• Image analysis

One of the more popular claims currently being made for most cosmeceuticals is that they are ‘antiaging and help restore a more youthful skin’, or words to that effect. One highly desirable outcome of such a treatment would be to reduce the appearance of facial wrinkles, such as those in the crow’s feet region. Although such changes can be documented by standardized clinical photographs, it is more desirable to cast a replica of the skin surface by using a silicone rubber impression material, such as Silflo. Figure 5.1 shows representative specimens obtained from individuals with varying degrees of photodamage; the differences in wrinkle depth are readily appreciated. However, by using optical profilometry one can objectively measure changes in skin surface topography due to effective cosmeceutical treatments. This technique involves computer imaging techniques in which a digitized image is taken of the replica that is illuminated from a fixed low angle. This causes various surface features to be highlighted or shadowed in such a way that a graphic representation of the surface topography can be generated and subsequently analyzed for wrinkling, roughness, and other textural features (Fig. 5.2).

Fig. 5.1 Representative skin specimens obtained from individuals with varying degrees of photodamage

Fig. 5.2 Digitizing skin replicas can create a reliable noninvasive method for wrinkle assessment

This is but one example of how computerized image analysis can be used to objectively extract quantitative information from images. Box 5.2 provides a listing of some of the more common applications of image analysis that have been used to study skin structure and function. The basic rule seems to be that anything that can be seen by the unaided eye can easily be measured. Moreover, by using specialized lighting techniques such as Wood’s lamp illumination, things that cannot be directly visualized can be detected and measured in the specially created images.

• Skin coloration

Another important visual clue to the condition of the skin after cosmeceutical application is its color, which depends upon a number of factors, including pigmentation, blood perfusion, and desquamation patterns. Experienced dermatologists frequently use color information in several ways. First, they can certainly appreciate the distribution of erythema and/or pigmented lesions on the basis of color. Moreover, by evaluating changes in the hue and/or intensity of color over time, they will be able to tell if patients are responding to treatment or not. Although the human eye is very sensitive, especially in detecting very subtle differences in contrast, the evaluation of color is still highly subjective. Color measuring devices offer the advantages of objectivity and quantification on a continuous scale that can be referenced to color standards.

The devices that are currently being employed in experimental dermatology, skin pharmacology, toxicology, and cosmetic science to measure skin color changes fall into two distinct types of instrument as shown in Box 5.3. In one category we have the tristimulus colorimeters, which are based on the three-dimensional L*a*b* color space (CIELAB). L*a*b* allows any color to be mathematically described by its hue (position on the color wheel), value (lightness), and chroma (saturation). These would include the Minolta ChromaMeter and the MicroColor of Dr. Bruno Lange GmbH which have seen widespread use for the quantification of erythema in the study of irritant dermatitis due to exposure to detergents, topical corticoid activity in the vasoconstriction test, and for measuring the percutaneous penetration of vasodilators such as nicotinic acid.

Other types of instrument are the DermaSpectrophotometer (Cortex Technology) and the Erythema Meter (Dia-Stron Ltd) which are based on the two-wavelength method of Diffey et al. These instruments emit green and red light and measure the reflected light from the skin surface. Because changes in skin redness will greatly affect the absorption of green light but will have very little effect on that of red light, an erythema index can be calculated. Since increased melanin pigmentation will lead to an increased absorption of both red and green light, a melanin index can be computed in a similar fashion.

• Dermal firmness/elasticity

Another characteristic change which is well documented in photoaged skin is the loss of firmness and elasticity due to structural changes in collagen and elastin. Over the years, a wide variety of instruments have been developed to objectively measure the biomechanical properties of the skin. Box 5.4 lists some of the more popular devices. Although there are fundamental differences in how they interact with the skin, the basic approach is the same for all of them, i.e. load the skin in a standard fashion and measure the subsequent deformation. These changes in deformation represent a change in skin elasticity and firmness, which can be altered by cosmeceuticals.

• Blood flow

As previously mentioned, increased blood flow generally leads to increased skin surface redness that can be visually assessed by either the dermatologist or the patient as well as by instrumentally measuring a color change. Blood flow can be analyzed by using laser Doppler velocimetry. This instrument utilizes the Doppler effect to determine the speed of blood flow.

• Transepidermal water loss

Another physiologic process that has been extensively measured with instrumentation, which has no visual or tactile counterparts, is transepidermal water loss (TEWL). Measurement of TEWL rates through human skin can be used to noninvasively monitor changes in stratum corneum barrier function (Fig. 5.3). In normal healthy skin, the barrier is quite effective and water loss rates are typically very low. If the barrier is compromised due to pathologic processes or damaged by physical or chemical agents, there will be a corresponding increase in water loss rate that directly relates to the degree of impairment. Conversely, there will be a corresponding decrease in TEWL as the barrier is restored. This means that monitoring changes in TEWL over time not only allows one to evaluate the therapeutic response to different treatments, but also to determine the effectiveness of various prophylactic strategies that might prevent or lessen the injury. Thus, it is not surprising that there is considerable literature dealing with TEWL measurements. Indeed, TEWL measurements were the very first to be reviewed by the Standardization Group of the European Society of Contact Dermatitis.

Fig. 5.3 A collecting chamber with two humidity meters is used to assess cutaneous transepidermal water loss

• Skin hydration

There are many biophysical methods available for measuring the relative hydration state of the stratum corneum. Most, such as those shown in Box 5.5, are based on the electrical properties of the skin surface. It has been shown most notably by Obata and Tagami that the ability of an alternating current to flow through the stratum corneum is an indirect measure of its water content. Higher water content translates into increased electrical conduction.

• High-frequency ultrasound

Another useful technique that allows characterization of the physical properties of the skin is high-frequency ultrasound images, such as the DermaScan C (Cortex Technology). As sound waves travel through the skin they generate ‘echoes’ at tissue interfaces where acoustic impedance changes. In the A mode display, the echo information is presented as an amplitude-modulated oscilloscope trace versus time of flight. Since only one spatial domain is displayed, the diagnostic information is limited. The B mode display is a two-dimensional image of each echo-producing interface and thus is equivalent to a radar screen. By using a graphic presentation that shows the intensity of the echoes at each location, it is possible to gain additional insights into skin structure and function without taking a biopsy. This technique allows better visualization of skin tumors. It has also revealed the existence of an echo-poor band that seems to be characteristic of photodamaged skin.