Proof-of-Concept of Electromechanical Impedance Sensing for Non-Destructive Monitoring of Viscosity Changes in Cosmetic Gels

Viscosity is a key quality parameter in cosmetic manufacturing, yet conventional rheological measurements require direct contact with the sample and are not suitable for continuous monitoring of the same specimen. This study investigates the feasibility of electromechanical impedance (EMI) sensing as a proof-of-concept approach for non-destructive monitoring of viscosity changes in cosmetic gels. Hydroxyethyl cellulose (HEC)-based model gels with HEC concentrations ranging from 0.0 to 1.0 wt% were prepared, providing viscosities between 1 and 794 cP. An acrylic-coated piezoelectric (PZT) sensor embedded in each gel was used to measure the electrical admittance spectra. The resonance peak conductance decreased progressively with increasing viscosity, whereas the resonance frequency remained nearly constant, indicating that resonance peak conductance is sensitive to viscosity-related changes in the surrounding gel. Continuous monitoring over 24 h under naturally varying temperature conditions further demonstrated that the EMI response changed consistently with the thermal behavior of the gel. These findings demonstrate the feasibility of EMI sensing as a non-destructive technique for continuously monitoring viscosity-related changes in cosmetic gels and provide a foundation for future studies using practical cosmetic formulations.