Explicit finite element implementation of a shape memory alloy constitutive model and associated analyses

Shape memory alloys (SMA) represent an important class of smart metallic materials employed in various innovative applications thanks to their unique thermomechanical behavior. Since the 1980s, several SMA constitutive models have been proposed and implemented into both commercial and academic finite element analysis software tools. Such models have demonstrated their reliability and robustness in the design and optimization of a wide variety of SMA-based components. However, most models are implemented using implicit integration schemes, thus limiting their applicability in highly nonlinear analyses. The objective of this work is to present a novel explicit integration scheme for the numerical implementation of the three-dimensional Souza-Auricchio model, a phenomenological model able to reproduce the primary SMA responses (i.e., pseudoelasticity and shape memory effect). The model constitutive equations are formulated by adopting the continuum thermodynamic theory with internal variables, following a plasticity-like approach. An elastic predictor-inelastic corrector scheme is here used to solve the time-discrete non-linear constitutive equations in the explicit framework. The proposed algorithm is investigated through several benchmark boundary-value problems of increasing complexity, considering both pseudoelastic and shape memory response in quasi-static conditions; a comparison with an implicit integration scheme is also performed. Such numerical tests demonstrate the ability of the algorithm to reproduce key material behaviors with effectiveness and robustness. Particularly, the analysis of SMA cables demonstrates the effectiveness of the explicit algorithm to solve complex problems involving widespread nonlinear contact, which prevent the convergence of the implicit scheme. Details such as mass-scaling options are also discussed

Multi-objective optimization of nitinol stent design

Nitinol stents continuously experience loadings due to pulsatile pressure, thus a given stent design should possess an adequate fatigue strength and, at the same time, it should guarantee a sufficient vessel scaffolding. The present study proposes an optimization framework aiming at increasing the fatigue life reducing the maximum strut strain along the structure through a local modification of the strut profile.The adopted computational framework relies on nonlinear structural finite element analysis combined with a Multi Objective Genetic Algorithm, based on Kriging response surfaces. In particular, such an approach is used to investigate the design optimization of planar stent cell.The results of the strut profile optimization confirm the key role of a tapered strut design to enhance the stent fatigue strength, suggesting that it is possible to achieve a marked improvement of both the fatigue safety factor and the scaffolding capability simultaneously. The present study underlines the value of advanced engineering tools to optimize the design of medical devices

Lamotrigine versus valproic acid as first-line monotherapy in newly diagnosed typical absence seizures: an open-label,randomized, parallel-group study

Purpose: To compare the efficacy of lamotrigine (LTG) and valproic acid (VPA) in newly diagnosed children and adolescents with typical absence seizures. Methods: A randomized, open-label parallel-group design was used. After undergoing an awake video-EEG recording, which included one to two trials of 3 min of hyperventilation and intermittent photic stimulation, eligible patients were randomized to receive LTG or VPA. LTG was initiated at a daily dose of 0.5 mg/kg for 2 weeks in two divided doses, followed by 1.0 mg/kg/day for an additional 2 weeks. Thereafter, doses were increased in 1-mg/kg/day increments every 5 days until seizures were controlled, intolerable adverse effects occurred, or a maximum dose of 12 mg/kg/day had been reached. VPA was equally uptitrated according to clinical response, starting at 10 mg/kg and increasing by 5 mg/kg/24 h every 3 days, if required, to a maximum of 30 mg/kg/day in three divided doses. Patients were seen in the clinic every month for ≤12 months.The primary efficacy end point at each visit was seizure freedom, defined as lack of clinically observed seizures since the previous visit and lack of electroclinical seizures during ambulatory 24-h EEG testing and a video-EEG session with hyperventilation. Results: Thirty-eight children (17 boys, 21 girls), aged from 3 to 13 years (mean, 7.5 years), all newly diagnosed with childhood or juvenile typical absence seizures, were enrolled. After 1 month of treatment, 10 (52.6%) of 19 children taking VPA and one (5.3%) of 19 taking LTG were seizure free (p = 0.004). By the 3-month follow-up, 12 (63.1%) children taking VPA and seven (36.8%) taking LTG were controlled (p = 0.19). After 12 months, 13 children taking VPA (dose range, 20–30 mg/kg/day; mean serum level, 76.8 mg/L; range, 51.4–91 mg/L) and 10 taking LTG (dose range, 2–11 mg/kg/day; mean serum level, 8.1 mg/L; range, 1.1–18 mg/L) were seizure free (p=0.51). Side effects were mostly mild and transient and were recorded in two (10.6%) children treated with VPA and in six (31.8%) treated with LTG. Conclusions: Both VPA and LTG can be efficacious against absence seizures, although VPA shows a much faster onset of action, at least in part because of its shorter titration schedule. KeyWords: Lamotrigine—Valproic acid—Typical absences— Monotherapy. Valproic acid (VPA) and ethosuximide (ESM) have been shown to be equally effective as monotherapy for typical absence seizures (1,2), and, at present, they are generally considered first-choice drugs for this seizure type. VPA controls absences in∼75% of patients, in addition to being effective against generalized tonic–clonic seizures (70%) and myoclonic seizures (75%). However, its use may involve safety risks for postmenarchal women (3). ESM produces complete control of absences in 70% of treated patients (4,5), but it is unsuitable as monotherapy Accepted Ma

Stress-Free Two-Way Shape Memory Effect of Poly(ethylene glycol)/ Poly(epsilon-caprolactone) Semicrystalline Networks

In this work, poly(ethylene glycol) (PEG)/poly(epsilon- caprolactone) (PCL) semicrystalline networks were prepared by photo-cross-linking of methacrylated macromonomers with different molecular weights and in different proportions to obtain amphiphilic materials capable of displaying properly designed shape memory effects. Networks based on PCL 10 kDa and PEG 3 kDa showed suitable thermal and mechanical properties with well-separated crystallization and melting regions to achieve a self-standing two-way shape memory effect. Particularly, after the application of a specific thermomechanical history, these materials are capable of cyclically changing their shape between two configurations upon cooling-heating cycles in the absence of any external load applied. The effect of the composition of the networks and of the employed thermomechanical parameters, such as the applied strain and the actuation temperature, was investigated to shed light on the shape memory mechanism for this class of materials, which are considered promising for applications in the biomedical field and as reversible actuators for soft robotics

Reversible Stress-Driven and Stress-Free Two-Way Shape Memory Effect in a Sol-Gel Crosslinked Polycaprolactone

The two-way shape memory effect is the ability of a material to change its shape between two configurations upon application and removal of a stimulus, and, among shape memory polymers, it is featured only by few systems, such as semicrystalline networks. When studied under tensile conditions, it consists of elongation-contraction cycles along cooling and heating across the crystallization and melting region, typically under the application of a constant load. However, recent studies on crosslinked semicrystalline co-polymers demonstrate that also a completely stress-free, or self-sustained, two-way effect may be achieved through specific thermomechanical cycles. This effect is currently regarded with interest for the development of intrinsically reversible sensors and actuators, and it may also be displayed by simpler materials, as homopolymer-based semicrystalline networks. Only seldom articles investigate this possibility, therefore in this work the two-way shape memory behavior is studied on a poly(e-caprolactone) system, crosslinked by means of a sol-gel approach. The effect is studied both under stress-driven and stress-free condition, by applying properly set-up thermo-mechanical histories. The results allow to describe the effect as a function of temperature, to reveal the dependence on specific testing parameters and to compare the extent of the reversible strain variation under these two conditions

A novel layered topology of auxetic materials based on the tetrachiral honeycomb microstructure

Microstructured honeycomb materials may exhibit exotic, extreme and tailorable mechanical properties, suited for innovative technological applications in a variety of modern engineering fields. The paper is focused on analysing the directional auxeticity of tetrachiral materials, through analytical, numerical and experimental methods. Theoretical predictions about the global elastic properties have been successfully validated by performing tensile laboratory tests on tetrachiral samples, realized with high precision 3D printing technologies. Inspired by the kinematic behaviour of the tetrachiral material, a newly-design bi-layered topology, referred to as bi-tetrachiral material, has been theoretically conceived and mechanically modelled. The novel topology virtuously exploits the mutual collaboration between two tetrachiral layers with opposite chiralities. The bi-tetrachiral material has been verified to outperform the tetrachiral material in terms of global Young modulus and, as major achievement, to exhibit a remarkable auxetic behaviour. Specifically, experimental results, confirmed by parametric analytical and computational analyses, have highlighted the effective possibility to attain strongly negative Poisson ratios, identified as a peculiar global elastic property of the novel bi-layered topology

447. AP20187-Inducible Insulin-Like Effects in Diabetic Muscle and Liver Transduced with AAV

Diabetes Mellitus, characterized by insulin deficiency (type I) or resistance (type II), derives from insulin action impairments in hormone target tissues: muscle, liver and adipocytes. Insulin regulates metabolism and glucose homeostasis through binding to a specific membrane receptor (IR) with tyrosine kinase activity. Induction of the insulin receptor signaling in hormone target cells may represent a tool to rescue glucose homeostasis in both insulin and insulin receptor deficiencies. Recently we have described that homodimerization of the chimeric insulin receptor LFv2IRE induced by the small dimerizer drug AP20187 results in insulin like actions in hepatocytes trasduced with adeno-associated viral vectors (AAV)