Low cost angular displacement sensors for biomechanical applications - a review

In the general scientific quest for increased quality of life a natural ambition is to know more about human body kinematics. Varied knowledge can be extracted from sensors placed on human body and through associated biomechanical parameter evaluation the causal connection between different biomechanical parameters and medical conditions can be inferred. From a biomechanical point of view, one of the most important parameters within the human body is the amplitude of angular movements of joints. Although many angular sensors are used in industry, particular characteristics such as small size, flexibility and appropriate attachment methods must be taken into consideration when estimating the amplitude of movement of human joints. This paper reviews the existing low cost easy to manipulate angular sensors listed in the scientific literature, which currently are or could be used in rehabilitation engineering, physiotherapy or biomechanical evaluations in sport. The review is carried out in terms of a classification based on the sensors’ working principles and includes resistive, capacitive, magnetic and piezoresistive sensors

Flexible Graphene Transistor Architecture for Optical Sensor Technology.

Ph.D. Thesis. University of Hawaiʻi at Mānoa 2017

Liquid Crystal Anchoring Control and its Applications in Responsive Materials

Liquid crystals (LCs), owing to their anisotropy in molecular ordering, are of interests not only in the display industry, but also in the soft matter community, e.g., to direct colloidal assembly and phase separation of surfactants, and to actuate two-dimensional (2D) sheets into three-dimension (3D). The functionality and performance of LC materials extensively rely on the molecular ordering and alignment of LCs, which are dictated by LC anchoring at various boundaries. Therefore, this thesis focuses on the study of LC anchoring from both small molecule LCs and liquid crystal monomers (LCMs), which in turn guides my design of surface topography and surface chemistry to control formation of uniform LC defect structures over cm2 samples under complex boundary conditions. The ability to precisely embed defect structures in a LC material also allows me to exploit the responsiveness of LCs to create actuators and scaffolds to (dis)assemble nano- and micro-objects. Specifically, by exploiting the bulk disclinations formed in the nematic phase of 4-octyl-4’-cyanobiphenyl (8CB) surrounding the micropillar arrays, we demonstrate (dis)assembly of gold nano-rods (AuNRs) for dynamic tuning of surface plasmon resonance (SPR). Due to the highly temperature-sensitive elastic anisotropy of 8CB, the bulk disclinations and consequently the AuNR assemblies and SPR properties can be altered reversibly by heating and cooling the LC system. Then we design and synthesize a new type of nematic LCMs with a very large nematic window. Therefore, they can be faithfully aligned at various boundary conditions, analogous to that of small molecule LCs. After crosslinking LCMs into liquid crystal polymers (LCPs), we are able to study the LC assembly, director field, and topological defects using scanning electronic microscopy (SEM) at the 100 nm resolution. We then turn our attention to direct LCM alignment through controlling of surface chemistry and topography. We demonstrate the essential role of surface chemistry in the fabrication of liquid crystal elastomer (LCE) micropillar arrays during soft lithography. A monodomain LCM alignment is achieved in a poly(2-hydroxyethyl methacrylate) coated polydimethylsiloxane (PDMS) mold. After crosslinking, the resultant LCE micropillars display a large radial strain (~30%) when heated across the nematic-isotropic phase transition temperature (TNI). The understanding of surface alignment in LCMs is then transferred to LCEs with embedded topological defects. On micron-sized one-dimensional channels with planar surface chemistry, LCMs can be faithfully oriented along the local channel direction. After crosslinking, the 2D LCE sheets show pre-programmed shape transformation to complex 3D structures through bending and stretching of local directors when heated above TNI. Last, we control LC alignment and defect formation on a flat surface simply by using chemical patterns. Planar anchored SU8 is photopatterned on homoetropically anchored dimethyloctadecyl[3-(trimethoxysilyl)propyl] ammonium chloride (DMOAP) coated glass. By exploiting the pattern geometry, thus, boundary conditions, in combination with anisotropy of LC elasticity, we show that LC orientation can be precisely controlled over a large area and various types of topological defects are generated. Such defect structures can be further used to trap micro- and nanoparticles

Investigations on the development of a novel hybrid sensor for environmental monitoring applications.

Heavy metal toxicity is a major environmental problem world-wide. Increased spreading and high concentration levels of the toxic heavy metals in water environments have posed a severe threat to human health and the ecosystem. Over the years, to improve the drinking water quality standards, safe threshold concentrations of these highly toxic pollutants are constantly being lowered by the governmental and environmental bodies. Current instrumental techniques used to detect these low levels of heavy metal ions are laboratory based, use sophisticated instruments, expensive, time consuming and require trained personnel. There is a constant need for the development of in-situ, rapid, highly sensitive and selective sensors to monitor the very low concentration levels. Various approaches for improving sensitivity and selectivity have been investigated over the years involving multiple detection techniques. In general, optical approaches provide higher sensitivity along with simplicity while electrochemical sensors provide better selectivity. In the last decade, nanomaterials have emerged as a key element in their sensitivity improvement. Combining all these advantages, a novel hybrid sensor has been envisaged integrating optical and electrical fields in addition to nanomaterials. This thesis reports investigations on enhancing the sensitivity/selectivity through optical, nanomaterials and electrochemical routes, and then integrating these to realise a hybrid sensor. A novel optical sensor has been developed using the phenomena of evanescent waves in optical fibre with dithizone to detect heavy metal ions. A U-bent sensor geometry has been investigated to enhance the optical sensitivity of the sensor through higher evanescent field near the surface. Further, optical field confinement to the surface has been investigated through thin film coating to improve the sensitivity. A new inverted trench design based sensor has been developed, and sensitivity enhancement has been achieved through this novel design and confirmed using modelling work accompanied by experimental results. Large surface to volume ratio of nanomaterials, such as ZnO nanowires, on the sensor surface can provide enhanced surface interactions leading to higher sensitivity. But, surfaces modified with ZnO nanostructures tend to be hydrophobic in nature. A new remote and non-contact method to tune the wettability of the ZnO nanostructures using LEDs has been developed. Higher sensitivity has been achieved by tuning the wettability of ZnO nanowires using the developed method. An electrochemical sensor has been developed in order to understand the potential effects of the electric field on the near surface molecular dynamics and thereby, effects on the optical detection. Effects of parameters such as deposition time, scan frequency, concentration, electrode materials and their surface area have been investigated to improve the sensitivity and selectivity. Multi-ions selectivity has been achieved by simultaneous detection of copper, mercury and lead ions in buffer solution. Higher sensitivity has been obtained by modifying the gold electrode using graphene flakes. Further, to integrate the optical field with this sensor to realize the hybrid sensor, thickness of the gold electrode has been optimised to allow the penetration of evanescent field onto the electrode surface. Under optimised conditions evanescent field resonantly couples to the surface plasmons of the gold electrode. Computational investigations have been carried out to study the effect of number of graphene layers on the sensitivity of the surface plasmon resonance (SPR) based optical sensor integrated with the electrochemical sensor. Preliminary investigations on the developed hybrid sensor show that the electric field complements the optical field. Investigations have shown that application of electric field enhances the sensitivity for optical detection by attracting more ions on the electrode and also, provides the multi-ion selectivity. These investigations have opened up new possibilities for the real-time monitoring of highly sensitive and selective molecular interactions, showing strong potential in a range of applications areas such as environmental sensing, biosensing and agricultural sensing

Controlled Cooperative Wetting Enabled Heterogeneous Structured 3D Morphing Transducers

A unique microfluidics approach for functional hydrogel patterning with multilayered heterogeneous structures is presented. Prepolymer solution droplets with differentiated sodium acrylate concentrations are dispensed/printed in a wetting‐controlled “two‐parallel plate” (TPP, like a Hele‐Shaw Cell) system. The gelation within the system enables hydrogel bilayer structures with reconfigurable 3D deformations driven by in‐plane and through‐thickness heterogeneity under stimuli‐responsive mask‐less swelling/deswelling. The cooperation between swelling mismatch of functional groups results in a higher complexity of 3D reconfiguration in responding to discrete levels of stimulation inputs. This facile patterning technology with an in‐built ionic hierarchy can be scaled up/down with advanced transducing functionalities in various fields

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

Strain engineering as a method for manufacturing micro- and; nano- scale responsive particles

Strain engineering is used as a means of manufacturing micro- and nano- scale particles with the ability to reversibly alter their geometry from three dimensional tubes to two dimensional flat layers. These particles are formed from a bi-layer of two dissimilar materials, one of which is the elastomeric material polydimethylsiloxane (PDMS), deposited under stress on a sacrificial substrate. Upon the release of the bi-layer structure from the substrate, interfacial residual stress is released resulting in the formation of tubes or coils. These particles possess the ability to dramatically alter their geometry and, consequently, change some properties that are reversible and can be triggered by a stimulus. This work focuses on the material selection and manufacturing of the bi-layer structures used to create the responsive particles and methods for characterizing and controlling the responsive nature of the particles. Furthermore, the potential of using these particles for a capture/release application is explored, and a systematic approach to scale up the manufacturing process for such particles is provided. This includes addressing many of the problems associated with fabricating ultra-thin layers, tuning the size of the particles, understanding how the stress accumulated at the interface of a bi-layer structure can be used as a tool for triggering a response as well as developing methods (i.e. experiments and applications) that allow the demonstration of this response.M.S.Committee Chair: Dr. Kyriaki Kalaitzidou; Committee Member: Dr. Karl Jacob; Committee Member: Dr. Peter Hesket

Specialty Fibers for Terahertz Generation and Transmission: A Review

Terahertz (THz) frequency range, lying between the optical and microwave frequency ranges covers a significant portion of the electro-magnetic spectrum. Though its initial usage started in the 1960s, active research in the THz field started only in the 1990s by researchers from both optics and microwaves disciplines. The use of optical fibers for THz application has attracted considerable attention in recent years. In this paper, we review the progress and current status of optical fiber-based techniques for THz generation and transmission. The first part of this review focuses on THz sources. After a review on various types of THz sources, we discuss how specialty optical fibers can be used for THz generation. The second part of this review focuses on the guided wave propagation of THz waves for their transmission. After discussing various wave guiding schemes, we consider new fiber designs for THz transmission