# Micromechanical Characterization of Diatom Frustules of Multiple Origin
## 2 State-of-the-Art
### Mechanical Strength of Diatom Frustules and Atomic Force Microscopy AFM  
AFM allows for the obtaining of high-resolution images from changes in the height of surfaces, making it possible to recognize objects at the nanoscale. Studies applying this technique to diatoms have increased significantly since 1992. AFM is applied for studies of taxonomy, biomineral formation, ultrastructure, mucilaginous layers, deformation, adhesion, Young's modulus, and nano and micromechanical properties, among others [20; 21]. An AFM consists of a micromachined elastic cantilever with a sharp tip perpendicular to the cantilever. When a force is applied to the tip, the cantilever deflects proportionally. The cantilever should have a sufficiently low \"spring constant k\" so that large deflections can be obtained when small forces are applied. These deflections are detected by a laser system reflecting on the cantilever. The AFM is suitable for studies in humid environments (water or other solutions). In addition to imaging, stress and strain records can be obtained from the AFM, even on a single-molecule scale [23]. The advantages of AFM over SEM microscopy are high image resolution, 3D representation, and the possibility of measuring physical and structural properties in a desired spatial environment. The accuracy of the quantitative results of nanoindentation and elastic measurement of AFM has been the subject of discussion, but the relative micromechanical information still turns out to be of great importance [22]. Concerning diatoms, AFM has been implemented from two perspectives. The first involves the total assessment of the cell through the progressive application of loads on its frustule, up to the point of reaching breakage. The second corresponds to evaluating the local properties, i.e., the resistance of the elements that make up the structure. The modulus values obtained in various investigations (for the few species studied) have shown considerable differences, even of an order of magnitude. This situation is associated with the variation in the silicification levels and the conditions of execution of the measurements [51]. Another line of research has been the nanoindentation of the surface, from which the results of depth and area make it possible to calculate the hardness of the frustules. This magnitude is a function of the geometry of the indenter. It is not always possible to differentiate whether the results of an indentation correspond exclusively to local properties (elastic deformations) or whether the influence of the total geometry of the frustule is present [22]. Although current technological advances do not allow the development of artificial siliceous structures with the level of precision and ordering (hierarchical structure) that diatoms do, nor do they allow the reproduction of the stages of biomineralization, atomic force microscopy at least opens the possibility of studying these organisms \"in vivo\", with a high level of spatial resolution and with the possibility of evaluating the mechanical behavior of both the frustules and the adhesive substances excreted [23; 52]. Ref. [48] used AFM to investigate the nanostructure, morphology, physical properties and the relationship of the outermost organic layer to the silica frustule under natural hydration conditions. AFM images revealed that the silica walls are coated by a thin mucilaginous material covering most of the surface, which is non-adhesive, soft, non-soluble and compressible. From the AFM on the diatoms of the _P. viridis_ species, it was recognized that the frustules are composed of packed silica spheres with diameters close to 44.8 nm in the valves and 40.3 nm in the bands. For the species _Hantzschia ambiaxys_, spheres with sizes close to 37.1 nm in valves and 38.1 in bands were found. It can be concluded that the sphere size does not change significantly between the place of arrangement within the same frustules, whereas the change is considerable when the species varies [48].