Fine-tuning the degree of stem cell polarization and alignment on ordered arrays of high-aspect-ratio nanopillars.

Nanobiomaterials are introducing new capabilities to coordinate cell selection, growth, morphology, and differentiation. Herein, we report that tuning the geometry of ordered arrays of nanopillars (NP) elicits specialized morphologies in adherent cells. Systematic analysis of the effects of the NP radius, height, and spacing reveals that stem cells assume either flattened, polarized, or stellate morphologies in direct response to interpillar spacing. Notably, on NPs of pitch near a critical spacing (d(crit) ≈ 2 µm for C3H10T1/2 cells), cells exhibit rounding of the cell body, pronounced polarization, and extension of narrow axon-like cell projections aligned with the square lattice of the NP array and extending hundreds of micrometers. Furthermore, increasing the NPs' aspect ratio from 12:1 to 50:1 to produce NPs with a corresponding reduction in the NP bending stiffness of 2 orders of magnitude amplified the cellular response and resulted in a previously unseen degree of cell polarization and alignment. The rapid morphological transformation is reproducible on surfaces that maintain key parameters of the NP geometry and spacing, is influenced by the cell seeding density, and persists for different stem cell lines and primary mesenchymal stem cells. The demonstrated ability to support various morphogenetic trends in stem cells by simply tuning the geometry of the NP substrates provides a stepping-stone for the future design of scaffolds where cellular morphology and alignment are crucial.

Numerical modeling of oxygen distributions in cortical and cancellous bone: oxygen availability governs osteonal and trabecular dimensions.

Whereas recent work has demonstrated the role of oxygen tension in the regulation of skeletal cell function and viability, the microenvironmental oxemic status of bone cells remains unknown. In this study, we have employed the Krogh cylinder model of oxygen diffusion to predict the oxygen distribution profiles in cortical and cancellous bone. Under the assumption of saturation-type Michaelis-Menten kinetics, our numerical modeling has indicated that, under steady-state conditions, there would be oxygen gradients across mature osteons and trabeculae. In Haversian bone, the calculated oxygen tension decrement ranges from 15 to 60%. For trabecular bone, a much shallower gradient is predicted. We note that, in Haversian bone, the gradient is largely dependent on osteocyte oxygen utilization and tissue oxygen diffusivity; in trabecular bone, the gradient is dependent on oxygen utilization by cells lining the bone surface. The Krogh model also predicts dramatic differences in oxygen availability during bone development. Thus, during osteon formation, the modeling equations predict a steep oxygen gradient at the initial stage of development, with the gradient becoming lesser as osteonal layers are added. In contrast, during trabeculum formation, the oxygen gradient is steepest when the diameter of the trabeculum is maximal. Based on these results, it is concluded that significant oxygen gradients exist within cortical and cancellous bone and that the oxygen tension may regulate the physical dimensions of both osteons and bone trabeculae.

Tunable liquid optics: electrowetting-controlled liquid mirrors based on self-assembled Janus tiles.

In this paper, we describe a tunable, high-reflectivity optofluidic device based on self-assembly of anisotropically functionalized hexagonal micromirrors (Janus tiles) on the surface of an oil droplet to create a concave liquid mirror. The liquid mirror is deposited on a patterned transparent electrode that allows the focal length and axial position to be electrically controlled. The mirror is mechanically robust and retains its integrity even at high levels of vibrational excitation of the interface. The use of reflection instead of refraction overcomes the limited available refractive-index contrast between pairs of density-matched liquids, allowing stronger focusing than is possible for a liquid lens of the same geometry. This approach is compatible with optical instruments that could provide novel functionality-for example, a dynamic 3D projector, i.e., a light source which can scan an image onto a moving, nonplanar focal surface. Janus tiles with complex optical properties can be manufactured using our approach, thus potentially enabling a wide range of novel optical elements.

Oxygen tension regulates preosteocyte maturation and mineralization.

Oxygen availability is a critical signal for proper development of many tissues, however there is limited knowledge of its role in the maturation of bone cells. To test the hypothesis that low pO2 regulates bone cell mineralization, MLO-A5 and MLO-Y4 cells were cultured in monolayer and three-dimensional alginate scaffolds in hypoxia (2% O2) or normoxia (20% O2). Hypoxia reduced mineralization and decreased alkaline phosphatase activity of preosteocyte-like MLO-A5 cells in both monolayer and alginate cultures. Similar changes in osteogenic activity were seen when the were subjected to chemical hypoxia. Likewise, Osteocyte-like MLO-Y4 cells also exhibited reduced osteogenic activity in hypoxia relative to normoxic controls. Based on these observations, it is concluded that a low pO2 decreased the mineralization potential of bone cells at both early and late stages of maturation. Since the oxemic state is transduced by the transcription factor, HIF-1alpha, experiments were performed to determine if this protein was responsible for the observed changes in mineral formation. It was noted that when HIF-1alpha was silenced, mineralization activities were not restored. Indeed, in hypoxia, in relationship to wild type controls, the mineralization potential of the knockdown cells was further reduced. Based on these findings, it is concluded that the osteogenic activity of preosteocyte-like cells is dependent on both the O2 tension and the expression of HIF-1alpha.