Shaping surface acoustic waves for cardiac tissue engineering

The heart is a non-regenerating organ that gradually suffers a loss of cardiac cells and functionality. Given the scarcity of organ donors and complications in existing medical implantation solutions, it is desired to engineer a three-dimensional architecture to successfully control the cardiac cells in vitro and yield true myocardial structures similar to native heart. This thesis investigates the synthesis of a biocompatible gelatin methacrylate hydrogel to promote growth of cardiac cells using biotechnology methodology: surface acoustic waves, to create cell sheets. Firstly, the synthesis of a photo-crosslinkable gelatin methacrylate (GelMA) hydrogel was investigated with different degree of methacrylation concentration. The porous matrix of the hydrogel should be biocompatible, allow cell-cell interaction and promote cell adhesion for growth through the porous network of matrix. The rheological properties, such as polymer concentration, ultraviolet exposure time, viscosity, elasticity and swelling characteristics of the hydrogel were investigated. In tissue engineering hydrogels have been used for embedding cells to mimic native microenvironments while controlling the mechanical properties. Gelatin methacrylate hydrogels have the advantage of allowing such control of mechanical properties in addition to easy compatibility with Lab-on-a-chip methodologies. Secondly in this thesis, standing surface acoustic waves were used to control the degree of movement of cells in the hydrogel and produce three-dimensional engineered scaffolds to investigate in-vitro studies of cardiac muscle electrophysiology and cardiac tissue engineering therapies for myocardial infarction. The acoustic waves were characterized on a piezoelectric substrate, lithium niobate that was micro-fabricated with slanted-finger interdigitated transducers for to generate waves at multiple wavelengths. This characterization successfully created three-dimensional micro-patterning of cells in the constructs through means of one- and two-dimensional non-invasive forces. The micro-patterning was controlled by tuning different input frequencies that allowed manipulation of the cells spatially without any pre- treatment of cells, hydrogel or substrate. This resulted in a synchronous heartbeat being produced in the hydrogel construct. To complement these mechanical forces, work in dielectrophoresis was conducted centred on a method to pattern micro-particles. Although manipulation of particles were shown, difficulties were encountered concerning the close proximity of particles and hydrogel to the microfabricated electrode arrays, dependence on conductivity of hydrogel and difficult manoeuvrability of scaffold from the surface of electrodes precluded measurements on cardiac cells. In addition, COMSOL Multiphysics software was used to investigate the mechanical and electrical forces theoretically acting on the cells. Thirdly, in this thesis the cardiac electrophysiology was investigated using immunostaining techniques to visualize the growth of sarcomeres and gap junctions that promote cell-cell interaction and excitation-contraction of heart muscles. The physiological response of beating of co-cultured cardiomyocytes and cardiac fibroblasts was observed in a synchronous and simultaneous manner closely mimicking the native cardiac impulses. Further investigations were carried out by mechanically stimulating the cells in the three-dimensional hydrogel using standing surface acoustic waves and comparing with traditional two-dimensional flat surface coated with fibronectin. The electrophysiological responses of the cells under the effect of the mechanical stimulations yielded a higher magnitude of contractility, action potential and calcium transient

Surface acoustic wave enhanced electroanalytical sensors

In the last decade, miniaturised “lab-on-a-chip” (LOC) devices have attracted significant interest in academia and industry. LOC sensors for electrochemical analysis now commonly reach picomolar in sensitivities, using only microliter-sized samples. One of the major drawbacks of this platform is the diffusion layer that appears as a limiting factor for the sensitivity level. In this thesis, a new technique was developed to enhance the sensitivity of electroanalytical sensors by increasing the mass transfer in the medium. The final device design was to be used for early detection of cancer diseases which causes bleeding in the digestive system. The diagnostic device was proposed to give reliable and repeatable results by additional modifications on its design. The sensitivity enhanced-sensor model was achieved by combining the surface acoustic wave (SAW) technology with the electroanalytical sensing platform. The technique was practically tested on a diagnostic device model and a biosensing platform. A novel, substrate (TMB) based label-free Hb sensing method is developed and tested. Moreover, the technique was further developed by changing the sensing process. Instead of forming the sensitive layer on the electrodes it was localised on polystyrene wells by a rapid one-step process. Results showed that the use of acoustic streaming, generated by SAW, increases the current flow and improves the sensitivity of amperometric sensors by a factor of 6 while only requiring microliter scale sample volumes. The heating and streaming induced by the SAW removes the small random contributions made by the natural convection and temperature variation which complicate the measurements. Therefore, the method offers stabilised conditions for more reliable and repeatable measurements. The label-free detection technique proved to be giving relevant data, according to the hemoglobin concentration. It has fewer steps than ELISA and has only one antibody. Therefore, it is quick and the cross-reactivity of the second antibody is eliminated from the system. The additional modifications made on the technique decreased the time to prepare the sensing platform because the passivation steps (i.e., pegylation), prior to structuring a sensitive layer were ignored. This avoidance also increased the reliability and repeatability of the measurements

Deposição de filmes do diamante para dispositivos electrónicos

This PhD thesis presents details about the usage of diamond in electronics. It presents a review of the properties of diamond and the mechanisms of its growth using hot filament chemical vapour deposition (HFCVD). Presented in the thesis are the experimental details and discussions that follow from it about the optimization of the deposition technique and the growth of diamond on various electronically relevant substrates. The discussions present an analysis of the parameters typically involved in the HFCVD, particularly the pre-treatment that the substrates receive- namely, the novel nucleation procedure (NNP), as well as growth temperatures and plasma chemistry and how they affect the characteristics of the thus-grown films. Extensive morphological and spectroscopic analysis has been made in order to characterise these films.Este trabalho discute a utilização de diamante em aplicações electrónicas. É apresentada uma revisão detalhada das propriedades de diamante e dos respectivos mecanismos de crescimento utilizando deposição química a partir da fase vapor com filament quente (hot filament chemical vapour deposition - HFCVD). Os detalhes experimentais relativos à otimização desta técnica tendo em vista o crescimento de diamante em vários substratos com relevância em eletrónica são apresentados e discutidos com detalhe. A discussão inclui a análise dos parâmetros tipicamente envolvidos em HFCVD, em particular do pré-tratamento que o substrato recebe e que é conhecido na literatura como "novel nucleation procedure" (NNP), assim como das temperaturas de crescimento e da química do plasma, bem como a influência de todos estes parâmetros nas características finais dos filmes. A caracterização morfológica dos filmes envolveu técnicas de microscopia e espetroscopia.Programa Doutoral em Engenharia Eletrotécnic

Novel Optical Applications Based on Photon-phonon Interactions

The phenomena of photon-phonon interactions can be found in all forms of matters including gases, plasma, liquids and solids. The applications based on such interactions, including Raman scattering, Bragg Scattering, polariton resonance, phonon-assisted Antistoke photoluminescence, etc. has been intensively investigated. In this dissertation, we present our study of three novel applications in the field of THz generation, hot phonons in transistors, and optical refrigeration. In Chapter 1, we studied the backward propagating Terahertz (THz) generation using optical rectification in periodically poled LiNbO3 and LiTaO3 samples with ultrafast laser pulse excitation. With the LiNbO3 sample, we have generated the highest frequency at 4.8 THz at the poling period of 7.1 µm, corresponding to an output wavelength of 62.5 µm. We have observed an enhancement factor as large as 61 in the output power comparing to that generated from bulk LiNbO3, which was attributed to the phonon polariton resonance-enhanced nonlinear optical coefficients. For the LiTaO3 samples, we have reached the highest output power of nearly 100 µW. Based on our study, the effective second-order nonlinear coefficient of LiTaO3 are enhance by factors of from 3.7 to 23, leading to the enhancement of THz output powers. The enhancement is rooted in a polariton resonance at the frequency of 127 cm-1, which can be induced by the nonlinear mixing of two transverse-optical phonons due to strong anharmonicity of LiTaO3. We also designed a second wafer with significantly shorter poling periods, and indeed we have observed the entire resonant peak. In Chapter 2, we studied the hot phonon behavior of GaN high electron mobility transistors (HEMT). We mainly investigated our effort on two methods utilizing Raman scattering to measure the phonon temperature, i.e. the hot phonon population of GaN HEMT device under operation. The ultimate goal was to employ these methods on the study of isotope disorder introduced GaN device and verify whether its phonon behavior is optimized than that in normal devices. The first method extracts phonon temperatures from the ratio of Antistokes and Stokes Raman signal intensities, which requires complex experimental procedures and tendency to wrong temperature deductions. The second method is based on the fitting of phonon temperature to the shift of Stokes Raman peak model, which leads to simple and fast measurement while sophisticated analysis with strong dependence to sample material properties. Comparing two methods, we believe the second one is advantageous due to our limited experimental condition, and it can be improved with proper calibration of the model. In Chapter 3, we studied the upconverstion of photoluminescence (PL) from both a free-standing bulk GaN sample and a GaN nanowire sample. When the excitation energy is in the tail of bandgap edge, the PL upconverstion can be attributed to phonon-assisted Antistokes photoluminescence (ASPL). We explored the potential of laser cooling based on such a phenomena with the analysis of PL intensity trending with pump power, excitation wavelength, and temperature. Such analysis proves the fact that the ASPL we measured is originated in single photon process assisted by phonons

Investigating surface acoustic waves and fluorescence techniques for lab-on-a-chip diagnostics

There is an emerging need for low-cost medical diagnostics for both high and middle to low-resource settings. Surface acoustic waves microfluidics are emerging lab-on-a-chip technologies which have the potential to provide all-in-one solution to actuate liquids and sense biomarkers, thus enabling point-of-care bioassays. DNA has become a key biomarker for a range of medical conditions, including infectious diseases, as it provides critical information on the pathogen or the response of the patients to particular treatment, on a personalized basis. This thesis will examine the effects of surface acoustic waves on DNA hybridization, with a view to integrate molecular diagnostic assays onto acousto-fluidic devices. The work used fluorescence to characterize the binding of DNA in a range of conditions, and revealed nucleobase-specific quenching (NB-S Quench) of fluorophores when attached to DNA as a double strand. This latter effect was examined as a replacement for common analytical markers used in standard techniques, such as melting curves, which typically rely on dyes which recognize DNA strands non-specifically (such as groove binders). The technique has been shown to be suitable for determine the concentration of DNA, performing DNA amplification and identifying the presence and melting temperature of target DNA. This work will have an impact on research into low-cost medical diagnostics, and improve the understanding of fluorescence of DNA modified with fluorophores, contributing to the understanding of future work in these areas

Feasibility study of an integrated optic switching center

The design of a high data rate switching center for a satellite tracking station is discussed. The feasibility of a switching network using an integrated switching matrix is assessed. The preferred integrated optical switching scheme was found to be an electro-optic Bragg diffraction switch. To ascertain the advantages of the integrated optics switching center, its properties are compared to those of opto-electronic and to electronics switching networks

The resonant acousto-optic effect

This dissertation is theoretical investigation of the resonant acousto-optic effect in ionic crystals and thin metal foils. The optical properties of these types of materials, in the presence of coherent acoustic pump excitation, are numerically modelled and compared with analytical results. The resonant acousto-optic effect in bulk ionic materials is shown to be dependent on the coupling of a bulk acoustic wave to the TO-phonon component of a TO-phonon polariton. This requires that the material used is not only an ionic crystal but also has a strongly anharmonic interatomic potential. It is also demonstrated that the process “TO phonon ± one (two) transverse acoustic phonon(s)→ TO phonon” is responsible for the cubic (quartic) resonant acousto-optic effect. The role of acoustic intensity and frequency in the optical properties of CuCl and TlCl is considered. Higher order transitions are also investigated. It is shown that, in the ferroelectric material LiNbO3, both cubic and quartic scattering channels are sufficiently strong enough to consider the resonant acousto-optic effect associated with them on an equal footing. The coupling strength of both scattering channels is estimated to the nearest order of magnitude. The cubic coupling is found be σ3 = 5 meV and the quartic coupling strength is found to be σ4 = 0.3 meV both for the acoustic intensity Iac = 25 kWcm−2. The effect the phase difference between the two anharmonic terms has on the optical properties of LiNbO3 is then investigated. A tunable THz filter is proposed, based on the resonant acousto-optic effect in LiNbO3. A numerical method is developed to calculated the partial wave amplitudes and optical properties of metal foils with acoustically excited, propagating sinusoidally corrugated surfaces. It is then used on a system of a thin acoustically perturbed Au foil on a glass substrate. The effects of varying the angle of incidence, acoustic wavevector, corrugation amplitude and foil thickness are investigated. The numerical method is shown to remain stable even for strong coupling between the acoustic wave and surface plasmon polariton

Lattice Engineering with Surface Acoustic Waves

Quantum simulators are special purpose quantum computers designed to examine problems that are impossible to test with classical computers. They can be used to simulate problems in atomic physics, condensed matter physics, cosmology, high energy physics, nuclear physics and quantum chemistry, among other topics. In condensed matter physics, the simulators are composed of particles and a lattice structure containing the particles. This thesis considers the use of exciton-polaritons as particles and surface acoustic waves to form the lattice potential. Exciton-polaritons are quasiparticles formed by the coherent mixture of photons and excitons in a semiconductor, with properties that are a combination of both components. Lattices of exciton-polaritons can be made by controlling the potential landscape for either the excitons or the photons. This thesis covers two main topics: Surface acoustic waves are mechanical waves that propagate along the surface of a material. They can be simply generated via the inverse piezoelectric effect with interdigital transducers (IDTs) and can be designed to operate at frequencies, from several MHz to several GHz. They modulate the bandgap of the underlying material through the strain field that they carry, which can be used to trap excitons. The formation of potential landscapes for exciton-polaritons with surface acoustic waves has been demonstrated by another group, although their lattices were limited in scope. 1) I designed IDT layouts used to produce lattices that have not been previously demonstrated with surface acoustic waves. Numerical calculations were used to verify the validity of the design. Several pieces of test equipment both for the electrical and optical measurements were designed and built. 2) The designed devices were fabricated using both electron-beam and optical lithography and the quality of the resulting IDTs was quantified by S-parameter measurements. Observation of the lattices formed by the designed devices and the next steps required in implementing a quantum simulator are discussed

Acoustoelectric Properties of Graphene and Graphene Nanostructures

The acoustoelectric effect in graphene and graphene nanoribbons (GNRs) on lithium niobate surface acoustic wave (SAW) devices was studied experimentally. Monolayer graphene produced by chemical vapour deposition was transferred to the SAW devices. The photoresponse of the acoustoelectric current (Iae) was characterised as a function of SAW frequency and intensity, and illumination wavelength (using 450 nm and 735 nm LEDs) and intensity. Under illumination, the measured Iae increased by more than the measured decrease in conductivity, while retaining a linear dependence on SAW intensity. The latter is consistent with the piezoelectric interaction between the graphene charge carriers and the SAWs being described by a relatively simple classical relaxation model. A larger increase in Iae under an illumination wavelength of 450 nm, compared to 735 nm at the same intensity, is consistent with the generation of a hot carrier distribution. The same classical relaxation model was found to describe Iae generated in arrays of 500 nm-wide GNRs. The measured acoustoelectric current decreases as the nanoribbon width increases, as studied for GNRs with widths in the range 200 – 600 nm. This reflects an increase in charge carrier mobility due to increased doping, arising from damage induced at the nanoribbon edges during fabrication. 2 Lastly, the acoustoelectric photoresponse was studied as a function of graphene nanoribbon width (350 – 600 nm) under an illumination wavelength of 450 nm. Under illumination, the nanoribbon conductivity decreased, with the largest percentage decrease seen in the widest GNRs. Iae also decreased under illumination, in contrast to the acoustoelectric photoresponse of continuous graphene. A possible explanation is that hot carrier effects under illumination lead to a greater decrease in charge carrier mobility than the increase in acoustoelectric attenuation coefficient. This causes the measured decrease in Iae