Mechanical and Compositional Implications of Gallium Ion Milling on Epoxy Resin.

Focused Ion Beam (FIB) is one of the most common methods for nanodevice fabrication. However, its implications on mechanical properties of polymers have only been speculated. In the current study, we demonstrated flexural bending of FIB-milled epoxy nanobeam, examined in situ under a transmission electron microscope (TEM). Controllable displacement was applied, while real-time TEM videos were gathered to produce morphological data. EDS and EELS were used to characterize the compositions of the resultant structure, and a computational model was used, together with the quantitative results of the in situ bending, to mechanically characterize the effect of Ga+ ions irradiation. The damaged layer was measured at 30 nm, with high content of gallium (40%). Examination of the fracture revealed crack propagation within the elastic region and rapid crack growth up to fracture, attesting to enhanced brittleness. Importantly, the nanoscale epoxy exhibited a robust increase in flexural strength, associated with chemical tempering and ion-induced peening effects, stiffening the outer surface. Young's modulus of the stiffened layer was calculated via the finite element analysis (FEA) simulation, according to the measurement of 30 nm thickness in the STEM and resulted in a modulus range of 30-100 GPa. The current findings, now established in direct measurements, pave the way to improved applications of polymers in nanoscale devices to include soft materials, such as polymer-based composites and biological samples.

Nonlinear multiscale analysis of coronary atherosclerotic vulnerable plaque artery: fluid-structural modeling with micromechanics.

A unique three-dimensional (3D) computational multiscale modeling approach is proposed to investigate the influence of presence of microcalcification particles on the stress field distribution in the thin cap layer of a coronary atherosclerotic vulnerable plaque system. A nested 3D modeling analysis framework spanning the multiscale nature of a coronary atherosclerotic vulnerable plaque is presented. At the microscale level, a micromechanical modeling approach, which is based on computational finite-element (FE) representative unit cell, is applied to obtain the homogenized nonlinear response of the calcified tissue. This equivalent response effectively allows the integration of extremely small microcalcification inclusions in a global biomechanical FE model. Next, at the macroscale level, a 3D patient-based fluid-structure interaction FE model, reconstructing a refined coronary artery geometry with calcified plaque lesion, is generated to study the mechanical behavior of such multi-component biomechanical system. It is shown that the proposed multiscale modeling approach can generate a higher resolution of stress and strain field distributions within the coronary atherosclerotic vulnerable plaque system and allow the assessment of the local concentration stress around the microcalcifications in plaque cap layers. A comparison of stress field distributions within cap layers with and without inclusion of microcalcifications is also presented.

Hyperelastic mechanical behavior of chitosan hydrogels for nucleus pulposus replacement-experimental testing and constitutive modeling.

Chitosan hydrogels (CHs) have been considered as a potential implant material for replacement and repair of the Nucleus Pulposus (NP) within the intervertebral disk. The nonlinear mechanical behavior of a CH material is investigated experimentally and computationally in this study. A series of confined and unconfined compression tests are designed and conducted for this hydrogel. Hyperelastic strain energy density functions (SEDFs) are calibrated using the experimental data. A hyperelastic constitutive model is selected to best fit the multi-axial behavior of the hydrogel. Its general prediction ability is verified using finite element (FE) simulations of hydrogel indentation experiments conducted using a spherical tip indentor. In addition, digital image correlation (DIC) technique is also used in the indentation test in order to process the full-field surface strains where the indentor contacts the hydrogel. The DIC test results in the form of top-surface strains compared well with those predicted by the FE model. Results show repeatability for the examined specimens under the applied tests. Confined and unconfined test results are found to be sufficient to calibrate the SEDFs. The Ogden model was selected to represent the nonlinear behavior of the CH material which can be used in future biomechanical simulations of the spine.