Quantification of gravity-induced skin strain across the breast surface

Background Quantification of the magnitude of skin strain in different regions of the breast may help to estimate possible gravity-induced damage whilst also being able to inform the selection of incision locations during breast surgery. The aim of this study was to quantify static skin strain over the breast surface and to estimate the risk of skin damage caused by gravitational loading. Methods Fourteen participants had 21 markers applied to their torso and left breast. The non-gravity breast position was estimated as the mid-point of the breast positions in water and soybean oil (higher and lower density than breast respectively). The static gravity-loaded breast position was also measured. Skin strain was calculated as the percentage extension between adjacent breast markers in the gravity and non-gravity loaded conditions. Findings Gravity induced breast deformation caused peak strains ranging from 14 to 75% across participants, with potentially damaging skin strain (>60%) in one participant and skin strains above 30% (skin resistance zone) in a further four participants. These peak strain values all occurred in the longitudinal direction in the upper region of the breast skin. In the latitudinal direction, smaller-breasted participants experienced greater strain on the outer (lateral) breast regions and less strain on the inner (medial) breast regions, a trend which was reversed in the larger breasted participants (above size 34D). Interpretation To reduce tension on surgical incisions it is suggested that preference should be given to medial latitudinal locations for smaller breasted women and lateral latitudinal locations for larger breasted women

The use of digital image correlation in the biomechanical area: a review

This paper offers an overview of the potentialities and limitations of digital image correlation (DIC) as a technique for measuring displacements and strain in biomechanical applications. This review is mainly intended for biomechanists who are not yet familiar with DIC. This review includes over 150 papers and covers different dimensional scales, from the microscopic level (tissue level) up to macroscopic one (organ level). As DIC involves a high degree of computation, and of operator- dependent decisions, reliability of displacement and strain measurements by means of DIC cannot be taken for granted. Methodological problems and existing solutions are summarized and compared, whilst open issues are addressed. Topics addressed include: preparation methods for the speckle pattern on different tissues; software settings; systematic and random error associated with DIC measurement. Applications to hard and soft tissues at different dimensional scales are described and analyzed in terms of strengths and limitations. The potentialities and limitations of DIC are highlighted, also in comparison with other experimental techniques (strain gauges, other optical techniques, digital volume correlation) and numerical methods (finite element analysis), where synergies and complementarities are discussed. In order to provide an overview accessible to different scientists working in the field of biomechanics, this paper intentionally does not report details of the algorithms and codes used in the different studies

Characterization of biaxial mechanical properties of rubber and skin

textBreast cancer is one of the most frequently diagnosed cancers affecting women in the United States. An ongoing objective of many research groups is to develop a biomechanical breast model for different applications, ranging from surgical outcome predictions for patients undergoing breast reconstruction surgery, to image registration for planning plastic surgery. Achieving the goal of developing a physics based biomechanical model of the human breast requires the determination of material properties of the various tissues constituting the breast. The objective of this thesis is to develop an appropriate hybrid experimental-numerical technique to enable the calibration of material parameters of skin for different constitutive models (commonly used for skin). The quantification of the material parameters thus obtained validates the bulge test method to be used in testing soft tissue specimens like skin. A bulge test device was custom-built for this work; it consists of a pressure chamber, two digital cameras, and a syringe pump as its main components. The syringe pump provides a constant flow rate of water into the pressure chamber and results in the bulging of specimens with a diameter between 45 mm and 80 mm. Three-dimensional Digital Image Correlation technique is used to obtain full field displacement measurements of the three dimensional shape of the bulge. Tests were performed on commercial rubber sheets of different thickness and on porcine skin specimens; in these tests, the bulge shape was measured at monotonically increasing and decreasing pressure levels, as well as during cyclic loading allowing determination of the deformation and strain fields over the specimen surface. In order to extract the material properties, a hybrid experimental-numerical method was used: the experiment was modeled numerically using the finite element analysis software Abaqus, imposing the commonly used Mooney-Rivlin model for isotropic materials and the Gasser-Ogden-Holzapfel model for anisotropic materials. A comparison between the experimentally measured and numerically simulated bulge shapes was used to determine the optimized material parameters under biaxial loading conditions over a large range of stretch levels.Biomedical Engineerin

Shear-promoted drug encapsulation into red blood cells: a CFD model and μ-PIV analysis

The present work focuses on the main parameters that influence shear-promoted encapsulation of drugs into erythrocytes. A CFD model was built to investigate the fluid dynamics of a suspension of particles flowing in a commercial micro channel. Micro Particle Image Velocimetry (μ-PIV) allowed to take into account for the real properties of the red blood cell (RBC), thus having a deeper understanding of the process. Coupling these results with an analytical diffusion model, suitable working conditions were defined for different values of haematocrit

marker tracking for local strain measurement in mechanical testing of biomedical materials

Local strain measurement is one of the key aspects in tensile tests of biomedical materials and biological tissues, especially if aimed at developing appropriate constitutive formulations to describe mechanical behavior. The measurement of strain as the ratio between the current and the initial length of the sample can be coupled with markers recognition via non-contact video extensometer for characterizing the local mechanical behavior. A crucial point in video extensometer measurement is the selection of the most appropriate markers and technique of their application on the sample surface. This work promotes understanding the effect of markers on material mechanical response. Different solutions were taken into account, as paint markers, namely a commercial lacquer and an acrylic paint, or physical markers attached with the use of adhesives, i.e. cyanoacrylate or medical spray band. Tensile tests revealed that markers can modify the mechanical response of the tested materials, inducing a local stiffening of the samples. The use of cyanoacrylate, as marker adhesive, affects not only the local but also the overall mechanical response, at least for the sample size considered in this work. These effects are more pronounced with higher material compliance. Based on these results, caution is recommended with the use of cyanoacrylate for attaching markers on biomedical polymers

Hyperelastic Ex Vivo Cervical Tissue Mechanical Characterization

This paper presents the results of the comparison between a proposed Fourth Order Elastic Constants (FOECs) nonlinear model defined in the sense of Landau’s theory, and the two most contrasted hyperelastic models in the literature, Mooney–Rivlin, and Ogden models. A mechanical testing protocol is developed to investigate the large-strain response of ex vivo cervical tissue samples in uniaxial tension in its two principal anatomical locations, the epithelial and connective layers. The final aim of this work is to compare the reconstructed shear modulus of the epithelial and connective layers of cervical tissue. According to the obtained results, the nonlinear parameter A from the proposed FOEC model could be an important biomarker in cervical tissue diagnosis. In addition, the calculated shear modulus depended on the anatomical location of the cervical tissue (mepithelial = 1.29 0.15 MPa, and mconnective = 3.60 0.63 MPa) .Ministry of Education DPI2017-83859-R DPI2014-51870-R UNGR15-CE-3664Ministry of Health - Turkey DTS15/00093Junta de Andalucía PI16/00339 PI-0107-2017 PIN-0030-201

Mechanical characterization of animal derived starting materials for tissue engineering

Animal derived starting materials are well established in the production of Tissue Engineered Medical Devices. Porcine specifically can be found in products ranging in application from hernia repair to breast reconstruction. Although this material has been largely accepted in the Tissue Engineering industry, little is known of its material properties and mechanical characteristics. A review of the scientific literature describes limited mechanical measures only on uncontrolled research grade material. The objective of this work is to mechanically characterize porcine starting material used in the medical device industry. Porcine skin is provided by Midwest Research Swine, LLC (MRS) an established supplier to Medical Device companies. The experiments are established to evaluate if the skin’s mechanical characteristics vary by location and direction. The porcine skin samples are marked for their location (Back and Neck) as well as Orientation (Head and Spine). A custom die is used to prepare uniaxial tensile samples parallel, perpendicular, and at 45 degrees from the Spine landmark. An MTS load frame and Digital Image Correlation (DIC) measurement system is used to acquire the stress-stretch relationship. Mechanical indices from the stress-stretch relationship is analyzed by first separating it into a toe and linear region through a bilinear curve fitting method, apply the Ogden hypereslastic material model to the Toe, and linear model to the linear region. The Ogden fit in the toe region reveals anisotropic behavior that varied by location on the porcine skin, where the Back region behaves anisotropically and the Neck isotopically. The Gasser-Ogden-Holzepfel (GOH) structural model is explored to unify the tissue’s directional properties. The GOH model requires that the microstructural element’s (e.g. collagen fiber) arrangement within the macroscopic tissue is known beforehand. In the literature this is accomplished through histological measurements on the tissue itself. This requirement limits the ability of the GOH model to be used in the real time analysis of experimental work, which is needed in both academia and the tissue engineering industry. A method is developed to determine the microstructural arrangement (angle and dispersion) by utilizing the mechanical response at two orientations. This developed Microstructural Arrangement Determination Method (MADM) is verified by reproducing a GOH ABAQUS model from the literature. MADM is then applied to porcine skin experimental data, revealing a potential limitation of the GOH model in its treatment of transverse strain