Mechanical Properties of Collagenous Tissues

In contrast to soft biologies, whose mechanical properties primarily depend upon the orientation of collagen fibers, the mechanical properties of mineralized tissues, or hard biologies, are more complicated. Factors such as density, mineral content, fat content, water content, and sample preservation and preparation play important roles in mechanical property determination. Specimen orientation also plays a key role, since most hard biologies such as bone are composite structures. For the most part, we will concentrate on the average properties of these materials and will relate these values to those of important, man-made replacement materials. 

Parry DAD. The molecular and fibrillar structure of collagen and its relationship to mechanical properties of connective tissue. Biophys Chem. 1988 29 195. 

We have discussed the mechanical properties of macromolecules and tissues and introduced the concept of viscoelasticity and the complexity that is introduced by this type of behavior. Although the components of ECMs are limited to cells, ions, water, collagen and elastic fibers, proteoglycans, and smooth muscle, the variation in arrangement of these components leads to wide variation in the mechanical properties of these tissues. It is important to understand the physical basis for the mechanical behavior of tissues. This is a complex task because of the time dependence as well as variations by which the components are attached. Below we attempt to address the physical basis of the mechanical behavior. 

Achilli M, Mantovani D (2010) Tailoring mechanical properties of collagen-based scaffolds for vascular tissue engineering the effects of pH, temperature and ionic strength on gelation. Polymers (Basel) 2 664—680. doi 10.3390/polym2040664... 

Yamamura N, Sudo R, Ikeda M, Tanishita K (2007) Effects of the mechanical properties of collagen gel on the in vitro formation of microvessel networks by endothelial cells. Tissue Eng 13 1443-1453. doi 10.1089/ten.2006.0333... 

Bone tissue, for example, is a nanocomposite composed of rigid hydroxyapatite (HA) nanocrystals (60%) precipitated onto coUagen fibers (30%) (Figure 40.1) [1]. Hydroxyapatite, which occurs as small plates that are tens of nanometers in length and width and 2 3 run in depth, impart compressive strength to bone. Collagen fibrils (1.5 3.5 nm in diameter) form triple hehces and bundle into fibers (50-70 nm diameter) responsible for the unique tensile properties of composite bone tissue [2]. The unique and complex mechanical properties of bone tissue arise from the interaction of these two components in the nanoscale [3]. 

FIGURE 40.1 Nanocomposite structure of bone. The interaction between coiiagen fibers and HA nanocrystals in the nanoscale gives rise to the complex mechanical properties of bone tissue observed in the macroscale. Cells operating on this collagen/hydroxyapatite nanocomposite continually remodel bone on the microscale. 

Although there is a strong molecular and nanoscopic driving force behind our understanding of the functionality of collagen fibrillar structures, the mesoscopic and macroscopic properties of the tissue are where the structural integrity manifests itself. One of the differences in mechanical resistive properties between tissues such as skin and tendon is the feltwork nature of fibril distribution in skin. This allows resistance to strain to occur within a two-dimensional plane. In contrast, tendon is required (normally) to resist strain only along its axis. 

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