Published papers

© 2019 Optical Society of America. Strain sensing was demonstrated by utilizing electrically conductive silicon-carbide (β-SiC) fabricated by femtosecond-laser-based direct modification of polydimethylsiloxane (PDMS). Depending on the laser scanning direction used for the fabrication procedure, the fabricated structures showed different sensitivity to strain and this difference was discussed by observing the surface morphology at various bending radii using scanning electron microscopy (SEM). The change in electrical conductance at the flat state after repeated bending was also investigated. Furthermore, preliminary demonstration of human motion sensing was performed using the fabricated structures. The presented method will open doors to novel electronic device applications using PDMS.

© The Royal Society of Chemistry 2020. Laser-based modification of polymer materials has been emerging as a versatile and efficient technique to simultaneously form and pattern electrically conductive materials. Recently, it has been revealed that native polydimethylsiloxane (PDMS) can be modified into electrically conductive structures using femtosecond laser irradiation; however, the details regarding the structures formed by this method have yet to be revealed. In this work, structures were fabricated by focusing and scanning femtosecond laser pulses onto the surface of PDMS. Raman Spectroscopy and Transmission Electron Microscopy (TEM) analyses revealed the formation of silicon carbide (SiC) nanocrystals, as well as multilayer graphitic carbon, in the modified regions of PDMS. The state of the formed material differed depending on the distance from the focal spot, suggesting that photo-thermal effects contributed to the degradation of PDMS into conductive material. Electrical conductivity measurements, in addition to Raman results, indicated that the amount of disorder in the formed graphitic carbon contributes to the electrical conductivity of the fabricated structures.

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. In this paper, we experimentally demonstrate femtosecond laser direct writing of conductive structures on the surface of native polydimethylsiloxane (PDMS). Irradiation of femtosecond laser pulses modified the PDMS to black structures, which exhibit electrical conductivity. Fourier-transform infrared (FTIR) and X-ray diffraction (XRD) results show that the black structures were composed of β-silicon carbide (β-SiC), which can be attributed to the pyrolysis of the PDMS. The electrical conductivity was exhibited in limited laser power and scanning speed conditions. The technique we present enables the spatially selective formation of β-SiC on the surface of native PDMS only by irradiation of femtosecond laser pulses. Furthermore, this technique has the potential to open a novel route to simply fabricate flexible/stretchable MEMS devices with SiC microstructures.

© 2018 by the authors. Biodegradable polymers have attracted increasing attention in tissue engineering and drug delivery systems owing to their high biocompatibility and biodegradability. Among the various methods for shape forming and modification of biodegradable polymers, laser processing has advantages in a dry processing approach that can process complex-shaped surfaces without using toxic chemical components. This review provides an overview of femtosecond laser processing of biodegradable polymers, especially in the last decade. The interaction mechanism of femtosecond laser pulse and biodegradable polymers, e.g., bond dissociation after laser irradiation, affects the degradable property of biodegradable polymers, which has the potential to control the degradation and sustainability of a structure. Applied studies on controlling cell behavior, tissue scaffolding, and drug release are also described.

© 2020 Optical Society of America. All Rights Reserved. The fabrication of accentuated gold microstructures is demonstrated by multi-photon photoreduction inside an anionic fluorophore-containing hydrogel. We attempted to facilitate gold-ion photoreduction near the focal point of laser pulses, expecting the donation of electrons by the oxidation of fluorophores in the vicinity of gold ions. The presence of anionic FITCdextran also inhibited the spontaneous reduction in untargeted zones, which is attributed to the coordination of gold ions and the anionic FITC-dextran. Simultaneous facilitation and inhibition are promising for the fabrication of dense metal microstructures in the targeted zone while maintaining the hydrogel's light permeability.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. The fabrication of hollow channels surrounded by gold nanoparticles in poly(ethylene glycol) diacrylate (PEGDA) is demonstrated. The absorption spectra show that gold nanoparticles were formed at the periphery of the focus by reduction of gold ions. The microscope observation and Raman spectroscopy analyses indicate that the center of the channels were void of PEGDA, which can be attributed to the femtosecond laser-induced degradation of the hydrogel. Since both the hydrogel and gold nanoparticles are biocompatible, this technique of fabricating hollow channels surrounded by gold nanoparticles is promising for tissue engineering, drug screening, and lab-on-a-chip devices.