Biomedical subjects

S H Cheung

Publications and source records attributed to S H Cheung.

6 recordsLinked to original sources

PubMed2006

Simultaneous determination of key bioactive components in Hedyotis diffusa by capillary electrophoresis.

A capillary zone electrophoresis (CZE) method based on systematic one-variable-at-a time approach was developed for the analysis of four important bioactive components (geniposidic acid, ursolic acid, quercetin and p-coumaric acid) in the extract of Hedyotis diffusa (HD). Separations were carried out in a fused-silica capillary tube with peak detection at 214 nm. Good separation was achieved using a 20 mM borate buffer containing 5% acetonitrile as organic modifier and pH adjusted to 10.0. Operating voltage was 15 kV and temperature was maintained at 25 degrees C while hydrodynamic injection was 5s. A good linearity, with correlation coefficients in the ranges of 0.997-0.999 was obtained in the calibration curves of each standard. Relative standard deviation (R.S.D.) of migration time was between 0.32 and 0.70% and deviation of corrected peak area was between 8.84 and 11.99%. These results indicate that this method could be used for rapid and simultaneous analysis of the bioactive components in HD and other herbal products.

Buffers↗

PubMed2005

Role of defect sites and Ga polarization in the magnetism of Mn-doped GaN.

We report a study of the Mn local structure, magnetism, and Ga moments in molecular beam epitaxy grown Mn-doped GaN films. Using x-ray absorption spectroscopy and magnetic circular dichroism, we find two distinct Mn sites and a Ga moment antiparallel to Mn. First-principles calculations reproduce this phenomenology and indicate that Mn preferentially populates Ga sites neighboring N split interstitial defects. These results show that defects may strongly affect the Mn ordering and magnetism, and that the GaN valence band is polarized, providing a long-range ferromagnetic ordering mechanism for Ga1-xMnxN.

Journal Article↗

PubMed2005

Selected growth of cubic and hexagonal GaN epitaxial films on polar MgO(111).

Selected molecular beam epitaxy of zinc blende (111) or wurtzite (0001) GaN films on polar MgO(111) is achieved depending on whether N or Ga is deposited first. The cubic stacking is enabled by nitrogen-induced polar surface stabilization, which yields a metallic MgO(111)-(1 x 1)-ON surface. High-resolution transmission electron microscopy and density functional theory studies indicate that the atomically abrupt semiconducting GaN(111)/MgO(111) interface has a Mg-O-N-Ga stacking, where the N atom is bonded to O at a top site. This specific atomic arrangement at the interface allows the cubic stacking to more effectively screen the substrate and film electric dipole moment than the hexagonal stacking, thus stabilizing the zinc blende phase even though the wurtzite phase is the ground state in the bulk.

Journal Article↗

PubMed1996

Simultaneous confidence intervals for pairwise multiple comparisons in a two-way unbalanced design.

Turkey's (1953, The Problem of Multiple Comparisons, unpublished report, Princeton University) procedure is widely used for pairwise multiple comparisons in one-way ANOVA. It provides exact simultaneous pairwise confidence intervals (SPCI) for balanced designs and conservative SPCI for unbalanced designs. In this paper, we will extend Turkey's procedure to two-way unbalanced designs. Both the exact and the conservative methods will be introduced. The application of the new procedure is illustrated with sample data from two experiments.

Algorithms↗

PubMed1994

A step-down procedure for multiple tests of treatment versus control in each of several groups.

Cheung and Holland extended Dunnett's treatment versus control multiple comparison procedure to the case of several groups. Dunnett and Tamhane provided a step-down procedure more powerful than that of Dunnett for situations where the researcher's interest is in testing and not in simultaneous confidence interval estimation. In this paper we show how one can use the approach of Dunnett and Tamhane to devise a testing procedure for the multi-group problem with power greater than that of Cheung and Holland. We illustrate the new procedure with reanalyses of published data from a clinical experiment.

Confidence Intervals↗

PubMed1993

Characterization of a human keratinocyte cell line immortalized by human papillomavirus 16 DNA.

A human keratinocyte cell line was established by transfecting neonatal foreskin keratinocytes of a Chinese with human papillomavirus (HPV) 16 DNA. As evidenced by the prolonged life span, the clone formation from a single cell, the piling up after prolonged culturing without passage and the chromosomal aneuploidy, this cell line possesses the biological characteristics of immortalization. The reason for obligatory growth requirements on epidermal growth factor (EGF) is not clear. The partial growth requirement on hydrocortisone for this immortalized cell line suggests that the glucocorticoid responding element of HPV 16 may play a role in cell immortalization. The constant over-expression of keratin 19 in this and other HPV 16 immortalized squamous epithelia indicates that it may serve as a useful marker for the potential malignant transformation of squamous epithelial cells. This immortalized cell line provides a model for investigating the factors and cofactors involved in carcinogenesis and differentiation of human epithelial cells.

Cell Division↗