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The objective of the current study is to elucidate the influence of freeze storage on the tensile properties of tendons by testing specimens at numerous adjacent durations to eliminate the influence of biological variation and experimental errors. Chicken flexor digitorum profundus tendons were stored intact in the digits at -20°C and specimens were tensile tested at 33 durations over 360 days. Scanning electron microscopy was done on fresh specimens and specimens freeze-stored for 30, 233 and 427 days to investigate microstructure change after freeze storage. The tensile properties of tendons, as represented by ultimate tensile strength, strained at ultimate tensile strength. The elastic modulus did not deviate significantly (p>0.01) after freeze storage while the collagen fiber density of specimens stored for 233 and 427 days decreased with porosity growth. In contrast, no obvious microstructure difference was observed between fresh tendons and tendons stored for 30 days. These findings suggest that the postmortem degradation incurred a minor effect on the structural integrity of the tendons and therefore did not affect the overall tensile properties. We conclude that freeze storage at -20°C does not alter the tensile properties of tendons for as many as 360 days.

Measurements of cross-sectional areas (CSAs) of soft tissues such as tendons and ligaments allow for the evaluation of the biomechanical properties of the tissue. Underlying in vitro techniques are data reduction approaches for determining the average thickness of the tissue and the assumption of the geometry of the cross-section, i.e. circular or elliptical. However, tissue distortions, sagging, and concavities could affect the reliability of these techniques, since these features may not be accounted for adequately. To address some of the concerns faced by these techniques, a non-contact (non-destructive) laser scan technique has been developed. In this technique, a laser scans along the axis of the tissue, a coordinate measuring machine simultaneously locates the corresponding point on the tissue based on the detection of reflected (attenuated) intensity, and, finally, computerized image analysis reconstructs the morphology of the tissue. This technique was applied to patellar tendons (PTs) from New Zealand rabbits. The scanning time for each PT was less than 2 minutes. Reconstructed three-dimensional surface plots revealed microconcavities consistent with images seen under optical microscopy. CSAs of these PTs were determined for repeatability and precision; results from a conventional approach which estimated the corresponding CSAs based on the average thickness and the assumption of ellipsoidal cross-sectional geometry were also determined for the purpose of comparison. Based on the standard cuboid model, the error between the laser technique and the conventional approach was within 0.4%; the reproducibility of the laser technique was within 2%.

Dynamic soft tissue characterization is of importance to robotic-assisted minimally invasive surgery. The traditional linear regression method is unsuited to handle the non-linear Hunt–Crossley (HC) model and its linearization process involves a linearization error. This paper presents a new non-linear estimation method for dynamic characterization of mechanical properties of soft tissues. In order to deal with non-linear and dynamic conditions involved in soft tissue characterization, this method improves the non-linearity and dynamics of the HC model by treating parameter $p$ as independent variable. Based on this, an unscented Kalman filter is developed for online estimation of soft tissue parameters. Simulations and comparison analysis demonstrate that the proposed method is able to estimate mechanical parameters for both homogeneous tissues and heterogeneous and multi-layer tissues, and the achieved performance is much better than that of the linear regression method.

High strain rate compression of soft tissues has recently gained attention due to its application in computational simulation of traumatic injuries. To understand high rate tissue behavior, a comparative study is needed to examine the biomechanical responses of multiple soft tissues. We hypothesized that the underlying mechanisms of soft tissue high rate compression is dependent upon water, microstructural organization, and extracellular matrix (ECM). Porcine brain, liver, and tendon, which have similar material density (brain: 1.05g/cm³, liver: 1.06g/cm³, and tendon: 1.12g/cm³) and water content (brain: $\sim$78%, liver: $\sim$71–75%, and tendon: $\sim$70%) but different cellularity and ECM properties, were subjected to polymeric split Hopkinson pressure bar (PSHPB) testing. Hydrated brain tissue, due to its high microstructural cavities (cavity area $\mathrm{\text{fraction}}=0.49\pm 0.26$) and low ECM content, had a high initial stress spike of $0.22\pm 0.12$MPa. Hydrated liver, with moderate microstructural cavities (cavity area $\mathrm{\text{fraction}}=0.30\pm 0.07$) and moderate ECM content, had a moderate stress spike of $0.10\pm 0.04$MPa. Hydrated tendon had low microstructural cavities (cavity area $\mathrm{\text{fraction}}=0.03\pm 0.02$) and high ECM content and had a minimal initial stress spike of $0.02\pm 0.01$MPa. Electron microscopy of the tissues’ microstructural cavities revealed a first-order estimation of water content that was not bound to ECM (e.g., intracellular water). Linear regression analysis showed that the initial spike was highly correlated ($r^2=0.9532$) with intracellular water. To understand the role of water in each tissue’s response to high deformation, each soft tissue was completely dehydrated and subjected to the same PSHPB compression test. After removal of all the water, neither the brain, liver, nor tendon revealed an initial stress spike, implicating the essential nature of water in the initial responses of high-rate compression. These results suggest that cellular water and ECM content play a critical role in the biomechanical responses to high strain rate compression.

Ballistic injury refers to the interaction of a projectile and the human body, resulting in penetration or blunt trauma. In order to consider both consequences, a hydrodynamic elastoplastic constitutive law was implemented in a numerical FE model of the human torso to simulate soft tissues behavior and to evaluate their injury risk. This law, derived from 20% ballistic gelatin, was proven to be very efficient and biofidelic for penetrating ballistic simulation in soft tissues at very high velocity. In this study, the ability of the hydrodynamic law to simulate blunt ballistic trauma is evaluated by the replication of Bir et al.’s (2004) experiments, which is a reference test of the literature for nonpenetrating ballistic impact. Lung injury criteria were also investigated through the Bir et al.’s experiments numerical replication. Human responses were evaluated in terms of mechanical parameters, which can be global (acceleration of the body, viscous criteria and impact force) or local (stress, pressure and displacement). Output results were found to be in experimental corridors developed by Bir et al., and the maximum pressure combined with the duration of the peak of pressure in the lungs seems to be a good predictor for lung injury.

Heating analysis of soft tissue at finite deformation is of importance during treatment planning process for thermal ablation. This paper presents a method by combining non-Fourier bio-heat transfer and nonlinear biomaterial model to simulate thermal and mechanical responses of soft tissues under combined thermal and mechanical loads. At the same time, anisotropic feature of biological tissue is also concerned by including the stored energy of collagen fiber bundles into the traditional neo-Hookean free energy function. Simulation results demonstrate that in spite of relatively small thermal-induced displacement, the resultant thermal strain and stress contribution is nonnegligible and can be used as theoretical basis to measure tissue pain during thermal ablation process.

This paper focuses on thick-walled spherical shells and cylindrical tubes made of the soft tissues and the rubber-like materials. These materials are characterized by high deformability in which their stress–stretch curves are arranged in the range of S-shaped to J-shaped forms. From the continuum viewpoint, a strain energy density function is postulated for modeling the behavior of these materials. In order to fulfill the main aims of this paper, among all existing energy functions including polynomial, power law, logarithmic and exponential functions, or a linear combination of them, we deduced to evaluate the performance of an Ogden-type model with only integer powers for the mechanical behavior modeling of the S-shaped to J-shaped materials. Most of all, this strain energy function because of its mathematical form can play a constructive role in presentation of the analytical closed-form solutions for the boundary value problems in the field of the finite deformation elasticity. This constitutive model due to the high performance in constitutive modeling and the simplicity of its mathematical form is applied to pressurized thick-walled spherical shells and cylindrical tubes in order to find a closed-form analytical solution for their analysis. Using these analytical solutions, a comprehensive study is done on vanishing circumstance of the snap-through instability that occurs in the inflation of internally pressurized spherical shells and cylindrical tubes. It was observed that the parameters such as shell thickness, the elastic material properties specially the materials with J-shaped mechanical behaviors and the absence and presence of axial forces in cylindrical tubes have significant influence on vanishing of the snap-through instability in the thick-walled pressurized spherical shells and cylindrical tubes.

While finite elements has been considered during the last decades as the universal tool to perform simulations in biomechanics, a recently developed wide family of methods, globally coined as meshless methods, has emerged as an attractive choice for an increasing variety of engineering problems. They present some key advantages such as the absence of a mesh in the traditional sense, particularly important in domains of very complex geometry, a less sensitivity to the nodal distribution and therefore to the implicit mesh distorsion what is especially interesting to handle problems under finite strains and large displacements in a Lagrangian framework. Here, we analyze the convenience and possible advantages of using meshless methods in numerical simulations of soft biological tissues. Biological tissues are usually nonlinear, anisotropic, inhomogeneous, viscoelastic, and undergo large deformations, so these methods seem to be an appealing possibility for this type of applications. In particular, we discuss the use of one of these methods, the so-called natural element method that has specific and important features as interpolatory character, easy handling of geometry, and essential boundary conditions via the so-called alpha-NEM extension, well-defined mathematical properties and a simple computer implementation. Different examples are solved using this approach including the human cornea, the temporo-mandibular joint, knee ligaments, and the passive behavior of the heart.