# Nanostructured Biosilica of Diatoms: From Water World to Biomedical Applications
## 5 Diatom as a Promising Scaffold for Cell Proliferation and Differentiation  
Bioactive mesoporous silica materials (MSNs) have been clinically investigated as scaffolds for cell attachment, proliferation, and differentiation, on account of their specific surface area, porous structure, and tunable pore size [80; 81]. Nevertheless, the synthesis of the MSNs is often expensive, since it involves toxic reagents, time-consuming and complicated processes. On the other hand, diatoms are an alternative source of silica with a naturally self-assembled 3D porous structure. This multifunctional material with unique mechanical and molecular transport properties is a promising material for regenerative medicine [82]. It is important to remember that surface chemistry plays a crucial role in the fabrication of platforms for biomedical applications. Hence, special consideration should be given to the possibility of attaching biomolecules to the scaffold surface. Diatom frustules offer a large surface area that could be modified with osteoinductive agents (e.g., peptides or growth factors), which represent attractive signals for bone cells and should promote the bone regeneration process. Chemical modifications of diatom frustules for bone cell adhesion and proliferation were proposed by [PERSON] et al., who studied the adhesion and growth of normal human dermal fibroblasts (NHDF) and human osteosarcoma cell line (Saos-2) on bare and silane-coated frustules [83]. In this study, nanoporous microcapsules 10-15 \(\upmu\)m in size were obtained from the pelagic centric diatoms _T. weissflogii_ and functionalized with APTES, a popular modification we have previously mentioned on account of its ability to facilitate the attachment of organic compounds to silica substrates. The authors also carried out the grafting of silanes containing mercapto (SH) groups by self-assembling monolayers of 3-mercaptoethanolpropyl-trimethoxysilane (MPTMS) on the surface of the microcapsules. The cytocompatibility of bare and modified microcapsules was tested by studying NHDF and Saos-2 cell viability, spreading, and proliferation. These cell lines are generally used for testing the biocompatibility of silicate-derived scaffolds [84]. Both cell lines adhered to the mesostructured bare frustules showing improved vitality and morphological shapes compared to glass substrates. Bare diatoms stimulated cells to emit long _filopodia_ and large _lamellipodia_, which extended connections to the substrate and played fundamental roles in cell migration. Cells showed higher vitality on bare and mercapto-modified frrustules between 24 and 96 h. The SEM investigations of single frustule-cell interactions showed that cells reached maximal stretching on bare and mercapto-modified frustules during the adhesion process. On the contrary, frrustules modified with amino groups hindered cell adhesion and proliferation. Amino groups have been reported to reduce cell adhesion and proliferation due to a supposed massive albumin subtraction from the cell media [85]. These results highlight the need to find the right functionalization strategy to promote cell growth on diatom-derived substrates and encourages the study of this material for further in vivo applications. Fused diatom frrustules (FDF) from _Cyclostephanos_ sp. were tested by [PERSON] et al. for their ability to support the colony growth of a variety of cell types including osteoblasts, HEK cells, and breast cancer cells [86]. The interconnected pore network of FDF provided topographical irregularity that improved osteogenic differentiation and mineralization of pre-osteoblasts (MC3T3) cells presumably through mechanical interlocking [87]. Fused diatom frrustules resulted to be non-toxic to cells and showed in all cases a cell adherence stronger than NUNC surface culture plates, enabling a high-density cell growth over 21 days. The high porous structure allowed cells to exchange gases and turn nutrients into usable energy through the 3D-structure of the biomaterial.