Pierre-Yves Joubert

A wireless multifrequency resonator (WMFR) based on an original combination of several flexible passive radiofrequency transmission-line resonators, is used as an inductive multi-sensor for the non-contact monitoring of kiwifruit ripening. In this paper, the complex dielectric property changes of a kiwifruit sample were monitored during a two-week ripening period, with a three-frequency WMFR in the 100 MHz-300 MHz bandwidth. Thanks to an equivalent electrical circuit modelling the coupling between the WMFR and the monitored fruit, three sensor output parameters were defined and assessed at each frequency, to efficiently monitor the dielectric changes induced by the structural and biochemical modifications of the fruit during ripening.

The dielectric properties of tissues have been widely used to detect and monitor different pathologies. One of the remaining challenges is to timely and accurately characterize the evolution of the dielectric properties of tissues in a non-invasive and contactless way, with a simple and portable monitoring system. This paper proposes investigating the use of a loop-shaped transmission line passive resonators (TLR) to sense organic tissue changes in the radiofrequency bandwidth (in the hundreds of MHz bandwidth), through inductive coupling with the tissue. This kind of sensor can be wirelessly excited, and is able to distantly detect the dielectric modifications in the targeted tissue through the changes of the transmitted electromagnetic field. TLR-based sensors are therefore very promising for the non-invasive, wearable and continuous monitoring of tissues. In this paper, a first study is carried out to monitor the decomposition of a beef muscle sample for six consecutive days with two different TLR-based sensors featuring two investigation frequencies (160 MHz and 350 MHz). The obtained results confirmed the ability of such sensors to follow the modifications of an organic tissue through the assessment of both the conductivity and the relative permittivity of the investigated sample. Results also confirmed that the investigation frequency, for which the loss factor within the tissue is around unity, is particularly well suited to sense changes within the tissue under investigation. A second study was realized with other soft matter samples (water, cottage cheese, water/gelatin mix) to determine the ability of TLR-sensors to discriminate between soft matter of various nature. Thanks to the ability of the TLR-based sensor to assess the loss factor of the monitored samples, it was found that i) the proposed sensor is relevant to discriminate between the considered soft matter samples and ii) that this discrimination can be made particularly efficient when using the appropriate investigation frequency. Furthermore, the benefits of the use of several investigation frequencies were also demonstrated for enhanced tissue characterizations. TLR-based sensors are therefore good candidates for the non-invasive, low-cost and sensitive sensing devices dedicated to the monitoring of pathologies such as wound healing and cancer detection.

The dielectric properties of organic tissues are related to their biological composition and structure. Monitoring those properties in a non-invasive and non-contact way is still a major challenge in biomedical applications. This paper presents a multifrequency inductive sensor based on an original combination of several passive radiofrequency inductive resonators, called WMFR. To assess the ability of the WMFR to monitor the dielectric properties of tissues, the decomposition of a beef muscle tissue sample was sensed during 6 days. The dielectric properties of the tissue were estimated during decomposition, and the WMFR was found able to sense the changes over time at 7 different frequencies ranging in the 20-350 MHz bandwidth. This study opens the way to a new generation of non-contact and wearable sensors dedicated to dielectric monitoring applications in the biomedical field.

This paper reports on the study of microporous polydimethylsiloxane (PDMS) foams as a highly deformable dielectric material used in the composition of flexible capacitive pressure sensors dedicated to wearable use. A fabrication process allowing the porosity of the foams to be adjusted was proposed and the fabricated foams were characterized. Then, elementary capacitive pressure sensors (15 × 15 mm² square shaped electrodes) were elaborated with fabricated foams (5 mm or 10 mm thick) and were electromechanically characterized. Since the sensor responses under load are strongly non-linear, a behavioral non-linear model (first order exponential) was proposed, adjusted to the experimental data, and used to objectively estimate the sensor performances in terms of sensitivity and measurement range. The main conclusions of this study are that the porosity of the PDMS foams can be adjusted through the sugar:PDMS volume ratio and the size of sugar crystals used to fabricate the foams. Additionally, the porosity of the foams significantly modified the sensor performances. Indeed, compared to bulk PDMS sensors of the same size, the sensitivity of porous PDMS sensors could be multiplied by a factor up to 100 (the sensitivity is 0.14 %.kPa⁻¹ for a bulk PDMS sensor and up to 13.7 %.kPa⁻¹ for a porous PDMS sensor of the same dimensions), while the measurement range was reduced from a factor of 2 to 3 (from 594 kPa for a bulk PDMS sensor down to between 255 and 177 kPa for a PDMS foam sensor of the same dimensions, according to the porosity). This study opens the way to the design and fabrication of wearable flexible pressure sensors with adjustable performances through the control of the porosity of the fabricated PDMS foams.

A flat circular transmission line-based 300 MHz resonator was implemented for the non-contact assessment of burn depths in biological tissues. Used as a transmit-and-receive sensor, it was placed at a 2 mm distance from organic material test samples (pork fillet samples) which were previously burned on their surface in various heating conditions involving different temperatures, durations, and procedures. Data extracted from the sensor by means of a distant monitoring coil were found to clearly correlate with the depth of burn observed in the tissue samples (up to 40% sensor output changes for a 7 mm burn depth) and with the heating conditions (around 5% sensor output changes observed in samples burned with identical heating procedures but at two different temperatures—75 °C and 150 °C—and around 40% sensor output changes observed between samples heated at the same temperature but with different heating procedures). These results open the way for the development of easy-to-implement assessment and monitoring techniques for burns, e.g., integrated in wearable medical dressing-like monitoring devices.

Purpose: The purpose of this paper is to propose an approach, which is easy to implement, for estimating the thickness of the air layer that may separate metallic parts in some aeronautical assemblies, by using the eddy current method. Design/methodology/approach: Based on an experimental study of the coupling of a magnetic cup core coil sensor with a metallic layered structure (consisting of first metal layer/air layer/second metal layer), which is confirmed by finite element modelling simulations, an inversion technique relying on a polynomial forward model of the coupling is proposed to estimate the air layer thickness. The least squares and the nonnegative least squares algorithms are applied and analysed to obtain the estimation results. Findings: The choice of an appropriate inversion technique to optimize the estimation results is dependent on the signal-to-noise ratio of measured data. The obtained estimation error is smaller than a few percent, for both simulated and experimental data. The proposed approach can be used to estimate both the air layer thickness and the second metal layer thickness simultaneously/separately. Originality/value: This model-based approach is easy to implement and available to all types of eddy current sensors.

Flat circular transmission-line based 300 MHz resonator is implemented for the noncontact assessment of burn depths in biological tissue. Used as a transmit-and-receive sensor, it is placed here at a 2 mm distance from organic material test samples (pork fillet samples) which have been previously heated on one face in various heating conditions involving various temperatures, durations and procedures. Data extracted from the sensor by means of a distant monitoring coil were found to clearly correlate with the depth of burn observed on the tissue samples (up to 40% sensor output changes for a 7 mm burn depth) and with the heating conditions (around 5% sensor output changes for 5.5 mm burn depth obtained at 75 °C or 150 °C). These results open the way to the development of easy to implement burn assessment and monitoring techniques, which could be integrated in wearable medical dressing-like monitoring devices.