Smart Steerable Biomedical Catheter by Femtosecond Laser Micropatterning of Polypyrrole Actuator and 3D Fibre-Optic Shape Sensor
Keywords: Active steerable catheter
Advanced laser processing
Bragg grating waveguide
Cladding photonics
Fast-actuating polypyrrole actuator
Fibre-optic shape sensor

Femtosecond laser processing was applied to enhance microactuators and enable optical sensing and thereby advance the development of smart steerable biomedical catheters. The advantages of ultrafast direct laser ablation were exploited to enhance the strain bending response of polypyrrole (PPy), a conjugated conducting polymer, by surface texturing hexagonal arrays of micron-sized holes. These microstructures overcame the ion diffusion rate-limited actuation mechanism in thick actuators that was necessary for maximum catheter bending force. Secondly, three-dimensional (3D) femtosecond direct laser writing of optical components in a single coreless optical fibre was developed to provide a temperature-compensated fibre-optic 3D shape sensor. An appropriate design is presented for in vivo catheter shape sensing to provide positional and azimuthal feedback without fluoroscopy. Polypyrrole exhibits large electrochemical strain with low driving voltage that is suitable for steerable catheter designs. However, a major limitation towards application development is PPy's slow speed of electrochemical actuation, which is proportional to the longest ion diffusion path to the polymer's inner volume. Ablative laser microstructuring was investigated as means to shorten such diffusion path, and, in turn, increase the actuation rate. Numerous combinations of design variables including the hole diameter, depth, pitch, and pattern were examined and applied to finite element (FE) techniques to model both the electron transport and the ion diffusion rates for various microstructured PPy surfaces. Optimization of the microhole array structural design investigated the ion diffusion response time while also retaining stiffness for maximum bending. PPy films were bonded to polyimide films to form bi-layer bending actuators. The bending radius and response time of both textured and non-textured actuators were recorded and compared to profiles predicted by the FE model. The results were used to infer the prospects towards improving previous generation PPy-actuated catheters. Fibre shape sensing was demonstrated with laser-written optical and Bragg grating waveguides that were distributed axially and radially inside the coreless fibre. Efficient light coupling between the laser-written optical circuit elements and a standard single mode fibre (SMF) was obtained for the first time by 3D laser writing of a 13 directional coupler to meet with the core waveguide in the fusion-spliced SMF. Simultaneous interrogation of nine Bragg gratings, distributed along three laterally offset waveguides, was presented through a single waveguide port and fused to a SMF for spectral interrogation at 1-kHz sampling rate to follow the Bragg wavelength shifts in real-time. In this way, one could infer the shape and temperature profile unambiguously along the fibre length. The sensor was inserted into a commercial steerable catheter and tested in vitro using a flow phantom, followed by an animal trial to determine the catheters 3D orientation in vivo with the objective to minimize X-ray dosage in minimally invasive surgical procedures.