Mechanical Characterization of PDMS Films for the Optimization of Polymer Based Flexible Capacitive Pressure Microsensors

This paper reports on the optimization of flexible PDMS-based normal pressure capacitive microsensors dedicated to wearable applications. The operating principle and the fabrication process of such microsensors are presented. Then, the deformations under local pressure of PDMS thin films of thicknesses ranging from 100 μm to 10 mm are studied by means of numerical simulations in order to foresee the sensitivity of the considered microsensors. The study points out that, for a given PDMS type, the sensor form ratio plays a major role in its sensitivity. Indeed, for a given PDMS film, the expected capacitance change under a 10 N load applied on a 1.7 mm radius electrode varies from a few percent to almost 40% according to the initial PDMS film thickness. These observations are validated by experimental characterizations carried out on PDMS film samples of various thicknesses (10 μm to 10 mm) and on actual microsensors. Further computations enable generalized sensor design rules to be highlighted. Considering practical limitations in the fabrication and in the implementation of the actual microsensors, design rules based on computed form ratio optimization lead to the elaboration of flexible pressure microsensors exhibiting a sensitivity which reaches up to 10%/N

Recent Advances in Printed Capacitive Sensors

In this review paper, we summarize the latest advances in the field of capacitive sensors fabricated by printing techniques. We first explain the main technologies used in printed electronics, pointing out their features and uses, and discuss their advantages and drawbacks. Then, we review the main types of capacitive sensors manufactured with different materials and techniques from physical to chemical detection, detailing the main substrates and additives utilized, as well as the measured ranges. The paper concludes with a short notice on status and perspectives in the field.H2020-MSCA-IF-2017-794885-SELFSEN

Polymeric Microsensors for Intraoperative Contact Pressure Measurement

Biocompatible sensors have been demonstrated using traditional microfabrication techniques modified for polymer substrates and utilize only materials suitable for implantation or bodily contact. Sensor arrays for the measurement of the load condition of polyethylene spacers in the total knee arthroplasty (TKA) prosthesis have been developed. Arrays of capacitive sensors are used to determine the three-dimensional strain within the polyethylene prosthesis component. Data from these sensors can be used to give researchers a better understanding of component motion, loading, and wear phenomena for a large range of activities. This dissertation demonstrates both analytically and experimentally the fabrication of these sensor arrays using biocompatible polymer substrates and dielectrics while preserving industry-standard microfabrication processing for micron-level resolution. An array of sensors for real-time measurement of pressure profiles is the long-term goal of this research. A custom design using capacitive-based sensors is an excellent selection for such measurement, giving high spatial resolution across the sensing surface and high load resolution for pressures applied normal to that surface while operating at low power

Modular integration and on-chip sensing approaches for tunable fluid control polymer microdevices

228 p.Doktore tesi honetan mikroemariak kontrolatzeko elementuak diseinatu eta garatuko dira, mikrobalbula eta mikrosentsore bat zehazki. Ondoren, gailu horiek batera integratuko dira likido emari kontrolatzaile bat sortzeko asmotan. Helburu nagusia gailuen fabrikazio arkitektura modular bat frogatzea da, non Lab-on-a-Chip prototipoak garatzeko beharrezko fase guztiak harmonizatuz, Cyclic-Olefin-Polymer termoplastikozko mikrogailu merkeak pausu gutxi batzuetan garatuko diren, hauen kalitate industriala bermatuz. Ildo horretan, mikrogailuak prototipotik produkturako trantsizio azkar, erraz, errentagarri eta arriskurik gabeen bidez lortu daitezkeenetz frogatuko da

Microfabrication of a MEMS piezoresistive flow sensor - materials and processes

Microelectromechanical systems (MEMS) based artificial sensory hairs for flow sensing have been widely explored, but the processes involved in their fabrication are lithography intensive, making the process quite expensive and cumbersome. Most of these devices are also based on silicon MEMS, which makes the fabrication of out-of plane 3D flow sensors very challenging. This thesis aims to develop new fabrication technologies based on Polymer MEMS, with minimum dependence on lithography for the fabrication of piezoresistive 3D out-of-plane artificial sensory hairs for sensing of air flow. Moreover, the fabrication of a flexible sensor array is proposed and new materials are also explored for the sensing application. Soft lithography based approaches are first investigated for the fabrication of an all elastomer device that is tested in a bench top wind tunnel. Micromolding technologies allow for the mass fabrication of microstructures using a single, reusable mold master that is fabricated by SU-8 photolithography, reducing the need for repetitive processing. Polydimethylsiloxane (PDMS) is used as the device material and sputter deposited gold is used as both the piezoresistive as well as the electrode material for collection of device response. The fabrication results of PDMS to PDMS metal transfer micromolding (MTM) are shown and the limitations of the process are also discussed. A dissolving mold metal transfer micromolding process is then proposed and developed, which overcomes the limitations of the conventional MTM process pertinent to the present application. Testing results of devices fabricated using the dissolving mold process are discussed with emphasis on the role of micro-cr  acking as one failure mode in elastomeric devices with thin film metal electrodes. Finally, a laser microfabrication based approach using thin film Kapton as the device material and an electrically conductive carbon-black elastomer composite as the piezoresistor is proposed and demonstrated. Laminated sheets of thick and thin Kapton form the flexible substrate on which the conductive elastomer piezoresistors are stencil printed. Excimer laser ablation is used to make the micro-stencil as well as to release the Kapton cantilevers. The fluid-structure interaction is improved by the deposition of a thin film of silicon dioxide, which produces a stress-gradient induced curvature, strongly enhancing the device sensitivity. This new approach also enables the fabrication of backside interconnects, thereby addressing the commonly observed problem of flow intrusion while using conventional interconnection technologies like wire-bonding. Devices with varying dimensions of the sensing element are fabricated and the results presented, with smallest devices having a width of 400 microns and a length of 1.5 mm with flow sensitivities as high as 60 Ohms/m/s. Recommendations are also proposed for further optimization of the device.M.S.Committee Chair: Allen, Mark; Committee Member: Allen, Sue Ann Bidstrup; Committee Member: Wong, C.P

Smart polymeric temperature sensors – for biological systems

The damaged brain is vulnerable to increase in brain temperature after a severe head injury. Continuous monitoring of intracranial temperature depicts functionality essential to the treatment of brain injury Many innovations have been made in the biomedical industry relying on electronic implants in treating condition such as traumatic brain injury (TBI) or other cerebral diseases. Hence, a methodical and reliable way to measure the temperature is crucial to assess the patient’s situation. In this investigation, an analysis of various approaches to detect the change in the temperature due to resistance, current-voltage characteristics with respect to time has been evaluated. Also, studies describing various materials used in sensors, their working principles and the results anticipated in these discrete procedures are presented. These smart temperature sensors have provided the accuracy and the stability compared to earlier methods used to detect the change in brain temperature since temperature is one of the most important variables in brain monitoring

Simulation and Fabrication of Three Novel Micromechanical Sensors

This work focuses on the simulation, fabrication and characterization of novel microdevices for chemical and biological sensors for improved sensitivity, enhanced performance and applicability. Specifically, microbridge and microcoil sensors have been fabricated via advanced microfabrication technologies. Due to the potential application in chemical and biological sensing, the growth of gold and platinum nanowires during an electrolysis process have also been investigated. A microbridge can be considered as the head-to-head fusion of two cantilevers and the middle of the bridge would deform in a way similar to a microcantilever. The microbridge sensing device is more stable than the microcantilever, especially in turbulent or vibrational conditions, since both ends are fixed. The trade-off is the low ΔR/R change (sensitivity) of the microbridge compared to that of the microcantilever. Simulation of the microbridge has been conducted via Finite Element Analysis (FEA). The width, thickness and doping level of the piezoresistor play an important part in the sensitivity of the microbridge. Based on the simulation results and following standard microfabrication technology, microbridges have been fabricated. The detection of Hg2+ based on the microbridge platform was investigated for sensor validation. The microcoil hygrometer can be used as a universal tool for the detection of chemical species by depositing a chemical specific coating on one side of the coil. The coil movement can be readily observed by the human eye and it advances as a cost-effective and power-free device. A micro- or nano-scale sized coil provides an outstanding sensor platform with improved dynamic response, greatly reduced size, and the integration of micromechanical components with on-chip electronic circuitry. Following standard microfabrication techniques, an SiO2/Si/SU-8 microcoil has been fabricated. After surface modification by treating the coil with aminopropyltriethoxysilane (APS), the microcoil was exposed to acetic acid vapor in air for characterization. This microcoil device has a potential to be used as a novel microsensor for the detection of chemical and biological species both in air and in solutions. A self-assembled approach to grow gold and platinum nanowires across the gap of two electrodes on a surface using an electrolysis process has been investigated. In this process, the anode electrode is oxidized to form nanowires on the cathode. The DC offset, AC signal frequency and the space between the two electrodes all play important roles in the growth of the nanowires

Experimentation and Multiphysical Modeling of Bioanalytical Microdevices

Bioanalytics involves quantitative measurements of complex biological samples that contain metabolites, DNA, RNA, and proteins. Efficient sample preparation for downstream analysis and sensitive detection of analytes can be achieved via bioanalytical microdevices. Fully realizing the potential of these devices requires tool characterization and bioprocess optimization, in addition to understanding device physics. Therefore, this thesis introduces multiphysical modeling and experimentation of microdevices, with applications to diabetes care and single-cell analysis. To understand the physics of viscometric glucose microsensors, this thesis presents a model of the sensor, which couples the fluid flow with vibrating diaphragms. The model is used to predict the sensor response to glucose via theory of squeeze-film damping and vibrations of pre-stressed plate. A first-principle-based model resulting from the theory can be evaluated from the device's geometric and material properties, and quantitatively determines the device response to vibrational excitations at varying glucose concentrations. Next, this thesis introduces a theoretical model for viscometric glucose microsensors that employ harmonic microcantilever oscillation in the sensing liquid. The presented model associates the unsteady Stokes equation with the motion of a bounded viscous liquid to understand the hydrodynamic impact on the cantilever. With a proper consideration of the viscosity and bounded geometry of liquid media, the model relaxes the thin-film assumption required for the diaphragm-based model, enabling an accurate representation of fluid-structure interactions based on fundamental structural vibration and fluid flow equations. Next, this thesis presents an experimental exploration of a hydrogel-based affinity microsensor for glucose monitoring via dielectric measurements. The microsensor incorporates a synthetic hydrogel that is attached to the device surface via in situ polymerization, which eliminates mechanical moving parts required in the viscometric glucose sensors. Changes in the dielectric properties of the hydrogel when binding reversibly with glucose molecules have been measured using a MEMS capacitive transducer to determine the glucose concentration. Experimental results demonstrate that in a glucose concentration range of 0–500 mg/dL and with a resolution of 0.35 mg/dL or better, the microsensor exhibits a repeatable and reversible response, and can potentially be useful for continuous glucose monitoring in diabetes care. Additionally, this thesis presents a microfluidic preprocessing method that integrates single-cell picking, lysing, reverse transcription and digital polymerase chain reaction to enable the isolation, tracking and gene expression analysis at single-cell level for individual cells. The approach utilizes a photocleavable bead-based microfluidic device to synthesize and deliver stable complementary DNA for downstream gene expression analysis, thereby allowing chip-based integration of multiple reactions and facilitating the minimization of sample loss or contamination. Finally, this thesis ends with concluding remarks and directions of future work towards continuous glucose monitoring and high-throughput single-cell genetic analysis

Gas Sensors on Plastic Foil with Reduced Power Consumption for Wireless Applications

Recently, there is a growing interest in developing so-called "smart" RFID tags for logistic applications. These smart tags incorporate sensing devices to monitor environmental parameters such as humidity and temperature throughout the supply chain. To fulfill these requirements cost-effectively, RFID tags were produced on plastic foil through large scale manufacturing techniques. To benefit from sensing capabilities on these systems, the integration of gas sensors directly produced on plastic foil was explored. Their gas sensing performances were investigated when fabricated on same polymeric substrates than the labels. To be compatible with wireless applications, all sensors were designed to operate in the sub-milliwatt power range. The integration of three different transducers on plastic foil for the detection of different gaseous species was investigated. First, the direct use of the PET or PEN foil as an optical waveguide for the fabrication of a selective colorimetric ammonia gas sensor was carried out. It led to a simplified processing based on additive fabrication techniques compatible with large scale manufacturing. Second, the impact of miniaturization on drop-coated metal-oxide gas sensors when fabricated on polyimide foil on their sensing performances was investigated. They took advantage from the low thermal conductivity of the substrate to reduce the power consumption with a simplified processing. The detection of oxidizing and reducing gases was achieved at low power consumption when pulsing the sensors. Lastly, the benefits brought by the gas absorption in a polyimide foil were exploited with the design of a simple capacitive structure. By operating it in a differential mode with a second functionalized capacitor, the discrimination between low-concentrations of volatile organic compounds and humidity was achieved. The design and fabrication of these sensors were developed with a vision of their future production performed by large scale manufacturing techniques. The gas sensing performances of all three transducers were assessed and revealed sensitivities comparable to standard devices made on silicon. Each sensor was associated with low-power electronics targeting an integration on wireless systems. The concept of a smart gas sensing system was demonstrated with the interfacing of a capacitive humidity sensor on a passive RFID label