Designing Hydrogels by ATRP - In-Situ Gelling Polymers: For Biomedical Applications - page 89

hydrophobic poly( ʵ -caprolactone) (PCL) segment was synthesized by ring open-
ing polymerization (ROP) followed by ATRP of hydrophilic OEOMA. The (PCL-
b-POEOMA) 4 star block copolymers formed micelles at room temperature, and
underwent a sol-gel transition below body temperature, demonstrating the ability
to form an injectable hydrogel with a biodegradable core [ 227 ]. The rate of drug
release can then be tuned through diffusion of active compounds through the gel
as well as the degradation rate of the PCL chains. In addition to multi-arm star
block copolymers, polymers with grafted stimuli-responsive chains can be used to
form hydrogels with low critical gelation concentrations due to extra chain entan-
glement from dangling chains [ 228 ].
8 One-Component Supersoft Gels
Hydrogels in a swollen state are very soft materials with moduli in a range of
1 kPa. However, they become much harder after water evaporation. Thus, it would
be very desirable to develop hydrogel-like single-component materials that would
never dry. Such materials should act as supersoft elastomers.
Bulk polymers in a glassy state are hard materials with moduli in the range of
1 GPa. Above their glass transition temperature they soften and eventually flow.
This depends on the molecular weight and nature of the polymers, i.e. entangle-
ment MW. If the chains are slightly crosslinked they form rubbers or elastomers
with typical moduli in the range of 100-1,000 kPa. Such crosslinked polymers
can swell and, depending on degree of crosslinking, can reach softness typical for
hydrogels. But after they dry, moduli return to the original values.
A new concept has been recently introduced to dilute a loose network of poly-
mer chains not with a solvent (water) but with multiple short covalently attached
unentangled side chains. In such a case, they could never evaporate and would
provide a permanent non-leachable diluent that would reduce the moduli to a value
of ca 1 kPa, forming a new class of stable supersoft elastomers [ 229 , 230 ].
Scheme 6 illustrates two routes for the preparation of such supersoft elasto-
mers. The left pathway relies on formation of bottlebrush macromolecules with
Scheme 6 Two pathways to supersoft elastomers