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Abstract:The surface of implantable biomaterials is in direct contact with the host tissue and plays a critical role in determining biocompatibility. In order to improve the integration of implants, it is desirable to control interfacial reactions such that nonspecific adsorption of proteins is minimized and tissue-healing phenomena can be controlled. In this regard, our goal has been do develop a method to functionalize oxidized titanium surfaces by the covalent immobilization of bioactive organic molecules. Titanium first was chemically treated with a mixture of sulfuric acid and hydrogen peroxide to eliminate surface contaminants and to produce a consistent and reproducible titanium oxide surface layer. An intermediary aminoalkylsilane spacer molecule was then covalently linked to the oxide layer, followed by the covalent binding of either alkaline phosphatase or albumin to the free terminal NH 2 groups using glutaraldehyde as a coupling agent. Surface analyses following coating procedures consisted of X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Enzymatic activity of coupled alkaline phosphatase was assayed colorimetrically, and surface coverage by bound albumin was evaluated by SEM visualization of colloidal gold immunolabeling. Our results indicate that the linkage of the aminoalkylsilane to the oxidized surface is stable and that bound proteins such alkaline phosphatase and albumin retain their enzymatic activity and antigenicity, respectively. The density of immunolabeling for albumin suggests that the binding and surface coverage obtained is in excess of what would be expected for inducing biological activity. In conclusion, this method offers the possibility of covalently linking selected molecules with known biological activity to oxidized titanium surfaces in order to guide and promote the tissue healing that occurs during implant integration in bone and soft tissues.

We report an experimental study of liquid drops moving against gravity, when placed on a vertically vibrating inclined plate, which is partially wet by the drop. Frequency of vibrations ranges from 30 to 200 Hz, and above a threshold in vibration acceleration, drops experience an upward motion. We attribute this surprising motion to the deformations of the drop, as a consequence of an up/down symmetry-breaking induced by the presence of the substrate. We relate the direction of motion to contact angle measurements. This phenomenon can be used to move a drop along an arbitrary path in a plane, without special surface treatments or localized forcing. PACS numbers:A drop of liquid on an inclined substrate will slide downward due to gravity, unless the drop is pinned by contact angle hysteresis [1,2]. Since the contact angle hysteresis is reduced by vertical vibrations [3,4], one might expect that sufficiently strong shaking will always make the drop come loose and provoke it to slide. Here we report for the first time that on the contrary, sufficiently strong harmonic shaking in the vertical direction will always cause the drop to climb up the slope, regardless of system parameters.We attribute the upward force to a combination of the broken symmetry caused by the inclination of the substrate with respect to the applied acceleration and the nonlinear frictional force between the drop and the substrate. During the downward acceleration phase, the drop becomes taller and thus more compliant to lateral forcing. Hence, the maximum value of the contact angle attained on the upper side ( Fig. 1(d)) is greater than the maximum value attained on the lower side ( Fig. 1(b)), and the drop thus experiences a net upward force [5]. However, for a purely linear frictional force the net force on the drop would average to zero over one period; hence some nonlinearity in the interaction between the drop and the substrate is needed. This key issue is illustrated by a model calculation below.In our experiments a drop of a glycerol-water mixture, of volume V between 0.5 and 20 µl was deposited on a plexiglass substrate inclined to the horizontal with an angle α up to 85 o . The resulting sessile drop was between 1 and 3 mm in diameter, and pinned in the absence of shaking. The substrate was oscillated vertically using an electromagnetic shaker with acceleration up to 50g where g is the acceleration due to gravity, and frequencies between 30 Hz and 200 Hz. The acceleration was monitored with a single-axis accelerometer; the acceleration due to unwanted lateral motion did not exceed 3% of the vertical acceleration.The kinematic viscosity ν of the various mixtures * Electronic address: p.brunet@bristol.ac.uk ranged between 31 and 55 mm 2 /s. For lower viscosities the drop can break up before the onset of climbing; for higher viscosities, drops move slower and thus their dynamics is more difficult to access. The surface tension γ was equal to 0.066 N/m, the density ρ at 20• C ranged from 1190 kg/m 3 for ν= 31 mm 2 /s to 1210 kg/m 3 for ν=...

A drop of water deposited on a cold plate freezes into an ice drop with a pointy tip. While this phenomenon clearly finds its origin in the expansion of water upon freezing, a quantitative description of the tip singularity has remained elusive. Here we demonstrate how the geometry of the freezing front, determined by heat transfer considerations, is crucial for the tip formation. We perform systematic measurements of the angles of the conical tip, and reveal the dynamics of the solidification front in a Hele-Shaw geometry. It is found that the cone angle is independent of substrate temperature and wetting angle, suggesting a universal, self-similar mechanism that does not depend on the rate of solidification. We propose a model for the freezing front and derive resulting tip angles analytically, in good agreement with the experiments.