Résumé: © 2019 Elsevier B.V. We consider a point-like observer that moves in a medium illuminated by noise sources with Lorentz-invariant spectrum. We show that the autocorrelation function of the signal recorded by the observer allows it to perceive its environment. More precisely, we consider an observer with constant acceleration (along a Rindler trajectory) and we exploit the recent work on the emergence of the Green's function from the cross correlation of signals transmitted by noise sources. First we recover the result that the signal recorded by the observer has a constant Wigner transform, i.e. a constant local spectrum, when the medium is homogeneous (this is the classical analogue of the Unruh effect). We complete that result by showing that the Rindler trajectory is the only straight-line trajectory that satisfies this property. We also show that, in the presence of an obstacle in the form of an infinite perfect mirror, the Wigner transform is perturbed when the observer comes into the neighborhood of the obstacle. The perturbation makes it possible for the observer to determine its position relative to the obstacle once the entire trajectory has been traversed.

Résumé: © 2019 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Background and objective: Different biological models have shown how mechanical stimulation may induce physiological responses from solicited cells, tissues, or organs. In models of cultured skin cells, the frequency of the mechanical stress appears to be a paramount parameter, generating a biological response in some cells, particularly from dermal fibroblasts. Our objective was to explore in ex vivo human skin explants the effects of mechanical stimulation. Materials and methods: Mechanical stimulations were provided by a torque test device, with different end effectors, able to generate cyclic strains at different frequencies (from 40 to 120 Hz). Skin explant samples were stimulated twice daily by the device for one minute, over 10 days. Results: At days 0, 5, and 10, samples were processed by immunohistological procedures, allowing some structural dermal proteins to be quantified (fluorescence). As compared to untreated skin explant samples, the stimulation procedure clearly led some proteins of the dermal-epidermal and some dermal proteins to be overexpressed. This stimulation was found to be frequency-dependent, with the greatest overall increases occurring at 60 and 90 Hz. Conclusion: For the first time, ultrafast ultrasound imaging in vitro (phantom mimicking skin mechanical properties) was used to analyze mechanical waves transmitted to the skin layers as a function of end effector shape.