3D fiber-deposited scaffolds for tissue engineering: Influence of pores geometry and architecture on dynamic mechanical properties

Eight structural elements in biological materials are identified as the most common amongst a variety of animal taxa. These are proposed as a new paradigm in the field of biological materials science as they can serve as a toolbox for rationalizing the complex mechanical behavior of structural biological materials and for systematizing the development of bioinspired designs for structural applications. They are employed to improve the mechanical properties, namely strength, wear resistance, stiffness, flexibility, fracture toughness, and energy absorption of different biological materials for a variety of functions (e.g., body support, joint movement, impact protection, weight reduction). The structural elements identified are: fibrous, helical, gradient, layered, tubular, cellular, suture, and overlapping. For each of the structural design elements, critical design parameters are presented along with constitutive equations with a focus on mechanical properties. Additionally, example organisms from varying biological classes are presented for each case to display the wide variety of environments where each of these elements is present. Examples of current bioinspired materials are also introduced for each element.

/pdf/structural-design-elements-in-biological-materials-3kuidj7v00.pdf

Structural Design Elements in Biological Materials: Application to Bioinspiration

http://www.fizjoterapeutom.pl/files/5/Biomechanics%20Of%20Knee%20Ligaments.pdf

Biomechanics of knee ligaments: injury, healing, and repair

Electromagnetic effects - From cell biology to medicine.

Tear resistance is of vital importance in the various functions of skin, especially protection from predatorial attack. Here, we mechanistically quantify the extreme tear resistance of skin and identify the underlying structural features, which lead to its sophisticated failure mechanisms. We explain why it is virtually impossible to propagate a tear in rabbit skin, chosen as a model material for the dermis of vertebrates. We express the deformation in terms of four mechanisms of collagen fibril activity in skin under tensile loading that virtually eliminate the possibility of tearing in pre-notched samples: fibril straightening, fibril reorientation towards the tensile direction, elastic stretching and interfibrillar sliding, all of which contribute to the redistribution of the stresses at the notch tip.

/pdf/on-the-tear-resistance-of-skin-1mbaj6axnk.pdf

On the tear resistance of skin

A physiologic constitutive expression is presented in algorithmic format for the nonlinear elastic response of wavy collagen fibrils found in soft connective tissues. The model is based on the observation that crimped fibrils in a fascicle have a three-dimensional structure at the micron scale that we approximate as a helical spring. The symmetry of this wave form allows the force/displacement relationship derived from Castigliano's theorem to be solved in closed form: all integrals become analytic. Model predictions are in good agreement with experimental observations for mitral-valve chordae tendinece.

Elastic model for crimped collagen fibrils

https://onlinelibrary.wiley.com/doi/pdf/10.1016/j.orthres.2005.03.015

Pulsed electromagnetic field treatments enhance the healing of fibular osteotomies

Knowledge of strain-rate sensitivity of soft tissue viscoelastic and nonlinear elastic properties is important for accurate predictions of biomechanical behavior and for quantitative assessment of the effects of disease or surgical/pharmaceutical intervention. Soft tissues are known to exhibit mild rate sensitivity, but experimental artifacts related to testing system control can confound estimation of these effects. “Perfect” ramp-and-hold stress-relaxation tests become difficult at high strain rates because of problems related to undershoot/overshoot error and vibrations. These errors can introduce unwanted bias into parameter estimation methods that rely on idealizations of the applied ramp-and-hold displacement. To address these problems, we describe a new method for estimating quasilinear viscoelastic (QLV) parameters that directly fits the QLV constitutive model to the actual point-wise stress–time history of the test, using an adaptive grid refinement (AGR) global optimization algorithm. This new method significantly improves the accuracy and predictivity of QLV parameter estimates for heart valve tissues, compared to traditional methods that use idealized displacement data. We estimated QLV parameters for aortic valve tissue over a range of physiologic displacement rates, finding that the viscoelastic content parameter (C) increased slightly with increasing strain rate, but the fast (τ1) and slow (τ2) time constants were strain rate insensitive.

The effect of strain rate on the viscoelastic response of aortic valve tissue: a direct-fit approach.

BACKGROUND Quasilinear viscoelasticity (QLV) theory has been widely and successfully used to describe the time-dependent response of connective tissues. Difficulties remain, however, particularly in material parameter estimation and sensitivities. In this study, we introduce a new alternative: the fractional order viscoelasticity (FOV) theory, which uses a fractional order integral to describe the relaxation response. FOV implies a fractal-like tissue structure, reflecting the hierarchical arrangement of collagenous tissues. METHOD OF APPROACH A one-dimensional (I-D) FOV reduced relaxation function was developed, replacing the QLV "box-spectrum" function with a fractional relaxation function. A direct-fit, global optimization method was used to estimate material parameters from stress relaxation tests on aortic valve tissue. RESULTS We found that for the aortic heart valve, FOV had similar accuracy and better parameter sensitivity than QLV, particularly for the long time constant (tau2). The mean (n = 5) fractional order was 0.29, indicating that the viscoelastic response of the tissue was strongly fractal-like. RESULTS SUMMARY: mean QLV parameters were C = 0.079, tau1 = 0.004, tau2 = 76, and mean FOV parameters were beta = 0.29, tau = 0.076, and rho = 1.84. CONCLUSIONS FOV can provide valuable new insights into tissue viscoelastic behavior Determining the fractional order can provide a new and sensitive quantitative measure for tissue comparison.

Fractional order viscoelasticity of the aortic valve cusp: an alternative to quasilinear viscoelasticity.

BACKGROUND AND AIM OF THE STUDY The aortic valve cusp is commonly described as a three-layered structure containing circumferentially aligned fiber bundles. Little is known, however, regarding fiber bundle sizes, branching patterns, or how they are connected. This is because previous morphological studies relied primarily on histological sectioning and staining techniques, which tend to affect all of the collagen, regardless of structure or orientation. METHODS To address this problem, a novel system was developed for the visualization and analysis of the intermediate-scale 'mesostructures' of aortic valve cusps. Mesostructures are defined as the branching fiber bundle and membrane structures that make up the valve. This system uses elliptically polarized light to provide contrast between collagen mesostructures without the need for embedding, staining, or other contrast-enhancing techniques. Using this system, high-resolution images of 42 whole porcine aortic valve cusps were acquired in an unloaded (i.e. resting) condition and during application of controlled manipulation. Image-processing algorithms were developed to quantify fiber bundle morphological features and produce detailed maps of the fiber bundle patterns. RESULTS Fiber bundle sizes and patterns were found to be significantly different for each of the three cusps. The non-coronary cusp had a significantly smaller bundle diameter (0.9 +/- 0.07 mm) than the left and right coronary cusps (1.1 +/- 0.08 mm). The left and non-coronary cusps appeared to be mirror images of each other, whereas the right coronary cusp was self-symmetric. When applying controlled loads to the cusp specimens, thin, overlapping, collagenous membranes were often found which connected the fiber bundles. Interesting pinnate fiber branching patterns were also found. CONCLUSION These morphological results were strikingly different than the currently accepted three-layer description, and may provide valuable insight into aortic valve structure-function relationships.

Todd C. Doehring

Papers

Elastic model for crimped collagen fibrils

Pulsed electromagnetic field treatments enhance the healing of fibular osteotomies

The effect of strain rate on the viscoelastic response of aortic valve tissue: a direct-fit approach.

Fractional order viscoelasticity of the aortic valve cusp: an alternative to quasilinear viscoelasticity.

Mesostructures of the aortic valve.