Calcium Orthophosphates as Bioceramics - Calcium Orthophosphates: Applications in Nature, Biology and Medicine - page 388

Many fabrication techniques are available to produce porous
calcium orthophosphate scaffolds (Table 4.1) with varying
architectural features (for details, see sections 4.3.3 Forming and
shaping and 4.4.4 Porosity). In order to achieve the desired properties
at minimum expenses, the production process should be optimized
[688]. With the advent of tissue engineering, the search is on for the
ultimate option—a “tissue engineered bone substitute,” consisting
of a synthetic calcium orthophosphate scaffold impregnated with
cells and growth factors. Figure 4.16 schematically depicts a possible
fabrication process of such item that, afterwards, will be implanted
into a living organism to induce bone regeneration [43, 56].
From the structural perspectives, a degree of scaffold porosity is
responsible for regulating the bioactivity of bone graft substitutes as
a function of its influence on structural permeability, which controls
the initial rate of bone regeneration and the local mechanical
environment, which mediates the equilibrium volume of new bone
within the repair site. Parameters such as pore interconnectivity,
pore geometry, strut topography and strut porosity all contribute
to modulate this process of osteogenesis and act synergistically to
promote or screen the osteoconductive or osteoinductive potential
of bone graft substitutes [455, 689, 690]. However, since bones have
very different structures depending on their functions and locations,
the same pore sizes and shapes may not be ideal for all potential uses.
Therefore, bioceramic scaffolds of various porosities are required.

4.7.3 Bioceramic Scaffolds from Calcium

Orthophosphates

Philosophically, the increase in life expectancy requires biological
solutions to orthopedic problems, which were previously managed
with mechanical solutions. Therefore, since the end of 1990s, the
biomaterials research focuses on tissue regeneration instead of
tissue replacement [691]. The alternatives include use hierarchical
bioactive scaffolds to engineer

living cellular constructs for
transplantation or use bioresorbable bioactive particulates or porous
networks to activate

in vitro

the mechanisms of tissue regeneration
[692, 693]. Thus, the aim of calcium orthophosphate bioceramics is
to prepare artificial porous scaffolds able to provide the physical and
chemical cues to guide cell seeding, differentiation and assembly
into 3D tissues of a newly formed bone [648, 694-700]. Particle

in vivo