Mechanical properties of ester- and ether-DPhPC bilayers: A molecular dynamics study.

In addition to its biological importance, DPhPC lipid bilayers are widely used in droplet bilayers, study of integral membrane proteins, drug delivery systems as well as patch-clamp electrophysiology of ion channels, yet their mechanical properties are not fully measured. Herein, we examined the effect of the ether linkage on the mechanical properties of ester- and ether-DPhPC lipid bilayers using all-atom molecular dynamics simulation. The values of area per lipid, thickness, intrinsic lateral pressure profile, order parameter, and elasticity moduli were estimated using various computational frameworks and were compared with available experimental values. Overall, a good agreement was observed between the two. The global properties of the two lipid bilayers are vastly different, with ether bilayer being stiffer, less ordered, and thicker than ester bilayer. Moreover, ether linkage decreased the area per lipid in the ether lipid bilayer. Our computational framework and output demonstrate how ether modification changes the mechano-chemical properties of DPhPC bilayers.

Dynamical modeling of manipulation process in Trolling-Mode AFM.

Dynamical lumped modeling of Trolling-mode AFM in manipulation of bio-samples is presented. The combination of high accuracy and compatibility with physiological conditions makes AFM a unique tool for studying biological materials in liquid medium. However, AFM microcantilever suffers from severe sensitivity degradation and noise intensification while operating in liquid; the large hydrodynamic drag between the cantilever and the surrounding liquid overwhelms the tip-sample interaction forces that are important in controlling the process. Therefore, an appropriate nanoneedle should be long enough to keep the cantilever out of liquid medium and short enough to be able to transmit the required force to push nanoparticle. Nonetheless, a long nanoneedle may deflect under the pushing force; therefore, its bending deflection should be accounted for in governing equations. Moreover, analytical and finite element stress analysis of nanoneedle and cantilever is performed to assure about their selected material and geometry. JKR theory is utilized to model contact mechanics between the needle/surface and the particle. Drag and meniscus forces are used to model the liquid media. Governing equations are solved using ODE45 and the system behavior is simulated. Critical conditions of sliding including critical time and force are obtained and changes of pushing force, needle deflection and indentation depths are illustrated. Also, effects of velocity variations are observed. Then, different heights for nanoneedle are tested and an appropriate one is selected for our purpose (to keep the needle out of liquid and transmit the force appropriately). The simulation is repeated for various biological particles and their behaviors are studied. At the end, the present simulation is validated through comparing the results with a previous work. This comparison shows that the simulation is reliable for the intended purpose.

Finite element modeling of trolling-mode AFM.

Trolling mode atomic force microscopy (TR-AFM) has overcome many imaging problems in liquid environments by considerably reducing the liquid-resonator interaction forces. The finite element model of the TR-AFM resonator considering the effects of fluid and nanoneedle flexibility is presented in this research, for the first time. The model is verified by ABAQUS software. The effect of installation angle of the microbeam relative to the horizon and the effect of fluid on the system behavior are investigated. Using the finite element model, frequency response curve of the system is obtained and validated around the frequency of the operating mode by the available experimental results, in air and liquid. The changes in the natural frequencies in the presence of liquid are studied. The effects of tip-sample interaction on the excitation of higher order modes of the system are also investigated in air and liquid environments.

Dynamic modeling of trolling-mode AFM: Considering effects of cantilever torsion, nanoneedle flexibility and liquid-nanoneedle interactions.

Trolling mode atomic force microscope (TR-Mode AFM) significantly reduces the hydrodynamic drag generated during operation in liquid environments. This is achieved by utilizing a long nanoneedle and keeping the cantilever out of liquid. In this research, a continuous mathematical model is developed to study TR-Mode AFM dynamics near a sample submerged in the liquid. Effects of cantilever torsion, nanoneedle flexibility, and liquid-nanoneedle interactions are considered in the model. In order to derive the equations of motion, Hamilton's principle and assumed mode method are used. System operation in dynamic mode is numerically simulated and the accuracy of the results is verified by comparison with the results of the finite element method. Displacements of different components of the system are also compared with each other and the dominant displacements are determined.

Smart Micro/Nano-robotic Systems for Gene Delivery.

BACKGROUND: Small scale robotics have attracted growing attention for the prospect of targeting and accessing cell-sized sites, necessary for high precision biomedical applications and drug/gene delivery. The loss of controlled gene therapy, inducing systemic side effects and reduced therapeutic efficiency, can be settled utilizing these intelligent carriers. METHODS: Newly proposed solutions for the main challenges of control, power supplying, gene release and final carrier extraction/degradation have shifted these smart miniature robots to the point of being employed for practical applications of transferring oligonucleotides (pDNA, siRNA, mRNA, etc.) in near future. CONCLUSION: In this paper, different scenarios and their endeavors to address the vital working demands and steps, in particular, carrier attachment and release, cell internalization, manipulation concerns as well as actuation systems are discussed.This review highlights some promising experimental results showing controlled gene release of robotic systems in comparison with current non-specific gene delivery methods.

Dynamics of the nanoneedle probe in trolling mode AFM.

Atomic force microscopy (AFM), as an indispensable tool for nanoscale characterization, presents major drawbacks for operation in a liquid environment arising from the large hydrodynamic drag on the vibrating cantilever. The newly introduced 'Trolling mode' (TR-mode) AFM resolves this complication by using a specialized nanoneedle cantilever that keeps the cantilever outside of the liquid. Herein, a mechanical model with a lumped mass was developed to capture the dynamics of such a cantilever with a nanoneedle tip. This new developed model was applied to investigate the effects of the needle-liquid interface on the performance of the AFM, including the imaging capability in liquid.

Influence of the tip mass on the tip-sample interactions in TM-AFM.

This paper focuses on the influences of the tip mass ratio (the ratio of the tip mass to the cantilever mass), on the excitation of higher oscillation eigenmodes and also on the tip-sample interaction forces in tapping mode atomic force microscopy (TM-AFM). A precise model for the cantilever dynamics capable of accurate simulations is essential for the investigation of the tip mass effects on the interaction forces. In the present work, the finite element method (FEM) is used for modeling the AFM cantilever to consider the oscillations of higher eigenmodes oscillations. In addition, molecular dynamics (MD) is used to calculate precise data for the tip-sample force as a function of tip vertical position with respect to the sample. The results demonstrate that in the presence of nonlinear tip-sample interaction forces, the tip mass ratio plays a significant role in the excitations of higher eigenmodes and also in the normal force applied on the surface. Furthermore, it has been shown that the difference between responses of the FEM and point-mass models in different system operational conditions is highly affected by the tip mass ratio.

Effects of higher oscillation modes on TM-AFM measurements.

The finite element method and molecular dynamics simulations are used for modeling the AFM microcantilever dynamics and the tip-sample interaction forces, respectively. Molecular dynamics simulations are conducted to calculate the tip-sample force data as a function of tip height at different lateral positions of the tip with respect to the sample. The results demonstrate that in the presence of nonlinear interaction forces, higher eigenmodes of the microcantilever are excited and play a significant role in the tip and sample elastic deformations. Using comparisons between the results of FEM and lumped models, how some aspects of the system behavior can be hidden when the point-mass model is used is illustrated.

Robust adaptive backstepping control of uncertain Lorenz system.

In this paper, a novel robust adaptive control method is proposed for controlling the Lorenz chaotic attractor. A new backstepping controller for the Lorenz system based on the Lyapunov stability theorem is proposed to overcome the singularity problem that appeared in using the typical backstepping control method. By exploiting the property of the system, the resulting controller is shown to be singularity free and the closed loop system is globally stable. Due to unavailability of system states measurement in practice, the controller is selected such that only one system state is needed. To overcome the problem of parameter uncertainty, an additional term to Lyapunov function is added and an identification scheme is adopted to have a negative definite Lyapunov function derivative. The simulation results demonstrate the effectiveness of the proposed controllers and approaches.