Chang Quan Lai

Uni-, bi-, and tri-directional wetting caused by nanostructures with anisotropic surface energies.

Wetting is a pervasive phenomenon that governs many natural and artificial processes. Asymmetric wetting along a single axis, in particular, has generated considerable interest but has thus far been achieved only by the creation of structural anisotropy. In this paper, we report that such directional wetting can also be achieved by anisotropically coating nanostructure surfaces with materials that modify the nanostructure surface energy, a phenomenon that has not been observed in natural or artificial systems thus far. Moreover, by combining this newfound chemical influence on wetting with topographic features, we are able to restrict wetting in one, two and three directions. A model that explains these findings in terms of anisotropy of the pinning forces at the triple phase contact line is presented. Through the resulting insights, a flexible method for precise control of wetting is created.

Creation of nanostructures by interference lithography for modulation of cell behavior

Emerging evidence of the striking differences that can be induced in the behavior of biological cells through topographical modulation of physically and chemically patterned nanostructured surfaces provides a great impetus for developing novel cellular-scale and sub-cellular-scale nanopatterned substrates and for employing them for exciting new applications in life and medical sciences and biotechnology. However, the lack of availability of cost-effective, large-surface-area nanofabricated substrates of appropriate dimensions and features has proved to be a major impediment for research in this area. Here, we demonstrate a simple and cost-effective method based on interference lithography to produce spatially precise and wide-surface-coverage silicon- and polymer-based nanostructures to study how cells react to nanoscale structures or surfaces.

Fluid mechanics of blood flow in human fetal left ventricles based on patient-specific 4D ultrasound scans

The mechanics of intracardiac blood flow and the epigenetic influence it exerts over the heart function have been the subjects of intense research lately. Fetal intracardiac flows are especially useful for gaining insights into the development of congenital heart diseases, but have not received due attention thus far, most likely because of technical difficulties in collecting sufficient intracardiac flow data in a safe manner. Here, we circumvent such obstacles by employing 4D STIC ultrasound scans to quantify the fetal heart motion in three normal 20-week fetuses, subsequently performing 3D computational fluid dynamics simulations on the left ventricles based on these patient-specific heart movements. Analysis of the simulation results shows that there are significant differences between fetal and adult ventricular blood flows which arise because of dissimilar heart morphology, E/A ratio, diastolic–systolic duration ratio, and heart rate. The formations of ventricular vortex rings were observed for both E- and A-wave in the flow simulations. These vortices had sufficient momentum to last until the end of diastole and were responsible for generating significant wall shear stresses on the myocardial endothelium, as well as helicity in systolic outflow. Based on findings from previous studies, we hypothesized that these vortex-induced flow properties play an important role in sustaining the efficiency of diastolic filling, systolic pumping, and cardiovascular flow in normal fetal hearts.

Fluid mechanics of human fetal right ventricles from image-based computational fluid dynamics using 4D clinical ultrasound scans

There are 0.6-1.9% of US children who were born with congenital heart malformations. Clinical and animal studies suggest that abnormal blood flow forces might play a role in causing these malformation, highlighting the importance of understanding the fetal cardiovascular fluid mechanics. We performed computational fluid dynamics simulations of the right ventricles, based on four-dimensional ultrasound scans of three 20-wk-old normal human fetuses, to characterize their flow and energy dynamics. Peak intraventricular pressure gradients were found to be 0.2-0.9 mmHg during systole, and 0.1-0.2 mmHg during diastole. Diastolic wall shear stresses were found to be around 1 Pa, which could elevate to 2-4 Pa during systole in the outflow tract. Fetal right ventricles have complex flow patterns featuring two interacting diastolic vortex rings, formed during diastolic E wave and A wave. These rings persisted through the end of systole and elevated wall shear stresses in their proximity. They were observed to conserve ∼25.0% of peak diastolic kinetic energy to be carried over into the subsequent systole. However, this carried-over kinetic energy did not significantly alter the work done by the heart for ejection. Thus, while diastolic vortexes played a significant role in determining spatial patterns and magnitudes of diastolic wall shear stresses, they did not have significant influence on systolic ejection. Our results can serve as a baseline for future comparison with diseased hearts.

Influence of nanoscale geometry on the dynamics of wicking into a rough surface

The dynamics of imbibition into the roughness of a surface was investigated with hexagonal arrays of anisotropic nanofins fabricated with interference lithography and metal assisted chemical etching. It was found that viscous drag caused by the nanofins is similar to that caused by open nano-channels of equal length and height containing the same volume of liquid. In addition, the energy dissipated by form drag for a given driving pressure was determined to be directly proportional to the volume of fluid between nanofin planes that are flat and normal to the imbibition direction.

A near-superhydrophobic surface reduces hemolysis of blood flow in tubes

The use of an external mechanical pump to sustain the circulation in a body, also known as extracorporeal circulation, is an integral part of many medical procedures such as hemodialysis and cardio-pulmonary bypass. However, the damage to red blood cells caused by the flow-induced shear stresses in the flow circuit has remained an intractable problem for many years, limiting the operational duration of extracorporeal circulation. In this study, near-superhydrophobic surfaces were investigated as a potential solution to mitigate the hemolysis of blood during extracorporeal pumping through the use of a proof-of-concept flow circuit. It was found that the thin layer of air trapped by the near-superhydrophobic surface due to the Cassie–Baxter state reduced the wall shear stress exerted on the blood flow, resulting in a corresponding decrease in the rate of hemolysis. For blood that undergoes an oscillatory flow, this reduction in the hemolysis was shown to be directly related to the mean shear rate and shear rate amplitude of the flow.

Versatile fabrication and applications of dense, orderly arrays of polymeric nanostructures over large areas

Dense arrays of nanostructures were fabricated in polymer surfaces over large areas (1 cm × 1 cm) using laser interference lithography and low power CF4/O2 plasma etching. The dependence of the etch rate and etch anisotropy on plasma composition was studied in detail for polystyrene and 4 distinct regimes were identified. In each of these regimes, the polystyrene nanostructures exhibit characteristic variations of etch rate, etch anisotropy and surface chemistry that were found to be closely related to the level of fluorination and polymerization on the substrate surface. A new technique, stitch etching, was developed and utilized in conjunction with low power plasma etching to increase the height of nanostructures without loss of array density. These nanofabrication techniques are shown to be versatile enough to be applied to a variety of polymers. The polymeric nanostructures were found to exhibit a number of useful properties including superhydrophobicity (directional effect, lotus leaf effect and rose petal effect), structural stiffness and biocompatibility, which were shown to be useful in applications such as self-cleaning surfaces, nanoimprinting molds and biocompatible substrates for neurite guidance.

Nonlinear Flow Rate Response to Pumping Frequency and Reduced Hemolysis in the Drastically Under-Occluded Pulsatile Roller Pump

Roller pumps are widely used in many medical procedures including cardiopulmonary bypass, left/right ventricular assist, and hemodialysis. However, to date, the problem of the roller pumping mechanism causing significant hemolysis remains unresolved. It has been shown that with under-occlusion of the roller pump, hemolysis can be reduced, but significant reduction of the mean flow rate also takes place due to backflow through the under-occlusion. We performed an investigation of the flow dynamics of an under-occluded roller pump which featured significantly higher amount of under-occlusion than previously investigated. Our results showed that the mean flow rate produced by the pump has a strong, nonlinear dependence on pumping frequency. Mean flow rate generally increases with the pumping frequency and the degree of maximum occlusion except at certain frequencies where sharp reductions were observed. These frequencies coincide with the fundamental frequency of the system and its harmonics, bearing resemblance to the impedance pump, suggesting that the drastically under-occluded roller pump is a unique device that employs the pumping mechanisms of both roller pumping and impedance pumping. At the appropriate frequencies, this under-occluded roller pump could sustain sufficiently high flow rates for clinical uses. Blood damage potential of the under-occluded roller pump was compared to a fully occluded roller pump via the assay of free-plasma hemoglobin, and it was found that the under-occlusion reduced hemolysis by about half for any given flow rate. The drastically under-occluded roller pumping reported in this study, therefore, has the potential of being translated into an improved clinical blood pump.

Effects of structural and chemical anisotropy of nanostructures on droplet spreading on a two dimensional wicking surface

The dynamics of droplet spreading on two-dimensional wicking surfaces were studied using square arrays of Si nanopillars. It was observed that the wicking film always precedes the droplet edge during the spreading process causing the droplet to effectively spread on a Cassie-Baxter surface composed of solid and liquid phases. Unlike the continual spreading of the wicking film, however, the droplet will eventually reach a shape where further spreading becomes energetically unfavorable. In addition, we found that the displacement-time relationship for droplet spreading follows a power law that is different from that of the wicking film. A quantitative model was put forth to derive this displacement-time relationship and predict the contact angle at which the droplet will stop spreading. The predictions of our model were validated with experimental data and results published in the literature.

Bacterial Attachment, Aggregation, and Alignment on Subcellular Nanogratings

Recent investigations on the interactions of bacteria with micro/nanostructures have revealed a wide range of prokaryotic responses that were previously unknown. Despite these advances, however, it remains unclear how collective bacterial behavior on a surface would be influenced by the presence of anisotropic nanostructures with subcellular dimensions. To clarify this, the attachment, aggregation, and alignment of Pseudomonas aeruginosa on orderly subcellular nanogratings with systematically varied geometries were investigated. Compared with a flat surface, attachment and aggregation of bacteria on the nanogratings were reduced by up to 83 and 84% respectively, whereas alignment increased by a maximum of 850%. Using a semiempirical quantitative model, these results were shown to be caused by a lowering of physicochemical attraction between the substrate and bacteria, possible disruption to cell communication, and physical isolation of bacteria that were entrenched in the nanogratings by capillary action. Furthermore, the bacterial attachment level was generally found to be exponentially related to the contact area between the substrate and bacterial cells, except when there were significant deficits in the available contact area, which prompted the bacterial cells to employ their appendages to maintain a minimum attachment rate. Because the contact area for adhesion is strongly dependent on the geometry of the surface features and orientation of the bacterial cells, these results indicate that the conventional practice of using roughness parameters to draw quantitative relationships between surface topographies and bacterial attachment could suffer from inaccuracies due to the lack of shape and orientation information provided by these parameters. On the basis of these insights, design principles for generating maximal and minimal bacterial attachment on a surface were also proposed and verified with results reported in the literature.