Showing papers by "Rama B. Bhat published in 2013"

Quantification of cellular penetrative forces using lab-on-a-chip technology and finite element modeling

Abstract: Tip-growing cells have the unique property of invading living tissues and abiotic growth matrices. To do so, they exert significant penetrative forces. In plant and fungal cells, these forces are generated by the hydrostatic turgor pressure. Using the TipChip, a microfluidic lab-on-a-chip device developed for tip-growing cells, we tested the ability to exert penetrative forces generated in pollen tubes, the fastest-growing plant cells. The tubes were guided to grow through microscopic gaps made of elastic polydimethylsiloxane material. Based on the deformation of the gaps, the force exerted by the elongating tubes to permit passage was determined using finite element methods. The data revealed that increasing mechanical impedance was met by the pollen tubes through modulation of the cell wall compliance and, thus, a change in the force acting on the obstacle. Tubes that successfully passed a narrow gap frequently burst, raising questions about the sperm discharge mechanism in the flowering plants.

Quantification of the Young's modulus of the primary plant cell wall using Bending-Lab-On-Chip (BLOC).

Abstract: Biomechanical and mathematical modeling of plant developmental processes requires quantitative information about the structural and mechanical properties of living cells, tissues and cellular components. A crucial mechanical property of plant cells is the mechanical stiffness or Young's modulus of its cell wall. Measuring this property in situ at single cell wall level is technically challenging. Here, a bending test is implemented in a chip, called Bending-Lab-On-a-Chip (BLOC), to quantify this biomechanical property for a widely investigated cellular model system, the pollen tube. Pollen along with culture medium is introduced into a microfluidic chip and the growing pollen tube is exposed to a bending force created through fluid loading. The flexural rigidity of the pollen tube and the Young's modulus of the cell wall are estimated through finite element modeling of the observed fluid-structure interaction. An average value of 350 MPa was experimentally estimated for the Young's modulus in longitudinal direction of the cell wall of Camellia pollen tubes. This value is in agreement with the result of an independent method based on cellular shrinkage after plasmolysis and with the mechanical properties of in vitro reconstituted cellulose-callose material.

PDMS Microcantilever-Based Flow Sensor Integration for Lab-on-a-Chip

Abstract: In this paper, a simple practical method is presented to fabricate a high aspect ratio horizontal polydimethylsiloxane (PDMS) microcantilever-based flow sensor integrated into a microfluidic device. A multilayer soft lithography process is developed to fabricate a thin PDMS layer involving the PDMS microcantilever and the microfluidics network. A three-layer fabrication technique is explored for the integration of the microflow meter. The upper and lower PDMS layers are bonded to the thin layer to release the microcantilever for free deflection. A 3-D finite element analysis is carried out to simulate fluid-structure interaction and estimate cantilever deflection under various flow conditions. The dynamic range of flow rates that is detectable using the flow sensor is assessed by both simulation and experimental methods and compared. Limited by the accuracy of the 1.76- μm resolution of the image acquisition method, the present setup allows for flow rates as low as 35 μL/min to be detected. This is equal to 0.8-μN resolution in equivalent force at the tip. This flow meter can be integrated into any type of microfluidic-based lab-on-a-chip in which flow measurement is crucial, such as flow cytometry and particle separation applications.

Modelling, validation and analysis of a three-dimensional railway vehicle–track system model with linear and nonlinear track properties in the presence of wheel flats

Abstract: This paper presents dynamic contact loads at wheel–rail contact point in a three-dimensional railway vehicle–track model as well as dynamic response at vehicle–track component levels in the presence of wheel flats. The 17-degrees of freedom lumped mass vehicle is modelled as a full car body, two bogies and four wheelsets, whereas the railway track is modelled as two parallel Timoshenko beams periodically supported by lumped masses representing the sleepers. The rail beam is also supported by nonlinear spring and damper elements representing the railpad and ballast. In order to ensure the interactions between the railpads, a shear parameter beneath the rail beams has also been considered into the model. The wheel–rail contact is modelled using nonlinear Hertzian contact theory. In order to solve the coupled partial and ordinary differential equations of the vehicle–track system, modal analysis method is employed. Idealised Haversine wheel flats with the rounded corner are included in the wheel–rail contact...

Semi-active control of aircraft landing gear system using H-infinity control approach

Abstract: The landing of an aircraft is one of the most critical operations because it directly affects the passenger safety and comfort. During landing, the aircraft fuselage undergoes excessive vibrations that cause the safety and the comfort problem and hence need to be suppressed quickly. A semi-active control system of a landing gear suspension by using Magnetorheological damper can solve the problem of excessive vibrations effectively. In this paper, a switching technique is developed in the simulation of the landing procedure which enables the system to switch from the single degree of freedom to three degrees of freedom system in order to simulate the sequential touching of the two wheels of the main landing gears and the nose landing gear wheels with the ground. A semi-active Magnetorheological damper is developed using two different controllers namely linear quadratic regulator and the H∞. Spencer model is used to predict the dynamic behavior of the Magnetorheological damper. The results of the designed controllers are compared to study the performance of the controllers in reducing the overshoot of the bounce response as well as the bounce rate response. The simulation results validated the improved performance of the robust controller compared to the optimal control strategy when the aircraft is subjected to the disturbances during landing.

In Vitro Study of Oscillatory Growth Dynamics of Camellia Pollen Tubes in Microfluidic Environment

Abstract: A hallmark of tip-growing cells such as pollen tubes and fungal hyphae is their oscillatory growth dynamics. The multiple aspects of this behavior have been studied to identify the regulatory mechanisms that drive the growth in walled cells. However, the limited temporal and spatial resolution of data acquisition has hitherto prevented more detailed analysis of this growth behavior. To meet this challenge, we employed a microfluidic device that is able to trap pollen grains and to direct the growth of pollen tubes along microchannels filled with liquid growth medium. This enabled us to observe the growth behavior of Camellia pollen tubes without the use of the stabilizer agarose and without risking displacement of the cell during time lapse imaging. Using an acquisition interval of 0.5 s, we demonstrate the existence of primary and secondary peak frequencies in the growth dynamics. The effect of sucrose concentration on the growth dynamics was studied through the shift in these peak frequencies indicating the pollen tube's ability to modulate its growth activity.

Robust Control of Collaborative Manipulators - Flexible Object System

Abstract: In many manufacturing and automobile industries, flexible components need to be positioned with the help of coordinated operations of manipulators. This paper deals with the robust design of a control system for two planar rigid manipulators moving a flexible object in the prescribed trajectory while suppressing the vibration of the flexible object. Dynamic equations of the flexible object are derived using the Hamiltonian principle, which is expressed as a partial differential equation (PDE) with appropriate boundary conditions. Then, a combined dynamics is formulated by combining the manipulators and object dynamics without any approximation. The resulting dynamics are thus described by the PDEs, having rigid as well as flexible parameters coupled together. This paper attempts to develop a robust control scheme without approximating the PDE in order to avoid measurements of flexible coordinates and their time derivatives. For this purpose, the two subsystems, namely slow and fast subsystems, are identified by using the singular perturbation technique. Specific robust controllers for both the subsystems are developed. In general, usage of the singular perturbation technique necessitates exponential stability of both subsystems, which is evaluated by satisfying Tikhnov's theorem. Hence, the exponential stability analysis is performed for both subsystems. Focusing on two three-link manipulators holding a flexible beam, simulations are performed and simulation results demonstrate the versatility of the proposed robust composite control scheme.

Equivalent area nonlinear static and dynamic analysis of electrostatically actuated microstructures

Abstract: Nonlinear dynamic investigation of electrostatically actuated micro-electro-mechanical-system (MEMS) microcantilever structures is presented. The nonlinear analysis aims to better quantify, than the linear model, the instability threshold associated with electrostatically actuated MEMS structures, where the pull-in voltage of the microcantilever is determined using a phase portrait analysis of the microsystem. The microcantilever is modeled as a lumped mass-spring system. The nonlinear electrostatic force is incorporated into the lumped microsystem through an equivalent area of the microcantilever for a given electrostatic potential. Electro-mechanical force balance plots are obtained for various electrostatic potentials from which the static equilibrium positions of the microcantilever are obtained and the respective conservative energy values are determined. Subsequently, phase portrait plots are obtained for the corresponding energy values from which the pull-in voltage is estimated for the microsystem. This pull-in voltage value is in good agreement with the previously published results for the same geometric and material parameters. The results obtained for linear electrostatic models are also presented for comparison.