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Finite element simulation is used to analysis stent designs, extension as well as interaction between a stent and a vessel. In this paper, two different stents with different geometries have been simulated. One is Zilver stent and the other one is Navalis stent. The aim of this study is to determine the effect of stents deployment with various designs that are made of shape memory alloy (SMA) on the distribution of vessel wall stresses by using computational modeling approach. The constitutive model which described the behavior of SMA is based on Microplane model. In addition, SMA stents have been simulated under torsion loading to compare the flexibility of various designs under different conditions. The superelastic behavior and shape memory effect of SMA stents are investigated in this paper. The numerical simulation results show the different geometries of stents have significant effect on the arterial wall. The results show the Navalis stent causes less stress on the arterial wall and it is more flexible than the Zilver stent under the same torsion loading. [ABSTRACT FROM AUTHOR]

Transcatheter aortic valve implantation is a minimally invasive treatment for severe symptomatic aortic valve stenosis. Nitinol stents are proposed for aortic stenosis patients at high risk. In the present study, at different implantation depths in the aortic valve, the crimping and performance of Nitinol stents are investigated. To do so, a constitutive model based on Microplane theory is utilized and implemented through the finite element to express the constitutive characteristics of Nitinol. The self-expanding stent made of NiTi is designed and simulated using the finite element method. To validate the developed model, the obtained results using beam and solid finite element models are compared with those reported in the literature. Superelastic behavior as well as shape memory effect of the Nitinol stent is studied during crimping and deployment. The simulated results show that the produced radial force increases by increasing the implantation depth in a cardiac cycle. [ABSTRACT FROM AUTHOR]

Magnesium (Mg) and its alloys can degrade gradually up to complete dissolution in the physiological environment. This property makes these biomaterials appealing for different biomedical applications, such as bone implants. In order to qualify Mg and its alloys for bone implant applications, there is a need to precisely model their degradation (corrosion) behavior in the physiological environment. Therefore, the primary objective develop a model that can be used to predict the corrosion behavior of Mg-based alloys in vitro, while capturing the effect of pitting corrosion. To this end, a customized FORTRAN user material subroutine (or VUMAT) that is compatible with the finite element (FE) solver Abaqus/Explicit (Dassault Systèmes, Waltham, MA, USA) was developed. Using the developed subroutine, a continuum damage mechanism (CDM) FE model was developed to phenomenologically estimate the corrosion rate of a biocompatible Mg-Zn-Ca alloy. In addition, the mass loss immersion test was conducted to measure mass loss over time by submerging Mg-Zn-Ca coupons in a glass reactor filled with simulated body fluid (SBF) solution at pH 7.4 and 37 °C. Then, response surface methodology (RSM) was applied to calibrate the corrosion FE model parameters (i.e., Gamma (γ), Psi (ψ), Beta (ψ), and kinetic parameter (Ku)). The optimum values for, ψ, ψ and Ku were found to be 2.74898, 2.60477, 5.1, and 0.1005, respectively. Finally, given the good fit between FE predictions and experimental data, it was concluded that the numerical framework precisely captures the effect of corrosion on the mass loss over time. [ABSTRACT FROM AUTHOR]

Biocompatible stent implants that have been made of shape memory alloys (SMA) are being used in arteries which are restoring normal blood flow. A high failure rate of stent implants in femoral artery causes the investigation of the mechanical behavior of stent implants, and its design and manufacturing, a necessity study. In this paper, two different stent designs, with different geometries, have been simulated. One is a Diamond-shaped profile stent and the other one is a V-shaped profile stent. These stents have been simulated under different loadings such as tensile and bending loadings that are very similar to the loading environment imposed by the arterial wall and blood flow. Two different temperatures have been selected to investigate superelasticity as well as shape memory effect of NiTi stents. During unloading for the shape memory case, the residual strains are recovered by heating the stents. The verified model based on microplane model is numerically considered for simulation of the stents. Numerical results show that the V-shaped stent design stretches more than the Diamond-shaped stent design under tensile loading. In addition, the V-shaped stent bends more than Diamond-shaped stent under the same bending loading, which shows more flexibility. [ABSTRACT FROM AUTHOR]

High-temperature shape memory alloys have remarkably solved the limitation of conventional shape memory alloys working at low levels of temperature and stress. The focus of this study is on a semi-analytical constitutive model for high-temperature shape memory alloys to predict their mechanical behavior. The model is based on Gibbs free energy, and the equations are simplified for NiTiHf at different loading conditions. A uniaxial compression case is used to validate the model with the experimental results. Superelastic behavior and also heating and cooling at different stress levels are compared to the experimental data reported in the literature. The nonlinear equations are solved using finite difference method, which is capable to calculate the distribution of strain and phase transformation along the cross section for various loading modes. Several more case studies on pure torsion and multiaxial loading are also investigated to show the capabilities of this approach. [ABSTRACT FROM AUTHOR]

In this study, a three-dimensional thermomechanical constitutive model based on the microplane theory is proposed to simulate the behavior of shape memory alloy tubes. The three-dimensional model is implemented in ABAQUS by employing a user material subroutine. In order to validate the model, the numerical results of this approach are compared with new experimental findings for a NiTi superelastic torque tube under tension, pure torsion, and proportional tension–torsion performed in stress- and strain-controlled manners. The numerical and experimental results are in agreement indicating the capability of the proposed microplane model in capturing the behavior of shape memory alloy tubes. This model is capable of predicting both superelasticity and shape memory effect by providing closed-form relationships for calculating the strain components in terms of the stress components. [ABSTRACT FROM PUBLISHER]

Gelatin films were prepared from gelatin solutions (10% w/v) containing carvacrol (1%, 2%, 3%, 4% and 5% w/w), glycerol (25% w/w) as plasticizer and glutaraldehyde (1% w/w) as cross-linker. The mechanical, water solubility, water swelling, water uptake, water vapor permeability, antioxidant and antibacterial properties of the films were obtained according to American Society for Testing and Materials. Gelatin films exhibited good tensile strength, elongation at break, water solubility, swelling, water uptake and water vapor permeability. Incorporation of carvacrol into the gelatin films caused a significant decrease in tensile strength, increase in elongation at break, increase in water solubility, decrease in swelling and water uptake, and increase in water vapor permeability of the films. Gelatin films exhibited low antioxidant activity while, gelatin films incorporated with carvacrol exhibited excellent antioxidant properties. The films incorporated with carvacrol also exhibited excellent antibacterial properties against both gram-positive and gram-negative bacteria. Practical Application Development of biodegradable films based on the protein polymer has become a very attractive option and production of protein based packaging with strong antioxidant and antibacterial activities is gradually obtained extensive concern in the world. In this study, the improvement of gelatin film properties incorporated with carvacrol as a potential antibacterial active packaging was investigated. Suitable physico-mechanical properties along with strong antioxidant and antibacterial effects make gelatin films incorporated with carvacrol as suitable food packaging material. [ABSTRACT FROM AUTHOR]

A three-dimensional phenomenological model based on microplane theory is extended to capture the coupling effects between tension and torsion in complex multiaxial loadings. Inelastic strain in a microplane approach is a component of transformation strain and anisotropic strain. Since the anisotropy effect is induced during martensitic transformation, anisotropic strain is defined as a function of transformation strain. Out-of-plane strain is induced in simple tension and pure torsion in free-end conditions. Anisotropy tensor is experimentally extracted and is used in the proposed model to predict the behavior in multiaxial loading. The ability of this extended microplane model to predict the tension-torsion coupling effects as well as the induced transformation anisotropic behavior of NiTi shape memory alloys is demonstrated. In addition, the microplane model is modified to capture the asymmetric behavior in tension and compression during uniaxial and multiaxial loadings. To this end, numerical correlations between the results of the modified microplane model are compared with experimental results to demonstrate the validity of the extended model. [ABSTRACT FROM AUTHOR]