the detection of chemical changes occurring in skin tissue. Skin surface conductance, transepidermal water loss and a replica of skin surface were measured to determine their possible influence on NIR spectra. Measurements obtained in specific wave number ranges were analysed by Principal Component Analysis (PCA) and this was found to be important in distinguishing between ageing and photoageing. The method is believed to offer insight into chemical changes taking place in skin tissues below the skin surface due to damage caused by different factors. Changes in alkyl group composition observed may explain physiological ageing which is known to cause thinning of dermis and a decrease in collagen content. It was also found that photoageing caused a significant change in amide chemistry, which could be related to degradation of collagen protein, resulting in lowering of the molecular weight.
ISP has been using infrared imaging (IR) spectroscopy as a tool for measuring a number of parameters, most recently to investigate the process of corneocyte maturation in the stratum corneum (SC), and particularly using IR imaging to monitor changes in the natural moisturising factor (NMF) within the SC, developing a new semi-quantitative skin imaging measurement. NMF provides the SC with its water holding capacity and is therefore critical to skin health and hydration. The method involves IR imaging of tape-stripped corneocytes from different layers of the SC, enabling the measurement of the relative changes in NMF concentration that occurs with corneocyte maturation in the SC. It also detects changes in NMF levels as a result of skin cleansing; washing skin results in loss of NMF from corneocytes and this can be measured directly and traced with IR imaging of tape-stripped corneocytes.
Nikkol has developed a non-invasive and sensitive method to measure catalase activity in the SC using areas only 6mm in diameter. The method should enable clarification of the relationship between melanin synthesis and oxidant stress by measuring catalase activity in small areas of lentigo/vitiligo skin. Catalase activity in the SC is sensitive to solar UV which indicates that it is a useful parameter for UV-induced skin damage. Nikkol has proposed the following index (USD) as a parameter of UV-induced skin damage.
catalase activity in sun non-exposed skin
USD = non-exposed skin catalase activity in sun exposed skin
The characterisation of photodamaged skin (photoageing and wrinkle formation) by scanning acoustic microscopy (SAM) has been investigated by Kanebo. SAM enables examination of microscopic changes in physical properties. It is said to reveal that the disappearance of the anchoring structure of oxytalan fibres with intrinsic ageing may decrease the elasticity in the papillary dermis and also that fibrosis in the Grenz zone, a histochemical hallmark of photoageing, may compensate for the strength of tissue under the progress of solar elastosis in photoaged skin.
Global parameters
LVMH has developed a new global approach for evaluating apparent facial skin age based on image processing analysis. While there are a number of different signs of ageing that are usually assessed, LVMH was looking to find a single global parameter representing the main changes in the face due to age, using a skin image analysis process. To find this single parameter, two selected criteria were obtained by image analysis of photos of the face and their correlation with chronological age was studied. The two chosen parameters were: the average of the four variance of grey level (statistical parameter representative of the homogeneity of pixel colour) calculated on each part of the face; and the average of the four areas of skin imperfections calculated on each part of the face. To describe the relationship between the real age and the parameters studied (average of the variances and average of the areas) the best-fitting line on a graph was calculated using simple regression analysis. The researchers found that regarding the global area parameter there was a weaker correlation with age, probably due to the presence of dilated pores and acne spots on young women’s faces. But mean variance criteria was found to be a good parameter representative of visual face ageing damages and is expected to be very useful for evaluating anti-ageing products.
Skin sagging is one of the topographical changes that occurs on the face during normal ageing and various methods including visual grading of 2D photos, live view evaluation and more recently 3D imaging have been used to measure sagging. P&G has recently evaluated the use of 3D imaging more thoroughly to quantify facial sagging and to measure the changes in facial sagging with age. To establish the significance of a genetic component women from three generations of families were included in the study. Using the 3D image scanner, two parameters were extracted from the image data and were found to correlate significantly with the visible perception of skin sagging. These sagging parameters were shown to change with age. The biophysical measurement of skin elasticity correlated significantly with the 3D imaging sagging parameters. The decrease in elasticity seemed to precede the increase in sagging (nasolabial fold and eye bag areas). If the decline in elasticity is directly responsible for the increase in sagging, the data suggests that it may take several years before the elasticity changes become manifest visually. A genetic component was also confirmed.
Human sebum is a complex mixture, its composition varying with skin conditions and from person to person. It includes triglycerides, fatty acids, wax esters, squalene, cholesterol and cholesterol esters. Most research into its physical properties has been limited to bulk properties and has been difficult because of sampling. Unilever has explored the potential of Langmuir monolayers as routine studies of physical behaviour of human sebum, using Langmuir isotherms with Brewster angle, fluorescence and atomic force microscopy. Isotherms of human sebum from different people were found to be strikingly similar but completely different from the artificial sebum isotherm. The Langmuir monolayer technique therefore appears to offer a promising tool for studying physical properties of limited samples of human sebum.
Skinspace Sorbent Tapes (SST) is a new sampling method that has been introduced as a convenient method for in vivo sampling of markers at skin surface. It is based on a sorption mechanism that permits highly reproducible sampling in vivo. Chanel has demonstrated its potential with a cosmetic application related to the assessment of skin shininess and the company says it has been able to study the efficacy of a mattifying cosmetic product that was previously impossible. The performance of SST (reproducibility, sensitivity etc) is said to offer great opportunities for the in vivo investigation of tracers related to skin ageing, skin hydration etc on skin.
Material analysis
Looking analytically beyond the skin to the raw materials and skin cream products, Unilever has developed methods to characterise free fatty acid conjugated linoleic (CLA) isomers present in conjugated sunflower and safflower oils. Gas chromatographic (GC) separation procedures were developed to determine CLA isomer profiles and derivatisation procedures were developed to convert conjugated free fatty acids to methyl ester derivatives suitable for GC analysis without changes in positional and geometric isomers. Solvent-based extraction systems were also developed for the isolation of conjugated linoleic acid isomers from skin cream formulations prior to derivatisation and analysis by GC.
But how long does product actually stay on the skin? Kosé has investigated retentivity, ie how long formulations are held in the skin, looking for a safe method of assessing this and achieving results consistent with previous models. Retentivity obviously has a strong impact on the efficacy of the product. Even if a formulation contains highly functional materials it can’t be effective unless the active materials are retained long enough in the skin to do what they’re there for. Kosé focused on determining the accumulation of material in the SC, as the SC functions as a reservoir for the skin. Biotin, a water soluble vitamin-like compound, was selected as the most suitable marker. By measuring the amount of biotin in the SC after a certain time following application of a formulation containing biotin it was possible to monitor the activity under use conditions. Formulations containing biotin, either in a liposome or in the external phase, were used. Tape samples found higher biotin content from the liposome formulation than from the control. Various types of formulations were evaluated with the method to reveal factors that promote retentivity. Kosé believes its new method will become a useful standard tool for evaluating skin care formulations in vivo.