Randall Tagg

Photodynamic therapy for treatment of solid tumors--potential and technical challenges.

Photodynamic therapy (PDT) involves the administration of photosensitizer followed by local illumination with visible light of specific wavelength(s). In the presence of oxygen molecules, the light illumination of photosensitizer can lead to a series of photochemical reactions and consequently the generation of cytotoxic species. The quantity and location of PDT-induced cytotoxic species determine the nature and consequence of PDT. Much progress has been seen in both basic research and clinical application in recent years. Although the majority of approved PDT clinical protocols have primarily been used for the treatment of superficial lesions of both malignant and non-malignant diseases, interstitial PDT for the ablation of deep-seated solid tumors are now being investigated worldwide. The complexity of the geometry and non-homogeneity of solid tumor pose a great challenge on the implementation of minimally invasive interstitial PDT and the estimation of PDT dosimetry. This review will discuss the recent progress and technical challenges of various forms of interstitial PDT for the treatment of parenchymal and/or stromal tissues of solid tumors.

Primary instabilities and bicriticality in flow between counter‐rotating cylinders

The primary instabilities and bicritical curves for flow between counter‐rotating cylinders have been computed numerically from the Navier–Stokes equations assuming axial periodicity. The computations provide values of the Reynolds numbers, wavenumbers, and wave speeds at the primary transition from Couette flow for radius ratios from 0.40–0.98. Particular attention has been focused on the bicritical curves that separate (as the magnitude of counter‐rotation is increased) the transitions from Couette flow to flows with different azimuthal wavenumbers m and m+1. This lays the foundation for further analysis of nonlinear mode interactions and pattern formation occurring along the bicritical curves and serves as a benchmark for experimental studies. Preliminary experimental measurements of transition Reynolds numbers and wave speeds presented here agree well with the computations from the mathematical model.

Transient turbulence in Taylor-Couette flow.

Recent studies have brought into question the view that at sufficiently high Reynolds number turbulence is an asymptotic state. We present direct observation of the decay of turbulent states in Taylor-Couette flow with lifetimes spanning five orders of magnitude. We also show that there is a regime where Taylor-Couette flow shares many of the decay characteristics observed in other shear flows, including Poisson statistics and the coexistence of laminar and turbulent patches. Our data suggest that for a range of Reynolds numbers characteristic decay times increase superexponentially with increasing Reynolds number but remain bounded in agreement with the most recent data from pipe flow. Our data are also consistent with recent theoretical predictions of lifetime scaling in transitional flows.

Theoretical estimation of retinal oxygenation in chronic diabetic retinopathy

This paper uses computer modeling to estimate the progressive decline in oxygenation that occurs in the human diabetic retina after years of slowly progressive ischemic insult. An established model combines diffusion, saturable consumption, and blood capillary sources to determine the oxygen distribution across the retina. Incorporating long-term degradation of blood supply from the retinal capillaries into the model yields insight into the effects of progressive ischemia associated with prolonged hyperglycemia, suggesting time-scales over which therapeutic mitigation could have beneficial effect. A new extension of the model for oxygen distribution introduces a feedback mechanism for vasodilation and its potential to prolong healthy retinal function.

Book Review: Biodesign: The process of innovating medical technologies

Biodesign: The process of innovating medical technologies Stefanos Zenios, Josh Makower, Paul Yock, Todd J. Brinton, Uday N. Kumar, Lyn Denend, Thomas M. Krummel and Christine Kurihara (eds.) Cambridge University Press, Cambridge, UK, 2009, hardback, 778pp., ISBN: 978-0521517423

Instability of Taylor-Couette Flow Subjected to a Coriolis Force

Taylor-Couette flow subjected to a Coriolis force is a rich pattern-forming system.1–5 The Coriolis force is applied by rotating a Couette apparatus so that the axis of rotation for the system is. orthogonal to the common axis of the cylinders (see figure 1). This orientation produces a nonaxisymmetric Coriolis force through the interaction of the rotation of the system with the (primarily) azimuthal base flow. Experimentally, a small Coriolis force is found to stabilize the base flow against Taylor vortex formation. In other words, the Coriolis force increases the critical value of the Reynolds number R c for the onset of instability in the base flow. At the transition to secondary flow, the Taylor vortices are tilted out of the plane normal to the cylinders’ axis. But the flow is still time-independent. This tilted Taylor vortex flow is an example of a novel pattern which arises as a result of the nonaxisymmetric Coriolis force. Several other novel patterns also arise. At somewhat large system rotation rates, there is a direct transition from the base flow to strong spatiotemporal turbulence. At small values of Ω (where Ω is a dimensionless measure of the angular frequency of the rotating system) there is nonhysteretic re-emergent order at Reynolds numbers not far from the threshold for instability.3 In a medium gap system, there is a Hopf bifurcation from the base flow to axially travelling tilted Taylor vortices.4