Published papers

This paper presents an additive manufacturing method of an electrothermally controllable 4D-printed bimorph microactuator. We fabricated an electrothermal 4D bimorph microactuator equipped with an electrical heater through a two-step additive process: two-photon polymerization (TPP) 4D printing and liquid metal microfluidic injection. First, a 3D thermal bimorph microstructure including a microchannel was built using TPP printing. Second, the printed microstructure was electrically wired by injecting a liquid metal into the microchannel. To validate this method, we fabricated and characterized a vertical-spiral-shaped electrothermal 4D rotary microactuator with a large angular displacement. The fabricated electrothermal microactuator successfully controlled its temperature and angular displacement via electricity. Furthermore, we experimentally demonstrated the unique ability of the electrothermal 4D bimorph microactuator to produce repeatable large angular displacements of more than 10° at 386 mV. Accordingly, the proposed method is expected to enhance the spatial design freedom of electrothermal microactuators with potential applications in soft microrobotics.

A novel blood flow sensor enabling a precise measurement in an endoscope has been developed. In this paper, a device design and a validity of an electrothermal actuator for the lens control are reported.

A large stroke electrothermal MEMS actuator with small device footprint and low driving voltage has been proposed for motion tracking blood flow measurement. This paper reports this kirigami-inspired device design and simulation results of the proposed actuator.

This paper reports the design of a bi-directional VO2-KIRIGAMI electromechanical microactuator with a large millimeter stroke and low power consumption. Using Finite Element Method (FEM) simulation, it has been demonstrated that the proposed VO2-KIRIGAMI design can achieve a remarkable stroke of 2.2 mm with a power consumption of only 54 mW. This design study has enabled the application of electrothermal microactuators in long-range lens focusing for endoscopic procedures.

A kirigami electrothermal MEMS actuator using the phase transition material VO2 has been proposed. This paper reports the fabrication process and mechanical characteristics of the actuator. The actuator was fabricated by MEMS processes with VO2 annealing method. Through the environmental heating, the actuator exhibited the bi-directional actuation due to the thermal expansion and phase transition shrinkage of VO2, resulting in a net downward displacement of 219.5 μm.

This work reports on a flexible thermal transport MEMS device driven by the capillary force of micropillars. In particular, it aims to enhance the device performance by improving liquid and gas sealing properties while introducing a quantitative method to evaluate thermal performance in bended positions. In the improved design, capillary force generated at micropillars are employed in the flexible evaporator as micropumps to drive the working fluid. The use of Parylene, paraxylene-based polymer, in the fabrication process improves sealing properties, enhancing overall device performance. A heat load test in the curved posture was performed, and heat transport performance of 3.0 W was demonstrated.

This paper reports the fabrication method for a soft thermal bimorph microactuator via femtosecond laser surface carbonization. This method advances the application of thermal bimorph actuators in photothermal soft micro robotics. In this study, the surficial layer of a PDMS microcantilever, printed via micro-stereolithography, was carbonized through femtosecond laser irradiation, resulting in a bimorph microactuator with different thermal expansion coefficients. A fast response of 40 milliseconds and reproducible actuation were achieved through laser heating. Furthermore, the proposed fabrication method was applied to a 3D-printed insect-inspired microstructure, demonstrating its potential for soft microrobotics applications.

This paper reports the rotational morphing capability of a low-power electrothermal kirigami MEMS actuator driven by low-cost pulse-width modulation (PWM) heating, advancing the application of electrothermal MEMS actuators in multi-degree-of-freedom (multi-DoF) robotics. PWM-controlled voltages were applied to each of the actuator's four quadrant-based Joule heaters. By dynamically adjusting the duty cycles of the voltages applied to each heater, localized heating regions within the actuator were rotated, resulting in mechanical rotational morphing. The actuator exhibited continuous rotational motion while maintaining a tilting angular displacement of approximately 8°, with a power consumption of only 76mW.

This paper reports on a moisture-harvesting silicone rubber composite capable of repeatable large actuation by liquid-gas phase transition, as a promising material for soft thermal actuators. The composite, fabricated by dispersing silica gel particles within a silicone rubber matrix, absorbed ambient moisture, resulting in a mass increase of approximately 1.6% after 2 h in a humidified environment. The composite exhibited repeatable displacements (δ/L=0.18, displacement-to-length ratio). These results demonstrate its ability to harvest environmental moisture as a fuel for actuation and to repeat large actuations, highlighting its potential for aquatic soft actuator applications.

Abstract Kirigami design principles have been widely applied to develop thermally responsive shape-morphing devices across various scales. However, the multi-degree-of-freedom (multi-DoF) morphing capabilities of microelectromechanical-systems (MEMS)-scale kirigami devices under localized Joule heating remain largely unexplored. This paper presents a quadrant kirigami-type electrothermal MEMS actuator with multi-DoF morphing capabilities, demonstrating both one-dimensional piston motion and two-dimensional (2D) tilting motion. The actuator, fabricated using surface and bulk micromachining techniques, features four independent electrical circuits, creating four electrothermally separated quadrants within the MEMS-scale kirigami structure. The temperature distribution within each quadrant of the actuator, subjected to localized Joule heating, was experimentally examined using thermography, revealing distinct temperature contrasts. The actuator demonstrated multi-DoF morphing capabilities, achieving a 2D tilting angular displacement of approximately 10° at 70 mW and a vertical piston displacement of 1.5 mm at 135 mW. These experimental results validate that the electrothermal quadrant design, leveraging localized Joule heating, enhances the DoF morphing capabilities of kirigami-type electrothermal MEMS actuators.

This study presents an additive and subtractive fabrication method for a bimorph thermal microactuator using TPP 3D printing and femtosecond laser ablation. First, a microcantilever with a thickness of 30 μm was printed by TPP 3D printing. Second, femtosecond laser ablation was utilized to reduce the thickness to 15 μm, which facilitated the creation of a bimorph structure. The cantilever-shaped bimorph microactuator demonstrated stable cyclic actuation, achieving a tip displacement of 11.5 μm through laser heating.

A fabrication method of a soft photothermal microactuator using two-photon polymerization and vacuum filling is proposed. Proposed microactuator was fabricated through two processes: A two-photon polymerization (TPP) and a composite injection by a vacuum filling. First, a PDMS microcantilever with a microfluidic channel was printed by TPP. Second, a PDMS composite containing thermally expandable microspheres was filled in the microfluidic channel. To validate this method, a microactuator with dimensions of 1 mm × 130 μm × 100 μm was fabricated. We demonstrated the ability of the soft photothermal microactuator to produce displacement of approximately 50 μm by laser heating.

Endoscopic autofluorescence lifetime imaging is a promising technique for making quantitative and non-invasive diagnoses of abnormal tissue. However, motion artifacts caused by vibration in the direction perpendicular to the tissue surface in a body makes clinical diagnosis difficult. Thus, this paper proposes a robust autofluorescence lifetime sensing technique with a lens tracking system based on a laser beam spot analysis. Our optical setup can be easily mounted on the head of an endoscope. The variation in distance between the optical system and the target surface is tracked by the change in the spot size of the laser beam captured by the camera, and the lens actuator is feedback-controlled to suppress motion artifacts. The experimental results show that, when using a lens tracking system, the standard deviation of fluorescence lifetime is dramatically reduced. Furthermore, the validity of the proposed method is experimentally confirmed by using a bio-mimicking phantom that replicates the shape, optical parameters, and chemical component distribution of the cancerous tissue.