Published papers

The pulsed laser viscometer (PLV) measures viscosity and surface tension using nanoscale liquid surface deformation generated by two-beam interference of a pulsed heating laser and detects damping oscillation from the first-order diffracted intensity of a probing laser. PLV features (1) non-contact and in situ measurement, (2) high spatial resolution of 10 to 100 μm, (3) high time resolution of microseconds to milliseconds, (4) small sample volume of microliters to milliliters, and (5) a wide viscosity range of 0.1 to 104 mPa·s. We established the PLV theory by solving the Navier‒Stokes‒Thermoelastic equation for volumetric heating of a liquid by a weakly absorbed pulsed laser. A new PLV experimental apparatus was developed using a pulsed YAG laser with a wavelength of 1064 nm as the heating laser, and experiments used six liquid samples (acetone, toluene, water, ethanol, 2-propanol, and 1-hexanol) at room temperature to verify that the instrument operates according to the theory. Measured viscosities agree with the literature values with an average deviation of 3.2 %. Measured surface tension agrees with the literature values with an average deviation of 6.2 %. Uncertainties are estimated at 2 % for viscosity and 3 % for surface tension. The temperature rise of the sample during the experiment was less than 0.01 K. Furthermore, the technique was found to be sensitive only to viscosity and surface tension, and insensitive to the values ​​of other thermophysical properties required for processing the data.

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.

Forming capillary networks with predefined geometries is a critical challenge in engineering complex three-dimensional tissues in vitro. While bioprinting and microfluidic technologies have enabled vascular tissue fabrication, precise control over capillary-scale vascularization remains limited. In this study, we investigated vascular formation process along hydrogel microchannels to elucidate mechanisms governing capillary-scale lumen formation. Microchannels were fabricated by femtosecond laser ablation in collagen and fibrin hydrogels. We optimized multiphoton lithography parameters to fabricate microchannels within these hydrogels and analyzed vascular formation along the channels. Luminal vascular structures formed readily in 50-μm channels, while vascular formation failed in 20-μm channels under static conditions, suggesting a significant shift in endothelial organization at the cellular scale. Flow stimulation significantly promoted vessel formation through collective endothelial cell migration and adhesion, whereas static conditions induced endothelial-to-mesenchymal transition. These findings provide key insights into capillary-scale vascularization and contribute to the development of more complex architectures with predefined shapes.

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 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.

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.

Liposomal products enable new therapies through the protection and controlled release of encapsulated medicines. The diffusion coefficient of a liposome is associated with its size and surface properties, and its measurement is important in the evaluation of the product and design of new drug delivery systems. We propose a rapid and label-free diffusion sensing method using light-induced dielectrophoresis for high-throughput analysis of liposomal products. In this study, the diffusion coefficient of liposomes with a diameter of approximately 100 nm was measured by observing the mutual diffusion within a few seconds. Prior to the measurement, a parametric study of the excitation conditions was performed to achieve highly efficient manipulation of liposomes by light-induced dielectrophoresis. The validation of the proposed measurement method for liposomes was demonstrated by comparing the result of observation by the transmission electron microscope. The effect of the surface functional groups of the liposomes was quantitatively evaluated using diffusion coefficient measurements. Furthermore, the thermo-sensitive liposomes were measured to demonstrate their sensitivity to phase transition during temperature change.

This study proposes the design of an ultra-far-infrared broadband metamaterial radiator for space cryogenic applications. To achieve a flexible broadband radiator without significant structural complexity, the proposed radiator employs a flexible glass sheet as an intermediate dielectric layer and a QR code-like pixel-patterned periodic metal structure. A genetic algorithm is used to design the radiator, followed by fabrication using microfabrication techniques. The results of the reflectance spectrum measurements by FTIR were in general agreement with the analytical results, thereby confirming the broadband radiation characteristics. The total hemispherical emittance, which is an evaluation index for space radiators, was estimated to be 0.57 at 20 K. Compared with the conventional radiator material of 80-μm black paint, the total hemispherical emittance was markedly enhanced, despite a film thickness ratio of 19.9 % and an area density ratio of 53.7 %. Therefore, the proposed radiator is expected to facilitate the development of cryogenic radiators with reduced weight and flexibility while minimizing the risk of contamination. This is a significant consideration for infrared astronomy missions that require cryogenic temperatures.

Polyethylene glycol diacrylate (PEGDA) is a photosensitive polymer offering self-healing capabilities through chemical or structural modifications, making it suitable for enhancing the reliability of micro/nano robotics. This study introduces a 3D microdroplet printing approach using direct laser writing (DLW) via two-photon polymerization with a femtosecond laser. Adding DI water can reduce the viscosity of PEGDA, and the nL droplets with T-junction can be created. On the basis, some kinds of metal powder can be added into PEGDA to make composite resin droplets. And I can also do 3D printing by using the different composite resin to create a multilayer structure of hundreds of microns. This work demonstrates a novel method for precise microdroplet-based 3D fabrication, advancing micro/nano manufacturing technologies.

This study presents a method for applying multi-photon absorption to fabricate micro-scale scaffold structures for blood vessel formation within hydrogels, using a femtosecond laser. The structures were fabricated via femtosecond laser ablation and cavitation enabling the replication of complex micro-scale microvascular systems. A pressure control system was designed to reproduce variations in channel diameter observed in natural blood vessels. By adjusting the pressure, changes in diameter were confirmed demonstrating the influence of pressure on cavitation. Fabricated structures using the two modes were also evaluated by seeding endothelial cells and subsequent blood vessel formation was observed.

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.

This study proposes the design of a focus-tunable freeform microlens printed on a MEMS actuator. The proposed freeform microlens, designed with its varying surface curvature along the azimuthal direction, can adjust its focal length through the tilt motion of the MEMS actuator. The designed freeform microlens was printed onto a glass substrate, and demonstrated focus tunability ranging from 0.31 mm to 3.72 mm.

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.

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.

(Accepted)

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. We propose a motion-robust laser Doppler flowmetry (LDF) system that can be used as a non-contact blood perfusion sensor for medical diagnosis. Endoscopic LDF systems are typically limited in their usefulness in clinical contexts by the need for the natural organs to be immobilized, as serious motion artifacts due to the axial surface displacement can interfere with blood perfusion measurements. In our system, the focusing lens moves to track the motion of the target using a low-frequency reference signal in the optical data, enabling the suppression of these motion artifacts in the axial direction. This paper reports feasibility tests on a prototype of this system using a microfluidic phantom as a measurement target moving in the direction of the optical axis. The frequency spectra detected and the perfusion values calculated from those spectra show that the motion tracking system is capable of suppressing motion artifacts in perfusion readings. We compared the prototype LDF system’s measurements with and without motion feedback, and found that motion tracking improves the fidelity of the perfusion signal by as much as 87%.

© 2018 International Heat Transfer Conference. All rights reserved. For endoscopic Laser Doppler flowmetry (LDF), motion artifacts present a serious challenge for clinical applications. We propose a motion-robust LDF method that eliminates motion artifacts by controlling the lens positon with a feedback signal representing tissue motion. This method could be applied to an endoscopic device with micro-electro-mechanical-systems (MEMS) technology. To demonstrate the feasibility of the proposed method, a bench-top apparatus was constructed, and a blood flow phantom was used as the measurement target. Experimental tests shows that the controlled lens was moved to the target position under phantom movement, and the detected perfusion signal were stabilized by the focus control.

© 2018 IEEE. A novel micro optical diffusion sensor enabling a high-speed and a small sample volume measurement has been developed. The proposed electrothermal actuators can achieve a miniaturization of the optical diffusion sensor. In this paper, a design of MEMS mirror with a long working distance is reported, and the validity of the mirror is discussed.