Pre-clinical development of a self-propelled biliary microrobot
Antoine Martin (Le Kremlin-Bicêtre, France), Quentin François (Paris, France), Victor Braga (Paris, France), Johan Quispe (Paris, France), Ali Oulmas (Paris, France), Ismaël Trouiller (Paris, France), Bertrand Duplat (Paris, France), Isabelle Boytchev (Le Kremlin-Bicêtre, France), Sinan Haliyo (Paris, France)
Background

ERCP is the reference technique for accessing the bile ducts. Its main limitation is the indirect visualization of the bile ducts in two-dimensional fluoroscopy. As a result, access to a specific biliary area is in most cases blind, using a soft-tipped guide wire. A cholangioscope introduced via ERCP provides a direct view of the bile ducts, with navigation under visual control. It is currently limited by its diameter (3.5mm) and angulation (30°), making exploration of the distal bile ducts difficult. The aim of this study was to design a biliary microrobot model with its own mechanical propulsion. 

Method

An in vitro test bench was developed. The viscosity and pH of eight patient bile samples heated to 37°C were measured: two purulent, two hematic and four healthy. A single-convergence bile duct phantom was developed using 3D printing (FormLabs Elastic resin), respecting the plastic properties of bile ducts. Several microrobot models with motorized rotary propulsion were developed and tested on this in vitro model. The rotary motors used were industrially manufactured. The structure and propeller of the microrobot were 3D printed. An external wired controller was used to operate the microrobot.

Results

The average bile pH was 7, and the fluidic behavior of healthy or hematic bile at 37°C was close to that of water at room temperature. Between October 2023 and September 2024, eight microrobot models were developed and tested in the 3D-printed bile duct phantom immersed in a vat of water. The diameter of the last microrobot model was 3mm, its overall length 14.5mm. The total length of the biliary phantom was 12cm, with a diameter of 8mm. A magnetic activation navigation system was integrated into the head of this latest version, enabling a maximum angulation of 45°. It was possible to move the microrobot forwards and backwards by self-propulsion, with an average speed of 7 mm/s. The microrobot was also able to move on an upward inclined plane.

Conclusion

The biliary microrobot was able to navigate in all 3 planes of space in this in vitro bile duct model. The next steps are to miniaturize and optimize the device's navigation, before integrating an operating channel and camera, and carrying out tests on animals. 

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