Ultrasonic microrheology for ex vivo skin explants monitoring: A proof of concept.

As an answer to alternative non-animal testing, biosensors dedicated to the ex vivo skin explants monitoring are a challenge to study physiological-like behavior and optimize new topical products. Because of the skin viscoelastic behavior, mechanical tests are commonly based on macroscopic measurement and give global descriptors of its state. Other techniques, including photoacoustic ones, are more focused on the molecular scale. There is a gap to fill in the mesoscopic range to get information about the microstructure of the skin. This article presents the proof-of-concept of a biosensor coupling a thickness shear-mode transducer with human skin explants kept in life-like state for a week. Thanks to a multifrequency analysis of the transducer impedance, this biosensor is able to monitor the viscoelastic properties of the skin. To extract the complex shear modulus and the microstructural evolutions, a mechanical model based on fractional calculus is used. As a preliminary results, the sensitivity of the sensor to probe the skin viscoelasticity in lifelike state and the impact of its culture medium are presented. A suitable microstructural coefficient is also extracted in order to identify mechanical breaches in the skin barrier after the application of peeling products.

Chirp-Z analysis for sol-gel transition monitoring.

Gelation is a complex reaction that transforms a liquid medium into a solid one: the gel. In gel state, some gel materials (DMAP) have the singular property to ring in an audible frequency range when a pulse is applied. Before the gelation point, there is no transmission of slow waves observed; after the gelation point, the speed of sound in the gel rapidly increases from 0.1 to 10 m/s. The time evolution of the speed of sound can be measured, in frequency domain, by following the frequency spacing of the resonance peaks from the Synchronous Detection (SD) measurement method. Unfortunately, due to a constant frequency sampling rate, the relative error for low speeds (0.1 m/s) is 100%. In order to maintain a low constant relative error, in the whole speed time evolution range, Chirp-Z Transform (CZT) is used. This operation transforms a time variant signal to a time invariant one using only a time dependant stretching factor (S). In the frequency domain, the CZT enables us to stretch each collected spectrum from time signals. The blind identification of the S factor gives us the complete time evolution law of the speed of sound. Moreover, this method proves that the frequency bandwidth follows the same time law. These results point out that the minimum wavelength stays constant and that it only depends on the gel.

Wide-range viscoelastic measurement using resonating sensors.

We have developed a specially designed sensor with its associated instrumentation using a so called acoustical near-field technique based on small resonating horns. Contrary to the conventional measuring devices used in the rheology industry, this sensor enables wide-range and continuous viscosity measurements, including liquid to solid material transition. The sensor is composed of a small piezoelectric element which enables the horn to resonate. Shear waves are generated by means of the thinner part of the sensor (dipped into the material) confining the acoustic field to the tip to ensure local measurements. The frequency is scanned across the resonating frequencies in order to perform these measurements with the maximum of speed and efficiency. The shear moduli G' and G'' of the material is deduced from the electrical impedance measurement at the resonance frequencies. In case of rapidly changing materials, such as quick setting cements or polyesters, we have developed a specific fast instrumentation based on the impulse response of the sensor, which realizes very fast measurements, typically 30 measurements per second. Modelization and experiments are reported.