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In this study, we present a two-dimensional dielectric grating which allows achieving high absorption in a monolayer graphene at visible and near-infrared frequencies. Dielectric gratings create guided-mode resonances that are exploited to effectively couple light with the graphene layer. The proposed structure was numerically analyzed through a rigorous coupled-wave analysis method. Effects of geometrical parameters and response to the oblique incidence of the plane wave were studied. Numerical results reveal that light absorption in the proposed structure is almost insensitive to the angle of the impinging source over a considerable wide angular range of 20&deg, This may lead to the development of easy to fabricate and experimentally viable graphene-based absorbers in the future.

Optogenetics promises spatiotemporal precise control of neural processes using light. However, the spatial extent of illumination within the brain is difficult to control and cannot be adjusted using standard fiber optics. We demonstrate that optical fibers with tapered tips can be used to illuminate either large brain volumes or dynamically selectable subregions. Remotely adjusting the light input angle to the fiber varies the light-emitting portion of the taper over several millimeters without movement of the implant. We use this mode to activate dorsal versus ventral striatum of individual mice and reveal different effects of each manipulation on motor behavior. Conversely injecting light over the full numerical aperture of the fiber results in light emission from the entire taper surface, achieving broader and more efficient optogenetic activation of neurons when compared to the standard flat-faced fiber stimulation. Thus, tapered fibers permit focal or broad illumination that can be precisely and dynamically matched to experimental needs.

Dielectric-like degradation of thin-film conductors is simulated up to final breakdown within a biased percolation model. As relevant indicators we take the damage pattern, current distribution, resistance variation, failure lifetime and relative resistance fluctuations. The results show that biased percolation predicts well several known features taking place close to the abrupt failure of thin films in close agreement with available experimental results.

There is huge research activity in the development of flexible and biocompatible piezoelectric materials for next-generation compliant micro electro-mechanical systems (MEMS) transducers to be exploited in wearable devices and implants. This work reports for the first time on the development of flexible ScxAl(1-x)N films deposited by sputtering technique onto polyimide substrates, assessing their piezoelectricity and biocompatibility. Flexible ScxAl(1-x)N films have been analyzed in terms of morphological, structural, and piezoelectric properties. ScxAl(1-x)N layer exhibits a good surface roughness of 4.40 nm and moderate piezoelectricity with an extracted effective piezoelectric coefficient (d33eff) value of 1.87 ± 0.06 pm/V, in good agreement with the diffraction pattern analysis results. Cell viability assay, performed to study the interaction of the ScxAl(1-x)N films with human cell lines, shows that this material does not have significant effects on tested cells. Furthermore, the ScxAl(1-x)N layer, integrated onto a flexible device and analyzed by bending/unbending measurements, shows a peak-to-peak open-circuit voltage (VOC) of 0.32 V and a short-circuit current (ISC) of 0.27 μA, with a generated power of 19.28 nW under optimal resistive load, thus demonstrating the potential of flexible ScxAl(1-x)N films as active layers for next-generation wearable/implantable piezoelectrics.

This work aims at developing a new and unconventional Sacrificial Stencil Mask (SSM) technology by exploiting Two-Photon Polymerization (2PP) in an IP-L/SU-8 double layer resist system. The process consists of the sequential deposition of two different resists, such as SU-8 and IPL, onto the same glass substrate, followed by 2PP lithography and distinct development processes. The 2PP writing process was used to polymerize structures inside the top and bottom resist layers to form, in one single exposure process, both SSM and a permanent polymeric structure, in our case a plain pedestal. The top IPL resist was developed using Isopropyl Alcohol (IPA), which does not affect either exposed or un-exposed SU-8 regions. In this way, structures written into the bottom layer remained latent, while exposed areas of the top IPL resist, including the stencil mask, were developed. The realization of 3D stencil masks, designed to be anchored inside the un-exposed bottom layer, was combined with metal evaporation to demonstrate the deposition of a plain metal line through the stencil mask. The final development of the bottom layer led to the lift off of the sacrificial stencil mask, uncovering the underlying, permanent polymer-metal structure. The combination of sacrificial polymer structures with permanent ones opens new possibilities in 3D MEMS design, enabling the integration of distributed electronic transducers in flexible polymeric structures. Keywords: Stencil lithography, Stencil mask, 3D metallization, Two-photon polymerization lithography

Disposable surgical face masks are usually used by medical/nurse staff but the current Covid-19 pandemic has caused their massive use by many people. Being worn closely attached to the people's face, they are continuously subjected to routine movements, i.e., facial expressions, breathing, and talking. These motional forces represent an unusual source of wasted mechanical energy that can be rather harvested by electromechanical transducers and exploited to power mask-integrated sensors. Typically, piezoelectric and triboelectric nanogenerators are exploited to this aim; however, most of the current devices are too thick or wide, not really conformable, and affected by humidity, which make them hardly embeddable in a mask, in contact with skin. Different from recent attempts to fabricate smart energy-harvesting cloth masks, in this work, a wearable energy harvester is rather enclosed in the mask and can be reused and not disposed. The device is a metal-free hybrid piezoelectric nanogenerator (hPENG) based on soft biocompatible materials. In particular, poly(vinylidene fluoride) (PVDF) membranes in the pure form and with a biobased plasticizer (cardanol oil, CA) are electrospun onto a laser-ablated polyimide flexible substrate attached on a skin-conformable elastomeric blend of poly(dimethylsiloxane) (PDMS) and Ecoflex. The multilayer structure of the device harnesses the piezoelectricity of the PVDF nanofibers and the friction triboelectric effects. The ultrasensitive mechanoelectrical transduction properties of the composite device are determined by the strong electrostatic behavior of the membranes and the plasticization effect of cardanol. In addition, encapsulation based on PVDF, PDMS, CA, and parylene C is used, allowing the hPENG to exhibit optimal reliability and resistance against the wet and warm atmosphere around the face mask. The proposed device reveals potential applications for the future development of smart masks with coupled energy-harvesting devices, allowing to use them not only for anti-infective protection but also to supply sensors or active antibacterial/viral devices.

Thin-film piezoelectric materials are currently employed in micro- and nanodevices for energy harvesting and mechanical sensing. The deposition of these functional layers, however, is quite challenging onto non-rigid/non-flat substrates, such as optical fibers (OFs). Besides the recent novel applications of OFs as probes for biosensing and bioactuation, the possibility to combine them with piezoelectric thin films and metallic electrodes can pave the way for the employment of novel opto-electro-mechanical sensors (e.g., waveguides, optical phase modulators, tunable filters, energy harvesters or biosensors). In this work the deposition of a thin-film piezoelectric wurtzite-phase Aluminium Nitride (AlN), sandwiched between molybdenum (Mo) electrodes, on the curved lateral surface of an optical fiber with polymeric cladding, is reported for the first time, without the need of an orientation-promoting interlayer. The material surface properties and morphology are characterized by microscopy techniques. High orientation is demonstrated by SEM, PFM and X-ray diffraction analysis on a flat polymeric control, with a resulting piezoelectric coefficient (d33) of ∼5.4 pm/V, while the surface roughness Rms measured by AFM is 9 ÷ 16 nm. The output mechanical sensing capability of the resulting AlN-based piezo-optrode is investigated through mechanical buckling tests: the peak-to-peak voltage for weakly impulsive loads increases with increasing relative displacements (up to 30%), in the range of 20 ÷ 35 mV. Impedance spectroscopy frequency sweeps (10 kHz-1 MHz, 1 V) demonstrate a sensor capacitance of ∼8 pF, with an electrical Q factor as high as 150. The electrical response in the long-term period (two months) revealed good reliability and durability.

There is a high demand for novel flexible micro-devices for energy harvesting from low-frequency and random mechanical sources. The research of new functional designs is required to strategically enhance the performances and to increase the control on mechanical flexibility. In this work we report the fabrication and characterization of bi-stable and statically balanced thin-film piezoelectric transducers based on Aluminum Nitride (AlN). The device consists of a piezoelectric layer sandwiched between two thin Molybdenum electrodes that were deposited on a Kapton substrate by reactive sputtering and patterned by UV lithography. In order to improve the out-of-plane flexibility, the mechanical design is distinguished by a post-buckled flexure that introduces a negative stiffness to compensate the otherwise positive stiffness of the system. The buckling was introduced by a new method, called Package-Induced Preloading (PIP) where the mechanisms are laminated over a package with a geometry extending out-of-plane. The induced buckling resulted in bi-stable and statically balanced mechanisms which demonstrated an enhanced voltage output during a triggered snapping step. A preliminary study shows potential for the statically balanced designs and the PIP method for wind energy harvesting, revealing prospective applications and future improvements for the development of energy harvesters.

This work shows a preliminary microfabrication route for a novel directional hydrophone based on a cross-shaped design of piezoelectric cantilevers. A thin layer of aluminum nitride (AlN) using Molybdenum (Mo) thin film as electrodes will be exploited as piezoelectric functional layer for the microfabrication of a cantilever-based ultrasonic micro electro mechanical system (MEMS) hydrophone. A parameterized simulation based on length of these cantilevers between 100 and 1000 μm allowed to set the first resonant mode between 20 kHz and 200 kHz, the desired underwater ultrasonic acoustic range. The microsystem was designed with cantilevers facing each other in a cross configuration in order to have novel MEMS hydrophone with an omnidirectional response. In order to investigate the first resonance frequency mode and displacement measurements, a Laser Doppler Vibrometer was used and good agreement between simulations and experimental results was achieved. Responsivity and directionality measurements of the piezoelectric MEMS cantilevers were performed in water. Maximum sensitivity up to −153 dB with omnidirectional directivity pattern was achieved by fabricated MEMS sensor.

Triboelectric nanogenerators (TENGs) have recently become a powerful technology for energy harvesting and self-powered sensor networks. One of their main advantages is the possibility to employ a wide range of materials, especially for fabricating inexpensive and easy-to-use devices. This paper reports the fabrication and preliminary characterization of a novel flexible triboelectric nanogenerator which could be employed for driving future low power consumption wearable devices. The proposed TENG is a single-electrode device operating in contact-separation mode for applications in low-frequency energy harvesting from intermittent tapping loads involving the human body, such as finger or hand tapping. The novelty of the device lies in the choice of materials: it is based on a combination of a polysiloxane elastomer and a poly (para-xylylene). In particular, the TENG is composed, sequentially, of a poly (dimethylsiloxane) (PDMS) substrate which was made porous and rough with a steam-curing step, then, a metallization layer with titanium and gold, deposited on the PDMS surface with an optimal substrate&ndash, electrode adhesion. Finally, the metallized structure was coated with a thin film of parylene C serving as friction layer. This material provides excellent conformability and high charge-retaining capability, playing a crucial role in the triboelectric process, it also makes the device suitable for employment in harsh, wet environments owing to its inertness and barrier properties. Preliminary performance tests were conducted by measuring the open-circuit voltage and power density under finger tapping (~2 N) at ~5 Hz. The device exhibited a peak-to-peak voltage of 1.6 V and power density peak of 2.24 mW/m2 at ~0.4 M&Omega, The proposed TENG demonstrated ease of process, simplicity, cost-effectiveness, and flexibility.

This paper presents an innovative, flexible and biocompatible Ultra High Frequency meander antenna, operating at about 800 MHz, realized by means of sputtering on a Polyethylene Naphthalate substrate, and by means of a multi-material 3D printer. The fabricated antennas were characterized in terms of scattering parameters, showing a good impedance matching and a bandwidth in the order of tens of megahertz, gain and 3D radiation patterns. A numerical model was also introduced to investigate the limits of the proposed technologies in terms of metal thicknesses. The fabricated antenna could be efficiently integrated with Surface Acoustic Wave resonators to realize compact, wireless, wearable and battery-less sensing platforms.

Transdermal drug delivery offers several advantages over other conventional modalities for drug administration, such as injections and oral administrations. It can be activated by exogenous energy sources, including electric and magnetic fields, or by light and ultrasound (US). Ultrasound can induce the degradation and deformation of a polymeric matrix, thus promoting the release of entrapped drugs and the topical administration through the skin. Our aim is to create a wearable patch composed of an ultrasound-responsive polymeric system encapsulating specific drugs, which are released on demand by the application of ultrasound. In this work, we investigate the effect of ultrasound on square poly(lactic-co-glycolic acid) (PLGA) microparticles loaded with curcumin (CURC), called curcumin-microplates (CURC-μPLs), realized using a top-down fabrication process. Using Finite Element Method (FEM) simulations, we identified the resonant frequencies and the mode shape of the μPLs. Guided by this simulation, we applied ultrasound stimulus at frequency of 1 MHz, close to the first mechanical resonance frequency of the microplates, leading to a 200% increase in CURC release rate after 30 min of ultrasonic treatment. This approach can be generalized to any ultrasound-sensitive matrix filled with specific drugs and with piezoelectric compliant transducers embedded in the smart patch for generating ultrasonic waves.

Colorimetric and electrochemical (bio)sensors are commonly employed in wearable platforms for sweat monitoring; nevertheless, they suffer from low stability of the sensitive element. In contrast, mass-(bio)sensors are commonly used for analyte detection at laboratory level only, due to their rigidity. To overcome these limitations, a flexible mass-(bio)sensor for sweat pH sensing is proposed. The device exploits the flexibility of piezoelectric AlN membranes fabricated on a polyimide substrate combined to the sensitive properties of a pH responsive hydrogel based on PEG-DA/CEA molecules. A resonant frequency shift is recorded due to the hydrogel swelling/shrinking at several pH. Our device shows a responsivity of about 12 kHz/pH unit when measured in artificial sweat formulation in the pH range 3–8. To the best of our knowledge, this is the first time that hydrogel mass variations are sensed by a flexible resonator, fostering the development of a new class of compliant and wearable devices.

This paper reports the fabrication and the characterization of novel reusable Flexible Dry Electrodes (FDEs), which can be employed in wearable devices for medical applications, such as bioelectrical impedance analysis (BIA). Two different FDEs were fabricated exploiting two flexible substrates, i.e. polydimethylsiloxane (PDMS) and polyethylene terephthalate (PET), and the deposition of the conductive layers made by silver paste and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Performance tests were conducted by measuring the conductivity at the skin interface and the reliability was assessed after several cycles with different disinfectant solutions and artificial sweat as corrosive agent. FDEs were then used in BIA measurements and compared with the commercial single use silver/silver chloride (Ag/AgCl) gel electrodes. The BIA values measured by the fabricated FDEs were comparable with those obtained by commercial electrodes. The results envision the promising and reliable long-term employment of the proposed FDEs in wearable and portable devices.

Biodegradable cellulose-based hydrogels are attracting increasing interest in the academic and industrial fields thanks to their high swelling capacity and reproducibility, which allow many novel applications. These properties are enabled by amplification effect of their sensitiveness on a molecular level, translated into macroscopic effects such as a change in swelling degree. The monitoring of the hydrogel state is a crucial step for understanding the response of the hydrogel to external environment. Accordingly, the major aim of this study is to exploit ultrasound to characterize the swelling and degradation of cellulose-based hydrogel with different blend of molecular weight and degree of substitutions. The ultrasonic sensor used herein relies on the determination of a Pulse-echo time of flight. This technique provides dimensional information, thanks to its capability of monitoring the thickness of the swollen/unswollen hydrogel during sorption mechanism. Furthermore, by combining these data with a rheological characterization, the degree of crosslink and its modification during multiple swelling/deswelling cycles (due to ion strength variation) has been monitored. This technique could be an effective, alternative, fast and non-destructive method for real-time hydrogel characterization. Graphical Abstract: [Figure not available: see fulltext.]. © 2018, Springer Science+Business Media B.V., part of Springer Nature.

Piezoelectric energy harvesters are promising transducers for supplying low-power mobile/portable electronics, wearable and implantable devices, or for autonomous wireless sensors and microsystems, which are of increasing importance for monitoring environmental and structural health, sensing in automotive applications, human health and wellness. One of the main requirements for such applications is the ability to develop piezoelectric materials and devices generating enough voltage/power even from tiny mechanical or biomechanical excitations at extremely low frequencies and out-of-resonance. In addition to high performance, wearable devices and medical implants require biocompatibility of materials/devices in the long term. This paper reviews recent advances on biocompatible, flexible, and compliant energy harvesters based on piezoelectric thin films scavenging energy from (bio)mechanical movements or flows induced deformations. © 2002-2012 IEEE.

Optogenetic control of neural activity in deep brain regions ideally requires precise and flexible light delivery with non-invasive devices. To this end, Tapered Optical Fibers (TFs) represent a versatile tool that can deliver light over either large brain volumes or spatially confined sub-regions, while being sensibly smaller than flat-cleaved optical fibers. In this work, we report on the possibility of further extending light emission length along the taper in the range 0.4 mm-3.0 mm by increasing the numerical aperture of the TFs to NA = 0.66. We investigated the dependence between the input angle of light (θin) and the output position along the taper, finding that for θin > 10° this relationship is linear. This mode-division demultiplexing property of the taper was confirmed with a ray tracing model and characterized for 473 nm and 561 nm light in quasi-transparent solution and in brain slices, with the two wavelengths used to illuminate simultaneously two different regions of the brain using only one waveguide. The results presented in this manuscript can guide neuroscientists to design their optogenetic experiments on the base of this mode-division demultiplexing approach, providing a tool that potentially allow for dynamic targeting of regions with diverse extension, from the mouse VTA up to the macaque visual cortex.

Electronic devices used for marine applications suffer from several issues that can compromise their performance. In particular, water absorption and permeation can lead to the corrosion of metal parts or short-circuits. The added mass due to the absorbed water affects the inertia and durability of the devices, especially for flexible and very thin micro-systems. Furthermore, the employment of such delicate devices underwater is unavoidably subjected to the adhesion of microorganisms and formation of biofilms that limit their reliability. Thus, the demand of waterproofing solutions has increased in recent years, focusing on more conformal, flexible and insulating coatings. This work introduces an evaluation of different polymeric coatings (parylene-C, poly-dimethyl siloxane (PDMS), poly-methyl methacrylate (PMMA), and poly-(vinylidene fluoride) (PVDF)) aimed at increasing the reliability of piezoelectric flexible microdevices used for sensing water motions or for scavenging wave energy. Absorption and corrosion tests showed that Parylene-C, while susceptible to micro-cracking during prolonged oscillating cycles, exhibits the best anti-corrosive behavior. Parylene-C was then treated with oxygen plasma and UV/ozone for modifying the surface morphology in order to evaluate the biofilm formation with different surface conditions. A preliminary characterization through a laser Doppler vibrometer allowed us to detect a reduction in the biofilm mass surface density after 35 days of exposure to seawater.

This work describes a method to inject light into tapered optical fibers (TFs) in order to achieve site-selective and wide-volume optical control of neural activity with a single optical setup. The method relies on a Galvanometric - Resonant mirrors scan head, employed to inject light into the fiber to: (i) select a specific subset of guided modes obtaining localized illumination or (ii) to scan between the different guided modes at high-rate to obtain full NA-like light injection on time scales >0.2 ms. This is shown by analysing the light emission profiles from the TF with different voltage biases applied to the Galvanometric and the Resonant mirrors, and comparing them with a standard Full NA light-injection. With the aim to find the experimental parameters to obtain efficient Full NA - like emission profile, the Emission Length and the First Emission Diameter of TFs were extracted and compared with an ideal wide-volume emitter. Both Galvanometric and Resonant scanners methods shown a good agreement with the reference values of the standard Full NA injection, allowing neuroscientists to switch from site-selective to wide-volume illumination using a single optical setup. © 2019 IEEE.

The possibility to optically control and monitor neural activity with optogenetic methods has generated the need for new implantable devices to deliver and collect light from neural tissues. This is being accompanied by a set of methods that allow to numerically predict and experimentally test how light emitted by implanted optoelectronic devices, semiconductor waveguides and optical fibers behave in a highly scattering medium like the brain tissue. After discussing the most common scattering models, this work focuses on how emission and collection properties of optical fibers implanted the brain can be estimated. To assess photometry efficiency fields of an optical fiber implanted in a turbid medium, we combine numerically evaluated light emission and collection fields in presence of scattering. This approach can help to complement current knowledge on the influence of tissue scattering on the optical properties of implanted photonics devices, providing additional information for the design of optical bidirectional neural interfaces. © 2019 IEEE.

This paper presents a new sponge-like electrode (SLE) material structured with porous gold (PG). The fabrication process is simple and no specific equipment is required. Notably, the use of liquid metal particles enables the direct growth of PG into the pores of a flexible conductive support matrix. With a SLE sample 13 mm long, 6 mm wide and 1.5 mm thick immersed in a 10 mM glucose solution, we demonstrate that a volumetric power density of 2.4 mW·cm −3 at ≈5 mA·cm −3 and 0.48 V can be reached without using any enzymes. Because the process presented is versatile and scalable, we envision SLEs with long-term stability that could to meet the power budget of various wearable/bioelectronic devices.

Two-photon lithography is a laser writing technique that can produce 3D microstructures with resolutions below the diffraction limit. This review focuses on its applications to study mechanical properties of cells, an emerging field known as mechanobiology. We review 3D structural designs and materials in the context of new experimental designs, including estimating forces exerted by single cells, studying selective adhesion on substrates, and creating 3D networks of cells. We then focus on emerging applications, including structures for assessing cancer cell invasiveness, whose migration properties depend on the cell mechanical response to the environment, and 3D architectures and materials to study stem cell differentiation, as 3D structure shape and patterning play a key role in defining cell fates. © 2018 Elsevier Ltd

In this work, we propose an optimized nanoimprint protocol for the fabrication of a two-dimensional (2D) array of polymethyl-methacrylate (PMMA) nano-pillars deposited on different sputtered configurations (bilayer and multi-layer) of copper (Cu) and aluminum nitride (AlN) slabs supported by a silicon dioxide (SiO2) substrate. Both the Cu/AlN bilayer and multilayer thin films were deposited by a sputtering technique. The sub-micron PMMA pillars were realized by using nanoimprint lithography (NIL). In order to optimize the NIL process, several tests were performed by varying temperature and pressure, allowing us to achieve uniform and high-resolution pillars. The fabricated periodic array enabled the phase-matching of the incident plane wave exciting optical resonances. All the fabricated devices were then optically characterized by means of an ad hoc setup, where the reflected light from the sample was analyzed. The fabricated nano-pillars are mechanically stable, and they could be fully exploited for the realization of novel metallo-dielectric core/shell structures for sensing, surface-enhanced Raman spectroscopy, and light&ndash, matter interactions.

In this paper, the design, fabrication and mechanical characterization of a novel polydimethylsiloxane (PDMS) soft probe for delivering and sensing forces in biological systems is proposed. On the basis of preliminary finite element (FEM) analysis, the design takes advantage of a suitable core geometry, characterized by a variable spring-like ring. The compliance of probes can be finely set in a wide range to measure forces in the micronewton to nanonewton range. In particular, this is accomplished by properly resizing the ring geometry and/or exploiting the mixing ratio-based elastic properties of PDMS. Fabrication by the thermal imprint lithography method allows fast and accurate tuning of ring sizes and tailoring of the contact section to their targets. By only varying geometrical parameters, the stiffness ranges from 1080 mNm&minus, 1 to 50 mNm&minus, 1, but by changing the base-curing agent proportion of the elastomer from 10:1 to 30:1, the stiffness drops to 37 mNm&minus, 1. With these compliances, the proposed device will provide a new experimental tool for investigating force-dependent biological functions in sensory systems.

Optogenetics sets new experimental para-digms that can reveal cell type-specific contributions on the neural basis of behavior. Since most of the available systems for this purpose are based on approaches that tether animals to a set of cables, recent research activities have been focused on minimizing external factors that can alter animal movements. Current wireless optogenetic systems are based on waveguide-coupled light-emitting diodes (LED) and implanted μLEDs. However, each configuration separately suffers from significant limitations, such as low coupling efficiency, penetration depth, and invasiveness of waveguide-coupled LED, and local heat generated by implanted μLEDs. This work presents a novel wireless head-mountable stimulating system for a wide-volume light delivery. The device couples the output of a semiconductor laser diode (LD) to a tapered optical fiber (TF) on a wireless platform. The LD-TF coupling was engineered by setting up far-field analysis, which allows a full exploitation of mode division demultiplexing properties of TFs. The output delivered light along the tapered segment is capable of stimulating structures of depths up to ∼2 mm. TFs are tapered to a gradual taper angle (Ψ ∼ 2° to Ψ ∼ 10°) that ends with a sharp tip (∼500 nm) for smooth insertion and less invasiveness. Thus, the proposed system extends the capabilities of wireless optogenetic by offering a novel solution for wide volume light delivery in deep brain regions. © 1964-2012 IEEE.

The huge mechanical energy available in the environment, mostly in form of kinetic energy of fluids such as wind, ocean and river currents or waves, is currently harvested by cumbersome, low efficiency and high environmental impact technologies. New approaches are needed for producing more compact and distributed mechanical energy converters. Nanogenerators and related micro and nanotechnologies can help in developing new environmentally friendly and biocompatible technologies. To face this challenge, new conversion physical principle, device structures and system architectures are currently being studied and developed. This work reviews the most recent advances on nanogenerators for harvesting energy transported by liquids in the environment such as water motion in rivers and marine environments and kinetic energy in rain. It discusses the most common physical transduction mechanisms, with a focus on piezoelectric and triboelectric nanogenerators (PENG/TENG), the requirements for producing flexible devices for effective conversion and the system architectures for optimizing the fluid-device interaction for producing large and fast oscillations, such as flapping, fluttering or galloping, from quasi-static or quasi-laminar fluid motion. Additionally, the work encompasses challenges such as waterproofing and antibiofouling, important issues in sub-marine and underwater environment. © 2018 Elsevier Ltd

In this work we report the design, the fabrication and the characterization of an innovative soft tactile micro-actuator, also called TAXEL (TActile piXEL), which is developed to be integrated in a portable tactile display for providing text content and graphical information to visually impaired people through the sense of touch. It exploits a thermo-active approach, by taking inspiration from common thermometers: the actuator is activated by the thermal deformation of an active material, namely the metallic alloy Galinstan© determined by heating the alloy through an underlying metallic resistor designed to work as a heater. The microfabrication of TAXELs is achieved in several steps consisting in heater fabrication, in SU8 micro chambers fabrication, in the deposition of Galinstan® inside and sealing by a PDMS membrane. Measurements of the TAXEL deformation have been accomplished by measuring the displacement of the PDMS sealing membrane, which is promoted by the expansion of the heated Galinstan® drop. These measurements have been achieved by using the Laser Doppler Vibrometer in the “topography mode “and revealed a total displacement of 50 μm when a tension of 2.4 V is applied at taxel terminals and, according to the Joule's law, a power converted from electrical energy to thermal energy of 7,2 W. © 2018 Elsevier B.V.

Vascular grafts are artificial conduits properly designed to substitute a diseased blood vessel. However prosthetic fail can occur without premonitory symptoms. Continuous monitoring of the system can provide useful information not only to extend the graft’s life but also to optimize the patient’s therapy. In this respect, various techniques have been used, but all of them affect the mechanical properties of the artificial vessel. To overcome these drawbacks, an ultrathin and flexible smart patch based on piezoelectric Aluminum Nitride (AlN) integrated on the extraluminal surface of the prosthesis is presented. The sensor can be conformally wrapped around the external surface of the prosthesis. Its design, mechanical properties and dimensions are properly characterized and optimized in order to maximize performances and to avoid any interference with the graft structure during its activity. The sensorized graft is tested in vitro using a pulsatile recirculating flow system that mimics the physiological and pathological blood flow conditions. In this way, the ability of the device to measure real-time variations of the hemodynamics parameters has been tested. The obtained high sensitivity of 0.012 V Pa−1 m−2, joint to the inherent biocompatibility and non-toxicity of the used materials, demonstrates that the device can successfully monitor the prosthesis functioning under different conditions, opening new perspectives for real-time vascular graft surveillance.

The development of wearable technology increasingly requires bendable sensing devices operating across multiple domains for opto-electro-mechanical and biochemical transduction. Piezoelectric materials integrated into flexible and transparent device architectures can enable multiple-sensing platforms. It is shown that flexible and compliant surface-acoustic-wave (SAW) piezoelectric devices include all these features and can be applied to the human body. A flexible and transparent aluminum-nitride-(AlN)-based SAW device on a thermoplastic polyethylene naphthalate (PEN) substrate, fabricated by low-temperature sputtering deposition of a multilayered AlN-based stack, is reported for the first time. Two resonant modes, corresponding to Rayleigh and Lamb wave propagation, are shown and compared with a control SAW device on a silicon substrate. A large transmission-signal amplitude, up to 20 dB, is achieved for the Lamb resonance mode around 500 MHz at an acoustic velocity of 10 500 m s−1. The technology is applied to the fabrication of a wearable temperature sensor. Compared to the same piezoelectric stack and SAW technology onto silicon substrates, the AlN/PEN SAW shows better performance and a temperature coefficient frequency as high as ≈810 ppm °C−1. The potential of this flexible SAW device as a wearable temperature sensor based on Rayleigh modes is demonstrated. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In this article, we report the measurement of underwater aerophobicity, through the captive-bubble method, for different polymeric coatings employed to protect microscale and nanoscale flexible electronic devices for seawater applications. Controlling the morphology and wettability of the coating, in particular with the incorporation of nanoparticles of fluorinated polymers, allows to adjust the hydrophilic/hydrophobic (aerophobic/aerophilic) character of the surface in order to achieve a more insulating and antibiofouling behavior. Morphological analysis (roughness) and wettability measurements in sessile-drop and captive-bubble methods were provided for some properly selected polymeric coatings. We found that parylene C decorated with poly(vinylidene fluoride) nanoparticles at a higher dispersion concentration (5 mg/mL) exhibits the best compromise between morphology, hydrophobicity, and underwater aerophobicity, with sessile-drop water contact angle of 95.1 ± 2.9° and captive-air-bubble contact angle of 133.1 ± 5.9°. © The Author(s) 2019.

Fiber photometry is used to monitor signals from fluorescent indicators in genetically-defined neural populations in behaving animals. Recently, fiber photometry has rapidly expanded and it now provides researchers with increasingly powerful means to record neural dynamics and neuromodulatory action. However, it is not clear how to select the optimal fiber optic given the constraints and goals of a particular experiment. Here, using combined confocal/2-photon microscope, we quantitatively characterize the fluorescence collection properties of various optical fibers in brain tissue. We show that the fiber size plays a major role in defining the volume of the optically sampled brain region, whereas numerical aperture impacts the total amount of collected signal and, marginally, the shape and size of the collection volume. We show that ~80% of the effective signal arises from 105 to 106 μm3 volume extending ~200 μm from the fiber facet for 200 μm core optical fibers. Together with analytical and ray tracing collection maps, our results reveal the light collection properties of different optical fibers in brain tissue, allowing for an accurate selection of the fibers for photometry and helping for a more precise interpretation of measurements in terms of sampled volume. © 2019 Pisanello, Pisano, Hyun, Maglie, Balena, De Vittorio, Sabatini and Pisanello.

Flexible electronics based on piezoelectric/triboelectric devices is an attractive technology for human sensing. Their hybridization overcomes the limitations of single components, resulting in compliant skin sensors with enhanced performances and applicability. Such hybrid devices are typically based on wide-area scarcely durable polymers or lead-containing piezoelectric materials; they are often not biocompatible and poorly skin-adaptable, lacking in multifunctionality. In this work, a novel compliant, conformal hybrid piezoelectric-triboelectric ultra-thin wearable sensor made of biocompatible materials is reported. The device is in contact with skin through an ultra-soft patch covered on both sides by a thin friction parylene film. Its working principle is unprecedently based on three simultaneous, complementary and mutually enhancing effects: piezoelectric, skin-contact-actuation, and piezo-tribo hybrid contact. The device can detect, with high sensitivity and wide measurement range, both the impulsiveness of sudden motions and the slower micro-friction phenomena due to skin deformations, ensuring a stable and repeatable identification of bio-signals typical of body movements. The device multifunctionality is shown for identifying gait walking, distinguishing hand gestures with a 5-sensor system on the hand back, and monitoring human joints motions (neck, wrist, elbow, knee, ankle). The assessed energy harvesting capabilities demonstrate the suitability for fabrication of more complex self-powered sensing systems.

In this paper, we report on the characterization of the sensitivity and the directionality of a novel ultrasonic hydrophone fabricated by microelectromechanical systems (MEMS) process, using aluminum nitride (AlN) thin film as piezoelectric functional layer and exploiting a stress-driven design. Hydrophone structure and fabrication consist of four piezoelectric cantilevers in cross configuration, whose first resonant frequency mode in water is designed between 20 kHz and 200 kHz. The MEMS fabricated structures exploit 1 &micro, m and 2 &micro, m thick piezoelectric AlN thin film embedded between two molybdenum electrodes grown by DC magnetron sputtering on silicon (Si) wafer. The 200 nm thick molybdenum electrodes thin layers add a stressgradient through cantilever thickness, leading to an outofplane cantilever bending. A water resistant parylene conformal coating of 1 &micro, m was deposited on each cantilever for waterproof operation. AlN upward bent cantilevers show maximum sensitivity up to &minus, 163 dB. The cross configuration of four stressdriven piezoelectric cantilevers, combined with an opportune algorithm for processing all data sensors, permits a finer directionality response of this hydrophone.

We propose a feedback-assisted direct laser writing method to perform laser ablation of fiber optic devices in which their light-collection signal is used to optimize their properties. A femtosecond-pulsed laser beam is used to ablate a metal coating deposited around a tapered optical fiber, employed to show the suitability of the approach to pattern devices with a small radius of curvature. During processing, the same pulses generate two-photon fluorescence in the surrounding environment and the signal is monitored to identify different patterning regimes over time through spectral analysis. The employed fs beam mostly interacts with the metal coating, leaving almost intact the underlying silica and enabling fluorescence to couple with a specific subset of guided modes, as verified by far-field analysis. Although the method is described here for tapered optical fibers used to obtain efficient light collection in the field of optical neural interfaces, it can be easily extended to other waveguide-based devices and represents a general approach to support the implementation of a closed-loop laser ablation system of fiber optics.

This communication introduces a new flexible elastomeric composite film, which can directly convert the chemical energy of glucose into electricity. The fabrication process is simple, and no specific equipment is required. Notably, the liquid metal Galinstan is exploited with a two-fold objective: (i) Galinstan particles are mixed with polydimethylsiloxane to obtain a highly conductive porous thick film scaffold; (ii) the presence of Galinstan in the composite film enables the direct growth of highly catalytic gold structures. As a first proof of concept, we demonstrate that when immersed in a 20 mM glucose solution, a 5 mm-long, 5 mm-wide and 2 mm-thick sample can generate a volumetric power density up to 3.6 mW·cm − 3 at 7 mA·cm − 3 and 0.51 V without using any enzymes.

Background: Flow stimuli in the natural world are varied and contain a wide variety of directional information. Nature has developed morphological polarity and bidirectional arrangements for flow sensing to filter the incoming stimuli. Inspired by the neuromasts found in the lateral line of fish, we present a novel flow sensor design based on two curved cantilevers with bending orientation antiparallel to each other. Antiparallel cantilever pairs were designed, fabricated and compared to a single cantilever based hair sensor in terms of sensitivity to temperature changes and their response to changes in relative air flow direction.Results: In bidirectional air flow, antiparallel cantilever pairs exhibit an axially symmetrical sensitivity between 40 μV/(m s−1) for the lower air flow velocity range (between ±10–20 m s−1) and 80 μV/(m s−1) for a higher air flow velocity range (between ±20–32 m s−1). The antiparallel cantilever design improves directional sensitivity and provides a sinusoidal response to flow angle. In forward flow, the single sensor reaches its saturation limitation, flattening at 67% of the ideal sinusoidal curve which is earlier than the antiparallel cantilevers at 75%. The antiparallel artificial hair sensor better compensates for temperature changes than the single sensor.Conclusion: This work demonstrated the successive improvement of the bidirectional sensitivity, that is, improved temperature compensation, decreased noise generation and symmetrical response behaviour. In the antiparallel configuration, one of the two cantilevers always extends out into the free stream flow, remaining sensitive to directional flow and preserving a sensitivity to further flow stimuli.

We present a Ray Tracing model of tapered optical fibers (TF), recently proposed for both wide-volume and site- selective optical control of neural activity [1]. Light-delivery properties of TFs are computationally investigated by identifying the emitting region of the taper and the resulting output angles. This is done in two alternative light coupling modes:i) injecting light within the entire acceptance angle of the optical fiber, generating light output from almost the entire taper, andii) selecting a specific input angle, therefore allowing light delivery from a specific portion of the TF. In both cases a good agreement with previously reported experimental data has been obtained, letting us envision that the proposed approach can be used to study the mode-division demultiplexing properties of TFs, representing an important tool to design optogenetics experimentsin vivowith TFs.

The increasing demand of piezoelectric energy harvesters for wearable and implantable applications requires biocompatible materials and careful structural device design, paying special attention to the conformability characteristics, properly tailored to scavenge continuously electrical energy even from the tiniest body movements. This paper provides a comprehensive study on a flexible and biocompatible aluminum nitride (AlN) energy harvester based on a new alternative fabrication approach, exploiting a thin polyimide (PI) substrate, prepared by spin coating of precursors solution. This strategy allows manufacturing substrates with adjustable thickness to meet conformability requirements. The device is based on a piezoelectric AlN thin film, sputtered directly onto the soft PI substrate, without poling/annealing processes and patterned by simple and low cost microfabrication technologies. AlN active layer, grown on soft substrate, exhibits good morphological and structural properties with roughness root mean squared (Rrms) of 6.35 nm, columnar texture and (002) c-axis orientation. Additionally, piezoelectric characterization has been performed and the extracted piezoelectric coefficient value of AlN thin film resulted to be 4.93 ± 0.09 pm/V. The fabricated flexible AlN energy harvester generates an output peak-to-peak voltage of ∼1.4 V and a peak-to-peak current up to 1.6 μA, under periodical deformation, corresponding to a current density of 2.1 μA/cm2, and providing a maximum generated power of 1.57 μW under optimal resistive load. Furthermore, the AlN energy harvester exhibits high elasticity and resistance to mechanical fatigue. High quality AlN piezoelectric layers on elastic substrates with tunable thicknesses pave the way for the development of a straightforward technological platform for wearable/implantable energy harvesters and biomechanical sensors. Copyright © 2018 American Chemical Society.

Wind and fluid flow represent some of the most attractive renewable energy sources for addressing climate change, pollution and energy insecurity issues. Wind harvesting technologies, in particular, are the fastest-growing electric technologies in the world because of their efficiency and lower environmental impact with respect to traditional energy sources, despite exhibiting major drawbacks such as big infrastructure investment and environment invasiveness, producing high levels of noise and requiring the need of large areas for their installation. A single wind turbine can produce megawatts of power and they have the potential to cover the entire world's energy demand in the next few years, but they have a technological limit in a cut-in wind speed of about 4 m s-1, below which the turbines do not operate, excluding them as an energy source for slow air flows. At the same time most of the wind available in the environment is below the turbines' threshold speed. In this paper we show that small flags, made by piezoelectric thin film on flexible polymers and whose shape resembles the dry leaves of trees, can efficiently act as harvesters of energy from wind at extremely low speed, such as from a gentle blow or breath. We demonstrate that piezoelectricity on flexible polymers is achievable by depositing a thin film of piezoelectric aluminium nitride (AlN), sandwiched between metal electrodes with columnar grains coherent through the polycrystalline layers, on Kapton substrates. The prototype flags have a natural curling due to the release of the residual stress of the layers. While the curling is essential for the activation of the maximum flag oscillation, this system is so elastic and light that oscillations start at a cut-in flow speed of 0.4 m s-1, the lowest reported so far, with an open circuit peak to peak voltage of 40 mV. The voltage increases to 1.2 V when the flag is flattened and parallel to the fluid flow lines, with a generated power of 0.257 mW cm-3. This journal is

Piezoelectric MEMS energy harvesters based on thin films are compact and cost-effective microgenerators for scavenging environmental vibrations. This technology is promising for the replacement of electrochemical batteries in low power autonomous sensors and microdevices capturing vibrations in the μW-mW range. Most of piezoelectric MEMS devices, reported in the last few years, exhibit low generated power/voltage and are not suitable for practical applications. This work reviews the current status of MEMS energy harvesters based on piezoelectric thin films, highlighting approaches/strategies to face the two main challenges to be addressed for high performance devices, namely generated power and frequency bandwidth. The paper introduces the theoretical principles and the main figures of merit of energy conversion in piezoelectric thin films and devices. After an overview on piezoelectric thin films for energy harvesting applications, highlighting their key properties, the manuscript reports a comprehensive survey on the state of the art for this device technology. The last section summarizes the review, highlighting key issues to be addressed and providing an insight into the future outlook to realize devices for practical applications.

We report a very simple and fast method to develop a fluidic system on Parylene C coated paper, based on fabrication of surface-tension-confined channels. As described here, the proposed method can be applied to fast fabrication of reusable and cheap analytical devices, as an alternative to disposable fluidic devices on paper. © 2017 IEEE.

Cells are highly dynamic elements, continuously interacting with the extracellular environment. Mechanical forces sensed and applied by cells are responsible for cellular adhesion, motility, and deformation, and are heavily involved in determining cancer spreading and metastasis formation. Cell/extracellular matrix interactions are commonly analyzed with the use of hydrogels and 3D microfabricated scaffolds. However, currently available techniques have a limited control over the stiffness of microscaffolds and do not allow for separating environmental properties from biological processes in driving cell mechanical behavior, including nuclear deformability and cell invasiveness. Herein, a new approach is presented to study tumor cell invasiveness by exploiting an innovative class of polymeric scaffolds based on two-photon lithography to control the stiffness of deterministic microenvironments in 3D. This is obtained by fine-tuning of the laser power during the lithography, thus locally modifying both structural and mechanical properties in the same fabrication process. Cage-like structures and cylindric stent-like microscaffolds are fabricated with different Young's modulus and stiffness gradients, allowing obtaining new insights on the mechanical interplay between tumor cells and the surrounding environments. In particular, cell invasion is mostly driven by softer architectures, and the introduction of 3D stiffness “weak spots” is shown to boost the rate at which cancer cells invade the scaffolds. The possibility to modulate structural compliance also allowed estimating the force distribution exerted by a single cell on the scaffold, revealing that both pushing and pulling forces are involved in the cell–structure interaction. Overall, exploiting this method to obtain a wide range of 3D architectures with locally engineered stiffness can pave the way for unique applications to study tumor cell dynamics. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The response to different force load ranges and actuation at low energies is of considerable interest for applications of compliant and flexible devices undergoing large deformations. We present a review of technological platforms based on nitride materials (aluminum nitride and silicon nitride) for the microfabrication of a class of flexible micro-electro-mechanical systems. The approach exploits the material stress differences among the constituent layers of nitride-based (AlN/Mo, Si x N y /Si and AlN/polyimide) mechanical elements in order to create microstructures, such as upwardly-bent cantilever beams and bowed circular membranes. Piezoresistive properties of nichrome strain gauges and direct piezoelectric properties of aluminum nitride can be exploited for mechanical strain/stress detection. Applications in flow and tactile sensing for robotics are described.

Two-photon lithography (2PL) is a direct laser writing approach for micro- and nanofabrication of 3D polymeric structures for a variety of applications. Due to its high resolution (

Spatial-resolved light delivery in the living mammalian brain is of utmost importance in optogenetics experiments, since it allows for combining cell-type specificity with spatial selective optical control of neural activity. This can be achieved at sub-cellular resolution in first cortical layers with two-photon microscopy, while sub-cortical regions can be accessed by using waveguide-based devices. Tapered and nanostructured optical fibers have been recently proposed as viable devices to dynamically switch light stimuli between different points of deep-brain structures, by exploiting both mode division demultiplexing operated by the fiber taper and excitation of well-defined subsets of guided modes into the fiber [Pisanello et al., Neuron, vol. 82, p.1245 (2014); Pisanello et al., Biomed. Opt. Express, vol. 6, p. 4014 (2015)]. This latter is achieved byselecting the injection angle of a high-quality Gaussian beam, therefore exciting guided modes with defined transversal propagation constants. In this contribution we analyze the effectiveness of this technique as a function of the laser beam profile used to guide light into the fiber, complementing experimental results with ray tracing simulations. Two different commercial laser sources, with sensibly different beam qualities, were used to determine the extent of the injected modal subsets in a 0.22 N.A. optical fiber. This was obtained by measuring the transversal component kt of the wavevector through far-field imaging. We show that even poor quality beams might return satisfactory selection accuracy, provided that care is taken in the design of the coupling strategy. © 2016 IEEE.

We present an innovative design for a flexible probe to study mechanisms of biological force sensing and force generation in the piconewton to micronewton range. Made of polydimethylsiloxane (PDMS) and employing a novel ring-spring section with adjustable size, the device works both as a force sensor and force actuator by precise calibration of its tunable stiffness and optical measurement of ring deformation. In addition, the tip geometry of the probe can be properly shaped to fit the anatomical profile of the sensory receptor of interest and to reproduce the in vivo stimulation. Finally, use of Finite Element Method (FEM) modal analysis confirms that the resonance frequencies of probes are outside the frequency range of interest for many sensory systems. © 2015 IEEE.

The progress in medical field has provided fast remedies for the most of diseases affecting today's society. Now, the next step is to propose a fast way of diagnosis, avoiding long waiting time and expensive laboratory assays. Thanks to the recent research findings and progress in nano-technological fabrication techniques, new biosensors have been developed with the purpose to introduce portable and user-friendly instruments for medical analysis aimed to improve life quality and lower healthcare costs. In this work, we propose a proof-of-concept of an immunosensor based on a two-dimensional silicon nitride photonic crystal membrane able to efficiently detect the concentration of Interleukin-6 in a buffer solution. Refereed/Peer-reviewed

Biosensors are devices able to reduce costs and time in biological assays. They are efficient and useful tools also in biochemical assays, providing high performance in sensitivity and selectivity of measurements. In this work, we developed a label-free biosensor based on a two-dimensional photonic crystal patterned on a large area silicon nitride membrane that operates in the visible spectrum for the detection of proteins. The biosensor has been preliminarily tested in solutions of different refractive index, showing as expected a shift of the resonance in the reflection spectrum Refereed/Peer-reviewed

This work presents a promising ultrasound wearable technology based on a piezoelectric transducer, realized on flexible highly oriented Aluminum Nitride, with significant mechanical displacement in spite of being attached on a rigid support. Circular membranes with different radius sizes, based on 1-μm-thick AlN thin film as the active piezoelectric layer, are designed, fabricated, and characterized. The AlN is deposited on kapton HN substrate with a low sputtering deposition temperature that allows the integration and the compatibility with flexible electronics. Mechanical and thermal stability of kapton makes this polyimide based sheet a potential substrate for flexible piezoelectric thin film technology. The actuation at low voltage (1-10 V) of the AlN membranes is studied in air in the range of ultrasound frequencies, from 0 Hz up to 2 MHz; the voltage amplitude, the shape and displacement of the flexure mode (0, 1) is studied by a Laser Doppler Vibrometer to characterize the mechanical properties of the device. © 2014 Elsevier B.V. All rights reserved.