Jaroslaw Drelich

Zinc Exhibits Ideal Physiological Corrosion Behavior for Bioabsorbable Stents

Zinc is proposed as an exciting new biomaterial for use in bioabsorbable cardiac stents. Not only is zinc a physiologically relevant metal with behavior that promotes healthy vessels, but it combines the best behaviors of both current bioabsorbable stent materials: iron and magnesium. Shown here is a composite image of zinc degradation in a murine (rat) artery.

The significance and magnitude of the line tension in three-phase (solid-liquid-fluid) systems

The excess energy of the three-phase system per unit length of the three-phase contact line is attributed to the line tension, a one-dimensional analog of the surface tension. The interpretation and magnitude of the line tension in three-phase systems that include a solid phase, and experimental techniques used for line tension determination are reviewed. The significance of the line tension in three-phase systems is discussed in relation to contact angle measurements for a liquid at a solid substrate of varying surface quality. Also discussed is the contribution of the linear free energy to the energy of attachment (detachment) of fine particles at (from) an interface.

Superhydrophilic and superwetting surfaces: definition and mechanisms of control.

The term superhydrophobicity was introduced in 1996 to describe water-repellent fractal surfaces, made of a hydrophobic material, on which water drops remain as almost perfect spheres and roll off such surfaces leaving no residue. Today, superhydrophobic surfaces are defined as textured materials (and coatings) on (nonsmooth) surfaces on which water forms contact angles 150° and larger, with only a few degrees of contact angle hysteresis (or sliding angle). The terms superhydrophilicity and superwetting were introduced a few years after the term superhydrophobicity to describe the complete spreading of water or liquid on substrates. The definition of superhydrophilic and superwetting substrates has not been clarified yet, and unrestricted use of these terms sometimes stirs controversy. This Letter briefly reviews the superwetting phenomenon and offers a suggestion on defining superhydrophilic and superwetting substrates and surfaces.

Wetting characteristics of liquid drops at heterogeneous surfaces

Abstract Well-defined heterogeneous surfaces consisting of hydrophobic and hydrophilic regions were prepared on gold (a 2000 A gold film supported on an Si/SiO2/Ti substrate) by patterning self-assembled monolayers (SAMs), using an elastomer stamp. One surface was composed of alternating and parallel hydrophobic (2.5 μm) and hydrophilic (3 μm) strips, and the second surface consisted of alternating hydrophilic squares (3 μm × 3 μm) separated by hydrophobic strips (2.5 μm). The wetting characteristics of these well-defined heterogeneous solid surfaces were examined by contact angle measurements. The contact angles for water drops, which varied in pH from 5.8 to 10.0, were measured with the strips both tangential to and normal to the three-phase contact line. The experimental contact angles are in good agreement with theory as calculated from the Cassie equation when the three-phase contact line is non-contorted (i.e. the three-phase contact line is situated along the hydrophobic strip). On the other hand, when the strips are normal to the drop edge, corrugation of the three-contact line affects the contact angle significantly. Contact angles, measured with the strips normal to the drop edge, were lower by 7–16° than those calculated from the Cassie equation. Analysis of these measurements, together with contact angle/drop size measurements for fully hydrophobic and hydrophilic surfaces, demonstrate the validity of a modified Cassie equation that includes a term describing the line tension contribution.

Origins of Thermodynamically Stable Superhydrophobicity of Boron Nitride Nanotubes Coatings

Superhydrophobic surfaces are attractive as self-cleaning protective coatings in harsh environments with extreme temperatures and pH levels. Hexagonal phase boron nitride (h-BN) films are promising protective coatings due to their extraordinary chemical and thermal stability. However, their high surface energy makes them hydrophilic and thus not applicable as water repelling coatings. Our recent discovery on the superhydrophobicity of boron nitride nanotubes (BNNTs) is thus contradicting with the fact that BN materials would not be hydrophobic. To resolve this contradiction, we have investigated BNNT coatings by time-dependent contact angle measurement, thermogravimetry, IR spectroscopy, and electron microscopy. We found that the wettability of BNNTs is determined by the packing density, orientation, length of nanotubes, and the environmental condition. The origins of superhydrophobicity of these BNNT coatings are identified as (1) surface morphology and (2) hydrocarbon adsorbates on BNNTs. Hydrocarbon molecules adsorb spontaneously on the curved surfaces of nanotubes more intensively than on flat surfaces of BN films. This means the surface energy of BNNTs was enhanced by their large curvatures and thus increased the affinity of BNNTs to adsorb airborne molecules, which in turn would reduce the surface energy of BNNTs and make them hydrophobic. Our study revealed that both high-temperature and UV-ozone treatments can remove these adsorbates and lead to restitution of hydrophilic BN surface. However, nanotubes have a unique capability in building a hydrophobic layer of adsorbates after a few hours of exposure to ambient air.

Superhydrophobicity of Boron Nitride Nanotubes Grown on Silicon Substrates

Partially vertical aligned boron nitride nanotubes (BNNTs) on Si substrates are found to be superhydrophobic in contrast to boron nitride (BN) thin films. While the hexagonal-phase BN films are partially wetted by water with advancing contact angle of about 50 degrees , partially vertically aligned BNNTs can achieve superhydrophobic state with advancing water contact angle exceeding 150 degrees . Our results show that the pH value of water does not affect the wetting characteristics of BNNTs. Since BN is chemically inert, resistive to oxidation up to 900 degrees C, and transparent to visible-UV light, BNNTs could potentially be useful as self-cleaning, transparent, insulating, anticorrosive coatings under rigorous chemical and thermal conditions.

A simplified in vivo approach for evaluating the bioabsorbable behavior of candidate stent materials

Metal stents are commonly used to revascularize occluded arteries. A bioabsorbable metal stent that harmlessly erodes away over time may minimize the normal chronic risks associated with permanent implants. However, there is no simple, low-cost method of introducing candidate materials into the arterial environment. Here, we developed a novel experimental model where a biomaterial wire is implanted into a rat artery lumen (simulating bioabsorbable stent blood contact) or artery wall (simulating bioabsorbable stent matrix contact). We use this model to clarify the corrosion mechanism of iron (≥99.5 wt %), which is a candidate bioabsorbable stent material due to its biocompatibility and mechanical strength. We found that iron wire encapsulation within the arterial wall extracellular matrix resulted in substantial biocorrosion by 22 days, with a voluminous corrosion product retained within the vessel wall at 9 months. In contrast, the blood-contacting luminal implant experienced minimal biocorrosion at 9 months. The importance of arterial blood versus arterial wall contact for regulating biocorrosion was confirmed with magnesium wires. We found that magnesium was highly corroded when placed in the arterial wall but was not corroded when exposed to blood in the arterial lumen for 3 weeks. The results demonstrate the capability of the vascular implantation model to conduct rapid in vivo assessments of vascular biomaterial corrosion behavior and to predict long-term biocorrosion behavior from material analyses. The results also highlight the critical role of the arterial environment (blood vs. matrix contact) in directing the corrosion behavior of biodegradable metals.

Hydrophobicity of ion-plated PTFE coatings

Abstract The influence of roughness on the wetting properties of ion-plated poly(tetrafluroethylene) (PTFE) coatings has been investigated using atomic force microscopy (AFM) and contact angle goniometry techniques. PTFE coatings with different surface roughness have been obtained by applying different substrate bias DC voltages during ion plating. Surface roughness has been characterized from the surface morphology studies carried out using atomic force microscopy. It was found that the majority of surface asperities for rough coatings prepared in this study have dimensions of a few nanometers, 6–13 nm. For such coatings, high water contact angles, 150–160°, were observed. Importantly, the measured contact angles correlated with surface texture (roughness) as determined by AFM. These measurements provide additional proof on the effect of nanometer-size surface asperities on the wetting characteristics of hydrophobic coatings.

Metallic zinc exhibits optimal biocompatibility for bioabsorbable endovascular stents

Although corrosion resistant bare metal stents are considered generally effective, their permanent presence in a diseased artery is an increasingly recognized limitation due to the potential for long-term complications. We previously reported that metallic zinc exhibited an ideal biocorrosion rate within murine aortas, thus raising the possibility of zinc as a candidate base material for endovascular stenting applications. This study was undertaken to further assess the arterial biocompatibility of metallic zinc. Metallic zinc wires were punctured and advanced into the rat abdominal aorta lumen for up to 6.5months. This study demonstrated that metallic zinc did not provoke responses that often contribute to restenosis. Low cell densities and neointimal tissue thickness, along with tissue regeneration within the corroding implant, point to optimal biocompatibility of corroding zinc. Furthermore, the lack of progression in neointimal tissue thickness over 6.5months or the presence of smooth muscle cells near the zinc implant suggest that the products of zinc corrosion may suppress the activities of inflammatory and smooth muscle cells.

The Effect of Drop (Bubble) Size on Contact Angle at Solid Surfaces

Abstract Examples of experimental contact angle data for varying drop and bubble volumes on different solids whose surfaces are smooth and homogeneous, rough and homogeneous, smooth and heterogeneous, and covered with unstable organic films are presented. The ideas and theoretical models as proposed in the literature for the interpretation of contact angle/drop (bubble) size relationships are critically reviewed. It is shown that major factors affecting the contact angle variation with drop (bubble) size such as surface heterogeneity, roughness, and stability, have been identified in the literature. However, there is still a need for experimental work with well-defined and well-characterized solid surfaces. Theoretical models that have been proposed in the literature are still inadequate. Advanced modeling of liquid behavior at heterogeneous and rough surfaces is required to understand further, and to predict, the contact angle/drop (bubble) size relationships at imperfect surfaces.