Biomechanics of Damage, Growth and Remodelling

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Biological tissues constitute the organs of living beings, and are comprised of cells of the same type embedded in a ground material called extracellular matrix. The extracellular matrix can be described as a porous material, reinforced by collagen fibres and saturated by a fluid in which several chemical substances are dissolved. ***The greatest difference between engineering materials and biological tissues is that the latter are living materials that interact with and sense stimuli from the surrounding environment. Because of these interactions, they undergo processes of growth, remodelling and, in unfortunate cases, injury and damage. Growth is defined as the increase (accretion) or decrease (resorption) of mass, remodelling as the rearrangement of the internal structure, and damage as the loss of structural integrity. ***Our research is aimed at developing a unified continuum mechanical model of biological tissues accounting for damage, growth and remodelling, as well as for the interaction among the three phenomena. We seek for an energy-based model, i.e., a model in which virtually all information about the behaviour of the tissue can be included in a single mathematical function, the (Helmholtz) free energy. The derivation of the mathematical expression of the free energy comes from considerations made on the microstructure of the tissue. Specifically, we are interested in the electromechanical interactions among the macromolecules in the matrix, collagen fibre reorientation under external stimuli, which accounts for a good part of the remodelling, and collagen fibre failure, which ultimately determines tissue damage. Emphasis will be placed on the study of blood vessels and articular cartilage, and experiments will be performed to validate the individual components of the unified model.***Such a general approach has never been attempted before and will give us the possibility to obtain information on the complex interaction among these phenomena, which is otherwise virtually impossible to quantify experimentally. Specific experiments will be performed on simple geometries to validate the damage and remodelling aspects of the model, individually.***The long-term objectives of this research are: ***1) To model the molecular mechanisms of damage in soft tissues, and how damage triggers and interacts with growth and remodelling;***2) To design techniques aimed at mechanically, chemically, electrically stimulating the biosynthetic response of the cells in the tissue in order to promote a specific response, as well as tissue-engineered materials capable of replacing irremediably damaged tissue, as is the case for articular cartilage. ***This programme will strongly contribute to the training of graduate students and postdocs in the foundations of Continuum Mechanics and its applications to Soft Tissue Biomechanics, as well as in the experimental techniques necessary for model validation.