Amplified Responsiveness of Multilayered Polymer Grafts: Synergy between Brushes and Hydrogels

The responsive properties of surface-grafted polymer films in aqueous media can be amplified by covalently layering thermosensitive brushes and hydrogels. This was demonstrated by synthesizing layers of linear poly(N-isopropylacrylamide) (PNIPAM) brushes, alternating with cross-linked, poly(hydroxyethyl)methacrylate (PHEMA) hydrogels via sequential surface-initiated atom-transfer radical polymerization (SI-ATRP) steps. Below the lower critical solution temperature (LCST) of PNIPAM, brush/hydrogel multilayered films swell similarly to linear PNIPAM homopolymer brushes, as measured by liquid ellipsometry. In contrast, above the LCST, the PHEMA hydrogel interlayer acts as stiffening element within the collapsed multilayered film, as monitored by atomic force microscopy (AFM) nanoindentation and lateral force microscopy (LFM). This translates into a 10-fold increase in Young's modulus by the collapsed, layered films compared to 2 4 PNIPAM homopolymer analogues. The (macro)molecular continuity between the brush main chains and hydrogel constituents thus enables a chemically robust layering to form graded, quasi-3D grafted polymer architectures, which display a concerted and amplified temperature-triggered transition

Stem-Cell Clinging by a Thread: AFM Measure of Polymer-Brush Lateral Deformation

If the binding strength of adhesive cues to the extracellular matrix (ECM) and the mechanisms involved in cell adhesion are synergistically correlated via a mechanical feedback, engineering of cue presentation at the ECM by designer macromolecules can enable control over cell-matrix interaction. Here, polymer brushes supporting fibronectin (FN) and presenting different grafted-chain length to modulate cell interaction at ECM cell-binding sites are exploited. Application of friction force microscopy allows us to estimate the lateral deformability and friction of oligoethylene glycol-containing brushes. These parameters are demonstrated to regulate the adhesion of human mesenchymal stem cells (hMSCs), which adopt their morphology and form focal adhesions (FAs) responding to FN brush-tether characteristics. Across a brush-thickness gradient presenting uniform FN exposure, thin brushes stimulate cell spreading and the development of FAs. Conversely, thick and more laterally deformable polymer grafts induce a decrease in cell spreading and FA formation. A correlation between frictional forces experienced at the (macro)molecular scale and the behavior of stem cells has been found. This interaction can be clarified by exploring novel aspects of FFM, which demonstrates a powerful tool to dynamically probe the ECM environment and indirectly suggest a way to structure ECM in order to trigger specific cell responses