Polymeric Supramolecular Hydrogels as Materials for Medicine - In-Situ Gelling Polymers: For Biomedical Applications - page 159

Fig. 7 Investigation of the elastic shear modulus, G ′, during formation of alginate-magnesium
hydrogels along with the gelation time, t g , depending on the concentration of precursor solutions
of alginate, c alg , and crosslinking magnesium ions, c Mg . Open circles represent the elastic shear
modulus after 24 h, whereas filled circles represent the gelation time as a function of the amount
of Mg 2 + added to 5 wt% alginate ( left ) and the amount of alginate supplemented by 100 mmol
L − 1 of Mg 2 + ( right ). The marked areas show concentrations where no gel formation is observed.
Reprinted from Groll et al. [ 112 ]. Copyright 2012 RSC Publishing Group
batch that had a close-to equal ratio of guluronic to mannuronic acid. To investi-
gate the impact of the different acids, the experiments were repeated using alginate
batches composed of either more guluronic or more mannuronic acid. This assess-
ment shows that gluronic acid has a higher affinity towards magnesium ions than
mannuronic acid, because hydrogels could be formed at lower concentrations of
ions and alginate than with batches of alginate containing higher contents of man-
nuronic acid, which did not form gels at all.
For biomedical applications, hydrogels should be biodegradable. Therefore,
hydrogels of the latter kind were stored within water, and the time of hydrogel
erosion was measured. In these tests, hydrogels crosslinked by magnesium ions
display full dissolution on a timescale of 2.5-5 h, depending on the concentra-
tion of alginate. Increasing the concentration of magnesium ions does not increase
the stability of the hydrogels towards dissolution in water, but it increases their
short-term elastic modulus. Groll and coworkers argue that the weak interaction of
magnesium ions with the alginate chains and the slow exchange of these ions lead
to an increased osmotic pressure; this causes gel swelling and finally rupture of
the hydrogel. To overcome the fast degradation of the gel, hydrogels were treated
with a calcium-ion solution, leading to additional crosslinking on the surface of
the hydrogel; after 24 h in aqueous medium, the hydrogels treated with calcium
did not dissolve. SEM micrographs of a magnesium-crosslinked hydrogel show a