@phdthesis{Frensemeier2016,
  author    = {Mareike Frensemeier},
  title     = {Switchable microtopographies based on the two-way shape memory effect in nickel-titanium alloys},
  doi       = {10.22028/D291-23165},
  url       = {https://nbn-resolving.org/urn:nbn:de:bsz:291:415-2563},
  pages     = {XVI, 154},
  year      = {2016},
  abstract  = {Nickel-titanium (NiTi) shape memory alloys are functional materials that are capable of undergoing a reversible temperature-induced shape change. Specifically in martensitic NiTi alloys, a reversible two-way shape memory effect can be induced using indentation techniques enabling a temperature-induced change in topography. Combining switchable topographies with nano- or microstructures could expand the properties of functional surfaces, and in addition make the surfaces responsive to their environment. For example, it would be possible to change the adhesive properties of surfaces with switchable dry adhesive microstructures or to control celladhesion on implant materials with specific nano- and micro-switchable structures. In this study, the indentation induced two-way shape memory effect was investigated in different NiTi alloys. In particular, the effects of alloy microstructure, deformation parameters (training) and thermal treatments on switchability were explored. In an austenitic NiTi alloy a specific thermal treatment led to the formation of coherent precipitates, which were shown to be crucial for the two-way shape memory behavior; exceeding the phase transformation temperature considerably decreased the switchability of the topography. At higher temperatures the stabilized martensite, which is required for an oriented phase transformation and consequently for the two-way shape memory behavior, transforms to austenite. An embossing and an electrochemical forming process were developed to prepare switchable topographies on larger areas. Both methods led to surface arrays on NiTi with two-way shape memory topographies. Finally, two approaches were presented, which use the switchable topographies to enable switching of a formerly passive surface function. In combination with bioinspired dry adhesive structures, the switchable NiTi topography led to a reversible, temperature-induced change of the adhesive properties of the surface. Secondly, the two-way shape memory effect was transferred to an alloy system with a phase transformation temperature near body temperature and a small width of hysteresis. By this, a switchable topography was induced, which is controllable within a physiological temperature range. The only issue impeding the use of this switchable surface for experiments on cell-surface interactions is an increased leakage of harmful copper ions. Therefore, surface passivation through oxidation is presented as a method to reduce the ion leakage.},
  language  = {en}
}