Cwj Cees Oomens

A numerical‐experimental method to characterize the non‐linear mechanical behaviour of human skin

Background/aims: Human skin is a complex tissue consisting of several distinct layers. Each layer consists of various components with a specific structure. To gain a better insight into the overall mechanical behaviour of the skin, we wish to study the mechanical properties of the different layers. A numerical‐experimental method was developed to characterize the non‐linear mechanical behaviour of human dermis.

A mixture approach to the mechanics of skin

Skin can be considered to be a mixture of a solid and a fluid. A general theory for the description of the behaviour of mixtures is presented and applied to a mixture of a solid and a fluid. A numerical procedure is presented to solve the non-linear field equations describing such a mixture. The abilities of the procedure are demonstrated by means of a confined compression test.

The Wrinkling of Thin Membranes: Part I—Theory

A method to describe the stress situation in a wrinkled membrane is presented. In this paper it will be shown that a special deformation tensor can be chosen which leads to the correct stress state of a membrane after wrinkling when it is substituted in the constitutive equation. The method can be used for anisotropic membranes in geometrically and physically nonlinear analysis. The case of simple shear and stretching of a membrane is considered to illustrate the potency of the method.

Can Loaded Interface Characteristics Influence Strain Distributions in Muscle Adjacent to Bony Prominences

Pressure distributions at the interface between skin and supporting tissues are used in design of supporting surfaces like beds, wheel chairs, prostheses and in sales brochures to support commercial products. The reasoning behind this is, that equal pressure distributions in the absence of high pressure gradients is assumed to minimise the risk of developing pressure sores. Notwithstanding the difficulty in performing reproducible and accurate pressure measurements, the question arises if the interface pressure distribution is representative of the internal mechanical state of the soft tissues involved. The paper describes a study of the mechanical condition of a supported buttock contact, depending on cushion properties, relative properties of tissue layers and friction. Numerical, mechanical simulations of a buttock on a supporting cushion are described. The ischial tuberosity is modelled as a rigid body, whereas the overlying muscle, fat and skin layers are modelled as a non-linear Ogden material. Material parameters and thickness of the fat layer are varied. Coulomb friction between buttock and cushion is modelled with different values of the friction coefficient. Moreover, the thickness and properties of the cushion are varied. High shear strains are found in the muscle near the bony prominence and the fat layer near the symmetry line. The performed parameter variations lead to large differences in shear strain in the fat layer but relatively small variations in the skeletal muscle. Even with a soft cushion, leading to a high reduction of the interface pressure the deformation of the skeletal muscle near the bone is high enough to form a risk, which is a clear argument that interface pressures alone are not sufficient to evaluate supporting surfaces.

Passive transverse mechanical properties of skeletal muscle under in vivo compression.

The objective of the present study is to determine the passive transverse mechanical properties of skeletal muscle. Compression experiments were performed on four rat tibialis anterior muscles. To assess the stress- and strain-distributions in the muscle during the experiment, a plane stress model of the cross section was developed for each muscle. The incompressible viscoelastic Ogden model was used to describe the passive muscle behaviour. The four material parameters were determined by fitting calculated indentation forces on measured indentation forces. The elastic parameters, mu and alpha, were 15.6+/-5.4 kPa and 21.4+/-5.7, respectively. The viscoelastic parameters, delta and tau, were 0.549+/-0.056 and 6.01+/-0.42 s. When applying the estimated material parameters in a three-dimensional finite element model, the measured behaviour can be accurately simulated.

Determination of the elasto-plastic properties of aluminium using a mixed numerical-experimental method

Abstract A mixed numerical–experimental method is used to determine the parameters in elasto-plastic constitutive models. Aluminium plates of non-standard geometry are mounted in a uniaxial tensile-testing machine. On the surfaces of the plates retro-reflective markers are placed. The displacements of these markers are measured optically. These measurements are used to determine yield stresses in the isotropic Von Mises and the orthotropic Hill yield criterion. The models are evaluated by examining the estimated parameters and the residual displacements.

Predicting local cell deformations in engineered tissue constructs: a multilevel finite element approach

A multilevel finite element approach is applied to predict local cell deformations in engineered tissue constructs. Cell deformations are predicted from detailed nonlinear FE analysis of the microstructure, consisting of an arrangement of cells embedded in matrix material. Effective macroscopic tissue behavior is derived by a computational homogenization procedure. To illustrate this approach, we simulated the compression of a skeletal muscle tissue construct and studied the influence of microstructural heterogeneity on local cell deformations. Results show that heterogeneity has a profound impact on local cell deformations, which highly exceed macroscopic deformations. Moreover, microstructural heterogeneity and the presence of neighboring cells leads to complex cell shapes and causes non-uniform deformations within a cell.

Deep tissue injury: how deep is our understanding?

Deep pressure ulcers, necessarily involving deep tissue injury (DTI), arise in the muscle layers adjacent to bony prominences because of sustained loading. They represent a serious type of pressure ulcer because they start in underlying tissues and are often not visible until they reach an advanced stage, at which time treatment becomes problematic. Underlying mechanisms of DTI require further investigation if appropriate preventive measures are to be determined. The present commentary illustrates a hierarchic research approach selected to study these mechanisms. To differentiate between the individual roles of deformation and ischemia in the onset of skeletal muscle damage, 2 complementary approaches have been selected. In an in vivo animal model, the effects of ischemia combined with deformation and ischemia per se were studied. An in vitro muscle model was used to study the separate effects of deformation and several aspects of ischemia, including hypoxia, glucose depletion, and tissue acidification, in more detail. Based on the results of both models a sequence of events leading to cell necrosis is proposed. Deformation levels exceeding a threshold value can result in rapid tissue damage that may persist, whereas ischemia has a more gradual effect as a result of glucose depletion and tissue acidification.

Compression Induced Cell Damage in Engineered Muscle Tissue: An In Vitro Model to Study Pressure Ulcer Aetiology

AbstractThe aetiology of pressure ulcers is poorly understood. The complexity of the problem, involving mechanical, biochemical, and physiological factors demands the need for simpler model systems that can be used to investigate the relative contribution of these factors, while controlling others. Therefore, an in vitro model system of engineered skeletal muscle tissue constructs was developed. With this model system, the relationship between compressive tissue straining and cell damage initiation was investigated under well-defined environmental conditions. Compression of the engineered muscle tissue constructs revealed that cell death occurs within 1–2 h at clinically relevant straining percentages and that higher strains led to earlier damage initiation. In addition, the uniform distribution of dead cells throughout the constructs suggested that sustained deformation of the cells was the principle cause of cell death. Therefore, it is hypothetised that sustained cell deformation is an additional mechanism that plays a role in the development of pressure ulcers.

Linear viscoelastic behavior of subcutaneous adipose tissue

Subcutaneous adipose tissue contributes to the overall mechanical behavior of the skin. Until today, however, no thorough constitutive model is available for this layer of tissue. As a start to the development of such a model, the objective of this study was to measure and describe the linear viscoelastic behavior of subcutaneous adipose tissue. Although large strains occur in vivo, this work only focuses on the linear behavior to show the applicability of the described methods to adipose tissue. Shear experiments are performed on porcine samples on a rotational rheometer using parallel plate geometry. In the linear viscoelastic regime, up to 0.1% strain, the storage and loss modulus showed a frequency- and temperature-dependent behavior. The ratio between the two moduli, the phase angle, did not show any dependency on temperature and frequency. The shear modulus was found to be 7.5 kPa at 10 rad/s and 37 degrees C. Time-temperature superposition was applicable through shifting the shear modulus horizontally. A power-law function model was introduced to describe both the frequency dependent behavior at constant temperature and the stress relaxation behavior. In addition, the effect of snap freezing as a preservation method was analyzed. Histological examination demonstrated possible tissue damage after freezing, but the mechanical properties did not change. Since results were reproducible, it is concluded that the methods we used are most probably suited to explore the non-linear behavior of subcutaneous adipose tissue.