Development and Application of Naturally Renewable Scaffold Materials for Bone Tissue Engineering - The Nanoscience and Technology of Renewable Biomaterials - page 298

for expansion, growth kinetics, and differentiation as BMMSCs depending on the source
and site of tissue (41).

11.3.2 Natural Renewable Materials Used for Bone Tissue Engineering

Tissue regeneration schemes have revolved around the use of progenitor cell populations
and bioactive molecules to catalyze neo-bone formation. Previously stated, this type of
therapy requires a scaffold material for cellular organization and to impart the correct
physiological matrix for success. The following section will address the use of naturally
renewable materials used for bone tissue engineering and focus on specific examples of
such scaffolding with progenitor and mesenchymal stem cell populations.

11.3.3 Naturally Occurring Polysaccharide Materials in BTE

Polysaccharide materials offer many benefits over other scaffolding materials, mainly
due to their abundance in nature. Chitosan, the deacetylated derivative of chitin, is
similar in structure to glycosaminoglycans within the mammalian extracellular matrix.
Chitosan is the treated form of chitin, the second most abundant polysaccharide obtained
from crustaceans' exoskeletons, after is has been demineralized by HCl, deproteinized by
NaOH, and deacetylated by 50% or more. Besides being biocompatible, biodegradable,
and bioresorbable, chitosan exhibits a cationic nature and is hydrophilic, aiding in cellu-
lar processes. Chitosan can be formed into numerous structures based on its method of
preparation and includes porous spheres, films, fibers, or injectable solutions (78). Due
to its high molecular weight and electrolytic properties, chitosan is highly insoluble at
neutral pH and is usually dissolved in weak acids such as acetic acid. Based on chi-
tosan's ability to form many different structures and possess a wide range of porosities,
much research has focused on its use in composite scaffold applications including com-
bination with natural and synthetic polymers, and inorganic materials (79-81). When
chitosan has been coupled with inorganic materials such as hydroxyapatite, investigators
have reported significant increases in osteogenic markers such as calcium deposition,
alkaline phosphatase (ALP) activity, and increased gene expression of bone sialoprotein,
osteopontin, and osteocalcin (82). Ge et al . investigated large weight percentages (25,
50, 75%) of hydroxyapatite in chitin films via lyophilization and were able to show
good biocompatibility with tissue ingrowth in a rabbit femur model after 2 months (83).
Histological analysis showed that scaffolds seeded with mesenchymal stem cells greatly
influenced tissue ingrowth compared to cell-free controls, and minimized inflammatory
response noted by the paucity of inflammatory cells (83). Work by Malafaya et al .
developed a unique approach to assemble micron sized chitosan particles into a macro-
scopic scaffold capable of filling a large bone defect void (84). Chitosan particles were
precipitated by drop-casting in a 1 M NaOH bath and thermally-pressing into the desired
mold shape. They reported that the scaffold was highly biocompatible and allowed cell
ingrowth into the porous structure (84). Gravel et al . experimented with a combination
of chitosan and coralline and determined that the composite allowed distinct cell mor-
phology displaying osteoblastic phenotypes for mesenchymal stem cells at higher ratios
of coralline:chitosan compared to pure chitosan alone (85). This was attributed to the
crystalline component and calcium content of the coralline.