Biomedical Engineering Reference
In-Depth Information
response of calcium orthophosphate bioceramics of
different porosity was investigated and a hardly any effect of
macropore dimensions (~150, ~260, ~510 and ~1220 μm) was
observed [452]. In another study, a greater differentiation of
mesenchymal stem cells was observed when cultured on ~200
μm pore size HA scaffolds when compared to those on ~500 μm
pore size HA [453]. The latter finding was attributed to the fact
that a higher pore volume in ~500 μm macropore scaffolds might
contribute to a lack of cell confluency leading to the cells proliferating
before beginning differentiation. Besides, the authors hypothesized
that bioceramics having a less than the optimal pore dimensions
induced quiescence in differentiated osteoblasts due to reduced
cell confluency [453]. Already in 1979, Holmes suggested that
the optimal pore range was 200-400 μm with the average human
osteon size of ~223 μm [92]. In 1997, Tsurga and coworkers implied
that the optimal pore size of bioceramics that supported ectopic
bone formation was 300-400 μm [454]. Thus, there is no need to
create calcium orthophosphate bioceramics with very big pores;
however, the pores must be interconnected [95, 384, 397, 398].
Interconnectivity governs a depth of cells or tissue penetration into
the porous bioceramics, as well as it allows development of blood
vessels required for new bone nourishing and wastes removal [455,
456].
Bioceramic microporosity (pore size < 10 μm), which is defined
by its capacity to be impregnated by biological fluids [455], results
from the sintering process, while the pore dimensions mainly depend
on the material composition, thermal cycle and sintering time. The
microporosity provides both a greater surface area for protein
adsorption and increased ionic solubility. For example, embedded
osteocytes distributed throughout microporous rods might form a
mechanosensory network, which would not be possible in scaffolds
without microporosity [457]. HA bioceramics with nanodimensional
(<100 nm) pores might be fabricated as well [458]. Differences in
porogens influence the macroporosity, while differences in sintering
temperatures and conditions affect the percentage of microporosity.
Usually, the higher the sintering temperature, the lower both the
microporosity content and the specific surface area of bioceramics.
Namely, HA bioceramics sintered at ~1200°C shows significantly less
microporosity and a dramatic change in crystal sizes, if compared
with that sintered at ~1050°C (Fig. 4.8). Furthermore, the average
In vivo
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