# Nanostructured Biosilica of Diatoms: From Water World to Biomedical Applications
## 5 Diatom as a Promising Scaffold for Cell Proliferation and Differentiation
Contrary to many artificial bone graft materials in which porosity is formed through techniques that could be a source of potential contamination [39; 40], frrustules show naturally designed porous architectures. Additionally, diatom frustules can be reused subsequently to a series of washings with 96% ethanol and PBS. Afterward, a sterilization process is compulsory to seed new cells on the scaffold without conditioning their survival rate. Silicon plays a crucial role in the bone regeneration process and influences bone mineral density. Silicon induces calcium phosphate precipitation in the early stage of bone mineralization and improves cell proliferation. Scaffolds incorporating amorphous silica, among which silica-collagen scaffolds [88; 89], and silk-silica composites [90], enhance osteoconductivity, alkaline phosphatase activity (ALP), matrix deposition levels, and osteocalcin synthesis. The fabrication of synthetic silica scaffolds involves the use of chemicals and surfactants whose residues might have toxic effects on cells. For this reason, there is a great interest in diatom frrustules as a novel, safe source of biogenic silicon. [PERSON] et al. [91] have recently demonstrated that diatom particles obtained from acid-purified raw diatomite powder can undergo degradation in an aqueous environment and release silicon ions in the medium. Later, [PERSON] et al. [92] reported the osteoinductive potentiality of micro-nano frustules incorporated into a multifunctional 3D fibrous scaffold. Micro-nano frustules obtained from broken frustules were incorporated into a matrix of poly (hydroxybutyrate-co-hydro-xyvalerate)/poly(\(\varepsilon\)-caprolactone) (PHBV/PCL) fibers and pullulan (PUL), which prevented any possible toxic effect on cells. The co-electrospinning method was successful to create a random mesh of nano and microfibers, which created a high surface for cell interaction and enabled mechanical support. Osteocompatibility tests, performed by incubating Saos-2 cells with the silica-doped scaffold, revealed improved cell viability after 4 and 7 days of incubation. The entrapment of micro-nano particles into the fibrous scaffold increased calcium deposition up to 20% compared to the undoped PHBV/PCL control scaffold after 7 days, which outlined the positive effect of silica-enriched substrate on Saos-2 ALP activity. The developed silica-doped scaffold improved cell distribution and spreading thanks to the synergistic effect of biosilica and polymers; silica structures promoted cell bioactivity, whereas PUL fibers improved cell spreading by increasing overall hydrophilicity. Moreover, Saos-2 cells seeded on the doped scaffold showed a remarkable elongated morphology compared to the undoped substrate, in which cells had a round morphology which indicates low interaction with fibers (Figure 5). One of the disadvantages of diatom frustules is that silica has low biodegradability in biological fluids. That leads to its toxic gathering in the human body, particularly in the liver. Accumulation of silica particles in the liver has been reported in previous studies as a result of particle uptake by macrophages in the reticuloendothelial system [93,94]. To overcome these limitations, [PERSON] et al. converted diatom silica into degradable silicon replicas for intraocular therapy [95]. Silica (SiO\({}_{2}\)) turned into silicon replicas through a three-steps magnesium process in which the magnesium source was melted and caused the reduction of silica to silicon replicas. Silicon replicas showed a better degradation rate compared to silica in vitro. The 20% of silicon dissolved within 30 days, whereas less than 1% of diatom silica degraded the same timeframe. Comparative SEM imaging revealed significant changes in silicon replicas structure, showing that the silicon core was completely dissolved after 30 days. Therefore, the conversion from diatom silica to silicon can be considered a valid strategy to prevent particle accumulation in the body fluids.