Chi-Mon Chen

Directed Water Shedding on High‐Aspect‐Ratio Shape Memory Polymer Micropillar Arrays

A reconfigurable, droplet-directing surface is developed based on high-aspect-ratio shape-memory polymer (SMP) pillars. The water droplet on the original or recovered SMP pillars can slide off the surface at a finite angle of inclination while being fully pinned on the deformed pillar array. This wettability contrast allows directed water shredding from the straight pillars to the deformed ones.

Curvature Sorting of Peripheral Proteins on Solid-Supported Wavy Membranes

Cellular membrane deformation and the associated redistribution of membrane-bound proteins are important aspects of membrane function. Current model membrane approaches for studying curvature sensing are limited to positive curvatures and often require complex and delicate experimental setups. To overcome these challenges, we fabricated a wavy substrate by imposing a range of curvatures onto an adhering lipid bilayer membrane. We examined the curvature sorting of several peripheral proteins binding to the wavy membrane and observed them to partition into distinct regions of curvature. Furthermore, single-molecule imaging experiments suggested that the curvature sensing of proteins on low-curvature substrates requires cooperative interactions.

Enhanced cell adhesion and alignment on micro-wavy patterned surfaces.

Various micropatterns have been fabricated and used to regulate cell adhesion, morphology and function. Micropatterns created by standard photolithography process are usually rectangular channels with sharp corners (microgrooves) which provide limited control over cells and are not favorable for cell-cell interaction and communication. This paper proposes a new micropattern with smooth wavy surfaces (micro-waves) to control the position and orientation of cells. To characterize cell growth and responses on the micro-patterned substrates, bovine aortic endothelial cells were seeded onto surfaces with micro-grooves and micro-waves for 24 h. As a result, the cells on the micro-wavy pattern appeared to have a lower death rate and better alignment compared to those on the micro-grooved pattern. In addition, flow-induced shear stress was applied to examine the adhesion strength of cells on the micro-wavy pattern. Results showed that cells adhered to the wavy surface displayed both improved alignment and adhesion strength compared to those on the flat surface. The combination of increased alignment, lower death rate and enhanced adhesion strength of cells on the micro-wavy patterns will offer advantages in potential applications for cell phenotype, proliferation and tissue engineering.

Guided wrinkling in swollen, pre-patterned photoresist thin films with a crosslinking gradient

A new thin film buckling system was developed from photosensitive SU-8 thin films doped with UV light absorbing dye, resulting in a depth-wise gradient modulus after photocrosslinking. When the film was swollen by an organic solvent, a wide range of wrinkling patterns was obtained, including lamella, peanut and semi-hexagonal patterns. Both the morphology and wavelength were found to be dependent on the original film thickness. By leveraging the thermoplasticity of SU-8, we imprinted one-dimensional (1-D) patterns on the dyed SU-8 with variable pitch and height from 1 μm to 20 μm and 15 nm to 2 μm, respectively. We then swelled the patterned films and investigated the interactions between the intrinsic buckling waves (both size and morphology) and the pre-patterns. As the pre-pattern pitch decreased, the swollen film in the patterned region evolved from isotropic wrinkles to out-of-phase, anisotropic waves, which further became in-phase when the pre-pattern pitch was smaller than the intrinsic wrinkle wavelength. For the latter, the aligned wrinkle morphology varied dramatically when the pre-pattern height decreased: from perpendicular to the pre-pattern wavevector to dual orientation with one set of wrinkles remained perpendicularly ordered and the other set of local buckling patterns aligned in parallel to the pre-pattern, and finally back to isotropic ones. Since the pre-patterns of different size and shape could be readily prepared, the combination of physical confinement together with controlled swelling in a graded thin film offers a new approach to access a wide range of controllable hierarchical patterns.

Capillarity induced instability in responsive hydrogel membranes with periodic hole array

We report capillary force induced instability from drying water swollen poly(2-hydroxyethyl methacrylate) (PHEMA) based hydrogel membranes with micron-sized holes in a square array. When the PHEMA membrane was exposed to deionized-water, the size of the holes became smaller but retained the shape, so-called breathing mode instability. However, during the drying process, the square pore array buckled into a diamond plate pattern. The deformed pattern could be recovered upon re-exposure to water. The instability mechanism was confirmed by comparing the observations from optical and scanning electron microscopy (SEM) images with theoretical prediction. When thermoresponsive poly(N-isopropylacrylamide) was introduced to the PHEMA gel, the poly(2-hydroxyethyl methacrylate-co-N-isopropylacrylamide) (PHEMA-co-PNIPAAm) membrane underwent pattern transformation only if dried below the lower critical solution temperature of PNIPAAm. Along the pattern transformation, we observed a dramatic change of the optical property of the film, from colourful reflection to transparent window.

Programming Tilting Angles in Shape Memory Polymer Janus Pillar Arrays with Unidirectional Wetting against the Tilting Direction

By coating a thin layer of metal, including gold and gold-palladium alloy, of different thickness on the deformed shape memory polymer (SMP) pillars, we manipulate the degree of recovery of the SMP pillars. Pillars of different tilting angles were obtained as a result of balancing the strain recovery energy of the SMP pillars that favor the original straight state and the elastic energy of the metal layers that prefer the bent state. With this selective coating of a metal layer on the tilted pillars, we report a unique anisotropic liquid spreading behavior, where the water droplet is fully pinned in the direction of pillar tilting but advances in the reverse direction. This phenomenon is explained by the interplay of the surface chemistry and topography.

Micro-patterned surface for efficient capturing of circulating tumor cells

This work aims to design, fabricate, and characterize a micro-patterned surface that will be integrated into microfluidic devices to enhance particle and rare cell capture efficiency. Capture of ultralow concentration of circulating tumor cells in a blood sample is of vital importance for early diagnostics of cancer diseases. Despite the significant progress achieved in development of cell capture techniques, the enhancement in capture efficiency is still limited and often accompanied with drawbacks such as low throughput, low selectivity, pre-diluting requirement, and cell viability issues. The goal of this work is to design a biomimetic surface that could significantly enhance particle/cell capture efficacy through computational modeling, surface patterning, and microfluidic integration and testing. A PDMS surface with microscale ripples is functionalized with epithelial cell adhesion molecule (EpCAM) antibody to capture prostate cancer PC3 cells. Our microfluidic chip with micropatterns has shown significantly higher cell capture efficiency and selectivity compared to the chips with plane surface or classical herringbone-grooves.