Capacitive sensor to monitor the level of drug inside an injector pen

Diabete is a disease which is spreading faster than ever, impacting now a larger population. Patients suffering from this disease need the injection of insulin to control extracellular sugar levels. Pharmaceutical companies developed injector pens to allow patients to inject themself the right amount of drug. But the dosage depends on many parameters as the previous meal, current blood sugar level... and a wrong dosage can lead to health problem e.g., loss of conciousness at short terms and severe diseases like cardiac pathologies at long terms... To prevent these situations, a smart pen cap has been developed at Valtronic Technologies to follow and log the individual injections by measuring the level of drug in the pen, the temperature, and the relative humidity to allow the patient and the medical staff to have an history of the injections and the climatic conditions.Injector pen and smart pen cap have been firstly presented with a non-exhaustive list of different physics which can be used to measure a level of liquid. For need of precision, the capacitance technique is the best technology choice. Therefore, the smart pen cap is based on a capacitive sensor. Different capacitance converter topologies have been compared through a bibliographic study. It appears that the Lock-in converter gives the best sensitivity and Charge Sensitivity Amplifier has the lowest power consumption. A trade-of was to use a capacitance-to-digital-converter. A chip from Analog device has been selected due to its high resolution below the femto-farad level, its simple interface and its availability on the market which allows a fast development and industrialisation. Then, switch and electronic structure are presented. First tests with an empty device have been conducted to find the noise floor, which is below 0.1fF.Subsequently, a study of different electrodes configuration has been performed. Firstly, an analysis on two parallel flat electrodes has been conducted using theoretical and simulation approaches. Then, two semi-cylindrical electrodes have been analysed. A comparison has been made between two theoretical methods which are: considering an infinite number of parallel flat electrodes following the shape of the semi-cylindrical electrodes and the conformal mapping methods. This last method gives coherent results when comparing real measurements and simulation. The device is equipped with semi-cylindrical electrodes which allow for a small package and a good correlation between capacitance and drug level with a convertion slope of 0.5fF/µL. Tests have been performed which give an error of +-1 IU and +-4 IU for large injection (set point equal to 72 IU).Finally, the study of some parasitic effects is presented in the last section. An important effect is the climatic effect which affects the capacitance due to variation of electric permittivity linked to temperature change and water absorption of certain materials as plastics. To compensate for this effect, reference electrodes and a climatic sensor have been implemented. Another important effect comes from the cartridge position in the non-uniform electric field of the smart pen cap. A four electrodes device has been realized to allow a perpendicular rotation of the electric field to obtain a more constant averaged electric field. As a consequence, the position impact of the cartridge has been reduced. Other effects have been considered. The device can detect a minimum injection of 52uL.

Conductivity, dielectric and piezoelectric properties of SrBi4Ti4O15

Strontium Bismuth Titanate is a very promising material for high temperature piezoelectric applications, its elevated ferroelectric phase transition (530°C), linear piezoelectric properties under low field and relatively low room temperature conductivity (compared to others Bismuth Titanates) make it very attractive for precision sensors. However, under severe conditions (low frequency, high field, high temperature or low oxygen partial pressure) some of those advantages disappear. Piezoelectric response is dominated by charge drift in general becomes unstable. Above all, at high temperature and low oxygen partial pressure, a large conductivity increase reduces the piezoelectric efficiency of the material, in this work, electrical conductivity, piezoelectric properties and dielectric permittivity of SrBIT ceramic have been investigated in conditions of high temperature, low oxygen partial pressure, low to moderate driving field and frequency. As a result of this research, a better understanding of SrBIT properties was achieved. Thanks to a careful study of SrBIT processing as a bulk ceramic, a reproducible route was established. Many basic mechanisms leading to both SrBIT crystallization and sintering have been identified. It was demonstrated that a detailed knowledge of the exact processing conditions is required in order to achieve high quality material. DC conductivity measurements were carried out as a function of temperature, oxygen partial pressure and dopant concentration. It was found that the apparent activation energy for conduction for undoped SrBIT was 1 eV between 140 and 220°C and 1.5 eV between 450 and 700°C. Decrease of the activation energy in the lower temperature range has been discussed considering grain boundary conductivity, as a transition from electronic to ionic conduction or as consequence of small polarons conduction. It was shown that lower activation energy resulted from Manganese doping (0.5 eV), this was interpreted as either growing influence of grain boundary conductivity as dopant concentration increases or as shallow hole trap formation. DC conductivity measurements and acceptor/donor doping experiments demonstrated p-type conductivity in the low temperature range (up to 220°C) as donor doping decreases conductivity, while oxygen partial pressure controlled measurements indicated n-type conductivity at higher temperature (above 700°C). An electrical impedance analysis was performed with several equivalent circuits. The aim of these models was to simulate the impedance of SrBIT. The best approximation was found with a distributed element of Havriliak-Negami type. However, as the physical justification for this circuit was not clear, the investigation of the grain, grain boundary and electrode impedances was performed with multiple discrete parallel RC elements. With temperature, grain size and oxygen activity variations, the identification of three separate contributions as grain, grain boundary and electrode was realized. The anisotropy of conductivity and permittivity was demonstrated with textured material and both DC and AC analysis. With the master curves built for the electrical modulus, it was found that the impedance probably related to Bismuth oxide layers produces an additional high frequency are. From this finding and by comparing characteristic relaxation frequencies of undoped, 2 mol.% Mn and 4 mol.% Nb SrBIT, it was determined that conductivity is higher in the ab plane direction than in the c direction within both perovskite units and Bismuth oxide layers. With conductivity measurements under controlled oxygen partial pressure, it was found that an acceptor-based (intrinsic or extrinsic) model could be used to describe the electrical conductivity under controlled oxygen partial pressure of both undoped and 2 mol.% Mn doped SrBIT. However, as neither a pO2-1/6 region (intrinsic oxygen vacancies compensated by electrons) for undoped SrBIT nor a pO2-1/4 region (oxygen vacancies compensated by singly ionized acceptors) for Mn doped SrBIT were seen, it was concluded that the acceptorcontrolled model is not sufficient for a complete description of SrBIT. For this reason and in order to include the low oxygen partial pressure behavior of undoped SrBIT, a donor-based (intrinsic) model was also considered. The source of intrinsic donors would be in that case Bi3+ cations sitting on Sr2+ sites in the perovskite sublattice. Considering Bismuth vacancies as the negative compensating species, both pO2-1/4 and pO2-independent regions could be predicted with the model. However, even if the donor-controlled model seems to better match conductivity measurements in the full PO2) range, rejecting the acceptor-based model would be an error. It is actually not demonstrated that in SrBIT the concentration of exchanged Bismuth cations is always (all temperature, pressure) larger than the natural acceptor impurity concentration. It is very likely that the cation exchange is dependent of the oxygen partial pressure. It is also not proved as suggested in the literature that direct compensation between exchanged Strontium and Bismuth exists, reducing the net donor-excess. With the acceptor-controlled model, the mass-action constants for reduction and for ionization of intrinsic carriers across the band gap were determined. The band gap of SrBIT was estimated to be 3.5 eV. The ionic conductivity of SrBIT was determined at high temperature with measurements under controlled oxygen partial pressure. It was found that the electrical conductivity of SrBIT is probably mixed (electronic and ionic) as the estimation of the transference number provided quite large values (t=0.8 at 800°C). From electronic and ionic conductivity data, mixed conduction can actually be predicted in a large temperature range (above room temperature). The piezoelectric measurements using direct effect demonstrated that it is actually possible to unlock piezoelectrically active ferroelectric domain walls and create non-linear piezoelectric properties in undoped SrBIT. This occurs above a threshold elastic field, which is thermally activated. With a piezoelectric composite, it was demonstrated that the electromechanical coupling between two different phases creates a piezoelectric relaxation. This one could be positive or negative depending on the respective properties of each composite's component. It was shown experimentally that a small temperature change is sufficient to transform a positive relaxation into a negative one. While these experiments did not provide a detailed microstructural explanation for the piezoelectric relaxation observed in 2 mol.% Mn doped SrBIT, they gave a first insight into an original phenomenological approach. Microstructure and piezoelectric properties were related with the calculation of a piezoelectric relaxation composite made of two textured samples. This demonstrated that a piezoelectric relaxation may occur just because of anisotropy. Microstructural reproduction of this coupling is actually an important feature of Aurivillius phases.

EPFL2000

Infrared properties of the Si-SiO2 interface from first principles

Feliciano Giustino

We study the infrared properties of the Si-SiO2 interface within a first-principles approach. In order to provide an atomic-scale description of the dielectric permittivity (both high-frequency and static) and of the infrared absorption at the interface, we introduce two theoretical schemes of general validity. First, we develop a method for investigating atomic-scale dielectric permittivity profiles across interfaces between insulators. From the microscopic charge density induced by an applied electric field, we calculate a local permittivity which describes variations of the dielectric response over length scales of the order of interatomic distances. In order to establish a relation between the dielectric response and the underlying microscopic structure, the local permittivity is further analysed in terms of maximally localized Wannier functions. Second, we develop a method for calculating from first principles both the transverse-optical and longitudinal-optical infrared absorption spectra at surfaces and interfaces. We derive expressions for the total absorption spectra of the system under consideration, and then define a spatial decomposition which provides the evolution of the infrared activity across that system. Such a decomposition is particularly suited for associating specific spectral features to the underlying local bonding arrangements. By using the first method, we determine the profile of the local permittivity across several structural models of the Si-SiO2 interface. We are able to show that the dielectric transition from the silicon to the oxide occurs within a width of only a few angstroms, and that the interfacial layer carries an enhanced permittivity with respect to bulk vitreous silica. Correspondingly, the equivalent oxide thickness of the interfacial oxide is found to be smaller than the corresponding physical thickness by 0.2–0.3 nm, with beneficial consequences for the scaling of Si-based electronic devices. By using the second method, we solve a long-standing controversy related to the red shifts of the high-frequency peaks observed in the infrared spectra of ultrathin oxides on silicon with decreasing thickness. By calculating the transverseoptical and the longitudinal-optical absorption spectra across a realistic model of the Si(100)-SiO2 interface, we are able to assign the microscopic origin of these shifts to the lengthening of the Si-O bonds in the interfacial substoichiometric oxide.

EPFL2005

Increased electromechanical sensitivity of polysiloxane elastomers by chemical modification with thioacetic groups

Yee Song Ko

Chemically cross-linked polydimethylsiloxane (PDMS) elastomers have gained importance as a dielectric in elastic capacitor actuators, which elongate when electrically charged. Common PDMS elastomers have a dielectric permittivity of only about 3 and thus elongations interesting for real applications occur only at high voltages. In this work, a new class of silicone-based elastomers with increased permittivity are synthesized starting from polymethylvinylsiloxane and thioacetic acid via a photo-induced thiol-ene reaction, whereby polar thioacetate groups are introduced at every siloxy unit. The silanol end-groups of the formed polymer are subsequently used for cross-linking into thin films using a condensation reaction with poly(methylhydrosiloxane-co-dimethylsiloxane). These films show good mechanical properties and have a low glass transition temperature (58 degrees C), which allows for a wide range of use temperatures. Further they have an increased permittivity (epsilon' = 4.7) and an increased sensitivity to electric field as compared to regular polydimethylsiloxane elastomers. Our new elastomers are easily accessible and can be processed into very thin films, which allows access to actuators operable way below 1000 V. An actuation strain of 12.8% at an electric field of 21.5 V/mu m was measured. (c) 2019 The Authors. Published by Elsevier Ltd.

2020

Infrared properties of the Si-SiO2 interface from first principles

Feliciano Giustino

EPFL2005

Conductivity, dielectric and piezoelectric properties of SrBi4Ti4O15

EPFL2000