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

Miniature untethered robots, actuated by remote energy sources like magnetic fields, offer significant potential for biomedical applications. Their ability to navigate precisely to target sites and carry payloads (drugs, genes, hydrogels, cells) for therapeutic purposes is well-established. However, current applications are primarily limited to mechanical interactions with the environment. Many biomedical procedures require other forms of interaction, especially localized heat generation for tasks like devitalization, coagulation, and cutting. Various remote heating methods exist, including thermochemical, acoustic, photothermal, and magnetic methods. Thermochemical methods, while precise, are invasive and lack localized heat control. Focused ultrasound is non-invasive but limited by its inability to penetrate high-density materials or air-liquid interfaces. Photothermal methods also offer precision but are limited to shallow depths. Magnetic methods are best suited for controllable, on-demand, non-invasive, and targeted remote heating deep within the body. Remote heating using alternating magnetic or radio-frequency (RF) fields can be achieved through Joule heating or hysteresis losses. While hysteresis losses have been studied extensively, they generate significantly less heat compared to Joule heating. Joule heating requires rigid metallic materials for consistent electrical conductivity and geometrical properties, which compromises the compliance of soft robots. This trade-off between effective remote heating and compliance is addressed by a pangolin-inspired design, where overlapping scales allow for rigid structures without compromising locomotion. This work introduces this overlapping scaled design, enabling simultaneous on-demand thermal functionalities alongside shape-morphing and locomotion capabilities.

The literature extensively explores the use of magnetic fields for actuating miniature robots in biomedical applications. Studies demonstrate precise navigation in various environments, payload delivery, and adaptive locomotion strategies. However, a gap exists in integrating non-magnetic stimuli sources, particularly remote heating, into these robots. While existing methods such as thermochemical, acoustic, photothermal, and magnetic hysteresis offer remote heating, they have limitations in terms of invasiveness, localization, and penetration depth. The use of magnetic hysteresis for heating has been studied in robotic and hyperthermia applications, but it's less efficient than Joule heating. The use of nanoparticles for magnetic hysteresis heating allows for soft materials, but heating is not the primary function. RF resonant circuits are also considered but lack robustness and reliable, controllable heating in complex environments. This paper addresses the need for a method that combines efficient Joule heating with the flexibility of soft robots.

The research involved optimizing Joule heating per scale while minimizing the robot's load. Equation 1 (P_in = ρ V c_p ΔT/Δt + H_L) describes the relationship between input power, material properties, temperature change rate, and heat losses. The optimization focused on electrical conductivity (σ), length (L), and thickness (w) of the metallic scales, as these factors significantly affect input power and heat loss. Characterization experiments and simulations explored the effects of these parameters. The optimal electrical conductivity was determined for different scale thicknesses. Simulations and experiments validated that tin provided the best heating performance for 100 µm thick scales. The study also investigated the influence of scale thickness and length. Thinner scales enhanced heating performance, but excessively thin scales reduced the maximum temperature. The impact of scale overlap was examined, revealing that overlapping scales increased the effective heating volume without significantly increasing surface area and heat loss. This design was inspired by pangolin scales. Three-point bending tests assessed the mechanical deformation performance of the robot with different scale sizes, overlaps, and materials. The flexural compliance was evaluated using stress-strain curves and the flexural chord modulus of elasticity (E_rc). The bending response to external magnetic fields was also characterized. The effect of scale length, overlap, material properties, and thickness on deflection angles was examined. The study involved fabrication of robots using laser-cut metal scales bonded to a magnetic PDMS base. The fabrication process included laser cutting the metal sheet to create scale patterns, separating and assembling the metal arrays, bonding them to the magnetic PDMS, and finally, removing the tape. The heating performance was characterized by measuring temperature changes using an infrared camera. The mechanical properties were characterized using a three-point bending test and by measuring the deflection angles under different magnetic fields. Biological and ex vivo demonstrations included selective cargo release (using beeswax as an adhesive), in situ demagnetization (by heating above the Curie temperature of the magnetic particles), bleeding mitigation in an ex vivo porcine stomach, and hyperthermia treatment of tumour spheroids. Biocompatibility assays were also performed to evaluate the safety of using aluminium scales in the human body.

The pangolin-inspired design successfully achieved significant Joule heating (ΔT > 70 °C) at distances >5 cm within <30 s. Optimization studies determined that for 100 µm thick scales, tin provided optimal heating performance. Thinner scales improved the rate of temperature rise, while overlapping scales enhanced the final temperature by compensating for the decrease in temperature caused by dividing a larger scale into smaller scales. The overlapping design, inspired by pangolin scales, proved advantageous by increasing effective heating volume while minimizing surface area and heat loss. Mechanical characterization showed that the overlapping design significantly improved the robot's compliance, matching that of the unscaled magnetic polymer at 75% overlap. The robot demonstrated advanced functionalities: in situ demagnetization by heating above the Curie temperature of the magnetic particles, allowing for a change in locomotion strategy; and selective cargo release by utilizing the different heating rates of scales with varying thicknesses. Ex vivo experiments showed the robot's ability to mitigate bleeding in a porcine stomach and destroy tumor spheroids through hyperthermia. Biocompatibility assays indicated acceptable short-term biocompatibility of the aluminium scales.

This research successfully addressed the trade-off between efficient remote heating and the inherent flexibility of soft robots. The pangolin-inspired design provided a bio-inspired solution, demonstrating that the overlapping scale design is beneficial for both mechanical compliance and remote heating. The key findings support the feasibility of using this design for minimally invasive procedures requiring localized heating in hard-to-reach areas. The demonstrated functionalities – in situ demagnetization and selective cargo release – significantly enhance the capabilities of untethered magnetic soft robots compared to existing designs. The successful ex vivo demonstrations of bleeding mitigation and hyperthermia suggest the robot's potential clinical applications. The results pave the way for developing novel minimally invasive therapies in the gastrointestinal tract and other hard-to-reach areas.

This work introduced a novel pangolin-inspired design for untethered magnetic soft robots capable of on-demand remote heating. This design successfully overcomes the limitations of existing approaches by combining efficient Joule heating with the desired flexibility. The demonstrated functionalities, including in situ demagnetization, selective cargo release, bleeding mitigation, and hyperthermia, showcase the significant potential of this technology for minimally invasive medical procedures. Future research should focus on integrating actuation and heating into a single magnetic system, enhancing heating efficiency, conducting in vivo safety studies, and exploring biocompatible and biodegradable materials for improved safety and longevity.

The study primarily focused on ex vivo experiments. In vivo studies are crucial to validate the findings and assess long-term biocompatibility and efficacy. The beeswax used for cargo release is biocompatible for gastrointestinal applications but re-solidifies upon cooling; future work should investigate suitable thermally degradable adhesives. While short-term biocompatibility was demonstrated, long-term effects of aluminium exposure on tissues need further investigation. The current design requires separate systems for actuation and heating; future work should explore integrated systems for improved control and reduced complexity.