368529 From Oil Recovery to Transport through Skin: Then and Now
Each of the authors has a connection to Professor Clay Radke and the University of California, Berkeley. Annette was one of Clay’s first graduate students. Around the same timeframe, Mark was a graduate student studying at Berkeley under the direction of Professor John Newman. As Mark served as a teaching assistant for one of Clay’s classes, he can claim to be a distant relative in the Radke family tree. Erick is Clay's academic "grandson", being Annette’s last graduate student. Erick was co-advised by Annette and Mark.
When Annette started as a new Assistant Professor in 1981, oil prices were high, and she thought her PhD research on enhanced oil recovery would sustain a long career. She focused on finding a niche that would distinguish her research from Clay’s work. That proved unnecessary because all too soon the oil boom went bust and funding for oil recovery research vanished. Her search for a new research area eventually led to studies of mass transfer through skin. This presentation will describe Annette’s transition from oil recovery to dermal absorption and then our recent collaborative studies on the electrochemical impedance of skin.
Impedance spectroscopy is a transient electrochemical technique in which an oscillating potential is imposed on a system and the resulting current is measured. The ratio of the input over the output is called the impedance. The frequency of oscillation may be varied, and the resulting impedance will change according to the imposed frequency. For impedance measurements on skin, the frequency ranges from 0.1 Hz to 10 kHz. In our work on skin, the impedance response can be attributed largely to the dielectric response of skin. Our measurements were performed in vitro, but similar in vivomeasurements could also be performed as the small amplitude of the potential signal makes the experiment non-invasive. A large value of impedance at low frequency is characteristic of intact skin; whereas, a small value is characteristic of damaged skin. The high-frequency response may be analyzed to obtain stratum corneum thickness or averaged dielectric constant. In the skin literature, single-frequency impedance measurements are often used, but our work shows that it is much better to use the full spectrum of frequencies.
Dimethyl sulfoxide (DMSO) is a solvent that damages skin. In our work on the influence of 100% DMSO, the impedance response after 0, 0.25, 0.5, 0.75 and 1 hour of exposure showed that the damage occurred along a moving front instead of by creating and expanding channels. The work on mechanical damage was used to prove that DMSO did not open discrete pathways in the skin by showing that the impedance of DMSO had features that were not observed in the impedance of punctured skin.
This work represents an application of the fundamental principles that Clay emphasized in his research group. Exciting discoveries often appear at the boundaries between fields, revealed by an emphasis on fundamentals.