# Ion Beam Nanopatterning of Biomaterial Surfaces
## 2 Nanoscale Pattern Formation on Solid Surfaces during Ion Beam Irradiation
### Pattern Transfer  
In order to minimize the impact of ion-induced alterations in surface chemistry and crystallinity or to adjust surface chemistry and surface topography independently, the fabricated nanopatterns can also be transferred into other materials with only minor variations in their surface topography. The most straightforward way to achieve this is the coating of the nanopatterned substrates with continuous thin films. In the literature, this approach has mostly been applied to thin metal films but can also be extended to non-metallic films [27]. In order to achieve high conformity between the substrate and the film surface, the film deposition parameters have to be carefully selected. In general, normal-incidence deposition is preferred, as deposition under an oblique angle of incidence may result in material accumulation on the pattern crests, leading to non-conformal columnar growth [99]. If the material to be deposited does not wet the substrate surface, application of an intermediate buffer layer may be necessary to ensure perfect replication of the substrate pattern [100; 101]. Furthermore, in the case of polycrystalline films, the disorder between the different grains in the film may result in a loss of conformity already at low film thicknesses. In such a case, magnetron sputter deposition with an applied negative bias may be used to achieve dense and highly conformal films with no apparent surface texture other than the replicated nanopattern (see Figure 3a) [98; 102]. If the nanopatterned substrate surface is crystalline, highly conformal films can also be grown epitaxially, if the lattice constants between film and substrate material are a match (see Figure 3b) [94]. Finally, various layer stacks [69; 101; 103] and multilayers [27; 104] have also been grown on nanopatterned substrate that replicated the substrate topography very well (see Figure 3c).  
Pattern transfer into polymeric materials is more complex. Two strategies have been reported so far. [PERSON] et al. employed thermal nanoimprinting lithography via an intermediate mold [105]. This intermediate mold was fabricated by UV crosslinking of a perfluoropolyether (PFPE) resin on a nanorippled Ge surface with about 300 nm periodicity. After stripping the intermediate PFPE mold from the Ge template, it was imprinted at elevated temperature into a cyclic olefin copolymer (COC) foil. After cooling down and removal of the PFPE mold, the COC foil surface exhibited a high-quality replica of the Ge nanopattern. [PERSON] et al. used a nanorippled glass substrate with about 200 nm periodicity as the template [64]. Polydimethylsiloxane (PDMS) solution was deposited on the template surface and left to solidify at room temperature. After a final annealing step, the PDMS replica was removed from the template, exhibiting the negative of the original template pattern on its surface. These transfer approaches are particularly interesting for biomaterials applications because they enable the transfer of a given nanopattern from a potentially toxic material such as GaAs to an established biocompatible polymer material. Furthermore, a large number of identical samples can be produced in this way using only a single nanopatterned substrate as the template. This is particularly advantageous for cell culture studies, which typically require very large numbers of identical substrates. However, it is not clear so far whether such imprinting approaches also allow for the transfer of smaller patterns with periodicities below 100 nm.