Ferroelectret nanogenerators for the development of bioengineering systems.

Bioengineering devices and systems will become a practical and versatile technology in society when sustainability issues, primarily pertaining to their efficiency, sustainability, and human-machine interaction, are fully addressed. It has become evident that technological paths should not rely on a single operation mechanism but instead on holistic methodologies that integrate different phenomena and approaches with complementary advantages. As an intriguing invention, the ferroelectret nanogenerator (FENG) has emerged with promising potential in various fields of bioengineering. Utilizing the changes in the engineered macro-scale electric dipoles to create displacement current (and vice versa), FENGs have been demonstrated to be a compelling strategy for bidirectional conversion of energy between the electrical and mechanical domains. Here we provide a comprehensive overview of the latest advancements in integrating FENGs in bioengineering systems, focusing on the applications with the most potential and the underlying current constraints.

Measuring vibrations on a biofidelic brain using ferroelectret nanogenerator.

Our knowledge of traumatic brain injury has been fast growing with the emergence of new markers pointing to various neurological changes that the brain undergoes during an impact or any other form of concussive event. In this work, we study the modality of deformations on a biofidelic brain system when subject to blunt impacts, highlighting the importance of the time-dependent behavior of the resulting waves propagating through the brain. This study is carried out using two different approaches involving optical (Particle Image Velocimetry) and mechanical (flexible sensors) in the biofidelic brain. Results show that the system has a natural mechanical frequency of [Formula: see text] 25 oscillations per second, which was confirmed by both methods, showing a positive correlation with one another. The consistency of these results with previously reported brain pathology validates the use of either technique, and establishes a new, simpler mechanism to study brain vibrations by using flexible piezoelectric patches. The visco-elastic nature of the biofidelic brain is validated by observing the the relationship between both methods at two different time intervals, by using the information of the strain and stress inside the brain from the Particle Image Velocimetry and flexible sensor, respectively. A non-linear stress-strain relationship was observed and justified to support the same.

Flexible, self-powered sensors for estimating human head kinematics relevant to concussions.

The present work demonstrates the development of a flexible, self-powered sensor patch that can be used to estimate angular acceleration and angular velocity, which are two essential markers for predicting concussions. The device monitors the dynamic strain experienced by the neck through a thin, polypropylene-based ferroelectret nanogenerator that produces a voltage pulse with profile proportional to strain. The intrinsic property of this device to convert mechanical input to electrical output, along with its flexibility and [Formula: see text] 100 [Formula: see text]m thickness makes it a viable and practical device to be used as a wearable patch for athletes in high-contact sports. After processing the dynamic behavior of the produced voltage, a correspondence between the electric signal profile and the measurements from accelerometers integrated inside a human head and neck substitute was found. This demonstrates the ability of obtaining an electronic signature that can be used to extract head kinematics during collision, and creates a marker that could be used to detect concussions. Unlike accelerometer-based current trends on concussion-detection systems, which rely on sensors integrated in the athlete's helmet, the flexible patch attached to the neck would provide information on the dynamics of the head movement, thus eliminating the potential of false readings from helmet sliding or peak angular acceleration.

Piezoelectric Lateral-Extensional Mode Resonators With Reconfigurable Electrode and Resonance Mode-Switching Behavior Enabled by a VO2 Thin-Film.

This article presents the first two-port lateral-extensional mode zinc oxide (ZnO) piezoelectric resonator with a reconfigurable bottom electrode that is enabled by embedding a vanadium dioxide (VO2) thin film. The insulator-to-metal phase transition of VO2 is triggered by substrate heating that translates to abrupt changes in electric field patterns and piezoelectrically transduced modal vibrations, thus allowing mode-switching of piezoelectric resonators at specific frequencies. Finite element method (FEM) analysis was used to model the broadband frequency response, while frequency characteristics of the corresponding two-port resonator were measured over a temperature range between 20 °C and 95 °C with a specific focus on two resonances at 88 and 148 MHz. By leveraging the hysteretic behavior of VO2 thin film during a heating/cooling cycle, a change in both the capacitive feedthrough and resonance signal levels was observed, due to the abrupt change in the conductivity of VO2 during its phase transition. The unique switch- ON behavior of the resonance at 88 MHz starts at 70 °C during the heating cycle, while the switch- OFF transition begins at 60 °C during the cooling cycle. On the other hand, when the temperature is increased from 20 °C to 60 °C, a decrease in the insertion loss and resonance frequency of 12 dB and 0.28 MHz, respectively, were observed for the resonance at 148 MHz. Meanwhile, a resonance frequency increase of 0.42 MHz was observed during a temperature increase from 60 °C to 95 °C, which can be ascribed to VO2 phase transition from monoclinic to rutile phase. The hysteresis loops for insertion loss and resonance frequency indicate a different critical temperature for phase transition from the monoclinic (insulator) phase and rutile (metallic) phase and vice versa. The substantial variation in the temperature coefficient of frequency can be largely ascribed to electrode reconfiguration enabled by VO2 phase transition.

Thin Film Piezoelectric Nanogenerator Based on (100)-Oriented Nanocrystalline AlN Grown by Pulsed Laser Deposition at Room Temperature.

In wearable or implantable biomedical devices that typically rely on battery power for diagnostics or operation, the development of flexible piezoelectric nanogenerators (NGs) that enable mechanical-to-electrical energy harvesting is finding promising applications. Here, we present the construction of a flexible piezoelectric nanogenerator using a thin film of room temperature deposited nanocrystalline aluminium nitride (AlN). On a thin layer of aluminium (Al), the AlN thin film was grown using pulsed laser deposition (PLD). The room temperature grown AlN film was composed of crystalline columnar grains oriented in the (100)-direction, as revealed in images from transmission electron microscopy (TEM) and X-ray diffraction (XRD). Fundamental characterization of the AlN thin film by piezoresponse force microscopy (PFM) indicated that its electro-mechanical energy conversion metrics were comparable to those of c-axis oriented AlN and zinc oxide (ZnO) thin films. Additionally, the AlN-based flexible piezoelectric NG was encapsulated in polyimide to further strengthen its mechanical robustness and protect it from some corrosive chemicals.

VO2-based micro-electro-mechanical tunable optical shutter and modulator.

VO2-based MEMS tunable optical shutters are demonstrated. The design consists of a VO2-based cantilever attached to a VO2-based optical window with integrated resistive heaters for individual mechanical actuation of the cantilever structure, tuning of the optical properties of the window, or both. Optical transmittance measurements as a function of current for both heaters demonstrates that the developed devices can be used as analog optical shutters, where the intensity of a light beam can be tuned to any value within the range of VO2 phase transition. A transmittance drop off 30% is shown for the optical window, with tuning capabilities greater than 30% upon actuation of the cantilever. Unlike typical mechanical shutters, these devices are not restricted to binary optical states. Optical modulation of the optical window is demonstrated with an oscillating electrical input. This produces a transmittance signal that oscillates around an average value within the range off VO2's phase transition. For an input current signal with fixed amplitude (fel= 0.28 Hz), tuned to be at the onset of the phase transition, a transmittance modulation of 14% is shown. Similarly, by modulating the DC-offset, a transmittance modulation of VO2 along the hysteresis is obtained.

Multimodal Artificial Neurological Sensory-Memory System Based on Flexible Carbon Nanotube Synaptic Transistor.

As the initial stage in the formation of human intelligence, the sensory-memory system plays a critical role for human being to perceive, interact, and evolve with the environment. Electronic implementation of such biological sensory-memory system empowers the development of environment-interactive artificial intelligence (AI) that can learn and evolve with diversified external information, which could potentially broaden the application of the AI technology in the field of human-computer interaction. Here, we report a multimodal artificial sensory-memory system consisting of sensors for generating biomimetic visual, auditory, tactile inputs, and flexible carbon nanotube synaptic transistor that possesses synapse-like signal processing and memorizing behaviors. The transduction of physical signals into information-containing, presynaptic action potentials and the synaptic plasticity of the transistor in response to single and long-term action potential excitations have been systematically characterized. The bioreceptor-like sensing and synapse-like memorizing behaviors have also been demonstrated. On the basis of the memory and learning characteristics of the sensory-memory system, the well-known psychological model describing human memory, the "multistore memory" model, and the classical conditioning experiment that demonstrates the associative learning of brain, "Pavlov's dog's experiment", have both been implemented electronically using actual physical input signals as the sources of the stimuli. The biomimetic intelligence demonstrated in this neurological sensory-memory system shows its potential in promoting the advancement in multimodal, user-environment interactive AI.

Measurement of suction pressure dynamics of sea lampreys, Petromyzon marinus.

Species-specific monitoring activities represent fundamental tools for natural resource management and conservation but require techniques that target species-specific traits or markers. Sea lamprey, a destructive invasive species in the Laurentian Great Lakes and conservation target in North America and Europe, is among very few fishes that possess and use oral suction, yet suction has not been exploited for sea lamprey control or conservation. Knowledge of specific characteristics of sea lamprey suction (e.g., amplitude, duration, and pattern of suction events; hereafter 'suction dynamics') may be useful to develop devices that detect, record, and respond to the presence of sea lamprey at a given place and time. Previous observations were limited to adult sea lampreys in static water. In this study, pressure sensing panels were constructed and used to measure oral suction pressures and describe suction dynamics of juvenile and adult sea lampreys at multiple locations within the mouth and in static and flowing water. Suction dynamics were largely consistent with previous descriptions, but more variation was observed. For adult sea lampreys, suction pressures ranged from -0.6 kPa to -26 kPa with 20 s to 200 s between pumps at rest, and increased to -8 kPa to -70 kPa when lampreys were manually disengaged. An array of sensors indicated that suction pressure distribution was largely uniform across the mouths of both juvenile and adult lampreys; but some apparent variation was attributed to obstruction of sensing portal holes by teeth. Suction pressure did not differ between static and flowing water when water velocity was lower than 0.45 m/s. Such information may inform design of new systems to monitor behavior, distribution and abundance of lampreys.

Quick self-assembly of bio-inspired multi-dimensional well-ordered structures induced by ultrasonic wave energy.

Bottom-up self-assembly of components, inspired by hierarchically self-regulating aggregation of small subunits observed in nature, provides a strategy for constructing two- or three-dimensional intriguing biomimetic materials via the spontaneous combination of discrete building blocks. Herein, we report the methods of ultrasonic wave energy-assisted, fast, two- and three-dimensional mesoscale well-ordered self-assembly of microfabricated building blocks (100 µm in size). Mechanical vibration energy-driven self-assembly of microplatelets at the water-air interface of inverted water droplets is demonstrated, and the real-time formation process of the patterned structure is dynamically explored. 40 kHz ultrasonic wave is transferred into microplatelets suspended in a water environment to drive the self-assembly of predesigned well-ordered structures. Two-dimensional self-assembly of microplatelets inside the water phase with a large patterned area is achieved. Stable three-dimensional multi-layered self-assembled structures are quickly formed at the air-water interface. These demonstrations aim to open distinctive and effective ways for new two-dimensional surface coating technology with autonomous organization strategy, and three-dimensional complex hierarchical architectures built by the bottom-up method and commonly found in nature (such as nacre, bone or enamel, etc.).

Flexible Carbon Nanotube Synaptic Transistor for Neurological Electronic Skin Applications.

There is an increasing interest in the development of memristive or artificial synaptic devices that emulate the neuronal activities for neuromorphic computing applications. While there have already been many reports on artificial synaptic transistors implemented on rigid substrates, the use of flexible devices could potentially enable an even broader range of applications. In this paper, we report artificial synaptic thin-film transistors built on an ultrathin flexible substrate using high carrier mobility semiconducting single-wall carbon nanotubes. The synaptic characteristics of the flexible synaptic transistor including long-term/short-term plasticity, spike-amplitude-dependent plasticity, spike-width-dependent plasticity, paired-pulse facilitation, and spike-time-dependent plasticity have all been systematically characterized. Furthermore, we have demonstrated a flexible neurological electronic skin and its peripheral nerve with a flexible ferroelectret nanogenerator (FENG) serving as the sensory mechanoreceptor that generates action potentials to be processed and transmitted by the artificial synapse. In such neurological electronic skin, the flexible FENG sensor converts the tactile input (magnitude and frequency of force) into presynaptic action potential pulses, which are then passed to the gate of the synaptic transistor to induce change in its postsynaptic current, mimicking the modulation of synaptic weight in a biological synapse. Our neurological electronic skin closely imitates the behavior of actual human skin, and it allows for instantaneous detection of force stimuli and offers biological synapse-like behavior to relay the stimulus signals to the next stage. The flexible sensory skin could potentially be used to interface with skeletal muscle fibers for applications in neuroprosthetic devices.

Electrode Effects on Flexible and Robust Polypropylene Ferroelectret Devices for Fully Integrated Energy Harvesters.

This work presents a characterization study of the electrode interface in polypropylene ferroelectret nanogenerators. An emphasis is made on the comparison of carbon nanotube fiber electrodes with traditional metallic thin film electrodes. Multiple experiments were performed on samples with the same electrode dimensions for a range of applied pressures. Results showed higher open-circuit voltage peak values for the thin film metal electrodes, regardless of the applied pressure. Interestingly, the difference in short-circuit current values between metal and carbon nanotube-based fiber electrodes was not as significant. The carbon nanotube fiber electrode was further investigated by post-treating the fiber with acetone and comparing the results with untreated carbon nanotube film electrodes and thin film metal electrodes. In an effort to enable a monolithic integration of ferroelectret energy harvesters with flexible energy storage elements, this work also presents studies on generation and leakage of induced free charge in the electrodes of flexible ferroelectret energy harvesters. It was found the current leakage through parasitic elements is a faster process than dipole relaxation in the polypropylene film. Finally, an electrode reliability study shows no significant difference in the electrical output of the devices with metallic thin film electrodes after single folding but shows a significant deterioration after crumpling; meanwhile, these processes had no effect on the performance of similar devices with carbon nanotube fiber-based electrodes.

Ferroelectret Nanogenerators for Loudspeaker Applications: A Comprehensive Study.

A ferroelectret nanogenerator (FENG) was recently developed as a flexible energy harvesting device with bi-directional capability between electrical and mechanical energy domains, and its use as a loudspeaker/microphone was demonstrated. Dependencies of Sound Pressure Levels (SPLs) generated by FENG due to an AC voltage stimulus, surface area, geometric shape, and addition of layers are presented here. Also, the relation between the sound output to the electrical input is studied and shown to be linear, which demonstrates that these flexible loudspeakers have low distortion within the human audible range of 20 Hz to 20 kHz. A study for ultrasonic frequencies up to 40 kHz is also presented. A theoretical model relating the electrical and acoustical domain of the FENG is developed based on the experimental observations made and using Boundary Element Methods (BEM) to accurately mimic the testing environment for simulation purposes. The comparison between this model and the actual behavior is presented under several cases and observed to be closely correlated.

Flexible Ferroelectret Polymer for Self-Powering Devices and Energy Storage Systems.

Applying flexible materials for energy scavenging from ambient mechanical vibrations is a clean energy solution that can help alleviate electrical power demands in portable devices and wearable electronics. This work presents fundamental studies on a flexible ferroelectret polymer with a strong piezoelectric effect and its interface with self-powered and energy storage systems. A single-layered device with a thickness of 80 µm was used for characterizing the device's output voltage, current, transferred charge, and energy conversion efficiency. The potential capability of harvesting mechanical energy and delivering to system load is demonstrated by integrating the device into a fully integrated power management system. The theory for determining the harvested energy that is ultimately delivered to external electronic loads (or stored in a battery) is discussed. The maximum power delivery is found to be for a 600 MΩ load, which results in a device power density of 14.0 W/m3 for input mechanical forces with a frequency around 2 Hz.

Nanogenerator-based dual-functional and self-powered thin patch loudspeaker or microphone for flexible electronics.

Ferroelectret nanogenerators were recently introduced as a promising alternative technology for harvesting kinetic energy. Here we report the device's intrinsic properties that allow for the bidirectional conversion of energy between electrical and mechanical domains; thus extending its potential use in wearable electronics beyond the power generation realm. This electromechanical coupling, combined with their flexibility and thin film-like form, bestows dual-functional transducing capabilities to the device that are used in this work to demonstrate its use as a thin, wearable and self-powered loudspeaker or microphone patch. To determine the device's performance and applicability, sound pressure level is characterized in both space and frequency domains for three different configurations. The confirmed device's high performance is further validated through its integration in three different systems: a music-playing flag, a sound recording film and a flexible microphone for security applications.

Maximizing the performance of photothermal actuators by combining smart materials with supplementary advantages.

The search for higher-performance photothermal microactuators has typically involved unavoidable trade-offs that hinder the demonstration of ubiquitous devices with high energy density, speed, flexibility, efficiency, sensitivity, and multifunctionality. Improving some of these parameters often implies deterioration of others. Photothermal actuators are driven by the conversion of absorbed optical energy into thermal energy, which, by different mechanisms, can produce mechanical displacement of a structure. We present a device that has been strategically designed to show high performance in every metric and respond to optical radiation of selected wavelength bands. The device combines the large energy densities and sensitivity of vanadium dioxide (VO2)-based actuators with the wavelength-selective absorption properties of single-walled carbon nanotube (SWNT) films of different chiralities. SWNT coatings increased the speed of VO2 actuators by a factor of 2 while decreasing the power consumption by approximately 50%. Devices coated with metallic SWNT were found to be 1.57 times more responsive to red light than to near-infrared, whereas semiconducting SWNT coatings resulted in 1.42 times higher responsivities to near-infrared light than to red light. The added functionality establishes a link between optical and mechanical domains of high-performance photoactuators and enables the future development of mechanical logic gates and electronic devices that are triggered by optical radiation from different frequency bands.

The nature of photoinduced phase transition and metastable states in vanadium dioxide.

Photoinduced threshold switching processes that lead to bistability and the formation of metastable phases in photoinduced phase transition of VO2 are elucidated through ultrafast electron diffraction and diffusive scattering techniques with varying excitation wavelengths. We uncover two distinct regimes of the dynamical phase change: a nearly instantaneous crossover into an intermediate state and its decay led by lattice instabilities over 10 ps timescales. The structure of this intermediate state is identified to be monoclinic, but more akin to M2 rather than M1 based on structure refinements. The extinction of all major monoclinic features within just a few picoseconds at the above-threshold-level (~20%) photoexcitations and the distinct dynamics in diffusive scattering that represents medium-range atomic fluctuations at two photon wavelengths strongly suggest a density-driven and nonthermal pathway for the initial process of the photoinduced phase transition. These results highlight the critical roles of electron correlations and lattice instabilities in driving and controlling phase transformations far from equilibrium.

Electronically-Controlled Beam-Steering through Vanadium Dioxide Metasurfaces.

Engineered metamaterials offer unique functionalities for manipulating the spectral and spatial properties of electromagnetic waves in unconventional ways. Here, we report a novel approach for making reconfigurable metasurfaces capable of deflecting electromagnetic waves in an electronically controllable fashion. This is accomplished by tilting the phase front of waves through a two-dimensional array of resonant metasurface unit-cells with electronically-controlled phase-change materials embedded inside. Such metasurfaces can be placed at the output facet of any electromagnetic radiation source to deflect electromagnetic waves at a desired frequency, ranging from millimeter-wave to far-infrared frequencies. Our design does not use any mechanical elements, external light sources, or reflectarrays, creating, for the first time, a highly robust and fully-integrated beam-steering device solution. We demonstrate a proof-of-concept beam-steering metasurface optimized for operation at 100 GHz, offering up to 44° beam deflection in both horizontal and vertical directions. Dynamic control of electromagnetic wave propagation direction through this unique platform could be transformative for various imaging, sensing, and communication applications, among others.

Bolometric-Effect-Based Wavelength-Selective Photodetectors Using Sorted Single Chirality Carbon Nanotubes.

This paper exploits the chirality-dependent optical properties of single-wall carbon nanotubes for applications in wavelength-selective photodetectors. We demonstrate that thin-film transistors made with networks of carbon nanotubes work effectively as light sensors under laser illumination. Such photoresponse was attributed to photothermal effect instead of photogenerated carriers and the conclusion is further supported by temperature measurements. Additionally, by using different types of carbon nanotubes, including a single chirality (9,8) nanotube, the devices exhibit wavelength-selective response, which coincides well with the absorption spectra of the corresponding carbon nanotubes. This is one of the first reports of controllable and wavelength-selective bolometric photoresponse in macroscale assemblies of chirality-sorted carbon nanotubes. The results presented here provide a viable route for achieving bolometric-effect-based photodetectors with programmable response spanning from visible to near-infrared by using carbon nanotubes with pre-selected chiralities.

Photoinduced insulator-to-metal transition and surface statistics of VO2 monitored by elastic light scattering.

Measurements of ultrafast light scattering within a hemisphere are performed for statistical analysis of nonequilibrium processes in VO2 epitaxial film. A Gerchberg-Saxton error reduction algorithm is applied for accurate calculation of a surface autocorrelation function from light scattering data and for partial reconstruction of a power spectral density function. Upon ultrafast photoinduced phase transition of VO2, the elastic light scattering reveals anisotropic grain-size-dependent dynamics. It was found that the transition rate depends on the optical absorption and orientation of VO2 grains with respect to polarization of the pump pulse. An observed stepwise evolution of surface autocorrelation length and transient anisotropy of the scattering field presumably originates from complex multistage transformation of VO2 lattice on a subpicosecond time scale.

Increasing efficiency, speed, and responsivity of vanadium dioxide based photothermally driven actuators using single-wall carbon nanotube thin-films.

Vanadium dioxide (VO2)-based actuators have demonstrated great performance in terms of strain energy density, speed, reversible actuation, programming capabilities, and large deflection. The relative low phase transition temperature of VO2 (∼68 °C) gives this technology an additional advantage over typical thermal actuators in terms of power consumption. However, this advantage can be further improved if light absorption is enhanced. Here we report a VO2-based actuator technology that incorporates single-wall carbon nanotubes (SWNTs) as an effective light absorber to reduce the amount of photothermal energy required for actuation. It is demonstrated that the chemistry involved in the process of integrating the SWNT film with the VO2-based actuators does not alter the quality of the VO2 film, and that the addition of such film enhances the actuator performance in terms of speed and responsivity. More importantly, the results show that the combination of VO2 and SWNT thin films is an effective approach to increase the photothermal efficiency of VO2-based actuators. The integration of SWNT films in VO2 devices can be easily applied to other VO2-based phototransducers as well as to similar devices based on other phase-change materials. While adding a sufficiently thick layer of some arbitrary material with high absorption for the light used for actuation (λ = 650 nm wavelength in this case) could have improved conversion of light to heat in the device, it could also have impeded actuation by increasing its stiffness. It is noted, however, that the low effective Young's modulus of SWNT film coating used in this work does not impair the actuation range.