Breakup and then makeup: a predictive model of how cilia self-regulate hardness for posture control.

Functioning as sensors and propulsors, cilia are evolutionarily conserved organelles having a highly organized internal structure. How a paramecium's cilium produces off-propulsion-plane curvature during its return stroke for symmetry breaking and drag reduction is not known. We explain these cilium deformations by developing a torsional pendulum model of beat frequency dependence on viscosity and an olivo-cerebellar model of self-regulation of posture control. The phase dependence of cilia torsion is determined, and a bio-physical model of hardness control with predictive features is offered. Crossbridge links between the central microtubule pair harden the cilium during the power stroke; this stroke's end is a critical phase during which ATP molecules soften the crossbridge-microtubule attachment at the cilium inflection point where torsion is at its maximum. A precipitous reduction in hardness ensues, signaling the start of ATP hydrolysis that re-hardens the cilium. The cilium attractor basin could be used as reference for perturbation sensing.

Effect of surface nanoscale topography on elastic modulus of individual osteoblastic cells as determined by atomic force microscopy.

Mechanical stimulation of osteoblasts by fluid flow promotes a variety of pro-differentiation effects and improving the efficiency of these mechanical signals could encourage specific differentiation pathways. One way this could be accomplished is by altering mechanical properties of osteoblasts. In this study, murine osteoblastic MC3T3-E1 cells were cultured on surfaces covered with nanometer-sized islands to examine the hypothesis that the elastic modulus of osteoblastic cells is affected by nanoscale topography. Nanoislands were produced by polymer demixing of polystyrene and poly(bromostyrene), which leads to a segregated polymer system and formation of nanometer-sized topographical features. The elastic modulus of MC3T3-E1 cells was determined using atomic force microscopy in conjunction with the Hertz mathematical model. Osteoblastic cells cultured on nanotopographic surfaces (11-38 nm high islands) had a different distribution of cellular modulus values, e.g., the distribution shifted toward higher modulus values, relative to cells on flat control surfaces. There were also differences in cell modulus distribution between two flat controls as surface chemistry was changed between polystyrene and glass. Taken together, our results demonstrate that both surface nanotopography and chemistry affect the mechanical properties of cells and may provide new methods for altering the response of cells to external mechanical signals.

The regulation of integrin-mediated osteoblast focal adhesion and focal adhesion kinase expression by nanoscale topography.

An important consideration in developing physical biomimetic cell-stimulating cues is that the in vivo extracellular milieu includes nanoscale topographic interfaces. We investigated nanoscale topography regulation of cell functions using human fetal osteoblastic (hFOB) cell culture on poly(l-lactic acid) and polystyrene (50/50 w/w) demixed nanoscale pit textures (14, 29, and 45nm deep pits). Secondary ion mass spectroscopy revealed that these nanotopographic surfaces had similar surface chemistries to that of pure PLLA because of PLLA component surface segregation during spin casting. We observed that 14 and 29nm deep pit surfaces increased hFOB cell attachment, spreading, selective integrin subunit expression (e.g., alphav relative to alpha5, beta1, or beta3), focal adhesive paxillin protein synthesis and paxillin colocalization with cytoskeletal actin stress fibers, and focal adhesion kinase (FAK) and phosphorylated FAK (pY397) expression to a greater degree than did 45nm deep pits or flat PLLA surfaces. Considering the important role of integrin-mediated focal adhesion and intracellular signaling in anchorage-dependent cell function, our results suggest a mechanism by which nanostructured physical signals regulate cell function. Modulation of integrin-mediated focal adhesion and related cell signaling by altering nanoscale substrate topography will have powerful applications in biomaterials science and tissue engineering.

Osteoblast adhesion on poly(L-lactic acid)/polystyrene demixed thin film blends: effect of nanotopography, surface chemistry, and wettability.

Biomaterial surface characteristics are critical cues that regulate cell function. We produced a novel series of poly(l-lactic acid) (PLLA) and polystyrene demixed nanotopographic films to provide nonbiological cell-stimulating cues. The increase in PLLA weight fraction (phi) in blend solutions resulted in topography changes in spin-cast films from pit-dominant to island-dominant morphologies having nanoscale depth or height (3-29 nm). Lower molecular weight PLLA segregated to the top surface of demixed films, as observed by X-ray photoelectron spectroscopy and secondary ion mass spectroscopy (SIMS). For phi > or = 0.5, the topmost film layer was predominantly filled with PLLA (>96% by SIMS at 20-A depth). Nanotextured substrata stimulated osteoblastic cell adhesion to a greater degree than did flat PLLA (phi = 1), and this effect was more pronounced for nanoisland (phi = 0.7 and 0.9) relative to nanopit topographies (phi = 0.5). Demixed films having relatively lower water contact angles generally enhanced cell adhesion and spreading. Our results reveal that cell adhesion is affected by surface chemistry, topography, and wettability simultaneously and that nanotextured surfaces may be utilized in regulating cell adhesion.

Human foetal osteoblastic cell response to polymer-demixed nanotopographic interfaces.

Nanoscale cell-substratum interactions are of significant interest in various biomedical applications. We investigated human foetal osteoblastic cell response to randomly distributed nanoisland topography with varying heights (11, 38 and 85 nm) produced by a polystyrene (PS)/polybromostyrene polymer-demixing technique. Cells displayed island-conforming lamellipodia spreading, and filopodia projections appeared to play a role in sensing the nanotopography. Cells cultured on 11 nm high islands displayed significantly enhanced cell spreading and larger cell dimensions than cells on larger nanoislands or flat PS control, on which cells often displayed a stellate shape. Development of signal transmitting structures such as focal adhesive vinculin protein and cytoskeletal actin stress fibres was more pronounced, as was their colocalization, in cells cultured on smaller nanoisland surfaces. Cell adhesion and proliferation were greater with decreasing island height. Alkaline phosphatase (AP) activity, an early stage marker of bone cell differentiation, also exhibited nanotopography dependence, i.e. higher AP activity on 11 nm islands compared with that on larger islands or flat PS. Therefore, randomly distributed island topography with varying nanoscale heights not only affect adhesion-related cell behaviour but also bone cell phenotype. Our results suggest that modulation of nanoscale topography may be exploited to control cell function at cell-biomaterial interfaces.