Published papers

Graphene quantum dots (GQDs) are fluorescent materials whose fluorescence intensity changes in response to external stimuli. In this study, we demonstrated the fabrication of a semi-cylindrical conductive microstructure encircled by GQDs on polydimethylsiloxane in a single step by laser carbonization. By adjusting laser parameters, structures with different resistance values were selectively fabricated. Under the voltage application to the structure, Joule heating in low-resistance structures led to a decrease in GQDs fluorescence intensity, while high-resistance structures showed minimal changes. Combining these structures enabled voltage-responsive anti-counterfeiting labels, readable only under excitation light and voltage. The proposed method enables one-step fabrication of voltage-responsive structures by laser irradiation, thereby paving the way for the development of voltage-controlled anti-counterfeiting technology.

In this talk, our study on carbonization and graphitization of polymers by laser irradiation including the direct writing of graphene quantum dots structures, fabrication of supercapacitors and triboelectric nanogenerators will be described.

The patterning of conductive graphitic carbon planar structures on hydrogels was demonstrated by laser irradiation onto the hydrogels. Furthermore, the patterned conductive structure was applied to the conductive material of a microstrip patch antenna.

For the treatment of irreversible, extensive skin damage, artificial skins or cultured skins are useful when allogeneic skins are unavailable. However, most of them lack vasculature, causing delayed perfusion and hence delay or failure in engraftment of the tissues. We previously developed a prevascularized three-dimensional (3D) cultured skin based on the layer-by-layer cell coating technique (LbL-3D skin), in which cells are seeded and laminated on a porous polymer membrane for medium supply to the thick cultured tissue. Recent animal studies have demonstrated that LbL-3D skin can achieve rapid perfusion and high graft survival after transplantation. However, there were practical issues with separating LbL-3D skins from the membranes before transplantation and the handling separated LbL-3D skins for transplantation. To address these problems, in this study, we examined the use of biodegradable porous polymer membranes that enabled the transplantation of LbL-3D skins together with the membranes, which could be decomposed after transplantation. Thin films made from poly (lactic-co-glycolic acid) (PLGA) were irradiated with femtosecond laser pulses to create micro through-holes, producing porous membranes. We designed and fabricated culture inserts with the PLGA membranes and cultivated LbL-3D skins with 2 × 106 neonatal normal human dermal fibroblasts and 1 × 104 human umbilical vein endothelial cells in the dermis of 20 cell layers and 1 × 105 neonatal human epidermal keratinocytes in the epidermis. Histological analyses revealed that the skins cultured on the PLGA membranes had thickness of about 400 μm and that there were no defects in the quality of the skins cultured on the PLGA membranes when compared with those cultured on the conventional (nonbiodegradable) commercial membranes. The cultured LbL-3D skins were then transplanted together with the PLGA membranes onto full-thickness excisional wounds in mice. At 7 days posttransplantation onto a mouse, the tissues above and below the membrane were connected through the holes with collagen-positive fibers that appeared to migrate from both the host and donor sides, and favorable reepithelization was observed throughout the transplanted skin region. However, insufficient engraftment was observed in some cases. Thus, further optimization of the membrane conditions would be needed to improve the transplantation outcome.

Thermoresponsive hydrogels, which exhibit changes in their optical properties and volume due to temperature variations, are promising candidates for applications in soft devices. In this study, we demonstrate the modulation of transmittance in a thermoresponsive hydrogel through light stimulation employing gold microstructures fabricated via multiphoton photoreduction. The spatial integration of photoresponsiveness, attributed to high-density gold nanoparticles within the thermoresponsive hydrogel, was accomplished through the high-speed laser scanning of femtosecond laser pulses. The temperature measurement during the fabrication of the gold microstructure revealed that the high-speed and multiple scanning over the same path effectively reduced the temperature in the irradiated area of femtosecond laser pulses. The present approach enabled the mitigation of thermal effects during the fabrication, resulting in minimizing distortion in the fine lines of the structures. Upon exposure to stimulus light, a rapid change in the transmittance of the region where the structures were fabricated was prominently observed. The present method unveils a promising avenue for the advancement of light-responsive soft devices.

Laser-induced carbonization, which allows for the facile generation of graphitic carbon, is considered a promising technique for fabricating arbitrary conductive microstructures. The morphology and crystallinity of the resulting product are acknowledged to be significantly influenced by laser irradiation conditions. However, unlike discussions pertaining to furnaces where detailed considerations of applied temperature and resulting products are common, discussions on the process of laser-induced carbonization are limited. In recent years, reports have shown that using polydimethylsiloxane (PDMS) as a precursor material not only produces graphitic carbon but also results in the formation of silicon carbide. In this study, we utilized a thermographic camera to measure temperature changes during laser-induced carbonization, aiming to elucidate the correlation between PDMS temperature fluctuations and the morphology and crystallinity of the resulting graphitic carbon. The results demonstrate that the morphology and crystallinity of the graphitic carbon formed through laser-induced carbonization are not solely determined by the maximum temperature in the laser-irradiated area. The temperature changes during laser irradiation play a crucial role in the selective generation of these materials.

Laser direct writing of 3D carbon structures onto flexible polymer substrates offers potential of rapid roll-to-roll manufacturing for a variety of key applications, including large area sensors, flexible electronics, robotics, energy storage/conversion, and other consumer applications. The specific formation mechanism of the carbon structures has been an issue of debate for many years with the prevailing notion of a simple photothermal conversion reaction that mainly depends on the total energy input. However, this view has been shown to be inconsistent with experimental observations of nonlinear changes in the resulting structures when multiple processing parameters are simultaneously changed. In this study, we propose a formation mechanism based on the nucleation and growth of laser-induced defects, which is experimentally validated by irradiating a continuous wave laser beam onto polydimethylsiloxane. The model is further validated by intentionally introducing controlled defects by femtosecond laser irradiation, and indicate the implications of a two-laser laser direct writing technique to go beyond the current processing limits. These results clarify the previously ambiguous mechanisms by which carbon structures form under laser irradiation and provide a deeper understanding of how to control photothermal processes for advanced material processing.

To advance the development of flexible materials for soft robotics applications, it is crucial to enhance the elastic modulus and breaking the stress of soft materials, such as hydrogels. Double network hydrogels (DN gels) have displayed remarkable mechanical strength owing to their unique network structure composed of two types of polymer networks. However, current fabrication methods for DN gels entail cross-linking two distinct hydrogel polymers throughout the entire hydrogel matrix. In this study, we focused on employing multi-photon polymerization (MPP) with femtosecond laser pulses as a cross-linking method for hydrogels to achieve spatially selective formation of DN gel structures at the micrometer scale, along with the consequent improvement in local mechanical strength. We assessed the mechanical properties of the fabricated structures and confirmed that the mechanical strength varied within the regions where DN gel was locally formed.

Hydrogels have emerged as promising supporting materials for wearable or implantable electronic devices, owing to their high biocompatibility. Among the many techniques for patterning conductive structures on supporting materials, laser-based graphitization allows the simultaneous synthesis and patterning of conductive structures. Here, it has been demonstrated for the first time, the direct patterning of conductive structures on hydrogels by laser-based graphitization, and the application of this method to the fabrication of hydrogel-based supercapacitors. Conductive graphitic carbon is formed on the hydrogel along the trajectory of laser scanning. One-step fabrication of supercapacitors is achieved by laser irradiation of hydrogels containing both lignin and NaCl electrolyte. Notably, the areal capacitance of the fabricated supercapacitor is retained after drying and swelling, indicating its reusability. The method realizes rapid fabrication of hydrogel-based supercapacitors that can be stored in a dry state, providing a facile method for storing and delivering energy-storage devices.

Laser direct-write (LDW) methods offer the rapid, on-demand, and high-resolution patterning of a variety of nanocrystals (NCs). Out of the many LDW methods, the laser-induced modification method is an attractive method as NCs can be directly patterned on polymers without supplementary preparation steps. Previously, it has been indicated that the laser irradiation of polydimethylsiloxane results in composite structures composed of graphitic carbon crystals (GCs) and silicon carbide NCs (SiC-NCs). However, material properties distinctive of SiC-NCs have not been reported for the resulting structures owing to the low formation content relative to those of GCs. In this study by utilizing a high-repetition femtosecond laser, we demonstrate the controlled formation of GCs and SiC-NCs to achieve a composite structure exhibiting a measurable semiconducting behavior for the first time. Moreover, photoluminescence emissions distinctive of fluorescent molecular-sized SiC-NCs were observed from the resulting structures. The presented results will trigger a wave of ideas utilizing ultrashort-pulsed lasers for the laser-induced modification of polymers toward the fabrication of a wide range of electronic and photonic devices, such as memory storage devices, photovoltaics, and optical sensors.

Plasmonic sensing using metal nanoparticles has received considerable attention because of its high sensitivity for detection. In this study, gold nanoparticles (AuNPs) were formed near the focal point of femtosecond laser pulses inside a hydrogel containing gold chloride and glutamine for plasmonic sensing. The presence of glutamine suppressed the formation of AuNPs in the region outside of the laser focal point. The proposed method for the formation of AuNPs enables spatially selective fabrication of structures with optical properties attributed to localized surface plasmon resonance. Typical biomolecules present in sweat were detected using hydrogels with the fabricated structures. Among the biomolecules, the peaks of the absorbance spectra of the AuNPs inside hydrogels exhibited a blue shift only for urea. This result indicates that urea can be detected using hydrogels with fabricated structures. The method presented in this study facilitates spatially selective fabrication of biosensing structures inside the bulk of hydrogels.

In this work formation of conductive lines on the surface of AlN ceramic induced by laser radiation is presented. The transformation to conductive state is done be ablation of the ceramic surface using nanosecond Nd:YAG laser. The dependences of the processing conditions as applied laser fluence and the pulse overlapping on the resistance value are obtained for different surface modifications including straight lines, ∟, and ┼ shapes. The formation of conductive structures is also studied for application of laser irradiation at the second (532 nm), third (355 nm) and fourth (266 nm) harmonics of the laser system. It is found that the resistance strongly depends on the processing conditions, as at appropriate choice it can be varied in a range of order of magnitude. Processing parameters windows are defined where the structure resistance has a minimal value. The performed analyses based on TEM, SEM, EDX, and Raman spectroscopy indicate that formation of aluminium layer, its oxidation and morphology define the processed areas conductivity. The role of oxidation is also confirmed by analyses of structures fabricated in vacuum, where significant reduction of the resistance is observed. It is found however, that a decrease of the ambient air pressure to 10 Torr is sufficient to obtain structures resistance of the same order as for processing in vacuum. Calculated data about the thickness of the conductive layer is also presented. The obtained results could be used in the design of microelectronic components, resistive elements, and novel optical materials.

Lasers enable modification of living and non-living matter with submicron precision in a contact-free manner which has raised the interest of researchers for decades. Accordingly, laser technologies have drawn interest across disciplines. They have been established as a valuable tool to permeabilize cellular membranes for molecular delivery in a process termed photoinjection. Laser-based molecular delivery was first reported in 1984, when normal kidney cells were successfully transfected with a frequency-multiplied Nd:YAG laser. Due to the rapid development of optical technologies, far more sophisticated laser platforms have become available. In particular, near infrared femtosecond (NIR fs) laser sources enable an increasing progress of laser-based molecular delivery procedures and opened up multiple variations and applications of this technique. This review is intended to provide a plant science audience with the physical principles as well as the application potentials of laser-based molecular delivery. The historical origins and technical development of laser-based molecular delivery are summarized and the principle physical processes involved in these approaches and their implications for practical use are introduced. Successful cases of laser-based molecular delivery in plant science will be reviewed in detail, and the specific hurdles that plant materials pose will be discussed. Finally, we will give an outlook on current limitations and possible future applications of laser-based molecular delivery in the field of plant science.

© COPYRIGHT SPIE. Stimuli-responsible controlled release of growth factors from a tissue scaffold has gained significant interest for efficient tissue engineering. Laser-triggered methods for the molecular release have the advantage of spatial and temporal controllability. In this study, we demonstrate laser-triggered molecular release from biodegradable polymer microcapsules incorporated in gelatin hydrogel for the aim of efficient tissue engineering. The microcapsules, which fluorescent molecules were encapsulated, were fabricated using a dual-coaxial nozzle system. The microcapsule suspension was mixed with gelatin solution, followed by cross-linking to fabricate a gelatin hydrogel. Femtosecond laser pulses were focused onto the hydrogel to release fluorescent molecules from the microcapsules in the gelatin hydrogel.

© 2018 The Japan Society of Applied Physics. We previously developed a compact acoustic-resolution photoacoustic (PA) imaging system with optical fiber-based illumination, by which blood vessels in the rat skin were visualized in vivo. However, there were issues to be improved in the imaging characteristics: blurring in the out-of-focus region of the ultrasound sensor and degraded resolution due to low-frequency components in the PA signals. These are common limitations in acoustic-resolution PA imaging methods. In this study, we attempted to solve these problems by introducing signal processing methods based both on the synthetic-aperture focusing technique, which was applied only to signals in the detection zone of the sensor in the horizontal direction that was determined in advance in a phantom study, and on digital acoustic signal filtering with three specific frequency bands. Ex vivo and in vivo PA imaging of blood vessels in the rat skin and subcutaneous tissue showed the validity of the present signal processing methods.

© COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only. By femtosecond-laser-based modification of polydimethylsiloxane (PDMS), we fabricated an electrically conductive structure composed of β-silicon carbide (β-SiC) on a PDMS thin film for strain sensing. With an increase in applied strain, i.e. smaller bend radii of the conductive structures, the conductivity of the structure decreased significantly. The result indicates that the resistance of the conductive structure has high sensitivity to strain, leading to potential applications such as real-time monitoring of human motion.

Copyright © 2019 SPIE. We experimentally demonstrated formation of electrically conductive structures by modifying native polydimethylsiloxane (PDMS) with femtosecond laser. By irradiating femtosecond laser pulses to native PDMS, black structures with electrical conductivity were formed. Analyses using X-ray diffraction (XRD) show that the formed structures were composed of β-silicon carbide (β-SiC). Our technique enables the spatially selective formation of β-SiC on the surface of PDMS, leading to open a novel route to develop a simple method to fabricate flexible or stretchable MEMS devices with SiC microstructures.

© The Author(s) 2018. Advanced gold (Au) and silver (Ag) nanostructures were produced by laser techniques on printer paper substrate. Surface-enhanced Raman spectroscopy (SERS) analyses of the fungicide mancozeb (Dithane DG) and insecticide thiamethoxam (Aktara 25 BG) in quantities smaller than usually applied in agricultural medicine were performed for the first time assisted by the structures fabricated. The investigations and results show an easy alternative and cheap way to detect small amounts or residue of harmful environmental pollutants, which has a direct bearing on food quality and thus on human health.

© 2019 SPIE. In this work results on laser assisted formation of silver nanoparticles in glass are presented. The sample material used in the experiments is Ag doped borosilicate glass fabricated by conventional melt quenching method. The glass samples are irradiated by nanosecond laser pulses at wavelength of 266 nm with a wide range of fluences and number of applied pulses. It is found that the laser radiation can lead to specific yellow coloration of the irradiated areas. The performed analyses show formation of silver nanoparticles in these zones. The optical properties of the irradiated areas are found to depend on the laser processing parameters and the silver concentration. The mechanism of nanoparticle formation is also discussed. The presented method can be used for formation of nanoparticles inside transparent materials and can be used for fabrication of novel materials with application in photonics.