Reply to von der Mark et al.: Looking further into the effects of nanotube dimension on stem cell fate

S. Oh, K. S. Brammer, Y. S. J. Li, D. Teng, A. J. Engler, S. Chien, S. Jin
2009 Proceedings of the National Academy of Sciences of the United States of America  
In their Letter to the Editor, von der Mark et al. (1) stated that they found adhesion, proliferation, migration, and osteogenic differentiation of rat bone marrow mesenchymal stem cells (MSCs) to be highest on 15-nm TiO 2 nanotubes and to be dramatically decreased on 70-and 100-nm nanotubes (2, 3). These findings are contrary to our results with human mesenchymal stem cells on a range of nanotubes with 30-to 100-nm diameter, where cell stretching and expression of osteogenic differentiation
more » ... differentiation markers was highest on 100 nm nanotubes (4). Park et al. (2, 3) illustrated that the optimum length scale for cell vitality and differentiation was shown to be the small-diameter nanotubes (Ϸ15 nm), in contrast to the large diameter of 100nm in our study (4). The opposite results found in the refs. 2 and 3 are interesting, and we agree that further discussion and experimentation are needed for their resolution. First, we would like to point out that the Park et al. (2, 3) studies and our study (4) used MSCs from different species and origins: rat bone marrow stem cells (2, 3) vs. human mesenchymal stem cells (4). Second, Park et al. (2, 3) used osteogenic induction media, whereas we only used regular cell growth media without any osteogenic inducing media, osteogenic supplements, or growth factors. Third, there are differences in the nature of the TiO 2 nanotubes used. The nanotubes used by Park et al. were as-anodized amorphous phase, whereas those used in our study were heat treated and crystallized, anatase phase; we have previously demonstrated that crystallized TiO 2 nanotubes are superior in osteoblast cell adhesion and growth (5, 6) and that the anatase phase plays an important role in proliferation and cell morphology. Furthermore, Park et al.'s (2, 3) amorphous nanotube samples can exhibit 5% fluorine at the sample surface (3), whereas our sample (4) surface was high-temperature annealed at 500°C to thermally decompose and remove most of the fluorine (XPS showing only 0.4%). It has been demonstrated that flu-orine has various degrees of effects on implant surfaces (7-9), and different nanotube pore openings may lead to different degrees of phosphoric or hydrofluoric acid residues which may be beneficial or induce toxicity to cells. The discussion above points to several key differences in the experimental conditions used in the Park studies (2, 3) and our study (4). Although these differences make it difficult to directly compare the results of these studies, the findings of opposite effects of nanotube diameter on osteogenesis of MSCs showed the striking variabilities and opportunities to choose different substrate topography for the purpose of influencing and controlling stem cell fate and motivate more systematic studies on the effects of TiO 2 nanotube geometry, materials, processing parameters, surface chemistry, crystal structure, and other differentiation approaches on behaviors of different types of stem cells. This will help us achieve the ultimate goal of establishing the optimum microenvironment, including that presented by nanotubes, for the control of stem cell fate.
doi:10.1073/pnas.0904869106 fatcat:bfbf67qp7nh7zogctqruccfppi