A Review of Smart Materials in Tactile Actuators for Information Delivery

As the largest organ in the human body, the skin provides the important sensory channel for humans to receive external stimulations based on touch. By the information perceived through touch, people can feel and guess the properties of objects, like weight, temperature, textures, and motion, etc. In fact, those properties are nerve stimuli to our brain received by different kinds of receptors in the skin. Mechanical, electrical, and thermal stimuli can stimulate these receptors and cause different information to be conveyed through the nerves. Technologies for actuators to provide mechanical, electrical or thermal stimuli have been developed. These include static or vibrational actuation, electrostatic stimulation, focused ultrasound, and more. Smart materials, such as piezoelectric materials, carbon nanotubes, and shape memory alloys, play important roles in providing actuation for tactile sensation. This paper aims to review the background biological knowledge of human tactile sensing, to give an understanding of how we sense and interact with the world through the sense of touch, as well as the conventional and state-of-the-art technologies of tactile actuators for tactile feedback delivery

Performance Comparison of Phase Change Materials and Metal-Insulator Transition Materials for Direct Current and Radio Frequency Switching Applications

Advanced understanding of the physics makes phase change materials (PCM) and metal-insulator transition (MIT) materials great candidates for direct current (DC) and radio frequency (RF) switching applications. In the literature, germanium telluride (GeTe), a PCM, and vanadium dioxide (VO2), an MIT material have been widely investigated for DC and RF switching applications due to their remarkable contrast in their OFF/ON state resistivity values. In this review, innovations in design, fabrication, and characterization associated with these PCM and MIT material-based RF switches, have been highlighted and critically reviewed from the early stage to the most recent works. We initially report on the growth of PCM and MIT materials and then discuss their DC characteristics. Afterwards, novel design approaches and notable fabrication processes; utilized to improve switching performance; are discussed and reviewed. Finally, a brief vis-á-vis comparison of resistivity, insertion loss, isolation loss, power consumption, RF power handling capability, switching speed, and reliability is provided to compare their performance to radio frequency microelectromechanical systems (RF MEMS) switches; which helps to demonstrate the current state-of-the-art, as well as insight into their potential in future applications

Piezoelectric Transducers Based on Aluminum Nitride and Polyimide for Tactile Applications

The development of micro systems with smart sensing capabilities is paving the way to progresses in the technology for humanoid robotics. The importance of sensory feedback has been recognized the enabler of a high degree of autonomy for robotic systems. In tactile applications, it can be exploited not only to avoid objects slipping during their manipulation but also to allow safe interaction with humans and unknown objects and environments. In order to ensure the minimal deformation of an object during subtle manipulation tasks, information not only on contact forces between the object and fingers but also on contact geometry and contact friction characteristics has to be provided. Touch, unlike other senses, is a critical component that plays a fundamental role in dexterous manipulation capabilities and in the evaluation of objects properties such as type of material, shape, texture, stiffness, which is not easily possible by vision alone. Understanding of unstructured environments is made possible by touch through the determination of stress distribution in the surrounding area of physical contact. To this aim, tactile sensing and pressure detection systems should be integrated as an artificial tactile system. As illustrated in the Chapter I, the role of external stimuli detection in humans is provided by a great number of sensorial receptors: they are specialized endings whose structure and location in the skin determine their specific signal transmission characteristics. Especially, mechanoreceptors are specialized in the conversion of the mechanical deformations caused by force, vibration or slip on skin into electrical nerve impulses which are processed and encoded by the central nervous system. Highly miniaturized systems based on MEMS technology seem to imitate properly the large number of fast responsive mechanoreceptors present in human skin. Moreover, an artificial electronic skin should be lightweight, flexible, soft and wearable and it should be fabricated with compliant materials. In this respect a big challenge of bio-inspired technologies is the efficient application of flexible active materials to convert the mechanical pressure or stress into a usable electric signal (voltage or current). In the emerging field of soft active materials, able of large deformation, piezoelectrics have been recognized as a really promising and attractive material in both sensing and actuation applications. As outlined in Chapter II, there is a wide choice of materials and material forms (ceramics: PZT; polycrystalline films: ZnO, AlN; polymers and copolymers: PVDF, PVDF-TrFe) which are actively piezoelectric and exhibit features more or less attractive. Among them, aluminum nitride is a promising piezoelectric material for flexible technology. It has moderate piezoelectric coefficient, when available in c-axis oriented polycrystalline columnar structure, but, at same time, it exhibits low dielectric constant, high temperature stability, large band gap, large electrical resistivity, high breakdown voltage and low dielectric loss which make it suitable for transducers and high thermal conductivity which implies low thermal drifts. The high chemical stability allows AlN to be used in humid environments. Moreover, all the above properties and its deposition method make AlN compatible with CMOS technology. Exploiting the features of the AlN, three-dimensional AlN dome-shaped cells, embedded between two metal electrodes, are proposed in this thesis. They are fabricated on general purpose Kapton™ substrate, exploiting the flexibility of the polymer and the electrical stability of the semiconductor at the same time. As matter of fact, the crystalline layers release a compressive stress over the polymer, generating three-dimensional structures with reduced stiffness, compared to the semiconductor materials. In Chapter III, a mathematical model to calculate the residual stresses which arise because of mismatch in coefficient of thermal expansion between layers and because of mismatch in lattice constants between the substrate and the epitaxially grown films is adopted. The theoretical equation is then used to evaluate the dependence of geometrical features of the fabricated three-dimensional structures on compressive residual stress. Moreover, FEM simulations and theoretical models analysis are developed in order to qualitative explore the operation principle of curved membranes, which are labelled dome-shaped diaphragm transducers (DSDT), both as sensors and as piezo-actuators and for the related design optimization. For the reliability of the proposed device as a force/pressure sensor and piezo-actuator, an exhaustive electromechanical characterization of the devices is carried out. A complete description of the microfabrication processes is also provided. As shown in Chapter IV, standard microfabrication techniques are employed to fabricate the array of DSDTs. The overall microfabrication process involves deposition of metal and piezoelectric films, photolithography and plasma-based dry and wet etching to pattern thin films with the desired features. The DSDT devices are designed and developed according to FEM and theoretical analysis and following the typical requirements of force/pressure systems for tactile applications. Experimental analyses are also accomplished to extract the relationship between the compressive residual stress due to the aluminum nitride and the geometries of the devices. They reveal different deformations, proving the dependence of the geometrical features of the three-dimensional structures on residual stress. Moreover, electrical characterization is performed to determine capacitance and impedance of the DSDTs and to experimentally calculate the relative dielectric constant of sputtered AlN piezoelectric film. In order to investigate the mechanical behaviour of the curved circular transducers, a characterization of the flexural deflection modes of the DSDT membranes is carried out. The natural frequency of vibrations and the corresponding displacements are measured by a Laser Doppler Vibrometer when a suitable oscillating voltage, with known amplitude, is applied to drive the piezo-DSDTs. Finally, being developed for tactile sensing purpose, the proposed technology is tested in order to explore the electromechanical response of the device when impulsive dynamic and/or long static forces are applied. The study on the impulsive dynamic and long static stimuli detection is then performed by using an ad hoc setup measuring both the applied loading forces and the corresponding generated voltage and capacitance variation. These measurements allow a thorough test of the sensing abilities of the AlN-based DSDT cells. Finally, as stated in Chapter V, the proposed technology exhibits an improved electromechanical coupling with higher mechanical deformation per unit energy compared with the conventional plate structures, when the devices are used as piezo-actuator. On the other hand, it is well suited to realize large area tactile sensors for robotics applications, opening up new perspectives to the development of latest generation biomimetic sensors and allowing the design and the fabrication of miniaturized devices

MEMS tunable infrared metamaterial and mechanical sensors

Sub-wavelength resonant structures open the path for fine controlling the near-field at the nanoscale dimension. They constitute into macroscopic “metamaterials” with macroscale properties such as transmission, reflection, and absorption being tailored to exhibit a particular electromagnetic response. The properties of the resonators are often fixed at the time of fabrication wherein the tunability is demanding to overcome fabrication tolerances and afford fast signal processing. Hybridizing dynamic components such as optically active medium into the device makes tunable devices. Microelectromechanical systems (MEMS) compatible integrated circuit fabrication process is a promising platform that can be merged with photonics or novel 2D materials. The prospect of enormous freedom in integrating nanophotonics, MEMS actuators and sensors, and microelectronics into a single platform has driven the rapid development of MEMS-based sensing devices. This thesis describes the design and development of four tunable plasmonic structures based on active media or MEMS, two graphene-based MEMS sensors and a novel tape-based cost-effective nanotransfer printing techniques. First of all, we present two tunable plasmonic devices with the use of two active medium, which are electrically controlled liquid crystals and temperature-responsive hydrogels, respectively. By incorporating a nematic liquid crystal layer into quasi-3D mushroom plasmonic nanostructures and thanks to the unique coupling between surface plasmon polariton and Rayleigh anomaly, we have achieved the electrical tuning of the properties of plasmonic crystal at a low operating electric field. We also present another tunable plasmonic device with the capability to sense environmental temperature variations. The device is bowtie nanoantenna arrays coated with a submicron-thick, thermos-responsive hydrogel. The favorable scaling of plasmonic dimers at the nanometer scale and ionic diffusion at the submicron scale is leveraged to achieve strong optical resonance and rapid hydrogel response, respectively. Secondly, we present two MEMS -based tunable near-to-mid infrared metamaterials on a silicon-on-insulator wafer via electrically and thermally actuating the freestanding nanocantilevers. The two devices are developed on the basis of the same fabrication process and are easy-to-implement. The electrostatically driven metamaterial affords ultrahigh mechanical modulation (several tens of MHz) of an optical signal while the thermo-mechanically tunable metamaterial provides up to 90% optical signal modulation at a wavelength of 3.6 õm. Next, we present MEMS graphene-based pressure and gas flow sensors realized by transferring a large area and few-layered graphene onto a suspended silicon nitride thin membrane perforated with micro-through-holes. Due to the increased strain in the through-holes, the pressure sensor exhibits a very high sensitivty outperformed than most existing MEMS-based pressure sensors using graphene, silicon, and carbon nanotubes. An air flow sensor is also demonstrated via patterning graphene sheets with flow-through microholes. The flow rate of the air is measured by converting the mechanically deflection of the membrane into the electrical readout due to the graphene piezeroresistors. Finally, we present a tape-based multifunctional nanotransfer printing process based on a simple stick-and-peel procedure. It affords fast production of large-area metallic and dielectric nanophotonic sensing devices and metamaterials using Scotch tape

Piezoresistive Carbon Nanofiber-Based Cilia-Inspired Flow Sensor

Evolving over millions of years, hair-like natural flow sensors called cilia, which are found in fish, crickets, spiders, and inner ear cochlea, have achieved high resolution and sensitivity in flow sensing. In the pursuit of achieving such exceptional flow sensing performance in artificial sensors, researchers in the past have attempted to mimic the material, morphological, and functional properties of biological cilia sensors, to develop MEMS-based artificial cilia flow sensors. However, the fabrication of bio-inspired artificial cilia sensors involves complex and cumbersome micromachining techniques that lay constraints on the choice of materials, and prolongs the time taken to research, design, and fabricate new and novel designs, subsequently increasing the time-to-market. In this work, we establish a novel process flow for fabricating inexpensive, yet highly sensitive, cilia-inspired flow sensors. The artificial cilia flow sensor presented here, features a cilia-inspired high-aspect-ratio titanium pillar on an electrospun carbon nanofiber (CNF) sensing membrane. Tip displacement response calibration experiments conducted on the artificial cilia flow sensor demonstrated a lower detection threshold of 50 µm. Furthermore, flow calibration experiments conducted on the sensor revealed a steady-state airflow sensitivity of 6.16 mV/(m s−1) and an oscillatory flow sensitivity of 26 mV/(m s−1), with a lower detection threshold limit of 12.1 mm/s in the case of oscillatory flows. The flow sensing calibration experiments establish the feasibility of the proposed method for developing inexpensive, yet sensitive, flow sensors; which will be useful for applications involving precise flow monitoring in microfluidic devices, precise air/oxygen intake monitoring for hypoxic patients, and other biomedical devices tailored for intravenous drip/urine flow monitoring. In addition, this work also establishes the applicability of CNFs as novel sensing elements in MEMS devices and flexible sensors

Nematic Liquid Crystal Carbon Nanotube Composite Materials for Designing RF Switching Devices

Radio frequency microelectromechanical systems (RF MEMS) devices are microdevices used to switch or modify signals from the RF to millimeter wave (mmWave) frequency range. Liquid crystals (LCs) are widely used as electro-optic modulators for display devices. An electric field-induced electrical conductivity modulation of pure LC media is quite low which makes it difficult to use for RF MEMS switching applications. Currently, RF MEMS devices are characterized as an excellent option between solid-state and electromechanical RF switches to provide high isolation, low insertion loss, low power usage, excellent return loss, and large frequency band. However, commercial usage is low due to their lower switching speed, reliability, and repeatability. This research presents an electrical conductivity enhancement through the use of carbon nanotube (CNT) doping of LCs to realize a high-performance RF LC-CNT switching device. This thesis presents simulations of an RF switch using a coplanar waveguide (CPW) with a LC-CNT composite called 4-Cyano-4’-pentylbiphenyl multi-walled nanotube (5CB-MWNT) that is suitable for RF applications. The electrical conductivity modulation and RF switch performance of the 5CB-MWNT composite is determined using Finite Element Analysis (FEA). The simulations will present data on the coplanar waveguide’s s-parameters at the input and output ports S11 and S21 to measure return and insertion loss respectively, two key parameters for determining any RF switch’s performance. Furthermore, this thesis presents applications for improving tunable phased antenna arrays using the LC-CNT composite to allow for beam steering with high-gain and directivity to provide a broad 3D scannable coverage of an area. Tunable antennas are an important characteristic for 5G applications to achieve an optimal telecommunication system to prevent overcrowding of antennas and reduce overall system costs. This research investigates various device geometries with 5CB-MWNT to realize the best performing RF device for RF applications and 5G telecommunication systems. This research presents return and insertion loss data for three waveguide device configurations: CPW, coplanar waveguide grounded (CPWG), and finite ground coplanar waveguide grounded (FG-CPWG). The best results are shown using the CPW configuration. The return loss for the LC-CNT device showed a 5 dB improvement from -7.5 dB to -12.5 dB when using the LC-CNT signal line device. The insertion loss for this configuration showed a much more consistent 0 to -0.3 dB insertion loss value with much less noise when using the LC-CNT device compared to the -0.3 to -1 dB insertion loss value with heavy noise when using the Au signal line device. For the other two configurations the return loss and insertion loss value stayed the same indicating there is no loss in performance when using the LC-CNT switching mechanism. This is ideal due to the benefits that the LC-CNT switching mechanism provides like device reliability and increased switching speeds

Smart Bolometer: Toward Monolithic Bolometer with Smart Functions

The content of this chapter refers to uncooled resistive bolometers amd the challenge that consists in their integration into monolithic devices exhibiting smart functions. Uncooled resistive bolometers are the essential constitutive element of the majority of existing uncooled infrared imaging systems; they are referred to as microbolometer pixels in that type of application where matrixes of such elementary devices are used. uncooled bolometers represent more than 95% of the market of infrared imaging systems in 2010 (yole 2010) and infrared imaging systems are required for more and more applications

Development of a light-powered microstructure : enhancing thermal actuation with near-infrared absorbent gold nanoparticles.

Development of microscale actuating technologies has considerably added to the toolset for interacting with natural components at the cellular level. Small-scale actuators and switches have potential in areas such as microscale pumping and particle manipulation. Thermal actuation has been used with asymmetric geometry to create large deflections with high force relative to electrostatically driven systems. However, many thermally based techniques require a physical connection for power and operate outside the temperature range conducive for biological studies and medical applications. The work presented here describes the design of an out-of-plane bistable switch that responds to near-infrared light with wavelength-specific response. In contrast to thermal actuating principles that require wired conductive components for Joule heating, the devices shown here are wirelessly powered by near -infrared (IR) light by patterning a wavelength-specific absorbent gold nanoparticle (GNP) film onto the microstructure. An optical window exists which allows near-IR wavelength light to permeate living tissue, and high stress mismatch in the bilayer geometry allows for large actuation at biologically acceptable limits. Patterning the GNP film will allow thermal gradients to be created from a single laser source, and integration of various target wavelengths will allow for microelectromechanical (MEMS) devices with multiple operating modes. An optically induced temperature gradient using wavelength-selective printable or spinnable coatings would provide a versatile method of wireless and non-invasive thermal actuation. This project aims to provide a fundamental understanding of the particle and surface interaction for bioengineering applications based on a “hybrid” of infrared resonant gold nanoparticles and MEMS structures. This hybrid technology has potential applications in light-actuated switches and other mechanical structures. Deposition methods and surface chemistry are integrated with three-dimensional MEMS structures in this work. The long-term goal of this project is a system of light-powered microactuators for exploring cells\u27 response to mechanical stimuli, adding to the fundamental understanding of tissue response to everyday mechanical stresses at the molecular level

Nanocarbon/elastomer composites : characterization and applications in photo-mechanical actuation.

Materials that change shape or dimensions in response to external stimuli are widely used in actuation devices. While plenty of systems respond to heat, light, electricity, and magnetism, there is an emerging class of light-driven actuators based on carbon nanostructure/elastomer composites. The addition of nanomaterials to elastomeric polymers results not only in significant material property improvements such as mechanical strength, but also assists in creating entirely new composite functionalities as with photo-mechanical actuation. Efficient photon absorption by nanocarbons and subsequent energy transduction to the polymeric chains can be used to controllably produce significant amounts of pre-strain dependent motion. Photo-mechanical actuation offers a variety of advantages over traditional devices, including wireless actuation, electro-mechanical decoupling (and therefore low noise), electrical circuit elimination at point of use, massive parallel actuation of device arrays from single light source, and complementary metal–oxide–semiconductor / micro-electro-mechanical (CMOS/MEMS) compatible processing. Applications of photo-responsive materials encompass robotics, plastic motors, photonic switches, micro-grippers, and adaptive micro-mirrors. The magnitude and direction of photo-mechanical actuation responses generated in carbon nanostructure/elastomer composites depend on applied pre-strains. At low levels of pre-strains (3–9%), actuators show reversible photo-induced expansion while at high levels (15–40%), actuators exhibit reversible contraction. Large, light-induced reversible and elastic responses of graphene nanoplatelet (GNP) polymer composites were demonstrated for the first time, with an extraordinary optical-to-mechanical energy conversion factor (?M) of 7–9 MPa/W. Following this demonstration, similar elastomeric composite were fabricated with a variety of carbon nanostructures. Investigation into photo-actuation properties of these composites revealed both layer-dependent, as well as dimensionally-dependent responses. For a given carbon concentration, both steady-state photo-mechanical stress response and energy conversion efficiency were found to be directly related to dimensional state of carbon nanostructure additive, with one-dimensional (1D) carbon nanotubes demonstrating the highest responses (~60 kPa stress and ~5 × 10-3% efficiency at just 1 wt% loading) and three-dimensional (3D) highly ordered pyrolytic graphite demonstrating the lowest responses. Furthermore, development of an advanced dispersion technique (evaporative mixing) resulted in the ability to fabricate conductive composites. Actuation and relaxation kinetics responses were investigated and found to be related not to dimensionality, but rather the percolation threshold of carbon nanostructure additive in the polymer. Establishing a connective network of carbon nanostructure additive allowed for energy transduction responsible for photo-mechanical effect to activate carbon beyond the infrared (IR) illumination point, resulting in enhanced actuation. Additionally, in the conductive samples photoconductivity as a function of applied pre-strain was also measured. Photo-conductive response was found to be inversely proportional to applied pre-strain, demonstrating mechanical coupling. Following investigation into photo-mechanical actuation responses between the various carbon forms, use of these composite actuators to achieve both macroscopic as well as microscopic movement in practical applications was evaluated. Using dual GNP/elastomer actuators, a two-axis sub-micron translation stage was developed, and allowed for two-axis photo-thermal positioning (~100 µm per axis) with 120 nm resolution (limitation of the feedback sensor) and ~5 µm/s actuation speeds. A proportional-integral-derivative control loop automatically stabilizes the stage against thermal drift, as well as random thermal-induced position fluctuations (up to the bandwidth of the feedback and position sensor). Nanopositioner performance characteristics were found to be on par with other commercial systems, with resolution limited only by the feedback system used. A mathematical model was developed to describe the elastomeric composite actuators as a series of n springs, with each spring element having its own independent IR-tunable spring constant. Effects of illumination intensity, position, and amount of the composite actuator illuminated are discussed. This model provided several additional insights, such as demonstrating the ability to place not just one, but multiple stages on a single polymer composite strip and position them independently from one another, a benefit not seen in any other type of positioning system. Further investigation yielded interesting and novel photo-mechanical properties with actuation visible on macroscopic scales. Addition of a third component (thermally expanding microspheres), produced a new class of stimuli-responsive expanding polymer composites with ability to unidirectionally transform physical dimensions, elastic modulus, density, and electrical resistance. Carbon nanotubes and core-shell acrylic microspheres were dispersed in polydimethylsiloxane, resulting in composites that exhibit a binary set of material properties. Upon thermal or IR stimuli, liquid cores encapsulated within the microspheres vaporize, expanding the surrounding shells and stretching the matrix. Microsphere expansion results in visible dimensional changes, regions of reduced polymeric chain mobility, nanotube tensioning, and overall elastic to plastic-like transformation of the composite. Transformations include macroscopic volume expansion (\u3e500%), density reduction (\u3e80%), and elastic modulus increase (\u3e675%). Additionally, conductive nanotubes allow for remote expansion monitoring and exhibit distinct loading-dependent electrical responses. Compared to well-established actuation technologies, research into photo-mechanical properties of carbon-based polymer composites is still in its infancy. Results in this dissertation demonstrate some of the enormous potential of light-driven carbon-based composites for actuation and energy scavenging applications. Furthermore, mechanical response dependence to carbon nanostructure dimensional state could have significance in developing new types of carbon-based mixed-dimensional composites for sensor and actuator systems. As the fabrication processes used here are compatible with CMOS and MEMS processing, carbon-based polymer composites allow for not only scaling actuation systems, but also ability to pattern regions of tailorable expansion, strength, and electrical resistance into a single polymer skin, making these composites ideal for structural and electrical building blocks in smart systems. Continued development of carbon-based polymer composites will extend the promising potential of light-driven actuation technologies and will serve as a catalyst to inspire continued research into energy conversion devices and systems