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Guest Editorial

J. Med. Devices. 2017;11(2):020301-020301-1. doi:10.1115/1.4036591.

Cardiovascular devices interact with a complex physiologic environment that can severely challenge device performance and longevity. These challenges include biocompatibility issues such as hemolysis and thrombosis, changing contact conditions during each heartbeat, high-cycle fatigue-to-fracture, the need to accommodate the highly variable geometric, material, and hemodynamic environment encountered in the target population, and many other design and performance concerns. Simultaneously, cardiovascular devices are expected to deliver improved therapeutic benefits with each new product release. Thus, the cardiovascular device industry increasingly relies on computer modeling as a controlled and repeatable methodology for assessing device design-related factors. This allows reduction in number of in vitro, ex vivo, and in vivo experiments, leading to decreased expense. Clinicians are also adopting computer modeling as a pre-interventional planning tool that can confirm (or reject) a diagnosis, assist with surgical planning, and optimize treatment outcomes. And regulatory bodies are looking to computational modeling and improved in vitro testing to help ensure the safety and efficacy of approved devices.

Commentary by Dr. Valentin Fuster

Research Papers

J. Med. Devices. 2017;11(2):021001-021001-7. doi:10.1115/1.4036286.

A numerical analysis of a semi-enclosed tubular mechanical embolus retrieval device (MERD) for the treatment of acute ischemic stroke (AIS) is presented. In this research, the finite element analysis (FEA) methodology is used to evaluate mechanical performance and provide suggestions for optimizing the geometric design. A MERD fabricated from nickel–titanium alloy (Nitinol) tubing is simulated and analyzed under complex in vivo loading conditions involving shape-setting, crimping, deployment, and embolus retrieval. As a result, the peak strain of the shape-setting procedure is proved to be safe for the device pattern. However, the MERD shows poor mechanical behavior after crimping into a catheter, because the peak crimping strain obtains a value of 12.1%. The delivery and deployment step demonstrates that the artery wall has little risk of serious injuries or rupture. In addition, the process of simulation of embolus retrieval and device system migration inside the cerebral artery lumen provides useful information during the design process.

Commentary by Dr. Valentin Fuster
J. Med. Devices. 2017;11(2):021002-021002-7. doi:10.1115/1.4036334.

In order to evaluate the performance of stents used in transcatheter aortic valve implantation (TAVI), finite element simulations are setup to reconstruct patient-specific contact forces between implant and its surrounding tissue. Previous work used structural beam elements to setup a numerical model of the CoreValve stent used in TAVI and developed a procedure for implementing kinematic boundary conditions from noisy computer tomography (CT) scanning data. This study evaluates element size selection and quantitatively investigates the choice of a linear elastic constitutive model for the Nitinol stent under physiological loading conditions. It is shown that this simplification leads to reliable results and enables a huge reduction in computation time. Further, the procedure used to compensate for noisy postoperative CT data is tested by adding artificial noise. It is concluded that for physiologically relevant loading ranges, the procedure yields convergent results and successfully eliminates the influence of the noise.

Commentary by Dr. Valentin Fuster
J. Med. Devices. 2017;11(2):021003-021003-8. doi:10.1115/1.4035687.

Endovascular aneurysm repair (EVAR) is a clinically effective technique for treating anatomically eligible abdominal aortic aneurysms (AAAs), involving the deployment of an endograft (EG) that is designed to prevent blood leakage in the aneurysmal sac. While most EGs have equivalent operating principles, the hemodynamic environment established by different EGs is not necessarily the same. So, to unveil the post-EVAR hemodynamic properties, we need an EG-specific computational approach that currently lacks from the literature. Endurant and Excluder are two EGs with similar pre-installation designs. We assumed that the flow conditions in the particular EGs do not vary significantly. The hypothesis was tested combining image reconstructions, computational fluid dynamics (CFD), and statistics, taking into account the postimplantation position of the EGs. Ten patients with Endurant EGs and ten patients with Excluder EGs were included in this study. The two groups were matched with respect to the preoperative morphological characteristics of the AAAs. The EG models are derived from image reconstructions of postoperative computed tomography scans. Wall shear stress (WSS), displacement force, velocity, and helicity were calculated in regions of interest within the EG structures, i.e., the main body, the upper and lower part of the limbs. Excluder generated higher WSS compared to Endurant, especially on the lower part of the limbs (p = 0.001). Spatial fluctuations of WSS were observed on the upper part of the Excluder limbs. Higher blood velocity was induced by Excluder in all the regions of interest (p = 0.04, p = 0.01, and p = 0.004). Focal points of secondary flow were detected in the main body of Endurant and the limbs of Excluder. The displacement force acting on the lower part of the Excluder limbs was stronger compared to the Endurant one (p = 0.03). The results showed that two similar EGs implanted in similar AAAs can induce significantly different flow properties. The delineation of the hemodynamic features associated with the various commercially available EGs could further promote the personalization of treatment offered to aneurysmal patients and inspire ideas for the improvement of EG designs in the future.

Commentary by Dr. Valentin Fuster
J. Med. Devices. 2017;11(2):021004-021004-11. doi:10.1115/1.4036095.

This paper presents design optimization of a magnetic resonance imaging (MRI) actuated steerable catheter for atrial fibrillation ablation in the left atrium. The catheter prototype, built over polymer tubing, is embedded with current-carrying electromagnetic coils. The prototype can be deflected to a desired location by controlling the currents passing through the coils. The design objective is to develop a prototype that can successfully accomplish the ablation task. To complete the tasks, the catheter needs to be capable of reaching a set of desired targets selected by a physician on the chamber and keeping a stable contact with the chamber surface. The design process is based on the maximization of the steering performance of the catheter by evaluating its workspace in free space. The selected design is validated by performing a simulation of an ablation intervention on a virtual model of the left atrium with a real atrium geometry. This validation shows that the prototype can reach every target required by the ablation intervention and provide an appropriate contact force against the chamber.

Commentary by Dr. Valentin Fuster
J. Med. Devices. 2017;11(2):021005-021005-7. doi:10.1115/1.4036287.

This paper describes the use of analytical methods to determine machinable centrifugal impeller geometries and the use of computational fluid dynamics (CFD) for predicting the impeller performance. An analytical scheme is described to determine the machinable geometries for a shrouded centrifugal impeller with blades composed of equiangular spirals. The scheme is used to determine the maximum machinable blade angles for impellers with three to nine blades in a case study. Computational fluid dynamics is then used to analyze all the machinable geometries and determine the optimal blade number and angle based on measures of efficiency and rotor speed. The effect of tip width on rotor speed and efficiency is also examined. It is found that, for our case study, a six- or seven-bladed impeller with a low blade angle provides maximum efficiency and minimum rotor speed.

Commentary by Dr. Valentin Fuster
J. Med. Devices. 2017;11(2):021006-021006-15. doi:10.1115/1.4036299.

This research was directed toward quantitatively characterizing the effects of arterial mechanical treatment procedures on the stress and strain energy states of the artery wall. Finite element simulations of percutaneous transluminal angioplasty (PTA) and orbital atherectomy (OA) were performed on arterial lesion models with various extents and types of plaque. Stress fields in the artery were calculated and strain energy density was used as an explicit description of potential damage to the artery. The research also included numerical simulations of changes in arterial compliance due to orbital atherectomy. The angioplasty simulations show that the damage energy fields in the media and adventitia are predominant in regions of the lesion that are not protected by a layer of calcification. In addition, it was observed that softening the plaque components leads to a lower peak stress and therefore lesser damage energy in the media and adventitia under the action of a semicompliant balloon. Orbital atherectomy simulations revealed that the major portion of strain energy dissipated is concentrated in the plaque components in contact with the spinning tool. The damage and peak stress fields in the media and adventitia components of the vessel were significantly less. This observation suggests less mechanically induced trauma during a localized procedure like orbital atherectomy. Artery compliance was calculated pre- and post-treatment and an increase was observed after the orbital atherectomy procedure. The localized plaque disruption produced in atherectomy suggests that the undesirable stress states in angioplasty can be mitigated by a combination of procedures such as atherectomy followed by angioplasty.

Commentary by Dr. Valentin Fuster
J. Med. Devices. 2017;11(2):021007-021007-12. doi:10.1115/1.4035723.

The interest in biodegradable polymers for clinical and biomedical engineering applications has seen a dramatic increase in the last 10 years. Recent innovations include bioresorbable polymeric stents (BPS), which are temporary vascular scaffolds designed to restore patency and provide short-term support to a blocked blood vessel, before becoming naturally resorbed over time. BPS offer possibilities to overcome the long-term complications often observed with the permanent metallic stents, well established in the treatment of coronary and peripheral artery disease. From the perspective of designing next generation BPS, the bulk degradation behavior of the polymer material adds considerable complications. Computational modeling offers an efficient framework to predict and provide understanding into the behavior of medical devices and implants. Current computational modeling techniques for the degradation of BPS are either phenomenologically or physically based. In this work, a physically based polymer degradation model is implemented into a number of different computational frameworks to investigate the degradation of a number of polymeric structures. A thermal analogy is presented to implement the degradation model into the commercially available finite-element code, abaqus/standard. This approach is then applied to the degradation of BPS, and the effects of material, boundary condition, and design on the degradation rates of the stents are examined. The results indicate that there is a notable difference in the molecular weight trends predicted for the different materials and boundary condition assumptions investigated, with autocatalysis emerging as a dominant mechanism controlling the degradation behavior. Insights into the scaffolding ability of the various BPS examined are then obtained using a suggested general relationship between Young's modulus and molecular weight.

Commentary by Dr. Valentin Fuster
J. Med. Devices. 2017;11(2):021008-021008-8. doi:10.1115/1.4035724.

ISO 10993-4 in vivo thrombogenicity testing is frequently performed for regulatory approval of many blood-contacting medical devices and is often a key part of submission packages. Given the current state of in vivo thrombogenicity assays, a more robust and reproducible assay design, including in vitro models, is needed. This study describes an in vitro assay that integrates freshly harvested ovine blood containing minimal heparin in a closed pumped loop. To confirm the reproducibility of this assay, control materials were identified that elicited either a positive or a negative thrombogenic response. These controls demonstrated reproducibility in the resulting thrombogenicity scores with median scores of 5 and 0 for the positive and negative controls, respectively, which also demonstrated a significant difference (p < 0.0001). For a direct comparison of the in vitro blood loop assay to the traditional in vivo nonanticoagulated venous implant (NAVI) assay, seven sheep were used as blood donors for the loop and then as subjects for an NAVI assay. In each assay—loop or NAVI—three study articles were used: the positive and negative controls and a marketed, approved catheter. The resulting thrombogenicity scores were similar when comparing the loop to the NAVI results. For each study article, the median thrombogenicity scores were the same in these two different assays, being 0, 1, and 5 for the negative control, the marketed catheter, and the positive control, respectively. These data suggest that the in vitro assay performs similarly to the in vivo NAVI assay. This in vitro blood loop method has the potential to predict a materials' in vivo thrombogenicity, can substantially de-risk the materials or coating selection process, and may eventually be able to replace the in vivo models currently in use.

Commentary by Dr. Valentin Fuster
J. Med. Devices. 2017;11(2):021009-021009-11. doi:10.1115/1.4035791.

Nowadays, transcatheter aortic valve (TAV) replacement is an alternative to surgical therapy in selected high risk patients for the treatment of aortic stenosis. However, left ventricular contraction determines a severe cyclic loading for the implanted stent-frame, undermining its long-term durability. Technical standards indicate in vitro tests as a suitable approach for the assessment of TAV fatigue behavior: generally, they do not specify test methods but require to test TAV in the worst loading conditions. The most critical conditions could be different according to the specific valve design, hence the compartment where deploying the valve has to be properly identified. A fast and reliable computational methodology could significantly help to face this issue. In this paper, a numerical approach to analyze Nickel-Titanium TAV stent-frame behavior during in vitro durability tests is proposed. A simplified multistage strategy was adopted where, in each stage, only two of the three involved components are considered. As a proof-of-concept, the method was applied to a TAV prototype. Despite its simplifications, the developed computational framework gave useful insights into the stent-frame failures behavior during a fatigue test. Numerical results agree with experimental findings. In particular, the most dangerous condition was identified among a number of experimental tests, where different compartments and pressure gradients were investigated. The specific failure location was also correctly recognized. In conclusion, the presented methodology provides a tool to support the choice of proper testing conditions for the in vitro assessment of TAV fatigue behavior.

Commentary by Dr. Valentin Fuster
J. Med. Devices. 2017;11(2):021010-021010-9. doi:10.1115/1.4036391.

Overlapping stents are widely used in vascular stent surgeries. However, the rate of stent fractures (SF) and in-stent restenosis (ISR) after using overlapping stents is higher than that of single stent implantations. Published studies investigating the nature of overlapping stents rely primarily on medical images, which can only reveal the effect of the surgery without providing insights into how stent overlap influences the implantation process. In this paper, a finite element analysis of the overlapping stent implantation process was performed to study the interaction between overlapping stents. Four different cases, based on three typical stent overlap modes and two classical balloons, were investigated. The results showed that overlapping contact patterns among struts were edge-to-edge, edge-to-surface, and noncontact. These were mainly induced by the nonuniform deformation of the stent in the radial direction and stent tubular structures. Meanwhile, the results also revealed that the contact pressure was concentrated in the edge of overlapping struts. During the stent overlap process, the contact pattern was primarily edge-to-edge contact at the beginning and edge-to-surface contact as the contact pressure increased. The interactions between overlapping stents suggest that the failure of overlapping stents frequently occurs along stent edges, which agrees with the previous experimental research regarding the safety of overlapping stents. This paper also provides a fundamental understanding of the mechanical properties of overlapping stents.

Topics: stents , Pressure
Commentary by Dr. Valentin Fuster
J. Med. Devices. 2017;11(2):021011-021011-11. doi:10.1115/1.4035865.

Patients born with a single functional ventricle typically undergo three-staged surgical palliation in the first years of life, with the last stage realizing a cross-like total cavopulmonary connection (TCPC) of superior and inferior vena cavas (SVC and IVC) with both left and right pulmonary arteries (LPA and RPA), allowing all deoxygenated blood to flow passively back to the lungs (Fontan circulation). Even though within the past decades more patients survive into adulthood, the connection comes at the prize of deficiencies such as chronic systemic venous hypertension and low cardiac output (CO), which ultimately may lead to Fontan failure. Many studies have suggested that the TCPC’s inherent insufficiencies might be addressed by adding a cavopulmonary assist device (CPAD) to provide the necessary pressure boost. While many device concepts are being explored, few take into account the complex cardiac anatomy typically associated with TCPCs. In this study, we focus on the extra cardiac conduit (ECC) vascular graft connecting IVC and pulmonary arteries (PAs) as one possible landing zone for a CPAD and describe its geometric variability in a cohort of 18 patients that had their TCPC realized with a 20 mm vascular graft. We report traditional morphometric parameters and apply statistical shape modeling (SSM) to determine the main contributors of graft shape variability. Such information may prove useful when designing CPADs that are adapted to the challenging anatomical boundaries in Fontan patients. We further compute the anatomical mean 3D graft shape (template graft) as a representative of key shape features of our cohort and prove this template graft to be a significantly better approximation of population and individual patient’s hemodynamics than a commonly used simplified tube geometry. We therefore conclude that statistical shape modeling results can provide better models of geometric and hemodynamic boundary conditions associated with complex cardiac anatomy, which in turn may impact on improved cardiac device development.

Commentary by Dr. Valentin Fuster
J. Med. Devices. 2017;11(2):021012-021012-8. doi:10.1115/1.4035981.

The hemodynamic energy loss through the surgically implanted conduits determines the postoperative cardiac output and exercise capacity following the palliative repair of single-ventricle congenital heart defects. In this study, the hemodynamics of severely deformed surgical pathways due to torsional deformation and anastomosis offset are investigated. We designed a mock-up total cavopulmonary connection (TCPC) circuit to replicate the mechanically failed inferior vena cava (IVC) anastomosis morphologies under physiological venous pressure (9, 12, 15 mmHg), in vitro, employing the commonly used conduit materials: Polytetrafluoroethylene (PTFE), Dacron, and porcine pericardium. The sensitivity of hemodynamic performance to torsional deformation for three different twist angles (0 deg, 30 deg, and 60 deg) and three different caval offsets (0 diameter (D), 0.5D, and 1D) are digitized in three dimensions and employed in computational fluid dynamic (CFD) simulations to determine the corresponding hydrodynamic efficiency levels. A total of 81 deformed conduit configurations are analyzed; the pressure drop values increased from 80 to 1070% with respect to the ideal uniform diameter IVC conduit flow. The investigated surgical materials resulted in significant variations in terms of flow separation and energy loss. For example, the porcine pericardium resulted in a pressure drop that was eight times greater than the Dacron conduit. Likewise, PTFE conduit resulted in a pressure drop that was three times greater than the Dacron conduit under the same venous pressure loading. If anastomosis twist and/or caval offset cannot be avoided intraoperatively due to the anatomy of the patient, alternative conduit materials with high structural stiffness and less influence on hemodynamics can be considered.

Commentary by Dr. Valentin Fuster
J. Med. Devices. 2017;11(2):021013-021013-10. doi:10.1115/1.4035895.

Coronary stents made from degradable biomaterials such as magnesium alloy are an emerging technology in the treatment of coronary artery disease. Biodegradable stents provide mechanical support to the artery during the initial scaffolding period after which the artery will have remodeled. The subsequent resorption of the stent biomaterial by the body has potential to reduce the risk associated with long-term placement of these devices, such as in-stent restenosis, late stent thrombosis, and fatigue fracture. Computational modeling such as finite-element analysis has proven to be an extremely useful tool in the continued design and development of these medical devices. What is lacking in computational modeling literature is the representation of the active response of the arterial tissue in the weeks and months following stent implantation, i.e., neointimal remodeling. The phenomenon of neointimal remodeling is particularly interesting and significant in the case of biodegradable stents, when both stent degradation and neointimal remodeling can occur simultaneously, presenting the possibility of a mechanical interaction and transfer of load between the degrading stent and the remodeling artery. In this paper, a computational modeling framework is developed that combines magnesium alloy degradation and neointimal remodeling, which is capable of simulating both uniform (best case) and localized pitting (realistic) stent corrosion in a remodeling artery. The framework is used to evaluate the effects of the neointima on the mechanics of the stent, when the stent is undergoing uniform or pitting corrosion, and to assess the effects of the neointimal formation rate relative to the overall stent degradation rate (for both uniform and pitting conditions).

Commentary by Dr. Valentin Fuster

Technical Brief

J. Med. Devices. 2017;11(2):024501-024501-5. doi:10.1115/1.4035545.

As the connection at the proximal tip plays an important role for sensing guidewires, we compared various sensing guidewires with regard to their proximal connectors. The strengths and weaknesses of each are discussed and recommendations for future development are provided. A literature search limited to the English language for the time period from the 1960s to the 2010s has been performed on the USPTO database, Espacenet, and Web of Science. The results have been categorized on the basis of the connector design. A comprehensive overview and classification of proximal connectors for sensing guidewires used for cardiovascular interventions is presented. The classification is based on both the type of connector (fixed or removable) and the type of connection (physical, wireless, or a combination). Considering the complexity of the currently prototyped and tested connectors, future connector development will necessitate an easy and cost-effective manufacturing process that can ensure safe and robust connections.

Commentary by Dr. Valentin Fuster
J. Med. Devices. 2017;11(2):024502-024502-6. doi:10.1115/1.4035982.

In this article, the design and development of a real-time heart rate (HR) and respiratory rate (RR) monitoring device is reported. The proposed device is designed to impose minimum data acquisition hazards on the subject. In standard bedside monitors, HR and RR are derived from electrocardiogram (ECG) and respiration signals, respectively, and different electrodes are required for capturing the 12-lead ECG and respiration via a chest belt, which is cumbersome for patients and healthcare providers. Respiration signal has an impact on ECG due to anatomical proximity of the heart and lung, and ECG is modulated by respiration, a phenomenon known as respiratory sinus arrhythmia (RSA). In the proposed method, the ECG signal is acquired using clip electrodes at the wrists and the respiration signal is extracted from the ECG using an Arduino Uno microcontroller-based real-time processing of ECG. RR is then derived from ECG-derived respiration (EDR). The prototype is tested on healthy subjects and compared to measurements taken using a standard MP45 data acquisition device associated with a Biopac Student Lab (BSL). A mean percentage error of 5.54 ± 8.48% was observed under normal breathing conditions and an error of −3.41 ± 3.27% was observed for a single subject tested under a variety of breathing conditions, such as resting, stair-climbing, and paced breathing. The proposed algorithm can also be used in combination with standard ECG monitoring systems to measure HR and RR, without any data acquisition hazard to the subject.

Commentary by Dr. Valentin Fuster
J. Med. Devices. 2017;11(2):024503-024503-5. doi:10.1115/1.4035866.

The total product life cycle (TPLC) of medical devices has been defined by four stages: discovery and ideation, regulatory decision, product launch, and postmarket monitoring. Manufacturers of medical devices intended for use in the peripheral vasculature, such as stents, inferior vena cava (IVC) filters, and stent-grafts, mainly use computational modeling and simulation (CM&S) to aid device development and design optimization, supplement bench testing for regulatory decisions, and assess postmarket changes or failures. For example, computational solid mechanics and fluid dynamics enable the investigation of design limitations in the ideation stage. To supplement bench data in regulatory submissions, manufactures can evaluate the effects of anatomical characteristics and expected in vivo loading environment on device performance. Manufacturers might also harness CM&S to aid root-cause analyses that are necessary when failures occur postmarket, when the device is exposed to broad clinical use. Once identified, CM&S tools can then be used for redesign to address the failure mode and re-establish the performance profile with the appropriate models. The Center for Devices and Radiological Health (CDRH) wants to advance the use of CM&S for medical devices and supports the development of virtual physiological patients, clinical trial simulations, and personalized medicine. Thus, the purpose of this paper is to describe specific examples of how CM&S is currently used to support regulatory submissions at different phases of the TPLC and to present some of the stakeholder-led initiatives for advancing CM&S for regulatory decision-making.

Commentary by Dr. Valentin Fuster

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