Polymeric Supramolecular Hydrogels as Materials for Medicine - In-Situ Gelling Polymers: For Biomedical Applications - page 155

in material sciences, since the swelling degree is too low for application in the
medicinal sector.
The examples from the groups of Shoichet [ 87 ] and Zhang [ 91 ] are based on the
use of biopolymer blends. Whereas natural hydrogels based on biopolymers that are
crosslinked by hydrogen bonding are well known, successful synthetic approaches to
prepare such hydrogels are rare. To overcome this limitation, Meijer and coworkers
reported about tunable hydrogels based on poly(ethylene glycol) (PEG) chains that
are end-group functionalized with ureidopyrimidinone (UPy) [ 92 ]. UPy is a self-
complementary AADD-DDAA quadruple hydrogen-bonding motif (A = hydrogen-
bond acceptor, D = hydrogen-bond donor). Meijer and coworkers were able to form
hydrogels from these materials by dissolving dried polymers in an isotonic water
solution at 70 °C and cooling it down to room temperature. In dilute solution, the
polymers aggregate to form isolated nanofibers, as shown in Fig. 5 a. With increasing
concentration, these nanofibers form a transient network, as also shown in Fig. 5 a.
This process is tunable by the length of alkyl spacers between the UPy motif and the
PEG chain. When the chain length of PEG is increased and the overall number of
Fig. 5 Formation and characterization of hydrogels based on poly(ethylene glycol) ( PEG ) chains
that are end-group functionalized with ureidopyrimidinone ( UPy ). a The process of hydrogel for-
mation of Upy-modified PEG by hierarchical assembly of different structural units at 50 °C, com-
prising single chains, micelles, and fibers. ( i ) Upon cooling or increase of concentration, a soft
hydrogel forms. ( ii ) Formation of supramolecular crosslinks after 16-24 h. ( iii ) Bundling of fib-
ers, leading to phase-separating domains. b Rheological measurement ( left axis ) and DSC ( right
axis ) showing the thermal collapse of a UPy-PEG 10-kDa hydrogel ( G ′: storage modulus, G ″: loss
modulus, ʔ H : enthalpy). c Rhodamine B release from fresh and aged hydrogels depending on the
chain length of the polymer used to form the hydrogels ( red PEG 6 kDa, blue PEG 10 kDa, green
PEG 35 kDa). Reproduced from Meijer et al. [ 92 ]. Copyright 2012 Wiley VCH