Joon Heon Kim

Selective Surface Modification in Silicon Microfluidic Channels for Micromanipulation of Biological Macromolecules

Interactions between biological macromolecules and micrometer- and sub-micrometer-scale surface structures are directly influenced by the surface wettability, chemical reactivity and surface charge. Understanding these interactions is crucial for developing integrated microsystems for biological and biomedical processing and analysis. We report development of selective surface modification techniques based on microcontact printing and polyelectrolyte adsorption. These techniques were applied to lithographically patterned silicon microfluidic channels and flat silicon substrates to create surface microstructures with contrasting wetting properties and surface charges. These controls enabled us to devise various techniques for controlled loading and processing of biomaterials in the channels. Solutions containing long chain biological macromolecules DNA and microtubules were directly loaded into the microchannels by using a micromanipulator/microinjector system. Structural arrangements of these linear macromolecules, which were probed by using fluorescence and laser scanning confocal microscopy, were found to be quite different from bulk solutions. As expected, the filamentous molecules were observed to align linearly along the channels, with the degree of alignment dependent on channel width as well as the length of the molecule. This molecular alignment, which is induced by both the surface confinement effect and capillary flow during sample loading, may be used to enhance processing of biological materials in silicon biomedical microdevices. It also opens up the possibility of carrying out direct combinatorial structural characterization of proteins in the microchannels utilizing X-ray diffraction, which so far has not been possible.

In-situ observation of the inside-to-outside molecular transport of a liposome.

The first in-situ and real-time observation of the molecular transport from inside to outside of a liposome was shown by using the second harmonic generation (SHG) technique. The transport of an organic cationic molecule across the liposome bilayer could be switched on and off using the structural change of the lipid bilayer caused by temperature change. This approach can be helpful for the understanding and control of the molecular transport in the liposome vehicle.

Biological polyelectrolyte complexes in solution and confined on patterned surfaces

In the first part of this paper we describe recently discovered structures in self-assemblies of charged membranes complexed with the biological polyelectrolyte DNA. These complexes are currently used in medical applications of non-viral gene delivery. The second part of the paper will describe entirely new experiments where liquid crystal and biological polymers are confined on lithographically prepared patterned surfaces to produce oriented self-assemblies for structural studies and confinement induced new phases. While the focus of the paper will be on DNA, other important biological polyelectrolytes include filamentous actin (F-actin), intermediate filaments, and microtubules with varying physical and chemical molecular properties, are also discussed.

Low internal pressure in femtoliter water capillary bridges reduces evaporation rates

Capillary bridges are usually formed by a small liquid volume in a confined space between two solid surfaces. They can have a lower internal pressure than the surrounding pressure for volumes of the order of femtoliters. Femtoliter capillary bridges with relatively rapid evaporation rates are difficult to explore experimentally. To understand in detail the evaporation of femtoliter capillary bridges, we present a feasible experimental method to directly visualize how water bridges evaporate between a microsphere and a flat substrate in still air using transmission X-ray microscopy. Precise measurements of evaporation rates for water bridges show that lower water pressure than surrounding pressure can significantly decrease evaporation through the suppression of vapor diffusion. This finding provides insight into the evaporation of ultrasmall capillary bridges.

Lipid and lipid-polymer mixtures at an interface

The surface pressure (Π) and surface area/molecule (A) isotherms of a mixture of DMPC (DL-α-phosphatidylcholine, Dimyristoyl) and PEG-DMPE (1,2-Diacyl-sn-Glycero-3-Phosphoethanolamine-N-[Poly(ethylene glycol)5000]) system were measured at various compositions by the Langmuir surface balance technique at an air/water interface. In the range where the surface pressure is less than about 8 dynes/cm, a PEG polymer chain of PEG-DMPE molecules remains on the surface and the isotherm can be explained by the 2-D power law behavior of chains in a good solvent. In the range above 8 dynes/cm, a part of the PEG polymer segment is dissolved into the water phase, and the surface pressure can be explained as the sum of the 2-D component and 3-D component. Furthermore, the mixing energy is negative, which indicates an attractive interaction between DMPC and PEG-DMPE.