Comparative Analysis of the GaN Nonpolar Plane Morphology by Wet Treatment and Its Effect on Electrical Properties in Trench MOSFET.

The morphological characteristics of the GaN nonpolar sidewalls with different crystal plane orientations were studied under various TMAH wet treatment conditions, and the effect of different morphological features on device carrier mobility was modeled and analyzed. After TMAH wet treatment, the morphology of the a-plane sidewall presents multiplied zigzag triangular prisms along the [0001] direction, which consist of two adjacent m-plane and c-plane on top. While along the [112̅0] direction, the m-plane sidewall is represented by thin, striped prisms with three m-plane and a c-plane on the side. The density and size of sidewall prisms were studied by varying the solution temperature and immersion period. The prism density decreases linearly as the solution temperature rises. With increased immersion time, both a-plane and m-plane sidewalls show smaller prism sizes. Vertical GaN trench MOSFET with nonpolar a- and m-plane sidewall channels were fabricated and characterized. By properly treated in TMAH solution, transistors with an a-plane sidewall conduction channel exhibit higher current density, from 241 to 423 A cm-2@VDS = 10 V, VGS = 20 V, and higher mobility, from 2.9 to 2.0 cm2 (V s)-1, compared to those of m-plane sidewall devices. The temperature dependence on mobility is also discussed, and a modeling analysis for the difference in carrier mobility is then performed.

Controlling the strontium-doping in calcium phosphate microcapsules through yeast-regulated biomimetic mineralization.

Yeast cells have controllable biosorption on metallic ions during metabolism. However, few studies were dedicated to using yeast-regulated biomimetic mineralization process to control the strontium-doped positions in calcium phosphate microcapsules. In this study, the yeast cells were allowed to pre-adsorb strontium ions metabolically and then served as sacrificing template for the precipitation and calcination of mineral shell. The pre-adsorption enabled the microorganism to enrich of strontium ions into the inner part of the microcapsules, which ensured a slow-release profile of the trace element from the microcapsule. The co-culture with human marrow stromal cells showed that gene expressions of alkaline phosphatase and Collagen-I were promoted. The promotion of osteogenic differentiation was further confirmed in the 3D culture of cell-material complexes. The strategy using living microorganism as 'smart doping apparatus' to control incorporation of trace element into calcium phosphate paved a pathway to new functional materials for hard tissue regeneration.

Phosphatidylserine enhances osteogenic differentiation in human mesenchymal stem cells via ERK signal pathways.

Phosphatidylserine (PS) has been demonstrated to promote bone mineralization. It has also been used in bone repairing biomaterials as a functional molecule. However, the effect of PS on mesenchymal stem cells (MSCs) is not clear. In this study, we determined the effect of PS on the osteogenic differentiation of human MSCs (hMSCs) cultured in growth or osteogenic differentiation medium and the role of the ERK1/2 signaling pathway on PS activity. Cytotoxicity of PS was measured by MTT assay in growth medium for 5 days. Cell osteogenic differentiation was evaluated by alkaline phosphatase (ALP) activity analysis, Alizarin Red S staining and real-time PCR assay. Western blotting and ERK blocking assay were used to examine the role of ERK1/2 signaling pathway on PS activity. The results showed no cytotoxicity for the doses of PS administered. For 21 days, 50-100 µM PS increased ALP expression and mineralization of hMSCs. The expression of the osteogenic gene marker, ALP, osteocalcin (OC), and RUNX2 was enhanced by 50 µM PS treatment at day 14. Phospho-ERK was activated by 50 µM PS at 30 min and 1h in growth medium. In osteogenic medium, 50 µM PS extended phospho-ERK activation by osteogenic induction medium from 30 min to 8 h. U0126, an ERK inhibitor, suppressed the ALP expression induced by PS. Our data indicate that the ERK signal is potentially a mediator in the process of osteogenic differentiation of hMSCs induced by PS. PS, as a functional molecule, has high potential for use in bone repairing biomaterials and bone tissue engineering.

In vitro generation of osteochondral differentiation of human marrow mesenchymal stem cells in novel collagen-hydroxyapatite layered scaffolds.

Integrated, layered osteochondral (OC) composite materials and/or engineered OC grafts are considered as promising strategies for the treatment of OC damage. A novel biomimetic collagen-hydroxyapatite (COL-HA) OC scaffold with different integrated layers has been generated by freeze-drying. The capacity of the upper COL layer and the lower COL/HA layer to promote the growth and differentiation of human mesenchymal stem cells (hMSCs) into chondrocytes and osteoblasts respectively was evaluated. Cell viability and proliferation on COL and COL/HA scaffolds were assessed by the MTT test. The chondrogenic differentiation of hMSCs on both scaffolds was evaluated by glucosaminoglycan (GAG) quantification, alcian blue staining, type II collagen immunocytochemistry assay and real-time polymerase chain reaction in chondrogenic medium for 21 days. Osteogenic differentiation was evaluated by alkaline phosphatase activity assay, type I collagen immunocytochemistry staining, alizarin S staining and mRNA expression of osteogenic gene for 14 days in osteogenic medium. The results indicated that hMSCs on both COL and COL/HA scaffolds were viable and able to proliferate over time. The COL layer was more efficient in inducing hMSC chondrogenic differentiation than the COL/HA layer, while the COL/HA layer possessed the superiority on promoting hMSC osteogenic induction over either COL layer or pure HA. In conclusion, the layered OC composite materials can effectively promote cartilage and bone tissue generation in vitro and are potentially usable for OC tissue engineering.