Although
two-dimensional hydrogel thin films have been applied
across many biomedical applications, creating higher dimensionality
structured hydrogel interfaces would enable potentially improved and
more biomimetic hydrogel performance in biosensing, bioseparations,
tissue engineering, drug delivery, and wound healing applications.
Herein, we present a new and simple approach to control the structure
of hydrogel thin films in 2.5D. Hybrid suspensions containing cellulose
nanocrystals (CNCs) and aldehyde- or hydrazide-functionalized poly(oligoethylene
glycol methacrylate) (POEGMA) were spin-coated onto prestressed polystyrene
substrates to form cross-linked hydrogel thin films. The films were
then structured via thermal shrinking, with control over the direction
of shrinking leading to the formation of biaxial, uniaxial, or hierarchical
wrinkles. Notably, POEGMA-only hydrogel thin films (without CNCs)
did not form uniform wrinkles due to partial dewetting from the substrate
during shrinking. Topographical feature sizes of CNC–POEGMA
films could be tuned across 2 orders of magnitude (from ∼300
nm to 20 μm) by varying the POEGMA concentration, the length
of poly(ethylene glycol) side chains in the polymer, and/or the overall
film thickness. Furthermore, by employing adhesive masks during the
spin-coating process, structured films with gradient wrinkle sizes
can be fabricated. This precise control over both wrinkle size and
wrinkle topography adds a level of functionality that to date has
been lacking in conventional hydrogel networks.