## 1 Introduction  
A brain aneurysm is a localized enlargement of an intracranial artery, usually due to damaged smooth muscle cells and elastin in the arterial layers [1] but often exacerbated by localized perturbation of blood flow. The Brain Aneurysm Foundation reported that the worldwide annual mortality due to intracranial aneurysms is 500,000, with approximately 50% of these patients being younger than 50 years old [2]. Common treatments for intracranial aneurysms include embolic coiling and stent-assisted coiling, but these are associated with frequent recurrence, especially for wide-neck aneurysms [3; 4]. Stent-assisted coiling prevents coil protrusion into the parent artery lumen, a particular problem with wide-neck aneurysms, and reduces the recurrence risk [5]. Intra-aneurysm coils act by initiating and propagating thrombus that ultimately cause the aneurysm to heal by thrombotic occlusion. An emerging method to treat brain aneurysms is the use of flow diverter (FD) devices. Typically, FDs are tubular stents made of high-density braided mesh that minimizes blood perfusion through the stent wall into the aneurysm, producing blood stasis within the aneurysm that facilitates thrombus and thereby mitigates the need for coils [6]. All available FD devices in the market are designed for the treatment of sidewall aneurysms, but many aneurysms occur at a bifurcation. Y stenting is occasionally performed at a bifurcation to secure the coil and provide better neck coverage by deploying one stent from the parent artery to one daughter branch and then telescoping a second stent to the other (contralateral) branch [7; 8]. FDs are not well suited to this anatomy because delivery methods preclude complete bridging of the neck. A FD is implanted from the parent artery to one daughter branch, providing a \"neck narrowing\" function to retain coils while also partially redirecting flow away from the aneurysm. Both Y stenting and FDs result in only partial coverage of the neck of the bifurcation, impede access to side branches, and, in the case of FDs, reduce the flow into contralateral side branches with the risk of hypoperfusion [9]. Another method for treating intracranial bifurcation aneurysms is with intrasaccum devices (e.g., WEB, Woven EndoBridge, Microvention, Aliso Viejo, CA, USA) [10]. While these devices disrupt intrasaccum flow to initiate thrombus, they do not reduce flow entering the neck of the aneurysm, a factor that can result in shape modification and compaction of intrasaccum contents [11; 12]. eCLIPs (eCLIPsTM, eVasc Neurovascular, Vancouver, BC, Canada) implants are semicircumferential but non-tubular devices with two sections (Table 1): an anchor segment that is placed into one branch, and a leaf segment that crosses the neck of a bifurcation from one side branch to the other (contralateral) to effect coil retention without blocking the flow to the branches [13]. [PERSON] et al. [14] demonstrated the FD characteristics of the eCLIPs leaf section using computational flow dynamics (CFD). They compared the eCLIPs flow diversion against that of clinically accepted FDs, e.g., the Pipeline Embolization Device (PED, Medtronic, Minneapolis, MN, USA), in a bifurcation geometry. They concluded that eCLIPs is 10% less effective than braided mesh in reducing the inflow and the jet into the aneurysm, possibly due to the larger pore size, but does not impede flow into either side branch. Different numerical and experimental hemodynamic studies had shown that reducing the pore size leads to a greater flow diversion effect [15; 16; 17]. Particle image velocimetry (PIV) is an optical measurement technique that has been widely used for in vitro assessment of the intracranial aneurysm hemodynamics and flow diversion effect [16; 17; 18]. The current state of the art in the endovascular treatment of intracranial aneurysms does not offer a universally reliable and safe method to treat bifurcation aneurysms but suggests that FDs in some form may offer a solution.