# Ion Beam Nanopatterning of Biomaterial Surfaces
## 4 Conclusions  
As we have seen in this review, ion beam nanopatterning is an attractive technique for the fabrication of well-defined model surfaces for the investigation of the numerous effects that nanoscale topographic surface features may exert on interacting biomolecules and cells. In this regard, the possibility to precisely tune the pattern dimension from about 10 to about 100 nm is particularly interesting for biomaterials applications. While larger nanopatterns with periodicities of a few hundred nanometers have been found to have stronger effects on adhering cells than equivalent ones with periodicities below 100 nm, the latter may, nevertheless, affect the adsorption of various biomolecules at several levels. Furthermore, the fact that broad ion beam irradiation enables the homogeneous and parallel patterning of large surface areas with well-defined nanopatterns represents a significant advantage over several other and in particular serial techniques. However, since ion-induced pattern formation is a self-organized process, the generated patterns will always exhibit defects. While the density of pattern defects and pattern regularity in general can be adjusted  
Figure 11: (**a**) SEM images of different bacteria after 1 h contact with untreated BC (first row), nanopatterned CoBC (second row), and nanopatterned LgBC (third row). Yellow arrows indicate the rupture points. (**b**) Quantification of live and dead cells of the different bacteria cultured on the different surfaces for 24 h. Reprinted with permission from Ref. [137]. Copyright 2020 American Chemical Society. within certain limits, for instance, by tuning the irradiation conditions [78] or applying sequential irradiation steps [138], some degree of disorder will remain. While this is certainly a disadvantage compared to lithographic techniques that enable the fabrication of arbitrary patterns with precisely adjusted regularity or disorder [12], similar limitations are faced also by other methods based on self-organization or self-assembly such as colloidal lithography [23] or anodic oxidation [16]. With few exceptions [128, 129, 137], the studies discussed in this review utilized ion-induced ripple patterns, presumably because ripple patterns are the most universal and can be generated on almost any surface. However, from a fundamental point of view, comparing the effects of different pattern types may be a rather interesting future endeavor. For instance, the importance of anisotropy could be assessed by comparing ripple and dot patterns of comparable amplitudes and periodicities after transfer into a common, biocompatible material. The role of the particular shape of the employed patterns could also be investigated in detail, for example, by comparing quasi-sinusoidal and faceted ripple patterns generated on amorphous and crystalline surfaces, respectively. Similarly, positive and negative replicas of a given pattern may be produced using nanoimprinting lithography. Such experiments may provide viable insights into the mechanisms of topography sensing by cells. From a more applied point of view, the translation of ion beam nanopatterning from model surfaces to real-world medical implant surfaces faces two challenges. First, in order to generate well-defined nanopatterns, comparatively smooth substrate surfaces are required. For polycrystalline metal surfaces such as Ti6 Al4V, each grain presents atomically flat crystal facets that can be patterned individually with ripple patterns aligned along their dominant crystallographic directions by normal or near-normal incidence irradiation [93, 126]. This, however, will not result in a homogeneous surface pattern. For this, oblique incidence irradiation of highly polished surfaces is required to minimize the effects of geometric shadowing [93]. For ceramic materials such as alumina or bioglass, the situation is even more difficult. These materials are typically produced by particle sintering and, thus, have very rough and porous surfaces [139].