Biomimetic Mineralization of Hydrogel Biomaterials for Bone Tissue Engineering - Biomimetics: Advancing Nanobiomaterials and Tissue Engineering - page 60

3.4.3 ALP Entrapment

In the aforementioned studies, ALP was incorporated by addition before
gel formation and entrapped during formation of the polymer network
during gelation. This strategy of ALP entrapment during gelation is uni-
versally applicable to all hydrogels and avoids the use of potentially toxic
crosslinking agents that may also alter enzyme activity during chemical
immobilization of the enzyme in the gel. Avoidance of toxic crosslinkers
is also advantageous from a biomaterial application point of view. ALP's
molecular weight has been reported to be 185 kD [68], which makes it a
a larger molecule than many other bioactive molecules such as growth
factors. For comparison, commonly used growth factors such as platelet-
derived growth factor (PDGF), vascular endothelial growth factor (VEGF)
and transforming growth factor beta 1 (TGF- b 1) [69], have molecular
weights of 32 kD [70], 46 kD [71] and 25 kD [72], respectively. The larger
molecular weight of ALP hinders its release by diffusion, facilitating
physical entrapment. In the aformentioned studies where ALP release
was studied, it was shown that ALP is retained in hydrogels to a large
extent. Filmon et al. showed that less than 40% of total ALP was released
over 6 days from PHEMA gels containing ALP at an initial concentration
of 6 mg/ml, and that release had dropped to almost zero after 6 days [56].
Douglas et al. incorporated ALP into cPEG (50 mg polymer/ml gel), col-
lagen (3 mg/ml) and OPF (225 mg/ml) gels at a concentration of 2.5 mg
ALP/ml gel and found that ALP release from OPF and cPEG was negli-
gible, while 30% was released from collagen [60].

3.5 Enhancement of Hydrogel Mineralization Using

Biomacromolecules

A wide range of naturally occurring biomacromolecules has shown affi nity
for calcium phosphate, such as proteoglycans, glycosaminoglycans [73],
serum albumin [74] and lactoferrin [75]. Hence, numerous studies have
been performed on the addition of biomacromolecules to hydrogels to
promote their mineralization. Such studies fall into two categories, namely
those dealing with fundamental research into the ability of a particular
biomacromolecule to cause mineralization, and those involving biomac-
romolecule-enhanced mineralization for bone regeneration applications.

3.5.1 Systems to Test Mineralization-Inducing Potential of

Biomacromolecules

In studies in the fi rst category the hydrogel serves as an artifi cial extracellu-
lar matrix whose function is to immobilize the biomacromolecules and act