Biomedical Engineering Reference
In-Depth Information
should ideally match their uptake by the cells [63]. Common growth factors
used in musculoskeletal regeneration include transforming growth factors
(TGF-
) and bone morphogenetic proteins (BMP) [63]. In addition to their
role in cell phenotype, growth factors also influence the cell viability, pro-
liferation, differentiation, and matrix production by providing the necessary
soluble signals to the cells [22, 60, 70, 71, 162, 256]. The growth factors can
be incorporated directly into the scaffold through covalent bonding or by
absorption.
β
5.1
Hydrolytically Degradable Hydrogels
Most of the synthetic hydrogels are degraded through hydrolysis of ester or
amide linkages located within the polymer backbone [240, 257, 258]. The hy-
drolytically susceptible groups can either be inherently a part of the polymer
or may be externally incorporated onto the polymer scaffold [239, 259]. This
is achieved by introducing susceptible chemical groups such as esters onto the
existing polymer back bone [239], or by copolymerizing, blending, or graft-
ing degradable oligomers with nondegradable polymers [260, 261]. Here, the
hydrogel degradation kinetics can be tuned by varying the scaffold chemistry
as well as the molecular weight of the polymer segments. Various research
groups have attempted to fine-tune hydrogel degradation.
Anseth et al. have created biodegradable PEG and PVA-based hydrogels by
incorporating PLA moieties through copolymerization [260]. Wang et al. in-
troduced a phosphate group into the PEGDA backbone by reacting PEG and
phophoryl chloride [239], whereas Jo et al. introduced fumerate groups into
the PEG backbone by reacting PEG with fumaryl chloride [259] to render PEG
polymers hydrolytically degradable. The incorporation of phosphate groups
intothePEGDAbackboneallowsonetocreatebio-interactivehydrogelsfor
bone engineering, where the phosphate group facilitates the mineralization
by directly interacting with the calcium ions. In all the studies discussed in
this section, the degradation profile of the hydrogel has been manipulated via
the choice of the degradable link, and not by the environment.
5.2
Enzymatically Degradable Hydrogels
Hydrogels made out of natural polymers can easily be degraded by en-
zymes,andthisisoneoftheirmainadvantagesaspotentialscaffolding
materials. Hence a marriage between such natural polymers and synthetic
polymers can be creatively used to develop semi-synthetic degradable hydro-
gels [262, 263]. Recently, there has been a strong push towards incorporating
enzyme-susceptible groups into synthetic polymers to render them enzymat-
ically degradable [48, 264-267]. In these approaches, the rate of degradation
