am9b08853_si_002.mp4 (49.58 MB)
Reversible Hydrogel Photopatterning: Spatial and Temporal Control over Gel Mechanical Properties Using Visible Light Photoredox Catalysis
media
posted on 2019-06-17, 00:00 authored by Faheem Amir, Kevin P. Liles, Abigail O. Delawder, Nathan D. Colley, Mark S. Palmquist, Houston R. Linder, Scott A. Sell, Jonathan C. BarnesThere is a growing
interest in being able to control the mechanical
properties of hydrogels for applications in materials, medicine, and
biology. Primarily, changes in the hydrogel’s physical properties,
i.e., stiffness, toughness, etc., are achieved by modulating the network
cross-linking chemistry. Common cross-linking strategies rely on (i)
irreversible network bond degradation and reformation in response
to an external stimulus, (ii) using dynamic covalent chemistry, or
(iii) isomerization of integrated functional groups (e.g., azobenzene
or spiropyran). Many of these strategies are executed using ultraviolet
or visible light since the incident photons serve as an external stimulus
that affords spatial and temporal control over the mechanical adaptation
process. Here, we describe a different type of hydrogel cross-linking
strategy that uses a redox-responsive cross-linker, incorporated in
poly(hydroxyethyl acrylate)-based hydrogels at three different weight
percent loadings, which consists of two viologen subunits tethered
by hexaethylene glycol and capped with styrene groups at each terminus.
These dicationic viologen subunits (V2+) can be reduced
to their monoradical cations (V•+) through a photoinduced
electron transfer (PET) process using a visible light-absorbing photocatalyst
(tris(bipyridine)ruthenium(II) dichloride) embedded in the hydrogel,
resulting in the intramolecular stacking of viologen radical cations,
through radical–radical pairing interactions, while losing
two positive charges and the corresponding counteranions from the
hydrogel. It is shown how this concerted process ultimately leads
to collapse of the hydrogel network and significantly (p < 0.05) increases (by nearly a factor of 2) the soft material’s
stiffness, tensile strength, and percent elongation at break, all
of which is easily reversed via oxidation of the viologen subunits
and swelling in water. Application of this reversible PET process
was demonstrated by photopatterning the same hydrogel multiple times,
where the pattern was “erased” each time by turning
off the blue light (∼450 nm) source and allowing for oxidation
and reswelling in between patterning steps. The areas of the hydrogel
that were masked exhibited lower (by 1–2 kPa) shear storage
moduli (G′) than the areas that were irradiated
for 1.5 h. Moreover, because the viologen subunits in the functional
cross-linker are electrochromic, it is possible to visualize the regions
of the hydrogel that undergo changes in mechanical properties. This
visualization process was illustrated by photopatterning a larger
hydrogel (∼9.5 cm on its longest side) with a photomask in
the design of an array of stars.
History
Usage metrics
Categories
Keywords
shear storage modulihydrogel cross-linking strategyphotoinduced electron transfernetwork cross-linking chemistryReversible Hydrogel Photopatterningnetwork bond degradationGel Mechanical Propertiesviologen subunitsCommon cross-linking strategiesweight percent loadingsVisible Light Photoredox Catalysisdicationic viologen subunits