Soft robotic steerable microcatheter for the endovascular treatment of cerebral disorders

Abstract

Catheters used for endovascular navigation in interventional procedures lack dexterity at the distal tip. Neurointerventionists, in particular, encounter challenges in up to 25% of aneurysm cases largely due to the inability to steer and navigate the tip of the microcatheters through tortuous vasculature to access aneurysms. We overcome this problem with submillimeter diameter, hydraulically actuated hyperelastic polymer devices at the distal tip of microcatheters to enable active steerability. Controlled by hand, the devices offer complete 3D orientation of the tip. Using saline as a working fluid, we demonstrate guidewire-free navigation, access, and coil deployment in vivo, offering safety, ease of use, and design flexibility absent in other approaches to endovascular intervention. We demonstrate the ability of our device to navigate through vessels and to deliver embolization coils to the cerebral vessels in a live porcine model. This indicates the potential for microhydraulic soft robotics to solve difficult access and treatment problems in endovascular intervention.

Fig. 1.. Endovascular procedures that would benefit from sub-millimeter diameter steerable catheters.

Aneurysm embolization is the focus of this paper, but steerable sub-millimeter catheters are broadly useful across the human vasculature.

Fig. 2.. Engineering a steerable catheter.

a) Illustration of the cerebral arterial structures and the average diameter of the middle cerebral artery. b) Design and working principle of the hydraulically driven tip: when a channel is inflated (indicated in red), the tip deflects in the opposite direction. c) Fabrication of the catheter is a multi-step molding process. d) Plan and cross-section images of the as-fabricated steerable tip tubing, with a length of 15 mm, an outer diameter of 900 μm, an inner diameter of 400 μm, and four 50-μm channels in the tube wall. The relatively rigid coating upon the softer interior material that forms the hydraulically-driven tip is visible via scanning electron microscope cross-sectional images and is approximately 25 μm thick.

Fig. 3.. Design parameters of the steerable tip, and the consequent curvature as a function of input pressure.

Nonlinear finite deformable structural mechanics analysis was used to compute the relationship between the tip curvature and input pressure for three values of a) the channel radius, R, and the channels’ radial position, R_P. The b,c) curvature of the tip as a function of the input pressure produces substantially different computational results for the b) radius R with the radial position R_P at R_P = 325 μm, and c) radial channel position R_P for a channel radius R = 50 μm. The computed results are similar to (solid line in b,c) experimental results obtained with R = 50 μm and R_P = 325 μm. The experimental results are averaged for four channels; blue lines indicate error bars (standard deviation).

Fig. 4.. The hydraulic channels’ position and radius affects the relationship between the tip’s radial expansion and the input pressure.

Computed radial expansion of the steerable tip at its outer surface as a function of a) channel radius R with R_P = 325 μm, and b) channel radial position R_P with channel radius R = 50 μm, compared to experimental results for a tip with R = 50 μm and R_P = 325 μm. The experimental results are averaged for four channels; blue lines indicate error bars (standard deviation).

Fig. 5.. Fabrication of an ex vivo silicone model from anonymized patient data.

a) Axial, b) sagittal, and c) coronal views of computed tomography angiogram images with vascular segmentation for a single slice shown in green. This process produces a d) complete vascular model from the femoral artery to the aortic arch, including principal vascular branches, in particular the carotid arteries to the cerebrovasculature. Through 3D printing and lost ABS casting as detailed in the text, e) a functional, fluid-tight vascular model may be produced. (d,e) Scale bar = 25 cm.

Fig. 6.. Animal study.

Illustration of porcine (3D Pig anatomy illustration by Biosphera) a) arteries and b) cerebral arteries for reference. (c) Anteroposterior (AP) diagnostic subtraction angiogram of the right common carotid artery (CCA). (d) Spot AP roadmap fluoroscopic image of the right CCA. enlarged in (e). (f1) Position one, showing the steerable microcatheter prior to introduction of a hydraulic load. (f2) Position two, showing the micro-catheter after hydraulically steering the tip to access the ascending pharyngeal artery from the external carotid artery. (f3,f4) Position three, showing the hydraulic steering of the catheter tip to access the parotid artery from the ascending pharyngeal artery. (f5) Coils deployed in the parotid artery from the stabilized catheter tip.

Fig. 7. Steerable catheter tip fabrication process.

a) Circular glass rods (diameter 900 μm) were placed next to 450-μm-thick rectangular glass sheets and b) bonded to a microscope glass slide using UV epoxy. c) Smooth-Cast 327 was poured over the assembly and, upon curing, d) de-bonded to obtain one-half of the complete mold. The mold is formed by e,f) clamping two such halves facing each other with a custom glass capillary tube (outer diameter of 900 μm, inner diameter of 400 μm, four 50-μm channels in the wall) to provide alignment. Relatively stiff Dragon-Skin 10 SLOW is first injected into the mold and then blown out with air to produce a thin layer g) on the inside of this mold. A h) 400-μm glass capillary tube is likewise i) spin-coated with the same material and j) introduced into the mold with k) four wires that represent the fluid passages for tip steering. The vacant regions are filled with Dragon-Skin 10 SLOW mixed with Hexane. The result of this process is a l,m) multilayer soft polymer structure.

Fig. 8. A thin coating of relatively stiff polymer reduces radial expansion.

Radial expansion of the steerable tip with and without a relatively stiff 25-μm-thick layer of Dragon-Skin 10 SLOW upon Dragon-Skin-hexane to form the bulk of the steerable tip structure. The steerable tip fabricated with concentric layers of Dragon-Skin encompassed by Dragon-Skin mixed with hexane exhibits much less radial expansion (5 μm) relative to a steerable tip without the coating (20 μm).