Biomedical Engineering Reference
In-Depth Information
6.1 Introduction
Hydrogels are used widely as a type of biomaterial attractive to
matrices for regenerative medicine on account of their biocompat-
ibility and tissuelike properties, such as viscoelasticity, diffusive
transport, and low mechanical irritation to the surrounding tissues
when implanted
in vivo
,aswellasinterstitialflowproperties.
1
,
2
When injected or implanted in a defect site, the hydrogel temporar-
ilyplaystheroleoftheextracellularmatrix(ECM)intheearlystages
of tissue formation from cells.
3
As tissue formation progresses, the
hydrogel matrix should be certainly replaced by naturally secreted
ECMsandspacesforproliferationandthemigrationofcells.
4
There-
fore, the tissue-specific nature and biodegradation of hydrogels are
essential properties to be temporally and spatially controlled dur-
ing the process of tissue formation, along with various bioactivities
andmechanicalproperties.Inadditiontothehydrogelproperties,
in
situ
gelation that causes a sol-gel transition in time for practical use
has attracted considerable interest for injectable scaffolds. Over the
last two decades, many studies concentrated on hydrogels formed
in situ
for the purpose of developing injectable scaffolds.
5
−
8
In situ
gelationcanbeobtainedwhencertainmolecules(mainlypolymers)
are assembled, cross-linked, or polymerized to form a polymeric
network by the responses to external stimuli (i.e., temperature, pH,
ionic strength, etc.) or mild chemical reactions. At the early stage of
thisfield,manystudiesfocusedonstimuli(especiallytemperature)-
responsive hydrogels due to their nontoxic condition and reversible
transitions. However, these hydrogels exhibited many problems,
such as poor solubility, high viscosity, slow phase transition time,
pH changes during degradation, and weak mechanical strength,
even though the bioactive effect is promising. Hydrogels formed
in
situ
using mild chemical reactions, such as the Michael reaction,
also have similar problems to stimuli-responsive hydrogels. In this
chapter,enzyme-triggeredhydrogelsaresuggestedasanalternative
option for such hydrogels formed
in situ
. First, current studies and
problems of hydrogels formed
in situ
are introduced. Second, the
basic knowledge and current studies of enzyme-triggered hydro-
gelsarepresented.Finally,therecentexamplesandfutureprospects
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