Biomedical Engineering Reference
In-Depth Information
copolymerized with hydrophilic polymers such as poly(ethylene glycol) (PEG).
The formed copolymer, poly(propylene fumerate)-co-poly(ethyleneglycol) is
thermo-responsive, and hence it can be used in a minimally invasive man-
ner [254]. In this copolymer system, the propylene fumerate repeat unit
inherently contains a polymerizable vinyl group and a hydrolytically degrad-
able ester group, while the poly (ethylene glycol) segments increases the
hydrophilicity of the network so as to imbibe water. Poly(propylene fumerate)-
co-poly(ethylene glycol) hydrogels have been reported to degrade in 12 weeks
time both in vivo and in vitro [255]. All of these features make this polymer an
attractive hydrogel scaffold for tissue engineering. Fisher et al. have explored
the possibility of using this hydrogel for cartilage engineering [107].
Another fumerate-based hydrogel scaffold which has been studied for tis-
sue engineering is poly(ethylene glycol fumerate) (OPF). OPF is synthesized
by a reaction between poly(ethylene glycol) and fumaryl chloride. Temenoff
et al. investigated the possibility of using OPF as a scaffold for bone tissue
engineering [31, 43]. Their observations showed that the OPF hydrogels are
suitable as an injectable cell carrier for bone and guided tissue regeneration.
5
Biodegradable Hydrogels
Most of the first generation hydrogel scaffolds are successful in providing
structural support for the growth of the encapsulated cells in culture. How-
ever, only a few of the synthetic hydrogels are biodegradable. Biodegradability
is an essential criterion for designing hydrogel scaffolds for tissue engineer-
ing applications. The hydrogels which initially provide a three-dimensional
support to cells need to be degraded eventually as the cells differentiate
and produce matrix. This is essential in order to maintain cell activities
without any hindrance from the scaffold, as well as for creating the matrix
with the desired cytoarchitecture. The kinetics of the degradation process
needs to be tuned according to the adopted tissue engineering strategy as
well as the targeted tissue and organ. For example, cells that proliferate fast
and produce matrix rapidly require a fast degrading scaffold, whereas tissue
structures that need stability and physical strength require a slow degrading
scaffold [30, 230]. Ideally, the scaffold degradation rate should closely parallel
the rate of ECM production. One of the important factors regarding the use
of biodegradable hydrogel is that the degradation products must be nontoxic,
and need to be eliminated rapidly by metabolic degradation or excretion by
the kidney.
In addition to scaffolding, biodegradable hydrogels are also used to deliver
various growth factors, which play a pivotal role in tissue development to in-
crease the in vivo efficiency as well as the longevity of the proteins [62, 63]. In
this application, the degradation profile i.e. the release of bio-active molecules
