Interplay between yielding, 'recovery', and strength of yield stress fluids for direct ink writing: new insights from oscillatory rheology.

Formulation design and rheology are critical for successful manufacturing via direct ink writing (DIW), thus linking rheology and printability is a growing area of research amongst the DIW and rheology communities. This work provides an extensive rheological investigation into the material strength, yielding and 'recovery' properties of graphite (Gr)-hydrogel based formulations. Using state-of-the-art Large Amplitude Oscillatory Shear (LAOS) techniques, Fourier Transform (FT) rheology and sequence of physical process (SPP) analysis, and 3-step 'recovery' tests we provide new insights on the yielding phenomenon, energy transitions and microstructural changes that the formulations undergo. The insights from the rheology experiments are combined with in situ and continuous monitoring during the printing process. From these analyses, we select rheological metrics or descriptors to quantify flowability, recoverability, and material strength. There is a threshold concentration of Gr powders (30 wt%) at which there is a shift in the yielding process. Below this threshold (for the F127 hydrogel and mixtures with low Gr content), perfect plastic dissipation ratio (ϕ) values are close to 0 in the LVR and then steeply increase to close to 1 after the cross-over in a narrow strain (and stress) space. As Gr concentration increases, and print quality gets worse, ϕ values consistently increase in the LVR and at any given γ0, evidencing an increased energy dissipation throughout the flow transition region. Lissajous-Bowditch curves and SPP Cole-Cole plots illustrate these trends. The extent of the 'recovery' (quantified by the mutation time, λI, and the storage modulus 'recovered' after large deformations ) is also directly related to Gr content, with higher loading resulting in lesser recovery. Our findings provide a comprehensive set of metrics to characterise complex (yield stress) fluids for DIW using three property maps, one for each stage: flowability or yielding process, recoverability and material strength. The results demonstrate that considering these three maps holistically provides insightful trends to guide formulation design and assess performance in DIW.

Complications associated with deep brain stimulation for Parkinson's disease: a MAUDE study.

INTRODUCTION: Deep brain stimulation (DBS) is a common surgical option for the treatment of medically refractory Parkinson's disease (PD). Manufacturer and User Facility Device Experience (MAUDE), a United States Food and Drug Administration (FDA)-compiled database of adverse event reports related to medical devices, is a public resource that can provide insight into the relative frequency of complications and patient complaints. MATERIALS AND METHODS: We accessed the MAUDE database and queried for adverse reports for deep brain stimulators implanted for PD from January 1, 2009 to December 31, 2018. Complaints were classified into device malfunction, patient non-compliance, patient complaint, surgically managed complications (i.e. complications that are corrected via surgery), and death. Patient complaints were further stratified into ineffective stimulation, shock, overstimulation, battery-related problems, or pain at the pulse generator site. Surgically managed complications were classified as intraoperative complications, impedance, migration, erosion, infection, lead fracture, and lead disconnection. Each event could receive multiple classifications and subclassifications. RESULTS: A total of 4,189 adverse event reports was obtained. These encompassed 2,805 patient complaints. Within this group, 797 (28%) events were classified as ineffective stimulation. There were 1,382 surgically managed complications, 104 (8%) of which were intraoperative complications, 757 (55%) documented impedance issues, 381 (28%) infections, and 413 (30%) lead-related issues. There were 53 documented deaths. CONCLUSIONS: The MAUDE database has potential use as a real time monitor for elucidating the relative occurrence of complications associated with deep brain stimulation. It also allows for the analysis of device-related complications in specific patient populations. Although the database is useful in this endeavor, it requires improvements particularly in the standardization of reporting adverse events.

Low- and High-Drag Intermittencies in Turbulent Channel Flows.

Recent direct numerical simulations (DNS) and experiments in turbulent channel flow have found intermittent low- and high-drag events in Newtonian fluid flows, at Reτ=uτh/ν between 70 and 100, where uτ, h and ν are the friction velocity, channel half-height and kinematic viscosity, respectively. These intervals of low-drag and high-drag have been termed "hibernating" and "hyperactive", respectively, and in this paper, a further investigation of these intermittent events is conducted using experimental and numerical techniques. For experiments, simultaneous measurements of wall shear stress and velocity are carried out in a channel flow facility using hot-film anemometry (HFA) and laser Doppler velocimetry (LDV), respectively, for Reτ between 70 and 250. For numerical simulations, DNS of a channel flow is performed in an extended domain at Reτ = 70 and 85. These intermittent events are selected by carrying out conditional sampling of the wall shear stress data based on a combined threshold magnitude and time-duration criteria. The use of three different scalings (so-called outer, inner and mixed) for the time-duration criterion for the conditional events is explored. It is found that if the time-duration criterion is kept constant in inner units, the frequency of occurrence of these conditional events remain insensitive to Reynolds number. There exists an exponential distribution of frequency of occurrence of the conditional events with respect to their duration, implying a potentially memoryless process. An explanation for the presence of a spike (or dip) in the ensemble-averaged wall shear stress data before and after the low-drag (or high-drag) events is investigated. During the low-drag events, the conditionally-averaged streamwise velocities get closer to Virk's maximum drag reduction (MDR) asymptote, near the wall, for all Reynolds numbers studied. Reynolds shear stress (RSS) characteristics during these conditional events are investigated for Reτ = 70 and 85. Except very close to the wall, the conditionally-averaged RSS is higher than the time-averaged value during the low-drag events.