Multi-frequency detection of a dielectric object using flexible contactless RF sensors for tissue diagnosis.

The aim of this paper is to experimentally assess the capacity of radio-frequency flexible sensors to localize a dielectric object inside a fluid, which could be used as a non contact sensor detected to the detection or the monitoring a local modification of a tissue, such as a tumor or a lesion. The used sensor is an MRI-like antenna, which consists in a flat, flexible and low cost multi-turn split resonator (MTLR), which features a geometrically pre-determined resonance frequency. The MTLR is used here as a transmit and receive sensor monitored by a distant loop coil. The complex impedance changes observed at the ends of the monitoring coil is known to be linked to the dielectric properties of the resonator environment. If a dielectric object is placed close to the resonator, the complex impedance is altered. In this work, an experimental set up is used to assess the relevance of such a measurement scheme to detect the presence of a dielectric inclusion embedded in another dielectric medium. The setup includes a spherical object of 1.5 cm diameter filled with various NaCl solutions moved vertically inside a tank filled with deionized water by means of a three axis robotic arm, to create an electrical conductivity contrast between the inclusion and the media, and three 2 cm diameter MTLR sensors featuring 30, 47 and 70 MHz resonance frequencies, respectively. The sensors are operated through the use of monitoring coils connected to a network analyzer, and measurements are carried out at each position of the spherical object. The resulting sensor responses are plotted and used for the assessment of sensor performances. In addition, a method to combine the multi-frequency data provided by the three different sensors is proposed. Two different metrics regarding the spatial resolution (SR) and the peak signal to noise ratio (PSNR) are computed to characterize the single sensor performances, as well as the enhancement provided by the proposed multi-frequency approach.

Imaging of a dielectric inclusion using a contactless radio-frequency inductive probe for tissue diagnosis.

In this paper, a contactless radio-frequency (RF) inductive probe is used to spatially localize and characterize a complex dielectric organic inclusion in a fluid. The effect of dielectric properties (DP) of this organic material is investigated experimentally and by numerical computations.The used RF probe is a 135 MHz 3 cm diameter and 10 cm long, cylindrical bracelet resonator, placed close to a water tank filled with deionized water which includes a 1.5 cm diameter inclusion filled of air or NaCl solutions and placed in arbitrary positions. The water tank and the inclusion are used to model an organic material including a tumor. The RF probe is used as a transmit and receive sensor. It induces a magnetic field inside the water tank, which, by reciprocity, conveys information about the DP of the investigated material. The impedance changes at the end of the RF probe are directly related to the modifications of the magnetic field, and are measured by means of a network analyzer. A complex fit of the impedance frequency response around the resonance frequency gives access to two quantities proportional to the electrical conductivity and dielectric constant of the inclusion. The inclusion is moved into the water tank along the three axes by means of a robotic arm, so that two three dimensional maps of the equivalent dielectric quantities in function of the inclusion position are sensed by the probe. Then, the inclusion is filled with different conductive NaCl solutions from 0.1 to 1.1 S/m in order to test the ability of the probe to sense the modifications of the dielectric properties of the inclusion. Experimental as well as computation results obtained using the Distributed Point Source Method (DPSM) validate the ability of the proposed probe to localize the inclusion as deep as 1 cm into the water, and the ability of the probe to sense the dielectric property changes of the inclusion.

One-shot synthesis of a poly(N-isopropylacrylamide)/silica hybrid gel.

Class I hybrid poly(N-isopropylacrylamide)/silica hydrogels, PNIPAM/SiO2, were prepared by a new one shot synthesis. In this approach, the free-radical polymerization of vinyl groups of N-isopropylacrylamide (NIPAM) and the hydrolysis-condensation of alkoxy groups of tetramethoxysilane (TMOS) are performed concomitantly using sodium persulfate and 3-(dimethylamino)-propionitrile, a well-known couple to initiate the organic polymerization. The cross-linker is N,N-methylenebisacrylamide. The kinetic study of mechanical properties from the sol-to-gel state for different ratios of TMOS/NIPAM was investigated by rheological ultrasonic measurements. The thermoresponse of hybrid materials was investigated by differential scanning calorimetry and the measurements showed that hybrid gels present a lower critical solution temperature, which is similar with one of single organic hydrogel.

Lamb mode reflections at the end of a plate loaded by a viscoelastic material.

This paper deals with a study of the conversion of a Lamb wave at the loaded edge of a plate. An experimental study is performed to know the interaction of surface waves with a viscoelastic material. A Lamb wave is generated by a wedge. At the end of the plate, this wave gives rise to several reflected waves. A first study makes it possible qualitatively to know the Lamb modes present in reflection. Secondly, conversion phenomena are studied to know if surface waves are sensitive to evolution in the time of the viscoelastic material structure. The selected material is a sol-gel containing silica.

Ultrasonic monitoring of yoghurt formation by using AT-cut quartz: lighting of casein micelles interactions process during the acidification.

The behavior of weak gels during their formation singularly attracts attention of dairy products factories. In our study we investigate acidified pre-heated milk gels formation that are fairly often used to product yoghurts. The gel formation requires a tight control of the first step of micelles modification process and the kinetics reaction parameters. The most current rheological parameters used to achieve the monitoring are the storage G' and the loss G'' shear moduli and the gelation time. The study of these parameters is commonly performed at very low frequencies (1 Hz). Our technique uses a 6 MHz AT-cut quartz crystal immersed in an acidified milk solution kept at a constant temperature. This method is singularly effective to ensure a complete and a reliable follow-up of the viscoelastic parameters of casein gels. A suitable new model enables a complete follow-up of the micelles evolution from the viscoelastic properties. The experimental results of the G' and G'' moduli versus temperature and versus glucono-delta-lactone (GDL) added to milk are analyzed. In order to understand the micelles modifications, an analysis of the viscoelastic evolution try to explain the validity of the various models of micelles modification. In addition a new accurate kinetics characteristic time is proposed. This time corresponds to the moment for which the elastic effect of material becomes significant. From the kinetics study of casein gels at various temperatures, the Arrhenius relationship and a modified Flory-Stockmayer relationship give us access to the activation energy. By using the proposed technique and the suitable models developed, the structure thus quality of dairy products may be better controlled.

Kinetic study of silica gels by a new rheological ultrasonic investigation.

The last decades have seen the development of sol-gel (SG) process currently used to develop new materials in a wide range of scientific applications. The SG process leads to an oxide macromolecular network through a sol (liquid phase) to gel transition. To optimize this process, the control of the kinetic of the chemical reaction is required. This kinetic can be deduced from the temporal evolution of the viscoelastic parameters. Upto date no complete investigation during the SG formation can be achieved by a unique non-destructive technique. In this paper, we present an ultrasonic technique to measure the viscoelastic parameters (storage G' and loss G'' shear moduli) of the gel material during its formation. By using a suitable model which takes into account the mass loading on the surface, the viscoelastic parameters of these materials are accurately deduced. In order to study the efficiency of this technique, silica gels transition is monitored at various formation temperatures and for different initial hydrolysis molar ratio (h). In addition, the monitoring is performed at different oscillatory shear measurements in the 6-54 MHz frequency range to determine a new characteristic time t(vs) corresponding to the moment when the material is no more a newtonian liquid. This characteristic time is then compared to the gelation time t(g) determined by rheological or acoustic audible range methods. Thus the new characteristic time is also a good criterion to characterize earlier the SG matrix transition. Our AT-cut quartz technique using our model can also be used as a high frequency rheometer for the sol-gel materials.

Multi-turn split-conductor transmission-line resonators.

Multi-turn split-conductor transmission-line resonators (MSTR) can be designed to operate over a wide range of NMR operating frequencies without lumped tuning capacitors. This architecture constitutes an alternative solution to recent designs proposed for high-Q, thin-film, high-temperature superconducting NMR probes. An advantage is that the resonant frequency can be calculated in a relatively simple manner in terms of coil turns or total coil length, coil width, substrate thickness, and dielectric constant. Analytical calculation of the resonant frequency is provided. Also, a series of MSTRs was constructed on a double copper-clad substrate, and their resonant frequencies are noted. The results obtained were in good agreement with the predicted values.