Biomedical Engineering Reference
In-Depth Information
9.2 Basic concept of composite material
traditionally, a composite is a material composed of two or more phases.
For the sake of simplicity, focusing on a two component composite, a
continuous phase, a dispersed phase and an interface can be distinguished.
the continuous phase is generally referred as the matrix and polymers
represent the material mostly used as its components. the dispersed phase
can be discontinuous (i.e. platelet or filler) or continuous (i.e. fibres) and it is
generally stiffer than the matrix, thus it is considered to be the reinforcement
component of the composite since it enhances the mechanical properties of
the matrix (i.e. stiffness, strength). However, examples of a dispersed phase
softer than the matrix which enhance the toughness of the composite also
exist (i.e. rubber-like reinforcement in a ductile brittle polymer matrix or
metal reinforcement in a ceramic matrix).
the interface between the matrix and the reinforcement plays a major role
in determining the mechanical performance and environmental stability of
composite materials. the mechanical features of the composite rely on the
load transfer at the interface. the quality of adhesion between reinforcement
and matrix depends on the interactions between the composite constituents,
which can be chemical and/or mechanical. mechanical bonding strongly
depends on the surface topography or morphology of the reinforcement,
whereas chemical bonding is preferentially promoted by surface treatments
or coatings such as the silanization process (Arends, 1993). To a certain
extent, the definition given to composite materials depends upon the level
of analysis, as all materials may be considered heterogeneous if the scale of
interest is sufficiently small. However, connective tissues show a composite
structure at all hierarchical levels and the effects of the structure organization
on the mechanical properties are evident at a larger micro-mechanical scale.
moreover, living tissues manifest a marked viscoelastic behaviour. to date,
polymers represent the only technological solution to bio-inspired composite
(Barbucci, 2002)
9.2.1 Mechanical tailoring
Reinforced polymers have gained an increasingly important role in the
development of new generation biomedical materials since they can be
engineered more accurately than monolithic structures (single phase materials),
thus allowing the development of more effective tissue substitutes. the main
font of inspiration for designing new high performance and multifunctional
materials arises from the observation and study of biological materials. For
example, the basic constituents of connective hard tissues are the extracellular
matrix (i.e. collagen, apatite and water) and cells (namely osteocytes in bone
and odontoblast in dentine) which control and adapt the structure performance
