Published papers

© 2019, Springer-Verlag GmbH Germany, part of Springer Nature. The degradation rates of biodegradable polymers are necessary to be controlled for tissue scaffold applications. In this work, we demonstrate that the degradation rate of poly(lactic-co-glycolic acid) (PLGA) was accelerated by irradiating with femtosecond laser pulses under the condition where material removal, i.e., laser ablation had not occurred. Scanning electron microscopy observation showed that the degradation rate was accelerated without significant laser ablation. The analysis of Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy revealed that the chemical properties of the PLGA had changed by femtosecond laser pulses irradiation. Furthermore, we have investigated the degradation of PLGA when the femtosecond laser pulses were focused inside the material. After the degradation, the PLGA showed enhanced optical scattering region in the depth of 300 µm from the surface. The results indicate that high-aspect-ratio modification and degradation are possible by changing the depth of focus of femtosecond laser pulses under the conditions where the direct removal of the material was not occurred.

© 2019, Springer-Verlag GmbH Germany, part of Springer Nature. Polydimethylsiloxane (PDMS) is widely used for bio-medical, optical and microfluidic applications. Hence, PDMS layers on free-form surfaces on defined areas are needed. Conventional ways such as spin coating show drawbacks by long processing time and no sufficient application on free form surfaces. In this work, laser direct writing with UV-curable PDMS on a curved surface was performed. The coating technique using aerosol jet printing showed thin film thicknesses, compared to conventional spin coating techniques, obtaining controllable layer thickness down to 3.5 µm in 5 min on an area of 600 mm 2 and on non-flat surfaces. Deposition rate control achieved layer thicknesses between 3.5 and 25.7 µm. The combination of material deposition and x–y galvanometric mirror scanner-based laser direct writing leads to a hybrid approach aiming for several applications in the fields of surface functionalization, and bio-medical and sensory applications. In addition, the procedure is able to overcome the batch-based PDMS processing and introduce the continuous flow-based application.

© 2019 Indocyanine green lactosome (ICG-lactosome) is an attractive new-generation agent for photodynamic therapy (PDT) that is characterized by a near-infrared excitation wavelength and high stability in the bloodstream. Fluorescence imaging has been used to examine its pharmacokinetics in vivo, but no depth-resolved information can be obtained with this method. In this study, we applied photoacoustic (PA) imaging to visualize the depth distribution of ICG-lactosome in a mouse subcutaneous tumor model. With this method, the depth distribution of blood vessels can also be visualized, enabling detection of vascular shutdown effects due to PDT. We performed PA imaging of both the distributions of ICG-lactosome and blood vessels in a tumor before and after PDT, and we found that PA signals originating from ICG-lactosome were greatly increased at 18 h after drug injection but rapidly decreased after PDT. These results indicate efficient accumulation of ICG-lactosome and rapid photobleaching due to the PDT reaction in the tumor, respectively. After PDT, PA amplitudes of hemoglobin were significantly decreased, being attributable to vascular shutdown effects. These results show the usefulness of PA imaging for monitoring not only photosensitizer accumulation and bleaching but also vascular responses in PDT with ICG-lactosome. This method can be applied to the diagnosis of many types of PDT processes.

© 2019 IEEE. Soft materials, including elastic polymers and hydrogels, are promising materials for soft robots, implantable devices, and wearable devices owing to their flexibility and biocompatibility. In recent years, the scope of use of such soft materials is expanding further because of their integration with metal micro- and nanostructures, which exhibit attractive optical and electrical properties. In this study, we demonstrate the formation of metal microstructures inside a hydrogel by photoreduction of metal ion by using femtosecond laser. Because the hydrogels have permeability to fluids, metal ions are able to be added by simply immersing the hydrogel into the metal ion solution. Femtosecond laser pulses were focused and scanned in the hydrogel to fabricate metal microstructures [1, 2]. The optical diffraction pattern obtained with the silver grating inside the hydrogel showed spaced diffraction spots, which indicated that a regular, periodic silver grating was formed. Notably, the distance between the diffraction spots changed when the water content in the hydrogel was reduced. The grating period decreased when the hydrogel shrank owing to the loss of water (Fig. 1(a)), but the straight shapes of the line structures were preserved, which demonstrated the optical tunability of the fabricated structure.

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. Hydrogel waveguides have found increased use for variety of applications where biocompatibility and flexibility are important. In this work, we demonstrate the use of polyethylene glycol diacrylate (PEGDA) waveguides to realize a monolithic lab-on-a-chip device. We performed a comprehensive study on the swelling and optical properties for different chain lengths and concentrations in order to realize an integrated biocompatible waveguide in a microfluidic device for chemical sensing. Waveguiding properties of PEGDA hydrogel were used to guide excitation light into a microfluidic channel to measure the fluorescence emission profile of rhodamine 6G as well as collect the fluorescence signal from the same device. Overall, this work shows the potential of hydrogel waveguides to facilitate delivery and collection of optical signals for potential use in wearable and implantable lab-on-a-chip devices.

© 2019 Elsevier B.V. This paper describes the luminescence properties of small silver clusters formed by laser treatment of silver-doped borosilicate glass. Using irradiation by a Nd:YAG nanosecond laser system operating at the fourth harmonic wavelength (266 nm), areas are formed in the glass samples that emit bright luminescence peaking at about 600 nm when excited by UV light in the spectral range 300–390 nm. The influence is presented and discussed of the glass composition and the laser processing parameters on the emission characteristics. It is further shown that the luminescent properties are affected by the fluence of the excitation source, namely, a complete quenching of the emission is achieved above a certain value. The luminescence of the irradiated areas is explained by the formation of Ag clusters due to irradiation-induced reduction of Ag ions and increased atom mobility resulting from the local heating. The limited stability of the luminescent areas upon UV radiation can be related to heat-induced cluster decomposition and re-oxidation. The results obtained provide a new insight in the optical properties of noble-metal nanostructures and could form the basis of fabrication of complex optical systems and luminescent component used in the study of, e.g., biological systems.

© 2019 Elsevier B.V. In this work, we present fabrication and studies of pure and Pd-decorated ZnO nanostructures on Ag-coated substrates in view of gas-sensing application. The sensor elements consist of a dense structure of ZnO nanobelts. The chemical composition of the surface and gas-sensing properties of pure and Pd-doped nanostructures were studied and compared. On the surface of the Pd-decorated ZnO samples, the presence was found of Pd and Pd-Ag clusters, as well as isolated Pd2+ ions were incorporated into the ZnO lattice. The Pd-ZnO sensor element showed a better response to CO, NH3, and acetone compared to the pure ZnO sample. The Pd-ZnO sample exhibited its highest response to exposure to acetone at room temperature. We associate the improvement of sensing performance of the Pd-decorated ZnO nanostructures with the increased number of adsorption centers on the surface due to the Pd nanoparticles.

© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Controlling cell adhesion and cell differentiation is necessary to fabricate a tissue with arbitrary properties for tissue engineering applications. A substrate with a porous structure as a cell scaffold allows the diffusion of the cell culture medium through the scaffold. In this work, we show that the femtosecond laser fabricated micro through-holes in biodegradable polymer films, enhance myoblast adhesion, and accelerates proliferation and differentiation. ChR2-C2C12 and UT-C2C12 cells were seeded on the films with micro through-holes each fabricated by a single femtosecond laser pulse. Cell adhesion was enhanced on films with holes fabricated by laser irradiation. In addition, cell proliferation was accelerated on films with micro through-holes that penetrate the film, compared to on films with micro craters that do not penetrate the film. On films with arrays consisting of micro through-holes, cells aligned along the arrays and cell fusion was enhanced, indicating the acceleration of cell differentiation.

© 2020 IEEE. The fabrication of electrically conductive structures by femtosecond-laser irradiation of a cellulose nanofiber (CNF) film was demonstrated. Raman analyses indicated the formation of graphitic carbon, which is attributable to the electrical conductivity of the structures.

© 2021 American Chemical Society. Laser graphitization allows for the spatially targeted modification of biomasses into electrically conductive structures and is a promising technique for the development of sustainable electronic devices. Out of the many biomasses, cellulose nanofiber (CNF) is a sustainable biomass which exhibits unique properties, such as mechanical strength and optical transparency. In this study, we demonstrated the fabrication of highly conductive graphitic carbon from CNFs using a high-repetition femtosecond laser. A conductivity as high as 6.9 S/cm was obtained by single scanning of the laser beam, which is estimated to be more than 100 times higher than the conductivity reported previously. From the Raman analysis of the structure, the formation of highly crystalline graphitic carbon was indicated, which contributes to the high conductivity. In the case of high-repetition femtosecond laser irradiation, it is considered that strong optical effects, in addition to thermal effects, contribute to the degradation of CNFs, resulting in the formation of highly crystalline graphitic carbon. Our presented technique allows for the direct writing of highly conductive graphitic carbon on CNF films, and the development of sustainable electronic devices, using mechanically robust and transparent CNF films as the substrate, should be realized.

We demonstrated the fabrication of gold microstructures inside a PEG-based hydrogel with different charged fluorophores by multi-photon photoreduction. By adding an anionic charged FITC-dextran or a cationic charged Rhodamine110 to the metal ion solution, line width of the fabricated gold microstructures increased compared to that of the structures fabricated in fluorophore-free hydrogel. The photobleaching could enhanced the reduction of gold ions accompanying the oxidation of the fluorophores. Notably, the inhibition of reduction of gold ions at a site other than the focal point was observed with FITC-dextran, which may be attributable to the coordination of gold ions to FITC-dextran.

Porous carbon structures have attracted much interest for applications, such as catalyst carriers, energy storage devices, and chemical sensors, due to their chemical stability, high adsorptive property, and electrical conductivity. Among several methods to fabricate porous carbon structures, laser carbonization is a promising method for spatially-selective and high-resolution fabrication of electrically conductive porous carbon strucutres [1], [2]. Recently, the fabrication of porous carbon structures by laser carbonization using biomasses, materials derived from living or recently-living organisms, as the precursor have been reported [3]. In this study, we fabricated electrically conductive porous carbon structures by the femtosecond-laser carbonization of bamboo, which is a biomass with a well-connected three-dimensional porous structure by nature.

Graphene quantum dots (GQDs) have emerged as a promising new class of environmentally friendly quantum dots with unique properties. However, the limitations of synthesis and patterning methods have hindered GQDs from displaying their true potentials to date. Here, we demonstrate the simultaneous synthesis and patterning of GQDs for the first time inside a transparent polymer, polydimethylsiloxane (PDMS), using femtosecond laser pulses. By focusing and scanning femtosecond laser pulses, arbitrary fluorescent patterns such as a concealed fluorescent QR code can be readily patterned without pre- and/or post-treatment. In addition, the proposed method is applied to the fabrication of fluorescent three-dimensional structures inside a transparent polymer via multiphoton interactions. The proposed method realizes single-stepped and spatially selective patterning of GQDs directly inside polymer substrates and expands the possibilities of GQDs for applications in novel flexible three-dimensional optoelectrical devices.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement. The dual-wavelength-driven shrinkage of metal microstructures and hydrogel actuation are demonstrated by the fabrication of multi-metal microstructures in hydrogels by multiphoton photoreduction. Silver and gold microstructures were fabricated in a poly-Nisopropylacrylamide (PNIPAm) hydrogel. Because of the different optical resonances of the metals, wavelength-dependent shrinkage of metal microstructures was demonstrated concurrently with the volume change of the supporting hydrogel by light stimulation. Furthermore, the direction of actuation of the hydrogel was controlled by switching the wavelength of light stimulation. The results indicate the potential of multiphoton photoreduction for applications in light-driven optical components and micro-robots fabricated with soft materials.

The mechanical strength of hydrogel microstructures is crucial for obtaining the desired flexibility, robustness, and biocompatibility for various applications such as cell scaffolds and soft microrobots. In this study, we demonstrate the fabrication of microstructures composed of cellulose nanofibers (CNFs) and poly(ethylene glycol) diacrylate (PEGDA) hydrogels by multiphoton polymerization. The stress of the fabricated microstructure during tensile testing increased with an increase in the CNF concentration, indicating that the mechanical strength of the microstructure was enhanced by using CNFs as fillers. Moreover, the swelling ratio of the microstructure increased with increasing CNF concentration in the PEGDA hydrogel. Our results show the potential of the technique for the microfabrication of advanced cell scaffolds and soft microrobots with the desired mechanical strength.

© 2018 The Author(s). Stimulation of neuronal cells generally resorts to electric signals. Recent advances in laser-based stimulation methods could present an alternative with superior spatiotemporal resolution. The avoidance of electronic crosstalk makes these methods attractive for in vivo therapeutic application. In particular, nano-mediators, such as gold nanoparticles, can be used to transfer the energy from a laser pulse to the cell membrane and subsequently activate excitable cells. Although the underlying mechanisms of neuronal activation have been widely unraveled, the overall effect on the targeted cell is not understood. Little is known about the physiological and pathophysiological impact of a laser pulse targeted onto nanoabsorbers on the cell membrane. Here, we analyzed the reaction of the neuronal murine cell line Neuro-2A and murine primary cortical neurons to gold nanoparticle mediated laser stimulation. Our study reveals a severe, complex and cell-type independent stress response after laser irradiation, emphasizing the need for a thorough assessment of this approach's efficacy and safety.

© 2017 The Author(s). The integration of metal microstructures and soft materials is promising for the realization of novel optical and biomedical devices owing to the flexibility and biocompatibility of the latter. Nevertheless, the fabrication of three-dimensional metal structures within a soft material is still challenging. In this study, we demonstrate the fabrication of a silver diffraction grating inside a biocompatible poly(ethylene glycol) diacrylate (PEGDA) hydrogel by using a 522-nm femtosecond laser via multi-photon photoreduction of silver ions. The optical diffraction pattern obtained with the grating showed equally spaced diffraction spots, which indicated that a regular, periodic silver grating was formed. Notably, the distance between the diffraction spots changed when the water content in the hydrogel was reduced. The grating period decreased when the hydrogel shrank owing to the loss of water, but the straight shapes of the line structures were preserved, which demonstrated the optical tunability of the fabricated structure. Our results demonstrate the potential of the femtosecond laser-based photoreduction technique for the fabrication of novel tunable optical devices as well as highly precise structures.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement The spatially-targeted fabrication of bimetallic microstructures coexisting in the supporting hydrogel is demonstrated by multi-photon photoreduction. Microstructures composed of gold and silver were fabricated along a predefined trajectory by taking advantages of the hydrogel's ionic permeability. Different resonant wavelengths of optical absorption were obtained for gold, silver, and their bimetallic structures. Transmission electron microscopy and energy dispersive X-ray analysis revealed that the optical properties are attributable to the formation of bimetallic structure consisted of core-shell nanoparticles. The fabrication of dissimilar metal structures within hydrogel is a promising technique for optically driven actuators in soft robotics and sensing applications by allowing for site-selective optical properties.