Biomedical Engineering Reference
In-Depth Information
Waltimo et al. [51] synthesized nanometric bioactive glass 45S5 and compared the antimicrobial
activity to micron-sized bioactive glass against enterococci from root canal infections and found
higher increased killing efficacy with the nanometric glass. This was possibly due to a tenfold
increase in silica release corresponding to a tenfold increase in surface area of the nanosized glass.
Delivery of bioactive compounds is yet another mechanism by which silica nanoparticles could be
used as an antimicrobial. A recent study investigated a novel nitric oxide releasing silica nanoparti-
cle as an antimicrobial on biofilm-based microbial cells [52] . Nitric oxide (NO) has been reported
to have antimicrobial properties and in fact these NO containing particles resulted in
99% killing
of five common bacteria. Another study used N-halamine-functionalized silica core-shell nanoparti-
cles (
$
500 nm) and demonstrated increased antibacterial activity against both gram-positive
and -negative bacteria relative to bulk powder N-halamine [50] .
200
B
4.6 Skeletal applications of silica-based nanomaterials
Although the skeleton and dentition have obvious differences, they also share some common
features such as similarities in the cells that create the mineralized matrix, osteoblasts, odontoblasts,
and cementoblasts [69] . Osteoblasts are bone forming cells of the skeleton, cementoblasts form the
mineralized tissue of the tooth root [70,71] , and odontoblasts function to create dentin [72] all of
which are thought to derive from cranial neural crest mesenchymal cells [73] , at least for osteo-
blasts of craniofacial bones [74] . All three cell types are active throughout life and mature cells can
be differentiated from precursors when required. Several genetic disorders also effect the skeleton
and dentin including hypophosphatemic rickets and osteogenesis imperfecta among others [75] .
Genetic studies in mice have identified a number of genes that are important for both dentin and
bone formation, two examples are the alkaline phosphatase knockout mouse which presents with
malformed incisors and defective enamel [76] as well as skeletal mineralization defects [77] and
the dentin matrix protein (DMP1) knockout mouse which also presents with defects in dentin and
skeleton [78
80] .
4.6.1 Skeletal modeling and remodeling, osteoblast, and osteoclasts
The skeleton is a dynamic organ that undergoes continuous regeneration. During development and
growth, the skeleton is sculpted to achieve its shape and size by the removal of bone from one site
and deposition at a different one (modeling) [81] . In contrast to modeling, bone remodeling serves
to maintain mechanical integrity of the adult skeleton and provides a mechanism by which calcium
and phosphate ions may be released from or conserved within the skeleton, a process central to
metabolic and cellular functions. In healthy young adult bone, remodeling is homeostatic, i.e., the
amount of bone resorbed is equivalent to the amount of new bone formed, with no net change in
bone mass [81] .
The two main cell types involved in homeostasis of the adult skeleton are osteoblasts and osteo-
clasts. Osteoblasts are the bone building cells of the body and derive from pluripotent mesenchymal
stem cells [82] . Runt-related transcription factor-2 (Runx2) is a critical transcription factor that
drives the initial differentiation of precursors toward an osteoblast phenotype, while an additional
transcriptional
regulator, osterix,
is critical
for continued osteoblast development
[83
85] .
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