Fluorescence-based optical biosensors for clinical and environmental applications

The aim of this thesis was to investigate the feasibility of simultaneous utilisation of pH and oxygen-dependent fluorescent indicators for the development of a novel fibre-optical fluorescence-based bio sensor. This approach would be used to measure simultaneously changes in the two indicator species generated by a single enzyme-catalysed reaction in response to one analyte where both the indicators and the enzyme are immobilised in the same sol-gel matrix, and to offer more accurate and reliable results using this portable optical biosensor in the clinical and environmental fields. HPTS (1-hydroxypyrene-3,6,8-trisulfonic acid) and tris(2,2'-bipyridyl)ruthenium(II) chloride hexahydrate, respectively, were used as the target fluorescent indicators; these two indicators had no cross sensitivity separate or in the same solution and well-separated emission bands at 510 nm and 610 mn, respectively. The catalytic oxidation of glucose by the enzyme glucose oxidase was initially investigated using the two indicators, and subsequently the same principle was applied in other biocatalysed oxidations such as of lactate, xanthine and phenol. Substrate concentration was assessed by simultaneously measuring two parameters: oxygen consumption, through the reduction of the fluorescence intensity of tris(2,2'-bipyridyl) ruthenium(II) chloride hexahydrate; and the production of acid, through pH changes affecting the fluorescence intensity of HPTS.A thorough spectroscopic study of the enzymatic oxidation of glucose was performed using glucose oxidase in solution in a cuvette, in the presence of both indicators. A number of combinations of wavelengths of the indicators for excitation and fluorescence were utilised in order to establish calibration curves with the optimum performance for glucose detection in the diabetic range. Similarly results were taken from the kinetic studies of lactate oxidase, xanthine oxidase and polyphenol oxidase for the detection of lactate and xanthine in blood and phenol in water at ppb-levels, using the above principle. The application and characterisation of immobilisation techniques for the fluorescence-based blood-glucose b iosenor were carried out. The advantages of the microcapsulation sol-gel method over conventional immobilisation techniques for application in an optical biosensor, were elucidated and this immobilisation technique was implemented for glucose and phenol detection. Finally, additional solution studies were conducted and used to evaluate the implementation and performance of the above method when used for the detection and measurement of glucose concentration in biological samples such as human serum

Comparative assessment of different sacrificial materials for releasing SU-8 structures

A range of materials are tested and compared as sacrificial layers for releasing SU-8 structures from the substrate following their manufacture. Four metals (chromium, chromiun/gold, copper and aluminium) and three polymers (polymethylmethacrylate, polyimide and polystyrene) were investigated. Factors assessed included; quality of the SU-8 structures (by SEM examination before and after release), effect of changing etchant concentrations, monitoring of the undercutting process (through depth measurement with time), and the time taken for the successful release process. Overall, it was found that the metals were the best choice as sacrificial layers under SU-8 for structures up to 200 μm, whereas polymers were the better for larger structures (up to 600 μm in this work)

Utilisation of JSR and BCB resists for the construction of gray scale microstructures

This paper reports on the use of two well-known photoresists, the JSR and BCB for the construction of gray scale structures which have a wide range of applications in the semiconductor/electronics industry. Reactive ion etching experiments were carried out in order to define the etching rate of the resist JSR and cyclotene using a combination of two different gases. Argon and Oxygen were used separately in order to determine the etching rate of the photoresist and the cyclotene by changing the following parameters: concentration, gas pressure and power of the RF unit. New reactive ion etching experiments were performed using combinations of the two gases in order to establish the optimum ratio of the two gases for accomplishing the desirable gray scale structures

Wearable Insulin Biosensors for Diabetes Management: Advances and Challenges

We present a critical review of the current progress in wearable insulin biosensors. For over 40 years, glucose biosensors have been used for diabetes management. Measurement of blood glucose is an indirect method for calculating the insulin administration dosage, which is critical for insulin-dependent diabetic patients. Research and development efforts aiming towards continuous-insulin-monitoring biosensors in combination with existing glucose biosensors are expected to offer a more accurate estimation of insulin sensitivity, regulate insulin dosage and facilitate progress towards development of a reliable artificial pancreas, as an ultimate goal in diabetes management and personalised medicine. Conventional laboratory analytical techniques for insulin detection are expensive and time-consuming and lack a real-time monitoring capability. On the other hand, biosensors offer point-of-care testing, continuous monitoring, miniaturisation, high specificity and sensitivity, rapid response time, ease of use and low costs. Current research, future developments and challenges in insulin biosensor technology are reviewed and assessed. Different insulin biosensor categories such as aptamer-based, molecularly imprinted polymer (MIP)-based, label-free and other types are presented among the latest developments in the field. This multidisciplinary field requires engagement between scientists, engineers, clinicians and industry for addressing the challenges for a commercial, reliable, real-time-monitoring wearable insulin biosensor

Low Fluorescence Enzyme Matrices Based on Microfabricated SU-8 Films for a Phenol Micro-Biosensor Application

In this contribution, the possibility of using SU-8 photoresist, a polymer widely used in MEMS applications, for the development of inexpensive and disposable optical phenol micro-biosensors is explored. The immobilisation of the enzyme, the encapsulation of the indicator and the patterning of the SU-8 were accomplished simultaneously in a simple one step microfabrication process. The enzyme still showed activity after encapsulation in SU-8 although the process involved its embedding in a hard and rigid epoxy resin matrix. This was carried out by measuring the signal of an oxygen-sensitive indicator (ruthenium-complex) through monitoring of the enzymatic oxidation of phenol which consumes oxygen. Films without enzyme showed negligible variation in fluorescence intensity upon phenol addition, whereas films with encapsulated enzyme and oxygen-sensitive fluorescent indicators showed a very clear change in fluorescence intensity upon addition of phenol. The current work demonstrates a new concept of a low cost immobilisation technique in combination with the microfabrication process for biosensor technology

Recent Advances in Energy Harvesting from the Human Body for Biomedical Applications

Energy harvesters serve as continuous and long-lasting sources of energy that can be integrated into wearable and implantable sensors and biomedical devices. This review paper presents the current progress, the challenges, the advantages, the disadvantages and the future trends of energy harvesters which can harvest energy from various sources from the human body. The most used types of energy are chemical; thermal and biomechanical and each group is represented by several nano-generators. Chemical energy can be harvested with a help of microbial and enzymatic biofuel cells, thermal energy is collected via thermal and pyroelectric nano-generators, biomechanical energy can be scavenged with piezoelectric and triboelectric materials, electromagnetic and electrostatic generators and photovoltaic effect allows scavenging of light energy. Their operating principles, power ratings, features, materials, and designs are presented. There are different ways of extracting the maximum energy and current trends and approaches in nanogenerator designs are discussed. The ever-growing interest in this field is linked to a larger role of wearable electronics in the future. Possible directions of future development are outlined; and practical biomedical applications of energy harvesters for glucose sensors, oximeters and pacemakers are presented. Based on the increasingly accumulated literature, there are continuous promising improvements which are anticipated to lead to portable and implantable devices without the requirement for batteries

A hybrid microfluidic platform for energy harvesting based on piezoelectricity and reverse electrowetting for wearable biosensors

The continuous monitoring of human biomarkers in wearable biosensors requires constant energy supply and the usage of batteries introduces a number of limitations. A self-rechargeable biosensor would prove to be beneficial and vital in a timely medical diagnosis and prevention of health implications. This study explores the proof of concept of a novel microfluidics platform of simultaneous harvesting energy from an arterial wall pulsation through piezoelectricity and from the reverse electrowetting on dielectric (REWOD) phenomenon. Both physical principles are successfully employed in conventional designs as separate actuating and sensory units, yet their combined incorporation is often overlooked. A designed hybrid droplet microfluidics platform utilizes piezoelectric films to react to perturbations caused by pulsation of arteries and to press down droplets while electrodes around the microchannel collect energy from deformed droplets. This hybrid approach allows enhanced energy harvesting. A commercial multi-physics based Computational Fluid Dynamics (COMSOL) software was used to verify the posited hypothesis and to carry out a set of time-varying flow simulations with parametric variation of a number of physical and geometrical parameters. As a result, the interrelations between various physical parameters of the designed system, such as viscosity, surface tension, flow velocity, frequency and amplitude of pressure variations occurring on microchannel walls, droplet properties, droplet number and distribution, wetting and de-wetting frequency etc, were investigated and correlated with the produced electrical generation aiming towards its maximisation. The design and control of microfluidic parameters are highly important for the optimised performance of the prototype device. Furthermore, based on the analysis and quantification of the extracted energy results, a number of design recommendations are provided and a forecast of potential applications and innovations such as wearable and implantable biosensors, continuous monitoring of medical conditions for personalized medicine is outlined

A hybrid piezoelectric and electrostatic energy harvester for scavenging arterial pulsations

Implantable and wearable biomedical devices suffer from a limited lifespan of on-board batteries which require change causing physical discomfort. In order to overcome this, various energy harvesters have been developed as the human body possesses several types of energy available for scavenging through appropriately designed energy harvesting devices, while the cardiovascular system in particular represents a constant reliable source of mechanical energy from vibration. Most conventional energy harvesters exploit only a single phenomenon, such piezo- or triboelectricity, thus producing reduced power density. As an improvement, hybridisation of energy harvesters intends to negate this drawback by simultaneously scavenging energy by multiple harvesters. In the present work, the reverse electrowetting on dielectric (REWOD) phenomenon is combined with the piezoelectric effect in a proof-of-concept hybrid harvester for scavenging biomechanical energy from arterial or other type pulsations. A mathematical model of the harvester was developed; and, an investigation using computational fluid dynamics simulations was carried out using the COMSOL Multiphysics software. The effect of the materials of piezoelectric film and geometrical features of the harvester on parameters such as the displacement, the frequency of pulsations and the energy produced were studied. An experimental setup that could model the time-varying pressures and displacements caused from arterial pulsations was designed and the characteristics of the produced piezoelectrical energy were analysed. A comparison between experimental and computational data was carried out demonstrating a good agreement. The dependencies between geometrical parameters and electrical output were determined and recommendations on piezoelectric materials and design solutions were provided

Computational and Experimental Investigation of Microfluidic Chamber Designs for DNA Biosensors

A critical characteristic for continuous monitoring using DNA biosensors is the design of the microfluidics system used for sample manipulation, effective and rapid reaction and an ultra-low detection limit of the analyte. The selection of the appropriate geometrical design and control of microfluidic parameters are highly important for the optimum performance. In the present study, a number of different shapes of microchambers are designed and computationally assessed using a Multiphysics software. Flow parameters such as pressure drop, and shear rates are compared. Three-dimensional printing was used to construct the designs and an experimental investigation is underway for the validation of the computational results

A 3D-Printed Piezoelectric Microdevice for Human Energy Harvesting for Wearable Biosensors

The human body is a source of multiple types of energy, such as mechanical, thermal and biochemical, which can be scavenged through appropriate technological means. Mechanical vibrations originating from contraction and expansion of the radial artery represent a reliable source of displacement to be picked up and exploited by a harvester. The continuous monitoring of physiological biomarkers is an essential part of the timely and accurate diagnosis of a disease with subsequent medical treatment, and wearable biosensors are increasingly utilized for biomedical data acquisition of important biomarkers. However, they rely on batteries and their replacement introduces a discontinuity in measured signals, which could be critical for the patients and also causes discomfort. In the present work, the research into a novel 3D-printed wearable energy harvesting platform for scavenging energy from arterial pulsations via a piezoelectric material is described. An elastic thermoplastic polyurethane (TPU) film, which forms an air chamber between the skin and the piezoelectric disc electrode, was introduced to provide better adsorption to the skin, prevent damage to the piezoelectric disc and electrically isolate components in the platform from the human body. Computational fluid dynamics in the framework of COMSOL Multiphysics 6.1 software was employed to perform a series of coupled time-varying simulations of the interaction among a number of associated physical phenomena. The mathematical model of the harvester was investigated computationally, and quantification of the output energy and power parameters was used for comparisons. A prototype wearable platform enclosure was designed and manufactured using fused filament fabrication (FFF). The influence of the piezoelectric disc material and its diameter on the electrical output were studied and various geometrical parameters of the enclosure and the TPU film were optimized based on theoretical and empirical data. Physiological data, such as interdependency between the harvester skin fit and voltage output, were obtained