###### Abstract  
Ensuring effective integration between the material of an implant and bone is critical to orthopedic implants' success in the long term. A major issue with dense materials is the mechanical mismatch between them and the bone, which leads to improper osseointegration. Porous implants have presented a solution to this issue as they are able to retain material properties in addition to decreasing mismatches. In order to make implants more biomimetic and to match the micro-/nano hierarchy of bone, several surface modifications have been explored in the literature. Hydrothermal treatment in an alkali media on dense titanium has demonstrated higher differentiation of adipose-derived stem cells to osteogenic lineages. In this study, we fabricated nanostructures using hydrothermal treatment in an alkali medium on micro-porous titanium surfaces and evaluated the adhesion, proliferation, and differentiation of adipose derived stem cells to osteoblasts. The nanostructured-micro-porous titanium surfaces displayed enhanced osteogenic differentiation of adipose derived stem cells. Therefore, they have the potential to be used as surfaces for the fabrication of orthopedic implants.  
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Nanostructures, such as nanotubes, nanowires, nanoflowers, nanoribbons, etc. [14; 15; 16; 17], are fabricated with the goal of complementing micro-scale modification to achieve a hierarchy.  
Even after fabricating surfaces which mirror the micro-nano hierarchy of the bone, implants with dense surfaces also have to overcome a bio-mechanical mismatch of the elastic modulus and structural difference with the surface of bone [18]. The mismatch is due to the inherent mechanical properties of bulk titanium being significantly higher to than that of bone [19]. This leads to bone resorption and implant loosening [20]. Porous material surfaces offer a solution to both of these issues as porosity brings the elastic modulus closer to that of the bone and also mimics the inherent porous structure present in bone. Porosity also serves as a natural scaffold, allowing cell growth and differentiation [18]. The differentiation of cells is influenced by the three-dimensional structure, which provides channels for nutrient transport [20; 21]. However, the bioinertness of titanium also needs to be countered; hence, some of the surface modifications mentioned before have also been applied to porous titanium surfaces in other studies [22; 23] to increase their bioactivity.  
In this study, nanostructures were fabricated on micro-porous titanium surfaces using hydrothermal treatment in an alkali medium to fabricate a micro-nano hierarchy. Micro-porous surfaces have displayed enhanced differentiation of mesenchymal stem cells to osteogenic lineages through enhanced expression of the RUNX2 gene and osteoclastin [20]. Hydrothermal treatment is a simple, one-step technique which fabricates titania nanostructures, which have demonstrated enhanced osteoblast response when fabricated on dense titanium surfaces [24; 25]. Cell interactions with these surfaces were investigated by seeding adipose derived stem cells on them and characterizing cell growth using fluorescence microscopy and SEM. Cell differentiation was characterized using immunofluorescence microscopy, SEM, and protein expression assays. The results of this study indicate that nanostructured, micro-porous titanium demonstrates enhanced osteogenesis compared to titanium surfaces, thus making them a prospective surface for fabricating orthopedic implants.