Deflection Of Microcantilever Research Articles

Effects of Design Parameters on the Sensitivity of a Micro-Cantilever Based Biosensor

Piezo-resistive actuation of a microcantilever induced by biomolecular binding such as DNA hybridization and antibody-antigen binding is an important principle useful in biosensing applications. As the magnitude of the forces exerted is small, increasing the sensitivity of the microcantilever becomes critical. In this paper, we are considering to achieve this by geometric variation of the cantilever. The sensitivity of the cantilever was improved so that the device can sense the presence of antigen even if the magnitude of surface-stresses over the microcantilever was very small. We consider a `T-shaped' cantilver that eliminates the disadvantages while improving the sensitivity simultaneously. Simulations for validation have been performed using Intellisuite software (a MEMS design and simulation package). The simulations reveal that the T-shaped microcantilver is almost as sensitive as a thin cantilever and has relatively very low buckling effect. Simulations also reveal that with an increase in thickness of the cantilever, there is a proportional decrease in the sensitivity. This paper presents an analytical modeling and simulation studies of a piezoresistive cantilever used as MEMS based biosensor for the detection of cardiac markers. Diagnosis of Myocardial Infarction was achieved by the nanomechanical deflection of the microcantilever due to adsorption of the Troponin I complex. The deflection of the microcantilever was measured in terms of the piezoresistive changes by implanting boron at the anchor point where there is maximum strain due to the adsorption of the analyte molecules. The biochemical interactions between the Cardiac Troponin I (cTnl) complex and the immobilized antibodies would cause change in resistance of the piezoresistor integrated at the anchor point. A ‘T’ shaped microcantilever design was proposed for the study. The distal end of the device was coated with gold. The sensitivity of the cantilever was improved so that the device can sense the presence of antigen even if the magnitude of surface-stresses over the microcantilever was very small. To obtain an application specific optimum design parameter and predict the cantilever performance. The miniaturization of the cantilever-based biosensor leads to significant advantages in the absolute device sensitivity.

Interaction Between Tripeptide Gly-Gly-His and Cu2+ Probed by Microcantilevers

Gly-Gly-His tripeptide modified microcantilever was developed by carbodiimide attachment of the Gly-Gly-His tripeptide onto a 3-mercaptopropionic acid(MPA) modified gold surface. The interaction of peptide with Cu2+ ion was studied. At a relative high concentration of Cu2+, the cantilever bent toward the gold side initially as the N atom of imidazole ring and carboxyl group in different peptide coordinate with Cu2+, which results in a tensile surface stress. And then the reversed deflection of microcantilever was observed, which implies that the peptide-Cu2+ complex are formed with conformation transition. In another case, i.e., at a relative low concentration Of Cu2+, only the process of conformation transition was observed due to the coordination mode can not be formed initially. The influences of pH and salt concentration of the test solution on the performance of the sensor were studied. The results show that the maximum deflection was obtained at pH 7 and the bonding Of Cu2+ to the Gly-Gly-His tripeptide was inhibited due to the formation Of CuClx2-x.

Surface stress induced by interactions of adsorbates and its effect on deformation and frequency of microcantilever sensors

Surface stress is widely used to characterize the adsorption effect on the mechanical response of nanomaterials and nanodevices. However, quantitative relations between continuum-level descriptions of surface stress and molecular-level descriptions of adsorbate interactions are not well established. In this paper, we first obtain the relations between the adsorption-induced surface stress and the van der Waals and Coulomb interactions in terms of the physical and chemical interactions between adsorbates and solid surfaces. Then, we present a theoretical framework to predict the deflection and resonance frequencies of microcantilevers with the simultaneous effects of the eigenstrain, surface stress and adsorption mass. Finally, the adsorption-induced deflection and resonance frequency shift of microcantilevers are numerically analyzed for the van der Waals and Coulomb interactions. The present theoretical framework quantifies the mechanisms of the adsorption-induced surface stress, and thus provides guidelines to the analysis of the sensitivities, and the identification of the detected substance in the design and application of micro- and nanocantilever sensors.

Comparison between Deflection and Vibration Characteristics of Rectangular and Trapezoidal profile Microcantilevers

Arrays of microcantilevers are increasingly being used as physical, biological, and chemical sensors in various applications. To improve the sensitivity of microcantilever sensors, this study analyses and compares the deflection and vibration characteristics of rectangular and trapezoidal profile microcantilevers. Three models of each profile are investigated. The cantilevers are analyzed for maximum deflection, fundamental resonant frequency and maximum stress. The surface stress is modelled as in-plane tensile force applied on the top edge of the microcantilevers. A commercial finite element analysis software ANSYS is used to analyze the designs. Results show paddled trapezoidal profile microcantilevers have better sensitivity.

Deflection of a viscoelastic cantilever under a uniform surface stress: Applications to static-mode microcantilever sensors undergoing adsorption

The equation governing the curvature of a viscoelastic microcantilever beam loaded with a uniform surface stress is derived. The present model is applicable to static-mode microcantilever sensors made with a rigid polymer, such as SU-8. An analytical solution to the differential equation governing the curvature is given for a specific surface stress representing adsorption of analyte onto the viscoelastic beam’s surface. The solution for the bending of the microcantilever shows that, in many cases, the use of Stoney’s equation to analyze stress-induced deflection of viscoelastic microcantilevers (in the present case due to surface analyte adsorption) can lead to poor predictions of the beam’s response. It is shown that using a viscoelastic substrate can greatly increase sensitivity (due to a lower modulus), but at the cost of a longer response time due to viscoelastic creep in the microcantilever. In addition, the effects of a coating on the cantilever are considered. By defining effective moduli for the coated-beam case, the analytical solution for the uncoated case can still be used. It is found that, unlike the case of a silicon microcantilever, the stress in the coating due to bending of a polymer cantilever can be significant, especially for metal coatings. The theoretical results presented here can also be used to extract time-domain viscoelastic properties of the polymer material from beam response data.

Redox Actuation of a Microcantilever Driven by a Self-Assembled Ferrocenylundecanethiolate Monolayer: An Investigation of the Origin of the Micromechanical Motion and Surface Stress

The electrochemically induced motion of free-standing microcantilevers is attracting interest as micro/nanoactuators and robotic devices. The development and implementation of these cantilever-based actuating technologies requires a molecular-level understanding of the origin of the surface stress that causes the cantilever to bend. Here, we report a detailed study of the electroactuation dynamics of gold-coated microcantilevers modified with a model, redox-active ferrocenylundecanethiolate self-assembled monolayer (FcC(11)SAu SAM). The microcantilever transducer enabled the observation of the redox transformation of the surface-confined ferrocene. Oxidation of the FcC(11)SAu SAM in perchlorate electrolyte generated a compressive surface stress change of -0.20 +/- 0.04 N m(-1), and cantilever deflections ranging from approximately 0.8 microm to approximately 60 nm for spring constants between approximately 0.01 and approximately 0.8 N m(-1). A comparison of the charge-normalized surface stress of the FcC(11)SAu cantilever with values published for the electrochemical oxidation of polyaniline- and polypyrrole-coated cantilevers reveals a striking 10- to 100-fold greater stress for the monomolecular FcC(11)SAu system compared to the conducting polymer multilayers used for electroactuation. The larger stress change observed for the FcC(11)SAu microcantilever is attributable to steric constraints in the close-packed FcC(11)SAu SAM and an efficient coupling between the chemisorbed FcC(11)S- monolayer and the Au-coated microcantilever transducer (vs physisorbed conducting polymers). The microcantilever deflection vs quantity of electrogenerated ferrocenium obtained in cyclic voltammetry and potential step/hold experiments, as well as the surface stress changes obtained for mixed FcC(11)S-/C(11)SAu SAMs containing different populations of clustered vs isolated ferrocenes, have permitted us to establish the molecular basis of stress generation. Our results strongly suggest that the redox-induced deflection of a FcC(11)SAu microcantilever is caused by a monolayer volume expansion resulting from collective reorientational motions induced by the complexation of perchlorate ions to the surface-immobilized ferroceniums. The cantilever responds to the lateral pressure exerted by an ensemble of reorienting ferrocenium-bearing alkylthiolates upon each other rather than individual anion pairing events. This finding has general implications for using SAM-modified microcantilevers as (bio)sensors because it indicates that the cantilever responds to collective in-plane molecular interactions rather than reporting individual (bio)chemical events.

Crystallization-induced stress in thin phase change films of different thicknesses

We have studied crystallization-induced stress in phase change films (Ge2Sb2Te5) as a function of thickness and with and without a capping layer, by measuring the deflection of microcantilevers. The stress is found to increase with decreasing film thickness. A thin dielectric capping layer leads to a further increase in stress compared to uncapped films. This observation can be explained by the suppression of stress relaxation in the phase change film in the presence of a capping layer. High stress will affect device performance as the size of phase change memory cells decreases.

Quantitative and Label-Free Technique for Measuring Protease Activity and Inhibition using a Microfluidic Cantilever Array

We report the use of a SiN x based gold coated microcantilever array to quantitatively measure the activity and inhibition of a model protease immobilized on its surface. Trypsin was covalently bound to the gold surface of the microcantilever using a synthetic spacer, and the remaining exposed silicon nitride surface was passivated with silanated polyethylene glycol. The nanoscale cantilever motions induced by trypsin during substrate turnover were quantitatively measured using an optical laser-deflection technique. These microcantilever deflections directly correlated with the degree of protease turnover of excess synthetic fibronectin substrate ( K M = 0.58 x 10 (-6) M). Inhibition of surface-immobilized trypsin by soybean trypsin inhibitor (SBTI) was also observed using this system.

In-plane photonic transduction of silicon-on-insulator microcantilevers

We demonstrate an in-plane photonic transduction method for microcantilevers, which have been widely investigated for sensor applications. In our approach the microcantilever is etched to form a single mode rib waveguide. Light propagates down the microcantilever and crosses a small gap at the free end of the microcantilever, some of which is captured by an asymmetrical multimode waveguide that terminates in a Y-branch. The Y-branch outputs are used to form a differential signal that is monotonically dependent on microcantilever deflection. The measured differential signal matches simulation when microcantilever rotation is properly accounted for. The measured differential signal sensitivity is 1.4 x 10(-4) nm(-1) and the minimum detectable deflection is 0.35 nm.

A mathematical model for the motion analysis of embedded straight microcantilevers under a pressure-driven flow

A mathematical model that estimates the deflection of straight microcantilevers embedded in a microchannel under a pressure-driven flow at low Reynolds numbers is presented. The model makes use of the Schwarz–Christoffel mapping in order to couple the geometry of the structure and the flow passing around it. Therefore, it allows the determination of the most influential parameters and suitable modifications in order to achieve the desired performance. The model does not require specific knowledge of the flow conditions in the vicinity of the structure, which improves its practical use during the early stages of design. Estimations have been made for two straight cantilevers under a range of pressures. The results obtained show good agreement with measurements from experiments.

EFFECT OF SILICON CARBIDE PASSIVATION FILM ON THE RESONANCE CHARACTERISTICS OF CANTILEVER

ABSTRACT We have used silicon carbide (SiC) thin films as an insulating material of the PZT micro cantilevers for electrical and biological passivation. The use of SiC thin films as a passivation layer of the PZT microcantilevers is also seemingly viable to insure the high mass sensitivity as well as the stable passivation. In this study, we report the effect of SiC passivation layer on the performance of the PZT microcantilevers. The micromachined PZT microcantilevers having a structure of SiNx/Ta/Pt/PZT/Pt were fabricated through MEMS processes. In order to improve the mass sensitivity and the passivation, SiC thin films of the high elasticity material were deposited on the cantilever using plasma enhanced chemical vapor deposition (PECVD) at the temperature of 400°C. Plane-strain modulus of SiC thin film was measured by nanoindentation. We observed that SiC thin films showed higher Young's modulus than Si and SiO2. Before and after the deposition of SiC thin films, the end-tip deflection and the resonant frequency change of microcantilevers were measured by a confocal microscope and an impedance analyzer. It was confirmed that end-tip deflection of microcantilever was reduced by 13∼18% through the deposition of SiC thin films, indicating the stress relaxation of the microcantilevers.

Electrostatic Nanomechanics of Cantilever Biosensors

Interest in microcantilever based biosensors in the biomedical field has largely increased during the last years. Potentially, this kind of sensor can provide a considerable contribution to complex disease diagnosis, which requires the detection of biological molecules. Microcantilever biosensors allow the detection of complementary DNA fragment hybridization or specific antibody-antigen binding; it is known that adsorption of specific biological molecules upon the microcantilever surface induces cantilever deflection due to the interaction of the molecules with the surface. To date, the phenomena which determine the deflection mechanism are not completely known. The present work investigates the electrostatic field within the molecules and the forces consequently acting on the molecules and on the cantilever in order to provide a description of the deflection mechanism. The electrostatic potential of arrays of double strand DNA molecules immersed in an ionic solution was modelled by means of cylinders negatively charged at the surface and a FE (Finite Element) continuum electrostatics analysis was implemented in order to numerically solve the second order non-linear Poisson-Boltzmann equation. Then, a FE structural analysis of the cantilever was performed coupled with the continuum electrostatics analysis. In this way, the effects of the molecules’ electrostatic interactions on the cantilever deflection were taken into account. The model was run to describe the microcantilever deflection due to the electrostatic field under different design and operating conditions, and it was also set to compare hexagonal and square disposition of double strand DNA molecules.

Microcantilever mechanics in flowing viscous fluids

Microcantilevers are often deployed in flowing fluids to measure local flow velocities or to detect rapidly the nanomechanical binding of trace quantities of target analytes. The authors investigate the flow-induced mechanics of microcantilevers by deriving a semianalytical theoretical model for the nanoscale deflections of an elastic microcantilever due to a laminar viscous flow incident upon it. Conversely, the model allows for the estimation of the local flow velocities based on measured microcantilever deflection. Careful experiments performed on silicon microcantilevers in flowing nitrogen confirm the theoretical predictions up to a critical flow rate, beyond which unsteady flow-induced vibrations are seen to occur.

Roles of Interfacial Properties on the Stability of Emulsified Bitumen Droplets

Because of the deformable nature of emulsion droplets, it is imperative to consider the inter-droplet pressure in assessing the stability of a liquid-liquid dispersion. In a novel methodology, the pressure generated between droplets in axial compression is estimated through considerations of equilibrium shape mechanics. The applied compressive pressure is compared to the disjoining pressure, induced by the surface charges, resisting droplet-droplet interaction. The stability of bitumen droplets, emulsified in aqueous media, is assessed from surface mechanical and electrostatic perspectives. In support of the analysis, the tensions at, and zeta potentials near, the bitumen droplet surfaces are measured. The predicted trends of stability against coalescence are confirmed by novel droplet interaction experiments: Individual emulsified bitumen droplets, manipulated by suction micropipettes, are compressed against one another along a mutual axis at up to 100 nN force. The force is measured via the deflection of a sensitive microcantilever. Because of the statistical nature of the experimental observations, a probability of coalescence is quantified. The water chemistry dramatically influences bitumen droplet coalescence, which is enhanced in acidic environments and at high ionic strength.

Deflection of microcantilever by growing vapor bubble

This work investigates the mechanics of microcantilever deflection due to growth of a vapor microbubble. We construct a theoretical model to predict the elastic beam deflection due to the capillary pressure developed by the bubble growing between the beam and a substrate. The theory is in good agreement with our experimental results in both macro- and micro-scales. The deflection of a thin-solid structure due to growing bubbles can be used in such micromachines as microactuators, microgenerators and micropumps, which are operated as immersed in liquid.

CMOS-Based Monolithic Controllers for Smart Sensors Comprising Micromembranes and Microcantilevers

This paper presents design considerations and implementations of monolithic controllers for smart sensors that make use of micromachined structures such as micromembranes and microcantilevers. Five control techniques are reviewed, and the classic control technique is selected for the design of the controllers. The circuit implementations of an analog proportional controller and a digital proportional-integral-derivative controller are described in detail. The first controller is used for controlling the microhotplate (micromembrane) temperature of a monolithic metal-oxide gas sensor array. The second controller is used for controlling the microcantilever deflection of a monolithic atomic force microscope. The controllers and microsensors are fabricated in standard 0.8-mum CMOS technology combined with post-CMOS micromachining. Chemical measurements and contact-mode imaging are performed to confirm the excellent performance of these monolithic controllers

Using microcantilever deflection to detect HIV-1 envelope glycoprotein gp120

Microcantilevers have been used over the last decade to detect biomolecules from solution. Specific binding events on one surface of the microcantilever create a differential stress, resulting in measurable deflection. Here we use this principle to detect human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) gp120 from solution. We observed deflections approximately twice that of the baseline (in PBS) upon specific binding of gp120 to cantilevers decorated on one side with monoclonal antibodies (mAbs) A32 or T8. Subsequent incubation with mAb 17b (known to bind an A32-induced epitope on gp120) further increased deflection of A32- but not T8-presenting cantilevers. This work shows the capability of microcantilever deflection sensors to detect an induced-fit interaction at test concentrations of 8 μg/mL gp120 and 0.17 mg/mL 17b. Further development of this technique could lead to a portable, low-cost device for the effective detection of HIV-1.

Microcantilever Sensing and Actuation with End-Grafted Stimulus-Responsive Elastin-Like Polypeptides

Stimulus-responsive elastin-like polypeptides (ELPs) grafted onto surfaces are of significant technical interest because they can be exploited for force generation, in sensing applications, or as molecular switches with tunable properties. Changes in the conformational state of grafted ELPs, induced by a phase transition or changes in osmotic pressure, lead to significant changes in the surface stress in the ELP graft layer and translate into detectable changes in microcantilever deflection. In this study, we investigate the conformational mechanics of ELPs in response to changes in solution pH and ionic strength using atomic force microscopy (AFM) microcantilever deflection and quartz crystal microbalance (QCM) measurements. We show that the use of genetically encoded, surface-grafted ELPs is exciting for cantilever actuation and sensing because commonly available microfabricated cantilever springs offer a simple and nonintrusive way to detect changes in solvent type, temperature, and pH, promising great potential for sensing applications in microfluidic devices.

Comparison of microcantilever Hg sensing behavior with thermal higher order modes for as-deposited sputtered and thermally evaporated Au films

As-deposited sputtered and thermally evaporated Au films are studied for Hg sensing with rectangular Si microcantilevers. Microcantilever deflection and frequency shifts in the first four thermally induced flexural modes are measured for each type of Au film as a function of Hg exposure. All thermally induced higher order modes show shifts in their frequencies for Hg exposures when the Au is sputter deposited whereas typically only the third and fourth order modes show frequency shifts when the Au is thermally evaporated. A technique is employed in order to vary Au coverage along a microcantilever surface. The shifts in the thermal microcantilever modes upon Hg exposure are thus studied as a function of fractional Au coverage along the microcantilever surface. Microcantilevers having various coverages of either sputtered or thermally evaporated Au films show a distinct response to Hg exposure. From both the deflection and spectral shifts it is observed that sputtered films give better response to Hg than thermally evaporated films when the Au is coated along the entire length of the microcantilever.

Young's modulus measurement of thin-film materials using micro-cantilevers

The need for a simple and effective characterization technique for thin-film materials which are widely used in MEMS (micro-electro-mechanical systems), using commonly available equipment, has prompted consideration of cantilever beam-based methods. The advantages of this class of techniques which employ a scanning surface profiler to deform micro-cantilevers are simplicity, speed, cost and wide applicability. A technique for extracting Young's modulus from static deflection data is developed in this paper and validated in experiments on thin-film specimens of silicon nitride deposited on a silicon substrate under different conditions. Finite element analysis is used to assess the influence of factors affecting the bending of thin films, and thus guide the analysis of micro-cantilever deflection data for reliable characterization of the material.