Polymeric Supramolecular Hydrogels as Materials for Medicine - In-Situ Gelling Polymers: For Biomedical Applications - page 161

Fig. 9 Schematic of the adherence of mussels on surfaces by formation of catchol complexes.
( 1 ) Production and storage of byssal thread cells at pH 5 through monocatechol complexes.
( 2 ) Release of the threads from the ventral grove (indicated by the white dashed line ).
( 3 ) Formation of bis- and triscatechol complexes in the byssal threads after exposure to seawater
(pH 8). Reprinted from Waite et al. [ 121 ]. Copyright 2011 National Academy of Sciences of the
United States
Fig. 9 . Upon adherence to a surface, mussels excrete preassembled thread cells
at a pH of 5-6; upon contact with seawater (pH ≈ 8), the stability constant of
the complexes increases from the mono to the bis- and tris-complex. As a result,
hydrogels interconnected by these tris-complexes exhibit high extensibilities,
hardness, and most importantly self-healing [ 120 ].
Based on this model, Waite and coworkers reported a hydrogel complexed by
dihydroxy-phenyalanin (dopa) modified PEG and Fe 3 + ions [ 121 ]. Dopa is a metal-
complexing ligand, exhibiting a modestly lower force needed to rupture the metal-
dopa bond than a covalent bond, but it possesses the ability of self-healing [ 117 ].
This feature renders this motif attractive for use in supramolecular hydrogels. To
pursue this goal, Waite and coworkers modified 4-arm PEG with four dopa function-
alities. Upon mixing with iron ions (dopa:Fe = 3:1) at pH = 5, a green-blue fluid
evolved that formed a sticky purple gel after increase of the pH to 8. At pH = 12,
a red elastomeric gel was formed. UV-vis absorbance and Raman spectroscopies
could show that upon increase of pH, higher-ordered complexes are formed. When
comparing spectra taken of the native mussel cuticle to that of the triscatechol com-
plex, both complexes show the same bands with almost same intensities. Probing
the systems by dynamic oscillatory shear rheology showed that the elasticity of the
systems decreases with increasing pH; the elastic modulus ranged from 30 Pa at
pH 5 to 1,000 Pa at pH 12, as compiled in Fig. 10 a. For comparison, a covalently
crosslinked hydrogel was prepared by addition of sodium periodate to the modified
4-arm PEG, forming a covalent C-C bond between two catechol moieties. Probing
the physical and the chemical hydrogels by shear rheology shows almost the same