Biomedical Engineering Reference
In-Depth Information
(bone) and soft tissue (e.g. the tympanic membrane of the ear). The
dissolution products of soluble silica and calcium ions are accepted to
be responsible for Bioglass having high bioactivity, as they stimulate
cells to produce more bone. Calcium is common to all of the bioactive
ceramics mentioned above. It would be easy to consider bioceramics as
ideal materials for bone repair. However, ceramics and glasses are hard,
brittle materials, with little flexibility and toughness.
Softer and flexible materials with the bioactivity of bioceramics or
high affinity to either soft or hard tissue would be advantageous in
clinical applications. Alternatively, bioactivity and improved stiffness
can be provided to polymers through hybridization. Surgeons would
like materials that could be cut with their surgical tools and pushed into
a defect with their fingers. Ideally, the implant would then expand and
fill the defect. They would also like the implant to share the load with
the bone. This is important, as bone cells produce more natural and
more healthy bone under load. Bioceramic implants are likely to fail in
defects that are under cyclic loads due to their brittle nature. Implants
may also be designed to be porous to act as temporary (degradable) tem-
plates (scaffolds) for three-dimensional tissue regeneration (Chapter 12).
Toughness, flexibility, and bioactivity are also the desirable properties
for bone scaffolds.
Suppose a large bone defect is filled with bone graft or bioactive
ceramics granules are employed to fill the cavity. Fibrous tissue may
invade the cavity when no membrane is applied to cover the opening.
This leads to incomplete bone regeneration. Composite films from
collagen or poly(lactic acid) and HA nanoparticles have been prepared
for such purposes.
Medical devices that are designed to degrade are often made of
synthetic polyesters, as they have a proven track record as biodegradable
sutures; for example, poly(lactic acid) or its derivatives are frequently
employed as pins and screws, or even as bone replacement. Although
they are degradable and are not encapsulated in scar tissue, they are just
as bio-inert as polyethylene when it comes to bone bonding or active
stimulation of tissue repair. They do not form a hydroxycarbonate
apatite (HCA) layer in body fluid and therefore do not bond with bone.
So, the obvious way to introduce flexibility and toughness into bioac-
tive ceramics is to create a composite (Chapter 8). However, conventional
composites consist of bioactive glass or ceramic fibers embedded in
a polymer matrix (Figure 10.2). Mechanical properties can be engi-
neered to be an ideal mix of those of the polymer and those of the
ceramic or glass, but there are problems for biomedical applications that
