Open Access
Add to BibSonomy
Abstract
Hydroxyapatite-based micropatterns were fabricated via near-infrared femtosecond laser irradiation. The micropatterns had a hierarchical cross-section comprising an Ag-based core and an overlying thick layer of non-photosensitive hydroxyapatite nanoparticles. Such micropatterns were continuously formed by the translation of the laser focus in hydroxyapatite nanoparticle-containing AgNO3 solution. A pattern more than quadruple in width was obtained by adding nanoparticles to the solution, despite the same laser irradiation conditions. Raman spectroscopy indicated that the hydroxyapatite nanoparticles retained their crystalline nature even after laser printing. Furthermore, we demonstrate the fabrication of hydroxyapatite-clad patterns on the inner walls of glass capillaries using laser printing.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Hydroxyapatite (Ca10(PO4)6(OH)2, HA), a calcium phosphate-based ceramics, is recognized to be one of the essential inorganic materials for biomedical engineering. HA can provide surfaces suitable for cell adhesion, migration, and differentiation because of its structural similarities to human bones and its physical stability [1,2]. In particular, micropatterned HA can function as an effective bioscaffold for controlling the area-specific adhesion and orientation of cells. These different cell behaviors strongly depend on the width and depth of the micropatterns [3–6]. Although micropatterned HA provides highly controllable microenvironments, the microfabrication of HA remains difficult, partly because of its low compatibility with lithographic processes. HA is not suited for reactive ion etching because of its non-Si-based composition, resulting in limited lithographic fabrication routes. Other surface microstructuring processes, including inkjet printing and laser ablation, have been reported [7–11]. Inkjet printing has a low spatial resolution ranging from tens to hundreds of micrometers [9,10] and also requires careful tailoring of ink composition and viscoelasticity. Laser ablation is a removal process involving material ionization that results in rough surfaces and cracks caused by thermal effects and explosive ejection of the materials [11]. In contrast, we have previously reported femtosecond laser-reduction-triggered nanoparticle deposition [12]. This process is a type of direct laser microprinting that allows for additive micromanufacturing only by translating the laser focus. Various direct laser printing methods have been extensively reported for micro-/nanofabrication [13–16]. However, there are still strong limitations in the material choices. Thus, the target materials require appropriate light absorption and subsequent photochemical or physical reaction paths against the incident laser wavelength. Although thus far, various devices for electronics, photonics, and biotechnology have been fabricated using direct laser printing [17–22], all these have been confined to the use of photosensitive materials, including photocurable resins, photosensitive composites, and photoabsorptive metallic inks. By contrast, femtosecond laser-reduction-triggered nanoparticle deposition can potentially provide a broad range of materials in direct laser printing. Figure 1 shows a schematic of the femtosecond laser-reduction-triggered nanoparticle deposition process: (a) first, a low-intensity femtosecond laser was focused in a diluted AgNO3 solution containing the target nanoparticles. An Ag-based core was generated by the two-photon reduction of Ag ions near the laser focus. (b) Subsequent laser pulses with a high repetition rate heat the core, leading to the generation of microscale vapor bubbles around the laser focus. (c) A strong temperature gradient on the bubble surface induced Marangoni convection, which attracted the surrounding nanoparticles, followed by substantial deposition on the core. In this process, direct interactions between the laser pulse and nanoparticles, such as photopolymerization and thermal melting, are not necessarily required because of the use of flow-based nanoparticle transport, which in principle can offer a wide range of material choices [12].
Fig. 1. Schematic illustration of femtosecond laser-reduction-triggered nanoparticle deposition. (a) Ag precipitation by two-photon reduction at the focus, (b) microbubble generation by subsequent pulses, and (c) nanoparticle transport and deposition on the core via Marangoni convection.
In this study, we report the application of femtosecond laser-reduction-triggered nanoparticle deposition to the microprinting of HA-clad patterns. HA exhibits high transparency over a wide range of wavelengths, including that of the incident laser. Therefore, it is difficult to fabricate HA micropatterns using conventional laser microprinting. Here, we directly laser-printed micropatterns with thick HA clad layers and investigated their properties of printing using laser-reduction-triggered nanoparticle deposition. Despite the non-photosensitivity of HA, continuous micropatterns were obtained by this laser printing method.
2. Experimental details
A near-infrared femtosecond fiber laser (HP-780, Menlosystems Ltd.) was used as the light source for the laser printing. The central wavelength, pulse duration, and repetition rate are 780, 127, and 100 MHz, respectively. A colloidal solution was prepared by adding HA nanoparticles (Sigma-Aldrich Co.) and C2H5OH (99.5%; Nacalai Tesque, Japan) to an AgNO3 solution (Wako Co., Japan). The concentrations of HA and AgNO3 were 7.2 wt% and 0.2 M, respectively. The average diameter of HA nanoparticles in colloidal solution, measured from TEM images after removal of precipitates that resulted from the addition of AgNO3 solution, was 90 nm with a standard deviation of 33 nm. Laser pulses were focused on the solution through a coverslip using an objective lens with an N.A. of 0.50, as described in our previous report [12]. The solution in a Teflon holder was placed on a computer-aided 3 axis stage system. Nanoparticle films of HA were prepared by spin-coating a colloidal solution (Sigma-Aldrich Co.) onto SiO2 substrates or coverslip at a rotational speed of 1000 rpm for 30 s, followed by baking at 100°C for 60 s. Nanoparticle films of Ag were formed by spin-coating of Ag nanoparticle ink (Ag1 T, ULVAC Co.) diluted by toluene at the same rotation and baking conditions. Laser irradiated films were developed by toluene and ethanol after laser printing. Optical absorption measurements were performed using UV-VIS spectrophotometer (V-650, JASCO Co.). Raman spectra were measured using a microspectroscopy system (Micro-RAM300, Ramda-vision Co., Japan) with an excitation light source of 532 nm. Surface observations and energy-dispersive X-ray spectroscopy (EDS) were performed using an SEM system (JSM-7600FA, JEOL Co., Japan). Ultrathin films, approximately 200 nm thickness, of micropatterns for cross-sectional observation were prepared by focused ion beam etching (JEM-9320, JEOL Co., Japan). High-resolution images of the ultrathin films were obtained using a transmission electron microscope (JEM-2100, JEOL Co., Japan). Pt-Pd thin films for conductivity and C protection were deposited prior to the FIB process.
3. Results and discussion
3.1 Laser printing property of HA-clad patterns
Figure 2(a) presents a top view of the line structure printed by translating the laser focus in the AgNO3 solution with HA nanoparticles. For comparison, an SEM image of the Ag structure obtained using a nanoparticle-free AgNO3 solution, which was previously used as two-photon reduction-based printing [23], is shown in Fig. 2(b). The average laser power and printing speed were, respectively, 50 mW and 100 µm/s. In both cases, continuous structures were formed along the translation direction of the laser focus. The width of the Ag line was approximately 3.2 µm, which is close to the focal spot size of 2 µm. Compared with this case, the linewidth of the nanoparticle-containing solution was enhanced to 14 µm, which was more than four times that of the nanoparticle-free solution. From an enlarged view, particles approximately 100 nm in diameter were observed on the line surface of the nanoparticle-containing solutions. This surface was smoother than the rugged Ag line surface.
Fig. 2. SEM images of micropatterns printed using (a) HA nanoparticle-containing AgNO3 solution and (b) HA nanoparticle-free AgNO3 solution.
3.2 Cross-sectional observation
Figure 3 shows a cross-sectional TEM image of the line printed using the nanoparticle-containing solution. The average laser power and translation speed were 50 mW and 100 µm/s, respectively. The line has a hierarchical cross-section consisting of a core region (dark semicircular region) and a clad layer. The outermost layer was used as a protective film for FIB etching during ultrathin sample preparation. The total line width and height were 15.3 µm and 7.1 µm, respectively. A cladding layer approximately 2 µm thick was uniformly distributed on the core. In the enlarged view, the cladding layer is an aggregate of fine particles approximately 100 nm in diameter. In addition to the fine particles, dark spots ranging from a few hundred nanometers to approximately 1 micron in size were unevenly distributed in the core. From the elemental analysis shown in Fig. 4, the clad layer exhibited signals of Ca and P as fundamental elements of HA, whereas Ag was not detected. In contrast, the core contained Ag, in addition to both P and Ca. Considering that both Ca and P were detected, the particles packed in the clad layer were likely HA nanoparticles originally dispersed in the solution before irradiation.
Fig. 3. Cross-sectional TEM images of micropatterns printed by femtosecond laser irradiation using HA nanoparticle-containing solutions. (a) Overview, (b) enlarged view of the clad layer (red square), and (c) enlarged view of the core (white square).
Fig. 4. (a) Cross-sectional SEM image and chemical composition maps of (b) Ag, (c) Ca, and (d) P for the cross-section of the micropattern printed by laser irradiation in HA nanoparticle-containing solution. The dotted line in (b) indicates the border between the nanoparticle and the protection layers. A dashed line in (b) represents the border between the nanoparticle layer and the carbon protection layer.
Furthermore, the similarity in the diameters of the nanoparticles in the cladding and dispersed nanoparticles in the solution are consistent with this inference. The dark spots in the core are considered to be Ag microstructures. Figure 5(a) presents absorption spectrum of a HA nanoparticle film. The film exhibited Rayleigh scattering losses, with no discernible clear absorption bands. The spectrum of Ag nanoparticle film, recognized as a representative photosensitive material, was additionally plotted to compare the photosensitivity between HA nanoparticles and Ag nanoparticles. The incident laser light is absorbed by Ag nanoparticles, leading to aggregation or melting through photothermal conversion, which is a common mechanism for laser patterning of nanoparticle films [24–26]. Absorption band around 490 nm in wavelength, attributed to plasmon absorption, was observed. The absorption tail of the band extended into near-infrared region including the incident laser wavelength of 780 nm. Considering the scattering loss in the spectrum of the HA nanoparticle film, absorption intensity of Ag nanoparticle film at 780 nm in wavelength was at least two times greater than that of HA nanoparticle film. While continuous Ag micropatterns were easily printed for Ag nanoparticle films at the conditions of a laser translation speed of 100 µm/s and an average power of 50 mW, as shown in Fig. 5(b), only dot-like HA structures were obtained for HA nanoparticle films, as depicted in Fig. 5(c). This was the case even at an ultralow translation speed of 10 nm/s with a power of 55 mW. Moreover, no changes were observed in HA nanoparticle films at power below 50 mW. Therefore, the photochemical reactions directly with the HA nanoparticles were almost negligible under the irradiation condition for the clad layer formation. When a femtosecond laser is tightly focused in highly transparent oxide glasses, structural modifications, including defect formation and densification, have been reported to occur through multiphoton processes despite non-photosensitivity [27–30]. However, these direct interactions require a peak intensity higher than 1013 W/cm2 [29,30]. In contrast, the intensity of our irradiation conditions was on the order of 1010 W/cm2, which did not allow for modification and related reactions. On the other hand, the unique features of the cross-section consisting of an Ag-based core and a nanoparticle aggregation layer suggested that femtosecond laser-reduction-triggered nanoparticle deposition occurred in the solution with HA nanoparticles. This implies that the Ag-based core formation by two-photon reduction reactions near the laser focus in the early stages of this process was followed by nanoparticle transport onto the core by microbubble-driven convection. Haga et al. previously reported the generation of vapor microbubble in colloidal solution by laser irradiation, without moving the laser focus, to Au coated glass substrates [31,32]. In their study, ring-like microstructures consisting of nanoparticle aggregates appeared after turning off the laser. The formation of the ring-like aggregates was explained by Marangoni convection-driven assembly of nanoparticles, induced by strong gradient in surface tension on the bubble surface. In their experiments, Au films was used as photothermal converters for microbubble generation. In contrast, unlike their approach, we did not pre-prepare Au films before the irradiation in our printing process. However, Ag photo-generated around the laser focus should have served a similar function as a photothermal converter. Therefore, it can be inferred that HA nanoparticles have been drawn to the interface between the microbubble and the underlying Ag-based core through Marangoni convection, leading to the formation of HA nanoparticle layers on the core.
Fig. 5. (a) Optical absorption spectrum of HA nanoparticle film and Ag nanoparticle film, and SEM images of (b) laser printed Ag line (magnification ×5000) and (c) laser printed HA structures (magnification ×10000).
The compositional separation between the core and cladding layers suggests that the formation mechanism differed between the two regions. These results support the deposition of HA nanoparticles transported via laser-induced convection. In the cross-sectional image shown in Fig. 3(c), the Ag-based core contains not only patchy Ag structures but also nanoparticles. Such a composite structure can be understood as a result of the embedding of HA nanoparticles into the photogenerated Ag matrix in the focal volume. This composite structure may have functioned as a photothermal converter for microbubble generation, resulting in nanoparticle transport without direct interaction between the laser pulses and nanoparticles.
The width of Ag-based core of the hierarchical structure was 9.7 µm in Fig. 3(a), although Ag line width was 3.2 µm in case of nanoparticle-free AgNO3 solution as shown in Fig. 2(b). Figure 6(a) shows optical absorption spectrum of HA colloidal solution without AgNO3. The HA concentration was suppressed to be 7.2 × 10−2 wt% for the measurements. While there was absorption below 235 nm in wavelength, the solution displayed optical losses attributed to Rayleigh scattering over a wide range of wavelengths, resulting in white coloration of the solution. The refractive index of HA is 1.63, which is higher than that of the solution of 1.33. In addition, due to the large particle size of approximately 100 nm used, scattering is prone to occur against the incident wavelength of 780 nm. The larger core formed using nanoparticle-containing solution, shown in Fig. 3(a), likely caused an expansion of the Ag generation region because of the scattering by HA nanoparticles within the laser focal volume. In case of TiO2 nanoparticle-containing AgNO3 solution in our previous study, such expansion of Ag-based core was not observed [12]. Although the refractive index of TiO2 is similar to that of HA, its smaller particle size of 20 nm may have suppressed the scattering.
Fig. 6. (a) Optical absorption spectrum of HA colloidal solution without AgNO3, and (b) Raman spectra of a micropattern formed by femtosecond laser printing using HA nanoparticle-containing solution and HA nanoparticle film. The photograph in Fig. 6(a) is HA colloidal solution without AgNO3.
3.3 Raman spectroscopy
Figure 6(b) shows the Raman spectra of the individual laser-printed micropatterns formed using HA-nanoparticle-containing solutions. The average laser power and translation speed for laser printing were 50 mW and 100 µm/s, respectively. The HA nanoparticle film is shown as a reference. Raman peaks appeared at 424, 450, 580 and 960 cm-1 for the micropattern. Weak peaks at 424 cm-1, 450 cm-1, and 580 cm-1 were attributed to asymmetric (v2) and symmetric (v4) bending modes of O-P-O in PO43-, and the intense peak at 960 cm-1 was attributed to the symmetric stretching mode of PO43- (v1) [33–36]. The Raman peak positions of the micropatterns were similar to those of the films. The peak at approximately 960 cm-1 exhibited a red shift compared to that of the film, which was caused by the difference in residual stress due to the thermal history. This similarity indicated that the crystal structures of the HA nanoparticles were retained even after deposition.
3.4 Power dependence of line width and height
Figure 7 shows the power dependence of the width and height of the laser-printed lines using the nanoparticle-containing solutions. The laser translation speed was maintained at 100 µm/s. The average laser power was changed from 30 to 50 mW, which corresponded to the peak intensity of 83 GW/cm2 to 138 GW/cm2.Both width and height increased almost monotonically with power. A maximum width of 15 µm and height of 8 µm were obtained at a power of 50 mW. This increase can be attributed to the increased number of deposited nanoparticles with increasing laser power. The linewidth exhibited a relatively high variability at a laser power of 30 mW because of the unstable generation of the Ag-based cores. The rate of increase in the width with respect to the laser power was 0.25 µm/mW, which was approximately twice that at a height of 0.12 µm/mW. As shown in the inset, although nanoparticle deposition occurred on both sides of the core in the horizontal direction, it occurred only above the core in the vertical direction. Therefore, the ratio of approximately 2 between the increase rates of the width and height suggests that the nanoparticle deposition occurred relatively uniformly along the surface of the Ag-based core in the semi-circular cross-section. In fact, the thickness of the nanoparticle layer remained approximately constant at 2 µm for the cross-section in Fig. 3(a).
Fig. 7. Power dependence of the (a) widths and (b) heights of micropatterns printed by laser irradiation in HA nanoparticle-containing solutions.
3.5 Estimation of affected volume
Figure 8 shows a schematic illustration of a structure sliced from a nanoparticle cladding line. The sliced structure with a thickness of 1 µm had a total width of 14 µm and a clad layer thickness of 2 µm. Assuming that the interparticle distance is 100 nm in the clad layer, the total number of nanoparticles in the clad corresponds to that in a 15.8 µm cubic volume in the nanoparticle-containing solution. The volume where the incident light can directly react is limited to the two-photon reaction region with the AgNO3 solution because HA nanoparticles exhibit no reactivity at either one- or two-photon wavelengths. The cubic volume, that is, the solution volume affected by this laser process, was 135 times larger than the two-photon reaction region, which was roughly estimated from the (spot diameter)2 × focal length. The nanoparticle concentration was estimated to increase by a factor of 104 due to nanoparticle confinement in the clad.
Fig. 8. Hierarchical cross-sectional model for estimating the volume affected by laser-reduction-triggered nanoparticle deposition.
3.6 Laser printing in closed space
This laser-reduction-triggered nanoparticle deposition is a full-solution process that requires neither film deposition in a vacuum nor spin coating of the target materials before laser irradiation. Therefore, the nanoparticle deposition process is applied in closed and narrow spaces. Figure 9 shows SEM images of the arrayed lines with HA nanoparticle layers laser-printed on the inner wall of a glass capillary. For laser printing, an HA-nanoparticle-containing solution was inserted into the inner space of the glass capillaries. Then, the laser pulses were focused on the solution through the capillary wall from the outside. The laser power and the translation speed were 45 mW and 100 µm/s, respectively. Each line with a width of 12 µm in the array contained HA nanoparticle layers. This flexible microfabrication of HA structures in closed spaces will be useful for the development of microfluidic biodevices for biomedical applications.
Fig. 9. SEM images of arrayed lines printed on the inner wall of the glass capillary. (a)–(c) Micropatterns along the axis and (d)-(f) along the horizontal direction.
4. Conclusions
Femtosecond laser-reduction-triggered nanoparticle deposition, as additive micromanufacturing, was applied to the HA particles. Micropatterns with thick HA-based cladding layers were printed directly using low-intensity femtosecond laser irradiation in a nanoparticle-containing AgNO3 solution. The nanoparticle concentration increased by a factor of 104 by nanoparticle confinement in the clad. The crystalline nature of the nanoparticles was retained even after the laser printing. Microfabrication of the inner wall of a glass capillary was demonstrated. Direct laser printing of biocompatible ceramics is useful for the production of flexible bioscaffolds in biomedical engineering.
Funding
Ministry of Education, Culture, Sports, Science and Technology (19H02474, 22K18746, 23H01723) .
Acknowledgments
This work was partly supported by Grants-in-Aid Nos. 19H02474, 22K18746, and 23H01723 (KAKENHI) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.
Disclosures
The authors declare no competing interests.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. X. H. Shi, S. L. Wang, Y. M. Zhang, et al., “Hydroxyapatite-coated silicone rubber enhanced cell adhesion and it may be through the interaction of EF1β and χ-actin engineering applications: A focused review,” PLOS ONE 9(11), e111503 (2014). [CrossRef]
2. W. S. W. Harun, R. I. M. Asri, J. Alias, et al., “A comprehensive review of hydroxyapatite-based coatings adhesion on metallic biomaterials,” Ceram. Int. 44(2), 1250–1268 (2018). [CrossRef]
3. M. G. Holthaus, J. Stolle, L. Treccani, et al., “Orientation of human osteoblasts on hydroxyapatite-based microchannels,” Acta Biomater. 8(1), 394–403 (2012). [CrossRef]
4. M. Tagaya, T. Yamazaki, D. Tsuya, et al., “Nano/microstructural effect of hydroxyapatite nanocrystals on hepatocyte cell aggregation and Adhesion,” Macromol. Biosci. 11(11), 1586–1593 (2011). [CrossRef]
5. X. Lu and Y. Leng, “Comparison of the osteoblast and myoblast behavior on hydroxyapatite microgrooves,” J. Biomed. Mater. Res., Part B 90B(1), 438–445 (2009). [CrossRef]
6. X. Lu and Y. Leng, “Quantitative analysis of osteoblast behavior on microgrooved hydroxyapatite and titanium substrata,” J. Biomed. Mater. Res., Part A 66A(3), 677–687 (2003). [CrossRef]
7. M. G. Holthaus, L. Treccani, and K. Rezwan, “Comparison of micropatterning methods for ceramic surfaces,” J. Eur. Ceram. Soc. 31(15), 2809–2817 (2011). [CrossRef]
8. S. Michna, W. Wu, and J. A. Lewis, “Concentrated hydroxyapatite inks for direct-write assembly of 3-D periodic scaffolds,” Biomaterials 26(28), 5632–5639 (2005). [CrossRef]
9. J. A. Hondred, L. R. Stromberg, C. L. Mosher, et al., “High-resolution graphene films for electrochemical sensing via inkjet maskless lithography,” ACS Nano 11(10), 9836–9845 (2017). [CrossRef]
10. W. D. Chen, Y. H. Lin, C. P. Chang, et al., “Fabrication of high-resolution conductive line via inkjet printing of nano-palladium catalyst onto PET substrate,” Surf. Coat. Technol. 205(20), 4750–4756 (2011). [CrossRef]
11. H. Zhu, Z. Zhang, J. Xu, et al., “An experimental study of micro-machining of hydroxyapatite using an ultrashort picosecond laser,” Precis. Eng. 54, 154–162 (2018). [CrossRef]
12. H. Nishiyama, K. Umetsu, and K. Kimura, “Versatile direct laser writing of non-photosensitive materials using multi-photon reduction-based assembly of nanoparticles,” Sci. Rep. 9(1), 14310 (2019). [CrossRef]
13. S. Kawata, H.-B. Sun, T. Tanaka, et al., “Finer features for functional microdevices,” Nature 412(6848), 697–698 (2001). [CrossRef]
14. R. Wang, X. Duan, J. Yao, et al., “Processing-structure-property relationship in direct laser writing carbonization of polyimide,” J. Appl. Polym. Sci. 137(34), 48978 (2020). [CrossRef]
15. B. Kang, S. Han, J. Kim, et al., “One-step fabrication of copper electrode by laser-induced direct local reduction and agglomeration of copper oxide nanoparticle,” J. Phys. Chem. 115(48), 23664–23670 (2011). [CrossRef]
16. H. Nishiyama, S. Odashima, and S. Asoh, “Femtosecond laser writing of plasmonic nanoparticles inside PNIPAM microgels for light-driven 3D soft actuators,” Opt. Express 28(18), 26470–26480 (2020). [CrossRef]
17. H. Nishiyama, M. Mizoshiri, T. Kawahara, et al., “SiO2-based nonplanar structures fabricated using femtosecond laser lithography,” Opt. Express 16(22), 17288–17294 (2008). [CrossRef]
18. W. Zhou, S. Bai, Y. Ma, et al., “Laser-direct writing of silver metal electrodes on transparent flexible substrates with high-bonding strength,” ACS Appl. Mater. Interfaces 8(37), 24887–24892 (2016). [CrossRef]
19. J. Kwon, H. Cho, H. Eom, et al., “Low-Temperature Oxidation-Free Selective Laser Sintering of Cu Nanoparticle Paste on a Polymer Substrate for the Flexible Touch Panel Applications,” ACS Appl. Mater. Interfaces 8(18), 11575–11582 (2016). [CrossRef]
20. M. Suter, L. Zhang, E. C. Siringil, et al., “Superparamagnetic microrobots: fabrication by two-photon polymerization and biocompatibility,” Biomed. Microdevices 15(6), 997–1003 (2013). [CrossRef]
21. J. B. In, B. Hsia, J.-H. Yoo, et al., “Facile fabrication of flexible all solid-state micro-supercapacitor by direct laser writing of porous carbon in polyimide,” Carbon 83, 144–151 (2015). [CrossRef]
22. H. Nishiyama and Y. Saito, “Electrostatically tunable plasmonic devices fabricated on multi-photon polymerized three-dimensional microsprings,” Opt. Express 24(1), 637–644 (2016). [CrossRef]
23. T. Tanaka, A. Ishikawa, and S. Kawata, “Two-photon-induced reduction of metal ions for fabricating three-dimensional electrically conductive metallic microstructure,” Appl. Phys. Lett. 88(8), 081107 (2006). [CrossRef]
24. B. Kang, S. Ko, J. Kim, et al., “Microelectrode fabrication by laser direct curing of tiny nanoparticle self-generated from organometallic ink,” Opt. Express 19(3), 2573–2579 (2011). [CrossRef]
25. M. Aminuzzaman, A. Watanabe, and T. Miyashita, “Direct writing of conductive silver micropatterns on flexible polyimide film by laser-induced pyrolysis of silver nanoparticle-dispersed film,” J. Nanopart. Res. 12(3), 931–938 (2010). [CrossRef]
26. K. An, S. Hong, S. Han, et al., “Selective sintering of metal nanoparticle ink for maskless fabrication of an electrode micropattern using a spatially modulated laser beam by a digital micromirror device,” ACS Appl. Mater. Interfaces 6(4), 2786–2790 (2014). [CrossRef]
27. K. M. Davis, K. Miura, N. Sugimoto, et al., “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21(21), 1729–1731 (1996). [CrossRef]
28. Y. Kondo, K. Nouchi, T. Mitsuyu, et al., “Fabrication of long-period fiber gratings by focused irradiation of infrared femtosecond laser pulses,” Opt. Lett. 24(10), 646–648 (1999). [CrossRef]
29. B. Poumellec, M. Lancry, A. Chahid-Erraji, et al., “Modification thresholds in femtosecond laser processing of pure silica: review of dependencies on laser parameters,” Opt. Mater. Express 1(4), 766–782 (2011). [CrossRef]
30. K. Hirao and K. Miura, “Writing waveguides and gratings in silica and related materials by a femtosecond laser,” J. Non-Cryst. Solids 239(1-3), 91–95 (1998). [CrossRef]
31. S. Fujii, K. Kanaizuka, S. Toyabe, et al., “Fabrication and placement of a ring structure of nanoparticles by a laser-induced micronanobubble on a gold surface,” Langmuir 27(14), 8605–8610 (2011). [CrossRef]
32. S. Fujii, K. Kobayashi, K. Kanaizuka, et al., “Manipulation of single DNA using a micronanobubble formed by local laser heating on a Au-coated surface,” Chem. Lett. 39(2), 92–93 (2010). [CrossRef]
33. D. Yamini, G. Devanand Venkatasubbu, J. Kumar, et al., “Raman scattering studies on PEG functionalized hydroxyapatite nanoparticles,” Spectrochim. Acta, Part A 117, 299–303 (2014). [CrossRef]
34. G. Penel, G. Leroy, C. Rey, et al., “MicroRaman spectral study of the PO4 and CO3 vibrational modes in synthetic and biological apatites,” Calcif. Tissue Int. 63(6), 475–481 (1998). [CrossRef]
35. V. K. Mishra, S. B. Rai, B. P. Asthana, et al., “Effect of annealing on nanoparticles of hydroxyapatite synthesized via microwave irradiation: Structural and spectroscopic studies,” Ceram. Int. 40(7), 11319–11328 (2014). [CrossRef]
36. V. Nosenko, N. Strutynska, I. Vorona, et al., “Structure of biocompatible coatings produced from hydroxyapatite,” Nanoscale Res. Lett. 10(1), 464–1–7 (2015). [CrossRef]
