Jee-Wook Lee

Long-term clinical study and multiscale analysis of in vivo biodegradation mechanism of Mg alloy

Significance In the past decade, countless studies have been performed to control the mechanical and corrosion property of magnesium-based alloy, which degrades in the physiological environment, to overcome the flaws of the inert implant materials and shift the paradigm of conventional bone fixation devices. Controlled degradation of Mg-5wt%Ca-1wt%Zn alloy results in the formation of biomimicking calcification matrix at the degrading interface to initiate the bone formation process. This process facilitates early bone healing and allows the complete replacement of biodegradable Mg implant by the new bone within 1 y of implantation, as demonstrated in 53 cases of successful long-term clinical study. There has been a tremendous amount of research in the past decade to optimize the mechanical properties and degradation behavior of the biodegradable Mg alloy for orthopedic implant. Despite the feasibility of degrading implant, the lack of fundamental understanding about biocompatibility and underlying bone formation mechanism is currently limiting the use in clinical applications. Herein, we report the result of long-term clinical study and systematic investigation of bone formation mechanism of the biodegradable Mg-5wt%Ca-1wt%Zn alloy implant through simultaneous observation of changes in element composition and crystallinity within degrading interface at hierarchical levels. Controlled degradation of Mg-5wt%Ca-1wt%Zn alloy results in the formation of biomimicking calcification matrix at the degrading interface to initiate the bone formation process. This process facilitates early bone healing and allows the complete replacement of biodegradable Mg implant by the new bone within 1 y of implantation, as demonstrated in 53 cases of successful long-term clinical study.

Direct and accurate measurement of size dependent wetting behaviors for sessile water droplets

The size-dependent wettability of sessile water droplets is an important matter in wetting science. Although extensive studies have explored this problem, it has been difficult to obtain empirical data for microscale sessile droplets at a wide range of diameters because of the flaws resulting from evaporation and insufficient imaging resolution. Herein, we present the size-dependent quantitative change of wettability by directly visualizing the three phase interfaces of droplets using a cryogenic-focused ion beam milling and SEM-imaging technique. With the fundamental understanding of the formation pathway, evaporation, freezing, and contact angle hysteresis for sessile droplets, microdroplets with diameters spanning more than three orders of magnitude on various metal substrates were examined. Wetting nature can gradually change from hydrophobic at the hundreds-of-microns scale to super-hydrophobic at the sub-μm scale, and a nonlinear relationship between the cosine of the contact angle and contact line curvature in microscale water droplets was demonstrated. We also showed that the wettability could be further tuned in a size-dependent manner by introducing regular heterogeneities to the substrate.

Detecting changes in arthritic fibroblast‐like synoviocytes using atomic force microscopy

The morphological and quantitative differences between arthritic fibroblast‐like synoviocytes (FLS) and normal FLS were determined as an effective diagnostic tool for rheumatoid arthritis (RA), and confirmed using atomic force microscopy (AFM). Collagen‐induced arthritic (CIA) mice and normal mice were prepared and FLS were isolated by enzymatic digestion from the synovial tissue of sacrificed mice at 5‐week and 8‐week pathogenesis periods. Analysis of cell morphology using AFM revealed that the surface roughness around the nucleus and around the branched cytoplasm was significantly higher in CIA FLS (P < 0.05) than that in normal FLS. In addition, the roughness of two different sites on the arthritic FLS increased with an increase in the duration of pathogenesis. These results strongly suggest that AFM can be widely used as a diagnostic tool in cytopathology to detect the early signs of RA and various others diseases at the intercellular level. Microsc. Res. Tech. 78:982–988, 2015.

Magnesium removal from molten aluminum by Cl2 bubbling

The experiments were performed by introducing Cl2 gas bubbles into Al melt in order to obtain basic information on the removal of Mg component. Thermodynamic calculations were performed to confirm the feasibility of Mg removal from Al melt. A mixture of Ar and Cl2 gas with a mixing ratio of 10–50% in a total flowrate of 50–100 sccm was bubbled into Al melt at 1000–1100 K. Mg could be removed from Al melt by Cl2 gas bubbling and the rate of removal depended largely on the total gas flowrate and the Cl2 mixing ratio. A greater rate of decreasing Mg was observed with a higher Cl2 mixing ratio in the bubbling gas. It was estimated that the removal rate of Mg complied with zero order kinetics. The rate-controlling step was estimated to be the mass transfer in Al melt. The activation energy for Mg removal by Cl2 gas bubbling was determined to be 63.1 kJ/mol. The mechanism of Mg removal from Al melt by Cl2 gas bubbling was proposed. The aluminum chlorides were formed by Cl2 gas bubbling, and were expected to react with the Mg in the Al melt which resulted in the formation of MgCl2.

Co-deteriorations of anisotropic extracellular matrix arrangement and intrinsic mechanical property in c-src deficient osteopetrotic mouse femur

Osteopetrotic bone shows dissociation between bone mineral density (BMD) and bone strength. In this study, volumetric BMD; preferential orientation of the extracellular matrix (ECM), which is composed of collagen fibers and apatite crystals as bone material quality; and mechanical properties of the src-/- osteopetrotic and normal mouse femoral cortical bone were analyzed and compared with each other at a bone tissue level. The degree of preferential orientation of ECM along the femoral long axis was significantly decreased in the src-/- mice femur, suggesting deteriorated bone quality. Youngs modulus, as a tissue-level mechanical property analyzed by nano-indentation technique along the long bone direction, also was decreased in the src-/- mice cortical femur, in spite of the similar volumetric cortical BMD. To the best of our knowledge, this is the first report to demonstrate the synchronous deterioration of Youngs modulus and anisotropic ECM organization in the src-/- osteopetrotic mouse bone. These results indicate that the deterioration of the preferential ECM orientation is one major cause of the impaired mechanical property in the src-/- mouse bone.

Induction of Biological Apatite Orientation as a Bone Quality Parameter in Bone Regeneration Using Hydroxyapatite/Poly ɛ-Caprolactone Composite Scaffolds.

Changes in the biological apatite (BAp) c-axis orientation were investigated as a bone quality parameter in bone regeneration using hydroxyapatite/poly ɛ-caprolactone (HA/PCL) composite scaffolds. Three-dimensional (3D) HA/PCL composite scaffolds were fabricated using a layer manufacturing process in three grid sizes (200-, 600-, and 1000 μm) and grafted into the forearm ulna of New Zealand white rabbits. The cross-sectional areas of the bones regenerated from the scaffolds with 600- and 1000-μm grid sizes were significantly larger than those from the scaffold with 200-μm grid sizes, whereas bone mineral density in the regenerated regions did not differ between the three grid sizes. Moreover, the BAp c-axis orientation in the bones regenerated from the scaffolds with grid sizes of 600- and 1000 μm was not significantly different; however, both scaffolds showed enhanced BAp orientation, although the degree of BAp orientation was lower than that in intact bones. In conclusion, HA/PCL composite 3D scaffolds with 600- and 1000-μm grid sizes induced BAp c-axis orientation and showed good bone regeneration behavior in vivo.

Femtosecond Laser Ablation of Polymer Thin Films for Nanometer Precision Surface Patterning

Femtosecond laser ablation of ultrathin polymer films on quartz glass using laser pulses of 100 fs and centered at λ=400 nm wavelength has been investigated for nanometer precision thin film patterning. Singleshot ablation craters on films of various thicknesses have been examined by atomic force microscopy, and beam spot diameters and ablation threshold fluences have been determined by square diameter-regression technique. The ablation thresholds of polymer film are about 1.5 times smaller than that of quartz substrate, which results in patterning crater arrays without damaging the substrate. In particular, at a 1/e 2 laser spot diameter of 0.86 μm, the smallest craters of 150-nm diameter are fabricated on 15-nm thick film. The ablation thresholds are not influenced by the film thickness, but diameters of the ablated crater are bigger on thicker films than on thinner films. The ablation efficiency is also influenced by the laser beam spot size, following a w 0q -0.45 dependence.