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

The study addresses how to enable effective, controllable, and deep-tissue remote heating in untethered miniature magnetic soft robots without sacrificing their compliance and shape-morphing capabilities. While magnetic actuation is advantageous for biomedical robots due to safe tissue penetration and precise navigation, most miniature robots rely primarily on mechanical interactions for function. Many medical procedures require heat (e.g., devitalization, coagulation, cutting), yet existing remote heating modalities pose challenges: thermochemical methods are invasive and poorly localized; focused ultrasound is limited by acoustic windows and high-density structures or air interfaces; photothermal methods have shallow penetration (~1–2 cm). Magnetic methods can provide non-invasive, deep, and targeted heating. Remote magnetic/RF heating mechanisms include Joule heating and magnetic hysteresis; hysteresis generally produces far less heat than Joule heating at distance, and RF resonant circuits can be unreliable in heterogeneous biological environments due to coupling sensitivity. Reliable Joule heating requires rigid, conductive metallic elements, but adding rigid parts compromises soft robot compliance. Inspired by pangolin overlapping keratin scales that preserve flexibility, the authors propose a bi-layered, overlapping metallic-scale design that preserves mechanical compliance while enabling efficient RF-induced Joule heating at clinically relevant distances.

Prior work has advanced magnetic soft robots in navigation and payload delivery, including drugs, genes, hydrogels, and cells. Temperature-responsive locomotion, anchoring on complex surfaces, and multifunctional microrobots have been shown. Remote heating in small robots has often used magnetic nanoparticles for hysteresis-based hyperthermia or to retain softness, but this dissipates substantially less heat than Joule heating at distances relevant for in vivo use. Photothermal and focused ultrasound techniques offer precise heating but are limited by penetration depth or acoustic windows. RF resonant circuits have been explored but exhibit coupling inefficiencies and environmental sensitivity that limit robust control in heterogeneous tissues. Biological inspiration from overlapping scales (e.g., pangolins) suggests a strategy to integrate rigid elements without sacrificing flexibility. This work builds on RF induction heating literature on thin plates and adds an integrated analysis of geometry, conductivity, thickness, and scale overlap effects on heating efficiency, coupled with soft-robot mechanical performance.

Design: A pangolin-inspired bi-layered soft robot with overlapping metallic scales bonded to a magnetized PDMS (mPDMS) substrate was developed to enable RF-induced Joule heating while preserving flexibility. RF heating used a 338 kHz field produced by a coil positioned typically 3–5 cm away and driven up to 621.6 A, corresponding to ~34.6 kA m⁻¹ magnetic field intensity. Optimization framework: The heating model balances input power and heat losses: P_in = ρ V c_p (ΔT/Δt) + H_L. Analytical and COMSOL simulations evaluated how electrical conductivity (σ), scale thickness (w), length (L), and material properties influence induced current density, magnetic flux penetration (skin depth δ = √(1/(σ μπ f))), heat generation, and losses. Characterization of heating: 1 cm² samples (various metals and thicknesses) were fabricated (laser cut, cleaned, bonded to glass with silicone adhesive, topped with Kapton to normalize emissivity) and placed 5 cm from the RF coil. Infrared thermography tracked temperature rise over 60 s at 621.6 A. Comparisons were made among Joule vs hysteresis-based configurations (aluminium, eGaIn surface, iron oxide, mPDMS with nanoparticles) and across material conductivities, thicknesses (10–250 µm), scale lengths, and percentage overlaps. Mechanical testing: Three-point bending (ASTM D7264 adapted) of 20×10×0.2–0.25 mm mPDMS samples with/without scales (varied length, thickness, and overlap) quantified stress-strain and flexural chord modulus E_FC in different configurations (intrados/extrados; longitudinal/transverse). Bending deflection under uniform magnetic fields was assessed by clamping one end and measuring tip angles at different field strengths (23–55 mT). Pull-out force tests quantified adhesion between aluminium scales and PDMS. Functional demonstrations: 1) In situ demagnetization using non-overlapping 50 µm Al scales to raise mPDMS above particle Curie temperature (~219 °C), erasing a pre-programmed magnetization profile, followed by remagnetization at 1.8 T to change locomotion mode (rolling to tumbling). 2) Selective cargo release: beeswax-secured cargos over Al scales of different thicknesses (50 vs 80 µm) to exploit different heating rates and melt points (61–65 °C). 3) Medical-relevant ex vivo demos: a robot packed in a standard size O gelatin capsule for oral delivery; mitigation of bleeding in porcine stomach with simulated capillary bleeding (1 µL s⁻¹) using a 3 s RF pulse; ultrasound-guided operation in porcine small intestine; hyperthermia on HT-29 tumor spheroids contacting heated scales at 60 °C for 5–15 min. Fabrication: Scales produced by partial-depth laser cutting and manual assembly onto tape, then bonded to pre-magnetized mPDMS with PDMS and cured at 90 °C; overlapping percentage controlled by engraved guides. Robots fabricated from mPDMS sheets (NdFeB microparticles in PDMS), magnetized (1.8 T), then bonded with metal scales. Simulations used COMSOL 6.0 Magnetic Fields and Heat Transfer modules with convection and radiation boundary conditions. Biocompatibility and stability: Cell viability assays with BJ fibroblasts on 1 cm², 100 µm Al and with Al powders at varying concentrations; stability tests in simulated gastric/intestinal fluids and DMEM by absorbance at 300 nm.

- RF Joule heating superiority: At 5 cm from the coil, hysteresis-based samples (iron oxide and mPDMS) showed no measurable temperature rise, whereas Joule-heated samples (100 µm aluminium, eGaIn surface) did; 100 µm aluminium produced the largest ΔT at distance. - Long-range, rapid heating with compliance: Overlapping scale design enables ΔT > 70 °C within <30 s at distances >5 cm while maintaining soft-body bending compliance. - Optimal conductivity and thickness: For 100 µm scales, optimal σ ≈ 5×10⁶ S m⁻¹ (close to tin). As thickness increases 50→250 µm, optimal σ shifts lower (1×10⁷→2×10⁶ S m⁻¹). For aluminium with Δ² ≈ 7.1 mm², optimal thickness for a 10 mm scale was <50 µm; best experimental performance at 20 µm, with reduced rise time as thickness decreased (10–100 µm range). - Scale length and overlap trade-offs: Dividing a 1 cm² plate into smaller scales (fixed total area) reduced maximum temperature by up to 75% due to reduced magnetic flux per scale. Overlapping scales recover performance: 50% overlap increased final temperature after 60 s by up to 67% vs non-overlapping of the same total area, making 100 µm-thick 50% overlap perform like a 60 µm uncut plate. Heating repeatability varied <5% over 30 cycles; autoclaving degraded performance <5%. - Mechanical compliance: Introducing scales reduced peak bending stress by ~87.9% (8.3→1.0 MPa); decreasing scale length 5→1 mm lowered to 0.68 MPa, approaching mPDMS (0.18 MPa). Overlap further reduced stress to 0.15 MPa at 75% overlap, matching mPDMS. Flexural chord modulus E_FC decreased with smaller/overlapped scales and became dominated by geometry rather than material; deflection comparable to mPDMS for anti-clockwise bending; clockwise deflection limited by plate contact, improved with thinner scales. - Robust adhesion: Pull-out force of Al scales from PDMS ~800 mN, indicating stable attachment during actuation. - In situ demagnetization: Heating above Curie temperature (~219 °C) erased magnetization profile; robot became unresponsive until re-magnetized at 1.8 T, enabling reprogrammable locomotion (rolling→tumbling). - Selective cargo release: Using 80 µm vs 50 µm Al scales under beeswax-secured cargos yielded ~1 s earlier melting on the thicker, faster-heating scale, enabling selective release. - Ex vivo hemostasis: In porcine stomach with simulated capillary bleeding (1 µL s⁻¹), a 3 s RF pulse stopped bleeding after the robot navigated to the site. - Hyperthermia: HT-29 tumor spheroids in contact with heated scales at ~60 °C were destroyed after 5 min exposure; viability decreased with exposure time (5–15 min). - Environmental sensitivity: Heating performance more sensitive to magnetic flux distance than to convective losses; to achieve similar reduction in final temperature after 60 s, distance increased 3× vs convective heat transfer coefficient increased 1000×.

The study resolves the central challenge of integrating efficient deep-tissue heating with soft-robot compliance by adopting a pangolin-inspired overlapping metallic-scale architecture. This design leverages rigid, high-conductivity metals to achieve strong, controllable RF Joule heating at clinically relevant distances while preserving the deformability required for magnetic actuation and shape programming. The analytical-simulation framework clarifies how conductivity, skin depth, thickness, and scale length interact to set heating efficiency, while overlapping compensates for the heating losses caused by subdividing plates for compliance. Mechanical tests confirm that appropriate scale sizing and high overlap restore compliance near that of the underlying mPDMS, enabling practical locomotion even with the added metal. The ability to heat rapidly and locally unlocks new robotic functions, including in situ demagnetization for reprogrammable locomotion and selective cargo release using differential heating, as well as direct therapeutic actions like hemostasis and hyperthermia in ex vivo GI models. Sensitivity analyses indicate that magnetic flux (distance) dominates heating performance relative to convective losses, informing deployment and control strategies. Overall, the results demonstrate a viable pathway to multifunctional, minimally invasive soft robots that can actuate and deliver therapeutic heat deep within the body.

This work introduces a pangolin-inspired, overlapping metallic-scale design enabling untethered magnetic soft robots to perform rapid, localized RF Joule heating at distances >5 cm (ΔT>70 °C in <30 s) without sacrificing mechanical compliance. The authors provide design guidelines linking electrical conductivity, thickness, scale length, and overlap to heating and bending performance, validated through experiments and simulations. The integrated robots demonstrate advanced functions including in situ demagnetization, selective cargo release, and ex vivo hemostasis and hyperthermia, highlighting their biomedical potential. Future directions include: integrating low- and high-frequency magnetic systems into a single setup for seamless actuation and in situ reprogramming; enhancing RF heating efficiency via proximity effects or sidewall insulation (e.g., Parylene C); establishing safety limits and optimizing current levels through in vivo studies; and improving biocompatibility/biodegradability and bonding (e.g., Dermabond, alternative substrates such as FePt or hydrogels). These advances could enable a new class of minimally invasive, multifunctional medical robots for hard-to-reach sites.

- Trade-off between heating and compliance: Subdividing plates to improve flexibility reduces heating efficiency; overlap mitigates but does not eliminate the fundamental flux-per-scale limitation. - Directional deflection constraints: Bending in the direction of the scales is limited by plate contact; thinner scales alleviate but do not fully remove the constraint. - Environmental sensitivity: Heating performance is highly sensitive to coil distance (magnetic flux) compared to convective losses, constraining operational range; high currents (e.g., 621.6 A at 338 kHz) may be needed. - Adhesive and cargo release: Beeswax re-solidifies upon cooling and is mainly suitable for GI applications; identification of thermally degradable, biocompatible adhesives is needed for broader clinical use. - Biocompatibility and safety: Aluminium and bonding materials require further biocompatibility assessments for long-term in vivo use; in vivo safety limits for RF exposure and tissue heating must be established. - Heterogeneous in vivo conditions: Variability in tissue properties, presence of gas/liquid interfaces, and residual GI contents can affect locomotion and heating; RF resonant methods were avoided due to coupling variability. - Thermal uniformity: Non-overlapping configurations used for even heating (e.g., demagnetization) may reduce compliance; overlapping for compliance may introduce non-uniform heating across layers if not carefully designed. - Potential tissue interaction: Sharp scale edges could risk tissue puncture; adding fillets reduces risk without significant heating penalty, but requires thorough validation.