AUG.2005 - Present, Postdoctoral Fellow, Hospital for Special Surgery (Soft Tissue Laboratory), New York
- Characterization of the initiation and progression of cartilage Extracellular Matrix (ECM) damage caused by Excessive Mechanical Loads (EML): It is hypothesized that mechanical load attenuates enzyme kinetics through matrix deformation, inhibits or enhances enzymatic ECM degradation, and the EML will initiate the degradation via MMP-1, 3, &13. To test this hypotheses, the damage (molecular site) to aggrecan (by aggrecanases-1, 2, and MMP-3) and collagen (MMPs-1,3,13) in the ECM will be determined and correlated with matrix content and mechanical properties. EML degradation will be compared to inflammation-induced injury by IL-1 in which aggrecanase-1 is the primary enzyme in the early degradation process.
- Characterization of the effect of varying mechanical loads to modify the susceptibility of the ECM to enzyme degradation by EML: It is hypothesized that a normal load (physiological) will inhibit catalysis, as compared to no load and an overload (EML) which will enhance the degradation of the ECM by MMPs and aggrecanases. To test this hypothesis, how mechanical load influences the pathway(s) for enzyme degradation of the ECM, whether it is mediated by cellular function or only depends on matrix deformation, and whether changing the mechanical load influences the degradation process once it is initiated will be determined.
SEP.2000 - JUL. 2005, PhD at Columbia University (Musculoskeletal Biomechanics Laboratory, Mechanical Engineering), New York
- Interstitial fluid load support of human and bovine articular cartilage: It was verified from experiments and FEM analysis that interstitial fluid pressurization supports most of the load transmitted across articular layers. This result is consistent with the theoretical prediction that peak fluid load support is dependent on the ratio of the tensile and compressive moduli of the tissue, without which the interstitial fluid could not contribute significantly to load support and dynamic stiffening.
- Dynamic unconfined compression loading of bovine articular cartilage: Dynamic compressive moduli, phase angles, and strain magnitudes were characterized at physiological compressive stress level of ~6 MPa and loading frequencies of 0.1-40 Hz with the testing device I designed and built. The frequency dependence of the dynamic modulus and phase angle over a broad range of loading frequencies confirms the viscoelastic nature of articular cartilage in compression, and the observation of significant nonlinearity in the stress-strain response under physiological compressive stresses provides why cartilage is able to sustain the physiological stresses without crushing the articular layer.
- Dynamic unconfined compression loading with collagenase digestion: This study investigated that enzymatic digestion by collagenase significantly reduced dynamic moduli and increased compressive strains at physiological loading conditions. Moreover, the result that the dynamic modulus is less sensitive to collagenase digestion than the equilibrium Young's modulus suggests that functional dynamic response is more resilient to collagen degradation than equilibrium response. - In situ mechanical response of intact bovine shoulder joint: This study provided novel experimental findings on the physiological strain magnitude (~27%) and dynamic modulus (~23 MPa) achieved in intact articular layers under cyclical loading conditions (~6 MPa stress and 1 Hz frequency).
- Microscale mechanical and frictional response from Atomic Force Microscopy (AFM): Micro- and macroscale frictional coefficients of bovine articular cartilage were compared, and surface roughness and compressive modulus were investigated using Hertz contact analysis at the microscale with AFM. The primary finding of this study is that, unlike the macroscale, the microscale AFM friction coefficient remains essentially constant over time and statistically comparable to macroscale equilibrium friction coefficient. This result suggests that AFM friction measurements represent the frictional response in the absence of cartilage interstitial fluid pressurization.
- Flow-dependent and flow-independent (intrinsic) viscoelasticity in compression and tension: A constitutive model for the intrinsic viscoelasticity of cartilage in tension was formulated and experimentally validated by fitting to the tensile stress-relaxation response and successfully predicting the experimental cyclical response. The energy dissipation in tension was found to be significantly smaller than in compression and a very strong linear correlation was observed between the dynamic tensile and dynamic compressive moduli at various frequencies. The results of this study emphasize that fluid-flow dependent viscoelasticity dominates the compressive response of cartilage, whereas intrinsic solid matrix viscoelasticity dominates the tensile response. Yet the dynamic compressive modulus of cartilage is critically dependent upon elevated values of the dynamic tensile modulus.
AUG.1996 - SEP.1998, Research Engineer, Daewoo Motor Company (Advanced Research Department), Korea (South)
- Manual Transmission (MT): Design of MT components using CATIA
- Electronically Automatic Clutch (EAC): EAC ECU calibration through performance tests (driveability, steerability, and stability) of EAC mounted vehicle to achieve proper clutch engagement, and development of test device to investigate the durability of EAC components
- Continuously Variable Transmission (CVT): System modeling of CVT system and pattern contouring for gear shifting control of CVT mounted vehicle in order to make controls aimed at performance improvement for the entire powertrain system possible KAIST (Telerobotics and Control Laboratory) Korea (South) Research Assistant SEP. 1994 - AUG. 1996
SEP.1994 - AUG.1996, MS at KAIST (Telerobotics and Control Laboratory, Automation And Design Engineering), Korea
- Vehicle Traction Control: Vehicle wheel spinning control on slippery road conditions by suppressing the spinning of driven wheels, which occurs easily on slippery roads during excessive acceleration, through the throttle valve motor control using fuzzy logic algorithm
- Elevator Control: Elevator speed control (acceleration and deceleration) using PID algorithm
MAR.1990 - FEB.1994, BS at KAIST (Mechanical Engineering), Korea
