Corneometry

Corneometry exploits the fact that water has a far higher dielectric constant, around 80, than the proteins and lipids that make up dry skin, which sit in the low single digits. A probe pressed against the skin creates a weak electrical field whose capacitance rises with the water in it. The field penetrates only the outermost tens of micrometres, so the measurement is confined to the stratum corneum and reports on an arbitrary scale, not in grams of water.

The dominant instrument, the Corneometer CM 825, outputs values on a bounded arbitrary scale, low in dry and eczematous skin and high in well-hydrated skin. Because readings are sensitive to ambient conditions, trial protocols acclimatise subjects for twenty to thirty minutes in a room at controlled temperature and humidity, take readings in triplicate at marked sites, and require the site to be free of residual product. It is usually reported alongside transepidermal water loss, which measures a flux rather than a content.

What it supports is a within-subject, same-session, same-site comparison against baseline or against an untreated or vehicle-treated control site. What it does not support is comparing an absolute value to one from another laboratory or another device, because arbitrary units are not transferable. It also measures surface water only, saying nothing about dermal water or collagen.

The error that matters commercially is attributing a rise to the active. A humectant and occlusive base raises corneometer values within minutes of application, so a peptide serum with a good vehicle will show a large hydration increase that the peptide contributed nothing to; only a vehicle-controlled comparison separates them. Headline percentages are also misleading, since a percentage change in arbitrary units is not a percentage change in anything physical.