Z. Ilke Kalcioglu

Recent advances in contact mechanics have formalized approaches to distinguish between poroelastic and viscoelastic deformation regimes via load relaxation experiments, and to simultaneously extract the mechanical and transport properties of gels at the macroscale. As poroelastic relaxation times scale quadratically with contact diameter, contact radii and depths on the mm scale can require hours for a single load relaxation experiment to complete. For degradable materials such as biodegradable hydrogels and soft biological tissues, it is necessary to minimize the required experimental time. Here, we investigated the applicability of these methods at smaller (μm) length scales to shorten relaxation times. We conducted load relaxation experiments on hydrated polyacrylamide (PAAm) gels at the microscale via atomic force microscopy (AFM)-enabled indentation, as well as at the macroscale via instrumented indentation. We confirmed the approach as a reliable means to distinguish between viscoelastic and poroelastic relaxation regimes at the microscale: shear modulus G, drained Poisson's ratio νs, diffusivity D, and intrinsic permeability κ of the gels agreed well at the micro- and macroscale levels. Importantly, these properties were accessed accurately within seconds at the microscale, rather than within hours at the macroscale. Our results demonstrate the promise of contact-based load relaxation analysis toward rapid, robust characterization of mechanical and transport properties for poroelastic gels and tissues.

We present the layer-by-layer assembly of an electroactive polymer nanocomposite thin film containing cationic linear poly(ethyleneimine) (LPEI) and 68 vol % anionic Prussian Blue (PB) nanoparticles, which allow for electrochemical control over film thickness and mechanical properties. Electrochemical reduction of the PB doubles the negative charge on the particles, causing an influx of water and ions from solution to maintain electroneutrality in the film; concomitant swelling and increased elastic compliance of the film result. Reversible swelling upon reduction is on the order of 2-10%, as measured via spectroscopic ellipsometry and electrochemical atomic force microscopy. Reversible changes in the Young's elastic modulus of the hydrated composite film upon reduction are on the order of 50% (from 3.40 to 1.75 GPa) as measured with in situ nanoindentation, and a qualitative increase in viscous contributions to energy dissipation upon redox is indicated by electrochemical quartz crystal microbalance. Electrochemical stimuli maintain a mild operating environment and can be applied rapidly, reversibly, and locally. We maintain that electrochemical control over the swelling and mechanical behavior of polymer nanocomposites could have important implications for responsive coatings of nanoscale devices, including mechanically tunable surfaces to modulate behavior of adherent cells.