Sharif Digital Repository

We have studied the mechanical properties of a few-layer graphene cantilever (FLGC) using atomic force microscopy (AFM). The mechanical properties of the suspended FLGC over an open hole have been derived from the AFM data. Force displacement curves using the Derjaguin–Müller–Toporov (DMT) and the massless cantilever beam models yield a Young modulus of Ec ~ 37, Ea ~ 0.7 TPa and a Hamakar constant of ~ 3 × 10 − 18 J. The threshold force to shear the FLGC was determined from a breaking force and modeling. In addition, we studied a graphene nanoribbon (GNR), which is a system similar to the FLGC; using density functional theory (DFT). The in-plane Young's modulus for the GNRs were calculated... 

Dilational and shear viscoelasticities are important properties of interfacial layers. These quantities are particularly relevant in all systems which contain a huge internal interfacial area such as foams and emulsions. Therefore, also the 3D rheological behavior of foams or emulsions studied by respective methods is superimposed by the 2D interfacial rheology.We report on recent developments in dilational and shear rheology from an experimental point of view as well as discuss the state of the art of the underlying theories. Examples of most relevant experiments are also presented and discussed. Although not yet extensively investigated, the links between bulk rheology of foams and... 

Controlling topographic features at all length scales is of great importance for the interaction of cells with tissue regenerative materials. We utilized an indirect three-dimensional printing method to fabricate polymeric scaffolds with pre-defined and controlled external and internal architecture that had an interconnected structure with macro- (400-500 μm) and micro- (∼25 μm) porosity. Polycaprolactone (PCL) was used as model system to study the kinetics of tissue growth within porous scaffolds. The surface of the scaffolds was decorated with TiO2 and bioactive glass (BG) nanoparticles to the better match to nanoarchitecture of extracellular matrix (ECM). Micrometric BG particles were... 

Finite element analysis is an effective tool to evaluate the material properties of living tissue. For an interactive optimization procedure, the finite element analysis usually needs many simulations to reach a reasonable solution. The metamodel analysis of finite element simulation can be used to reduce the computation of a structure with complex geometry or a material with composite constitutive equations. The intervertebral disc is a complex, heterogeneous, and hydrated porous structure. A poroelastic finite element model can be used to observe the fluid transferring, pressure deviation, and other properties within the disc. Defining reasonable poroelastic material properties of the... 

In the present research, the introduction of multi-walled carbon nanotubes (MWCNTs) into the hydroxyapatite (HA) matrix and dip coating of nanocomposite on titanium alloy (Ti-6Al-4V) plate was conducted in order to improve the performance of the HA-coated implant via the sol-gel method. The structural characterization and electron microscopy results confirmed well crystallized HA-MWCNT coating and homogenous dispersion of carbon nanotubes in the ceramic matrix at temperatures as low as 500 C. The evaluation of the mechanical properties of HA and HA/MWCNT composite coatings with different weight percentages of MWCNTs showed that the addition of low concentrations of MWCNTs (0.5 and 1 wt.%)... 

Low back pain is a common reason for activity limitation in people younger than 45 years old, and was proved to be associated with heavy physical works, repetitive lifting, impact, stationary work postures and vibrations. The study of load transferring and the loading condition encountered in spinal column can be simulated by finite element models. The intervertebral disc is a structure composed of a porous material. Many physical models were developed to simulate this phenomenon. The confounding effects of poroelastic properties and loading conditions on disc mechanical responses are, nevertheless, not cleared yet. The objective of this study was to develop an axisymmetric poroelastic... 

In percutaneous applications, needle insertion into soft tissue is considered as a challenging procedure, and hence, it has been the subject of many recent studies. This study considers a model-based dynamics equation to evaluate the needle movement through prostate soft tissue. The proposed model estimates the applied force to the needle using the tissue deformation data and finite element model of the tissue. To address the role of mechanical properties of the soft tissue, an inverse dynamics control method based on sliding mode approach is used to demonstrate system performance in the presence of uncertainties. Furthermore, to deal with inaccurate estimation of mechanical parameters of... 

The aim of this study was to use finite element analysis (FEA) to assess the influence of microthread design at the implant neck on stress distribution in the surrounding bone. A commercially available implant with 3.5 mm diameter and 10.5 mm length was selected and used as a model. For the purpose of designing the microthread implant model, microthreads were added to the implant neck in a computerized model. A force measuring 100 N was then applied to the entire surface of the abutment in the vertical direction. The results showed that in both models, stress was mainly concentrated at the cortical bone adjacent to the neck of the implant. Maximum stress values in the cortical bone... 

The meshless element-free Galerkin method was generalized and an algorithm was developed for 3D dynamic modeling of deformable bodies in real time. The efficacy of the algorithm was investigated in a 3D linear viscoelastic model of human spleen subjected to a time-varying compressive force exerted by a surgical grasper. The model remained stable in spite of the considerably large deformations occurred. There was a good agreement between the results and those of an equivalent finite element model. The computational cost, however, was much lower, enabling the proposed algorithm to be effectively used in real-time applications  

In this paper, a 4-DOF robotic arm for tool handling in laparoscopic surgery is introduced. The robot provides sufficient force to handle endoscopic tools used for large organ manipulation and is capable of measuring the tool-tissue forces. The RCM constraint is achieved using a spherical mechanism and roll and insertion motions are provided using time pulley and spindle-drive, respectively. The forward and inverse kinematics of the robot was solved and the dimensions of its links were determined, using particle swarm optimization method, so that the maximum kinematic and dynamic performance could be achieved  

In this paper, the elastic properties of monomeric actin (G-actin) and the trimer nucleus (G-actin trimer) in different states of nucleotide binding are estimated using steered molecular dynamic (SMD) simulations. Three nucleotide binding states are considered: ADP- and ATP-bound actin and nucleotide-free actin assemblies. Our results show that nucleotide binding and the corresponding changes in structure have significant effects on the mechanical behaviors of actin assemblies. Simulations reveal that the deformation behavior of G-actin monomers is generally elastic up to engineering strains of 16 and 40% in the tension and shear tests, respectively. In addition, the G-actin trimers react... 

The aim of this study was to develop a more sophisticated model of the spleen tissue and investigate its interactions with a three-fingered laparoscopic grasper. The spleen tissue, modeled as a hyper viscoelastic material, was subjected to external loadings, imposed by rigid grasping jaws. The tissue deformation as well as the sliding occurrence between tissue and jaws was investigated using nonlinear finite element method. Results indicated that a grasping configuration which aimed a sufficiently large piece of spleen with small radius of curvature was more successful for effective grasping. The trends and magnitudes of the tooltissue interaction forces obtained during effective and... 

Four derivatives of hyaluronic acid (HA) bearing thiol (HA-SH), hydrazide (HA-hy), 2-dithiopyridyl (HA-SSPy), and aldehyde groups (HA-al) respectively were synthesized. Thiol and 2-dithiopyridyl as well as hydrazide and aldehyde make up two chemically orthogonal pairs of chemo-selective functionalities that allow in situ formation of interpenetrating (IPN) disulfide and hydrazone networks simultaneously upon the mixing of the above derivatives at once. The formation of IPN was demonstrated by comparing it with the formulations of the same total HA concentration but lacking one of the reactive components. The hydrogel composed of all four components was characterized by a larger elastic... 

Hydrogels made of natural polymers [chitosan (CS) and gelatin (G)] have been prepared having mechanical properties similar to those of muscle tissues. In this study, the effect of polymer concentration and scaffold stiffness on the behavior of seeded muscle-derived cells (MDCs) on the CS-G hydrogel sheets has been evaluated. Both variables were found to be important in cell viability. Viability was assessed by observation of the cell morphology after 1 day as well as a 14-day MTT assay. The CS-G hydrogels were characterized using Fourier transform infrared (FTIR) analysis, which revealed evidences of strong intermolecular interactions between CS and G. Hydrogel samples with intermediate... 

In this research, polypropylene (PP) nanocomposite films were prepared by melt intercalation method and their properties have been evaluated. To facilitate the formation of either intercalated or exfoliated nanocomposites, ethylene vinyl acetate copolymer (EVA) was used as a compatibiliser. Morphology of composites was determined by X-ray Diffraction (XRD) and transmission electron microscopy (TEM). XRD and TEM analyses confirmed that increasing of EVA content leads to achievement of intercalated nanocomposites. Furthermore, differential scanning calorimetry (DSC) measurement indicated a decrease in crystallinity, melting point and crystallization temperature. Also, permeability tests showed... 

The objective of this study was to propose a new analytical model for studying response of head impacts. Head is modeled by fluidfilled ellipsoidal shell of inconstant thickness impacted by a solid elastic sphere. Modeling the head as an ellipsoid is more realistic than modeling it as a sphere, the previous model existing in the literature [3]-[8]. In this model, the effect of Hertzian contact stiffness and local shell stiffness are combined to derive explicit equations for impact duration, the peak force transmitted to head, and the head injury criterion. One of the advantages of the model presented is sensitivity to the site of impact. A comparison between the present analytical results... 

In this article, a multiscale modeling procedure is implemented to study the effect of interphase on the Young's modulus of CNT/polymer composites. For this purpose, a three-phase RVE is introduced which consists of three components, i.e., a carbon nanotube, an interphase layer, and an outer polymer matrix. The nanotube is modeled at the atomistic scale using molecular structural mechanics. Moreover, three-dimensional elements are employed to model the interphase layer and polymer matrix. The nanotube and polymer matrix are assumed to be bonded by van der Waals interactions based on the Lennard-Jones potential at the interface. Using this Molecular Structural Mechanics/Finite Element... 

We have studied the mechanical properties of a few-layer graphene cantilever (FLGC) using atomic force microscopy (AFM). The mechanical properties of the suspended FLGC over an open hole have been derived from the AFM data. Force displacement curves using the Derjaguin-Müller-Toporov (DMT) and the massless cantilever beam models yield a Young modulus of Ec ∼ 37, Ea ∼ 0.7TPa and a Hamakar constant of ∼ 3 × 10 -18J. The threshold force to shear the FLGC was determined from a breaking force and modeling. In addition, we studied a graphene nanoribbon (GNR), which is a system similar to the FLGC; using density functional theory (DFT). The in-plane Young's modulus for the GNRs were calculated from... 

Molecular dynamics simulation was used to study of mechanical properties of NiTi/Cu bilayer by nanoindentation and tensile testing. A comparison has been made among mechanical properties measured and plastic deformation process at different copper thickness during nanoindnetation and tensile test of the samples. Embedded atom method potentials for describing of inter-Atomic interaction and Nose-Hoover thermostat and barostat are employed in the simulation at 400 K. The results showed that as the copper film thickness decreased, the maximum load and hardness values increased during nanoindetation. Saha and Nix model is used to describe reduced young's modulus behaviour of the bilayer system...