# Micromechanical Characterization of Diatom Frustules of Multiple Origin
## 2 State-of-the-Art
### Mechanical Strength of Diatom Frustules and Atomic Force Microscopy AFM
This leads to a comparison of the elastic recovery obtained in atomic microscopy (particularly when the loads are removed) with the elastic rebound observed in one-dimensional consolidation tests when the vertical stresses are removed in the unloading stage. This aspect applies to both intact and fractured frustules. Diatom-rich sediments can thus \"remember\" the in situ stress regime. However, these soils are not cohesive and can quickly lose or acquire water, which generally disturbs the samples and leads to stress memory loss [24]. Ref. [11] assessed the defensive capacity of the frustules as diatom armor against possible attacks by predators through the quantification of their mechanical resistance. Virtual(finite element analysis) and accurate load tests by atomic force microscopy using calibrated glass microneedles were carried out on a series of centric and pennate cells. The forces required to rupture live and individual cells of three diatom species, _Thalassioira punctigera_, _Coscinodiscus granii_ (centric) and _Fragiliaropsis kerguelensis_ (pennate), were measured. All species' shear strength was inversely proportional to diameter (See Table 1). From the previous results and finite element simulations, it was deduced that the ultimate compressive and tensile strength depends on the place where the force is applied, either on the faces or on the lateral bands. The latter presented the highest resistant stresses (up to 680 N/mm\({}^{2}\)) [11]. The behavior during the application of the loads indicated that the frrustules presented rigid failure. Verifications of fractures in pennate diatom frustules have shown that the crack path does not pass through but surrounds the silica spheres that form the frustule, leading to a greater separation length and energy required to generate the breakage [11]. Frustules perform several functions in diatoms, one of which is to provide mechanical stability. The presence of pores and ridges decreases the concentration of stresses on the frustule's structure. Through atomic force tests, it has been found that these elements can withstand stresses ranging from 1 to 7 N/mm\({}^{2}\) (100-700 t/m\({}^{2}\)) [11; 44]. From nanoindentation tests, different moduli of elasticity were obtained in frustules of _Coscinodiscus_ species, which ranged from 3.40 to 15.61 GPa, Refs. [10; 54]. The micromechanical properties of the frustules show significant variations depending on the layer that was analyzed. \"A clear correlation between mechanical properties and diatom features (e.g., porosity, shape, thickness and pore size) has not yet been established\" [55]. Ref. [56] studied the flexibility of four morphologically different diatom species. The flexural rigidity varies widely, even up to four orders of magnitude (1.7 \(\times\) 10\({}^{-13}\)-1.3 \(\times\) 10\({}^{-17}\) Nm\({}^{2}\)). Ref. [22] reports hardness values in diatoms of the species _Navicula pelliculosa_ between 1 and 12 GPa, although higher values are referenced in certain places of the frustule. Ref. [21] analyzed the changes in the mechanical properties and the impact on the population of some freshwater diatoms due to variations in environmental conditions (water acidity and temperature). Nanomechanical assessment was conducted using an atomic force microscope in a peak force touch mode. For the case of _Cyclotella meneghini_ species, [PERSON]'s modulus values were recorded in the frustules, between 7 and 25 GPa. Ref. [51] developed micro indentation experiments using the AFM to evaluate the local variations of [PERSON]'s modulus in both cells and junction structures that are part of diatom chains of marine origin of the _Lithodesmium undulatum_ species. From the results, it was possible to recognize the significant variability of the modulus and determine that the components of the frustule that are newly formed are softer than those associated with the parental cells. The hardening of the new parts may involve the incorporation of organic material.