Enhancing Physical Interactions of Untethered Miniature Magnetic Robots for Biomedical Applications
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2024
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Doctoral Thesis
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Abstract
Untethered miniature robots can harvest energy made available to them remotely, such as light, magnetic or acoustic energy, and convert them to other forms of energy, such as mechanical deformation. Magnetic actuation, in particular, has emerged as a promising method for robots in biomedical applications due to the magnetic field’s ability to penetrate human tissues safely. Advancements in the field have enabled these robots to navigate precisely to a target site on land or in fluid-filled confined environments, while carrying payloads, such as drugs, genes, hydrogel structures and even patient cells for therapeutic applications. Despite these advances, the use of miniature magnetic robots in clinical applications is severely limited because only one form of functionality, namely the shape-morphing capability of the robot for locomotion or cargo delivery to tissue surfaces, is typically utilised. This work aims to overcome this lack of diversity in functionalities by proposing several methods which can be readily adopted on existing untethered miniature magnetic robots, actively triggered on demand and independently controlled, to enable advanced physical interactions between the robot and tissue. Such capabilities pave the way for the development of next-generation magnetic miniature soft robots capable of addressing realworld clinical applications.
To achieve this, a two-pronged strategy is adopted. Firstly, the “fully soft” assumption of such robots is challenged. Currently, there exists an assumption that the robots have to be fully soft for enhanced safety, or for continuum motion to be achieved. However, this imposes a significant, and sometimes unnecessary, constraint on these robots – especially since rigid materials exhibit certain material properties that are desirable and superior to soft materials. Secondly, high-frequency magnetic fields are utilised, to expand the number of control, and hence design parameters. Although the profile and magnitude of magnetic fields have been extensively exploited to achieve different locomotion modes, the effects of changing the frequency of the magnetic fields have been less explored. At such frequencies, specifically in the kHz regime, the magnetic fields still remain safe for human exposure, whilst being too high for actuation, thereby avoiding any interference with the robot’s locomotion. As such, by relaxing these constraints, coupled with novel mechanical designs and implementation, advanced functionalities can be achieved to further enhance the capabilities of these robots. Three new functionalities for these robots are developed based on this strategy.
In the first part, we demonstrate a significant improvement in the force capabilities of these robots. Specifically, we propose a wireless spring-preloaded barbed needle release mechanism, which can provide up to 1.6 N of force to drive a barbed needle into soft tissues to allow robust on-demand anchoring on three-dimensional (3D) surfaces. The mechanism is wirelessly triggered using radiofrequency (RF) remote heating and can be easily integrated into existing untethered soft robotic platforms without sacrificing their mobility. Design guidelines aimed at maximising anchoring over the range of the most biological tissues (kPa range) and extending the operating depth of the device inside the body (up to 75%) are also presented. Enabled by these advances, we achieve robust anchoring on a variety of ex vivo tissues and demonstrate the usage of such a device when integrated with existing soft robotic platforms and medical imaging. Moreover, by simply changing the needle, additional functionalities such as controlled detachment and sub-surface drug delivery into 3D cancer spheroids are also demonstrated.
In the second part, we propose a design that enables substantial heat generation in an untethered miniature robotic system. Such a capability had not been developed because there is an inherent trade-off between effective remote heating at long distances and compliance. Specifically, rigid metallic body parts should be used for remote heating to ensure that the electrical conductivity and geometrical properties remain constant and stable for enhanced and reliable remote heating via Joule heating, yet the use of rigid materials inherently restricts and compromises the compliance of such untethered soft robots. The pangolin-inspired design introduced in this work allows users to achieve significant heating (ΔT > 70 °C) at large distances (> 5 cm) within a short period of time (< 30 s), thereby realising on-demand localised heating in tandem with shape-morphing capabilities. Endowed with this new capability, advanced robotic functionalities, such as selective cargo release, in situ demagnetisation, hyperthermia and mitigation of bleeding, are demonstrated on tissue phantoms and ex vivo tissues.
In the final part, we demonstrate how electricity can be directly harvested and utilised in an untethered miniature robotic system. Electricity is one of the most widely used and versatile form of energy but there exist few miniature robotic systems which possess such a functionality. The size and weight constraints makes it difficult to implement conventional electronics such as batteries on these robots. Even in cases where such systems are demonstrated, the proposed solutions suffered from various practical and feasibility issues. These limitations include, but are not limited to, a short working range or a requiring a specific set of environmental conditions, which greatly restricts their use to niche applications. To address this, we propose an untethered miniature robotic system which generates electricity by exploiting high-frequency magnetic fields. Magnetic fields can safely penetrate the body and can as such, allow power to be wirelessly transmitted across distances into the body. This in turn unlocks a wide range of potential applications for these robotic systems. While there has been extensive research into wireless power transfer systems, none of them had focused on their applicability to miniature untethered robotic systems.
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ETH Zurich
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Wireless medical robots; miniature robotics
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09726 - Sitti, Metin (ehemalig) / Sitti, Metin (former)