328 JOURNAL OF COSMETIC SCIENCE Figure 6. LSR calibration jig. displacement. The final slope is expressed in terms of ADC input per mm displacement. The following values are required: The LVDT calibration value from its certificate, L The excitation voltage measured with a NAMAS calibrated digital volt meter, V The correction factor for the digital volt meter taken from its certificate, C The slope value is derived as follows: output voltage for 1-mm displacement = (L * V ß C), and the ADC input reading for 1-mm displacement = (L * V * C * 2048)/10.

LINEAR SKIN RHEOMETER 329 In practice, these calculations are performed automatically by a simple software program, allowing rapid and simple calibration of absolute force and displacement. SKIN MEASUREMENT USING THEIR LSR For direct comparison with the GBE reproducibility data obtained by Maes eta/. (3), the reproducibility of the LSR was estimated by the same method. Forty consecutive iden- tical measurements were performed on the back of the hand of a female volunteer. Results were analyzed to determine the coefficient of variation of the measurement. To determine the ability of the LSR to measure sensitive changes in stratum corneum mechanics in response to simple hydration, the following study was performed: Two moisturizing formulae of differing hydration performance (products A and B hydration performance was determined by impedance measurements using a Nova TM Dermal Phase Meter 9003, see below) were applied to the back of the hands of 13 female subjects (aged 18-35). The dorsal surface of the hand was chosen for mechanical measurements (a) to conform to previous measurement sites using the GBE (3) and (b) because it is relatively simple to immobilize the hand effectively. The study was performed in a controlled- environment chamber (temperature 20 + IøC relative humidity 45 + 5%). The plastic stub on the end of the LSR wire probe was attached to skin on the back of the hand via a circular piece of double-sided tape (5 mm diameter). LSR measurements were then performed in triplicate. Baseline measurements were performed before product applica- tion. Test products were then applied at a rate of 2 l•l/cm 2 to the entire back of the hand according to a predetermined randomization schedule. LSR measurements were per- formed at one, three, and six hours after product application. As the whole dorsal surface of each hand was used for product treatment, inclusion of an untreated control was not possible. Results were, therefore, expressed as mean difference to initial pretreatment baseline. Hydration performance of products A and B was assessed by randomized application at the same rate as above (2 t•l/cm 2) to 5 x 5-cm sites on the volar forearms of 12 female subjects (aged 18-35 the volar forearm was chosen as the site for hydration measure- ments because of its smooth, hairless morphology and its utility as a standard in this type of testing (6). Each forearm also contained an untreated 5 x 5-cm control site. The study was performed within a controlled-environment chamber (temperature 20 + iøC rela- tive humidity 45 + 5%). Impedance measurements were performed using a Nova TM Dermal Phase Meter 9003 with the standard measuring probe DPM 9103 (Nova In- struments, USA) at one, two, four, and six hours after application, and results were expressed as mean difference to untreated control. RESULTS Forty consecutive measurements on the same subject and same site indicated that the coefficient of variation of the measurement was only 2.9% (Figure 7). This demonstrates very good reproducibility of the measurement technique and compares very favorably with the value of 3% obtained by Maes et al. (3) for the GBE. The variation measured is almost certainly due to movement of the subject during the probe cycle. This has always been the main source of error in these types of sensitive measurements, and