Kirchhoff's law-based velocity-controlled motion models to predict real-time cutting forces in minimally invasive surgeries.

A theoretical framework, united by a "system effect" is formulated to model the cutting/haptic force evolution at the cutting edge of a surgical cutting instrument during its penetration into soft biological tissue in minimally invasive surgery. Other cutting process responses, including tissue fracture force, friction force, and damping, are predicted by the model as well. The model is based on a velocity-controlled formulation of the corresponding equations of motion, derived for a surgical cutting instrument and tissue based on Kirchhoff's fundamental energy conservation law. It provides nearly zero residues (absolute errors) in the equations of motion balances. In addition, concurrent closing relationships for the fracture force, friction coefficient, friction force, process damping, strain rate function (a constitutive tissue model), and their implementation within the proposed theoretical framework are established. The advantage of the method is its ability to make precise real-time predictions of the aperiodic fluctuating evolutions of the cutting forces and the other process responses. It allows for the robust modeling of the interactions between a medical instrument and a nonlinear viscoelastic tissue under any physically feasible working conditions. The cutting process model was partially qualitatively verified through numerical simulations and by comparing the computed cutting forces with experimentally measured values during robotic uniaxial biopsy needle constant velocity insertion into artificial gel tissue, obtained from previous experimental research. The comparison has shown a qualitatively similar adequate trend in the evolution of the experimentally measured and numerically predicted cutting forces during insertion of the needle.

Atomistic modeling of the mechanical properties: the rise of machine learning interatomic potentials.

Since the birth of the concept of machine learning interatomic potentials (MLIPs) in 2007, a growing interest has been developed in the replacement of empirical interatomic potentials (EIPs) with MLIPs, in order to conduct more accurate and reliable molecular dynamics calculations. As an exciting novel progress, in the last couple of years the applications of MLIPs have been extended towards the analysis of mechanical and failure responses, providing novel opportunities not heretofore efficiently achievable, neither by EIPs nor by density functional theory (DFT) calculations. In this minireview, we first briefly discuss the basic concepts of MLIPs and outline popular strategies for developing a MLIP. Next, by considering several examples of recent studies, the robustness of MLIPs in the analysis of the mechanical properties will be highlighted, and their advantages over EIP and DFT methods will be emphasized. MLIPs furthermore offer astonishing capabilities to combine the robustness of the DFT method with continuum mechanics, enabling the first-principles multiscale modeling of mechanical properties of nanostructures at the continuum level. Last but not least, the common challenges of MLIP-based molecular dynamics simulations of mechanical properties are outlined and suggestions for future investigations are proposed.

Modeling neuron growth using isogeometric collocation based phase field method.

We present a new computational framework of neuron growth based on the phase field method and develop an open-source software package called "NeuronGrowth_IGAcollocation". Neurons consist of a cell body, dendrites, and axons. Axons and dendrites are long processes extending from the cell body and enabling information transfer to and from other neurons. There is high variation in neuron morphology based on their location and function, thus increasing the complexity in mathematical modeling of neuron growth. In this paper, we propose a novel phase field model with isogeometric collocation to simulate different stages of neuron growth by considering the effect of tubulin. The stages modeled include lamellipodia formation, initial neurite outgrowth, axon differentiation, and dendrite formation considering the effect of intracellular transport of tubulin on neurite outgrowth. Through comparison with experimental observations, we can demonstrate qualitatively and quantitatively similar reproduction of neuron morphologies at different stages of growth and allow extension towards the formation of neurite networks.

A first-principles and machine-learning investigation on the electronic, photocatalytic, mechanical and heat conduction properties of nanoporous C5N monolayers.

Carbon nitride nanomembranes are currently among the most appealing two-dimensional (2D) materials. As a nonstop endeavor in this field, a novel 2D fused aromatic nanoporous network with a C5N stoichiometry has been most recently synthesized. Inspired by this experimental advance and exciting physics of nanoporous carbon nitrides, herein we conduct extensive density functional theory calculations to explore the electronic, optical and photocatalytic properties of the C5N monolayer. In order to examine the dynamic stability and evaluate the mechanical and heat transport properties under ambient conditions, we employ state of the art methods on the basis of machine-learning interatomic potentials. The C5N monolayer is found to be a direct band gap semiconductor, with a band-gap of 2.63 eV according to the HSE06 method. The obtained results confirm the dynamic stability, remarkable tensile strengths over 10 GPa and a low lattice thermal conductivity of ∼9.5 W m-1 K-1 for the C5N monolayer at room temperature. The first absorption peak of the single-layer C5N along the in-plane polarization is predicted to appear in the visible range of light. With a combination of high carrier mobility, appropriate band edge positions and strong absorption of visible light, the C5N monolayer might be an appealing candidate for photocatalytic water splitting reactions. The presented results provide an extensive understanding concerning the critical physical properties of the C5N nanosheets and also highlight the robustness of machine-learning interatomic potentials in the exploration of complex physical behaviors.

A 3D nano scale IGA for free vibration and buckling analyses of multi-directional FGM nanoshells.

This article explores a three-dimensional solid isogeometric analysis (3D-IGA) approach based on a nonlocal elasticity theory to investigate size effects on natural frequency and critical buckling load for multi-directional functionally graded (FG) nanoshells. The multi-directional FG material uses a power law rule with three power exponent indexes concerning three parametric coordinates. Nanoshell's geometries include the square plate, cylindrical and spherical panels with the side length considered in a nanoscale with various thickness ratios. Because 3D-IGA utilizes an approximation of NURBS basic functions to integrate from geometry modeling to discretized domain, it does not require any hypotheses for deformations distribution and stress component through the plate's thickness. Therefore, the results from the 3D solution are obtained accurately with any thickness ratio of the shells. The numerical solutions are verified by those published in several pieces of literature to determine the current approach's accuracy and reliability. After a convergence solution is examined, a quartic NURBS basic function can yield ultra-converged and high-accurate results with a low computational cost. The findings show the size effect parameters which significantly impact the frequencies and the critical buckling factors of the multi-directional FG nanoshells. Generally, increases in the size effect parameters will cause declines in the frequencies and the critical buckling factors of the nanoshells.

First-Principles Multiscale Modeling of Mechanical Properties in Graphene/Borophene Heterostructures Empowered by Machine-Learning Interatomic Potentials.

Density functional theory calculations are robust tools to explore the mechanical properties of pristine structures at their ground state but become exceedingly expensive for large systems at finite temperatures. Classical molecular dynamics (CMD) simulations offer the possibility to study larger systems at elevated temperatures, but they require accurate interatomic potentials. Herein the authors propose the concept of first-principles multiscale modeling of mechanical properties, where ab initio level of accuracy is hierarchically bridged to explore the mechanical/failure response of macroscopic systems. It is demonstrated that machine-learning interatomic potentials (MLIPs) fitted to ab initio datasets play a pivotal role in achieving this goal. To practically illustrate this novel possibility, the mechanical/failure response of graphene/borophene coplanar heterostructures is examined. It is shown that MLIPs conveniently outperform popular CMD models for graphene and borophene and they can evaluate the mechanical properties of pristine and heterostructure phases at room temperature. Based on the information provided by the MLIP-based CMD, continuum models of heterostructures using the finite element method can be constructed. The study highlights that MLIPs were the missing block for conducting first-principles multiscale modeling, and their employment empowers a straightforward route to bridge ab initio level accuracy and flexibility to explore the mechanical/failure response of nanostructures at continuum scale.

A Multiscale Investigation on the Thermal Transport in Polydimethylsiloxane Nanocomposites: Graphene vs. Borophene.

Graphene and borophene are highly attractive two-dimensional materials with outstanding physical properties. In this study we employed combined atomistic continuum multi-scale modeling to explore the effective thermal conductivity of polymer nanocomposites made of polydimethylsiloxane (PDMS) polymer as the matrix and graphene and borophene as nanofillers. PDMS is a versatile polymer due to its chemical inertia, flexibility and a wide range of properties that can be tuned during synthesis. We first conducted classical Molecular Dynamics (MD) simulations to calculate the thermal conductance at the interfaces between graphene and PDMS and between borophene and PDMS. Acquired results confirm that the interfacial thermal conductance between nanosheets and polymer increases from the single-layer to multilayered nanosheets and finally converges, in the case of graphene, to about 30 MWm-2 K-1 and, for borophene, up to 33 MWm-2 K-1. The data provided by the atomistic simulations were then used in the Finite Element Method (FEM) simulations to evaluate the effective thermal conductivity of polymer nanocomposites at the continuum level. We explored the effects of nanofiller type, volume content, geometry aspect ratio and thickness on the nanocomposite effective thermal conductivity. As a very interesting finding, we found that borophene nanosheets, despite having almost two orders of magnitude lower thermal conductivity than graphene, can yield very close enhancement in the effective thermal conductivity in comparison with graphene, particularly for low volume content and small aspect ratios and thicknesses. We conclude that, for the polymer-based nanocomposites, significant improvement in the thermal conductivity can be reached by improving the bonding between the fillers and polymer, or in other words, by enhancing the thermal conductance at the interface. By taking into account the high electrical conductivity of borophene, our results suggest borophene nanosheets as promising nanofillers to simultaneously enhance the polymers' thermal and electrical conductivity.

Evaluation of the efficiency of various force configurations on scoliotic, lordotic and kyphotic curves in the subjects with scoliosis.

BACKGROUND: The efficiency of the braces designed for scoliotic subjects depends on configurations and also magnitudes of the forces used to stabilize and correct scoliotic curve. However, the effects of various force configurations on the spinal curves in sagittal plane should also be considered. The aim of this study was to determine the efficiency of various force configurations on scoliotic, lordotic and kyphotic curves. METHOD: A 3d model of spine was developed based on CT scan images of spine obtained from a scoliotic girl. The model was exported to Abaqus software to check the effects of various force configurations and magnitudes on spinal curves. The spinal curves in thoracic and lumbar, lordotic and kyphotic curves were evaluated in this study. Transverse forces, vertical forces and combination of transverse and vertical forces were selected in this study. RESULTS: The results of this study showed that use of transverse forces did not influence the scoliotic curve significantly. Vertical directed forces not only decreased scoliotic curves but also decreased lordotic and kyphotic curves. It seems that a combination of both transverse and vertical directed forces decreased scoliotic curves but did not influence spinal curves in sagittal plane. CONCLUSION: It is recommended to use a combination of transverse and vertical forces to decrease scoliotic curve without significant side effects on the spinal curves in sagittal plane. As this is a case study the outputs of the study should be used with caution.

Exploration of mechanical, thermal conductivity and electromechanical properties of graphene nanoribbon springs.

Recent experimental advances [Liu et al., npj 2D Mater. Appl., 2019, 3, 23] propose the design of graphene nanoribbon springs (GNRSs) to substantially enhance the stretchability of pristine graphene. A GNRS is a periodic undulating graphene nanoribbon, where undulations are of sinus or half-circle or horseshoe shapes. Besides this, the GNRS geometry depends on design parameters, like the pitch's length and amplitude, thickness and joining angle. Because of the fact that parametric influence on the resulting physical properties is expensive and complicated to examine experimentally, we explore the mechanical, thermal and electromechanical properties of GNRSs using molecular dynamics simulations. Our results demonstrate that the horseshoe shape design GNRS (GNRH) can distinctly outperform the graphene kirigami design concerning the stretchability. The thermal conductivity of GNRSs was also examined by developing a multiscale modeling, which suggests that the thermal transport along these nanostructures can be effectively tuned. We found that however, the tensile stretching of the GNRS and GNRH does not yield any piezoelectric polarization. The bending induced hybridization change results in a flexoelectric polarization, where the corresponding flexoelectric coefficient is 25% higher than that of graphene. Our results provide a comprehensive vision of the critical physical properties of GNRSs and may help to employ the outstanding physics of graphene to design novel stretchable nanodevices.

Evaluation of the efficiency of Boston brace on scoliotic curve control: A review of literature.

Context: Bracing is one of the most important treatment approaches that have been utilized in patients with scoliosis. Boston brace used to manage a scoliotic curve especially in lumbar and thoracolumbar areas. Objective: The aim of this review was to evaluate the efficiency of Boston brace to control the progression of the curve based on the available literature. Methods: A search was carried out using the following databases including Scopus, ISI Web of knowledge, PubMed, Ebsco, and Embasco. The key words used for the search were Boston brace, Boston orthosis which were used in combination with scoliosis. Articles identified were screened based on titles and abstracts. The quality of the studies was evaluated using Black and Down tool. Data were summarized based on PICO style. Results: Based on the aforementioned key words, 18 papers were selected, in which 7 studies focused on efficiency of Boston brace, 3 papers focused on quality of life, 5 papers on finite element analysis and 3 papers on comparison of efficiency of Boston with other available braces. The quality of the selected studies varied between 14 and 21. Conclusion: The results of most of the studies support the efficiency of this brace to control the progression of scoliotic curve, especially for the curve between T6 and L2. The efficiency of this brace may be due to its rigid structure and also location and direction of the straps.

An evaluation of the efficiency of endpoint control on the correction of scoliotic curve with brace. A case study.

PURPOSE: The use of braces is one of the conservative treatment approaches recommended for scoliotic subjects. However, the main question posted here is how to improve the efficiency of braces to control the scoliotic curve or to decrease its progression. The aim of this study was to evaluate the efficiency of various boundary conditions (endpoint control) of brace on the correction of scoliotic curves. METHOD: CT scan images of a scoliotic subject, with double lumbar and thoracic curves, was used to produce 3d model of spine. The correction of spine (decrease in scoliotic curves) was determined following the use of transverse (lateral-to-medial direction) and the combination of transverse and vertical (upward directed force, traction) forces on spine in Abaqus software. The effects of pelvic fixation (pelvic basket of a brace) on both sides (basket enclosed pelvic in both sides), on one side (basket enclosed the pelvic in only one side), and fixation of lumbar (part of the brace encircled the lumbar area) were evaluated in this study. RESULTS: The results of this study showed that the effect of vertical forces (traction) was more than that of transverse force. Moreover, the combination of vertical and transverse forces on lumbar and thoracic curves correction was more than that of other conditions (only transverse forces). The best correction was achieved with lumbar fixation and with combination of vertical and transverse forces. CONCLUSIONS: The use the combination of vertical and transverse forces may be suggested to correct the scoliotic curve. Moreover, the efficiency of lumbar fixation in frontal plane seems to be more than pelvic fixation to correct scoliotic curve. The outputs of this study can be used to design new braces for scoliotic subjects.

Sensitivity analysis for the mechanics of tendons and ligaments: Investigation on the effects of collagen structural properties via a multiscale modeling approach.

The effects of the stochasticity of collagen-related structural properties on the biomechanical properties of tendons and ligaments are investigated in this study. The tissue mechanics is modeled by means of a macroscale constitutive model based on a multiscale structural approach. This rationale allows to introduce model parameters directly associated with tissue structural and biochemical features, opening to physically motivated parametric studies. Variance and density-based global sensitivity analyses are employed, together with the quantification of output uncertainty due to stochastic variations of parameters. Novel insights on tissue structure-mechanics relationship are provided, quantifying the dependence between mechanical output quantities on specific collagen-related structural features. Moreover, the uncertainty quantification shows that model predictions provided by the multiscale structural approach are reliable with respect to inevitable uncertainties in tissue structure. Addressing rat tail tendons, the use of average values in tissue properties returns a constitutive response that fits well-available experimental data, and it is robust with respect to parameter stochasticity.

A nanoscale study of the negative strain rate dependency of the strength of metallic glasses by molecular dynamics simulations.

Compressive strength and deformation characteristics of a metallic glassy alloy related to strain rate are studied by molecular dynamics simulations. The negative strain rate dependency of strength is presented, i.e., compressive strength decreases with the increase of strain rate, which is well in line with the experimental results. The negative strain rate dependency of strength is explained from two aspects at the atomic scale of free volume and potential energy. Compressive strength is related to the free volume formation in a shear band, which is different from that in a metallic glass matrix. In addition, the relation of potential energy and temperature is also investigated, which indicates that thermal softening also plays an important role in the negative strain rate dependency of strength. The thermal-mechanical coupling mechanisms causing the negative strain rate dependency of the strength of the metallic glassy alloy are clarified. It is significant to explore the intrinsic deformation characteristics of the metallic glassy alloy under a high rate loading.

The effect of plaque eccentricity on blood hemodynamics and drug release in a stented artery.

Atherosclerosis in the coronary arteries is one of the leading causes of death in the world. Percutaneous coronary interventions (PCI) associated with the implantation of drug eluting stents (DES) is one of the most common forms of revascularization in patients with atherosclerotic coronary artery disease. The use of DES is considered as an effective tool to reduce restenosis after PCI. However despite all the progress made in DES procedures, the rate of restenosis remains relatively high. Mathematical modeling and numerical simulation are believed to play an essential role in identifying zones with a higher risk of in-stent restenosis. In this work the local delivery of a therapeutic agent, from a stent implanted in a coronary artery, is mathematically modeled and numerically simulated. The mathematical model includes the diffusion of the dissolved drug in the biodegradable polymeric coating of the stent, the diffusion and convection of the drug with reversible binding in the viscoelastic arterial wall with plaques of different morphology and the local hemodynamics. The study is an attempt to detect zones with a higher risk of in-stent restenosis and their relation to plaque eccentricity. The location of zones with highest risk of thrombosis and plaque rupture is also addressed. The results are in agreement with claims presented in clinical papers.

Scoliosis conservative treatment: A review of literature.

BACKGROUND: Scoliosis is defined as lateral curvature of the spine which is also associated with a change in the curves in sagittal plane and vertebral rotation. Various types of conservative treatment approaches have been recommended for the patients with scoliosis. The aim of this review article was to introduce the various methods of conservative treatment which can be used for the patients with scoliosis. METHODS: A search was done in some databases including PubMed, ISI Web of knowledge, Google scholar, Ebsco, Embasco, and Scopus. Some keywords such as conservative treatment, orthosis, brace, exercise, and physical therapy were used in combination with scoliosis. As the aim of this paper was to introduce the conservative methods, no quality assessment was done in this review study. RESULTS: Forty papers were found on various conservative treatments approaches which have been used for the patients with scoliosis, in which most of the papers focused on different designs of braces. There were a few studies on other interventions such as wedge, functional electrical stimulation, and yoga. CONCLUSION: Various treatment approaches have been used to treat scoliosis based on conservative approach. It is suggested that the efficiency of various methods be evaluated based on available literature.

Borophene hydride: a stiff 2D material with high thermal conductivity and attractive optical and electronic properties.

Two-dimensional (2D) structures of boron atoms, so-called borophene, have recently attracted remarkable attention. In a recent exciting experimental study, a hydrogenated borophene structure was realized. Motivated by this success, we conducted extensive first-principles calculations to explore the mechanical, thermal conduction, electronic and optical responses of borophene hydride. The mechanical response of borophene hydride was found to be anisotropic, with an elastic modulus of 131 N m-1 and a high tensile strength of 19.9 N m-1 along the armchair direction. Notably, it was shown that by applying mechanical loading the metallic electronic character of borophene hydride can be altered to direct band-gap semiconducting, very appealing for application in nanoelectronics. The absorption edge of the imaginary part of the dielectric function was found to occur in the visible range of light for parallel polarization. Finally, it was estimated that this novel 2D structure at room temperature can exhibit high thermal conductivities of 335 W mK-1 and 293 W mK-1 along the zigzag and armchair directions, respectively. Our study confirms that borophene hydride shows an outstanding combination of interesting mechanical, electronic, optical and thermal conduction properties, which are promising for the design of novel nanodevices.

Evaluation of the efficiency of the Chêneau brace on scoliosis deformity : A systematic review of the literature.

BACKGROUND: Scoliosis is a three-dimensional deformity of the spine and rib cage. Depending on the severity of this disease, various kinds of treatment methods have been used and bracing is among the most common. One of the braces which has been used for subjects with scoliosis is the Chêneau brace. The aim of this review was to evaluate the efficiency of the Chêneau brace on the scoliosis curve progression and control based on the available literature. METHOD: We conducted a Medline search via PubMed, Google Scholar, ISI Web of Sciences, Ebsco and Scopus. Keywords such as Chêneau brace, Chêneau light and CAD/CAM spinal brace were used in combination with scoliosis. The quality of the studies was evaluated by the Down and Black tool. RESULTS: Based on the aforementioned keywords, 55 papers were found. Finally based on the mentioned criteria 14 papers were selected for final analysis. The quality of the studies varied between scores of 13 and 25 using the Down and Black tool. The results of the selected studies confirmed that a good scoliotic curve correction can be achieved with the Chêneau brace. CONCLUSION: The Chêneau brace provides a 3-dimensional correction of the spinal deformity which not only influences the progression of scoliotic curve but also influences its natural history. It cannot be concluded that the Chêneau brace is superior to other available braces; however, it has been shown that this brace is effective to control the scoliotic curve progression especially in the lumbar and thoracolumbar regions.

Thermal and electronic transport characteristics of highly stretchable graphene kirigami.

For centuries, cutting and folding papers with special patterns have been used to build beautiful, flexible and complex three-dimensional structures. Inspired by the old idea of kirigami (paper cutting), and the outstanding properties of graphene, recently graphene kirigami structures were fabricated to enhance the stretchability of graphene. However, the possibility of further tuning the electronic and thermal transport along the 2D kirigami structures has remained original to investigate. We therefore performed extensive atomistic simulations to explore the electronic, heat and load transfer along various graphene kirigami structures. The mechanical response and thermal transport were explored using classical molecular dynamics simulations. We then used a real-space Kubo-Greenwood formalism to investigate the charge transport characteristics in graphene kirigami. Our results reveal that graphene kirigami structures present highly anisotropic thermal and electrical transport. Interestingly, we show the possibility of tuning the thermal conductivity of graphene by four orders of magnitude. Moreover, we discuss the engineering of kirigami patterns to further enhance their stretchability by more than 10 times as compared with pristine graphene. Our study not only provides a general understanding concerning the engineering of electronic, thermal and mechanical response of graphene, but more importantly can also be useful to guide future studies with respect to the synthesis of other 2D material kirigami structures, to reach highly flexible and stretchable nanostructures with finely tunable electronic and thermal properties.

A mechanistic model for drug release from PLGA-based drug eluting stent: A computational study.

Atherosclerosis in the coronary artery is one of the leading causes of death in the world. The stenting as a minimally invasive technique was considered as an effective tool to reduce the severity of atherosclerotic stenosis. In-stent restenosis is the main drawback of the stenting in the coronary artery. Understanding the mechanism of drug release from drug-eluting stents and drug uptake in the arterial wall and obtaining more information about their functionality using mathematical modeling and numerical simulation, could be considered as a predictive tool to investigate in-stent restenosis growth which is experimentally expensive to study. In this work, the local delivery of a therapeutic agent from a PLGA-based bioabsorbable stent implanted in a coronary artery to predict the drug release as well as spatio-temporal drug distribution in a coronary artery with a vulnerable plaque is mathematically modeled and numerically simulated. The effect of copolymer ratio on drug release has been also investigated.

Anisotropic mechanical and optical response and negative Poisson's ratio in Mo2C nanomembranes revealed by first-principles simulations.

Transition metal carbides include a wide variety of materials with attractive properties that are suitable for numerous and diverse applications. A most recent experimental advance could provide a path toward the successful synthesis of large-area and high-quality ultrathin Mo2C membranes with superconducting properties. In the present study, we used first-principles density functional theory calculations to explore the mechanical and optical response of single-layer and free-standing Mo2C. Uniaxial tensile simulations along the armchair and zigzag directions were conducted and we found that while the elastic properties are close along various loading directions, the nonlinear regimes in stress-strain curves are considerably different. We found that Mo2C sheets present negative Poisson's ratio and thus can be categorized as an auxetic material. Our simulations also reveal that Mo2C films retain their metallic electronic characteristic upon uniaxial loading. We found that for Mo2C nanomembranes the dielectric function becomes anisotropic along in-plane and out-of-plane directions. Our findings can be useful for the practical application of Mo2C sheets in nanodevices.