1,721,013 research outputs found
Constitutive modelling of skin mechanics
No full textThe objective of this chapter is to provide a structured review of constitutive models of skin mechanics valid for finite deformations, with special emphasis on state-of-the-art anisotropic formulations which are essential in most advanced modelling applications. The fundamental structural and material characteristics of the skin, necessary for understanding its mechanics and for the formulation of constitutive equations, are briefly presented
Mathematical and computational modelling of skin biophysics: a review
No full textThe objective of this paper is to provide a review on some aspects of the mathematical and computational modelling of skin biophysics, with special focus on constitutive theories based on nonlinear continuum mechanics from elasticity, through anelasticity, including growth, to thermoelasticity. Microstructural and phenomenological approaches combining imaging techniques are also discussed. Finally, recent research applications on skin wrinkles will be presented to highlight the potential of physics-based modelling of skin in tackling global challenges such as ageing of the population and the associated skin degradation, diseases and traumas
A mesostructurally-based anisotropic continuum model for biological soft tissues — decoupled invariant formulation
No full textCharacterising and modelling the mechanical behaviour of biological soft tissues is an essential step in the development of predictive computational models to assist research for a wide range of applications in medicine, biology, tissue engineering, pharmaceutics, consumer goods, cosmetics, transport or military. It is therefore critical to develop constitutive models that can capture particular rheological mechanisms operating at specific length scales so that these models are adapted for their intended applications.Here, a novel mesoscopically-based decoupled invariant-based continuum constitutive framework for transversely isotropic and orthotropic biological soft tissues is developed. A notable feature of the formulation is the full decoupling of shear interactions. The constitutive model is based on a combination of the framework proposed by Lu and Zhang [Lu, J., Zhang, L., 2005. Physically motivated invariant formulation for transversely isotropic hyperelasticity. International Journal of Solids and Structures 42, 6015–6031] and the entropic mechanics of tropocollagen molecules and collagen assemblies. One of the key aspects of the formulation is to use physically-based nanoscopic quantities that could be extracted from experiments and/or atomistic/molecular dynamics simulations to inform the macroscopic constitutive behaviour. This effectively couples the material properties at different levels of the multi-scale hierarchical structure of collagenous tissues. The orthotropic hyperelastic model was shown to reproduce very well the experimental multi-axial properties of rabbit skin. A new insight into the shear response of a skin sample subjected to a simulated indentation test was obtained using numerical direct sensitivity analyses
Investigating the influence of relative humidity on expression microwrinkles
No full textThe quest for efficient anti-wrinkle treatments has mainly focused on biochemical approaches aiming to mitigate or slow down the effects of both intrinsic and extrinsic ageing. However, the biophysical principles that govern the formation and evolution of wrinkles remain to be elucidated. Georges Limbert shares the findings of a study his computational biophysics group conducted with US researchers.The prospects and consequences of ageing are of concern to all, especially with regard to wrinkles. Wrinkles are not only a hallmark of ageing, with its various cosmetic and social implications, but also play a fundamental role in how people interact with many products and devices, from moisturisers and make-up, to adhesive plasters, incontinence products, razors and clothing fabrics. Unveiling the underlying biophysical principles that condition the morphologies and patterns of wrinkles are essential in evaluating, and ultimately, predicting, how ageing or aged skin interacts with its environment
A constitutive model of the posterior cruciate ligament
No full textFurther to our previous work on the development of a general constitutive framework for transversely isotropic viscohyperelasticity (Limbert, G, Middleton, J. A transversely isotropic viscohyperelastic material. Application to the modelling of biological soft connective tissues. Int J Solids Struct 2004;41(15):4237–60.), we propose a phenomenological constitutive law to describe the anisotropic viscohyperelastic behaviour of the human posterior cruciate ligament (PCL) at high strain rates. The mechanical formulation is based on the definition of a Helmholtz free energy function containing a hyperelastic and a viscous potential. The equations are valid for arbitrary kinematics and satisfy elemental thermodynamic principles. Identification of the constitutive model with experimental data obtained from human PCL specimens was performed and showed the ability of the model to capture accurately the mechanical characteristics of the PCL at various strain rates. Influence of the isotropic and directional viscous stress responses on the global mechanical response are discussed in connection with the modelling hypotheses. This work was motivated by the need to provide an accurate constitutive model of the PCL to be used in finite element analyses of human occupants in car crash simulations. Besides uniaxial tests along the natural fibre orientation of the PCL, additional tests such as equibiaxial, strip biaxial compression–tension and shear tests were also performed in order to assess the physical response of the model in different loading situations. It was found that the model performed as well in these conditions
A transversely isotropic viscohyperelastic material: application to the modelling of biological soft connective tissues
No full textA general transversely isotropic viscohyperelastic constitutive law including strain rate effects was proposed. It is based on a definition of a general Helmholtz free energy function which depends explicitly on the right Cauchy–Green deformation tensor, its material time derivative and a structural tensor characterizing the preferred direction from which anisotropy arises. The elastic and viscous potentials that defined the free energy function were assumed to be decoupled, thus facilitating the identification process. This law was valid for arbitrary kinematics and aimed at modeling the mechanical behavior of biological soft tissues at high strain rates and at the finite strain regime. This is of high relevance for dynamic analyses of human occupants in car crash simulations (finite element analyses) and for situations where dynamic loads are significant (sport injury, etc). Explicit expression of the stress, elasticity and viscosity tensors were established. As an application of the constitutive law, the general expressions of the stress tensors were particularized for a specific Helmholtz free energy function describing the mechanical characteristics of the human anterior cruciate ligament. The constitutive model was shown to capture the strain rate effects and other essential characteristics of ligaments such as finite strain, anisotropy and nearly incompressibility. The model was also tested for various multi-axial loading situations
Dataset for the thesis '3D image-based modelling of collagenous soft tissues'
No full textThis dataset contains:
A zip file including several txt files that store the extarcted nominal stress and Green-Lagrange strain data for Models 1ABC subjected to uniaxial tensile tests.
Also the maximum principal logarithmic strain and maximum principal stress data for Models 1A and 1C were written into two txt files respectively.
These data were extracted from .ODB files using Python script.</span
Rapid flapping and fiber-reinforced membrane wings are key to high-performance bat flight
No full textBats fly using significantly different wing motions than other fliers, stemming from the complex interplay of their membrane wings’ motion and structural properties. Biological studies show that many bats fly at Strouhal numbers, the ratio of flapping to flight speed, 50-150% above the range typically associated with optimal locomotion. We use high-resolution fluid-structure interaction simulations of a bat wing to independently study the role of kinematics and material/structural properties on aerodynamic performance and show that peak propulsive and lift efficiencies for a bat-like wing motion require flapping 66% faster than for a symmetric motion, agreeing with the increased flapping frequency observed in zoological studies. In addition, we find that reduced membrane stiffness is associated with improved propulsive efficiency until the membrane flutters, but that incorporating microstructural anisotropy arising from biological fiber reinforcement enables a tenfold reduction of the flutter energy whilst maintaining high aerodynamic efficiency. Our results indicate that animals with specialized flapping motions may have correspondingly specialized flapping speeds, in contrast to arguments for a universally efficient Strouhal range. Additionally, our study demonstrates the significant role that the microstructural constitutive properties of the membrane wing of a bat can have on its propulsive performance
Skin friction under pressure. The role of micromechanics
No full textThe role of contact pressure on skin friction has been documented in multiple experimental studies. Skin friction significantly raises in the low-pressure regime as load increases while, after a critical pressure value is reached, the coefficient of friction of skin against an external surface becomes mostly insensitive to contact pressure. However, up to now, no study has elucidated the qualitative and quantitative nature of the interplay between contact pressure, the material and microstructural properties of the skin, the size of an indenting-sliding object and the resulting measured macroscopic coefficient of friction. A mechanistic understanding of these aspects is essential for guiding the rational design of products intended to interact with the skin through optimally-tuned surface and/or microstructural properties.&#13; Here, an anatomically-realistic two-dimensional multi-layer finite element model of the skin was embedded within a computational contact homogenisation procedure. The main objective was to investigate the sensitivity of macrosocpic skin friction to the parameters discussed above, in addition to the local (i.e. microscopic) coefficient of friction defined at skin asperity level. This was accomplished via the design of a large-scale computational experiment featuring 312 analyses. Results confirmed the potentially major role of finite deformations of skin asperities on the resulting macroscopic friction. This effect was shown to be modulated by the level of contact pressure and relative size of skin surface asperities compared to those of a rigid slider. The numerical study also corroborated experimental observations concerning the existence of two contact pressure regimes where macroscopic friction steeply and non-linearly increases up to a critical value, and then remains approximately constant as pressure increases further.&#13; The proposed computational modelling platform offers attractive features which are beyond the reach of current analytical models of skin friction, namely, the ability to accommodate arbitrary kinematics, non-linear constitutive properties and the complex skin microstructure
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