Showing papers by "Keefe B. Manning published in 2017"

FDA Benchmark Medical Device Flow Models for CFD Validation.

Abstract: Computational fluid dynamics (CFD) is increasingly being used to develop blood-contacting medical devices. However, the lack of standardized methods for validating CFD simulations and blood damage predictions limits its use in the safety evaluation of devices. Through a U.S. Food and Drug Administration (FDA) initiative, two benchmark models of typical device flow geometries (nozzle and centrifugal blood pump) were tested in multiple laboratories to provide experimental velocities, pressures, and hemolysis data to support CFD validation. In addition, computational simulations were performed by more than 20 independent groups to assess current CFD techniques. The primary goal of this article is to summarize the FDA initiative and to report recent findings from the benchmark blood pump model study. Discrepancies between CFD predicted velocities and those measured using particle image velocimetry most often occurred in regions of flow separation (e.g., downstream of the nozzle throat, and in the pump exit diffuser). For the six pump test conditions, 57% of the CFD predictions of pressure head were within one standard deviation of the mean measured values. Notably, only 37% of all CFD submissions contained hemolysis predictions. This project aided in the development of an FDA Guidance Document on factors to consider when reporting computational studies in medical device regulatory submissions. There is an accompanying podcast available for this article. Please visit the journal's Web site (www.asaiojournal.com) to listen.

A resolved two-way coupled CFD/6-DOF approach for predicting embolus transport and the embolus-trapping efficiency of IVC filters

Abstract: Inferior vena cava (IVC) filters are medical devices designed to provide a mechanical barrier to the passage of emboli from the deep veins of the legs to the heart and lungs. Despite decades of development and clinical use, IVC filters still fail to prevent the passage of all hazardous emboli. The objective of this study is to (1) develop a resolved two-way computational model of embolus transport, (2) provide verification and validation evidence for the model, and (3) demonstrate the ability of the model to predict the embolus-trapping efficiency of an IVC filter. Our model couples computational fluid dynamics simulations of blood flow to six-degree-of-freedom simulations of embolus transport and resolves the interactions between rigid, spherical emboli and the blood flow using an immersed boundary method. Following model development and numerical verification and validation of the computational approach against benchmark data from the literature, embolus transport simulations are performed in an idealized IVC geometry. Centered and tilted filter orientations are considered using a nonlinear finite element-based virtual filter placement procedure. A total of 2048 coupled CFD/6-DOF simulations are performed to predict the embolus-trapping statistics of the filter. The simulations predict that the embolus-trapping efficiency of the IVC filter increases with increasing embolus diameter and increasing embolus-to-blood density ratio. Tilted filter placement is found to decrease the embolus-trapping efficiency compared with centered filter placement. Multiple embolus-trapping locations are predicted for the IVC filter, and the trapping locations are predicted to shift upstream and toward the vessel wall with increasing embolus diameter. Simulations of the injection of successive emboli into the IVC are also performed and reveal that the embolus-trapping efficiency decreases with increasing thrombus load in the IVC filter. In future work, the computational tool could be used to investigate IVC filter design improvements, the effect of patient anatomy on embolus transport and IVC filter embolus-trapping efficiency, and, with further development and validation, optimal filter selection and placement on a patient-specific basis.

Determination of Reynolds Shear Stress Level for Hemolysis.

Abstract: Reynolds shear stress (RSS) has served as a metric for the effect of turbulence on hemolysis. Forstrom (1969) and Sallam and Hwang (1984) determined the RSS threshold for hemolysis to be 50,000 and 4,000 dyne/cm, respectively, using a turbulent jet. Despite the order of magnitude discrepancy, the threshold by Sallam and Hwang has been frequently cited for hemolytic potential in blood pumps. We recreated a Sallam apparatus (SA) to resolve this discrepancy and provide additional data to be used in developing a more accurate hemolysis model. Hemolysis was measured over a large range of Reynolds numbers (Re) (Re = 1,000-80,000). Washed bovine red blood cells (RBCs) were injected into the free jet of phosphate buffered saline, and hemolysis was quantified using a percent hemolysis, Hp = h (100 - hematocrit [HCT])/Hb, where h (mg/dl) is free hemoglobin and Hb (mg/dl) is total hemoglobin. Reynolds shear stress was calculated using two-dimensional laser Doppler velocimetry. Reynolds shear stress of ≥30,000 dyne/cm corresponding to Re of ≥60,000 appeared to cause hemolysis (p < 0.05). This RSS is an order of magnitude greater than the RSS threshold that Sallam and Hwang suggested, and it is similar to Forstrom's RSS threshold. This study resolved a long-standing uncertainty regarding the critical values of RSS for hemolysis and may provide a foundation for a more accurate hemolysis model.

Computational predictions of the embolus-trapping performance of an IVC filter in patient-specific and idealized IVC geometries

Abstract: Embolus transport simulations are performed to investigate the dependence of inferior vena cava (IVC) filter embolus-trapping performance on IVC anatomy. Simulations are performed using a resolved two-way coupled computational fluid dynamics/six-degree-of-freedom approach. Three IVC geometries are studied: a straight-tube IVC, a patient-averaged IVC, and a patient-specific IVC reconstructed from medical imaging data. Additionally, two sizes of spherical emboli (3 and 5 mm in diameter) and two IVC orientations (supine and upright) are considered. The embolus-trapping efficiency of the IVC filter is quantified for each combination of IVC geometry, embolus size, and IVC orientation by performing 2560 individual simulations. The predicted embolus-trapping efficiencies of the IVC filter range from 10 to 100%, and IVC anatomy is found to have a significant influence on the efficiency results ( $$P < 0.0001$$ ). In the upright IVC orientation, greater secondary flow in the patient-specific IVC geometry decreases the filter embolus-trapping efficiency by 22–30 percentage points compared with the efficiencies predicted in the idealized straight-tube or patient-averaged IVCs. In a supine orientation, the embolus-trapping efficiency of the filter in the idealized IVCs decreases by 21–90 percentage points compared with the upright orientation. In contrast, the embolus-trapping efficiency is insensitive to IVC orientation in the patient-specific IVC. In summary, simulations predict that anatomical features of the IVC that are often neglected in the idealized models used for benchtop testing, such as iliac vein compression and anteroposterior curvature, generate secondary flow and mixing in the IVC and influence the embolus-trapping efficiency of IVC filters. Accordingly, inter-subject variability studies and additional embolus transport investigations that consider patient-specific IVC anatomy are recommended for future work.

Toward the Virtual Benchmarking of Pneumatic Ventricular Assist Devices: Application of a Novel Fluid-Structure Interaction-Based Strategy to the Penn State 12 cc Device.

Abstract: The pediatric use of pneumatic ventricular assist devices (VADs) as a bridge to heart transplant still suffers for short-term major complications such as bleeding and thromboembolism Although numerical techniques are increasingly exploited to support the process of device optimization, an effective virtual benchmark is still lacking Focusing on the 12 cc Penn State pneumatic VAD, we developed a novel fluid-structure interaction (FSI) model able to capture the device functioning, reproducing the mechanical interplay between the diaphragm, the blood chamber, and the pneumatic actuation The FSI model included the diaphragm mechanical response from uniaxial tensile tests, realistic VAD pressure operative conditions from a dedicated mock loop system, and the behavior of VAD valves Our FSI-based benchmark effectively captured the complexity of the diaphragm dynamics During diastole, the initial slow diaphragm retraction in the air chamber was followed by a more rapid phase; asymmetries were noticed in the diaphragm configuration during its systolic inflation in the blood chamber The FSI model also captured the major features of the device fluid dynamics In particular, during diastole, a rotational wall washing pattern is promoted by the penetrating inlet jet with a low-velocity region located in the center of the device Our numerical analysis of the 12 cc Penn State VAD points out the potential of the proposed FSI approach well resembling previous experimental evidences; if further tested and validated, it could be exploited as a virtual benchmark to deepen VAD-related complications and to support the ongoing optimization of pediatric devices

Asynchronous Pumping of a Pulsatile Ventricular Assist Device in a Pediatric Anastomosis Model

Abstract: Background:Both pulsatile and continuous flow ventricular assist devices are being developed for pediatric congenital heart defect patients. Pulsatile devices are often operated asynchronously with...

Impact of a Biomedical Engineering Undergraduate Research Program on Student and Faculty Perceptions of Creativity

Abstract: Immersive research experiences have been shown to significantly improve the research and communication abilities of students who participate in them, as well as increase the likelihood that these students will pursue higher education after the completion of their bachelor’s degrees. While Research Experiences for Undergraduates (REU) programs are widespread, the Cardiovascular Research: Engineering a Translational Experience (CREATE) REU program is unique in that it emphasizes the parallels between the creative process and research. Creativity, an attribute that most feel is important for aspiring engineers, is typically not emphasized in research programs or in the undergraduate curriculum. This study describes the impacts that emphasizing the creative process in the CREATE REU had both on student and faculty perceptions. Background and Literature Review The National Academy of Engineering (NAE) emphasizes that creativity is one of the important characteristics that engineering students should possess. As stated in the pivotal Engineer of 2020 manuscript, “Creativity...is an indispensable quality for engineering and given the growing scope of the challenges ahead and the complexity and diversity of the technologies of the 21 century, creativity will grow in importance” (p. 55). However, creativity is not typically emphasized in the traditional engineering curriculum and, rather, is relegated to design courses or entrepreneurship minors. Few core technical courses incorporate elements that require students to demonstrate aspects of the creative process in their assigned work. In fact, research has shown that both faculty and students feel that creativity, “is not valued in contemporary engineering education” (p. 762). This lack of focus on the creative process in the engineering curriculum has been hypothesized to be a factor in the retention of engineering students. Results of a cross-sectional study of students’ creative self-perceptions showed that senior students tended to feel that creativity was less expected of them in their engineering courses as compared to first-year students. In addition, seniors tended to identify themselves as being less creative than first-year students. The results of this study suggested that more creative students could potentially migrate out of engineering towards majors that better allowed them to utilize the creative process. These results were supported by Atwood and Pretz who demonstrated that students’ creative self-efficacy negatively correlated with students’ persistence in engineering. As the authors noted, “until engineering students see creativity rewarded in their grades and emphasized in their curriculum, the engineering field will miss the opportunity to retain and develop more creative engineers” (p. 555). Embedding creativity into the technical curriculum and courses can be challenging as course content can be restrictive, focusing on specific ABET outcomes. In addition, faculty may not feel comfortable integrating elements of the creative process in their courses if they do not feel trained to do so. This paper focuses on a National Science Foundation (NSF) funded REU program in biomedical engineering which emphasized the parallels between the creative process and the scientific method. While outside of the core engineering curriculum, REUs can afford students the opportunity to enhance skills that may be helpful in their professional careers, such as increasing their creative capacity. In addition, the creative process has some strong parallels with the scientific method. Emphasizing this parallel may be more appealing and intuitive to faculty. Research Experience for Undergraduates (REU) Programs The National Science Foundation (NSF) identifies REU programs as being, “...a major contributor to the NSF goal of developing a diverse, internationally competitive, and globallyengaged science and engineering workforce.” These programs aim to increase the involvement of undergraduate students, particularly underrepresented minorities, in research-based experiences. Among the reported benefits of REUs are increased recruitment of underrepresented groups, gains in research-based skills, clarification, refinement, and reinforcement of career and educational goals, an increased understanding of the research process, increased student retention in science and engineering, and an increase in students’ selfconfidence of ability and self-esteem. Prior work shows that students who have participated in REUs often describe their experiences as being a very important part of their undergraduate careers. These students perceived increases in their ability to understand scientific findings, communicate the results of their research, and understand scientific literature after completion of the REU, and were also more likely to pursue graduate education. Faculty perceptions of REU experiences have also been reported as reflecting significant educational benefits for students and the graduate students who serve as their mentors. Grimberg et al. found that students participating in the last four years of a seven year REU program reported greater levels of satisfaction when the program coordinators began to emphasize a broader theme, environmental sustainability, as compared to implementations of the program without the emphasis of the sustainability theme. In the context of the REU described by Grimberg, the students reported that the inclusion of the environmental sustainability theme made them more aware of the “value and greater context of their work.” Similarly, the REU being described in this paper emphasizes the creative process and its relationship with the scientific method as a broad theme of the program. To our knowledge, creativity has not been previously studied in the REU literature. We hypothesize that the incorporation of this theme has the potential to impact both student satisfaction and perceptions of the REU program. Parallels between Creative Process and Scientific Method Creativity was implemented into the REU described in this study by describing creativity to the students and faculty in the context of engineering research problems. The creative process used for this training was a model described by Mumford, and can be compared in parallel to the traditional scientific method. The processes included in Mumford’s model of creativity are described in Table 1. Table 1. Description of processes included in Mumford’s model of creativity. Process Description Problem Construction Defining the problem. This includes identifying goals, constraints, outcomes, key steps towards a solution, and important information that needs to be considered. Information Gathering Determining which information might contribute to solving the problem. Concept Selection Identifying which information you have gathered will be the most pertinent in constructing a problem solution. Concept Combination Combining and reorganizing the information chosen in the concept selection step to move towards generating novel ideas. Idea Generation Formally determining potential problem solutions. Idea Evaluation Determining the efficiency and appropriateness of the proposed solution. Implementation Planning Testing the chosen problem solution. Monitoring Searching for evidence to determine the problem solution’s level of success. By emphasizing the parallels between the creative process and the scientific method, faculty who had previously thought of creativity as being outside the bounds of technical engineering may now see how the creative process is used in research. Table 2 shows the alignment of the two processes, as mapped by the principal investigators for the proposal. The authors noted that there are some differences in the two processes, such as the level of detail relating to certain steps. For example, in Mumford’s creative process, individuals go through three processes relating to coming up with new ideas: concept selection, conceptual combination, and idea generation. While some of these steps may occur during scientific research, the model identified by Crawford and co-authors uses a broader category of forming an explanatory hypothesis. Similarly, the creative process refers to “idea evaluation,” while the scientific method breaks this down into how the hypothesis would be evaluated through experimentation, data collection, data analysis, and interpretation. Table 2. Parallels between the Creative Process and the Scientific Method. 8 Stages of the Creative Process 8 Steps of the Scientific Method 1 Problem Construction 1 Define the question 2 Information gathering 2 Gather information and resources (observe)