The development of ways of measuring cosmetic effects is driven by increasing pressure on cosmetic companies to provide solid evidence to support product claims. John Woodruff highlights the most important areas
Instrumental methods for efficacy testing of cosmetic products have long been of interest. The first Journal of the Society of Cosmetic Chemists published in 1947 contained an article on cosmetic efficacy testing although the only instrumental method quoted was the use of a spectrophotometer to measure UV absorbance of sunscreen agents.[1] It is interesting to note that the need to determine if these were subject to photodegradation was mentioned.
Papers on efficacy testing have appeared in almost every issue of the journal since that first edition but most methods are subjective. Instrumental methods other than those to measure physical parameters or analytical ones to measure ingredient concentrations of the cosmetic composition were sadly lacking until 1956 when a paper describing the measurements of percutaneous absorption using radioisotopes was published. These tests included in-vivo measurements on human volunteers.[2] Also published in the journal during 1956 was an in-vitro method using radioisotopes to measure absorption by hair[3] and the measurement of the hardness of keratin by in-vitro and in-vivo methods.[4] And 1956 was also the year that the first instrumental method of measuring static charge on human hair was published.[5]
Interest in instrumental techniques grew apace; the author was fortunate to attend the 2nd IFSCC Congress held in London in 1962 where the use of an instrument to measure lipids on the skin surface was described[6] and another presentation was on the measurement of perspiration.[7] The importance of proving cosmetic efficacy by methods of instrumental measurement was now firmly established and it only remained for engineers to design practical instruments that would give reproducible results that could be compared from one test centre to another.
The development of the means of measuring cosmetic effects is driven by increasing pressure on cosmetic companies to provide solid evidence to support claims made for certain types of products. Claims associated with the reversal of the signs of ageing such as reduced facial wrinkling are often subjected to intense scrutiny both from the regulators and competitors and instrumental evaluation is often used in an attempt to provide data to support claims. Other claims include increased moisture levels in skin, improvements in skin texture, elasticity and smoothness, and of regulation of sebum. Nor are claims restricted to cosmetic effects on skin. Hair properties are also measured, such as improvements in hair strength, improved hair conditioning and styling products that retain their properties even in conditions of high humidity.
However this is an area fraught with difficultly as often the limitations of the instruments are not understood. The differences between micro and macro topography of the skin surface and the instruments that evaluate one or the other are often not appreciated. A cosmetic scientist may have a different perception to that of a consumer of what is meant by the term moisturisation. Moisturisation to the consumer is a mixture of textural, frictional and visible factors such as softness, smoothness and lack of visible dryness. Cosmetic scientists regard it as the water content in the stratum corneum and have developed ways of measuring the amount in the skin surface.[8]
Hydration
An understanding of the instruments and their limitations is essential when considering the design of clinical trials to support a claim. The methodology and precautions that have to be taken when measuring skin hydration using the Corneometer were described in the February issue of SPC and on cosmeticsbusiness.com[9] and this instrument is probably the most commonly used of all skin measuring devices.
The measuring principle of the Corneometer is based on capacitance measurement of a dielectric medium and any change in the dielectric constant due to skin surface hydration is recognised by the instrument. Products applied to the skin only have minimal influence on the measurements and even slight changes in the hydration level can be measured. The design of the measuring head is such that the measurement depth is restricted to the first 10-20µm of the stratum corneum. This is important for investigation of epidermal hydration if the influence of deeper skin layers is to be avoided.
Although it is a measure of the water content of the skin, it is only an indirect measure of barrier function, which may be determined by measuring transepidermal water loss (TEWL) through the epidermal surface. The TEWL value is a measure of the rate of water lost through the skin and is an index of the extent of possible damage of the skin’s water-barrier function. Because water loss through the skin normally occurs by passive diffusion through the epidermis, higher TEWL values indicate greater water loss and are consistent with increased damage of the barrier function of the stratum corneum such as may occur during irritant exposure, self-excoriation, or atopic dermatitis.
Firmness & elasticity
Skin firmness is its ability to resist deformation and elasticity is its ability to return to its natural state. Skin elasticity can be measured by the suction method whereby a probe is applied to the test site and the skin is drawn into the aperture of the probe. Inside the probe the penetration depth is determined by a non-contact optical measuring system. The probe contains an elastic spring that provides constant pressure on the skin. The resistance of the skin to be sucked up by the negative pressure measures firmness and its ability to return to its original position measures elasticity.
An alternative method of measurement uses a ballistometer whereby a light pendulum is dropped from a fixed height onto the skin surface and the rebound, which is a measure of firmness and elasticity, is recorded.[10, 11] A different approach uses a device that applies a twisting motion to the skin surface. When the torque is released the time taken for the skin to return to its normal state is measured and this is related to skin elasticity and extensibility.
A new instrument from Courage + Khazaka (C+K) called the Reviscometer RV measures the resonance running time of an acoustical shockwave and determines the direction of collagen and elastin fibres as well as skin elasticity and firmness. The probe head contains two sensors which are placed on the skin. The first emits an acoustical shockwave and the other serves as receiver. Shockwaves propagate differently through the skin according to the state of the elastic fibres and the moisture content of the skin and the time the wave needs to travel from transmitter to receiver is measured. This and the extensive range of C+K instruments are described on its website.[12]
Wrinkles
Until relatively recently measuring skin hydration and TEWL, pH, firmness and elasticity was sufficient for most cosmetic investigations but for products with anti-ageing claims visualising changes in wrinkles has created a lot of interest. Confocal laser scanning microscopy provides the science behind the Mavig VivaScope. The confocal image shows each individual layer of skin in horizontal sections, equivalent to an optical biopsy, and depicting all skin structures in microscopic accuracy from the epidermis to the upper reticular dermis using non-invasive technology. Changes in the skin related to sun damage and skin ageing, pigmentation and epidermal thickness can be accurately documented and analysed and improvements due to cosmetic applications recorded.[13]
Ultrasound is defined as energy above 20 kHz, which represents the upper frequency limit of human hearing. Transducers, which are thin disc-shaped crystals made out of piezoelectric materials, generate acoustic energy when a voltage is applied to them. Wavelengths of higher frequencies allow better resolution of small objects located near the skin surface. With increasing frequency, the depth of penetration of ultrasound waves decreases and the high frequency transducers used to evaluate cutaneous structures function in the 20-50 MHz range. High frequency ultrasound is commonly used for diagnosing medical skin problems and many papers are available describing dermatological investigations, but they can also be used as a measuring device for recording improvements in skin condition.[14, 15, 16]
Pigmentation
There is increasing demand to measure the colour and pigmentation of skin. It enables cosmetics companies to make products that more closely match or complement the skin tone of clients. In pharmaceutical research and development, skin measurement can be used to determine the effects of sun tanning on skin and in the prevention of sunburn. The CM-SA is a handheld instrument from Minolta used in combination with a Konica Minolta spectrophotometer that enables highly accurate measurement of skin colour simultaneously with a numerical display of the melanin index, hemoglobin index, and hemoglobin oxygen saturation index.[17]
For measuring skin colouring and imperfections diffuse reflectance spectroscopy is used for determining the concentration of skin chromophores, melanin and haemoglobin by observing the absorption spectra. Fluorescence spectroscopy is similarly useful for ascertaining the presence of tryptophan, collagen cross-links, elastin cross-links and keratin. Ultraviolet photography may be used to visually enhance the appearance of pigmentation, and polarised light photography has also been developed to selectively enhance either surface or subsurface features of the skin.
Blue fluorescence imaging produces bright images of the distribution of coproporphyrin produced by the bacteria P. acnes and of the mixture of sebaceous lipids, keratinocytes, and sebocytes impacted in open comedones and blackheads on the skin. The use of different photographic techniques are described in patent USP 20060092315 and a variety of imaging systems are available from Canfield Imaging Systems.[18]
Molecular
In vivo confocal Raman spectroscopy can provide information on important molecules such as water and natural moisturising factors as well as exogenous molecules such as glycerol delivered from skin care products.[19] Claiming to be the world’s smallest handheld 3D in-vivo skin measurement device Primos Lite from GFM[20] is used in conjunction with comprehensive 3-D measurement and evaluation software and the sensor is a complete 3D system for flexible use in cosmetics and medicines.
Micro-relief
For micro-relief the skin Visiometer SV 600 from C+K reproduces the topography of the skin surface by light transmission of a very thin, special blue dyed silicone. A very viscous two-part silicone is mixed under vacuum to avoid bubbles and is applied to the skin to create a replica, which is then photographed by a special camera and a controlled light source. When the light penetrates the replica, it is absorbed according to the thickness of the silicone material. The replica reproduces the heights and depths of the skin as a negative; wrinkles are higher in the replica as the silicone is thicker in this area.
Deep lines & wrinkles
For deeper lines and wrinkles C+K recommends the use of its Visioscan VL65. This method is based on shadows on a silicon replica. The replica, which represents a negative of the skin, is illuminated uniformly with a defined light source mounted at a specified angle. The shadows visible on the replica created by the oblique light are captured with a high resolution camera mounted vertically to the replica. Software analyses different characteristics of the wrinkle in length, depth and shape. Silicon replica profilometry is also the basis of the Monaderm system.[21] As with the majority of these instrumental techniques, it relies on sophisticated software to analyse results and to present them in a pictorial fashion.
Many other aspects of skin condition are measured including sebum levels; skin temperature using infra-red thermometers; the measurement of cutaneous blood flow by a laser doppler method; 
and of skin pH using specially adapted pH probes. Analytical techniques are used to measure reactive oxygen species, the release of inflammatory compounds and other biochemical effects. Other tests are more related to the quality of the products such as hardness testing of lipsticks, flow characteristics of powders and emolliency and slip of creams and lotions.
The testing of SPF, UVA and antioxidant capacity of sunscreens is another important area that warrants its own article. The testing of hair products also lends itself to extensive instrumental testing and these will be discussed in the November issue of SPC.
Whatever parameters are being measured, as previously stated, an understanding of the instruments and their limitations is essential when considering the design of clinical trials to support a claim. The cost of the instruments, the difficulty of assembling a suitable panel of volunteers and of performing clinical trials is beyond the means and abilities of many companies so the use of a testing house is recommended. There are a number to choose from, many specialising in particular aspects of efficacy testing.
Third party testing
1. Cutest Systems is an independent testing laboratory established by Professor Ronnie Marks, Dr Peter Dykes and Tony Pearse in 1984. The founding directors have extensive experience in clinical dermatology and skin research and this forms the basis of the company approach to product testing. Cutest has modern skin testing equipment for measuring skin structure and function and assessing product performance.[22]
2. Leeds Skin is an independent commercial testing facility specialising in human skin microbiology, living skin equivalent tissue culture systems and human volunteer and clinical dermatology research. This includes testing for residual antimicrobial efficacy after application and human volunteer studies to screen products for moisturisation, irritancy and ageing and clinical service support for in vitro and in vivo testing of therapeutic agents.[23]
3. Dermatest is a German testing laboratory offering a wide range of services to determine product tolerance and to evaluate the effectiveness and safety of cosmetic ingredients. It offers determination of skin texture with a confocal laser microscope and dermatological ultrasound determinations as well as tests for skin elasticity, moisture levels and roughness.[24]
4. TRI/Princeton is a full service, independent laboratory in New Jersey, US, which provides a wide range of testing services for cosmetics suppliers. It has developed special protocols and procedures for studying the efficacy of finished products and their active ingredients. No two projects are ever the same since no two clients ever have the exact same objectives and claims. As a result, each project has a custom designed protocol.[25]
5. 4-Front Research is part of the Intertek group and offers efficacy testing using a wide variety of instruments to measure skin hydration, TEWL, elasticity, firmness and sebum levels and it visualises lines and wrinkles using Silflo Replica Analysis.[26]
6. Located in Brazil are the well-equipped laboratories of the Institute of Bioengineering of the Skin (IBP) [27] which performs most of the in-vitro and in-vivo instrumental tests mentioned in this feature. IBP also studies changes in skin biochemistry including research into the effects of ingredients and compositions on interleukins, growth factors, lipids and epidermal metabolism and hormones, neurotransmitters, opioids and regulatory enzymes.
There are many other laboratories offering efficacy studies, details of which may be found at cosmeticsbusiness.com.
References
1. Guiteras AF, Evaluation of efficacy and safety of cosmetics, J Soc Cosmet Chem, 1, 17-26 (1947)
2. Malkinson FD, Radioisotope techniques in the study of percutaneous absorption, J Soc Cosmet Chem, 7, 109-122 (1956)
3. White HJ & Underwood DLJ, The use of radiotracers to study absorption by hair, J Soc Cosmet Chem, 7, 198-204 (1956)
4. Peck SM & Glick AW, A new method for measuring the hardness of keratin; J Soc Cosmet Chem, 7, 530-540 (1956)
5. Mills CM, Ester VC & Henkin H, Measurement of static charge on hair, J Soc Cosmet Chem, 7, 466-475 (1956)
6. Roth K, The sebographical skin test, Proceedings of the 2nd Congress of the IFSCC, London, 21-27 (1962)
7. Serak L & Hybasek P, The measurement of perspiration insensibilis, Proceedings of the 2nd Congress of the IFSCC, London, 29-46 (1962)
8. Dykes P, What are meters measuring? Int J Cos Sci, 24, 241-245 (2002)
9. Khazaka D & Uhl C, Taking measurements in principle and in practice, SPC, February, 45-46 & www.cosmeticsbusiness.com (2010)
10. Jemec GB, Selvaag E, Agren M & Wulf HC, Measurement of the mechanical properties of skin with ballistometer and suction cup, Skin Res Technol, 2, 122-126 (2001)
11. http://www.diastron.com/diastron_instrumentation_skin_instruments.html
12. http://www.courage-khazaka.de/products/scientific_rd_prod.html
13. http://www.vivascope.de/en/confocal-laser-scanning-microscopy.html
14. http://www.cortex.dk/dermascan_c.htm
15. http://www.tpm-online.de
16. http://www.medical.siemens.com
17. http://www.konicaminolta.com
18. http://www.canfieldsci.com
19. http://www.gfm3d.com
20. Zhang SL et al, In vivo confocal raman microspectroscopy of the skin: effect of skin care products on molecular concentration depth-profiles, Microsc Microanal 11, suppl 2 (2005)
21. http://www.monaderm.com
22. http://www.cutest.co.uk
23. http://www.leedsskin.co.uk
24. http://www.dermatest.de
25. http://www.triprinceton.org
26. http://www.4frontresearch.co.uk
27. http://www.ibp-pi.com.br
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