Free-Standing Thin Hydrogels: Effects of Composition and pH-Dependent Hydration on Mechanical Properties

Abstract

We developed an approach to obtain nanothin free-floating poly(methacrylic acid) (PMAA) hydrogel films through the dissolution of a sacrificial SiO2 layer. The hydrogel films were produced by chemical crosslinking of PMAA in PMAA/poly(N-vinylpyrrolidone) (PVPON) hydrogen-bonded precursors assembled through spin-assisted (SA) multilayer assembly. Surface-anchored precursor films and hydrogels were released from silicon templates into an aqueous solution via etching the thick sacrificial SiO2 layer from the templates. We studied the impact of the release method on the surface morphology, thickness, and elasticity of the released (PMAA/PVPON)60 and (PMAA)60 films using atomic force microscopy (AFM) and demonstrated that these properties were affected neither by the film release nor by their following transfer onto Si wafers. We found that 16 h crosslinked (PMAA)60 hydrogels and their more swollen 8 h crosslinked counterparts (76% hydration at pH = 5) produced free-floating films with good mechanical integrity and strength. The elastic moduli of a 16 h crosslinked (PMAA)60 film decreased from 1.9 ± 0.1 GPa in the dry state to 77 ± 13 MPa at pH = 5 and then to 14 ± 3 MPa at pH = 6.5. We also showed that incorporating Zr(IV) into twice thinner (PMAA)30 hydrogels (∼60 nm dry thickness) improved film mechanical robustness and allowed their successful release and transfer onto Si wafers. In contrast, Zr-free (PMAA)30 were less tolerant to the release/transfer procedure due to mechanical fragility. The temporary coordination links between Zr(IV) and the hydrogel could be removed from the network by an EDTA chelator. The lift-off approach developed here is simple, versatile, and applicable to a wide range of polymer films. The free-floating hydrogels obtained by this method can be used as transferrable platforms to develop lab-on-a-chip systems, nanocomposite pressure sensing platforms, rapid optical biosensors, and responsive platforms for regulating cell adhesion and more effective cell sheet recoveries.