Laser-assisted processing of multilayer films for inexpensive and flexible biomedical microsystems

Flexible/stretchable electronics offer ideal properties for emerging health monitoring devices that can seamlessly integrate with the soft, curvilinear, and dynamic surfaces of the human body. The resulting capabilities have allowed novel devices for monitoring physiological parameters, improving surgical procedures, and human-machine interfaces. While the attractiveness of these devices are indubitable, their fabrication by conventional cleanroom techniques makes them expensive and incompatible with rapid large-scale (e.g., roll-to-roll) production. The purpose of this research is to develop inexpensive fabrication technologies using low-cost commercial films such as polyimide, paper, and metalized paper that can be utilized for developing various flexible/stretchable physical and chemical sensors for wearable and lab-on-chip applications. The demonstrated techniques focus on an array of laser assisted surfaces modification and micromachining strategies with the two commonly used CO2 and Nd: YAG laser systems. The first section of this dissertation demonstrates the use of localized pulsed CO2 laser irradiation to selectively convert thermoset polymer films (e.g., polyimide) into electrically conductive highly porous carbon micro/nanostructures.Thisprocessprovidesauniqueandfacileapproachfordirect writing of carbonized conductive patterns on flexible polyimide sheets in ambient conditions, eliminating complexities of current methods such as expensive CVD processes and complicated formulation/preparation of conductive carbon based inks used in ink jet printing. The highly porous laser carbonized layer can be transferred to stretchable elastomer or further functionalized with various chemical substances such as ionic solutions, nanoparticles, and chemically conductive polymers to create different mechanical and chemical sensors. The second section of this dissertation describes the use of laser ablation for selective removal of material from multilayer films such as ITO-coated PET, parchment paper, and metalized paper to create disposable diagnostic platforms and in-vitro models for lab-on-chip based studies. The ablated areas were analyzed using electrical, mechanical, and surface analysis tools to understand change in physical structure and chemical properties of the laser ablated films. As proof-of-concept demonstrations of these technologies, four different devices are presented here: mechanical, electrochemical, and environmental sensors along with an in-vitro cell culture platform. All four devices are designed, fabricated, and characterized to highlight the capability of commercial laser processing systems in the production of the next generation, low-cost and flexible biomedical devices

Comparative study of micromixers for laminar blood mixing

Miniaturization is the trend in analytical chemistry and life science. It has been emerging into the research field of microfluidics in the application of LOC. The application is used for biochemistry analysis and require a rapid mixing in small area. Due to laminar flow (Reynold Number < 1) passive micromixer is the best method in fluids mixing. Passive micromixer also depend on the channel geometry for mixing effectiveness. In this study, four different micromixers were evaluated based on the baseline control Y-micromixer. The micromixers are internal rib micromixer, patterned grooves micromixer, obstruction micromixer and slanted rib micromixer. These micromixer has 1000μm channel length, 150μm inlet length, 90o between inlets ports, width and depth are 40μm each. The fluids used for mixing were blood which has 1.0 × 10-6 kg/μms of viscosity and toluene which has low viscosity than blood (0.664 × 10-9 kg/μms). The fluids used to evaluate the differences in term of their visual performance based image’s standard deviation by plotting the graph and mixing efficiency by calculation. Based on these evaluations, the slanted rib micromixer is the best micromixer design with the highest mixing efficiency of 99.85% at the outlet of the channel

UWB antenna using indium tin oxide (ITO)

Recently, the need of high data rate communication, enormously fast speed and shorter distance, the engineers and scientists had have decided to work in the larger scale of frequency resulted in the usage of ultra wide band (UWB) frequency which ranged from 3.1 GHz to 10.6 GHz. The patch antennas offer a potential solution for narrowband and with some modifications and researches done before, it does applicable for the ultra-wide band applications. It cost, light weight, easy to feed and their attractive radiation characteristics. In this project, the performance of the UWB antenna characteristics using Indium Tin Oxide (ITO) and AgHT were investigated. Parametric studies were performed to quantify the effect of the ITO and AgHT characteristics as well as the PET (plastic polymer) coating. All those parametric studies are conducted and simulated using CST Microwave Studio. The reflection coefficient, radiation pattern, VSWR and gain values were compared and contrast between the two antennas. It can be concluded that the AgHT coated antenna works better than ITO in terms of the gain and radiation pattern while work as good as ITO in reflection coefficient and VSW

Glassy Materials Based Microdevices

Microtechnology has changed our world since the last century, when silicon microelectronics revolutionized sensor, control and communication areas, with applications extending from domotics to automotive, and from security to biomedicine. The present century, however, is also seeing an accelerating pace of innovation in glassy materials; as an example, glass-ceramics, which successfully combine the properties of an amorphous matrix with those of micro- or nano-crystals, offer a very high flexibility of design to chemists, physicists and engineers, who can conceive and implement advanced microdevices. In a very similar way, the synthesis of glassy polymers in a very wide range of chemical structures offers unprecedented potential of applications. The contemporary availability of microfabrication technologies, such as direct laser writing or 3D printing, which add to the most common processes (deposition, lithography and etching), facilitates the development of novel or advanced microdevices based on glassy materials. Biochemical and biomedical sensors, especially with the lab-on-a-chip target, are one of the most evident proofs of the success of this material platform. Other applications have also emerged in environment, food, and chemical industries. The present Special Issue of Micromachines aims at reviewing the current state-of-the-art and presenting perspectives of further development. Contributions related to the technologies, glassy materials, design and fabrication processes, characterization, and, eventually, applications are welcome

Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II

Wide bandgap (WBG) semiconductors are becoming a key enabling technology for several strategic fields, including power electronics, illumination, and sensors. This reprint collects the 23 papers covering the full spectrum of the above applications and providing contributions from the on-going research at different levels, from materials to devices and from circuits to systems

Graphene-based flexible sensors towards electronic wearables

Flexible electronics and wearable devices have attracted considerable attention because they produce mechanical liberty, in terms of flexibility and stretchability that can enable the possibility of a wide range of new applications. The term “wearable electronics” can be used to define devices that can be worn or mated with the sensed surface to continuously monitor signals without limitations on mechanical deformability of the devices and electronic performance of the functional materials. The use of polymeric substrates or other nonconventional substrates as base materials brings novel functionalities to sensors and other electronic devices in terms of being flexible and light weight. Conductive nanomaterials, such as carbon nanotubes and graphene have been utilized as functional materials for flexible electronics and wearable devices. Graphene has specifically been considered for producing next-generation sensors due to its impressive electrical and mechanical properties and a result, incorporation of flexible substrates and graphene-based nanomaterials has been widely utilized to form versatile flexible sensors and other wearable devices through use of different fabrication processes. Creation of a large-scale, simple, high-resolution and cost-effective technique that overcomes fabrication limitations and supports production of flexible graphene-based sensors with high flexibility and stretch ability is highly demanding. Soft lithography can be merged with a mechanical exfoliation process using adhesive tape followed by transfer printing to form a graphene sensor on a desired final substrate. In situ microfluidic casting of graphene into channels is another promising platform driving the rapid development of flexible graphene sensors and wearable devices with a wide dynamic detection range. Selective coating of graphene-based nanomaterials (e.g. graphene oxide (GO)) on flexible electrode tapes can, because of its flexibility and adhesive features, be used to track relative humidity (RH) variations at the surface of target surfaces. This thesis describes the design and development of flexible and wearable strain, pressure and humidity sensors based on a novel tape-based cost-effective patterning and transferring technique, an in situ microfluidic casting method, and a novel selective coating technique for graphene-based nanomaterials. First of all, we present a tape-based graphene patterning and transferring approach to production of graphene sensors on elastomeric substrates and adhesive tapes. The method utilizes the work of adhesion at the interface between two contacting materials as determined by their surface energies to pattern graphene on PDMS substrate and transfer it onto a target tape. We have achieved patterning and transferring method with the features of high pattern spatial resolution, thickness control, and process simplicity with respect to functional materials and pattern geometries. We have demonstrated the usage of flexible graphene sensors on tape to realize interaction with structures, humans, and plants for real-time monitoring of important signals. Secondly, we present a helical spring-like piezo resistive graphene sensor formed within a microfluidic channel using a unique and easy in situ microfluidic casting method. Because of its helical shape, the sensor exhibits a wide dynamic detection range as well as mechanical flexibility and stretch ability. Finally, we present a flexible GO-based RH sensor on an adhesive polyimide thin film realized by selectively coating and patterning GO at the surface of Au Interdigitated electrodes (IDEs) and subsequently peeling the device from a temporary PDMS film. Real-time monitoring of the water movement inside the plant has been demonstrated by installing GO-based RH sensor at the surfaces of different plant leaves

Grafeno em papel para dispositivos flexíveis: sensores e OLEDs

The interest in flexible electronics has been growing considerably due to the possibility of products and devices with novel functionalities and improved comfort in their utilization. Graphene, with a combination of properties, is a natural candidate for these applications. Simultaneously, paper electronics is proving itself as a potentially significant branch of flexible electronics. Thus, it is particularly interesting to investigate the combination of these two materials for the development of novel and disruptive applications. This work covers the development of two types of flexible devices based on gra-phene and paper: physical and electrochemical sensors and organic light emit-ting diodes (OLEDs). In the context of sensors, one of the most recent additions to the family of graphene-based materials is explored: laser-induced graphene obtained from paper (paper-LIG), a graphene foam synthesized by a fast and low-cost process. The sensibility of paper-LIG to mechanical stimuli (strain and bending), as well as to humidity and temperature (in the latter case also shown for laser-induced graphene obtained from xylan, a biopolymer similar to cellulose) is demonstrated. The development of these devices is accompanied by a study of the influence of the synthesis parameters on the obtained material, comprising a sizeable contribution to the description of this material and its applications in the literature. Additionally, a non-enzymatic paper-LIG transductor for the electrochemical detection and quantification of uric acid is presented, demonstrating its response capability in real human urine samples, with a sensitivity of 0.363 μA cm⁻² μM⁻¹ and a linear range that covers the clinically relevant concentration range for this physiological parameter. In the scope of OLEDs, an optimized graphene synthesis process by chemical vapour deposition is presented, with the goal of using this single-layer graphene as a transparent electrode. The issue of high surface roughness typical of paper is addressed by the use of cellulose nanocrystal membranes and transparent rolling papers as flexible, biodegradable substrates, accompanied by the development of modified graphene film transfer and stacking approaches. The properties of this material are improved by thermal evaporation of MoO3, allowing the construction of OLEDs with 0.34% external quantum efficiency. The development of these devices not only contributes to reaffirm the vast potential of graphene, but also serves to introduce novel approaches in the context of low-cost and biodegradable flexible devices.O interesse na eletrónica flexível tem crescido consideravelmente devido ao de-senvolvimento de produtos e dispositivos com novas funcionalidades e maior conforto na utilização dos mesmos. O grafeno, com uma combinação única de propriedades, surge como um candidato natural para este tipo de aplicações. Simultaneamente, a eletrónica em papel tem-se revelado como uma vertente potencialmente significativa na área da eletrónica flexível. Assim, torna-se parti-cularmente interessante investigar a combinação destes dois materiais para o desenvolvimento de novas e disruptivas aplicações. Este trabalho explora o desenvolvimento de dois tipos de dispositivos flexíveis à base de grafeno em papel: sensores físicos e eletroquímicos e díodos orgânicos emissores de luz (OLEDs). No contexto dos sensores é abordada uma das mais recentes adições à família dos materiais à base de grafeno: o grafeno induzido por laser obtido a partir do papel (paper-LIG), uma espuma de grafeno sintetizada por um processo rápido e de baixo custo. É demonstrada pela primeira vez a sensibilidade do paper-LIG a estímulos mecânicos (distensão e flexão), bem como à humidade e tempera-tura (neste último caso também para o grafeno induzido por laser obtido a partir de xilana, um biopolímero semelhante à celulose). O desenvolvimento destes dispositivos é acompanhado por um estudo da influência dos parâmetros de sín-tese no material obtido, constituindo uma contribuição significativa para a des-crição deste material e das suas aplicações na literatura. É ainda apresentado um transdutor não-enzimático de paper-LIG, para a deteção e quantificação ele-troquímica de ácido úrico, demonstrando a sua capacidade de resposta em amostras reais de urina humana, com uma sensibilidade de 0.363 μA μA cm⁻² μM⁻¹ e uma gama linear que abrange o intervalo de concentrações clinicamente rele-vante para este parâmetro fisiológico. No âmbito dos OLEDs, é apresentado um processo otimizado de síntese de grafeno monocamada por deposição química em fase vapor, com vista à sua utilização como elétrodo transparente. A questão da elevada rugosidade tipica-mente associada ao papel é colmatada pelo uso de membranas de celulose nanocristalina e de mortalhas transparentes como substratos flexíveis e biode-gradáveis, acompanhado pelo desenvolvimento de técnicas modificadas de transferência e empilhamento de múltiplas camadas de grafeno. As proprieda-des deste material são melhoradas pela evaporação térmica de MoO3, permi-tindo a construção de OLEDs com 0.34% de eficiência quântica externa. O desenvolvimento destes dispositivos não só contribui para reafirmar o vasto potencial do grafeno em conjugação com o papel, como serve também para introduzir novas abordagens no contexto de dispositivos flexíveis de baixo custo e biodegradáveis.Programa Doutoral em Nanociências e Nanotecnologi

Development of a low-cost graphene-based impedance biosensor

PhD ThesisThe current applicability and accuracy of point-of-care devices is limited, with the need of future technologies to simultaneously target multiple analytes in complex human samples. Graphene’s discovery has provided a valuable opportunity towards the development of high performance biosensors. The quality and surface properties of graphene devices are critical for biosensing applications with a preferred low contact resistance interface between metal and graphene. However, each graphene production method currently results in inconsistent properties, quality and defects thus limiting its application towards mass production. Also, post-production processing, patterning and conventional lithography-based contact deposition negatively impact graphene properties due to chemical contamination. The work of this thesis focuses on the development of fully-functional, label-free graphene-based biosensors and a proof-of-concept was established for the detection of prostate specific antigen (PSA) in aqueous solution using graphene platforms. Extensive work was carried out to characterize different graphene family nanomaterials in order to understand their potential for biosensing applications. Two graphene materials, obtained via a laser reduction process, were selected for further investigations: reduced graphene oxide (rGO) and laser induced graphene from polyimide (LIG). Electrically conductive, porous and chemically active to an extent, these materials offer the advantage of simultaneous production and patterning as capacitive biosensing structures, i.e. interdigitated electrode arrays (IDE). Aiming to enhance the sensitivity of these biosensors, a novel, radio-frequency (RF) detection method was investigated and compared with conventional electrochemical impedance spectroscopy (EIS) on a well-known biocompatible material: gold (standard). It was shown that the RF detection methods require careful design and testing setup, with conventional EIS performing better in the given conditions. The method was further used on rGO and LIG IDE devices for the electrochemical impedance detection of PSA to assess the feasibility of the graphene based materials as biosensors. The graphene-based materials were successfully functionalized via the available carboxylic groups, using the EDC-NHS chemistry. Despite the difficulty of producing reproducible graphene-based electrodes, highly required for biosensor development, extensive testing was carried out to understand their feasibility. The calibration curves obtained via successive PSA addition showed a moderate-to-high ii sensitivity of both rGO and LIG IDE. However, further adsorption and drift testing underlined some major limitations in the case of LIG, due to its complex morphology and large porosity. To enable low contact resistance to these biosensors, the electroless nickel coating process is shown to be compatible with various graphene-based materials. This was demonstrated by tuning the chemical nickel bath and method conditions for pristine graphene and rGO for nickel contacts deposition

III-Nitride Vertical-Cavity Surface-Emitting Lasers: Growth, Fabrication, and Design of Dual Dielectric DBR Nonpolar VCSELs

Vertical-cavity surface-emitting lasers (VCSELs) have a long history of development in GaAs-based and InP-based systems, however III-nitride VCSELs research is still in its infancy. Yet, over the past several years we have made dramatic improvements in the lasing characteristics of these highly complex devices. Specifically, we have reduced the threshold current density from ~100 kA/cm2 to ~3 kA/cm2, while simultaneously increasing the output power from ~10 µW to ~550 µW. These developments have primarily come about by focusing on the aperture design and intracavity contact design for flip-chip dual dielectric DBR III-nitride VCSELs. We have carried out a number of studies developing an Al ion implanted aperture (IIA) and photoelectrochemically etched aperture (PECA), while simultaneously improving the quality of tin-doped indium oxide (ITO) intracavity contacts, and demonstrating the first III-nitride VCSEL with an n-GaN tunnel junction intracavity contact. Beyond these most notable research fronts, we have analyzed numerous other parameters, including epitaxial growth, flip-chip bonding, substrate removal, and more, bringing further improvement to III-nitride VCSEL performance and yield. This thesis aims to give a comprehensive discussion of the relevant underlying concepts for nonpolar VCSELs, while detailing our specific experimental advances. In Section 1, we give an overview of the applications of VCSELs generally, before describing some of the potential applications for III-nitride VCSELs. This is followed by a summary of the different material systems used to fabricate VCSELs, before going into detail on the basic design principles for developing III-nitride VCSELs. In Section 2, we outline the basic process and geometry for fabricating flip-chip nonpolar VCSELs with different aperture and intracavity contact designs. Finally, in Section 3 and 4, we delve into the experimental results achieved in the last several years, beginning with a discussion on the epitaxial growth developments. In Section 4, we discuss the most noteworthy accomplishments related to the nonpolar VCSELs structural design, such as different aperture and intracavity contact developments. Overall, this thesis is focused on the nonpolar VCSEL, however our hope is that many of the underlying insights will be of great use for the III-nitride VCSELs community as a whole. Throughout this report, we have taken great effort to highlight the future research fronts that would advance the field of III-nitride VCSELs generally, with the goal of illuminating the path forward for achieving efficient CW operating III-nitride VCSELs