Biomedical subjects

C T Chan

Publications and source records attributed to C T Chan.

At least 19 recordsLinked to original sources

PubMed2006

Sleep disorders in end-stage renal disease: 'Markers of inadequate dialysis'?

Excessive daytime sleepiness and sleep disorders, including sleep apnea syndrome, restless legs syndrome, and periodic limb movement disorder, occur with increased frequency in patients with end-stage renal disease (ESRD). The detection and management of sleep disorders in ESRD patients is often challenging but may have significant clinical benefits. Some of the poor quality of life in ESRD may be attributed to the presence of concomitant sleep disorders, yet the classical symptoms of sleep disorders (poor concentration, daytime sleepiness, and insomnia) are often ascribed to the uremic syndrome itself. Conventional risk factors and screening tools used in the diagnosis of sleep disorders seem to have limited applicability in dialysis patients implicating the unique pathophysiology of sleep disorders in ESRD. Emerging evidence suggests that sleep apnea may contribute to the augmented cardiovascular event rates and to the accelerated development of atherosclerosis in ESRD. Whether treatment of sleep disorders in ESRD patients can affect the high morbidity and mortality of ESRD patients has yet to be elucidated. To date, conventional renal replacement therapies do not appear to have a significant impact on the treatment of sleep disorders in ESRD. The promising therapeutic effects of optimal uremia control in the forms of nocturnal hemodialysis and renal transplantation on sleep disorders require further mechanistic and clinical studies.

Atherosclerosis↗

PubMed2006

Localized vibrational modes in optically bound structures.

We show, through analytical theory and rigorous numerical calculations, that optical binding can organize a collection of particles into extended, periodic one-dimensional lattices. These lattices, as well as other optically bound structures, are shown to exhibit spatially localized vibrational eigenmodes. The origin of localization here is distinct from the usual mechanisms such as disorder, defect, or nonlinearity but is a consequence of the long-ranged nature of optical binding. For an array of particles trapped by an interference pattern, the stable configuration is often dictated by the external light source, but we observed that interparticle optical binding forces can have a profound influence on the dynamics.

Journal Article↗

PubMed2006

Highly spin-polarized field emissions induced by quantum size effects in ultrathin films of Fe on W(001).

Spin-polarized field emissions from Fe pseudomorphic ultrathin films on W(001) surfaces are studied by density functional calculations. We found that nearly completely spin-polarized field emission currents can be realized in two and four Fe layers on W(001) and that these systems have the additional advantages of thermal stability and low work functions. The unusually high spin polarizations of the field emission current is traced to the Fe film's quantum size effects leading to spin-polarized quantum well states and surface resonance states.

Journal Article↗

PubMed2006

Central venous catheter outcomes in nocturnal hemodialysis.

Central venous catheter (CVC) as hemodialysis (HD) access is associated with great morbidity and mortality in the end-stage renal disease population. Quotidian, nocturnal HD (NHD) is a novel dialysis modality associated with cardiovascular and quality of life benefits, yet there is a concern of a potential increase in vascular access-related complications through patient-directed access cannulation. We aimed to determine catheter incidence and prevalence in the NHD population and to compare rates of catheter-related: infection, thrombolytic administration, hospitalization, survival, and reasons for their loss before and after conversion to NHD. This observational cohort consisted of incident and prevalent NHD patients between 1 November 1993 and 31 May 2005. Rate comparisons were determined by Poisson analysis and catheter survival by Kaplan-Meier curves. Eighty-one CVCs in 33 patients accounted for 17 150 CVC days (CVCD); 40 CVCs were exclusively used for conventional three times weekly HD (CHD) and 25 CVCs were exclusively used during NHD. The incidence and prevalence of CVC use in our NHD population was 35 and 25%, respectively. Comparing CHD to NHD, no significant differences were seen in total rates of infection, thrombolytic administration, or access-related hospitalization. Catheter survival was superior in NHD vs CHD (P=0.03). Adverse terminal catheter events were higher during CHD than NHD (5.84 vs 2.92/1000 CVCD; P=0.03). CVC use and complications in NHD is comparable to that in CHD with the benefit of longer cumulative survival. More frequent CVC use should not be a deterrent to NHD.

Adult↗

PubMed2006

All-angle broadband negative refraction of metal waveguide arrays in the visible range: theoretical analysis and numerical demonstration.

In this Letter, we introduce a simple metal waveguide array for realizing all-angle wide frequency bandwidth negative refraction from the visible to infrared frequencies. Theoretical analysis from the rigorous coupled-wave theory reveals that the negative coupling constant resulting from the anomalous coupling of guided surface plasmon polariton modes contributes to the negative refraction. The analytical results are confirmed by finite-difference time-domain numerical simulations. Our result provides an alternative way to construct robust all-angle negative refractive materials operating in a wide range of frequency from the near-infrared to the visible range.

Journal Article↗

PubMed2006

Improvements in cognition in patients converting from thrice weekly hemodialysis to nocturnal hemodialysis: a longitudinal pilot study.

Cognitive impairment has been documented in uremia with partial improvement after dialysis. Nocturnal daily hemodialysis (NHD) is a novel dialysis modality with multiple benefits. Previous reports have shown marked improvements in quality of life, cardiac function, resolution of peripheral vascular disease, and reversal of central sleep apnea. We hypothesized that patients maintained on NHD would have better cognitive functioning than those receiving conventional therapy. Using a longitudinal study design, patients were tested at baseline and again after >or=6 months NHD. At each of the two time points, a battery of 10 neuropsychological tests were used to evaluate three domains of cognitive functioning--attention and working memory skills, psychomotor efficiency and processing speed, and learning efficiency. Clinical subjective symptoms for cognitive functioning and depression were measured using the Patients Assessment of Own Functioning inventory and the Beck Depression Index. Twelve patients (six males, six females) were recruited. Patients were aged 39.6+/-3.3 years at the time of first testing. Thirty-three percent were diabetic, with a mean Charlson comorbidity score of 3.5+/-2.0. Depression (defined as >16 on the Beck Depression Index score) was not seen in any patient. Over the 6-month period, a 22% reduction in cognitive symptoms (P=0.01), 7% improvement in psychomotor efficiency and processing speed (P=0.02), and 32% improvement in attention and working memory (P=0.04) was seen. Learning efficiency scores were unchanged. NHD may be associated with improved general cognitive efficiency as measured by psychomotor efficiency and attention and working memory.

Adult↗

PubMed2006

Diamagnetic response of metallic photonic crystals at infrared and visible frequencies.

We show analytically and numerically that diamagnetic response (effective magnetic permeability mue<1) at infrared and visible frequencies can be achieved in photonic crystals composed of metallic nanowires or nanospheres when the wavelength is much larger than the lattice constant a (lambda approximately 2000a). When lambda approximately100a, the metallic photonic crystals will exhibit strong diamagnetic response (mue<0.8), leading to many interesting phenomena such as the unusual Brewster angle for s waves and incident-angle-and-polarization-independent reflection and transmission.

Journal Article↗

PubMed2006

Frequency-selective absorption characteristics of a metal surface with embedded dielectric microspheres.

The coupling of the resonances of spherical particles embedded near the surface of a metal and the surface plasmon is investigated by calculations using a multiple scattering formalism. The system we considered is a periodic array of glass spheres buried just beneath the surface of an Ag substrate. The calculated absorption spectra show features that can be traced to the resonance frequency of the microsphere cavity or the surface mode of the metal. In general, the absorption peaks for both the cavity and the surface mode are relatively weak when their resonance frequencies are far from each other. However, if the system is designed such that the cavitylike mode and the surface mode have almost the same frequency, the coupling of the resonances will lead to enhanced frequency-selective absorption. Such systems can be employed to modify or control thermal emission.

Journal Article↗

PubMed2006

Enhanced chemo-responsiveness in patients with sleep apnoea and end-stage renal disease.

Although sleep apnoea is very common in patients with end-stage renal disease, the physiological mechanisms for this association have not yet been determined. The current authors hypothesised that altered respiratory chemo-responsiveness may play an important role. In total, 58 patients receiving treatment with chronic dialysis were recruited for overnight polysomnography. A modified Read rebreathing technique, which is used to assess basal ventilation, ventilatory sensitivity and threshold, was completed before and after overnight polysomnography. Patients were divided into apnoeic (n = 38; apnoea/hypopnoea index (AHI) 35+/-22 events.h(-1)) and nonapnoeic (n = 20; AHI 3+/-3 events.h(-1)) groups, with the presence of sleep apnoea defined as an AHI >10 events.h(-1). While basal ventilation and the ventilatory recruitment threshold were similar between groups, ventilatory sensitivity during isoxic hypoxia (partial pressure of oxygen (PO2) 6.65 kPa) and hyperoxia (PO2) 19.95 kPa) was significantly greater in apnoeic patients. Overnight changes in chemoreflex responsiveness were similar between groups. In conclusion, these data indicate that the responsiveness of both the central and peripheral chemoreflexes is augmented in patients with sleep apnoea and end-stage renal disease. Since increased ventilatory sensitivity to hypercapnia destabilises respiratory control, the current authors suggest this contributes to the pathogenesis of sleep apnoea in this patient population.

Adult↗

PubMed2006

Anomalous Doppler effects in phononic band gaps.

Doppler effects in periodic acoustic media were studied theoretically and experimentally. Analytical formulas are derived using the Green's function formalism. We found that a far field observer cannot hear the sound inside a band gap from a stationary source, but a moving source can be heard even if the frequency is inside the gap, and the Doppler shifts can be inverted or anomalously large.

Journal Article↗

PubMed2006

Surface reconstructions and stability of X-shaped carbon nanotube junction.

A complete surface reconstruction takes place after a local connection between two crossed tubes is established, leading to the creation of an extended X-shaped junction constituted by topological defects with smooth negative curvature. Molecular-dynamics simulations show that the surface reconstructions occur through (1) generalized Stone-Wales transformation and (2) the movement of sp and sp3 atoms and their transformation to sp2 atoms by bond rearrangement. Based on both the principle of energy minimization and a generalized Euler's rule, it is demonstrated that the most stable structure for X junctions contains only 12 heptagons. The annealing temperature influences the topological structure and stability of junctions.

Journal Article↗

PubMed2006

Daily hemodialysis 2006. State of the art.

The need to improve the dialysis outcomes, the negative results from the HEMO and ADEMEX studies as well reports of clinical benefits have rekindled the interest in daily hemodialysis. Although no randomized controlled studies have been published, a large number of manuscripts have described significant benefits from both the short or nocturnal forms of daily (quotidian) dialysis or hemo(dia)filtration. They include improved quality of life, hemodynamic stability, blood pressure control with minimal number of medications, anemia control, regression of cardiac hypertrophy and improved nutrition. Furthermore, quotidian nocturnal hemodialysis provides higher dialysis dose, and has been described to improve endothelial as well as endothelial progenitor cell function, heart rate variability, sleep and phosphate control while it offers unrestricted diet. Several studies have pointed to a lower overall cost and improved cost utility when treating patients using quotidian hemodialysis at home. The obstacles to widespread use are the reimbursement structure in most countries, the willingness and ability of the patients to do home hemodialysis and the availability of user-friendly machines. A prospective randomized controlled study sponsored by the National Institutes of Health (NIH) and US Centers for Medicare and Medicaid services (CMS) currently underway will be pivotal in confirming these benefits and convincing the stakeholders to promote the use of daily hemodialysis.

Costs and Cost Analysis↗

PubMed2005

Photonic band gap effect and structural color from silver nanoparticle gelatin emulsion.

We have fabricated planar structures of silver nanoparticles in monochromatic gelatin emulsion with a continuous spacing ranging from 0.15-0.40 micron using a two-beam interference of a single laser source. Our planar holograms display a colorful "rainbow" pattern and photonic bandgaps covering the visible and IR ranges. We model the planar silver nanoparticle-gelatin composite system using an effective medium approach and good agreement is obtained between theory and experiment.

Journal Article↗

PubMed2005

Refraction of water waves by periodic cylinder arrays.

We show that in the long wavelength limit, water waves propagate through an array of bottom-mounted vertical cylinders as if the water has an effective depth and effective gravitational constant that depends on the filling ratio of the cylinders, leading to refraction phenomena that can be described by analytic formulas. The results are obtained with rigorous homogenization techniques, as well as the multiple scattering formalism that gives full dispersion relationships. This phenomenon provides a mechanism to control the flow of ocean wave energy, as exemplified by a water-wave focusing lens.

Journal Article↗

PubMed2005

Strong optical force induced by morphology-dependent resonances.

We consider the resonant optical force acting on a pair of transparent microspheres by the excitation of morphology-dependent resonance (MDR). The bonding and antibonding modes of the MDR correspond to strong attraction and repulsion, respectively. The dependence of the force on separation and the role of absorption are discussed. At resonance, the force can be enhanced by orders of magnitude such that it will dominate other relevant forces. We found that a stable binding configuration can be induced by the resonant optical force.

Journal Article↗

PubMed2005

Relaxation mechanisms in three-dimensional metamaterial lens focusing.

We study the transient behaviors in focusing a point source with a metamaterial lens (epsilon = mu = -1 + delta). We find the time evolutions of image formation are dictated by two relaxation mechanisms, namely, surface wave transport and absorption, determined, respectively, by Re(delta) and Im(delta). We show that image oscillations are inevitable in this three-dimensional configuration when Re(delta) double dagger 0 and establish relationships among the relaxation time, the resolution enhancement, and delta.

Journal Article↗

PubMed2005

Two-dimensional sonic crystals with Helmholtz resonators.

We present a type of sonic crystal composed with an array of two-dimensional Helmholtz resonators, which in the long-wave regime have both a high relative acoustic refractive index n and at the same time, a small acoustic impedance Z mismatch with air for airborne sound. We analyze the n and Z of such sonic crystals by finite-difference time-domain simulations, and by mapping our results to a corresponding electromagnetic (EM) model, and we find that our Helmholtz resonant sonic crystal has a bigger effective magnetic permeability mu than the conventional rigid-cylinder sonic crystal in its EM counterpart. As a result, a thin convergent lens with very good focusing effect is demonstrated based on our crystal.

Journal Article↗

PubMed2005

Experimental and theoretical investigation of single Cu, Ag, and Au atoms adsorbed on Si(111)-(7x7).

Using scanning tunneling microscopy and first principles calculations, the adsorption sites of single Cu, Ag, and Au atoms on the Si(111)-(7x7) surface have been systematically investigated and identified. Despite their monovalence, these atoms were found to adsorb at high coordination sites, seeking to saturate the maximum number of dangling bonds. The stable adsorption sites were further observed to be distinctly different in the faulted and unfaulted half unit cells, showing an asymmetry that has never been observed for many other adsorbates.

Journal Article↗