Introduction
Mechanics of Living Tissues: Applications, Challenges and Methods

Cédric LAURENT

LEM3, Université de Lorraine, Metz, France

I.1. The mechanics of living tissues: what is the purpose?

The various chapters that constitute this book will focus on specific and often understudied tissues, attempting to draw up an inventory of current knowledge with regard to their anatomy and microstructure, their mechanical behavior or the various modeling proposals (analytic and/or numerical) that have been put forward. The challenges associated with these different aspects constitute a significant part of the current literature on living tissue mechanics, and the answers that are provided often depend on the specificities of the tissue studied, as well as the (often clinical) purpose of the research that is conducted. Given that this book is focused on the description of the mechanics of living tissues, it seems appropriate to begin by addressing its purpose, and drawing up some examples of problems that will be tackled, and which require a detailed understanding of the mechanics of living tissues.

I.1.1. Detecting and understanding pathologies

When an injury or pathology appears within a living tissue, its mechanical properties are often affected. While most methods related to the detection or diagnosis of such pathologies are outside the field of mechanics, characterization of the mechanical properties of tissues can nevertheless help to detect or monitor the evolution of a pathology as well as help to understand the underlying mechanisms. We can thus try to characterize the evolution of the properties of a tissue over time, as will be the case for the diagnosis of liver fibrosis (Chapter 1) or the monitoring of degenerative pathologies of bone (Chapter 9). We can also try to understand the interaction between living tissue and its environment, by trying to identify the many actions and feedback that exist between the composition of a tissue, its mechanical environment and its actual properties (as in the case of cardiovascular diseases in Chapter 8). Based on the modeling of the role of the tissue in its environment, it might also be worthwhile to understand the origin of a pathology such as the rupture of the anterior cruciate ligament (Chapter 7), and provide information on the prevention of such an injury or appropriate rehabilitation protocols, or try to interpret the measures that are made to characterize a pathological cornea more precisely (Chapter 3). In all cases, these approaches require a detailed understanding of the normal and pathological behavior of biological tissues, which is the objective of this book.

I.1.2. Developing implant designs or surgical procedures

For several decades, joint advances in medicine, biomedical engineering and materials science have made it possible to formulate solutions to many clinical problems related to tissue rupture, aging or damage. When it comes to developing such a solution (most often an implant), it is necessary to know the stresses to which it will be subjected in its physiological environment, as well as the native properties of the tissue it aims to replace.

In both cases, experimental campaigns obviously make it possible to characterize these mechanical properties: they nevertheless require long and expensive approaches, and are not without limitations, as will be conveyed below. Alternatively, different computational simulation approaches (and in particular the finite element method) make it possible both to attempt to simulate the mechanical environment of a tissue during a physiological activity, and to simulate the changes in this environment by replacing native tissue with a replacement solution. These simulations can also make it possible to predict post-implantation evolutions if additional and more complex phenomena (aging, degradation, remodeling, etc.) are taken into account in the models. In addition, and finally, such simulations can make it possible to study (and improve) certain parameters of a surgical procedure (position or properties of an implant, interaction with surrounding tissues, surgical access, etc.) without resorting to a trial-and-error approach, which is of course unsuitable in the field of health.

However, these different approaches also require a detailed understanding of the mechanics of living tissues. They will be discussed particularly with regard to the improvement of liver surgery (Chapter 1), reconstructive skin surgery (Chapter 2), to predict and control the functional impact of lingual surgery (Chapter 4), the development of alternatives for the disc of the temporomandibular joint (Chapter 5), the intervertebral disc (Chapter 6) or the cornea (Chapter 3), or improving the ligamentoplasty of the anterior cruciate ligament (Chapter 7).

I.2. The mechanics of living tissues: specificities

I.2.1. Defining external geometries and microstructures

When it is a question of studying a biological tissue or a substitute to replace it, it is necessary to define its external geometry (at the macroscopic scale of the organ), and possibly also its microstructure or constituents (at the mesoscopic or microscopic scale). In both cases, the use of different imaging methods (often complementary) has become unavoidable. Table I.1 summarizes and illustrates different imaging modalities that are commonly used to inspire work in the field of mechanics of living tissues, and which will be mentioned in the following chapters, without aiming to be exhaustive. The resolutions associated with these different modalities depend strongly on the equipment used, as well as the conditions of acquisition; it is difficult to draw up an inventory of achievable resolutions at a given moment with each of the methods, as these specifications evolve quickly over time. However, orders of magnitude are given as an indication in Table I.1. Some methods are very local and make it possible to characterize the microstructure, composition or surface roughness of a tissue (atomic force microscopy (AFM), confocal microscopy, Raman spectroscopy, micro-computed tomography (µCT)), while others are limited to the macroscopic scale but allow for large volumes of images to be acquired (magnetic resonance imaging (MRI), computed tomography (CT), three-dimensional (3D) scanner, dual-energy X-ray absorptiometry (DXA)).

Table I.1. Examples of imaging methods commonly used to characterize the external geometry and/or microstructure of biological tissues