Theoretical Analysis of Laterally Vibrating Hammerhead Microcantilever Sensors in a Viscous Liquid

Dynamically driven prismatic microcantilevers excited in the in-plane flexural mode have been investigated and used in liquid-phase sensing applications. However, the performance is restricted due to their limited surface sensing area and higher stiffness in shorter and wider prismatic microcantilevers. To increase the surface sensing area, and further improve sensing characteristics, it has been proposed to investigate symmetric hammerhead microcantilevers vibrating laterally in viscous liquid media. In this work, a theoretical model is proposed and the characteristics of the microcantilevers with symmetric shaped hammerheads (isosceles trapezoid, semi-circle, uniform rectangle and composite rectangle) are investigated. In the analysis, the stem of the structure is modeled as an Euler-Bernoulli beam while the head is modeled as a rigid body. Since the arbitrary, symmetric head has a varying width, 2b2(x), in the length direction, a new semi-analytical expression for the hydrodynamic function in terms of the Reynolds number, Re(x), and aspect ratio, h/[2b2(x)] is obtained and the resonance frequency, quality factor and mass sensitivity are investigated as a function of both the hammerhead microcantilever geometry and liquid media properties. For the investigated geometries, the results show that, for a hammerhead microcantilever with a fixed head area, as the mass center of the head moves towards the support end of the stem, the resulting resonance frequency and mass sensitivity will first increase and then decrease, because the total kinetic energy will first decrease and then increase. The quality factor will keep increasing, due to a more rapid decrease in the energy dissipation. It is also found that, hammerhead microcantilevers with wider heads tend to have higher quality factors. For instance, the highest quality factors are found for the hammerhead microcantilevers with the isosceles trapezoid-shaped, uniform rectangular and composite rectangular head as 140, 72 and 129, respectively, due to the possible shift of the mass center of the head towards the support end of the stem. Such trends can be used to optimize sensor device geometry and frequency stability. By further increasing the surface sensing area (additional mass), the resonance frequency and the mass sensitivity will significantly decrease. Such trade-offs must be considered when designing the geometry of the hammerhead microcantilever devices. For appropriately designed hammerhead microcantilevers, the improvement in the sensing area and quality factor are expected to yield much lower limits of detection in (bio) chemical sensing applications

Lateral-Mode Vibration of Microcantilever-Based Sensors in Viscous Fluids Using Timoshenko Beam Theory

Dynamic-mode microcantilever-based devices are well suited to biological and chemical sensing applications. However, these applications often necessitate liquid-phase sensing, introducing significant fluid-induced dissipative forces and reducing device quality factors (Q). Recent experimental and analytical research has shown that higher in- fluidQis achieved by exciting microcantilevers in the lateral flexural mode. However, the experimental results show that, for microcantilevers having larger width-to-length (b/L) ratios, the behaviors predicted by current analytical models differ from measurements. To more accurately model microcantilever resonant behavior in viscous fluids and to improve understanding of lateral-mode sensor performance, a new analytical model is developed, incorporating both viscous fluid effects and Timoshenko beam effects (shear deformation and rotatory inertia). Analytical solutions for the frequency response are obtained and verified by reduction to known special cases. Beam response is examined for two harmonic load types that simulate current actuation methods: tip force and support rotation. Results are expressed in terms of total beam displacement and beam displacement due solely to bending deformation, which correspond to current detection methods commonly used with these devices (laser and piezoresistive detection, respectively). Resonant frequencies (fres) and Q are determined from the theoretical beam response. The influences of the shear, rotatory inertia, and fluid parameters, as well as the load/detection scheme, on the resonant characteristics are investigated in detail. Results show that the new model reproduces the experimental trends in fres and Q for lateral- mode microcantilevers at higher b/L ratios (i.e., for the high-Q devices for which Euler- Bernoulli models prove inadequate). Over the practical ranges of system parameters considered, the results indicate that Timoshenko beam effects can account for a reduction in fres and Q of up to 23%, but are negligible (no more than 2% reduction) for length-to- width ratios of 7 and higher. Also derived is a simple analytical expression relating Q to system parameters while incorporating Timoshenko and fluid effects. Finally, to evaluate the influence of lateral-mode chemical sensor design parameters on performance, the results for fres andQ are related to the mass/chemical sensitivities and to the limit of detection (LOD), and illustrative calculations of sensitivity and LOD are presented

Integrated microcantilever fluid sensor as a blood coagulometer

The work presented concerns the improvement in mechanical to thermal signal of a microcantilever fluid probe for monitoring patient prothrombin time (PT) and international normalized ratio (INR) based on the physical measurement of the clotting cascade. The current device overcomes hydrodynamic damping limitations by providing an internal thermal actuation force and is realised as a disposable sensor using an integrated piezoresistive deflection measurement. Unfortunately, the piezoresistor is sensitive to thermal changes and in the current design the signal is saturated by the thermal actuation. Overcoming this problem is critical for demonstrating a blood coagulometer and in the wider field as a microsensor capable of simultaneously monitoring rheological and thermal measurements of micro-litre samples. Thermal, electrical, and mechanical testing of a new design indicates a significant reduction in the thermal crosstalk and has led to a breakthrough in distinguishing the mechanical signal when operated in moderately viscous fluids (2-3 cP). A clinical evaluation has been conducted at The Royal London Hospital to measure the accuracy and precision of the improved microcantilever fluid probe. The correlation against the standard laboratory analyser INR, from a wide range of patient clotting times(INR 0.9-6.08) is equal to 0.987 (n=87) and precision of the device measured as the percentage coefficient of variation, excluding patient samples tested < 3 times, is equal to 4.00% (n=64). The accuracy and precision is comparable to that of currently available point-of-care PT/INR devices. The response of the fluid probe in glycerol solutions indicates the potential for simultaneous measurement of rheological and thermal properties though further work is required to establish the accuracy and range of the device as a MEMS based viscometer

Thermal Bimorph Micro-Cantilever Based Nano-Calorimeter for Sensing of Energetic Materials

The objective of this study is to develop a robust portable nano-calorimeter sensor for detection of energetic materials, primarily explosives, combustible materials and propellants. A micro-cantilever sensor array is actuated thermally using bi-morph structure consisting of gold (Au: 400 nm) and silicon nitride (Si3N4: 600 nm) thin film layers of sub-micron thickness. An array of micro-heaters is integrated with the microcantilevers at their base. On electrically activating the micro-heaters at different actuation currents the microcantilevers undergo thermo-mechanical deformation, due to differential coefficient of thermal expansion. This deformation is tracked by monitoring the reflected ray from a laser illuminating the individual microcantilevers (i.e., using the optical lever principle). In the presence of explosive vapors, the change in bending response of microcantilever is affected by the induced thermal stresses arising from temperature changes due to adsorption and combustion reactions (catalyzed by the gold surface). A parametric study was performed for investigating the optimum values by varying the thickness and length in parallel with the heater power since the sensor sensitivity is enhanced by the optimum geometry as well as operating conditions for the sensor (e.g., temperature distribution within the microcantilever, power supply, concentration of the analyte, etc.). Also, for the geometry present in this study the nano-coatings of high thermal conductivity materials (e.g., Carbon Nanotubes: CNTs) over the microcantilever surface enables maximizing the thermally induced stress, which results in the enhancement of sensor sensitivity. For this purpose, CNTs are synthesized by post-growth method over the metal (e.g., Palladium Chloride: PdCl2) catalyst arrays pre-deposited by Dip-Pen Nanolithography (DPN) technique. The threshold current for differential actuation of the microcantilevers is correlated with the catalytic activity of a particular explosive (combustible vapor) over the metal (Au) catalysts and the corresponding vapor pressure. Numerical modeling is also explored to study the variation of temperature, species concentration and deflection of individual microcantilevers as a function of actuation current. Joule-heating in the resistive heating elements was coupled with the gaseous combustion at the heated surface to obtain the temperature profile and therefore the deflection of a microcantilever by calculating the thermally induced stress and strain relationship. The sensitivity of the threshold current of the sensor that is used for the specific detection and identification of individual explosives samples - is predicted to depend on the chemical kinetics and the vapor pressure. The simulation results showed similar trends with the experimental results for monitoring the bending response of the microcantilever sensors to explosive vapors (e.g., Acetone and 2-Propanol) as a function of the actuation current

Development of calix[4]arene-functionalized microcantilever array sensing system for the rapid, sensitive and simultaneous detection of metal ions in aqueous solutions

The work described in this thesis was conducted with the aim of: 1) investigating the binding capabilities of calix[4]arene-functionalized microcantilevers towards specific metal ions and 2) developing a new16-microcantilever array sensing system for the rapid, and simultaneous detection of metal ions in fresh water. Part I of this thesis reports on the use of three new bimodal calix[4]arenes (methoxy, ethoxy and crown) as potential host/guest sensing layers for detecting selected ions in dilute aqueous solutions using single microcantilever experimental system. In this work it was shown that modifying the upper rim of the calix[4]arenes with a thioacetate end group allow calix[4]arenes to self-assemble on Au(111) forming complete highly ordered monolayers. It was also found that incubating the microcantilevers coated with 5 nm of Inconel and 40 nm of Au for 1 h in a 1.0 M solution of calix[4]arene produced the highest sensitivity. Methoxy-functionalized microcantilevers showed a definite preference for Ca²⁺ ions over other cationic guests and were able to detect trace concentration as low as 10⁻¹² M in aqueous solutions. Microcantilevers modified with ethoxy calix[4]arene displayed their highest sensitivity towards Sr²⁺ and to a lesser extent Ca²⁺ ions. Crown calix[4]arene-modified microcantilevers were however found to bind selectively towards Cs⁺ ions. In addition, the counter anion was also found to contribute to the deflection. For example methoxy calix[4]arene-modified microcantilever was found to be more sensitive to CaCl₂ over other water-soluble calcium salts such as Ca(NO₃)₂ , CaBr₂ and CaI₂. These findings suggest that the response of calix[4]arene-modified microcantilevers should be attributed to the target ionic species as a whole instead of only considering the specific cation and/or anion. Part II presents the development of a 16-microcantilever sensor setup. The implementation of this system involved the creation of data analysis software that incorporates data from the motorized actuator and a two-axis photosensitive detector to obtain the deflection signal originating from each individual microcantilever in the array. The system was shown to be capable of simultaneous measurements of multiple microcantilevers with different coatings. A functionalization unit was also developed that allows four microcantilevers in the array to be coated with an individual sensing layer one at the time. Because of the variability of the spring constants of different cantilevers within the array, results presented were quoted in units of surface stress unit in order to compare values between the microcantilevers in the array

Probing extraordinary nanoscale energy transfer using bimaterial microcantilevers

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references.Nanostructured materials have recently drawn a great deal of attention in the field of energy research such as for solar photovoltaic, thermophotovoltaic and thermoelectric applications. The energy transport properties of nanostructures can differ greatly from their bulk counterparts because the characteristic dimensions of nanostructures are often comparable with the wavelength or the mean free path of energy carriers such as photons, phonons and electrons. Due to the small dimensions, probing energy transfer at the nanoscale is extremely challenging. By developing new experimental techniques based on the bi-material microcantilevers used in Atomic Force Microscopes (AFM), this thesis has studied several extraordinary energy transfer phenomena at the nanoscale including near-field radiation beyond Planck's law, high thermal conductivity polymers and the optical absorption of micro/nanostructures. First, surface phonon polaritons, which is one type of electromagnetic surface waves, are demonstrated to enhance the thermal radiation between two surfaces at small gaps by measuring radiation heat transfer between a microsphere and a flat surface down to a 30 nm separation. The corresponding heat transfer coefficients at nanoscale gaps are three orders of magnitude larger than that of the Planck's blackbody radiation limit. This work will have practical impacts in areas such as thermophotovoltaic energy conversion, radiative cooling, and magnetic data recording. Next, a new technique is developed to fabricate ultra-drawn polyethylene nanofibers. We demonstrated that these ultradrawn nanofibers can have a thermal conductivity as high as ~ 100 W/m.K, which is about a 3 orders of magnitude enhancement compared to that of bulk polymers. Such high thermal conductivity polymers can potentially provide a cheaper alternative to conventional metal-based heat transfer materials. Finally, an experimental setup is presented to directly measure the spectral absorption of individual micro/nanostructures in applications to solar photovoltaics. Further refinement on experimental technique and characterization using the platform will guide the optimization of dimension, shape, and materials selections of nanostructures in order to maximize the efficiencies of solar cells.by Sheng Shen.Ph.D

Development of microcantilever sensors for cell studies

Micro- and nano- electromechanical devices such as microcantilevers have paved the way for a large variety of new possibilities, such as the rapid diagnosis of diseases and a high throughput platform for drug discovery. Conventional cell assay methods rely on the addition of reagents, disrupting the measurement, therefore providing only the endpoint data of the cell growth experiment. In addition, these methods are typically slow to provide results and time and cost consuming. Therefore, microcantilever sensors are a great platform to conduct cell culturing experiments for cell culture, viability, proliferation, and cytotoxicity monitoring, providing advantages such as being able to monitor cell kinetics in real time without requiring external reagents, in addition to being low cost and fast, which conventional cell assay methods are unable to provide. This work aims to develop and test different types of microcantilever biosensors for the detection and monitoring of cell proliferation. This approach will overcome many of the current challenges facing microcantilever biosensors, including but not limited to achieving characteristics such as being low cost, rapid, easy to use, highly sensitive, label-free, multiplexed arrays, etc. Microcantilever sensor platforms utilizing both a single and scanning optical beam detection methods were developed and incorporated aspects such as temperature control, calibration, and readout schemes. Arrays of up to 16 or 32 microcantilever sensors can be simultaneously measured with integrated microfluidic channels. The effectiveness of these cantilever platforms are demonstrated through multiple studies, including examples of growth induced bending of polyimide cantilevers for simple real-time yeast cell measurements and a microcantilever array for rapid, sensitive, and real-time measurement of nanomaterial toxicity on the C3A human liver cell line. In addition, other techniques for microcantilever arrays and microfluidics will be presented along with demonstrations for the ability for stem cell growth monitoring and pathogen detection

A Thermally actuated microelectromechanical (MEMS) device for measuring viscosity

A thermally actuated non-cantilever-beam micro-electro-mechanical viscosity sensor is presented. The proposed device is based on thermally induced vibrations of a silicon-based membrane and its damping due to the surrounding fluid. This vibration viscometer device utilizes thermal actuation through an in-situ resistive heater and piezoresistive sensing, both of which utilize CMOS compatible materials leading to an inexpensive and reliable system. Due to the nature of the actuation, thermal analysis was performed utilizing PN diodes embedded in the silicon membrane to monitor its temperature. This analysis determined the minimum heater voltage pulse amplitude and time in order to prevent heat loss to the oil under test that would lead to local viscosity changes. In order to study the natural vibration behavior of the complex multilayer membrane that is needed for the proposed sensor, a designed experiment was carried out. In this experiment, the effects of the material composition of the membrane and the size of the actuation heater were studied in detail with respect to their effects on the natural frequency of vibration. To confirm the validity of these measurements, Finite Element Analysis and white-light interferometry were utilized. Further characterization of the natural frequency of vibration of the membranes was carried out at elevated temperatures to explore the effects of temperature. Complex interactions take place among the different layers that compose the membrane structures. Finally, viscosity measurements were performed and compared to standard calibrated oils as well as to motor oils measured on a commercial cone-and-plate viscometer. The experimentally obtained data is compared to theoretical predictions and an empirically-derived model to predict viscosity from vibration measurements is proposed. Frequency correlation to viscosity was shown to be the best indicator for the range of viscosities tested with lower error (+/- 5%), than that of quality factor (+/- 20%). Further viscosity measurements were taken at elevated temperatures and over long periods of time to explore the device reliability and drift. Finally, further size reduction of the device was explored

Current Research in Thin Film Deposition

Today, thin films are near-ubiquitous and are utilised in a very wide range of industrially and scientifically important areas. These include familiar everyday instances such as anti-reflective coatings on ophthalmic lenses, smartphone optics, photovoltaics, decorative, and tool coatings. A range of somewhat more exotic applications also exists, such as astronomical instrumentation (e.g., ultra-low loss dielectric mirrors and beam splitters in gravitational wave detectors, such as laser interferometer gravitational-wave observatory (LIGO)), gas sensing, medical devices and implants, and accelerator coatings (e.g., coatings for the large hadron collider (LHC), and compact linear collider (CLIC) experiments at European organization for nuclear research (CERN)). This Special Issue will provide a platform for researchers working in any area within this highly diverse field to share and exchange their latest research findings. The Special Issue contains novel studies encompassing material characterisation techniques, a range of thin-film coating deposition processes and applications of such technology