Published papers

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%.

Large-displacement microelectromechanical system (MEMS) scanners are in high demand for a wide variety of optical applications. Kirigami, a traditional Japanese art of paper cutting and folding, is a promising engineering method for creating out-of-plane structures. This paper explores the feasibility and potential of a kirigami-inspired electrothermal MEMS scanner, which achieves large vertical displacement by out-of-plane film actuation. The proposed scanner is composed of film materials suitable for electrothermal self-reconfigurable folding and unfolding, and microscale film cuttings are strategically placed to generate large displacement. The freestanding electrothermal kirigami film with a 2 mm diameter and high fill factor is completely fabricated by careful stress control in the MEMS process. A 200 μm vertical displacement with 131 mW and a 20 Hz responsive frequency is experimentally demonstrated as a unique function of electrothermal kirigami film. The proposed design, fabrication process, and experimental test validate the proposed scanner’s feasibility and potential for large-displacement scanning with a high fill factor.

This paper proposes an electrothermally-actuated circular pyramidal kirigami microscanner with a millimeter-range low-power lens drive for endoscopic biomedical applications. A variation of Japanese origami art, kirigami involves creation of out-of-plane structures by paper cutting and folding. The proposed microscanner is composed of freestanding kirigami film on which the spiral-curved thermal bimorphs are strategically placed. The kirigami microscanner is electrothermally transformed into an out-of-plane circular multistep pyramid by Joule heating. The circular pyramidal kirigami microscanner on a small footprint of 4.5 mm × 4.5 mm was fabricated by microelectromechanical system processes. A large four-step pyramidal actuation was successfully demonstrated, and a large 1.1-mm lens travel range at only 128 mW was achieved.

This paper presents a laser direct writing method for the fabrication of a laser-induced graphitic carbon (LIGC)/polydimethylsiloxane (PDMS) bimorph thermal microactuator via two-photon polymerization (TPP) and laser-induced graphitization (LIG). The fundamental parameters of femtosecond laser dotting, adopted as the writing mode for LIG, were investigated by observing the carbonized surface of the TPP-written PDMS plate using optical, scanning electron, and Raman microscopies. Subsequently, as a proof of concept for the proposed method, the LIGC/PDMS bimorph thermal microactuator was fabricated by modifying the surface of a PDMS microcantilever with a thickness of 25 μm and a length of 280 μm through femtosecond laser dotting. The directly written LIGC/PDMS bimorph thermal microactuator produced repeatable displacements of approximately 20 μm with the temperature rise of 66 ℃. Furthermore, the LIGC/PDMS bimorph microactuator exhibited a displacement of 27 μm at 10 mW and a response speed of 0.05 s in photothermal heating. The proposed direct writing fabrication method based on TPP and LIG demonstrates potential for advancing the development of PDMS-based bimorph photothermal microrobots.

This letter presents a micropillar-pumped polymer loop heat pipe (LHP) with potential applications in flexible electronics. A unique evaporator with a micro pillar wick was designed for a flexible polymer LHP. The polymer LHP was fabricated via simple and cost-effective soft lithography, omitting the need for a porous wick. This design and fabrication approach facilitated passive two-phase cooling in the polymer LHP, with up to 34 $^{\circ}$ C reduction in evaporator temperature when compared to a non-fluid-charged state in a horizontal orientation. 2023-0140

The thermal management of flexible electronic devices represents a major barrier to their applications. Thus, in this study, a parylene-coated polydimethylsiloxane (PDMS)-based loop heat pipe (LHP), parylene/PDMS-LHP, equipped with a micropillar evaporator was developed. A newly developed one-dimensional steady-state numerical model was used and the parylene/PDMS-LHP (thickness: 1.5 mm) was designed based on the model. Flow channel of PDMS was formed by a mold using conventional photolithography technique. Parylene is vapor-deposited on the inner surface of the flow channel to improve the liquid barrier properties and hydrophilicity. Ethanol was selected as the working fluid from the viewpoint of the contact angle with parylene surface. Heat input tests were conducted in six different orientations (five without bending and in different orientations, and one with bending) to verify the effect of operating direction on performance, assuming that the Parylene/PDMS-LHP will be mounted on flexible electronic devices. The maximum heat-transport of the parylene/PDMS-LHP was 3.0 W, and the heat source temperature decreased to ∼ 60°C compared with that of a non-fluid-charged device in a horizontal orientation. In tests in five orientations without bending and one orientation with bending, continuous heat transfer from 1.0 W to 2.5 W or 3.0 W was achieved in all orientations, confirming the applicability of the proposed Parylene/PDMS-LHP to wearable devices.