The purpose of this study is to investigate the effects of preconditioning on the deformation response of planar tissues measured by inflation tests. The inflation response of test specimens, including the bovine cornea, bovine and porcine sclera, and human skin, exhibited a negligible evolving deformation response when subjected to repeated pressure loading with recovery periods between cycles. Tissues obtained complete recovery to the reference state, and strain contours across the entire specimen were nearly identical at the maximum pressure of each load cycle. This repeatability was obtained regardless of strain history. These results suggest that negligible permanent change was induced in the microstructure by inflation testing. Additionally, we present data illustrating that a lack of a recovery period can result in an evolving deformation response to repeated loading that is commonly attributed to preconditioning. These results suggest that the commonly observed effects of preconditioning may be avoided by experimental design for planar tissues characterized by long collagen fibers arranged in the plane of the tissue. Specifically, if the test is designed to fully fix the specimen boundary during loading, adequate recovery periods are allowed after each load cycle, and loads are limited to avoid damage, preconditioning effects may be avoided for planar tissues.
Reinforcing
mechanically weak hydrogels with fibers is a promising
route to obtain strong and tough materials for biomedical applications
while retaining a favorable cell environment. The resulting hierarchical structure recreates
structural elements of natural tissues such as articular cartilage,
with fiber diameters ranging from the nano- to microscale. Through
control of properties such as the fiber diameter, orientation, and
porosity, it is possible to design materials which display the nonlinear,
synergistic mechanical behavior observed in natural tissues. In order
to fully exploit these advantages, it is necessary to understand the
structure–property relationships in fiber-reinforced hydrogels.
However, there are currently limited models which capture their complex
mechanical properties. The majority of reported fiber-reinforced hydrogels
contain fibers obtained by electrospinning, which allows for limited
spatial control over the fiber scaffold and limits the scope for systematic
mechanical testing studies. Nevertheless, new manufacturing techniques
such as melt electrowriting and bioprinting have emerged, which allow
for increased control over fiber deposition and the potential for
future investigations on the effect of specific structural features
on mechanical properties. In this review, we therefore explore the
mechanics of fiber-reinforced hydrogels, and the evolution of their
design and manufacture from replicating specific features of biological
tissues to more complex structures, by taking advantage of design
principles from both tough hydrogels and fiber-reinforced composites.
By highlighting the overlap between these fields, it is possible to
identify the remaining challenges and opportunities for the development
of effective biomedical devices.
This study investigates how the collagen fiber structure influences the enzymatic degradation of collagen tissues. We developed a micromechanical model of a fibrous collagen tissue undergoing enzymatic degradation based on two central hypotheses. The collagen fibers are crimped in the undeformed configuration. Enzymatic degradation is an energy activated process and the activation energy is increased by the axial strain energy density of the fiber. We determined the intrinsic degradation rate and characteristic energy for mechanical inhibition from fibril-level degradation experiments and applied the parameters to predict the effect of the crimped fiber structure and fiber properties on the degradation of bovine cornea and pericardium tissues under controlled tension. We then applied the model to examine the effect of the tissue stress state on the rate of tissue degradation and the anisotropic fiber structures that developed from enzymatic degradation.
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