Biomedical Engineering Reference
In-Depth Information
Table 9.1
Requirements for synthetic bone grafts.
1
Biocompatible
2
High porosity with an interconnected pore network to facilitate migration
of cells, enabling fluid flow for nutrient supply and the removal of cellular
waste products, and to permit vascular invasion
3
Suitable bioactivity to exploit the body's natural repair process, with
biological response similar to that achieved by bioactive glasses
4
Biodegradable with predictable biodegradation rate matched to the
formation rate of neo-tissue
5
Sufficient mechanical competence, (time-dependent) structural integrity and
easy to process into 3D complex porous shapes in a controllable manner
from the composite structure and hierarchy of porosity of bone in order
to develop improved synthetic composites.
Bioactive materials are an integral part of this strategy. These materials
react with physiological fluids and form tenacious bonds to hard tissue
through biological interdigitation of collagen fibres with HCA layers on
the material surface [1]. Thus these biomaterials can be used to transfer
loads to and from living bone. Bioactive glasses, as a special class of
bioactive ceramics, is the subject of this volume, and detailed descriptions
of the chemical composition, processing routes and properties of these
materials are included in this topic.
Like most ceramic materials (including bone minerals), the major
disadvantage of bioactive glasses is their low fracture toughness (i.e. brit-
tleness). Bioactive glass is therefore often used in composites combined
with polymers, similar to bone minerals combined with collagen in nat-
ural bone. Both stable polymers, for example poly(methyl methacrylate)
(PMMA), and biodegradable polymers, for example aliphatic polyesters,
have been applied in the fabrication of biocompatible composites. How-
ever, non-degradable materials are likely eventually to be rejected by
the body, owing to the cells in the body forming a fibrous capsule
around the implant. Therefore, wherever possible, surgeons prefer to
use biodegradable materials, as long as they complete their function
before degrading and allowing natural tissue repair.
Polymer-based composites are usually made with glass or ceramic
fibres dispersed in the polymer matrix (Figure 9.2) to reinforce the
polymer and increase its stiffness. This is the principle used in the
manufacture of lightweight high-performance components, for example,
in specialist aircraft fuselages, car bodies, bicycle frames or tennis rackets.
Just dispersing particles or short fibres in a matrix (Figure 9.2b) does
stiffen the matrix (increase in Young's modulus) but further increase
