**4. Diagnostics of barrier properties of the skin—TEWL (transepidermal water loss), TST (tape stripping test), ATR-FTIR (attenuated total reflection-Fourier transform infrared spectroscopy)**

There are wide varieties of methods available for analysing skin structure and other properties including its barrier function, such as Raman spectroscopy [35], X-ray diffraction [36], electron diffraction [37] and transmission electron microscopy [38]. Barrier properties of the stratum corneum can be confirmed by the transepidermal water loss (TEWL) test, which indicates water evaporation from the inner body through skin [21]. This test expresses the amount of water in grams evaporated per square metre in 1 h. Stratum corneum is a barrier against water diffusion and some other chemicals. The better the barrier function of the skin, the higher the water content and the lower the TEWL value. The tape striping test was described for the first time by Pinkus [39] in 1951. This method is based on removing of stratum corneum layers of the skin. The amount of removed stratum corneum is not constant and it depends on many parameters such as cohesion between cells, hydration or body sites [40]. This technique has been used for evaluation of the barrier function of skin, i.e. for investigating the depth of penetration of drugs [41], the influence of drug enhancers on stratum corneum [42], pH profiles [43] and many others. The tape stripping test (TST) is a representative method for estimating the barrier performance of skin and other properties of the stratum corneum [44]. When the Scotch tape is placed on skin and peeled off, stratum corneum layer is stuck to the surface of the tape. Therefore, the barrier performance is decreased. Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) is an effective method when a material which is not well transmissive is measured, and the method can be used for liquid or solid samples. The information tells us about several micrometres of surface; thus, it is also a useful method for analysing skin [45, 46]. ATR-FTIR spectra can give us information about hydration of skin [47], structure of proteins [48] and lipids [49] and about their changes during skin treatment by chemical or other drug enhancers [50, 51]. There are several vibrational bands associated with chemical functional groups. Vibrational bands at 2850 and 2920 cm−1 belong to CH2 symmetric and asymmetric stretching modes, respectively [52]. The absorbance of the spectrum decreases as the thickness of the skin decreases and the wavenumber indicates a change in chain conformation, especially a change in peak shape [49].
