Biomedical Engineering Reference
In-Depth Information
1.3 SCAFFOLD MATERIALS FOR BONE TISSUE ENGINEERING
The fi rst step in achieving a successful scaffold is to choose a suitable biomaterial. Natural bone
matrix is a composite of biological ceramic (a natural apatite) and biological polymer. Carbonated
hydroxyapatite Ca
10
(PO
4
)
6
(OH)
2
accounts for nearly two-thirds of the weight of a bone. The inor-
ganic component provides compressive strength to the bone. Roughly one-third of the weight of
a bone is from collagen fi bers. Collagen fi bers are tough and fl exible, and thus tolerate stretching,
twisting, and bending. It is not surprising that polymers, ceramics, or their composites have been
chosen for bone repair [16]. They can be either synthetic or naturally occurring ones. Table 1.2 lists
synthetic and natural scaffold biomaterials that have been most widely investigated for bone regen-
eration, some of which are well-established and clinically applicable. In this section, the biocompat-
ibility, biodegradability, and mechanical properties of these scaffold materials, which are the most
essential factors to be considered in the fabrication of bone regeneration scaffold, are reviewed
concisely. Particular attention is paid to a key issue that remains with almost all existing scaffold
biomaterials, that is, mechanically strong materials (in crystalline structure) tend to be bioinert, and
biodegradable materials (in amorphous structure) are, in general, mechanically weak. An excep-
tion, 45S5 Bioglass-derived glass-ceramic, is considered in more detail because the issue associated
with the two apparently irreconcilable properties (mechanical strength and biodegradability) have
been successfully addressed in this material [17].
1.3.1 B
IOCERAMICS
: C
ALCIUM
P
HOSPHATES
1.3.1.1 Biocompatibility
Since almost two-thirds of the weight of a bone is hydroxyapatite Ca
10
(PO
4
)
6
(OH)
2
, it seems logi-
cal to use this ceramic as a major component of scaffold materials for bone tissue engineering.
Actually, hydroxyapatite and related calcium phosphates (e.g., β-tricalcium phosphate [β-TC P])
have been intensively investigated [16,18,21]. As expected, calcium phosphates have an excellent
biocompatibility due to their close chemical and crystal resemblance to bone mineral [19,20].
Although they have not shown osteoinductive ability, they certainly possess osteoconductive prop-
erties as well as a remarkable ability to bind directly to bone [32-35]. A high number of
in vivo
and
in vitro
assessments have concluded that calcium phosphates, no matter which forms (bulk,
coating, powder, or porous) and which phases (crystalline or amorphous) they are in, always sup-
port the attachment, differentiation, and proliferation of cells (such as osteoblasts and mesenchy-
mal cells), with hydroxyapatite being the best among these scaffold materials [36]. Although the
excellent biological performance of hydroxyapatite and related calcium phosphates has been well-
documented, the slow biodegradation of their crystalline phases and the weak mechanical strength
of their amorphous states limit their application in engineering of new bone tissue, especially at
load-bearing sites.
1.3.1.2 Degradability
Typically, crystalline calcium phosphates have a long degradation time
in vivo
, often of the order of
years [37]. The dissolution rate of synthetic hydroxyapatite depends on the type and concentration
of the buffered or unbuffered solutions, pH of the solution, degree of the saturation of the solution,
solid and solution ratio, length of suspension in the solution, as well as composition and crystallinity
of the hydroxyapatite. In the case of crystalline hydroxyapatite, the degree of micro and macropo-
rosities, defect in the structure, and amount and type of other phases present also have signifi cant
infl uence [39]. Crystalline hydroxyapatite exhibits the slowest degradation rate, compared with
other calcium phosphates. The dissolution rate decreases in the following order [38]:
Amorphous hydroxyapatite
>
all other calcium phosphates (e.g., TCP)
>>
crystalline
hydroxyapatite.
