The Challenge of Attaching Dissimilar Materials - Structural Interfaces and Attachments in Biology - page 8

attachment of cartilage to bone (Chap. 5). These junctions effectively transfer load
between two dissimilar materials through complex transitions in composition,
architecture, and mechanical properties. They develop through a well-orchestrated
developmental process via gradients of biologic factors to produce efficient natural
attachments (Chaps. 6 and 11). Similar mechanisms of attachment have been
described at interfaces found in teeth (Chap. 7).
Numerous organisms have evolved novel attachment systems to interact with
their environments and to enhance their physiologic function. The attachment
between muscle and tendon in fruit flies is discussed in Chap. 6, with a discussion
of the origin and development of tendon in the embryo of the fly. Furthermore, the
myotendinous junction and the molecular pathways driving embryonic develop-
ment are considered.
A diverse set of material designs is described in Chap. 9 for organisms that need
to attach to surfaces in aqueous environments. By studying the attachment systems
of mussels, barnacles, and tubeworms, the mechanisms of underwater adhesion are
elucidated. A fascinating attachment system is exhibited by geckos, enabling them
to easily form and release attachments to surfaces (Chap. 10). Both attachment and
detachment are achieved by changing the pulling angles at an interface and
manipulating an anisotropic adhesion mechanism. In addition, it is shown that the
fractal gecko hair system is highly damage-tolerant, being able to withstand exten-
sive cracking.
Chapter 8 reviews the mechanics of cellular attachment to surfaces. As cell
adhesion is critical for many biological functions, including cell migration, prolif-
eration, differentiation, and growth, a better understanding of cell adhesion
mechanisms has broad implications in biology and biophysics.
In the current chapter, we present an introductory overview of several biologic
examples where dissimilar materials with a large mismatch in mechanical properties
attach over a relatively short distance. The cases considered include dental tissues,
metal-to-bone orthopedic interfaces, and tendon-to-bone attachments.
1.2.2.1 Dental Attachments
An archetypal example of the attachment of dissimilar materials is found at the
interface between the tooth and alveolar bone. As described in [ 9 ], this attachment
involves two distinct interfaces: one between the alveolar bone and the cementum
(through periodontal ligament (PDL) collagen fibers) and one between the cemen-
tum and the root dentin (through the cementum-dentin junction (CDJ)). The
attachment structure and the variation in modulus throughout the attachment are
shown in Fig. 1.7 . It is interesting to observe that both interfaces possess lower
stiffnesses than those of adjacent materials. A similar observation was made in the
course of research at the tendon-to-bone insertion site; this is discussed below,
where it is hypothesized that such a dip in the stiffness is beneficial for reducing
stress concentrations. Chap. 7 further explores the adhesive mechanisms in teeth
and Chap. 3 explores the mechanics of attaching tendon to bone.