Characterization of biological tissues, biomaterials

In the broad and multidisciplinary panorama of bioengineering, the study of biological tissues and biomaterials occupies a broad spectrum role both in their clinical and methodological relevance. In particular, the tissues are characterized by a multi-physical and mechano-biological behavior in which the mechanics are certainly a relevant part, especially considering the relationship between physio-pathological conditions and the properties of the tissues. Biological tissues are characterized by highly non-linear, viscoelastic, and generally anisotropic mechanical behavior. The properties of the tissues regulate, in turn, numerous pathophysiological mechanisms that influence the activity and functionality of biological macrostructures (e.g., bone tissue, articular cartilage, blood vessels). 

Alongside biological tissues, biomaterials used in biomedical devices (e.g., bone prostheses, implants, stents) are also subject to mechanical and multi-physical characterization in order to guarantee their functionality and biocompatibility. The conception, prototyping, and development of innovative materials and devices represent, in fact, a sector in continuous expansion and with promising prospects. The characterization, the modeling, and analysis of the mechanical behavior of biological tissues and biocompatible materials are, therefore, fundamental themes for the development of design and analysis methodology (experimental or computational) with clinical relevance. 

It is important to underline that these applications are strongly characterized by multi-scale mechanisms (e.g., the tissue response as a whole depends on the properties and spatial organization on all scales. It means from the molecular scale to the characteristic length of the entire organ; the biocompatibility of a material depends on the cellular response to the nanoscale) and multiphysics (e.g., fluid-structure interactions, poroelastic mechanisms, transport-reaction-diffusion mechanisms). It can be addressed through knowledge, methods, and strategies in common with numerous other fields of 'Engineering and, in particular, of Mechanics.

Within the multidisciplinary field of bioengineering, the research on biomaterials and tissues covers a central role for clinical and methodological approaches. In this framework, mechanics and biological processes (mechano-biology) strongly couple each other, determining the physiological behavior of biological structures, as well as the onset of pathologies, in a complex multiphysics environment. Biological tissues are characterized by highly nonlinear mechanical behavior, viscoelasticity, anisotropy as well as an adaptive process. These properties are strictly related to physio-pathologic mechanisms that affect the functions of tissues at all scales, which, in turn, determine the response of macro-biological structures (e.g., bone tissue, articular cartilage, blood vessels, etc.). Moreover, biomaterials used in biomedical devices (e.g., bone prosthesis, implants, stents, etc.) are also of paramount relevance, and their design is a challenging topic as they should be biocompatible and able to replace damaged tissue/structures with proper functionality. 

The above concepts show how the modeling of innovative materials and devices is a growing field of research with promising scientific and clinical perspectives. In particular, the characterization and the modeling of the mechanical behavior of tissues and biomaterials should address advanced and novel multi-scale. For example, the response of tissue highly depends on the arrangement of constituents, down from the molecular scale while the biocompatibility of materials depends on the cellular response at the nanoscale) and multiphysics. (e.g., fluid-structure interaction; poroelastic mechanisms; transport-reaction transport phenomena) approaches in a multi-disciplinary environment.

Tissue regeneration

Tissue regeneration techniques are also used in modern aesthetic medicine and current cosmetics, which provides for the replacement of conventional cosmetic products, built on the concept of masking imperfections. With new classes of compounds/formulations which is called cosmeceuticals, capable of eliciting responses, organic products that "naturally" revitalize the skin tissue, correcting the aging imperfections. 

In fact, the recent discovery of stem cells also in dermo-epidermal structures is giving new impetus to the development of cosmeceuticals aimed at activating this cell population. 

Numerous factors are involved in the differentiation and proliferation processes of the skin area, among all, those of growth for the epithelium plays a fundamental role not only in the epithelial district but above all in the supportive connective tissue, thus becoming primary factors for all healing mechanisms. The interactions between epithelium and connective tissue are fundamental for the survival of the tissue itself, and these interactions are determined by small signal molecules that can alter and/or control the homeostatic balance of the skin district itself.

The molecular correlation between these processes is still not fully known today and represents one of the most important open challenges in the biomedical field.

The regeneration of tissues and whole organs, starting from stem cells, is one of the most promising frontiers of biomedical research. It is on the perspectives that this field of research opens up, which trusts the scientific community to identify the cure for pathologies in which the selective death of a particular type of cells, such as in Parkinson's disease, heart disease, and diabetes. What are the mechanisms that regulate the fate of stem cells in maintaining tissue integrity? And how are these same mechanisms reprogrammed in pathological conditions, such as cancer and diabetes, or during the physiological wound healing process?

Throughout the adult life of the organism, there is a continuous need to produce new cells to replace those lost during differentiation, cellular aging, or tissue damage. Stem cells take care of ensuring homeostasis of the tissues. The cells are capable of self-renewal but also capable of generating the progenitors, which, by differentiating, give rise to tissues and organs. But it is not the only function of the stem cells. These particular cells are also the protagonists of the injury response.

Among the most common and best-studied lesions, epidermal wounds are high on the list. The skin is, in fact, a system with high regenerative capacities, a feature that makes it the ideal tissue for studying the regeneration and role of stem cells. Following an injury, the first response is inflammation that calls the immune system cells to the damaged site. A few days later, the second phase begins in which the stem cells of the epidermis multiply, giving rise to the progenitors, responsible for the formation of new tissue. The third phase, called remodeling, marks the end of the damage response: all the processes activated after the injury cease, and the normal structure of the tissue is restored. The Caspase-8, a protein classically known to be involved in apoptosis, has a key role in reduced in order for the tissue to repair itself. Defects in the regulation of this protein cause serious consequences: in diabetes, for example, the inability to heal from wounds correlates with the increased production of Caspase-8 and, on the other hand, a persistent reduction of its expression translates into a chronic inflammatory response with symptoms such as eczema.

Author: Vicki Lezama