# Ion Beam Nanopatterning of Biomaterial Surfaces
## 3 Ion Beam-Nanopatterned Surfaces in Contact with Biomolecules and Cells
### Cell Response
#### 3.2.2 Bacteria  
All the studies discussed above investigated ion beam-nanopatterned surfaces in contact with mammalian tissue cells, aiming at applications in reconstructive medicine and tissue engineering. Recently, however, the potential of ion beam nanopatterning in the fabrication of antimicrobial surfaces was explored as well. While it is well established that topographically nanopatterned surfaces can exhibit contact-killing properties by inducing the rupture of the bacterial plasma membrane, such patterns typically require comparatively high aspect ratios [134; 135]. To achieve such high aspect ratios, [PERSON] et al. irradiated dried bacterial cellulose (BC) hydrogels with 1 keV Ar\({}^{+}\) ions at normal incidence [136]. With increasing ion fluence, this resulted in a transition of the original BC morphology that is composed of interlaced ribbons to self-standing spike-like nanostructures 50-70 nm in width and about 400 nm in height. This was also accompanied by an increase in the effective Young's modulus. Nanostructure formation was attributed to a transition from chemical sputtering at low fluences to physical sputtering at high fluences. Most remarkably, these high-aspect-ratio nanostructures did not collapse or change their shape upon heat sterilization and immersion in aqueous environments, rendering them suitable for a wide range of in vitro and possibly in vivo applications. In a follow-up study from the same lab, different BC types were compared, namely commercial BC (CoBC) and lab-grown BC (LgBC), which had different ribbon widths [137]. These differences resulted in smaller  
Figure 10: Low- (**a**) and high-magnification (**b**) SEM images of SCs on a flat (left) and nanorippled surfaces with 270 nm (middle) and 290 nm periodicity (right). Scale bars are 100 μm (first row) and 2 μm (second row). White asterisks indicate parallel alignment of filopodia with regard to the ripple direction as indicated by the white arrows. Republished with permission of The Royal Society of Chemistry, from Ref. [133]; permission conveyed through Copyright Clearance Center, Inc.  
nanostructure dimensions for LgBC, despite maintaining a constant aspect ratio of about 5. The interaction of these nanostructured as well as untreated BC hydrogels with Gram-positive and Gram-negative bacteria was then investigated. In general, nanopatterned CoBC showed a stronger bactericidal activity than irradiated LgBC (Figure 11). Furthermore, it was found that both nanostructured BCs had a stronger bactericidal effect on the Gram-positive bacterium _Bacillus subtilis_ than Gram-negative _Escherichia coli_ (see Figure 11), despite _B. subtilis_ having a stiffer cell envelope. It was further observed that the geometry of the Gram-positive bacteria played an important role as well. In particular, rod-shaped _B. subtilis_ was more susceptible to the contact-killing nanopatterned BC hydrogels than spherical _Staphyloccus aureus_ (see Figure 11). Most importantly, however, the authors could demonstrate that these nanopatterned BC hydrogels have no effect on the viability of murine preosteoblasts, which is an important prerequisite for applications of antimicrobial coatings on medical implants.  
This work demonstrates the great potential of ion beam nanopatterning to serve not only as a tool for enhancing biocompatibility of implant surfaces but also to minimize their microbial colonization. Furthermore, it serves as an excellent example that the application of this well-established technique to novel and in particular organic materials systems may result in new nanopatterned surface morphologies with unexpected biofunctional properties.