Design and characterisation of low-cost thick-film piezoresistive force sensors for the 100 mN to 100 N range

Two simple low-cost thick-film force sensor designs are optimised, characterised and tested in this work, combining calculations of sensor element and joint stresses, joint strength characterisation and measurement of complete sensors. All sensors are based on standard hybrid substrates. Our results show that a single sensor geometry cannot cover such a wide range of forces in an optimal way. For small forces (100 mN to ca. 2 N), simple cantilever force sensors are an excellent solution, achieving reasonable precision with a very simple design and compatibility with an SMD (surface mount device) assembly process with solder or conductive glue. Characterisation of solder joint strength shows that such joints can reliably withstand the bending moments resulting form the loading of the cantilever sensor up to ca. 2 N. Above this force, both solder joint and cantilever strength become critical in the cantilever design. Therefore, a 3-point or 4-point bending beam geometry must be selected, thereby extending the force range to ca. 100 N.

Micro and nano tools for magnetic field imaging

Schahrazede-Lila Mouaziz

Scanning probe microscopy has become, nowadays, a branch of microscopy widely used to image surfaces of different specimens. Among the various applications, the magnetic imaging is one of the most exciting scanning probe microscopy techniques. It allows the study of topography of samples at the nanometer scale. The magnetic force microscopy is the best known and the most employed magnetic imaging technique. It has been successfully used for the imaging of various samples but with the major inescapable limitations that it is confined to the surface, non-quantitative and invasive. Techniques such as the scanning Hall probe microscopy (SHPM) and the magnetic resonance force microscopy (MRFM) have emerged, several years later, as promising approaches to overcome these limitations. In this thesis, we present the design, the fabrication and the characterization of, first, micro-Hall sensors on SU-8 cantilevers used for SHPM and, second, of ultrasensitive Si cantilevers used for MRFM experiments. A microfabrication technique has been developed to produce hybrid devices consisting of micro-nano Hall sensors integrated on SU-8 cantilevers. Two materials have been used to produce the Hall sensors. First, bismuth (Bi) was chosen for its low carrier charge density and its negligible surface charge depletion effects so that it should provide sensitive sensors. The second material tested was nickel (Ni), chosen for the dominant effect called the extraordinary Hall effect. The active areas of 5×5, 2×2 and 1×1 µm2 were realized and reproduced with the standard photolithography. Producing submicron Hall sensors required the use of the focused ion beam (FIB) that allowed a decrease of the size of the sensors down to 80×80 nm2. The delicate transferring of the nano sensors on the polymer cantilevers was successful. Bi micro-Hall sensors have shown a fragility making them delicate to handle. However, the Ni micro-Hall sensors had an excellent functionality. Their characterization has shown sensors with a low sensitivity compensated by the high applied current providing a high magnetic field resolution. The magnetic field images were carried out using the Ni micro-Hall sensors. They had a high stability during the imaging experiments. The spatial resolution was set by the sensors active area limited to the micro-meter scale. However, the magnetic field resolution is better than 1 T/√Hz a result comparable to the high resolution offered by the semiconductors. Single crystal silicon cantilevers have been fabricated for use in MRFM experiments. Also, cantilevers with integrated micro-magnets have been successfully produced. The developed microfabrication technique has allowed a high yield production (up to 70%). The softest fabricated cantilevers are 0.34 µm thick, 500 µm long and 10 µm wide. These cantilevers have been characterized under vacuum conditions (1×10-6 mbar). The resonance frequencies and the quality factors were measured using the frequency sweep or noise methods and both provided the same results. The Rayleigh method that we have adapted to the cantilevers geometry, made it possible to estimate the theoretical resonance frequencies with a good approximation and the calculated values fit well with the measured ones. All the cantilevers have a quality factor above 20000 and the force sensitivity achieved was between 10-16 and 10-17 N/√Hz A mathematical model has been developed based on the motion equation of a harmonic oscillator under the influence of a magnetic field. The model was studied to find the expression of the resonance frequency of cantilevers with integrated magnetic tips in the presence of a parallel or a perpendicular magnetic field. The shift in frequency due to the presence of the applied magnetic field depended on the magnetic tip magnetization. The measurement of the frequency shift should permit the calculation of the magnetization of these micro-magnets directly integrated on the cantilevers. The probes that we have realized represent micro and nano tools suitable for the magnetic field imaging that can be done with the SHPM and MRFM techniques.

EPFL2007

The Design and Control of the Multi-Modal Locomotion Origami Robot, Tribot

Mohsen Falahi, Jamie Paik, Zhenishbek Zhakypov

Origami robots (Robogamis) use architecture to strategically activate different sets and sequence of actuators to achieve large variety of reconfigurable forms. Tribot is a unique mobile origami robot that can simultaneously choose between two modes of locomotion: jumping and crawling. When assembled, Tribot measures 64 x 34 x 20 mm^3, weighs 4 g, crawls at 17% of its body length per gait cycle and jumps seven times its height repeatedly without needing to be reset. To optimize the practicality of the nominally 2D design, we made two different approaches to build the prototypes. For one of them, we used the "traditional", monolithic, layer-by-layer robogami fabrication method and the second, we printed out most parts using a multi-material 3D printer. By showing the performance of two prototypes side-by-side, we show that with the 3D printer, we can minimize the number of functional layers and reduce the fabrication time. The embedded sensors allow Tribot's crawling gait pattern and jumping height to be modulated with a closed loop control. We compare the expected gait step size and displacement to that of the presented prototype while describing the design and control parameters to achieve the experimental results. We also illustrated the preliminary graphical design tool platform developed to optimize the next design iteration of Tribot.

2015

Thermal microsensors for in vitro and in vivo monitoring of chemical and biological processes

Rima Padovani

Metabolism is a highly coordinated biochemical cellular activity indispensable for sustaining life, and is often related to mitochondrial functioning. Innovative technological approaches allowing the study of the thermal signature of chemical and metabolic processes are presented in this thesis. In particular, two solutions are proposed: (i) a nanocalorimetric platform enabling measurements of biochemical heat production in vitro, and (ii) a method for localized in vivo temperature measurements in mice using implantable miniaturized sensors. First, the design and development of the nanocalorimetric platform is presented. In particular, this platform is built around a commercial thermopile-based sensor chip, on which an open-well reservoir holding a sample with a volume of a few tens of microliter is directly positioned. Both components are embedded into an isothermal housing providing an excellent temperature stability (± 1 mK), which is a prerequisite for accurate heat power measurements. Furthermore, the platform is characterized by a fast thermalization time to the target temperature (less than 30 minutes) and a fast sensing response time (a few seconds). Numerical simulations, which allowed the optimization of the platform design, are also reported. Electrical calibration of the platform was carried out using resistive heaters positioned either on the sensor chip or in the sample reservoir. A heat power limit of detection of 70 nW was estimated. Then, two reactions for the validation of the nanocalorimetric platform were investigated: (i) the mixing of 1-propanol in water, and (ii) the oxidation of glucose catalyzed by glucose oxidase. The results showed good agreement with literature, confirming that our versatile platform may be applied to many thermochemical studies, including thermodynamic analysis and kinetic reaction analysis. Furthermore, localized in vivo temperature measurements in mice by means of implantable miniaturized sensors are presented. The aim was to monitor mouse metabolism during cold exposure, and to record possible temperature differences between the body temperature measured in the abdomen and the temperature of the brown adipose tissue (BAT) situated in the interscapular area. This approach is of biological interest as it may help unravelling the question whether biochemical activation of BAT is associated with local increase in metabolic heat production. Specifically, the preparation and calibration of miniaturized thermistor sensors are reported and the surgical procedure is also described. Temperature measurements, carried out on mice during cold exposure (6 °C) for a maximum duration of 6 hours, are presented and analyzed. Control measurements with a conventional probe confirmed the good performance of the implanted sensors. Moreover, analysis of the experimental results allowed the identification of two different mouse phenotypes, distinguishable in terms of their metabolic resistance to cold exposure. This difference was investigated from the thermal point of view by computational simulations. A simple physical model of the mouse body allowed to reproduce the global evolution of hypothermia and to explain qualitatively the temperature difference between abdomen and BAT locations. While with this approach, we have demonstrated the importance and feasibility of localized temperature measurements on mice, further optimization of this technique is foreseen to better identify local metabolism variations.