Cell forces on functionalised and nanostructured polymer substrates

Over the last years, the basic understanding of cell-matrix interaction became increasingly important in many research fields. Not only surface chemistry and morphology play a crucial role in this complex process, but also substrate stiffness was found to be an important factor [Engler et al., Cell, 126, 677-689, 2006]. Our aim is to measure cell forces on stiff substrates that are relevant for implantology (e.g. titanium, polyetheretherketone (PEEK)). By functionalising and micro-structuring of the substrate surface, we expect differences in cell forces, an important parameter to design future implants.

Cells do not attach directly to their substrate, but via extracellular matrix (ECM) proteins that cover the surface of the substrate. Cells exert their force to the underlying substrate by integrin mediated focal adhesions schematically shown in the figure below. Integrins connect the cytosceleton inside the cell with the surrounding extracellular matrix outside the cell. The right figure shows rat2 fibroblasts stained for filamentous actin (red), a component of the cytosceleton, and vinculin (green), a protein that binds to the integrin complex. Co-localisation (yellow) indicates focal adhesions (arrows). The actin fibers are mounted to integrin complexes on both ends, exerting contractile forces via stress fibers. Scale bar 10 ┬Ám.

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Left: scheme of focal adhesion. Right: rat2 fibroblasts stained for vinculin (green) and actin (red), focal adhesions appear yellow.

Cell force measurements are performed with a laser device setup (schema below) detecting deflection changes of cell-seeded cantilevers. Cells attach over night on the cantilevers to allow force generation, and therefore cantilever bending. By addition of a trypsin solution or an actin cytosceleton toxin, the cells relieve the exerted surface stress on the cantilever, resulting in a measurable deflection change.

The system is currently optimised with silicon cantilevers, followed by the use of micro-structured polymeric cantilevers.

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Scheme of the cantilever bending approach.

The cantilever bending approach presents a simple and versatile method to study the effects of surface functionalisation and microstructuring on contractile cell forces. Besides the basic study, the aim of this project is to investigate biomedically relevant systems, meaning different cells (e.g. fibroblasts, endothelial cells, mesenchymal stem cells) on different surfaces (e.g. titanium, PEEK) and evaluate the potential benefit of the contractile cell force measurements for future implant development. Furthermore, the acquired knowledge will be transferred to disposable polymeric microcantilevers, which are currently in the stage of development.

These research activities belong to the project 'DICANS', a collaborative initiative between the Biomaterials Science Center, Paul Scherrer Institute, University of Applied Sciences and Concentris GmbH.

Responsible author: Jasmin Althaus, PhD student