# Nanostructured Biosilica of Diatoms: From Water World to Biomedical Applications
## 6 In Vivo Evaluation of Biodistribution and Toxicity of Diatom Biosilica  
Animal research is indispensable for medical devices and novel therapies. Although there is considerable research on the effectiveness of biosilica in vitro, research about its safety and efficacy in vivo is still scarce. To investigate the possible side effects of injected frustules, [PERSON] et al. [17] administered genetically-engineered frustules in BALB/c mice via intravenous injection and studied their biodistribution for eight days. Frustules were mainly trapped by the kidney due to the particle accumulation in the mononuclear phagocyte system (MPS) and liver. Biosilica administration did not cause structural alterations of the main organs, including the brain, kidney, heart, lung, liver, and tail. However, the accumulation of the silica particles could be particularly dangerous in the case of repeated administration in organs with limited blood supply (e.g., vitreous body). Figure 5: Laser scanning confocal microscopy images of Saos-2 cells on PHBV/PCL (**a**), PHBV/PCL/DS (**b**), PHBV/PCL/CA (**c**), and PHBV/PCL/CA: PUL/DS (**d**) scaffold groups after 7 days of incubation, (**e**) Cells spread area. [PERSON] and red colors are representing Alexa Fluor 4. Reproduced from Reference [92], copyright (2019), with permission from Elsevier. Therefore, considering alternative routes of administration (e.g., topical, transdermal) may be useful to enable new therapeutic platforms that have unfavorable pharmacokinetic properties. That has led scientists to explore new strategies outside of the traditional oral or intravenous routes and create new opportunities for diatom applications. [PERSON]'s group [96] found that the topical application of diatoms and diatomite on injuries can help to manage bleeding from surgical surfaces. Topical hemostatic agents are generally required when surgical hemostasis is inadequate or impractical. [PERSON]'s group [96] compared the hemostatic property of chitosan-modified diatoms, diatomite, zeolite, and traditional gauze on amputated rat tail. Diatoms frustule provided efficient hemorrhage control, thanks to the ability of silanol groups to stimulate the coagulation factors. Biosilica concentrated platelets and coagulation factors in the local injury, thereby reducing the thrombin generation time and inducing activated platelets stick to form the blood clot. Therefore, the blood clot was formed by diatoms more rapidly than all other hemostatic agents. [PERSON] et al. found that the synergy between silanol anions and nano/microstructures on the frustule surface confers to diatoms hydrophilic and hemophilic properties [88]. Frustules from Melosira nummuloides exhibited interesting wettability and contact angles towards blood samples. Frustules applied to wounds formed a \"clay-like\" mixture able to respond as a viscoelastic material under shear stress, and to control hemorrhage in a model of liver bleeding by incision. Anionic charges on the frustule surface promoted the activation of factor XII to FXIIa and provoked plasma clotting. Contrary to traditional hemostatic agents, which have low efficiency and undesirable collateral effects, silica from diatom is a biocompatible and safe material. Hence, the topical administration of biosilica could represent a fascinating approach to treat skin diseases -without unfavorable side effects- and to avoid silica accumulation in the kidney. The choice of the appropriate animal model is crucial to evaluate in vivo biosilica effects. The complexity of diatoms and their frustules places higher demands on animal models. Recently, many researchers turned to less studied and more unusual systems, such as _Hydra_ polyps, for their studies. These systems can make it easier to answer questions that have remained unanswered to date. [PERSON] et al. [97] confirmed the safety of biosilica administration on the cnidarian freshwater polyp _Hydra Vulgaris.