Untethered miniature soft robots hold immense promise across biomedical and industrial domains due to their ability to access confined spaces and interact safely with humans. Existing untethered systems, however, often compromise actuation accuracy and force output compared to their tethered counterparts. This limitation significantly hinders their clinical and industrial translation. The challenge lies in developing a novel actuation method for untethered miniature soft robots that combines precise control, high force output (Newton-level), and multi-tasking capabilities, especially within deep tissues or liquid environments. A key obstacle is the need for an effective, spatially selective actuation method that enables a single robot to perform multiple functions depending on the task. This is critical for minimally invasive surgery, where selective use of different tools is essential. Current approaches, while demonstrating some progress in selective actuation, often increase material and structural complexity, requiring integration of diverse materials or mechanisms responsive to specific physical fields (e.g., light or magnetic fields). A simpler strategy is to use non-homogeneous physical fields for selective stimulation, but existing methods, such as light or microwave, often lack the spatial resolution needed for biological tissue applications. Focused ultrasound, offering both deep tissue penetration (up to tens of centimeters) and high resolution (millimeter-level), presents a promising solution for spatially selective actuation. Another major challenge is the requirement for substantial force output (Newton-level) in many biomedical applications, such as biopsy, stent deployment, or suturing. The limited power supply of current untethered actuation methods and the high energy dissipation of soft materials often prevent achieving such force levels. Wireless fluidic actuators based on phase transitions, offering considerable force output without the need for batteries, represent a potential solution, typically by controlling pressure changes within the soft actuator through external energy-triggered phase transitions of enclosed liquids, resulting in substantial deformation and force. This method is also MRI-compatible due to the absence of batteries. This research addresses these challenges by proposing a novel actuation method.

The existing literature highlights the substantial potential of untethered miniature soft robots in various applications, including manipulation, sensing, drug delivery, and minimally invasive surgery. The inherent advantages of safe human-machine interaction, environmental adaptability, and access to confined spaces have spurred significant interest in clinical and industrial translation. However, a critical trade-off exists between the improved accessibility of wireless systems and their limitations in reliable and accurate actuation and force output. While some advancements have been made in selective actuation using materials with distinct responses to light or magnetic fields, these methods increase complexity. Leveraging non-homogeneous physical fields like light or microwaves offers a more straightforward approach, but spatial resolution remains insufficient for biological tissue applications. Similarly, generating large force outputs (Newton-level) needed for many biomedical procedures presents a considerable challenge due to limited power supply and energy dissipation in soft materials. Existing methods, such as those employing phase transitions, have shown promise but need further development to address the limitations mentioned above.

This study introduces a focused ultrasound-controlled phase transition (FUPT) method for wirelessly actuating soft robots. The method leverages the acoustothermal effect of iron oxide nanoparticles (Fe3O4 NPs) doped elastomer and the phase transition of an enclosed low-boiling-point liquid (Novec 7000) to achieve controllable deformation and locomotion. A piezoelectric transducer array generates focused ultrasound waves, producing thermal energy via the acoustothermal effect. This localized temperature rise causes the low-boiling-point liquid to vaporize, increasing internal pressure and resulting in mechanical output (deformation and force). The FUPT method's efficacy is demonstrated through several design aspects: **Acoustic Field Analysis:** The acoustic field generated by the piezoelectric transducer array was analyzed using a piston source model to simulate the wave emitted by each transducer, calculating the total acoustic field by summing the contributions of each transducer. Simulations and experimental calibration established the relationship between acoustic pressure amplitude and applied voltage. **Influence Factors:** The influence of spatial resolution and acoustothermal effects on the phase transition were investigated. A transducer array was used to improve both the pressure amplitude and the spatial resolution of the acoustic field compared to a single transducer. The impact of Fe3O4 NPs doping on the acoustothermal effect was assessed by measuring the temperature change in elastomer samples with and without nanoparticles. The results demonstrated that Fe3O4 NPs doping accelerated heating and enhanced acoustic-to-thermal conversion efficiency. The relationship between actuation voltage, steady-state temperature, and internal pressure change was also examined. **Actuator Design and Fabrication:** The FUPT-based actuators were designed and fabricated using a 3D printing template method. Two actuator designs were created, one that facilitates expansion and another that facilitates elongation, with detailed finite element analysis (FEA) performed to predict and optimize their deformation characteristics under pressure. **Soft Robot Applications:** The FUPT method was demonstrated in several applications. An in-pipe robot, composed of expansion and elongation actuators, demonstrated locomotion and liquid cargo delivery capabilities. The robot's movement was analyzed, revealing a repeatable stride length. A multiunit capsule was created to showcase the spatially selective actuation of multiple units and on-demand liquid cargo delivery using the FUPT method. Furthermore, a soft robot was developed for tissue sampling and patching procedures. The robot incorporates a needle and bio-patch, activated using the FUPT method to achieve needle insertion, tissue acquisition, and bio-patch adhesion. The force output of the actuator was measured, demonstrating the capability to generate forces suitable for most minimally invasive surgeries. **Imaging and Control System:** A control system was developed integrating a focused ultrasound transducer array, ultrasound imaging probe, and robotic arm. An instance segmentation algorithm (YOLOv8) processed the ultrasound images to track the soft robot's position and orientation, guiding the robotic arm to align the focal point with the robot. Ultrasound imaging also allowed for on-off control of actuation, minimizing unnecessary exposure to acoustic radiation. The system's efficacy was tested in experiments involving biopsy tasks in the intestine.

This research successfully demonstrated the feasibility and efficacy of the focused ultrasound-controlled phase transition (FUPT) method for actuating untethered soft robots. Key findings include: * **Millimeter-level Spatial Resolution:** The FUPT method achieved high spatial resolution, enabling precise, selective actuation of soft robots, even within complex environments such as biological tissues. This capability is a significant improvement over existing methods, which often lack the required precision. * **Newton-level Force Output:** The FUPT method produced Newton-level forces, meeting the requirements of various biomedical applications like biopsy, stent deployment, and suturing. This substantial force generation capability is significantly enhanced compared to previously reported actuation methods for untethered soft robots. * **Multi-tasking Capabilities:** A single soft robot was successfully used to perform multiple tasks, such as liquid cargo delivery, tissue acquisition, and wound patching, showcasing the versatility of the FUPT method. This multifunctionality greatly increases the range of potential applications for untethered soft robots. * **Autonomous Control System:** The developed autonomous control system using ultrasound imaging effectively aligned the acoustic field with the soft robot, enabling precise, real-time control of actuation. This system reduces the need for manual adjustments and improves the robustness of the system. * **Biomedical Applications:** Successful demonstrations in in-vivo applications, such as liquid cargo delivery and tissue biopsy/patching within the intestine, validated the potential of this technology for minimally invasive procedures. The ability to penetrate tissue to reach the target area is a significant achievement. The large force output generated, over 5.5N in experiments, demonstrates the method's suitability for complex tasks. * **Tissue Penetration:** The method demonstrates successful actuation through up to 8 cm of tissue. This highlights the advantages of ultrasound in penetrating biological material. Future work could investigate using ultrasound contrast agents to improve penetration through complex or gas-filled mediums. Specific quantitative data points include the 5.5N force output, the 4.5 mm resolution between selectively actuated units in the multiunit capsule, and the 8 cm tissue penetration depth. The repeatable stride length of the in-pipe robot also demonstrates the reliable actuation of the system.

The FUPT method represents a significant advancement in the actuation of untethered soft robots. The combination of millimeter-level spatial resolution, Newton-level force output, and the ability to perform multiple tasks addresses key limitations of current technologies. The successful demonstrations of liquid cargo delivery, tissue biopsy, and wound patching underscore its potential for minimally invasive surgery and other biomedical applications. The integration of ultrasound imaging into the autonomous control system ensures accurate and safe operation within complex biological environments. The use of focused ultrasound also offers advantages in terms of thermal safety, as the heat is concentrated at the focal point and reduces the risk of damaging surrounding tissues. The FUPT method's compatibility with magnetic resonance thermometry provides an avenue for real-time temperature control, further enhancing its potential in medical applications. This technology opens doors for more sophisticated and minimally invasive medical interventions.

This study presented a novel focused ultrasound-controlled phase transition (FUPT) method for actuating untethered soft robots, demonstrating remarkable capabilities in spatial resolution, force output, and multi-tasking. Successful in-vivo experiments highlight its potential for various biomedical applications. Future research should focus on enhancing the environmental adaptability of FUPT-driven soft robots, particularly in scenarios involving numerous actuation cycles or long-term implantation, perhaps by investigating alternative matrix materials with improved vapor sealing properties. This work paves the way for more versatile and adaptable untethered soft robots in diverse biomedical and industrial fields.

While the FUPT method shows considerable promise, certain limitations exist. The current design may require optimization for scenarios involving a large number of actuation cycles or long-term implantation, possibly requiring modifications to reduce vapor permeability. Furthermore, the system's performance in highly heterogeneous media, such as those with significant air pockets, might require additional refinements, potentially through the use of ultrasound contrast agents. Further research is also needed to explore the long-term biocompatibility of the materials used and the potential effects of prolonged exposure to focused ultrasound.