324 JOURNAL OF COSMETIC SCIENCE all the principles of the original GBE, but contains none of the components. This instrument, designated the linear skin rheometer (LSR) is described in the following sections. MATERIALS AND METHODS INSTRUMENT HARDWARE AND DESIGN A schematic diagram of the new instrument is shown in Figure 3. A force-controlled miniature d.c. servo (Maxon 23-12, 0.5 W rating, supplied in U.K. by Trident Engi- neering), gearing, and leadscrew now replace the GBE solenoid arrangement, and drive the LSR probe. The original Schaevitz 050 HR LVDT in the GBE has been replaced with a unit of linearity 0.3% (15 lam) and sensitivity 0.01% (0.5 12m) (Solartron type DF2.5, Schlumberger Industries). The force exerted on the probe is now measured directly by a calibrated load beam (Minigram Beam Load Cell, type MBH50, rated 50 g, supplied in U.K. by RDP Electronics) with an overall accuracy of 20 rag. The load beam is mounted vertically within the instrument casing. All components fit into one casing measuring 20.0 x 14.8 x 6.9 cm, and the whole unit weighs 1.7 kg. The probe housing itself is a light-weight machined perspex chuck mounted on a low-friction swivel assembly allowing 360 ø movement (analogous to the GBE). This is protected from damage during routine usage by a metal collar. The chuck contains wire grips to allow the wire probes to be inserted or withdrawn by a simple, firm push or pull. A single lead connects the unit to a PC via a 25-pin D-type connector. Power for the LSR unit is taken from the PC via the connector. The unit can be seen in Figure 4. INSTRUMENT CONTROL An IBM (or compatible) PC is used to control the movement of the probe and to log Figure 3. Schematic diagram of the LSR sensing head: a, miniature motor b, load screw c, lead cell d, load cell sensing head e, LVDT f, probe g, skin surface.

LINEAR SKIN RHEOMETER 325 GøoclYer Figure 4. The linear skin rheometer. force and displacement data. Both force and displacement are monitored continuously at a rate of 1 KHz using a 12-bit ADC plug-in card (National Instruments MIO16). The motor is controlled with an analogue output signal also generated by the PC. The desired force/time cycle, which is normally a single sinusoid, is calculated initially and then stored in memory as a table of values. The actual force applied to the probe is compared with the desired value in the table 1000 times a second. A feedback loop is used to control the motor that moves the load cell in such a way as to minimize any discrepancy. The force applied thus follows the desired force/time cycle extremely closely. The control loop uses an algorithm with proportional and integral terms, whose relative weighting can be varied. The PC logs all the force and displacement values over a complete measurement cycle, which is usually set at 0.33 Hz, thus generating 3000 pairs of points over a three-second cycle. Two waveform plots are then obtained (Figure 5). Three parameters may be obtained from these curves: Fmax: the peak force that is applied to the skin surface Pmax: the peak displacement occuring as a result of that force T: the phase shift between the two signals The dynamic spring rate (DSR) of the stratum corneum is given simply by the formula Fm,,x/Pma x. Derivatives are calculated and expressed as g/mm, mm/N, and lam/g.