Kenneth Kar Ho Wong

Frequency-dependent viscoelasticity measurement by atomic force microscopy

We demonstrate a new technique for investigating viscoelastic properties of soft materials using the atomic force microscope. A small oscillatory voltage is added to the deflection signal of the atomic force microscope causing a vertical oscillatory sample motion. Monitoring the amplitude and phase of this motion allows determination of the viscous and elastic moduli of the sample as a function of frequency during contact imaging. This technique is applied to suspended poly(vinyl alcohol) nanofibers and poly(vinyl alcohol) hydrogels, giving results similar to those measured using traditional static methods. However, the moduli of both the fibers and the hydrogels show a significant frequency dependence. The Youngs modulus of the fibers increases with frequency, while for the viscoelastic hydrogels, the storage modulus dominates the mechanical response at low frequency whereas the loss modulus dominates at high frequency.

Genipin-Cross-linked Electrospun Collagen Fibers

The fabrication of a fibrous collagen scaffold using electrospinning is desirable for tissue-engineering applications. Previously, electrospun collagen fibers were shown to be unstable in aqueous environments and, therefore, cross-linking is essential to stabilize these fibers. In this study genipin, a significantly less cytotoxic cross-linking agent compared to glutaraldehyde, was used to cross-link electrospun collagen fibers. The significance of this research lies in the use of four alcohol/water solvent systems to carry out the crosslinking reaction to maintain fibrous morphology during cross-linking. The four cross-linking conditions established were: (1) ethanol, 5% water and 3 days, (2) ethanol, 3% water and 5 days, (3) ethanol, 5% water and 5 days, and (4) isopropanol, 5% water and 5 days at a genipin concentration of 0.03 M. Results illustrated that genipin-cross-linking was effective in maintaining collagen fiber integrity in aqueous and cell culture media environments for up to 7 days. In addition, it was shown that fiber swelling could be controlled by using different cross-linking conditions. Swelling of cross-linked fibers immersed in Dulbeccos modified eagle medium for 7 days ranged from 0 to 59 ± 4%. The cross-linked fibers were analyzed using scanning electron microscopy, Fourier transform infrared spectroscopy and ninhydrin assay. Finally, studies using primary human fibroblasts indicated good cell adhesion to these scaffolds. Overall, our data suggest that these stabilized fibrous collagen scaffolds provide a promising environment for tissue-regeneration applications.

Waveguide evanescent field fluorescence microscopy: waveguide mode scattering by non-uniform grating and defects in the wave guiding film

Waveguide evanescent field fluorescence microscopy is an evanescent field based microscopy to visualize cell-substrate contact regions and solid thin films. Despite some advantages of this method compared to other evanescent field based microscopes, non-uniform illumination source and background are reported to be the problems for producing nonuniform images. We noticed similar non-uniformities in our primary results with ultra- thin solid film and cell-substrate contact regions. We used various microscopy methods to demonstrate induced inhomogenities in the waveguide evanescent field fluorescence microscopy images by secondary patterns in the gratings, and defects in waveguides surface. We discuss their effects on the waveguide microscopy background, ultra-thin solid films, and cell-substrate contact region images. Defect-free waveguides with uniform gratings fabricated and used to image phase separated lipids monolayer LB films and cell-substrate contact regions.