Sensors & Actuators: A. Physical 340 (2022) 113536 Contents lists available at ScienceDirect Sensors and Actuators: A. Physical journal homepage: www.journals.elsevier.com/sensors-and-actuators-a-physical A flexible dipole antenna for direct transduction of microwave radiated power into DC mechanical deflection R. Ruiz *, J. Bonache, G. Abadal Departament d′ Enginyeria Electrònica, Universitat Autònoma de Barcelona, Bellaterra 08193, Spain A R T I C L E I N F O A B S T R A C T Keywords: A new device resulting from the merging of mechanical and electromagnetic properties of a couple of conductive RF-MEMS cantilevered beams is described in this paper. The device, which is based on two metallic clamped-free beams Wireless power transfer which, at the same time, are the arms of a radio frequency dipole antenna is capable to receive the electro­ Electrostatic actuation magnetic power radiated from an emitting antenna and transduce it directly in a dc mechanical actuation. A numerical model developed to describe the behavior of the device has been validated through the test of a millimeter scale demonstrator working in the microwave frequency band. 1. Introduction to around 1/3 of the capacitive transduction gap [19]. Several strategies based on a current source driving [20], a high frequency actuation [21], Multidisciplinary nature of microelectromechanical systems tech­ a series capacitor [22], or a levitation base electrode [23] have proved to nology (MEMS) has been proved along its recent history, through its overcome this limitation. impact on mature disciplines as optics, biology or microwave engi­ Resonant driving [24–27] is a very promising alternative to extend neering, giving rise to new application areas such as MOEMS [1], the operation range, which is based on using an LC circuit resonator to Bio-MEMS [2] or RF-MEMS [3]. In particular, works involving switching drive the electrostatic actuator. In this case, the frequency dependence networks [4], low noise oscillators [5], amplifiers [6], tunable capaci­ of the ac driving voltage introduces useful negative feedback that tors [7] and reconfigurable antennas [8] have been extensively reported compensates the pull-in instability. But, as an additional advantage, in the field of RF-MEMS. To excite and detect static displacements or resonant driving operated at the resonance frequency of the LC circuit dynamic vibrations of suspended moving elements, transducing mech­ allows to get an amplifying effect on the ac driving voltage, which allows anisms in RF-MEMS involve, in most of the cases, electrostatic actuation to consequently lower the excitation ac voltage. and detection through optimized comb-drive [9,10] or parallel plate In the work reported here we present a new concept derived from the [11,12] capacitive structures that are integrated using MEMS resonant driving strategy that makes possible to drive a capacitive technologies. MEMS structure without any on-chip locally applied voltage. The new Concerning driving and biasing of the MEMS capacitive structures, concept, called direct transduction from electromagnetic to mechanical controlled voltage signals as well as dc bias voltages that can be exter­ domain is sustained by a new device called MEMSTENNA. Such a nally applied, are preferably synthesized by on-chip integrated circuitry MEMSTENNA, which is a combination of an electrostatic MEMS [13], using hybrid technologies as CMOS-MEMS [14]. Thus, although capacitive transducer and a radio frequency antenna, consists of two low dc voltages are always needed as power supply of the active CMOS clamped-free metal beams that constitute, at the same time, the two circuitry [15], large bias voltages are usually required not only as arms of a dipole antenna. The free ends of both beams are separated driving signal [16,17], but also to generate detectable currents in the along an overlapping short length, so that a parallel plate capacitive readout capacitive MEMS structures [18]. Therefore, capacitive MEMS transducer is defined between both arms of the antenna. When an devices are intrinsically active since they need a dc voltage to work. electromagnetic radiation is received by the MEMSTENNA, an ac However, one of the main drawbacks of electrostatic transduction is voltage is generated in its feed point, i.e. the capacitive MEMS trans­ imposed by the pull-in instability, which comes from the need to ducer in this case, so that the induced ac electrostatic force will produce polarize the structures. It is well known that when a simple voltage a flexural actuation on both flexible arms of the MEMSTENNA. In the source is used to drive the structure, its range of displacement is limited present work we describe a numerical model of the MEMSTENNA * Corresponding author. https://doi.org/10.1016/j.sna.2022.113536 Received 25 November 2021; Received in revised form 7 March 2022; Accepted 29 March 2022 Available online 13 April 2022 0924-4247/© 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/). R. Ruiz et al. Sensors and Actuators: A. Physical 340 (2022) 113536 device, and we discuss the test results of a proof-of-concept prototype free end at the modified feeding point converted to the electrostatic designed and fabricated to validate the model. actuator and the anchors at the opposite end. Thus, when the MEM­ Potential applications of a MEMSTENNA are foreseen as the core STENNA is receiving an electromagnetic wave, an ac voltage is induced structure of a batteryless and wireless relay in, for instance, wake up at the modified feeding point and the electrostatic attractive force wireless circuits [28] which have promising applications in IoT tech­ induced in the MEMS capacitor will produce a deflection of both nologies [29]. Here, the MEMSTENNA could be configured in a relay clamped-free beams. Notice that since the frequency of the microwave structure that can be switched ON and OFF by a remote RF power radiation, typically in the GHz range, is usually orders of magnitude control pulse. Unlike standard wake up wireless solutions, a MEM­ larger than the resonance frequency of the beams (Hz-kHz), any vibra­ STENNA based relay would not only be activated remotely but it would tion can be excited directly on the cantilevers. However, because of the also be powered remotely since the mechanical switching function nonlinear dependence of the electrostatic force with its inducing voltage would be not only controlled but also powered wirelessly. However, as it (it is proportional to V2), a dc component on the force is present and a will be discussed later in this paper, the RF power levels needed to static deflection of the beams is produced. mechanically actuate our actual MEMSTENNA proof-of-concept dem­ To modelize the behavior of the MEMSTENNA, let us consider line­ onstrators are too high to be realistic for a practical application, and arly polarized incident electromagnetic plane waves propagating in the hence an optimization of the power efficiency of the MEMSTENNA de­ z-direction with the electric field, Einc, aligned along the longitudinal vice is mandatory for the success of future applications. direction of the antenna, x-axis, as shown in Fig. 1.c. As the dipole works in the receiving mode, when an electromagnetic wave excites the an­ 2. Working principle and modelization of the device tenna, a voltage, Vm, is generated at the antenna gap, i.e, the MEMS capacitive actuator. Consequently, an electrostatic force, Fee, is applied As described in Fig. 1, the MEMSTENNA device is composed of two in the z-direction at each of the plates of the MEMS capacitor producing metal arms of dimensions L, w and t (length, width and thickness), the mutual attraction of the cantilevers at its very end. which is, from an electromagnetic perspective, a half-wavelength dipole The MEMSTENNA, as a receiving antenna, is electrically modeled by receiver antenna [30] with a modified feeding point. The referred its Thévenin equivalent circuit [31,32], which, as shown in the left side modification consists of placing the antenna arms in parallel along a of Fig. 2, is defined by a series antenna’s impedance, Z(ω), given by: certain overlapping length, lm, and separated by a gap distance, g0. 1 Thereby, the standard feeding port of the antenna is converted into an Z(ω) = Ra (ω) + jX a (ω) = Ra (ω) + jωLa − j (1) ω Ca electrostatic MEMS actuator defined by the MEMS capacitor, Cm, which plates area is A=lm⋅w, and distance between plates is g0. On the other and an open-circuit voltage, VOC, that can be calculated [33] from the hand, the mentioned two arms are, at the same time and from a me­ effective length, Leff, and the incident electric field, Einc, as: chanical point of view, two clamped-free beams (cantilevers) having the Fig. 1. Lateral (a) and top (b) views of the MEMSTENNA structure based on a dipole antenna structure, with arms being mechanical beams in a clamped-free configuration. (c) Bending of the MEMSTENNA beam components produced by the electrostatic force, Fee. 2 R. Ruiz et al. Sensors and Actuators: A. Physical 340 (2022) 113536 Ra(ω) Xa(ω)j cantilever system is defined as: Vm ϵ0 ⋅A⋅V 2m k1 b1 Fee = (6) z1(t) 2⋅(g0 − z1 (t) − z2 (t))2 m1 VOC Cp If we consider each component of the double cantilever as identical, g0 m2 z2(t) then Eqs. (4), (5) and (6) can be reduced to a single equation as: k2 b1 d2 z(t) dz(t) ϵ0 ⋅A⋅V 2m m⋅ + b⋅ + k⋅z(t) = (7) dt2 dt 2⋅(g0 − 2⋅z(t))2 Fig. 2. Thévenin equivalent circuit of the MEMSTENNA as a dipole antenna Finally, Fig. 3 shows the block diagram of the model implementation. (left side) loaded by the MEMS capacitive actuator (right side), represented by In a first block, the movement equation of the cantilevers (Eq. 7) is its spring-mass-damper model. numerically solved and the time evolution of the cantilevers deflection, z (t), is obtained. In the second block, the MEMS capacitance, that changes VOC = Leff ⋅Einc (2) when the cantilevers move, Cm(z(t)), is calculated taking z(t) as an input. The effective length, Leff, for a half-wavelength dipole is defined in Also in this block, the antenna impedance components, Ra and Xa, are [33], where a uniform and sinusoidal current distribution of the dipole is calculated by means of the method of moments (MoM) [34]. From the considered. Therefore, an average value of 2/π = 0.64 for the maximum antenna and the MEMS capacitance impedances, the voltage at the sinusoidal current distribution has been considered. As the length of the MEMS capacitive actuator terminals, Vm, is determined. Then, to close antenna component of our MEMSTENNA device is L= λ/2, the effective the loop, the electrostatic force, Fee, is calculated in the third block, as of length is given by: the cantilevers deflection, z(t), and the MEMS actuator voltage, Vm. Leff = 0.64⋅L (3) 3. Test results of a proof-of-concept demonstrator. Model Additionally, right side of Fig. 2 shows the electromechanical model validation of the MEMS capacitive actuator, which is based on a pair of spring- mass-damper systems, associated to the double cantilever components 3.1. Prototype description of the MEMSTENNA. Eqs. (4) and (5) describe the dynamic behavior of the top and bottom cantilevers, respectively. A MEMSTENNA demonstrator has been fabricated as a proof-of- concept prototype to experimentally validate the model described in m1 ⋅ d2 z1 (t) + b1 ⋅ dz1 (t) + k1 ⋅z1 (t) = Fee (4) Section 2. The main characteristics of the prototype have been sum­ dt2 dt marized in Table 1. As predicted by the model, the MEMSTENNA dipole impedance d2 z2 (t) dz2 (t) m2 ⋅ + b2 ⋅ + k2 ⋅z2 (t) = Fee (5) shows a resonance frequency around 800 MHz, corresponding to a null dt2 dt of the reactance (Fig. 4.a). Below this frequency the antenna behavior is Here, Fee represents the electrostatic force, m1 and m2 are the capacitive (Xa(f<800 MHz) ≈ Ra+ (1/j⋅ω⋅Ca)), and it is inductive for effective masses of each beam component, k1 and k2 are their spring higher frequencies (Xa(f>800 MHz) ≈ Ra+j⋅ω⋅La). However, as it is constants and b1 and b2 are the damping factors. The time-dependent shown in Fig. 4.b, the capacitive load Cm of the MEMS is modifying the deflections of each beam at their very end are represented by z1(t) and original antenna reactance, Xa, producing an increase of the resonant z2(t). Cp is a parasitic capacitance which is in parallel with the active frequency of the dipole antenna when the gap is increasingly opening. MEMS capacitor, Cm, and Vm is the voltage induced in the feeding point of the MEMSTENNA which, as pointed out above, corresponds to the actuation voltage applied at the MEMS capacitive actuator terminals. 3.2. Measurement setup description The electrostatic force Fee acting on each component of the double The measurement set up is depicted in Fig. 5. On the transmission 3. MEMSTENNA DIPOLE 2. ELECTROSTATIC FORCE + +- x k - 1/m 1/s 1/s b - + v 1. ELECTROMECHANICAL ACTUATOR Fig. 3. Block diagram of the MEMSTENNA model. 3 R. Ruiz et al. Sensors and Actuators: A. Physical 340 (2022) 113536 Table 1 3.3. Test results and discussion Physical parameters of the MEMSTENNA proof-of-concept prototype. Parameter Symbol Value units 3.3.1. Switching transient response 3 In a first set of measurements, we have analyzed the time response Density (Steel) ρ 8050 kg/m Young Modulus (Steel) E 200 GPa during OFF-ON and ON-OFF switching transients at different excitation Length of the cantilever Length of the dipole lc/L 83/ mm electric fields and gap values for a fixed carrier frequency (fc=834 MHz). 88 As it is shown in Fig. 6, the cantilever response presents a small over­ Width w 5 mm shoot in both OFF-ON and ON-OFF transients that is almost mixed up Overlapping length lm 803 μm Thickness t 100 μm with the mechanical noise fluctuations observed along the stationary Static gap go 50 μm states. On the other hand, the average stationary vertical deflection Spring constant k 0,43 mN/ achieved after OFF-ON switching transient grows when the electric field m is increased. In particular, when the cantilevers are separated by a gap of Frequency of the 1st cantilever mode f0 6,8 Hz g0 = 54 µm (Fig. 6.a) the excitation electric field can be increased up to Frequency of the 1st dipole antenna mode(for fa 875 MHz go=50 µm) 200 V/m before the collapse between both structures, which occurs when the MEMSTENNA is excited at 210 V/m, as it is evidenced by the dramatic reduction of the noise fluctuations in the ON stationary state. side, an RF signal generator (HP 8647 A) followed by a RF 12 W power When the gap distance is increased to g0 = 67 µm (Fig. 6.b), excitation amplifier (RFPA RF101000–10) is connected to a λ/2 dipole emitting electric field can be increased up to 230 V/m and the structure is not antenna designed to resonate at 940 MHz. The generator and power collapsed. amplifier set can deliver a RF power slightly above 12 W to the emitting antenna, which radiates a carrier signal (the above-mentioned excitation 3.3.2. Stationary response dependence on the electric field electromagnetic wave) of frequency fc. The MEMSTENNA, aligned in the To precisely account for the dependence of the stationary vertical x-axis, is placed at a certain distance from the emitter dipole. The an­ deflection on the electric field, we have collected the data shown in chors of both MENTENNA’s cantilevers are attached to xyz micro­ Fig. 7. Here, the experimental data have been fit by calculated curves positioners to allow a fine adjustment of the MEMSTENNA gap, go, and from the model. In the model calculations best fit is obtained when Cp overlap length, lm. A diode laser beam (635 nm, 1.2 mW) is focused on = 0.8⋅Co. As it can be noticed, a good agreement between the experi­ the back side of the top cantilever and the deflection angle of the re­ mental points and the values predicted by the model is obtained. Thus, flected beam is measured by a Position Sensitive Device (PSD) (New the collapse effect observed at g0 = 54 µm that has been described Focus 2930). The vertical displacement of the top cantilever free end, previously is more clearly reproduced here. Besides, it can be observed represented by z2 in Fig. 5, is consequently measured through an oscil­ that a smooth linear dependence of the stationary vertical deflection on loscope (Tektronix MDO3024) connected to the PSD output. A zenithal the electric field is reproduced for large gap values (g0 =100 µm and g0 CCD camera is placed on top of the MEMSTENNA in the capacitive =140 µm). However, such a linear dependence turns into a saturation transduction zone to optically adjust the gap distance. In this set up we trend when gap values (g0 =67 µm) are reduced close to the collapse are considering that the two arms of the MEMSTENNA have the same conditions (g0 =54 µm). This dependence, which seems to be inconsis­ displacement (z1 =z2), and the RF incident wave is plane. To have a tent with the square power dependence of the electrostatic force with control of the excitation electric field through the signal generator, we the voltage (Eq. 6) and, consequently, with the electric field (Eq. 2), can have measured the electric field intensity at the memstenna position for be partly explained by the saturation of the RF power amplifier. Added different signal generator powers using an electric field probe. to this amplifier saturation effect, an intrinsic negative feedback effect due to the increase of the capacitive load is also present here and will be analyzed below. 250 1.4 Resistance, Ra Resistance (Ohms) 200 Reactance, Xa 1.2 Capacitive Area 150 1.0 Vm / VOC 100 0.8 50 0.6 g0 = 50 µm g0 = 75 µm 0 Inductive 0.4 g0 = 100 µm Area g0 = 150 µm -50 0.2 750 800 850 900 950 1000 750 800 850 900 950 1000 1050 1100 Frequency (MHz) Frequency (MHz) (a) ( b) Fig. 4. Frequency response of the MEMSTENNA dipole impedance (a) and MEMS actuator voltage at different static transducing gaps for a constant electric field excitation of Einc= 190 V/m (b). A parasitic parallel capacitance value of Cp = 0.7⋅Co is considered in the calculations. 4 R. Ruiz et al. Sensors and Actuators: A. Physical 340 (2022) 113536 3.3.3. Effect of the carrier frequency on the stationary response vs. carrier power dependence To bring out clearly the effect described in previous section, we have calculated how the stationary vertical deflection grows with the electric field intensity for different carrier frequencies and for an initial gap g0 = 50 µm, using the model equations. The resulting curves are plotted in Fig. 8. For carrier frequencies below the resonance frequency of the MEMSTENNA dipole (see Table 1), i.e. fc< 875 MHz (see Fig. 4.b), the cantilever collapses almost like when driven by a voltage source and the collapse position is close to g0/3 = 16.7 µm (see dashed curves in Fig. 8, corresponding to 0.7–0.85 GHz). In fact, collapse is produced earlier because when load capacitance is increased by the gap reduction, then resonance frequency of the MEMSTENNA dipole is shifted down (fres=1/ (L⋅C)1/2) and, consequently, the amplitude of the driving voltage Vm, grows, inducing an extra positive feedback effect, that allows collapsing the structure by exciting with lower electric field values. Indeed, the Fig. 5. Block diagram of the measurement setup. Used components: CCD: Digital camera, PSD: Position Sensitive Device, RFPA: RF Power Amplifier (12 W), ED: Emitter Dipole, RDL: Red Diode Laser (λ = 673.8 nm), MP: xyz Micro Positioner. Inset: Optical image of the capacitive transduction area of the MEMSTENNA with a gap g0 = 42 µm. Fig. 7. Experimental (symbol points) and calculated (solid curves) stationary response of the MEMSTENNA as a function of the electric field at the mem­ stenna, for different gap values at a constant carrier frequency of fc= 834 MHz. Best fit is obtained when Cp = 0.8⋅Co. Fig. 6. Time evolution of the vertical deflection of the top cantilever during OFF-ON and ON-OFF switching transients for g0 = 54 µm (a) and g0 = 67 µm (b), and different excitation electric field values. Carrier frequency is kept constant at fc= 834 MHz. 5 R. Ruiz et al. Sensors and Actuators: A. Physical 340 (2022) 113536 collapse electric field is clearly obtained when carrier frequency is increased from 820 MHz to 845 MHz. However, when the excitation is produced by a carrier signal with a frequency (fc=950 MHz) inside the negative feedback regime, the electric field needed to actuate the structure is increased and the collapse is practically cancelled, as the sigmoidal shape curve like the ones predicted by the model (solid curves in Fig. 8, 0.9–1.05 GHz) demonstrates. Considering the analyzed test results from the perspective of the above-mentioned wireless-batteryless relay future application, power efficiency can be optimized by adjusting the transduction gap and the carrier frequency into the positive feedback regime (0.7–0.85 GHz for the demonstrator case), so that the collapse of the structure occurs with the minimum electric field (or carrier power). Specifically, our test re­ sults show that collapse is produced at 210 V/m (Figs. 6 and 7) for g0 = 54 µm and fc= 834 MHz or at 185 V/m (Fig. 9) for g0 = 110 µm and fc= 848 MHz. These electric field values are obtained with carrier powers above 12 W and 7 W, respectively. Even in the best case pre­ dicted by the model (Fig. 8, g0 =50 µm, fc=700 MHz) the electric field Fig. 8. Calculated stationary vertical deflection of the MEMSTENNA as a needed to collapse is around 100 V/m, which is obtained with a carrier function of the incident electric field for different carrier frequencies. The gap is power of 1.25 W in our setup conditions. An additional improvement kept constant at g0 = 50 µm. that would allow to further decrease the carrier power below 1 W or, alternatively, to increase the distance between the MEMSTENNA and higher the carrier frequency of the dashed curves in Fig. 8 the emitter dipole antenna, can be achieved by downscaling the MEM­ (0–7–0.85 GHz), the lower the electric field needed for the structure to STENNA dimensions. Indeed, a dimensional analysis shows that if we collapse. Instead, the opposite feedback effect is produced when the downscale all the geometric dimensions of the MEMSTENNA by a scale starting point corresponds to a carrier frequency above the resonance factor SF, the collapse voltage frequency of the MEMSTENNA dipole, i.e. fc> 875 MHz (see Fig. 4.b). In √̅̅̅̅̅̅̅̅̅ this case, the amplitude of the driving voltage Vm, decreases as the gap is k⋅g30 closing and the collapse instability can even be avoided (see solid curves Vcollapse = (8) ϵ0 ⋅A in Fig. 8, corresponding to 0.9–1.05 GHz), as reported previously in the literature [25]. Finally, when the carrier frequency is far above the will also scale linearly with SF, since spring constant, k = 0.25⋅E⋅w⋅(t/ resonance frequency of the MEMSTENNA, i.e., fc> 1.05 GHz, the pre­ lc)3, scales like SF, g03 scales like SF3 and transduction area, A, scales like vious negative feedback effect is increasingly reduced, so that the SF2. On the other hand, the transduction voltage, which is proportional collapse of the structures is produced again at a gap position which is to the open circuit voltage, VOC (Eq. 2) scales like SF too. This means that also increasingly reduced as the carrier frequency is increased (see when we downscale the whole structure, the voltage needed to collapse, dashed-point curves in Fig. 8, corresponding to 1.0–1.2 GHz). Vcollapse, is reduced, but the available voltage is reduced too. Conse­ A sample of some of the model predicted curves shown in Fig. 8 have quently, a solution could consist of reducing the length of the MEM­ been experimentally reproduced and shown in Fig. 9. The measured STENNA by a scale factor smaller than the scale factor of the rest of points and model fit curves obtained at carrier frequencies fc= 820 MHz dimensions, so that the collapse voltage is reduced more than the open and fc= 845 MHz corresponds to the final part of the positive feedback circuit voltage. regime (dashed curves in Fig. 8, 0.7–0.85 GHz). A reduction of the 4. Conclusions The concept of MEMSTENNA has been defined and described in terms of structure, configuration and working principle, which is based on the resonant driving phenomenology associated to the LC of MEM­ STENNA dipole antenna. A complete model of the MEMSTENNA that includes the electromagnetics of the dipole antenna and its mutual interaction with the mechanics of the cantilevers components has been also described. A mm-scale proof-of-concept prototype of MEMSTENNA made of steel has been fabricated to validate the model. Its geometry has been designed in such a way that the dipole antenna associated to the prototype resonates at 875 MHz and the cantilever components have a spring constant of 0.43 mN/m, corresponding to a mechanical funda­ mental mode resonance of 6.8 Hz. Both, transient and stationary mea­ surements of the cantilever’s deflection have been performed at different transduction gap, carrier frequency and electric field condi­ tions. In particular, the two experimental characteristics families cor­ responding to cantilever deflection vs. electric field, parametrized by the transduction gap and carrier frequency, have been contrasted to the calculated curves obtained from model calculations. The degree of agreement between theoretical curves and experimental points allows to successfully validate the MEMSTENNA model. Fig. 9. Measured stationary vertical deflection of the MEMSTENNA as a function of the electric field and the corresponding model fit curves for three different carrier frequencies. The gap is kept constant at g0 = 110 µm. Best fit is obtained when Cp = 0.7⋅Co. 6 R. Ruiz et al. Sensors and Actuators: A. Physical 340 (2022) 113536 Declaration of Competing Interest [19] W.M. Zhang, H. Yan, Z.K. Peng, G. Meng, Electrostatic pull-in instability in MEMS/ NEMS: a review, Sens. Actuators A Phys. 214 (2014) 187–218. [20] L. Castañer, J. Pons, R. Nadal-Guardia, A. 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Micro Syst. 13 (2) (2004) 279–289. Autònoma de Barcelona. He has more than 18 years of experience in tasks of design and [10] S. Kavitha, R.J. Daniel, K. Sumangala, Design and analysis of MEMS comb drive verification of ASIC. At present he works at EKRAN SYSTEMS as ASIC/SoC consulting capacitive accelerometer for SHM and seismic applications, Measurement 93 engineer. His research interests include design, characterization and integration of micro (2016) 327–339. and nanoelectromechanical systems (MEMS and NEMS). He is currently developing his [11] C.H. Han, D.H. Choi, J.B. Yoon, Parallel-plate MEMS variable capacitor with PhD in MEMS direct transduction from electromagnetic to mechanical domain. superior linearity and large tuning ratio using a levering structure, J. Microelectromech. Syst. 20 (6) (2011) 1345–1354. Dr. Gabriel Abadal received his degree in physics in 1991 and his PhD in electrical en­ [12] G.N. Nielson, G. Barbastathis, Dynamic pull-in of parallel-plate and torsional gineering in 1997 from the UAB. Since 2002 he is an Associate Professor in the Electronics electrostatic MEMS actuators, J. Microelectromech. Syst. 15 (4) (2006) 811–821. Engineering Department of the UAB, where he currently leads the NANERG LAB group [13] J. Wang, J. Yang, D. Chen, L. Jin, Y. Li, Y. Zhang, F. Wu, Gas detection microsystem (http://grupsderecerca.uab.cat/nanerglab/). During the last 28 years he has participated with MEMS gas sensor and integrated circuit, IEEE Sens. J. 18 (16) (2018) in more than 30 national and European research projects, he has contributed to more than 6765–6773. 160 conferences and he has published more than 80 papers in journals (H-index: 21). Since [14] H. Baltes, O. Brand, A. Hierlemann, D.A.L.D. Lange, C.A.H.C. Hagleitner. CMOS 2006, he focused his activity in the application of MOEMS and NOEMS to harvest energy MEMS-present and future. In Technical Digest. MEMS 2002 IEEE International from ambient vibrations and electromagnetic radiation sources at the nanoscale. His actual Conference. Fifteenth IEEE International Conference on Micro Electro Mechanical interest is oriented towards the study of graphene and other 2D materials as basic com­ Systems (Cat. No. 02CH37266), 2002, January, pp. 459–466. ponents of the future nanoenergy harvesting strategies. [15] J. Verd, A. Uranga, J. Segura, N. Barniol. A 3V CMOS-MEMS oscillator in 0.35 μm CMOS technology. In 2013 Transducers & Eurosensors XXVII: The 17th International Conference on Solid-State Sensors, Actuators and Microsystems Dr. Jordi Bonache was born in Cardona (Barcelona), Spain, in 1976. He received the (TRANSDUCERS & EUROSENSORS XXVII). 2013 (pp. 806–809). IEEE. Physics and Electronics Engineering degrees and the Ph.D. in electronics engineering from [16] M. Pallay, R.N. Miles, S. Towfighian, Towards a high bias voltage MEMS filter the Universitat Autònoma de Barcelona, in 1999, 2001 and 2007, respectively. In 2000, he using electrostatic levitation, Mech. Syst. Signal Process. 150 (2021), 107250. joined the High Energy Physics Institute of Barcelona, where he was involved in the design [17] J.B. Muldavin, G.M. Rebeiz, Inline capacitive and DC-contact MEMS shunt and implementation of the control and monitoring system of the MAGIC telescope. In switches, IEEE Microw. Wirel. Compon. Lett. 11 (8) (2001) 334–336. 2001, he joined the Department of Electronics Engineering, Universitat Autònoma de [18] H. Yang J.S. Cho Y. Yang S. Kim. A load variation tolerant readout interface for Barcelona, where he is currently Professor of Electronics. From 2006–2009, he was the high linear MEMS capacitive microphones. In 2017 IEEE international symposium Executive Manager of CIMITEC, Bellaterra, Spain. His current research interests include on circuits and systems (ISCAS). 2017, May (pp. 1–4). IEEE. active and passive microwave devices, metamaterials, antennas and RFID. 7