Biomedical subjects

Hyunjung Lee

Publications and source records attributed to Hyunjung Lee.

5 recordsLinked to original sources

PubMed2005

Biomechanical analysis of silicon microelectrode-induced strain in the brain.

The ability to successfully interface the brain to external electrical systems is important both for fundamental understanding of our nervous system and for the development of neuroprosthetics. Silicon microelectrode arrays offer great promise in realizing this potential. However, when they are implanted into the brain, recording sensitivity is lost due to inflammation and astroglial scarring around the electrode. The inflammation and astroglial scar are thought to result from acute injury during electrode insertion as well as chronic injury caused by micromotion around the implanted electrode. To evaluate the validity of this assumption, the finite element method (FEM) was employed to analyze the strain fields around a single Michigan Si microelectrode due to simulated micromotion. Micromotion was mimicked by applying a force to the electrode, fixing the boundaries of the brain region and applying appropriate symmetry conditions to nodes lying on symmetry planes. Characteristics of the deformation fields around the electrode including maximum electrode displacement, strain fields and relative displacement between the electrode and the adjacent tissue were examined for varying degrees of physical coupling between the brain and the electrode. Our analysis demonstrates that when physical coupling between the electrode and the brain increases, the micromotion-induced strain of tissue around the electrode decreases as does the relative slip between the electrode and the brain. These results support the use of neuro-integrative coatings on electrode arrays as a means to reduce the micromotion-induced injury response.

Biomechanical Phenomena↗

PubMed2005

Chemical imaging in a surface forces apparatus: confocal raman spectroscopy of confined poly(dimethylsiloxane).

Confocal Raman spectroscopy has been implemented within the molecularly thin films of a surface forces apparatus. Applying this technique to an initial system, we investigate the confinement and shear-induced changes in the Raman spectra of poly(dimethylsiloxane) (PDMS) liquids confined between atomically smooth mica surfaces at thicknesses less than the unperturbed radius of gyration of the polymer. We focus on stretch vibrations of the PDMS methyl group, whose net orientation is perpendicular to the chain backbone. When PDMS was confined to a thickness of approximately the unperturbed radius of gyration (3.5 nm) but no shear, the Raman intensity of the methyl group was anisotropic in the x-y plane, signifying that chains oriented preferentially parallel to the confining surfaces. Relative to the bulk fluid, the relative intensity of the asymmetric to symmetric carbon-hydrogen stretch (2965 and 2907 cm-1, respectively) was enhanced, indicating that asymmetric vibration was enhanced by confinement. Measurements using polarized radiation showed coherent planar anisotropy in the x-y plane whose direction varied stochastically from experiment to experiment. It seems that although coherent in-plane alignment was favored, no preferential alignment direction was favored in the absence of shear. Application of shear caused the time-averaged polymer conformations to become more nearly isotropic in the plane of shear. These measurements are considered to represent the first chemical imaging of chemical species within the contact area of a surface forces apparatus.

Journal Article↗

PubMed2003

Zeolite-mediated photochemical charge separation using a surface-entrapped ruthenium-polypyridyl complex.

Employing the strategy of quaternization of the 2,2' N atoms of the conjugated bipyridine ligand 1,4-bis[2-(4'-methyl-2,2'-bipyrid-4-yl)ethenyl]benzene (L), a polypyridyl complex of ruthenium(II) was tethered on the surface of zeolite Y. Electrochemical and spectroscopic properties of the complex suggest that, upon visible photoexcitation of the MLCT band, the electron is localized on the conjugated ligand rather than the bipyridines. Electron transfer from the surface complex to bipyridinium ions (methyl viologen) within the zeolite was observed. Visible light photolysis of the ruthenium-zeolite solid ion-exchanged with diquat and suspended in a propyl viologen sulfonate solution led to permanent formation of the blue propyl viologen sulfonate radical ion in solution. The model that is proposed involves intrazeolitic charge transfer to ion-exchanged diquat followed by interfacial (zeolite to solution) electron transfer to propyl viologen sulfonate in solution. Because of the slow intramolecular back-electron-transfer reaction and the forward electron propagation via the ion-exchanged diquat, Ru(III) is formed. This Ru(III) complex formed on the zeolite is proposed to react rapidly with water in the presence of light, followed by reaction with the propyl viologen sulfonate, to form pyridones and regeneration of Ru(II), which then continues the photochemical process.

Journal Article↗

PubMed2003

Slippery questions about complex fluids flowing past solids.

Viscous flow is familiar and useful, yet the underlying physics is surprisingly subtle and complex. Recent experiments and simulations show that the textbook assumption of 'no slip at the boundary' can fail greatly when walls are sufficiently smooth. The reasons for this seem to involve materials chemistry interactions that can be controlled--especially wettability and the presence of trace impurities, even of dissolved gases. To discover what boundary condition is appropriate for solving continuum equations requires investigation of microscopic particulars. Here, we draw attention to unresolved topics of investigation and to the potential to capitalize on 'slip at the wall' for purposes of materials engineering.

Friction↗

PubMed2002

Poly(D,L-lactic acid-co-glycolic acid)-b-poly(ethylene glycol)-b-poly (D,L-lactic acid-co-glycolic acid) triblock copolymer and thermoreversible phase transition in water.

Novel thermoreversible gelation behavior of aqueous solutions of ABA-type triblock copolymers composed of the central polyethylene oxide (PEG) block and two poly(D,L-lactic acid-co-glycolic acid) side blocks was found. Phase transition characteristics, such as critical gel concentration (CGC) and lower and upper critical gel temperature (CGT), are closely related to the molecular structure of the triblock copolymers. The CGC and the lower CGT both increases with increasing PEG/PLGA molecular weight ratio. Increasing the GA content in PLGA block induces a somewhat higher CGC. The copolymer forms micelles with a PLGA loop core and a PEG shell in water. Also grouped micelles are identified seemingly due to the bridging of two micelles sharing two PLGA blocks of a block copolymer chain. As the temperature increases the association of micelles increases, which results in gelation. The ABA-type copolymers exhibit a relatively low CGC (<10%) and low sol-gel transition temperatures compared to BAB-type copolymers. As the temperature increases further gel-sol transition is observed, which would result from the shrinkage of micelles with temperature increase. The hydrodynamic size of the micelles is monitored by dynamic laser scattering, and a possible gelation mechanism was suggested.

Gels↗