# Mini-Review: Potential of Diatom-Derived Silica for Biomedical Applications
## 5 Silica for Biomedical Applications: Advantages
### Biocompatibility  
Diatom-derived biosilica has several advantages compared to other porous materials, in terms of high compatibility with biological systems [18; 26]. Biocompatibility tests were performed on various tumour cells, and some significant examples are reported below. An ATP-based luminescent assay aimed at detecting the short-time (6-24 h) detrimental effects on cells showed that DE particles had very low toxicity on the following three colon cancer cell lines: Caco-2, HT-29, and HCT-116 [94]. The effect of amino-modified DE nanoparticles on human lung epidermoid carcinoma cells (H1355) was evaluated by the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) assay. Different concentrations of diatom particles were tested for 24, 48 and 72 h, and the results showed very low cytotoxicity against the abovementioned tumour cells. This feature made functionalized DE particles useful carriers to transport small interfering ribonucleic acid (SiRNA) inside human lung epidermoid carcinoma cells (H1355), silencing gene expression [93]. Biocompatibility was also assessed on bone cells, such as normal human dermal fibroblasts (NDHS) and Saos-2 osteoblasts, by the functionalization of _Thalassiavia weisslogii_ frustrules with 3-mercaptoethanolpropyl-trimethoxysilane (MPTMS). The mercapto-coated biosilica successfully stimulated the growth of both cell lines, even more than bare cells [90]. The biological compatibility of silica-derived diatoms was also assessed in studies aimed at targeting the antiapoptotic factor B-cell lymphoma/leukemia 2 (Bcl2) with small interfering RNA (siRNA). Specifically, the amino groups of silanized silica particles were complexed with siRNA to downregulate the expression of tumour-associated genes. The target line was the A20 murine lymphoma, and no differences in cytotoxicity between the functionalised frustules and controls (e.g., untreated cells) were observed by applying the following three different methodologies: MTT, Cell-Titer GLO and propidium iodide assays [92]. Biocompatibility between functionalized DE particles and breast cancer cells (lines MCF-7 and MDA-MB-231) has also been proven. In this case, amino-modified particles were further improved by PEGylation (i.e., diatom-coating with polyethylene glycol) and cell-penetrating peptide (CPP) bioconjugation, to promote cell internalization through physical and biological changes in the silicon source. The biological compatibility was also evaluated with a luminescent cell viability assay based on the adenosine triphosphate concentration, and the results showed that the cytotoxicity of biosilica that underwent a double modification with PEG and CCP was lower than that of the bare material, as well as that of diatoms that had been amino-modified only [95]. Most cytotoxicity assays mentioned above were performed on short timescales. The effect of longer exposure times (21 days) was assessed on human embryonic kidney cells (HEK-293) and MDA-MB-231 breast cancer cells exposed to syntherized (e.g., fused at high temperature) diatoms. Biocompatibility was tested through viability assays with the dye Calcein-AM (its fluorescence intensity depends on the activity of cellular esterases, and thus of viable cells), and the results confirmed that natural silicon is not toxic. This suggested that fused diatom frustules could be a suitable alternative for synthetic bone graft substitutes [96]. In order to foresee the effects of long-term exposure of silica-based devices on biological systems, [PERSON] and co-workers [97] investigated the in vivo impact of diatomic particles on the model organism _Hydra vulgaris_. Untreated specimens and animals exposed to bare frustules and to diatom nanoparticles modified with the cell-penetrating peptide [(aminooxy)acetyl]-Lys-(Arg)9 (to enhance cellular uptake) were monitored for 14 days, and no detrimental effects in terms of growth rates and apoptosis were observed in all conditions.