Farzad Khademolhosseini

Nonlocal Continuum Modeling and Molecular Dynamics Simulation of Torsional Vibration of Carbon Nanotubes

This paper investigates the size effects in the dynamic torsional response of single-walled carbon nanotubes (SWCNTs) by developing a modified nonlocal continuum shell model. The purpose is to facilitate the design of devices based on CNT torsion by providing a simple, accurate, and efficient continuum model that can predict the frequency of torsional vibrations and the propagation speed of torsional waves. To this end, dispersion relations of torsional waves are obtained from the proposed nonlocal model and compared to classical models. It is seen that the classical and nonlocal models predict nondispersive and dispersive behavior, respectively. Molecular dynamics simulations of torsional vibrations of (6, 6) and (10, 10) SWCNTs are also performed, the results of which are compared with the classical and nonlocal models and used to extract consistent values of the nonlocal elasticity constant. The superiority and accuracy of the nonlocal elasticity model in predicting the size-dependent dynamic torsional response of SWCNTs are demonstrated.

Fabrication of robust hydrogel coatings on polydimethylsiloxane substrates using micropillar anchor structures with chemical surface modification.

A durable hydrophilic and protein-resistant surface of polydimethylsiloxane (PDMS) based devices is desirable in many biomedical applications such as implantable and microfluidic devices. This paper describes a stable antifouling hydrogel coating on PDMS surfaces. The coating method combines chemical modification and surface microstructure fabrication of PDMS substrates. Three-(trimethoxysilyl)propyl methacrylates containing C═C groups were used to modify PDMS surfaces with micropillar array structures fabricated by a replica molding method. The micropillar structures increase the surface area of PDMS surfaces, which facilitates secure bonding with a hydrogel coating compared to flat PMDS surfaces. The adhesion properties of the hydrogel coating on PDMS substrates were characterized using bending, stretching and water immersion tests. Long-term hydrophilic stability (maintaining a contact angle of 55° for a month) and a low protein adsorption property (35 ng/cm(2) of adsorbed BSA-FITC) of the hydrogel coated PDMS were demonstrated. This coating method is suitable for PDMS modification with most crosslinkable polymers containing C═C groups, which can be useful for improving the anti-biofouling performance of PDMS-based biomedical microdevices.

Fabrication and Patterning of Magnetic Polymer Micropillar Structures Using a Dry-Nanoparticle Embedding Technique

Previously, solvent casting techniques have been used for the fabrication of magnetic polymer micropillar structures. These techniques provide very limited control over magnetic-particle placement, and particle agglomeration limits their use with highly viscous polymers such as polydimethylsiloxane. We report a new technique for the fabrication of magnetic polymer micropillars to overcome the aforementioned limitations. In this technique, magnetic micro-/nanoparticles are applied to a mold in their dry particulate state, omitting the need for the use of solvents. We demonstrate magnetic micropillars with uniform properties using high-viscosity polymers and iron nanoparticles. We show that simple modifications to the dry-nanoparticle embedding technique allow the embedding of other functional (nonmagnetic) particles inside the polymer micropillars, and we demonstrate patterning of the device. We present experimental results for the material composition, the magnetic properties, and the bending performance of our magnetic micropillar arrays. Compared to previously fabricated magnetic micropillars of similar dimensions and using lower magnitudes of externally applied magnetic fields and magnetic field gradients (286 mT, 41.45 mT/mm), our 8-μm-diameter 18-μm-high pillars produce an estimated maximum horizontal tip force of 0.33 ±0.08 μN, larger than the values previously reported.

A dry nanoparticle embedding technique for fabrication of magnetic polymer micropillars

Polymeric micropillar devices have been used for cell studies. Previously, solvent-casting techniques have been used for fabrication of magnetic micropillar structures. However, aggregation of magnetic particles limits the application of solvent-casting techniques with highly viscous polymers, such as polydimethylsiloxane (PDMS). We report a new fabrication technique for magnetic polymer micropillars. In this technique magnetic micro/nano-particles are used in their dry particulate state. We demonstrate magnetic micropillars with uniform properties using high viscosity polymers and coated iron nanoparticles.

Application of periodic loads on cells from magnetic micropillar arrays impedes cellular migration

This paper presents a study on the application of active micropillar surfaces to control cell migration. We present experimental results on the migration of confluent sheets of cells subject to periodic mechanical forces from actuated magnetic polymer micropillars. We show that in contrast to passive micropillar surfaces which cause no significant alterations in cell migration rates, active micropillar surfaces actuated at a frequency of 1 Hz can decrease cell migration rates by 80%. The magnetic micropillar structures presented can be actuated remotely with small magnetic fields making them a viable candidate for the development of smart materials for in vivo tissue engineering applications.

A magnetically actuated cellular strain assessment tool for quantitative analysis of strain induced cellular reorientation and actin alignment

Commercially available cell strain tools, such as pneumatically actuated elastomer substrates, require special culture plates, pumps, and incubator setups. In this work, we present a magnetically actuated cellular strain assessment tool (MACSAT) that can be implemented using off-the-shelf components and conventional incubators. We determine the strain field on the MACSAT elastomer substrate using numerical models and experimental measurements and show that a specific region of the elastomer substrate undergoes a quasi-uniaxial 2D stretch, and that cells confined to this region of the MACSAT elastomer substrate undergo tensile, compressive, or zero axial strain depending on their angle of orientation. Using the MACSAT to apply cyclic strain on endothelial cells, we demonstrate that actin filaments within the cells reorient away from the stretching direction, towards the directions of minimum axial strain. We show that the final actin orientation angles in strained cells are spread over a region of compressive axial strain, confirming previous findings on the existence of a varied pre-tension in the actin filaments of the cytoskeleton. We also demonstrate that strained cells exhibit distinctly different values of actin alignment coherency compared to unstrained cells and therefore propose that this parameter, i.e., the coherency of actin alignment, can be used as a new readout to determine the occurrence/extent of actin alignment in cell strain experiments. The tools and methods demonstrated in this study are simple and accessible and can be easily replicated by other researchers to study the strain response of other adherent cells.