Novel magnetic technologies for endovascular applications

Traditional surgical procedures to treat coronary diseases are highly invasive and carry with them a high risk of morbidity. In contrast, minimally invasive procedures using guidewires and catheters have been developed to access deep and remote anatomic regions of the body. Such procedures can be as efficient as traditional surgical methods while being safer. Therefore, the use of catheters is becoming increasingly common in interventional procedures like stent placement, laser ablation, and diagnostics among others. However, working with traditional catheters and guidewires requires extensive skill and experience on the interventionist's part. Navigation in a tortuous environment is complicated as conventional devices lack active tip steering. With greater operator experience and skills, the success rate of endoluminal catheterization procedures can increase. To offset this need for skill and experience, robotic approaches and steerable catheters are proposed.

Robotic approaches can help in improving catheter navigation offering a more ergonomic system and greater precision to ease bypass of anomalies and prevent tissue damage.  While robotic approaches might achieve good navigation precision and accuracy, they suffer from high cost. There is also a setup time for the system before each procedure and additional staff training is necessary. On the other hand, the use of steerable catheters involves significantly lower cost and setup time while still significantly helping the interventionist with navigation and control. Additionally, soft bodied catheters can provide improved navigation even at acute angles. In light of their ability to easily deform and adapt to the environment, soft materials are particularly attractive when targeting medical applications, thanks to their inherent safety in interacting with body tissues. Among the range of stimuli to actuate soft materials, remote magnetic actuation is uniquely suited for this task due to its safety, controllability, and miniaturization capabilities. Combining the advantages offered by magnetic actuation and soft materials is therefore an attractive option. From the magnetically actuated catheters reported in literature, it was found that instead of embedding discrete permanent magnets in the catheter, it is more beneficial to have uniformly distributed magnetic microparticles throughout their entire body. The use of magnetic microparticles as distributed actuation sources can enable the potential miniaturization of magnetic catheters and guidewires to submillimeter scale, which would be impossible with other techniques. Controlling the direction of magnetization of the magnetic microparticles in the magnetic catheter or other soft magnetic structures might give the liberty to program the way they behave in a magnetic field. A novel fabrication paradigm that can combine and exploit the benefits of 3D printing and control of the structure's magnetization is reported in this thesis. In general, soft magnetic structures having a non-uniform magnetization profile can achieve multimodal locomotion that is helpful to operate in confined spaces. However, incorporating such magnetic anisotropy into their body is not straightforward. Existing methods are either limited in the anisotropic profiles they can achieve or are too cumbersome and time-consuming to produce.

This thesis demonstrates a 3D printing method allowing to incorporate magnetic anisotropy directly into the printed soft structure. This offers at the same time a simple and time-efficient magnetic soft robot prototyping strategy. The proposed process involves orienting the magnetized particles in the magnetic ink used in the 3D printer, by a newly designed electromagnetic coil system acting onto the particles while printing. The resulting structures were extensively characterized to confirm the validity of the process. The extent of orientation was determined to be between 92% and 99%. A few examples of remotely actuated small-scale soft robots that were printed through the developed method are also demonstrated. Just like 3D printing gives the freedom to print a large number of variations in shapes, the proposed method also gives the freedom to incorporate an extensive range of magnetic anisotropy.

A hollow catheter gives the ability to introduce guidewires and other functionalities through it which may be needed during the intervention. As far as the author is aware of, this work shows the first hollow soft magnetic catheter tip for targeting the coronary arteries fabricated using the developed 3D printing method. An External Permanent Magnet (EPM) that can be mounted on a robotic arm was used to actuate the fabricated magnetic catheter. The catheter was characterized in detail in terms of its bending hysteresis, bending forces, and dynamic response. The catheter showed <10% hysteresis on average and bending forces up to 0.8N. The magnetic catheter was then successfully guided (5 out of 6 times) using the EPM mounted on a robotic arm inside a realistic aortic phantom while being inserted using a robotic catheter driver. Each such successful trial took around 100s.

Further, this concept was extended to a robot-assisted catheterization platform. In conjunction with a larger base catheter that can anchor itself in the aorta and guide the magnetic catheters, 100% (10 trials) successful catheterization was achieved from certain anchor points in the aorta. Regions from where it was not feasible to perform successful catheterizations were also identified. The developed procedure in this thesis could be extended to other endovascular applications like ureteroscopy or cerebrovascular intervention as well.

While catheters consisting of discrete permanent magnets are not well suited for navigating through narrow lumen of the body, they are very well suited for filtering and retrieving therapeutic magnetic nanoparticles from the bloodstream. A nanoparticle retrieval catheter concept containing a magnetic module is presented. A high capture efficiency (50% up to 92%) for a varied particle size (10nm to 500nm) was numerically calculated. The proposed method could be a solution to the oft undiscussed problem of using magnetic micro- and nanoparticles for therapy inside the body.

Overall, it is believed that the field of novel magnetic technologies for endovascular applications will be advanced one step forward through this thesis.