Towards rapid 3D direct manufacture of biomechanical microstructures

The field of stereolithography has developed rapidly over the last 20 years, and commercially available systems currently have sufficient resolution for use in microengineering applications. However, they have not as yet been fully exploited in this field. This thesis investigates the possible microengineering applications of microstereolithography systems, specifically in the areas of active microfluidic devices and microneedles. The fields of micropumps and microvalves, stereolithography and microneedles are reviewed, and a variety of test builds were fabricated using the EnvisionTEC Perfactory Mini Multi-Lens stereolithography system in order to define its capabilities. A number of microneedle geometries were considered. This number was narrowed down using finite element modelling, before another simulation was used to optimise these structures. 9 × 9 arrays of 400 μm tall, 300 μm base diameter microneedles were subjected to mechanical testing. Per needle failure forces of 0.263 and 0.243 N were recorded for the selected geometries, stepped cone and inverted trumpet. The 90 μm needle tips were subjected to between 30 and 32 MPa of pressure at their failure point - more than 10 times the required pressure to puncture average human skin. A range of monolithic micropumps were produced with integrated 4 mm diameter single-layer 70 μm-thick membranes used as the basis for a reciprocating displacement operating principle. The membranes were tested using an oscillating pneumatic actuation, and were found reliable (>1,000,000 cycles) up to 2.0 PSIG. Pneumatic single-membrane nozzle/diffuser rectified devices produced flow rates of up to 1,000 μl/min with backpressures of up to 375 Pa. Another device rectified using active membrane valves was found to self-prime, and produced backpressures of up to 4.9 kPa. These devices and structures show great promise for inclusion in complex, fully integrated and active microfluidic systems fabricated using microstereolithography alone, with implications for both cost of manufacture and lead time

Micro- and nano-electrode arrays for electroanalytical sensing

A systematic investigation of the electrochemical behaviour of two sets of microelectrode arrays, fabricated by standard photolithographic and reactive-ion etching techniques, is presented. The first set of microelectrode arrays had a constant relative centre-centre spacing of 10r (where r is the electrode radius). As a value of r was decreased, the cyclic voltammograms recorded from the array became increasingly peak-shaped, due to merging of the diffusion fields of the individual electrodes. Furthermore, it was shown that the peak current densities obtained were largest for the arrays with the smallest individual electrodes, as were the signal-to-noise ratios (SNRs). Electroplating the individuals electrodes with platinum black was also shown to increase the peak currents and the SNRs, due to an increase in the effective surface area. Sigmoidal voltammograms, which are indicative of radial diffusion, were obtained for an individual electrode radius of 25 mm but not for arrays with smaller electrodes. To obtain radial diffusion for an array of 2.5 mm electrodes, it was shown (using a second set of microelectrode arrays) that a minimum relative centre-centre spacing of 40r is required. Further enhancement of the peak current densities were obtained by decreasing the size of the individual electrodes. A series of nanoelectrode arrays were fabricated using electron-beam lithography (EBL). The voltammograms obtained from these arrays exhibited a continual increase in the recorded peak current as the individual electrodes radius was decreased to a value of 110 nm. Since EBL is a slow and costly technique, nanoimprint lithography (NIL) was investigated as an alternative method of fabricating nanoelectrode arrays and comparable results were obtained from arrays produced by EBL and NIL. A dissolved oxygen and temperature sensor incorporating a working microelectrode array was also designed and fabricated. The sector comprised a densely packed array of 2.5 mm radius electrodes and a micro-reference electrode, both of which were covered with an agarose electrolyte gel enclosed in an SU8 chamber. A thermal resistor was included for temperature compensation of the dissolved oxygen measurements. The Ag|AgCl micro-reference electrode was found to be stable for approximately 80 hours in 0.1 M KCl, with 100 nA of current passing through it. Linear calibration curves were obtained from both temperature and dissolved oxygen measurement

DESIGN, FABRICATION, AND TESTING OF A PDMS MICROPUMP WITH MOVING MEMBRANES

This paper will discuss the design, fabrication, and testing of a Poly(dimethylsiloxane) (PDMS) microfluidic pump. PDMS is commonly described as a soft polymer with very appealing chemical and physical properties such as optical transparency, low permeability to water, elasticity, low electrical conductivity, and flexible surface chemistry. PDMS microfluidic device fabrication is done easily with the use of soft lithography and rapid prototyping. PDMS microfluidic devices make it easier to integrate components and interface devices with particular users, than using typically harder materials such as glass and silicon. Fabrication and design of single and multilayer PDMS microfluidic devices is much easier and straightforward than traditional methods. A novel design of a PDMS micropump with multiple vibrating membranes has been developed for application in drug delivery and molecule sorting. The PDMS micropump consists of three nozzle/diffuser elements with vibrating membranes, which are used to create pressure difference in the pump chamber. Preliminary analysis of the fluidic characteristics of the micropump was analyzed with ANSYS to investigate the transient responses of fluid velocity, pressure distributions, and flow rate during the operating cycle of the micropump. The design simulation results showed that the movement of the wall membranes combined with rectification behavior of three nozzle/diffuser elements can minimize back flow and improve net flow in one direction. To prove that the theoretical design is valid, the fabrication and testing process of the micropump has been carried out and completed. This paper will discuss in depth the design, fabrication, and testing of the PDMS micropump

Self-folding 3D micro antennas for implantable medical devices

Tese de Doutoramento em Engenharia Biomédica.Recent advances in device miniaturization have been enabling smart and small implantable medical devices. These are often powered by bulky batteries whose dimensions represent one of the major bottlenecks on further device miniaturization. However, alternative powering methods, such as electromagnetic waves, do not rely on stored energy and are capable of providing high energy densities per unit of area, thus increasing the potential for device miniaturization. Hence, we envision an implanted medical device with an integrated miniaturized antenna, capable of receiving a radiofrequency signal from an exterior source, and converting it to a DC signal, thus enabling remote powering. This thesis addresses the analysis, design, fabrication and characterization of novel 3D micro antennas that can be integrated on 500 × 500 × 500 μm3 cubic devices, and used for wireless power transfer purposes. The analysis is built upon the theory of electrically small antennas in lossy media, and the antenna design takes into consideration miniaturization techniques which are compatible with the antenna fabrication process. For the antenna fabrication, a methodology that combines conventional planar photolithography techniques and self-folding was used. While photolithography allows the easy patterning of virtually every desired planar antenna configuration with reproducible feature precision, and the flexibility to easily and precisely change the antenna geometry and size, self-folding allows assembly of the fabricated planar patterns into a 3D structure in a highly parallel and scalable manner. After fabrication, we characterized the fabricated antennas by measuring their S-parameters and radiation patterns, demonstrating their efficacy at 2 GHz when immersed in dispersive media such as water. This step required the development and test of multiple characterization setups based on connectors, RF probes and transmission lines and the use of an anechoic chamber. Moreover, we successfully show that the antennas can wireless transfer energy to power an LED, highlighting a proof of concept for practical applications. Our findings suggest that self-folding micro antennas could provide a viable solution for powering tiny micro devices.Os recentes avanços das tecnologias de miniaturização têm permitido o desenvolvimento de dispositivos médicos implantáveis inteligentes e mais pequenos. Estes são muitas vezes alimentados por baterias volumosas cujas dimensões limitam o nível de miniaturização alcançável por um micro dispositivo. No entanto, existem formas alternativas de alimentar estes dispositivos que não dependem de energia armazenada, tais como ondas eletromagnéticas, que são capazes de providenciar uma elevada densidade de energia por unidade de área, aumentando assim o potencial de miniaturização dos dispositivos. Desta forma, visionamos um dispositivo médico implantado, com uma antena miniaturizada e integrada, capaz de receber um sinal de rádio frequência a partir de uma fonte externa, e convertê-lo num sinal DC, permitindo assim a alimentação remota do aparelho. Esta tese apresenta a análise, desenho, fabrico e caracterização de micro antenas 3D, passíveis de serem integradas em micro dispositivos cúbicos (500 × 500 × 500 μm3), e utilizadas para fins de transferência de energia sem fios. A análise assenta na teoria das antenas eletricamente pequenas em meios com perdas, e o design da antena considera técnicas de miniaturização de antenas. Para o fabrico da antena foi utilizada uma metodologia que combina técnicas de fotolitografia planar e auto-dodragem (self-folding). Enquanto a fotolitografia permite a padronização de virtualmente todos os tipos de configurações planares de forma precisa, reprodutível, e com a flexibilidade para se mudar rapidamente a geometria e o tamanho da antena, o self-folding permite a assemblagem dos painéis planares fabricados numa estrutura 3D. Depois do fabrico, as antenas foram caracterizadas medindo os seus parâmetros S e diagramas de radiação, demonstrando a sua eficácia a 2 GHz quando imersas num meio dispersivo, tal como água. Esta etapa exigiu o desenvolvimento e teste de várias setups de caracterização com base em conectores, sondas de RF e linhas de transmissão, e ainda o uso de uma câmara anecóica. Além disso, mostramos com sucesso que as micro antenas podem receber e transferir o energia para um LED acendendo-o, destacando assim esta prova de conceito para aplicações práticas. Os nossos resultados sugerem que estas micro antenas auto-dobráveis podem fornecer uma solução viável para alimentar micro dispositivos implantáveis muito pequenos.Fundação para a Ciência e a Tecnologia (FCT) bolsa SFRH/BD/63737/2009

Diamond nanostructured devices for chemical sensing applications

Research in the area of CVD single crystal diamond plates of which only recently has been made commercially available saw significant advancements during the last decade. In parallel to that, detonation nanodiamond (DND) particles also now widely made accessible for requisition are provoking a lot of scientific investigations. The remarkable properties of diamond including its extreme hardness, low coefficient of friction, chemical inertness, biocompatibility, high thermal conductivity, optical transparency and semiconducting properties make it attractive for a number of applications, among which electronic and micro electrical-mechanical systems devices for chemical and biological applications are few of the key areas. A detailed knowledge of diamond devices at the prototypical stage is therefore critical. The work carried out encapsulated in this thesis describes the employment of the nanometer-scale diamond structures for the design, fabrication and testing of electronic devices and micro electrical-mechanical system (MEMS) structures for chemical sensing applications. Two major approaches are used to achieve engineering novelty. The first type being devices based on single crystal diamond substrates, which include state of the art δ-doped single crystal diamond Ion Sensitive Field Effect Transistor with an intrinsic layer capping the delta-doped layer for pH sensing and the fabrication and characterization of a triangular-face single crystal diamond MEMS. A comprehensive set of characterisations was systematically performed on the delta ISFET devices. Cyclic Voltammetry has been used to determine the devices’ potential window determining the limits of the applied potential for the Current-Voltage measurements. In solutions of different pH levels, an improved sensitivity of 55mV/pH compared to cap-less design in a previous study is taken as the salient figure of merit. Electrochemical Impedance Spectroscopy sheds some light on device performance in terms of flatband voltages and conduction pathways through circuit modelling. Improved ISFET characteristics such as lower flat-band voltage at 3.74V, simpler conduction paths and drain current saturation onsets show the chosen design is correct and advances delta-doped diamond ISFET research and development work. For the single crystal diamond cantilever, the theoretical modelling supports the triangular-face design to be a better option, generating 3x greater deflections in relation to the conventional rectangular-face design, when operated as a static mode sensor. Based on experimental characterisation methods such as Raman and Energy Dispersive Spectroscopy, the focusedion beam only milling technique inflicts minimum damage to the beam structure. In the second approach, a novel hybrid device idea was conceived and implemented using off-the-shelf silicon ISFETs and cantilevers with a coat of nanodiamond particles on the ‘active area’ surfaces of the respective devices. These nanodiamond-coated silicon devices exhibit high sensitivity for tracing threat signatures such as explosive precursors and analogues with the former in both liquid and vapour medium, and the latter in the vapour phase. The nanodiamond-gated ISFET shows a voltage response of a commendable maximum voltage shift of ~90 mV throughout 0 to 0.1M concentration range of NO2 - and ClO3 - solutions. In the vapour phase detecting 2,4-DNT, a sensitivity of ~20mV/0.4ppm is observed. The nanodiamond-coated silicon cantilever demonstrates a performance advantage of 7.4 Hz/ppb to 1.7 Hz/ppb in a previous study. Fourier Transform Infra-red spectroscopy was carried out on the nanodiamond surfaces hosted by potassium bromide (KBr) discs to ascertain the vapour chemisorption. With the fabrication technique simplified, commercialisation of these proof-of-concept devices should be less time consuming thus enabling quicker deployment of diamond-based surface sensing technology