Flow-Diverter Stent With an Incorporated Flow Sensor—The Integration of a Treatment and Diagnostic Device for Intracranial Aneurysms

The emergence of flow-diverting devices has allowed for safe and efficacious treatment of intracranial aneurysms that were previously thought to be unsuitable for endovascular therapy. Flow-diverting devices, by promotion of laminar flow through the parent artery and stasis of flow within the aneurysm, encourage thrombosis of the aneurysm. However, unlike surgical clipping where the aneurysm is completely excluded from circulation immediately following treatment, flow-diversion devices require time to induce aneurysm thrombosis. Therefore, extended imaging follow-up is required to determine that the flow-diversion treatment strategy was successful. Multiple studies have highlighted the importance of both short- and long-term follow-up to help minimize the postprocedural morbidity which has been reported to be as high as 10% in some series.1 Traditionally, this follow-up consists of multiple post-treatment angiograms, CTAs, and/or MRIs. CTA and MRI studies provide an imperfect assessment of the treated aneurysm due to artifact induced by the flow-diversion device. Follow-up angiograms subject the patient to procedure-associated risks and carry a significant financial cost. An alternative strategy to serial imaging follow-up would be the development of implanted flow sensors that provide real-time data regarding the hemodynamics of the aneurysm/parent vessel/flow-diverting device environment. Several such devices have been developed to assist in the follow-up for aortic aneurysms and other peripheral pathologies.2 Previously, however, the size and other technical limitations of these devices have precluded them from being effectively applied to smaller aneurysms and the challenging vascular architecture of the intracranial circulation. Howe et al3 have provided a critical stepping stone to overcoming these limitations through their construction of a soft, stretchable, flexible, low-profile sensor system integrated with a flow-diverting device. They describe a system that combines the already available Neuroform stent (Stryker Neurovascular, Stryker Corp, Kalamazoo, Michigan) with a flow sensor array consisting of hyperelastic thin film nitinol and a nanostructured capacitive ring-type flow-sensor sheathed by polyimide (PI; HD Microsystems, Parlin, New Jersey) and encased by a soft hemocompatible elastomeric membrane (Ecoflex, Smooth-On, Macungie, Pennsylvanisa). The resulting device has the necessary stretchability (500% radial stretching) and bendability (180° with 0.75 mm radius of curvature) for deployment in cerebrovascular vessels and combines the treatment potential of a flow-diverting device with an integrated flow-sensing monitor that can monitor treatment progress and possibly obviate the need for multiple follow-up angiograms. In this study, Howe et al3 describe the technical properties of their device and their rationale for the chosen materials and design. The authors discuss various experiments including computation and physical models to study the mechanical safety and integrity of their device as well as a hemocompatibility model to demonstrate the feasibility for long-term insertion in a blood vessel. The results of their computational modeling and quantitative experimental studies demonstrated that their sensor array design was stable under loading conditions of radial stretching up to 500% and radial bending up to 180°. Their hemocompatibility study, using utilizing ovine blood, demonstrated overall less platelet deposition relative to existing implantable materials. The authors further demonstrate that this sensor is capable of detecting the rate of blood flow into an aneurysm based on computational and in vitro models (Figure3). With the sensory array placed across the neck of the aneurysm, the nanostructure-based soft sensor detects the incoming blood flow to the aneurysm via mechanical deflection. The compliant membrane in the sensor experiences the capacitance change according to the amount of deflection induced by the incoming flow rate to the neck. The rate of flow can then be calculated based on the magnitude of capacitance change.FIGURE.: Fluid dynamic analysis. A, 3D computational model of a sensor on a stent backbone, together embedded in a parent blood vessel (5 mm in diameter) and aneurysm sac. B-D, Images showing velocity amplitude in an entire model and expanded cross-sectional profile view of the aneurysm. Applied mean velocity in the parent vessel is 0.5 m/s with steady flow conditions. B, Model without any intervention device (ie, open untreated aneurysm). C, Model with the flow diverter deployed to the aneurysm site, which displays a significant decrease of the incoming flow to the aneurysm sac. D, Model with the implanted flow diverter and sensor, showing effects of the added sensor to the system on intrasaccular flow. E, Images capturing sensor deflections in the neck of the aneurysm, caused by low flow (left; 0.1 m/s vessel mean blood velocity) and high flow (right; 0.5 m/s vessel mean blood velocity). Scale bars present the maximum deflection of the sensor. F, Simultaneous comparison between the computation results (maximum sensor deflection) and experimental measurements (normalized capacitance change in the sensor), caused by variations of flow rates. The experimental results validate the computational modeling with the well-agreed values. Reprinted with permission from Howe C, Mishra S, Kim YS, et al. Stretchable, Implantable, Nanostructured Flow-Diverter System for Quantification of Intra-aneurysmal Hemodynamics. ACS nano. Aug 28 2018;12(8):8706-8716.3 Copyright 2018 American Chemical Society.To construct their in vitro model, Howe et al3 utilized a porcine artery and fashioned an aneurysm sac from excess arterial vessel tissue. A flow-diverter device with their integrated flow-sensor array was placed inside the vessel. A pulsatile pump was employed to simulate blood flow. With the sensor positioned over the lumen of aneurysm neck, blood that passes through this lumen resulted in a mechanical deflection of sensor and subsequent change in capacitance. The rate of blood flow into the aneurysm was then able to be quantified based on these sensor data. The overall sensitivity of the system was notable, with a detection blood flow threshold as small as 0.032 m/s. Overall, this manuscript highlights exciting advances in technology development and describes the fabrication of a mechanically soft flow-diverter system with an integrated flow sensor array that may eventually offer an effective treatment for aneurysms coupled with real-time, active monitoring of intra-aneurysmal hemodynamics which might obviate the need for multiple follow-up angiograms. While there remain numerous hurdles prior to translation of this into humanclinical trials (need for integrated wireless transmission of data and power source/generator; expansion of sensor array width to ensure delivery over the aneurysm neck; multitude of safety studies, etc), the medical and scientific implications of a flow-sensor array that can be safely deployed in the cerebrovascular system are significant. Aside from real-time monitoring of flow-diverting device or stent-assisted coiling treated aneurysms, such devices could provide critical information on the development of intracranial aneurysms and their mechanisms of rupture, and allow for better risk stratification of patients with carotid stenosis or moyamoya for ischemic events. Further study and development of such technologies will dramatically enhance our understanding of cerebrovascular diseases and as well as our potential to better treat affected patients. Disclosure The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.