Characterization of the anisotropic mechanical properties of excised human skin

The mechanical properties of skin are important for a number of applications including surgery, dermatology, impact biomechanics and forensic science. In this study, we have investigated the influence of location and orientation on the deformation characteristics of 56 samples of excised human skin. Uniaxial tensile tests were carried out at a strain rate of 0.012 s(-1) on skin from the back. Digital Image Correlation was used for 2D strain measurement and a histological examination of the dermis was also performed. The mean ultimate tensile strength (UTS) was 21.6±8.4 MPa, the mean failure strain 54%±17%, the mean initial slope 1.18±0.88 MPa, the mean elastic modulus 83.3±34.9 MPa and the mean strain energy was 3.6±1.6 MJ/m(3). A multivariate analysis of variance has shown that these mechanical properties of skin are dependent upon the orientation of the Langer lines (P<0.0001-P=0.046). The location of specimens on the back was also found to have a significant effect on the UTS (P=0.0002), the elastic modulus (P=0.001) and the strain energy (P=0.0052). The histological investigation concluded that there is a definite correlation between the orientation of the Langer lines and the preferred orientation of collagen fibres in the dermis (P<0.001). The data obtained in this study will provide essential information for those wishing to model the skin using a structural constitutive model.     

Adhesive and mechanical properties of hydrogels influence neurite extension

Photopolymerizable polyethylene glycol (PEG) hydrogels conjugated with bioactive ligands were examined for their use as scaffolds in peripheral nerve regeneration applications. The bioactivity and mechanical properties of PEG hydrogels can be tailored through the integration of bioactive factors (adhesion ligands, proteolytic sites, growth factors) and the alteration of PEG concentrations, respectively. For peripheral nerve regeneration, it will be important to determine the type and concentration of the bioactive molecules required to improve neurite extension. In this study, cell adhesion ligands (RGDS, IKVAV, and YIGSR) were covalently attached to PEG hydrogels. Both the type and concentration of cell adhesion ligand used affected neurite extension. Extension from PC12 cells was greater on hydrogels with RGDS incorporated than IKVAV, and the optimal concentration for each ligand was different. Cells adhered to but did not extend neurites on hydrogels with YIGSR. Cells did not adhere to hydrogels containing RGES. Furthermore, different combinations of these ligands affected neurite extension to different degrees. The mechanical properties of the hydrogels also significantly affected neurite extension. PC12 cells grown on more flexible hydrogels exhibited the greatest degree of neurite extension. PEG hydrogels have thus been developed with varying biochemical and mechanical properties that may enhance nerve regeneration.     

Determination of the optical properties of anisotropic biological media using an isotropic diffusion model

We investigate anisotropic light propagation in biological tissue in steady-state and time domains. Monte Carlo simulations performed for tissue that consists of aligned cylindrical and spherical scatterers show that steady-state and time-resolved reflectance depends strongly on the measurement direction relative to the alignment of the cylinder axis. We examine the determination of optical properties using an isotropic diffusion model and find that in the time domain, in contrast to steady-state spatially resolved reflectance measurements, the obtained absorption coefficient does not depend on the measurement direction and is close to the true value. Contrarily, the derived reduced scattering coefficient depends strongly on the measurement direction in both domains. Measurements of the steady-state and time-resolved reflectance from bovine tendon confirm the theoretical findings.     

Differences in the mechanism of type 1 and type 2 diabetes-induced skin dryness by using model mice

Diabetes induces dry skin that may cause infective diseases. In this study, we aimed to clarify the mechanism of diabetes-induced skin dryness in animal models. We also examined the difference in the mechanism of skin dryness in type 1 and type 2 diabetes. We examined skin dryness in type 1 diabetes model mice (streptozotocin [STZ] induction), non-obesity type 2 diabetes model mice (newborn STZ injection), and obesity type 2 diabetes model mice (KK-Ay/TaJcl). An increase in transepidermal water loss was observed in the type 1 diabetes model mice, and reduced skin hydration was observed in the type 2 diabetes model mice. In the type 1 diabetes model mice, an increase in advanced glycation end products and matrix metalloproteinase-9 led to a decline in collagen IV level, inducing skin dryness. In the obesity type 2 diabetes model mice, an increase in the release of histamine and hyaluronidase by mast cells resulted in a decline in the level of hyaluronic acid, inducing skin dryness. However, in the non-obesity type 2 diabetes model mice, the main factors of skin dryness could not be clearly identified. Nevertheless, inflammatory cytokine levels increased. We hypothesize that inflammatory cytokines disrupt the collagen of the skin. Diabetes caused skin dryness in each mouse model, and the mechanism of skin dryness differed by diabetes type.     

Estimation of the viscous properties of skin and subcutaneous tissue in uniaxial stress relaxation tests

Knowledge of viscoelastic properties of soft tissues is essential for the finite element modelling of the stress/strain distributions in finger-pad during vibratory loading, which is important in exploring the mechanism of hand-arm vibration syndrome. In conventional procedures, skin and subcutaneous tissue have to be separated for testing the viscoelastic properties. In this study, a novel method has been proposed to simultaneously determine the viscoelastic properties of skin and subcutaneous tissue in uniaxial stress relaxation tests. A mathematical approach has been derived to obtain the creep and relaxation characteristics of skin and subcutaneous tissue using uniaxial stress relaxation data of skin/subcutaneous composite specimens. The micro-structures of collagen fiber networks in the soft tissue, which underline the tissue mechanical characteristics, will be intact in the proposed method. Therefore, the viscoelastic properties of soft tissues obtained using the proposed method would be more physiologically relevant than those obtained using the conventional method. The proposed approach has been utilized to measure the viscoelastic properties of soft tissues of pig. The relaxation curves of pig skin and subcutaneous tissue obtained in the current study agree well with those in literature. Using the proposed approach, reliable material properties of soft tissues can be obtained in a cost- and time-efficient manner, which simultaneously improves the physiological relevance.     

A study of the uniaxial mechanical properties of human dura mater preserved in glycerol

Heart valve replacements have now been made for some years from human dura mater. While the clinical results have been encouraging there is a considerable lack of information on the mechanical properties of the dura mater and the effects of the glycerol treatment employed. This study revealed that glycerol provided an adequate preserving medium for periods of up to 12 days, with no significant changes in the mechanical properties of dura mater. A wide range in mechanical properties was observed with, for example, the tensile strength varying from 3.28 N/mm² to 7.86 N/mm². Although no age-related deterioration in human dura mater could be shown, the results do suggest that dura mater in excess of fifty years of age should be avoided in order to prevent the selection of poor quality material.     

A model for the prediction of energy balance and body weight

A model for changes in energy balance and body weight is described which can be written as an iterative computer program. Medium and long-term changes in body weight resulting from random daily fluctuations in energy balance are quantitatively predicted. Body weight varies periodically about a mean value to to the extent of ± 1 kg over a span of a few weeks. Long-term weight stabilization is the result of a dynamic equilibrium between energy intake and output which depends on activity and tissue metabolism. A lean : fat tissue deposition ratio defines the metabolic type of individual. This ratio, which is constant for each individual, governs the proportions in which these tissues are both deposited and mobilized. In the model, the ratio has an important effect on the magnitude and composition of the weight gains resulting from over-eating. It is suggested that the differences in response which have been observed in over-eating experiments result from inter-individual differences in this ratio.     

A model for the prediction of energy balance and body weight

A model for changes in energy balance and body weight is described which can be written as an iterative computer program. Medium and long-term changes in body weight resulting from random daily fluctuations in energy balance are quantitatively predicted. Body weight varies periodically about a mean value to to the extent of +/- 1 kg over a span of a few weeks. Long-term weight stabilization is the result of a dynamic equilibrium between energy intake and output which depends on activity and tissue metabolism. A lean : fat tissue deposition ratio defines the metabolic type of individual. This ration, which is constant for each individual, governs the proportions in which these tissues are both deposited and mobilized. In the model, the ratio has an important effect on the magnitude and composition of the weight gains resulting from over-eating. It is suggested that the differences in response which have been observed in over-eating experiments result from inter-individual differences in this ratio.     

A comparative study on the mechanical properties of the healthy and varicose human saphenous vein under uniaxial loading

Abstract

Saphenous Vein (SV) due to fatness, age, inactiveness, etc. can be afflicted with varicose. The main reason of the varicose vein is believed to be related to the leg muscle pump which is unable to return the blood to the heart in contradiction of the effect of gravity. As a result of the varicose vein, both the structure and mechanical properties of the vein wall would alter. However, so far there is a lack of knowledge on the mechanical properties of the varicose vein. In this study, a comparative study was carried out to measure the elastic and hyperelastic mechanical properties of the healthy and varicose SVs. Healthy and varicose SVs were removed at autopsy and surgery from seven individuals and then axial tensile load was applied to them up to the failure point. In order to investigate the mechanical behaviour of the vein, this study was benefitted from three different stress definitions, such as 2nd Piola-Kichhoff, engineering and true stresses and four different strain definitions, i.e. Almansi-Hamel, Green-St. Venant, engineering and true strains, to determine the linear mechanical properties of the SVs. A Digital Image Correlation (DIC) technique was used to measure the true strain of the vein walls during load bearing. The non-linear mechanical behaviour of the SVs was also computationally evaluated via the Mooney-Rivlin material model. The true/Cauchy stress–strain diagram exhibited the elastic modulus of the varicose SVs as 45.11% lower than that of the healthy ones. Furthermore, by variation of the stress a significant alteration on the maximum stress of the healthy SVs was observed, but then not for the varicose veins. Additionally, the highest stresses of 4.99 and 0.65 MPa were observed for the healthy and varicose SVs, respectively. These results indicate a weakness in the mechanical strength of the SV when it becomes varicose, owing to the degradation of the elastin and collagen content of the SV. The Mooney-Rivlin hyperelastic and the Finite Element (FE) data were finally well compared to the experimental data.     

The effects of ovariectomy on the mechanical properties of skin in rats

OBJECTIVE: After menopause, observable changes occur in the physical characteristics of the human skin. These changes and their responses to various treatments can be assessed with non-invasive in-vivo mechanical tests. However, tests measuring breaking strength and tensile strengths can only be done ex-vivo, they require relatively higher quantities of skin and thus have generally been performed on animals. Mechanical changes in the skin of ovariectomized rats, an appropriate model for the study of postmenopausal period, have not been dealt with in the literature. In this study mechanical characteristics of the skin, such as breaking strength and tensile strength have been tested and studied histologically in ovariectomized rats. METHODS: Sixteen rats were divided into two groups, one undergoing ovariectomy and one control group undergoing a sham operation. Three months later, the rats were sacrificed and tensile properties of their back skins were tested with a tensometer and evaluated histologically. RESULTS AND CONCLUSION: Breaking strength, tensile strength and the Young's modulus have increased and the thickness of the subcutis has decreased in ovariectomized rats. This study should be tested by others, because of existence of some conflicts between available knowledge and the results, relating to postmenopausal skin changes.     

Influences of supra-physiological temperatures on microstructure and mechanical properties of skin tissue

Thermal therapies under supra-physiological temperatures are increasingly used to treat skin diseases (e.g., superficial melanoma, removal of port-wine stains pigmented and cutaneous lesions). The efficacy of these therapies depends on the thermal and mechanical loadings that skin experiences during the treatment process. Therefore, it is of great significance to better understand the role of thermally induced changes in skin mechanical behavior and microstructure. In this study, rabbit belly skin was thermally damaged by immersing skin samples into saline solutions with controlled temperatures. We investigated the effect of thermal damage on skin mechanical behavior. We quantified the changes in skin microstructure (i.e., fiber, fibril) using histological staining and transmission electron microscopy (TEM). The results indicate that (i) the elastic modulus of skin, obtained by the uniaxial tensile test, decreased with increasing heating temperature; (ii) the skin tensile behavior was correlated with its microstructure changes induced by thermal denaturation of collagen fibers under supra-physiological temperatures; (iii) skin thermal damage predicted using the Arrhenius burn integration quantitatively agrees well with the evolution of the microstructure (i.e., percentage of the collagen area in Hematoxylin and Eosin (H&E) staining results). This study provides a better understanding of the coupled bio-thermo-mechanical behavior of skin tissue that could help to improve clinical thermal therapies.     

利用应变能衰减率确定软组织单轴拉伸预调次数

目的 探讨利用应变能衰减率来确定软组织单轴拉伸的预调次数的方法。方法 以健康的新西兰大白兔的腹部皮肤为研究对象,裁取与腹白线平行（0°）、垂直（90°）两个方向上的条状试样,进行应变为15%的单轴拉伸预调实验。将获得的载荷 位移加载卸载实验数据、转换成相应的应力 应变数据,并利用MATLAB编程计算应变能衰减率η=Wn－Wn－1Wn－1×100%（Wn表示第n次预调的应变能损失）,获取η分别≤5%和10%条件下的预调次数n。结果5%衰减率下,0°和90°方向上的预调次数均值分别为11.11和13.67。10%衰减率下,0°和90°方向上的预调次数分别为6.67和7.78。结论 利用应变能衰减率可以量化软组织单轴拉伸预调次数,为软组织生物力学测试的标准化提供了研究基础。     

Refinements in the methods for the measurement of the mechanical properties of unwounded and wounded skin

One of the most important and least understood problems in medicine is that of what variables govern the rate and the quality of the wound healing process. It is highly desirable to know the effects of external or internal agents such as drugs, radiation, and physical irritants on the healing process with a view toward accelerating and improving it. Since mechanical strength is a basic criterion, it is important that standardized methods for the physical testing, analysis and interpretation of results be devised. This paper presents a basic technique for the determination of the mechanical properties of wounded and unwounded skin.     

Estimation of distributed arterial mechanical properties using a wave propagation model in a reverse way

To estimate arterial stiffness, different methods based either on distensibility, pulse wave velocity or a pressure-velocity loop, have been proposed. These methods can be employed to determine the arterial mechanical properties either locally or globally, e.g. averaged over an entire arterial segment. The aim of this study was to investigate the feasibility of a new method that estimates distributed arterial mechanical properties non-invasively. This new method is based on a wave propagation model and several independent ultrasound and pressure measurements. Model parameters (including arterial mechanical properties) are obtained from a reverse method in which differences between modeling results and measurements are minimized using a fitting procedure based on local sensitivity indices. This study evaluates the differences between in vivo measured and simulated blood pressure and volume flow waveforms at the brachial, radial and ulnar arteries of 6 volunteers. The estimated arterial Young's modulus range from 1.0 to 6.0 MPa with an average of (3.8 ± 1.7) MPa at the brachial artery and from 1.2 to 7.8 MPa with an average of (4.8 ± 2.2) MPa at the radial artery. A good match between measured and simulated waveforms and the realistic stiffness parameters indicate a good in vivo suitability.     

Experimental method for determining the 2-dimensional mechanical properties of living human skin

An experimental method for obtaining the 2-dimensional skin tension/extension-ratio characteristics of living human skin is described. This method incorporates a strain-gauged pretension device used to determine the naturally existing tension and deformation fields in a specific skin area and a suction-cup system that applies a homogeneous, plane strain field to the skin at controlled straining rates. A series of experiments conducted to validate the use of the method for obtaining quantitative measures of the mechanical behaviour of skinin vivo is also presented. This method is being used to investigate the variation of mechanical properties with such factors as age, sex, hormonal change, ultraviolet radiation and various disease states.     

The mechanical properties of the skin epidermis in relation to targeted gene and drug delivery

A challenge in combating many major diseases is breaching the skin's tough outer layer (the stratum corneum (SC)) and delivering drugs and genes into the underlying abundant immunologically sensitive viable epidermal cells with safe, practical physical technologies. To achieve this effectively and accurately, design information is needed on key skin mechanical properties when pushing into and through epidermal skin cells. We measure these important mechanical properties by penetrating through the intact SC and viable epidermis (VE) of freshly excised murine skin with a NANO-indenter, using custom tungsten probes fabricated with nominally 5 and 2 microm diameters (with nanoscale tips). We show the skin Young's modulus, storage modulus and stress all dramatically decreased through the SC. Also, for a given penetration depth, decreasing the probe size significantly increases the storage modulus. Biological variation in penetrating the skin was shown. These collective findings advance the rational design of physical approaches for delivering genes and drugs within key cells of the VE.     

Effect of adjuvant induced arthritis on the mechanical properties and thermal behavior of skin and tendon

In rats with adjuvant induced arthritis, the mechanical properties and thermal behavior of the skin and tendon were studied during the acute and chronic phase of the disease. Among the mechanical properties, the tensile strength and elastic modulus were found to decrease markedly both in the skin and tendon of arthritic rats while the extensibility did not show any significant change during the course of the disease. The thermal behavior, namely, shrinkage temperature, denaturation temperature, isometric tension and the temperature at isometric tension were decreased appreciably both in the skin and tendon of arthritic rats, the decrease being prominent during the chronic phase of arthritis. Both the mechanical properties and thermal behavior were correlated with biochemical parameters such as total collagen content and the ratio of the total activity of 14C-hydroxyproline of soluble collagens to insoluble collagen. The results support the concept of impaired collagen crosslinking in adjuvant induced arthritis. Prednisolone and L-thyroxine were administered to arthritic rats and the changes in the mechanical properties and thermal behavior of the skin and tendon of arthritic rats were studied. Both prednisolone and L-thyroxine increased the decreased values of tensile strength and elastic modulus of the skin and tendon in adjuvant induced arthritis. Similarly the decreased values in the thermal behavior of arthritic rats were found to be increased by treatment with prednisolone and L-thyroxine. Results indicated correlations with the beneficial action of prednisolone and L-thyroxine on the impaired collagen crosslinking in adjuvant induced arthritis.