Shape Memory Polymer-Based Endovascular Devices: Design Criteria and Future Perspective

Abstract

Devices for the endovascular embolization of intracranial aneurysms (ICAs) face limitations related to suboptimal rates of lasting complete occlusion. Incomplete occlusion frequently leads to residual flow within the aneurysm sac, which subsequently causes aneurysm recurrence needing surgical re-operation. An emerging method for improving the rates of complete occlusion both immediately after implant and in the longer run can be the fabrication of patient-specific materials for ICA embolization. Shape memory polymers (SMPs) are materials with great potential for this application, owing to their versatile and tunable shape memory properties that can be tailored to a patient's aneurysm geometry and flow condition. In this review, we first present the state-of-the-art endovascular devices and their limitations in providing long-term complete occlusion. Then, we present methods for the fabrication of SMPs, the most prominent actuation methods for their shape recovery, and the potential of SMPs as endovascular devices for ICA embolization. Although SMPs are a promising alternative for the patient-specific treatment of ICAs, there are still limitations that need to be addressed for their application as an effective coil-free endovascular therapy.

Keywords: 3D printing; endovascular embolization; intracranial aneurysms; personalized aneurysm treatment; shape memory polymers.

Figure 1

Schematic of the two primary aneurysm treatment methods: (i) surgical clipping (the metal clip not drawn to scale) (left); and (ii) endovascular embolization using Guglielmi detachable coils (GDCs) (right).

Figure 2

Schematic of two potential approaches to fabricate shape memory polymer (SMP)-based endovascular devices for ICA treatment: (a) SMP-coated coils; and (b) SMP coil-free foams with patient-specific geometries.

Figure 3

Schematic of different state-of-the-art endovascular devices for occluding (i) “bifurcation” aneurysms (e.g., GDCs, hydrogel coils, liquid embolic devices, and woven EndoBridge—WEB); and (ii) “side-walled” aneurysms (e.g., flow diverter).

Figure 4

Histological analysis of embolized aneurysms with bare coils vs. SMP-coated coils in a rabbit model. Staining was performed using Masson’s Trichrome at different timepoints (30, 60 and 180 days post-implantation) in this pre-clinical study. Scale bars$=1$ mm. (Images were adapted from Herting et al. [19] with permission of John Wiley & Sons).

Figure 5

Methods for the fabrication of porous SMPs: CO${}_2$ gas foaming (left), particle leaching (middle), and 3D printing (right).

Figure 6

Photos and thermography images (insets) of the shape recovery process of a CNT-infiltrated polyurethane SMP foam developed in our lab. This experiment was performed in an in-vitro bifurcation aneurysm model that mimics a patient-specific aneurysm geometry.