Pangolin-inspired untethered magnetic robot for on-demand biomedical heating applications

Abstract

Untethered magnetic miniature soft robots capable of accessing hard-to-reach regions can enable safe, disruptive, and minimally invasive medical procedures. However, the soft body limits the integration of non-magnetic external stimuli sources on the robot, thereby restricting the functionalities of such robots. One such functionality is localised heat generation, which requires solid metallic materials for increased efficiency. Yet, using these materials compromises the compliance and safety of using soft robots. To overcome these competing requirements, we propose a pangolin-inspired bi-layered soft robot design. We show that the reported design achieves heating > 70 °C at large distances > 5 cm within a short period of time <30 s, allowing users to realise on-demand localised heating in tandem with shape-morphing capabilities. We demonstrate advanced robotic functionalities, such as selective cargo release, in situ demagnetisation, hyperthermia and mitigation of bleeding, on tissue phantoms and ex vivo tissues.

Fig. 1. Proposed pangolin-inspired RF heating mechanism for untethered magnetic robots.

A Conceptual illustration of the pangolin-inspired robot operating in the small intestine. The robot is actuated with a low frequency magnetic field to the target location. Application of a high frequency magnetic field results in Joule heating of the metal plates. The heat energy can then be used to interact with the environment. Inset on the right shows potential medical conditions in which a miniature untethered magnetic robot with heating capabilities would have utility. Figure created with biorender.com. B Armour on pangolins consists of individual overlapping hard keratin scales attached to the body. This design allows for rigid structures to be attached for protection without compromising on locomotion. Scaled robot inspired by this overlapping design is shown on the right. Images of pangolins used under licence from Shutterstock. C Fabrication procedure. (I) The metal of correct thickness and material is selected. (II) The sheet is laser cut to form the scaled patterns. (III) The metal arrays are detached and assembled on a tape. (IV) The assembled structures on the tape are bonded to the magnetic PDMS with PDMS before the tape is removed.

Fig. 2. Characterisation of heating performance.

A Schematic of experimental setup. B Identified parameters affecting Joule heating performance. C Infrared camera images tracking the temperature change of a 100 µm-thick aluminium scale over 60 s. D Simulated temperature at t = 60 s of the metal scales with varying electrical conductivities and thicknesses. E Temperature change of 100 µm-thick scales made from different materials over time (n = 6). Source data are provided as a Source Data file. F Simulated graphs for the selection of geometrical and material properties for optimal heating at different scale lengths. $\frac{1}{\delta }$.on the y-axis represents the inverse of the skin depth. G Temperature change of aluminium scales with different thicknesses over time (n = 6). Source data are provided as a Source Data file. H Temperature change of 100 µm-thick aluminium scales with identical areas but composed of different scale lengths over time (n = 6). Source data are provided as a Source Data file. I Temperature change of 100 µm-thick aluminium scales with different percentage overlap over time (n = 6). Source data are provided as a Source Data file.

Fig. 3. Characterisation of mechanical performance.

A Schematic of experimental setup. Figure created with biorender.com. B Stress-strain curve for a 20 × 10 × 0.25 mm sample with different scale lengths (n = 3). Source data are provided as a Source Data file. C Stress-strain curve for a 20 × 10 × 0.2 mm sample bonded to 0.05 mm aluminium with different percentage overlap (n = 3). Source data are provided as a Source Data file. D Comparison of the flexural chord modulus of elasticity (E_FC) for different materials and thicknesses (n = 3). Source data are provided as a Source Data file. E Stress-strain curve for a 20 × 10 × 0.2 mm sample bonded to 0.05 mm aluminium at 50% overlap with different test configurations (n = 3). Source data are provided as a Source Data file. F Deflection of a 20 × 10 × 0.2 mm sample bonded to 0.1 mm Al at 50% overlap at different applied external magnetic fields. Inset shows the magnetisation profile of the robot. G Deflection angles for a 20 × 10 × 0.25 mm sample with different scale lengths (n = 3). Source data are provided as a Source Data file. H Deflection angles for a 20 × 10 × 0.2 mm sample bonded to 0.05 mm aluminium with different percentage overlap (n = 3). Source data are provided as a Source Data file. I Comparison of the deflection angles for different materials and thicknesses. Source data are provided as a Source Data file.

Fig. 4. Enhanced functionalities of untethered miniature robots.

A Schematic of the untethered magnetic robot which can perform in situ demagnetisation to switch the locomotion modes from Mode 1 – rolling to Mode 2 – tumbling. Inset shows the magnetisation profile and the response of the robot to an externally applied magnetic field. Figure created with biorender.com. B Deployment of the robot in a stomach phantom.

Fig. 5. Enhanced functionalities of untethered miniature robots.

A Schematic of the untethered magnetic robot which can perform selective cargo release. The selective cargo release is enabled by exploiting the different heating rates of aluminium of different thicknesses. It can also be enabled by using different materials. Figure created with biorender.com. B Deployment of the robot in a tissue phantom.

Fig. 6. Ex vivo demonstration directly utilising heat energy to mitigate blood loss.

A A 20 × 10 × 0.2 mm robot with 50 µm aluminium scales at 50% overlap inside a standard size “0” gelatine capsule (21.2 × 7.3 mm). B The integrated robot moves to the target location inside an ex vivo porcine stomach. Upon application of a 3 s RF pulse (frames circled in red), no more bleeding at the site was observed.

Fig. 7. Ex vivo demonstration directly utilising heat energy for hyperthermia.

A Ultrasound guided robot operating inside an ex vivo porcine small intestine with a simulated tumour. Small intestine is filled with DI water. (B) Representative fluorescence images of RF exposed HT-29 tumour spheroids stained with calcein-AM/ethidium homodimer−1 after 24 h of incubation. C Viability of HT-29 tumour spheroids after different durations of RF exposure (n = 3). Three spheroids were independently tested once for the experiments. Source data are provided as a Source Data file.