Fluid mechanical performance of ureteral stents: The role of side hole and lumen size

Abstract Ureteral stents are indispensable devices in urological practice to maintain and reinstate the drainage of urine in the upper urinary tract. Most ureteral stents feature openings in the stent wall, referred to as side holes (SHs), which are designed to facilitate urine flux in and out of the stent lumen. However, systematic discussions on the role of SH and stent lumen size in regulating flux and shear stress levels are still lacking. In this study, we leveraged both experimental and numerical methods, using microscopic‐Particle Image Velocimetry and Computational Fluid Dynamic models, respectively, to explore the influence of varying SH and lumen diameters. Our results showed that by reducing the SH diameter from 1.1 to 0.4mm the median wall shear stress levels of the SHs near the ureteropelvic junction and ureterovesical junction increased by over 150%, even though the flux magnitudes through these SH decreased by about 40%. All other SHs were associated with low flux and low shear stress levels. Reducing the stent lumen diameter significantly impeded the luminal flow and the flux through SHs. By means of zero‐dimensional models and scaling relations, we summarized previous findings on the subject and argued that the design of stent inlet/outlet is key in regulating the flow characteristics described above. Finally, we offered some clinically relevant input in terms of choosing the right stent for the right patient

Flow Dynamics in Stented Ureter

Urinary flow is governed by the principles of fluid mechanics. Urodynamic studies have revealed the fundamental kinematics and dynamics of urinary flow in various physiological and pathological conditions, which are cornerstones for future development of diagnostic knowledge and innovative devices. There are three primary approaches to study the fluid mechanical characteristics of urinary flow: reduced order, computational, and experimental methods. Reduced-order methods exploit the disparate length scales inherent in the system to reveal the key dominant physics. Computational models can simulate fully three-dimensional, time-dependent flows in physiologically-inspired anatomical domains. Finally, experimental models provide an excellent counterpart to reduced and computational models by providing physical tests under various physiological and pathological conditions. While the interdisciplinary approaches to date have provided a wealth of insight into the fluid mechanical properties of the stented ureter, the next challenge is to develop new theoretical, computational and experimental models to capture the complex interplay between the fluid dynamics in stented ureters and biofilm/encrustation growth. Such studies will (1) enable identification of clinically relevant scenarios to improve patients’ treatment, and (2) provide physical guidelines for next-generation stent design

An in vitro bladder model with physiological dynamics: Vesicoureteral reflux alters stent encrustation pattern.

In vitro models are indispensable to study the physio-mechanical characteristics of the urinary tract and to evaluate ureteral stent performances. Yet previous models mimicking the urinary bladder have been limited to static or complicated systems. In this study, we designed a simple in vitro bladder model to simulate the dynamics of filling and voiding. The physio-mechanical condition of the model was verified using a pressure-flow test with different bladder outlet obstruction levels, and a reflux test was performed to qualitatively demonstrate the stent associated vesicoureteral reflux (VUR). Finally, the setup was applied with and without the bladder model to perform encrustation tests with artificial urine on commercially available double-J stents, and the volumes of luminal encrustations were quantified using micro-Computed Tomography and image segmentation. Our results suggest that, VUR is an important factor contributing to the dynamics in the upper urinary tract with indwelling stents, especially in patients with higher bladder outlet obstruction levels. The influence of VUR should be properly addressed in future in vitro studies and clinical analyses

Sigmoid isostiffness-lines: An in-vitro model for the assessment of aortic stenosis severity.

Introduction The aortic valve opening area (AVA), used to quantify aortic stenosis severity, depends on the transvalvular flow rate (Q). The currently accepted clinical echocardiographic method assumes a linear relation between AVA and Q. We studied whether a sigmoid model better describes this relation and determined "isostiffness-lines" across a wide flow spectrum, thus allowing building a nomogram for the non-invasive estimation of valve stiffness. Methods Both AVA and instantaneous Q (Qinst) were measured at 10 different mean cardiac outputs of porcine aortic valves mounted in a pulsatile flow loop. The valves' cusps were chemically stiffened to obtain three stiffness grades and the procedure was repeated for each grade. The relative stiffness was defined as the ratio between LV work at grade with the added stiffness and at native stiffness grade. corresponding to the selected of the highest 3 and 5 cardiac output values was predicted in K-fold cross-validation using sequentially a linear and a sigmoid model. The accuracy of each model was assessed with the Akaike information criterion (AIC). Results The sigmoid model predicted more accurately (AIC for prediction of AVA with of the 3 highest cardiac output values: -1,743 vs. -1,048; 5 highest cardiac output values: -1,471 vs. -878) than the linear model. Conclusion This study suggests that the relation between AVA and Q can be better described by a sigmoid than a linear model. This construction of "isostiffness-lines" may be a useful method for the assessment of aortic stenosis in clinical echocardiography

Preventing Biofilm Formation and Encrustation on Urinary Implants: (Bio)molecular and Physical Research Approaches

Stents and catheters are used to facilitate urine drainage within the urinary system. When such sterile implants are inserted into the urinary tract, ions, macromolecules and bacteria from urine, blood or underlying tissues accumulate on their surface. We presented a brief but comprehensive overview of future research strategies in the prevention of urinary device encrustation with an emphasis on biodegradability, molecular, microbiological and physical research approaches. The large and strongly associated field of stent coatings and tissue engineering is outlined elsewhere in this book. There is still plenty of room for future investigations in the fields of material science, surface science, and biomedical engineering to improve and create the most effective urinary implants. In an era where material science, robotics and artificial intelligence have undergone great progress, futuristic ideas may become a reality. These ideas include the creation of multifunctional programmable intelligent urinary implants (core and surface) capable to adapt to the complex biological and physiological environment through sensing or by algorithms from artificial intelligence included in the implant. Urinary implants are at the crossroads of several scientific disciplines, and progress will only be achieved if scientists and physicians collaborate using basic and applied scientific approaches

Turbulence generated by multiscale inhomogeneous and anisotropic grids

In this thesis two classes of inhomogeneous multiscale grids are proposed and investigated experimentally. These studies provide further understandings of the idea to generate bespoke turbulence field using fractal grids as proposed by Vassilicos and colleagues. The first part of the work introduces the rectangular fractal grid (RFG). Due to the inhomogeneous grid geometry, the streamwise location of turbulence intensity peak appears different from the wake interaction length scale calculated from the grid bar dimensions, and a region of decreasing length scales with decreasing Reynolds number is observed. This region is shown to be inhomogeneous and anisotropic. Nevertheless, the non-equilibrium scaling is found in this region, where the ratio between the integral length scale and Taylor microscale remains constant. From the second part of the work, turbulent flows with different mean shear rates are studied using the new inhomogeneous multiscale grids. By designing the local blockage ratio and the bar dimensions, this new type of shear generating grid is capable of producing different mean velocity and turbulence intensity profiles at the same time. The mean velocity profiles are shown to match the predictive mean velocity model, and a scaling relation of turbulence intensity is proposed based on the wake interaction length scale. The streamwise evolution of the Reynolds stress is studied, and a new dimensionless time scale is proposed. Finally, the design and scaling methods of the turbulent shear flow generated by the inhomogeneous multiscale grid are tested in a low-fidelity engineering wind tunnel with different size and background flow quality, and the results are consistent. These results perhaps provide a general methodology to produce various types of turbulent flows through the design of one single passive grid, which is desirable for both fundamental studies of turbulence and engineering applications such as the wind engineering experiments.Open Acces

Perturbing spanwise modes in turbulent boundary layers

University of Minnesota M.S. thesis. January 2013. Major: Aerospace Engineering and Mechanics. Advisor: Ellen K. Longmire. 1 computer file (PDF); vi, 77 pages,The objective of the current study was to manipulate the coherent vortex packets in a turbulent boundary layer at Re_tau=2480 by inserting a small scale cylinder array and to improve the understanding of the downstream flow stability issues. The height of the cylinders was H/delta=0.2 (H+=500) with aspect ratio (AR=cylinder height/base diameter) of 4, and three cases were studied using single array of 0.2 delta, 0.4 delta and 0.6 delta spaced cylinders. Both fixed location data and flying data were acquired at z+=296 using PIV, and the spanwise scales of the packets and the wake-packet interactions downstream of the cylinder array were also discussed. The non-perturbed flow was studied first and the dominant spanwise scale of the vortex packets was found to be approximately 0.6 delta. From the flying data, the organization of vortex packets was found to persist over a streamwise distance of approximately 8 delta. The averaged results of the perturbed cases showed a spanwise variation of the streamwise velocity downstream of the cylinder array, and the spanwise scales of the low speed regions were most stable for the 0.6 delta spacing case. Also, distinct downwash behavior was observed directly behind each cylinder. The flying data showed frequent spanwise interactions of cylinder wakes in the 0.2 delta case and the downstream structures were affected greatly by the incoming flow condition. The 0.4 delta and 0.6 delta cases were discussed based on the relative spanwise location of the upstream vortex packets and the cylinders and it was concluded that the organization of flow structures was most stable when the perturbation scale was the same as the dominant spanwise mode of the non-perturbed flow

Fluid mechanical performance of ureteral stents: The role of side hole and lumen size

Abstract Ureteral stents are indispensable devices in urological practice to maintain and reinstate the drainage of urine in the upper urinary tract. Most ureteral stents feature openings in the stent wall, referred to as side holes (SHs), which are designed to facilitate urine flux in and out of the stent lumen. However, systematic discussions on the role of SH and stent lumen size in regulating flux and shear stress levels are still lacking. In this study, we leveraged both experimental and numerical methods, using microscopic‐Particle Image Velocimetry and Computational Fluid Dynamic models, respectively, to explore the influence of varying SH and lumen diameters. Our results showed that by reducing the SH diameter from 1.1 to 0.4mm the median wall shear stress levels of the SHs near the ureteropelvic junction and ureterovesical junction increased by over 150%, even though the flux magnitudes through these SH decreased by about 40%. All other SHs were associated with low flux and low shear stress levels. Reducing the stent lumen diameter significantly impeded the luminal flow and the flux through SHs. By means of zero‐dimensional models and scaling relations, we summarized previous findings on the subject and argued that the design of stent inlet/outlet is key in regulating the flow characteristics described above. Finally, we offered some clinically relevant input in terms of choosing the right stent for the right patient

Fluid mechanical modeling of the upper urinary tract

The upper urinary tract (UUT) consists of kidneys and ureters, and is an integral part of the human urogenital system. Yet malfunctioning and complications of the UUT can happen at all stages of life, attributed to reasons such as congenital anomalies, urinary tract infections, urolithiasis and urothelial cancers, all of which require urological interventions and significantly compromise patients' quality of life. Therefore, many models have been developed to address the relevant scientific and clinical challenges of the UUT. Of all approaches, fluid mechanical modeling serves a pivotal role and various methods have been employed to develop physiologically meaningful models. In this article, we provide an overview on the historical evolution of fluid mechanical models of UUT that utilize theoretical, computational, and experimental approaches. Descriptions of the physiological functionality of each component are also given and the mechanical characterizations associated with the UUT are provided. As such, it is our aim to offer a brief summary of the current knowledge of the subject, and provide a comprehensive introduction for engineers, scientists, and clinicians who are interested in the field of fluid mechanical modeling of UUT