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Abstract
Herein, three-dimensional (3D) incorporation of plasmonic Ag nanoparticles was performed inside temperature-responsive poly(N-isopropylacrylamide) microgels using near-infrared femtosecond laser multi-photon reduction. The nanoparticles, formed by laser writing at lower doses, exhibited intense plasmonic absorption in the gels around 420 nm wavelength. Light-induced local shrinking of up to 86%, under assumption of isotropic shrinkage, in volume was achieved by the efficient photothermal conversion of Ag nanoparticles. Such shrinkages and deformation speeds strongly depended on the geometric design and 3D layout of the laser writing patterns of Ag nanoparticles inside the microgels. In particular, femtosecond laser incorporation enhanced the recovery speed by more than twice in comparison with the gels containing nanoparticles over the entire region. Laser direct incorporation allows for the control of the 3D position and extent and response speeds of gel deformation.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Stimuli-responsive polymers have attracted much attention over the last decades due to a wide range of innovative applications in diagnostics, tissue engineering, and 4D-printed soft devices. Their physical properties, such as shapes, size, and hydrophobicity, can be changed using external stimuli, including thermal, electrical, magnetic, and ionic interactions [1–5]. So far, various applications have been demonstrated for micromachines [6,7], biomedical structures [8], inchworm-like motion [9], and soft actuator devices [10–21].
Combining nanomaterials and polymer matrices is an effective approach to enhance responsiveness. Such composite polymers exhibit not only mechanical flexibility but also unique responses to external stimuli, which mainly originated from nanomaterial size effects. For example, Au nanorods [22–24] or nanofilms [25] allowed for near-infrared (NIR) light-driven actuation of polymer gels. Motion or shape controls of microstructures containing magnetic nanoparticles has been demonstrated using modulation of external magnetic fields [26,27]. However, in most cases, nanomaterials have been dispersed uniformly in polymer matrices using chemical synthesis processes, which mitigates against the achievement of complex deformation controls. In response to external stimuli, the materials always exhibit the same deformation throughout and this limitation causes difficulty in achieve different modes of motion from a single composite material.
The most common way to overcome this problem is to offer the ability of response only into the specific areas of polymer matrices. Despite such requirements, localized incorporation of nanomaterials into soft materials, particularly wet materials such as hydrogels, remains challenging because of their low compatibility with conventional microfabrication techniques, including vacuum deposition and plasma processes. When femtosecond laser pulses are focused into metal ion solutions, metallic micro-, or sub-microparticles are known to be generated only near the focus, through the process of multi-photon reduction [28–30]. Furthermore, by translation of the laser focus in the solution, 3D metallic structures can be created. Various applications of this technique have been intensively studied so far [31–33]. Here, we have focused attention on the water-content ratio of hydrogels being much higher than other solids. Such abundant water can result in metal ion solution penetration deep into gels, allowing for direct formation of metallic heat sources by laser irradiation even inside the gels.
In this study, we report on the process of 3D incorporation of Ag particles inside temperature-responsive Poly(N-isopropylacrylamide) (PNIPAM) microgels, using NIR femtosecond laser multi-photon reduction, and on investigation of the visible light-induced deformation properties of microgels containing Ag nanoparticles. In the microgels, Ag nanoparticles worked as efficient photothermal converters, so that internal structures consisting of Ag nanoparticles provided a novel controllability mechanism for light-triggered shape changes of hydrogels. The 3D position, extent and response-speeds of gel deformation could be controlled by managing the femtosecond laser irradiation conditions.
2. Experimental details
PNIPAM gels were prepared from an aqueous solution containing N-isopropylacrylamide (NIPAM, 97%, Sigma-Aldrich Co.), N, N’-methylenebisacrylamide (BIS, Sigma-Aldrich Co.) and a photoinitiator (Darocur 1173, Ciba) by UV exposure. NIPAM of 1 M and BIS of 20 mM were stirred in purified H2O at room temperature for several hours. Darocur 1173 of 0.3 wt% was then added to the solution, and it was exposed to UV LED (UVLEDN-102CT, NS-Lighting Co., Ltd.), which delivered light with a wavelength of approximately 365 nm, for 20 s at 3°C. Finally, the gels were immersion in purified H2O for 2 h. PNIPAM gel microfibers was obtained by UV LED exposure through a Cr mask. PNIPAM gel plates and fibers with 1-mm diameter were synthesized by the exposure without a mask. The shrinkage and swelling ratio were almost the same between these samples. The phase-transition temperature and the water-content ratio of PNIPAM gels were measured to be 36°C and 89%, respectively.
Femtosecond fiber laser system (HP-780, Menlosystems Ltd.), which delivers laser pulses of 780-nm wavelength, 127-fs pulse duration with repetition rate of 100 MHz, was used. Linearly polarized laser pulses were focused into PNIPAM gels on cover glasses through an oil immersion objective lens of NA 1.42. Laser translation was performed by moving the laser focus using a computer-controlled, three-axis, piezo stage system. The laser focus was observed in real time with a CCD camera. In direct laser writing, gels were immersed into 0.2 M AgNO3 solution. Coumarin 120 of 0.01 wt% was added to the solution as a photosensitizer [28].
To evaluate gel responses to external stimuli, continuous wave laser of 450 nm wavelength (Civil laser, 100 mW) was used to irradiate to microgel fibers on a stage of inverted microscope Olympus IMT2. The focal spot diameter on the gel was measured as 18 μm. Gel deformation was observed in real time with a CCD camera.
Optical absorption spectra were measured using UV-VIS spectrophotometer (V-650, JASCO. Co., Japan). For the laser-irradiated region, the spectra were measured using an aperture with a diameter of 3 mm.
In TEM cross-sectional observations (JEM-2100, JEOL Co., Japan), sections of approximately 200 nm thickness were obtained from a line micropattern, using a FIB system (JEM-9320, JEOL Co., Japan). Elemental analysis was carried out using energy-dispersive X-ray spectroscopy (JSM-7600FA, JEOL Co., Japan).
3. Results and discussion
3.1 Incorporation property
Figure 1 is a schematic showing the process of incorporating Ag particles into PNIPAM gels. Firstly, the gel microfibers or plates were formed on cover glasses, and were then immersed in AgNO3 solution for 2 h before femtosecond laser irradiation. Femtosecond laser focus were translated inside the AgNO3 solution-filled gels, before irradiated gels were finally immersed into H2O, to remove the AgNO3 solution. No changes of the phase-transition temperature and shrink/swell behaviors were observed, either before or after immersion into AgNO3 solution.
Fig. 1. Incorporation of Ag particles inside PNIPAM gels by NIR femtosecond laser writing. (a) PNIPAM gel preparation on substrates, (b) immersion in AgNO3 solution, (c) femtosecond laser writing inside the gels and (d) immersion in H2O to wash away AgNO3 solution from the gels.
Fig. 2. Optical microscope images of arrayed lines fabricated by femtosecond laser writing inside PNIPAM gel plates. Top views of lines formed by writing at the laser translation speed of (a) 20 μm/s and (b) 100 μm/s. (c) Cross-sectional view of lines. Lines were formed 80 μm beneath the gel surface.
Figures 2(a) and 2(b) are optical microscope images of arrayed lines fabricated using femtosecond laser direct writing in gel plates. Two translation speeds, 20 and 100 μm/s, were used at laser power of 30 mW. The top views illustrate how the irradiated region became evenly brown at the laser translation speed at 100 μm/s, while in comparison, at 20 μm/s, the line was darker and uneven in color. The lines were 4 μm wide in both cases. From the cross-sectional image in Fig. 2(c), it can be seen that the lines were located 80 μm beneath the gel surface. That is, the lines were directly formed inside the gels. In addition, no damages were observed on the gel surfaces.
The TEM observation shown in Figs. 3(a) and 3(b) revealed that Ag particles with a diameter less than 80 nm existed in the laser irradiated region of the gels. Before this observation, the gel was dried for a few days at room temperature, after washing away the AgNO3 solution, after which, an ultrathin cross-section sample was obtained from the dried gel, using focused ion beam milling. No particles were observed at the laser unirradiated region. In spite of such photofabrication of Ag particles, there is almost no absorption around laser oscillation wavelength at 780 nm in these PNIPAM gels and AgNO3 solution [34]. Considering the peak laser intensity of 1010 W/cm2 of femtosecond laser, we considered that such 3D incorporation of Ag particles had been induced by photo-reduction, triggered from multi-photon absorption, only near the focal volume.
Fig. 4. (a) Absorption spectra of PNIPAM gels before and after femtosecond laser writing, (b) calculated wavelength dependence of normalized absorption cross-section σabs/V of Ag nanosphere and (c) logo formed inside a gel fiber.
Figure 4(a) shows absorption spectra for PNIPAM gels irradiated at different laser translation speeds. The size of the measured region was 3 mm2, which included 180 lines formed by laser irradiation. It was notable that a stronger absorption band around 420 nm clearly appeared at the higher laser translation speed, that is, with the lower irradiation dose, while conversely absorption in the longer wavelength region was lower at the higher translation speed.
Nanoscale Ag exhibits plasmonic absorption bands in various visible wavelengths, depending on its size and shape, whereas bulk Ag has extremely high reflectivity over a broad wavelength range. The absorption cross-section σabs of an Ag nanosphere, under light irradiation at different wavelengths, was estimated from Eq. (1) [35]:
3.2 Light-driven deformation property
Shrink/swell behavior of PNIPAM gel microfibers containing Ag nanoparticles at 0, 0.5, 5, 10 and 20 s after switching on a 450-nm wavelength light can be seen in Fig. 5. This irradiation was conducted for 5 s in H2O. The power was measured to be 35 mW. The widths and thicknesses of the fibers were both 60 μm before irradiation. The area of the Ag nanoparticle-incorporated region, which comprised 10 parallel lines and 5 μm inter-line distances, was approximately 50 μm × 50 μm. The lines were written by raster translation of the femtosecond laser focus in a single layer, just 25 μm beneath the gel surface. The femtosecond laser power and translation speed were 30 mW and 100 μm/s, respectively. The 450-nm wavelength light, as a light source for deformation, could be absorbed efficiently by Ag nanoparticles through the plasmonic band seen in Fig. 4(a).
Fig. 5. Optical microscope images of PNIPAM microgel fiber at (a) 0, (b) 0.5, (c) 5, (d) 10, and (e) 20 s after switching on a 450-nm wavelength light (see Visualization 1). Irradiation was conducted for 5 s in H2O. The Ag nanoparticle-incorporated region comprised 10 parallel lines and 5 μm inter-line distances. (f) a shrunk fiber of 1 mm-diameter,which contains Ag nanoparticle incorporated by UV exposure, after the irradiation of 450-nm wavelength light.
The Ag nanoparticle-incorporated region shrunk rapidly just after the irradiation commenced, and the gel color changed—with these changes gradually reverting to their original states once the laser was turned off. Almost no microfiber damage was seen to result from this irradiation, and the reversible shrinking and swelling were most likely due to discontinuous thermo-responsive phase transitions of the PNIPAM gels. In contrast, neither volume nor color changes were observed when light was irradiated to the region without Ag nanoparticles. A movie of typical shrink/swell behaviors is provided in Visualization 1. These results clearly indicated that Ag nanoparticles worked as efficient photothermal converters, sufficient for heating gels above the critical temperature of 36°C against the light of 450 nm wavelength. The microfibers exhibited almost no polarization dependence of 450-nm wavelength light in this deformation. Non-spherical nanometals such as ellipsoids have polarization-dependent absorption which originated from difference in the resonance length. The polarization independence indicates that the aspect ratios of Ag nanoparticles were not so large, which was consistent with TEM images in Fig. 3 and the single-peak absorption band of Fig. 4(a). Figures 6(a) and 6(b) show the time dependence of the shrunk region width and length, respectively, in the irradiation. Gel microfiber types G1, G2 and G3 were used, with each type including a different number of lines within the same 50 × 50 μm2 area. Respective inter-line distance, d, of G1, G2, and G3 were 5, 10 and 15 μm, respectively. Each layer was formed 25 μm beneath the surface. The laser power of 450 nm wavelength was 35 mW. Microfiber G1, which had the highest concentration of Ag nanoparticles, exhibited the largest shrinkage, with its width decreased to 52% of the original width within 5 s of irradiation, which corresponds to volume shrinkage of 86% (=1-(0.52)3) under assumption of isotropic shrinkage. The deformation rate was so fast that the shrinkage approached 83% of its maximum within 1 s of the laser being switched on. The microfiber was adhered to the substrate in this observation. More marked changes and quick responses may be induced in the case without such a constraint. Deformation of both the width and length were enhanced at increased Ag nanoparticle concentrations, with the G1 fiber width becoming deformed by 1.5 times more than that of G3, under the same irradiation conditions. We also observed that shrink speed increased with increased concentration. The maximum length of the deformed region reached 60 μm in G1, which was twice as long as that of G3, and more than thrice than the focal spot of the incident light. Such large areas indicated that the shrinkages had proceeded through thermal conductance, which stemmed from the Ag nanoparticles as heat sources.
Fig. 6. Time dependencies of (a) the widths and (b) lengths of PNIPAM gel microfibers containing different laser-written geometric designs. The pattern was inside a single layer. (c) UV exposure time dependence for nanoparticle incorporation and (d) 450-nm irradiation power dependence of the deformation of microfibers containing Ag nanoparticles incorporated by UV-LED exposure. Deformation was induced by 450 nm wavelength light irradiation.
Ag nanoparticles could also be incorporated into the microfiber by UV exposure. In this case, a UV LED was used instead of the femtosecond laser, as shown in Fig. 1(c). The exposed region size, light power, and exposure time were 50 μm2, 200 μW and 10 min, respectively. In this microfiber with a diameter of 61 μm, where the nanoparticles had been dispersed over the whole region, recovery time was 25 s after the 450-nm wavelength light had been switched off. It was also apparent that the swelling behavior was more moderate in the period following the 2 s after turn-off, whereas, in comparison, the recovery time was as short as 10 s for femtosecond laser-incorporated microfibers, regardless of the concentration.
Assuming that the heat diffusivity D of the gels is 10−3 cm2/s, the heat diffusion length (Dt)1/2 for 5 s is roughly estimated to be 700 μm. Furthermore, heat diffuses in the microfiber of 50-μm width within several tens of ms. Despite the high speed, the recovery time of the microfiber, especially for the ones incorporated by UV exposure, was much longer. This was likely due to the temperature increase of the surrounding solution beyond the phase-transition region during the irradiation. In fact, a fiber with a 1-mm diameter, which had Ag nanoparticles incorporated by immersion in AgNO3 solution for 24 h and subsequent UV exposure for 10 min, exhibited a shrunk region over 580 μm in diameter under similar conditions of 450-nm irradiation (Fig. 5(f)). Such temperature increase of the surroundings might have hindered the rapid cooling of the microfibers.
To improve the recovery speed for UV-incorporation, it is important to reduce the total amount of light absorption. This contributes to the suppression of the heated region in the solution. One method to obtain such suppression is to reduce the absorption of individual nanoparticles by decreasing the plasmonic absorption efficiency of the nanoparticles or reducing the incident power of the 450-nm light. In contrast, highly efficient absorption is necessary to heat particles above the phase-transition temperature. When the irradiated region contains a great number of particles, it is not easy to meet these conflicting demands. Even for lower plasmonic absorption of individual particles, the total amount of absorption energy becomes large. Conversely, the microfiber with a small number of nanoparticles should be advantageous for suppressing the total amount of absorption even for higher plasmonic absorption at each particle.
Figure 6(c) shows the changes of deformation property of the microfiber with Ag nanoparticles incorporated by different UV exposure time. The microfiber width and thickness were both 60 μm. The laser power and irradiation time of 450 nm wavelength for light stimulation were 35 mW and 5 s, respectively. The shrinkage ratio (w0-w)/w0 (w = minimal width of microfiber after irradiation; w0 = microfiber width before irradiation) kept to be zero in case of the exposure time less than 1 min, and then increased until 10 min. In the early stage, the plasmonic absorption of the nanoparticles was insufficient for heating the gel above the phase-transition temperature. The microfibers exhibited moderate swell behavior in any condition. Even in case of the microfiber at the UV exposure time of 1 min, the recovery speed was not high, considering small amount of shrinkage.
Figure 6(d) presents the effects of the 450-nm irradiation power on the deformation behaviors of the microfibers with nanoparticles incorporated by UV exposure for 10 min. The powers were 10, 15, 25 and 35 mW. Although no deformation was observed at the power of 10 mW, the amount of shrinkage was saturated over 25 mW. It was apparent that the recovery speed kept moderate by light-irradiation even at lower power. From these trends, it is not easy to obtain both large amount of shrinkage and rapid recovery for microfibers with Ag nanoparticles incorporated by UV exposure. That is, the UV exposure-incorporation process, which generates nanoparticles over the whole region, is suggested to be not always suited to balance the competing demands. Furthermore, another group previously reported chemical synthesis process for Au plasmonic nanorods in PNIPAM gel fibers with diameter of approximately 150 μm [38]. In their case, the gels, which contained the nanorods over the entire region, showed the moderate recovery similar to our results for UV-incorporated process. In contrast, femtosecond laser irradiation was useful for the incorporation of a small number of nanoparticles with high absorption properties. Although further investigation including detailed calculation is required, this compatibility, which was achieved only after 3D incorporation, might have allowed for both remarkable shrinkages and rapid recovery of microfibers.
For the microfibers with Ag nanoparticles incorporated by UV exposure, a gas bubble of several tens of micrometers diameter was often generated around the irradiated region, caused by increasing the temperature over the gas-liquid transition threshold. Bubble lifetime was from several minutes to several tens of minutes, which delayed subsequent laser irradiation. In comparison, for the case where the incorporation was achieved using femtosecond laser, gas bubbles were almost never observed under the same irradiation conditions, which meant that the total amount of light absorption was suppressed to be lower than that of UV exposure-incorporation.
Figure 7 presents light-driven deformation properties of the microfibers containing a different number of the laser-written layers. Gel microfiber types G4 and G5, respectively, contain 1 and 3 layers consisting of the same geometrical pattern as shown in the inset. The layer layout of G4 was the same as G1. Respective layer of G5 were formed 10, 30 and 50 μm beneath the surface. The length of the deformed region in G5 was enhanced by 1.5 times compared to the case of single layer, whereas the width shrunk were almost the same. This was because the transmitted light from the first layer were absorbed by the second and third layers, which generated multiple photothermal conversion. In spite of this enhancement, the recovery speed of G5 was kept to be as high as that of G4.
Fig. 7. Time dependencies of (a) the widths and (b) lengths of PNIPAM gel microfibers containing 1 and 3 layers. Each layer had the same geometric design. Deformation was induced by 450 nm wavelength light irradiation. This irradiation to the microfiber was conducted for 5 s in H2O.
Fig. 8. Cyclic deformation properties of the width of PNIPAM microfiber. Ag nanoparticle were incorporated by femtosecond laser direct writing. Deformation was induced by irradiation of 450-nm light. Each cycle comprised a 5 s irradiation-period followed by a 30 s interval time.
The cyclic deformation property of microfibers with Ag nanoparticles incorporated by femtosecond laser irradiation can be seen in Fig. 8. Each cycle comprised a 5 s irradiation-period, with 450-nm light, followed by a 30 s interval time. In any cycle, the microfiber exhibited the maximum shrinkage within 5 s, followed by the recovery to its original state within 10 s. This deformation behaviors were almost the same in all the cycles and no damage was observed. In comparison, incorporation by UV exposure led to much slower swelling behavior, and by the fourth cycle, visible damage was detectable at the irradiated region. Although femtosecond laser incorporation was more effective for light-driven actuations, even in this case, the deformation amount slightly decreased after further cyclic irradiation. This change suggests that minor damage was generated in the microfibers. This might have been caused by local alteration of the gel matrix, which resulted from the excess plasmonic heating of Ag nanoparticles. A similar amount of deformation could be induced by 450-nm irradiation at lower power. Therefore, the long-term reliability would be improved by optimizing the irradiation conditions.
The 3D incorporation of plasmonic Ag nanoparticles enabled us to control the position, extent and speed of the light-triggered responses of PNIPAM microgels reproducibly. By forming programmed Ag nanostructures inside the microgels, soft actuators with complex controllability could be achieved.
4. Conclusions
Plasmonic Ag nanoparticles were incorporated into PNIPAM microgels using femtosecond laser multi-photon reduction. The nanoparticles worked as efficient photothermal converters against visible light, which allowed for reversible changes as large as 86% in volume under assumption of isotropic shrinkage. Such microgel deformation exhibited excellent reproducibility, even under cyclic light irradiation. Femtosecond laser 3D incorporation enabled us to achieve both drastic shrinkage and rapid recovery of the microgels, compared to gels that had nanoparticles throughout their entirety. This flexible Ag nanoparticle incorporation process promises to be a powerful tool for developing of 4D-printed microdevices and highly controllable soft actuators.
Funding
Ministry of Education, Culture, Sports, Science and Technology (16H04240, 17K18849, 19H02474).
Disclosures
The authors declare no competing interests.
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