LINEAR SKIN RHEOMETER 323 Position Force Figure 2. Typical position vs force hysteresis loop produced by a one GBE measurement cycle. extension), mm/N or Fm/g (measures of stretching or compression of the skin in response to a given applied force). Such analysis yields information about the elastic properties of the skin. Analysis of the phase lag between force and displacement responses yields information about the viscous properties of the skin. After 20 years of experience with the GBE within our laboratories, we believe that the principle of the GBE measurement is still the best available for measuring sensitive changes in the mechanical properties of the human stratum corneum in vivo. Subtle though important changes in skin elasticity [dubbed "softness" by Maes eta/. (3)] in response to the application of moisturizing formulae have been measured, as have changes in skin "tightness" due to surfactant damage. Our experience has, however, also highlighted the drawbacks of employing the original Hargens GBE instrument in a modern laboratory. These are as follows: 1. Importantly, the instrument employs an "open-loop" method of control, i.e., during calibration of the instrument, and subsequently in routine operation, one assumes that an applied current equals a given force. As the GBE is calibrated on one point only (3 g), linearity is not guaranteed over the whole measurement range of the GBE. In addition, calibration drift over time is certain. 2. The components needed to run the GBE are bulky and dated (function generator, signal conditioner, storage oscilloscope, compressed gas/air). 3. The probe components are fragile and, in our experience, break easily and require excessive servicing when used routinely (for example, the fine copper wires connect- ing the armature to the body of the probe). 4. The air-bearing employed in the probe design is inherently susceptible to misalign- ment, soiling, and malfunction. In recent years, the cost of precision has improved greatly. We can now achieve with conventional technology what was achieved previously through Hargens' (1) consider- able ingenuity. We have designed and built a new instrument that retains and builds on

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.