Mechanotransduction in Polyelectrolyte Brushes - Towards Mechanically Addressable Smart Surfaces
Final Report Abstract
Mechanoresponsive materials are the subject of an uprising field in polymer research. These materials can convert a mechanical force into another type of physical signal, for instance, an optical response. Such a mechanoresponsive material was developed based on charged polymer brushes which are polymers tethered with one end to a solid (glass) surface. Having attached a fluorescent dye to the polymer, mechanical compression can be detected by the change in optical properties of the dye. The mechanically addressable surfaces were investigated by pressing a soft bead attached to an AFM cantilever onto the brush which allowed for applying low and well-defined forces. The bead deformed under compression and produced a large contact area which was visualised by a fluorescence microscope with high lateral resolution. Under compression, the fluorescence intensity of the mechanoresponsive brushes decreased which is related to a quenching mechanism. Conversely, when the bead is retracted, polymer strains at the edge of the contact area are stretched which results in enhanced fluorescence. This is one of the first reported examples of a force sensor responding to both compression and strain. The brushes showed high sensitivity towards compression with the lowest detectable pressure in the 10 kPa regime. This is the same sensitivity as human skin (a natural pressure sensor) has. The signal was reversible and could be quantified as a linear function of fluorescence intensity versus pressure (in the range of 10-100 kPa). This makes mechanoresponsive brushes very attractive as pressure sensors with high lateral resolution, the latter being mainly limited by the setup. A possible application of the sensors is an artificial skin (www.uni-bayreuth.de/blick-in-die-forschung and www.kompetenznetze.de/service/ nachrichten/2011, date 24/10/2011). As a proof-of-principle, the sensors were applied to study the forces occurring underneath a bio-inspired adhesion stamp. These stamps mimic the feet of animals such as geckos which can easily move on vertical walls. Thus, mechanoresponsive sensors may contribute to the understanding of what happens underneath these animal feet. This knowledge can open up routes towards strong reversible adhesives and devices that move up walls in a gecko-like fashion. In total, the project successfully combined chemistry (polymer brush synthesis) with physics (contact mechanics) and demonstrated how science can move from basic research to a first application of mechanoresponsive materials in sensors.
Publications
- “Polymer Brushes: Routes toward Mechanosensitive Surfaces”. Accounts of Chemical Research 2010, 43, 466‒474
Bünsow, J.; Kelby, T. S.; Huck, W. T. S.
- “Direct Correlation Between Local Pressure and Fluorescence Output in Mechanoresponsive Polyelectrolyte Brushes”. Chemie Int. Ed. 2011, 50, 9629-9632 [Highlighted in: Nature Materials 2011, 10, 724]
Bünsow, J., Erath, J., Biesheuvel, P. M., Fery, A., Huck, W. T. S.
