Alessandro Tocchio

A Resonant Microaccelerometer With High Sensitivity Operating in an Oscillating Circuit

A new micromachined uniaxial silicon resonant accelerometer characterized by a high sensitivity and very small dimensions is presented. The devices working principle is based on the frequency variations of two resonating beams coupled to a proof mass. Under an external acceleration, the movement of the proof mass causes an axial load on the beams, generating opposite stiffness variations, which, in turn, result in a differential separation of their resonance frequencies. A high level of sensitivity is obtained, owing to an innovative and optimized geometrical design of the device that guarantees a great amplification of the axial loads. The acceleration measure is obtained, owing to a properly designed oscillating circuit. In agreement with the theoretical prediction, the experimental results show a sensitivity of 455 Hz/ ( g being the gravity acceleration) with a resonant frequency of about 58 kHz and a good linearity in the range of interest.

Versatile fabrication of vascularizable scaffolds for large tissue engineering in bioreactor

Despite significant progresses were achieved in tissue engineering over the last 20 years, a number of unsolved problems still remain. One of the most relevant issues is the lack of a proper vascularization that is limiting the size of the engineered tissues to smaller than clinically relevant dimensions. Sacrificial molding holds great promise to engineered construct with perfusable vascular architectures, but there is still the need to develop more versatile approaches able to be independent of the nature and dimensions of the construct. In this work we developed a versatile sacrificial molding technique for fabricating bulk, cell-laden and porous scaffolds with embedded vascular fluidic networks. These branched fluidic architectures are created by highly resistant thermoplastic sacrificial templates, made of poly(vinyl alcohol), representing a remarkable progress in manufacturability and scalability. The obtained architecture, when perfused in bioreactor, has shown to prevent the formation of a necrotic core in thick cell-laden constructs and enabled the rapid fabrication of hierarchically branched endothelium. In conclusion we demonstrate a novel strategy towards the engineering of vascularized thick tissues through the integration of the PVA-based microfabrication sacrificial approach and perfusion bioreactors. This approach may be able to scale current engineered tissues to clinically relevant dimensions, opening the way to their widespread clinical applications.

Mechanical and Electronic Amplitude-Limiting Techniques in a MEMS Resonant Accelerometer

Two methods for limiting the oscillation amplitude in micromechanical resonators, typically used in many kinds of MEMS sensors, are discussed and compared. First, it is shown how the presence of parasitic capacitances sets several constraints on the design of the oscillating circuit gain and bandwidth. The paper specifically focuses on the case of a transimpedance based oscillator coupled to a clamped-clamped beam, that forms the sensing element of a resonant accelerometer. Experimental results then show that the oscillating amplitude can be limited either using an electronic limiting stage, or exploiting the mechanical nonlinearities of the beam for large displacements. Though the latter approach is advantageous in terms of power dissipation, it is shown that the sensitivity of the resonant accelerometer is strongly compromised.

Poly(amido-amine)-based hydrogels with tailored mechanical properties and degradation rates for tissue engineering

Poly(amido-amine) (PAA) hydrogels containing the 2,2-bisacrylamidoacetic acid-4-amminobutyl guanidine monomeric unit have a known ability to enhance cellular adhesion by interacting with the arginin-glycin-aspartic acid (RGD)-binding αVβ3 integrin, expressed by a wide number of cell types. Scientific interest in this class of materials has traditionally been hampered by their poor mechanical properties and restricted range of degradation rate. Here we present the design of novel biocompatible, RGD-mimic PAA-based hydrogels with wide and tunable degradation rates as well as improved mechanical and biological properties for biomedical applications. This is achieved by radical polymerization of acrylamide-terminated PAA oligomers in both the presence and absence of 2-hydroxyethylmethacrylate. The degradation rate is found to be precisely tunable by adjusting the PAA oligomer molecular weight and acrylic co-monomer concentration in the starting reaction mixture. Cell adhesion and proliferation tests on Madin-Darby canine kidney epithelial cells show that PAA-based hydrogels have the capacity to promote cell adhesion up to 200% compared to the control. Mechanical tests show higher compressive strength of acrylic chain containing hydrogels compared to traditional PAA hydrogels.

Operation of Lorentz-Force MEMS Magnetometers With a Frequency Offset Between Driving Current and Mechanical Resonance

The paper discusses the operation of Lorentz-force-based microelectromechanical magnetometers at a driving-current frequency slightly lower than the device resonance frequency. Among the advantages with respect to operation at resonance, there are a higher achievable signal to noise ratio (thanks to the lower permitted pressure and damping coefficient, which have now no influence on the maximum sensing bandwidth) and the possibility of driving more magnetometers in series through a single current source, enabling the fabrication of low-power 3-axis magnetic field sensors. A partial drawback is represented by a loss in gain-factor. Experimental results obtained on a sample device confirm the trade-off between gain-factor decrease and bandwidth increase. Guidelines for an optimized design of Lorentz force magnetometers are given together with a comparison with other state-of-the-art technologies through the introduction of a figure of merit. In particular, it is shown how Lorentz force devices can reach better performance in terms of minimum detectable magnetic flux density per unit current consumption and bandwidth.

Design Criteria of Low-Power Oscillators for Consumer-Grade MEMS Resonant Sensors

This paper discusses the constraints in the design of circuits for microelectromechanical systems (MEMS) resonant sensors in consumer applications, presents a novel integrated circuit implementation, and shows that this approach can be competitive with respect to the mostly used capacitive readout. From a circuit design perspective, it is shown how the large equivalent resistance typical of MEMS resonators, their operation close to mechanical nonlinearity, and the effect of feedthrough capacitances on the oscillating loop constrain the power requirements of the driving/readout electronics. As a case study, a resonant accelerometer built in an industrial process is coupled to a suitably designed transimpedance amplifier with a low-power “hard limiter.” The performance shown in terms of linearity across the measurement range (±8 g), minimum measurable acceleration (1 mg with a readout bandwidth of 100 Hz), and power consumption (≈ 100 μW per axis) is comparable to those of state-of-the-art capacitive inertial sensors.

A high sensitivity uniaxial resonant accelerometer

A new micro-machined uniaxial silicon resonant accelerometer characterized by a high sensitivity is presented. The geometrical setting is analytically optimized in order to improve the sensitivity and the linearity of the sensor. The high level of sensitivity is obtained at relatively low quality factors and keeping the dimensions very small, thanks to an innovative and optimized geometrical design of the device. The proposed accelerometer has been fabricated and the first experimental measurements are presented in the paper.

MEMS Motion Sensors Based on the Variations of the Fringe Capacitances

In this paper, a MEMS motion sensor is described which uses fringe field capacitances as sensing elements. Each capacitance basic cell is formed using thin strips of standard electrical paths, running side by side, and close one another in a plane parallel to the substrate: in this way, the capacitance between the paths is mainly formed by the fringe field streamlines. A grounded seismic mass, suspended in the air volume where the electric fringe field streamlines are concentrated, determines changes in the capacitance value when subject to an external force that makes it move. With respect to parallel plates capacitive sensors, this implementation has the advantage to be more immune from the pull-in instability. A test device is built using an industrial surface micromachining process. The experimental results show a sensitivity close to 1 fF of capacitance variation per unit of earth acceleration for a device taking up an area of (430 μm)2. A detailed analysis performed through FEM simulations predicts possible consistent sensitivity improvements.

A Versatile Instrument for the Characterization of Capacitive Micro- and Nanoelectromechanical Systems

This paper presents a laboratory prototype of an instrument developed for electromechanical characterization of capacitive micro- and nanoelectromechanical systems (M/NEMS). The instrument aims at filling the gap between commercial electrical instrumentation (impedance meters) and optical instrumentation: For most M/NEMS devices, impedance meters allow only quasi-stationary measurements (i.e., capacitance-voltage C-V curves) with a bandwidth limited to few hundreds of hertz; optical instrumentation allows dynamic characterization (Bode diagrams) but, besides being bulky and extremely costly, does not allow the measurement on packaged devices-and packaging is often an issue for the device performance and reliability. The proposed versatile characterization platform, controlled by LabVIEW libraries, monitors the capacitance variation, resulting from different kinds of electrical stimuli, via real-time capacitive sensing. The measurements are both stationary C-V curves and dynamic responses to input steps in the time domain, which are convertible into Bode plots in the frequency domain. Measurements can be done on bare or packaged and on wafer-level or diced devices, in a differential or single-ended configuration, with a good immunity to parasitic capacitances, a sensing resolution on the order of ≈1 aF/√(Hz), and a maximum testable device mechanical bandwidth around 100 kHz. A characterization of two sample structures, a micromachined magnetometer and a clamped-clamped beam resonator, is given as an example, followed by a discussion on future improvements.

Off-Resonance Low-Pressure Operation of Lorentz Force MEMS Magnetometers

This paper demonstrates bandwidth extension for Lorentz force microelectromechanical systems magnetometers operated at a frequency slightly lower than the mechanical resonance. This off-resonance mode of operation also minimizes the critical tradeoff between maximum sensing bandwidth and minimum measurable field, enabling low-pressure operation and the associated resolution improvement. The experimental verification is obtained using two identical devices packaged at two different pressures and tested with a suitable low-noise board-level electronics. The first device, packaged at 1 mbar, has a (mechanical) sensing bandwidth of ~50 Hz, and a resolution per unit bandwidth and driving current of 290 nT · mA/√Hz. With the second device, packaged at 0.25 mbar and operated off-resonance, roughly twice better resolution (160 nT · mA/√Hz) and four times larger bandwidth (>200 Hz) are obtained.