Focused ultrasound enables selective actuation and Newton-level force output of untethered soft robots

Untethered soft miniature robots offer advantages such as safe human–machine interaction, adaptability, and the ability to access narrow and deep regions, making them attractive for clinical and industrial applications. However, existing untethered actuation methods face a trade-off: while wireless actuation improves accessibility in hard-to-reach environments, it often compromises reliable, accurate, and selective control and limits force output to below the Newton-level required for many minimally invasive procedures. Key challenges include: (1) achieving selective actuation in confined and deep tissue environments so a single robot can perform multiple task-specific functions; and (2) generating large mechanical output forces (up to several Newtons) necessary for tasks such as biopsy, stent deployment, and suturing. Prior selective actuation approaches often require integrating heterogeneous materials or multiple mechanisms responsive to different stimuli, increasing complexity. Spatially non-homogeneous fields (e.g., light, microwaves) can provide selectivity but lack deep tissue penetration and sufficient spatial resolution in vivo. Focused ultrasound is promising due to its tissue penetration (centimeters) and millimeter-level spatial resolution. To address the above, the authors propose a focused ultrasound-controlled phase transition (FUPT) strategy that leverages ultrasound-induced heating of Fe3O4 nanoparticle-doped elastomers and a low-boiling-point liquid to produce controllable pressure changes and strong actuation, aiming to deliver selective, deep-tissue-capable, and forceful actuation for untethered soft robots.

The paper situates its contribution within soft robotics for biomedical applications, highlighting limitations of existing untethered actuators in precision and force. Selective actuation has been explored using heterogeneous material responses to different wavelengths of light or magnetic field patterns, but these approaches increase design complexity and are difficult to apply deep in tissue. Spatially selective stimulation with light or microwaves offers simpler structures but is limited by penetration and resolution in biological media. For high-force output, wireless fluidic/phase-transition actuators that exploit liquid–gas phase change have shown promise, enabling significant deformation and force without onboard power and maintaining MRI compatibility. Focused ultrasound has established biomedical utility for deep penetration and millimeter-scale resolution and has been used in contexts like thermoablation and drug delivery, motivating its adoption here for soft robot actuation. The paper builds on these strands by combining focused ultrasound with nanoparticle-enhanced acoustothermal conversion and phase change to achieve both selectivity and Newton-level force.

Core actuation principle (FUPT): Soft actuators/robots are fabricated from Ecoflex 00–30 elastomer doped with Fe3O4 nanoparticles (NPs) and internally filled with a low-boiling-point liquid (3M Novec 7000; boiling point ~34 °C at 1 atm). A focused ultrasound (US) field generated by a piezoelectric (PZT) transducer array induces localized heating via acoustothermal effects. The temperature rise vaporizes part of the internal liquid, increasing internal pressure and inflating the structure to generate mechanical work (deformation, locomotion, force). Upon cooling, the actuator recovers. Acoustic hardware and focusing: A spherical-array configuration (diameter 90 mm) of 1.7 MHz PZT elements (20 mm diameter, 1.2 mm thickness) oriented toward a common focal point is used. Each element behaves as a piston source; the total field is the superposition of individual complex pressures with appropriate phases. Calibration yields acoustic pressure amplitude ≈ 8.16 kPa/Vpp. Array focusing provides millimeter-scale lateral resolution and ~5 mm axial focal length (longitudinal resolution), with a ~7× pressure amplitude increase over a single transducer. Acoustic impedance and transmission: For efficient energy deposition, actuator acoustic impedance is matched to surrounding media. Transmission coefficients between Fe3O4-doped Ecoflex and water/tissue are computed as ≈98.7% and ≈96.8%, respectively, indicating high acoustic transparency in biofluids and tissues. Nanoparticle-enhanced acoustothermal conversion: Fe3O4 NPs increase effective thermal conductivity and acoustic attenuation, enhancing US-to-heat conversion. Measured heating rates under identical US fields show faster heating with Fe3O4-doped Ecoflex (~−14.9 °C/s initial slope) versus pure Ecoflex (~−8.0 °C/s). Attenuation coefficients at 1.7 MHz: pure Ecoflex 0.54 dB/mm; Fe3O4-doped 0.84 dB/mm. Higher excitation voltage increases steady-state temperature and internal pressure. Actuator/robot designs: (1) Expansion actuator: incorporates a central pillar to favor lateral expansion with minimal elongation. (2) Elongation actuator: employs varying cross-section to maximize axial extension with minimal lateral expansion. Finite element analysis (Ansys) models inflation by applying internal pressure to predict deformation and stress distributions. In-pipe robot: Integrates two expansion actuators (for anchoring) and one elongation actuator (for stride). Locomotion is achieved by sequentially focusing the US field on actuators A (anchor), B (extend), and C (anchor), then releasing A and B to complete one stride. Direction is controlled by changing activation sequence. Liquid cargo systems: A capsule unit with two chambers—one containing cargo, one containing low-boiling-point liquid—enables US-triggered jetting through tissue. A multiunit capsule (array of ~4×4×3.5 mm units) allows selective release by aligning the US focal spot to targeted units. Biopsy and patching actuators: A soft module with internal functional tools (needle or bio-patch) behind baffles remains closed in the initial state to prevent contamination. US-induced inflation extrudes the tool for tissue engagement; on cooling, the tool retracts. A composite robot combines biopsy and patching actuators with an axially magnetized cylindrical permanent magnet for orientation control and magnetic navigation. Imaging and control system: A robotic arm mounts both the focused US array and an US imaging probe. Control workflow: robotic scanning acquires US images; a YOLOv8 instance segmentation model detects the robot in images; masks are stacked into a 3D point cloud; ICP-based registration estimates orientation; coordinate transformation aligns the focal point with the robot. Real-time US imaging monitors actuation to implement on-off control, minimizing acoustic exposure. The system demonstrated actuation through ~8 cm of tissue. Experimental conditions and analyses: Temperature dynamics and steady-state temperatures are measured for doped vs undoped elastomers under various voltages. Internal pressure changes are recorded versus actuation voltage. Robot locomotion in straight/curved pipes is quantified (stride vs curvature radius). Output force is measured versus initial distance to a target plane. Thermal stability and sensitivity are evaluated from 25–45 °C to mimic physiological conditions. Safety considerations leverage focusing to confine higher temperatures inside the actuator while limiting surface temperature increases.

- Focused ultrasound focusing performance: Lateral FWHM resolution ~2.5 mm (single) vs ~2.6 mm (array); the array lacks lateral advantage but yields ~7× higher pressure amplitude and provides ~5 mm longitudinal focusing, which single elements lack. - Acoustic coupling: Transmission coefficients between Fe3O4-doped Ecoflex and water/tissue are ~98.7%/~96.8%, supporting efficient energy delivery in biological environments. - Acoustothermal enhancement by Fe3O4 NPs: Initial heating rate increases in doped Ecoflex (~−14.9 °C/s) vs pure (~−8.0 °C/s). Attenuation coefficients at 1.7 MHz: 0.84 dB/mm (doped) vs 0.54 dB/mm (pure). Higher excitation voltages yield higher steady temperatures and greater internal pressure (phase transition), enabling larger deformations and forces. - Actuator performance: Expansion and elongation actuators show controllable deformation rates scaling with actuation voltage; large, repeatable deformations are demonstrated and validated via FEA. - In-pipe robot locomotion: Sequential selective actuation of multiple units achieves robust upward (against gravity) and downward motion with consistent stride length across cycles and stable anchoring, including in soft biological pipes with mucus. Stride length decreases with smaller pipe curvature radius. - Selective actuation: Multiunit soft capsule with units spaced ~4.5 mm center-to-center enables selective activation and release of targeted liquid cargo via focal point alignment. - Through-tissue operation: Remote cargo release through tissue is demonstrated. FUPT actuation through ~8 cm tissue thickness is achieved, indicating strong penetration capability. - High force output: Biopsy/patching actuator generates a maximum output force of ~5.5 N at an initial distance of 5 mm, meeting Newton-level force requirements for many minimally invasive procedures. Sufficient force for tissue puncture is maintained even beyond 13 mm stand-off (sampling up to ~20 mm considering needle length). - Task execution timing: Tool extrusion occurs within <10 s of US activation; recovery to initial state takes ~220 s after deactivation (cooling dependent on environment and geometry). - Thermal stability and sensitivity: Actuators remain stable at body temperature; internal volume increases by ~50% from 25–37 °C, with successful actuation at ~42.5 °C and ~220% volume expansion at 45 °C. Focused heating confines higher temperatures within the actuator, limiting surface temperature rise and mitigating tissue damage risk. - Closed-loop imaging and control: A YOLOv8- and ICP-based control system performs automatic localization, alignment of the acoustic focus, and on-off control via US imaging feedback, enabling safe, precise actuation and successful in-intestine biopsy and patching demonstrations.

The study addresses the core challenges in untethered soft robotics—precise, selective actuation in deep tissue and sufficient force output—by combining focused ultrasound with phase-change-driven soft actuation. Focused ultrasound offers intrinsic deep penetration and millimeter-scale spatial resolution, enabling selective stimulation of one actuator among adjacent units and allowing a single robot to perform multiple functions in confined spaces. The acoustothermal effect in Fe3O4-doped elastomers accelerates heating and enhances energy conversion, providing rapid pressure buildup and strong actuation without onboard power, preserving softness and MRI compatibility. Demonstrations include robust in-pipe locomotion with selective anchoring/extension, targeted liquid cargo delivery (including through tissue), and clinically relevant tasks such as biopsy and tissue patching with Newton-level forces. Integration with ultrasound imaging and automated control (segmentation, 3D reconstruction, ICP registration) enables accurate localization and closed-loop actuation, reducing unnecessary acoustic exposure and improving safety. Collectively, these results indicate that FUPT bridges the gap between accessibility and actuation capability in untethered soft robots, broadening their applicability in biomedical and underwater settings.

This work introduces a focused ultrasound-controlled phase transition (FUPT) strategy that delivers millimeter-level selective actuation and Newton-level force in untethered soft robots. By leveraging Fe3O4 NP-enhanced acoustothermal conversion and low-boiling-point fluid phase change, the authors realize high-force, wire-free actuation compatible with tissue environments. Prototypes demonstrate selective multi-unit control for on-demand liquid release, robust in-pipe locomotion, and clinically relevant biopsy and tissue patching, all under a closed-loop imaging and control framework. The approach is compatible with integration of MRI thermometry for precise thermal control, further expanding medical applications. Future work should improve long-term reliability by lowering vapor permeability through alternative matrices or coatings, enhance environmental robustness (e.g., in high-speed fluidic vessels), and tailor robot designs to specific clinical scenarios to maximize safety, efficiency, and functionality.

- Long-term durability: Vapor permeability of the elastomer matrix may limit long-cycle or long-term implanted use; materials or barrier layers to reduce vapor loss are needed. - Environmental robustness: Locomotion robustness in soft, folded, or highly curved biological pipelines requires further optimization; high-speed fluid environments (e.g., blood vessels) impose mechanical and thermal loads needing tailored designs. - Thermal dynamics: Cooling/recovery times (e.g., ~220 s) may constrain cycle frequency; optimization of heat dissipation pathways, materials, and geometry is needed. - Inhomogeneous media: Air gaps or impedance mismatches in vivo can impair US delivery; adjuncts like ultrasound contrast agents or adaptive focusing may be required. - Spatial selectivity vs. unit spacing: Demonstrated selectivity at ~4.5 mm spacing may constrain packing density; higher-frequency arrays or improved focusing algorithms could enhance resolution. - Safety and monitoring: Although focusing limits surface heating, comprehensive in vivo thermal dosimetry and closed-loop thermometry (e.g., MR thermometry) are needed for clinical translation.