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-N-isopropylacrylamide (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.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Stimulus-responsive hydrogels exhibit changes in volume and properties with external stimuli such as temperature and pH. Shrinking, swelling, and deformation of the hydrogels can be induced at arbitrary times and positions in space by controlling the external stimuli, which is promising for actuators [1–3] and microfluidics [4,5] using soft materials. Light-based technologies have played a pivotal role in maximizing the potential of stimulus-responsive hydrogels. For example, multiphoton polymerization provides 3D structuring of the stimulus-responsible hydrogel down to sub-micrometer resolution [6,7], which is attractive for fabrication of soft micro-robots and micro-valves. Furthermore, using a laser, on-off switching of stimulation offers high temporal resolution as well as high-spatial-resolution non-contact stimulation.
Functionalization of polymers using optical stimulation can be classified into (1) chemical modifications and (2) methods using light-absorbing nanoparticles embedded in temperature-responsive polymers. In the former case, azobenzene  and spiropyran [9,10] have been used as photochromic dyes. Regarding a soft actuator, Zuo et al. reported a hierarchical structured liquid crystal elastomer material that responds to three wavelengths . In the case of a hydrogel, two photocycloadditions were used to induce dual-wavelength stiffening of the hydrogel matrix . In such structures, optical absorbers are distributed through the material, and the optical absorbance is relatively low. In the latter case, metal or carbon nanoparticles are embedded in a hydrogel to enhance the photothermal effect for stimulation of the temperature-responsive hydrogel. Optical absorbance can be significantly improved using nanoparticles; moreover, a wide range of wavelengths can be used for light stimulation because of the optical properties of nanoparticles. Several studies using carbon nanotubes , graphene oxide nanoparticles [5,14], and metal nanoparticles [15–17] have been reported. Sershen formed two valves with a thermally responsive hydrogel: one valve with gold colloids, and the other with gold nanoshells . Wavelength-selective flow in a microchannel was demonstrated by selective shrinking of a valve placed in the right and left channels, connected to a T-junction. Watanabe et al. reported dual-wavelength-responsive hydrogel actuation using neodymium (III) oxide and ytterbium (III) oxide nanoparticles . Nanoparticles were distributed through the material or layered structures containing nanoparticles were used for optical absorbing systems. A 3D fabrication method that enables spatial distribution of optical absorbing nanoparticles on the micrometer scale could help realize localized stimulation and hence advanced control of hydrogel actuation.
Multiphoton photoreduction is an exclusive method for fabricating three-dimensional metal microstructures by laser direct writing, in which the resolution can be reduced to tens of nanometers [20–22]. In the case of a hydrogel, metal microstructures are fabricated at the focal point of laser pulses inside a supporting hydrogel as a result of the permeation of metal ions into the hydrogel. Metal microstructures are sustained in the hydrogel after placing the hydrogel into water to remove residual ions by diffusion. Multiphoton photoreduction in gelatin  and a poly(ethylene glycol) diacrylate (PEGDA) hydrogel [24,25] have been reported previously. In addition, we have demonstrated the fabrication of two metal microstructures in a PEGDA hydrogel in a stepwise manner . The laser-based fabrication method of metal microstructures with high spatial targeting and resolution is useful for fabrication of light-driven micro-robots with soft materials.
In this paper, we demonstrate the fabrication of multi-metal microstructures by multiphoton photoreduction in a poly-N-isopropylacrylamide (PNIPAm) hydrogel that exhibits volume phase transition. The metal microstructures shrink upon light stimulation concurrently with the volume change of the supporting hydrogel. By taking advantage of the different optical absorbances attributable to the properties of silver and gold microstructures, wavelength-dependent shrinkage of the metal microstructure and actuation of the hydrogel are demonstrated.
2. Materials and methods
First, 100 mg of N-isopropylacrylamide (NIPAm, FUJIFILM Wako Pure Chemical Corp.), 10 mg of the photoinitiator Irgacure2959 (Sigma-Aldrich Co. LLC), and 10 mg of N,N’-methylene bisacrylamide (Sigma-Aldrich Co. LLC) were dissolved in 1 mL of pure water. The solution was placed in a mold, which was in contact with a refrigerant, and illuminated by 365-nm light from a UV lamp for 40 min to form PNIPAm by photo-cross-linking. PNIPAm is a temperature responsive polymer that shows volume phase transition at 32 °C .
The PNIPAm hydrogel was immersed in silver nitrate (Sigma-Aldrich Co. LLC) solution (40 mg/mL) or gold(III) chloride (Sigma-Aldrich Co. LLC) solution (0.4 mg/mL) for 10 min to allow metal ions to permeate the hydrogel. Metal microstructures were fabricated by multiphoton photoreduction of metal ions induced by femtosecond laser pulses. A femtosecond laser oscillator with a second harmonic generator unit (HighQ-2-SHG, Spectra-Physics, Inc.) operating at 522 nm with a 63-MHz repetition rate and 192-fs pulse duration was used for the fabrication of metal microstructures. A water-immersion objective lens (numerical aperture (NA) 1.0, working distance 2.0 mm, Olympus) was used to focus the femtosecond laser pulses in the hydrogel placed on a computer-controlled, three-axis encoded (XYZ) motorized stage. The stepwise procedure described in our previous paper  was used to fabricate both gold and silver microstructures in the same hydrogel. The fabricated microstructures inside the wet hydrogel were observed with an inverted transmission optical microscope (Eclipse Ti-E, Nikon). The width of a line microstructure was determined based on the gradient of brightness on both sides of the line in optical microscope images. The optical absorbance spectra of the hydrogel in which metal microstructures were fabricated were measured using a halogen lamp (TQ8111, Advantest Corp.) and a spectrometer (USB4000, Ocean Optics, Inc.).
The laser power of the femtosecond laser pulses and the scanning speed were fixed at 15 mW and 100 µm/s, respectively, to fabricate metal microstructures as optical absorbers in the light-stimulated shrinkage and actuation experiments. For the light stimulation, continuous wave (CW) laser diodes (LDs) with central wavelengths of 405 nm and/or 520 nm were used. The output power was 30 mW for both LDs, unless otherwise noted. The obliquely incident laser beam was loosely focused to 185 µm and 200 µm for 405 nm and 520 nm LDs, respectively, at the hydrogel. The incident angle of the laser beam was 45°. The PNIPAm hydrogel was observed using an optical microscope to evaluate the shrinkage and actuation. The swelling ratio of the metal microstructure was expressed as the area ratio to the area of the initial microstructure before light stimulation. The bending angle in the actuation experiment was based on the original angle of the hydrogel before light stimulation, where the clockwise direction was considered positive.
3. Results and discussion
Figures 1(a) and 1(b) show bright-field microscope images of the silver (a) and gold (b) line microstructures fabricated in the PNIPAm hydrogel, taken 80 µm from the surface of the hydrogel. The colors of the metal microstructures can be attributed to the surface plasmon resonances of the respective metal nanoparticles. As shown in Fig. 1(c), the width of the line microstructures increased with increasing laser power. The line width was larger than the size of the theoretical diffraction limit of the laser beam at the focal point, which may be explained by the diffusion of electrons and the growth of metal particles resulting from the multiple laser pulse irradiation. In addition, the heating effect caused by the high repetition of laser pulses could induce shrinkage of the supporting PNIPAm hydrogel. The metal structures fabricated in a shrunken hydrogel could be swelled after the thermal relaxation of the hydrogel. Gold line microstructures fabricated at laser powers higher than 20 mW show void structures in the center. Localized material removal may be induced by the linear optical absorption of succeeding laser pulses by the fabricated gold nanoparticles. The absorbance of laser pulses by the metal nanoparticles will be discussed in the next paragraph.
Figures 2(a)–2(h) show the metal grating microstructures fabricated in an area of 100 µm × 100 µm inside the hydrogel with different spacings between adjacent lines. The microstructure became significantly turbid in the hydrogel with a silver line spacing of 5 µm [Fig. 2(a)]. Excessive heating may be generated by the femtosecond laser pulses absorbed to the existing adjacent parallel line because the width of the silver-line microstructure fabricated at a laser power of 15 mW was larger than the line spacing of 5 µm, as shown in Fig. 1(c). Because the area of the microstructure was deformed in Fig. 2(a), irreversible denaturation of PNIPAm matrix may be occurred. Figure 2(i) shows the optical absorbance spectra of the fabricated gratings with a line spacing of 10 µm formed from silver or gold within the PNIPAm hydrogel. The absorbance peaks of the silver and gold gratings were observed at approximately 415 nm and 510 nm, respectively. The absorbance peak wavelength of the gold grating is comparable with the wavelength of the femtosecond laser pulses. The void formation at a laser power higher than 20 mW [Fig. 1(b)] can be explained by the significant linear optical absorption of femtosecond laser pulses by the formed gold nanoparticles during laser scanning because of the pulse overlaps caused by the 63-MHz repetition rate.
Light-stimulated shrinkage of metal microstructures in the PNIPAm hydrogel was performed with the experimental setup illustrated in Fig. 3(a). Considering the absorbance peaks for the metals, CW laser beams of 405 nm and 520 nm were used as light simulation for silver and gold microstructures, respectively. The dependence of the swelling ratios of the silver and gold microstructures immediately after 10 s of laser illumination on the line spacing are plotted in Fig. 3(b) (N=5). The swelling ratios of the silver and gold microstructures in the PNIPAm hydrogel were decreased by light stimulation, demonstrating the light-stimulated shrinkage of both metal microstructures. The metal microstructures inside the hydrogel acted as optical absorbers, resulting in the volume phase transition of the supporting PNIPAm hydrogel and concurrent shrinkage of the metal microstructures. The change in the swelling ratio of the silver microstructure for the line spacing of 5 µm was smaller than those with different line spacings, although the microstructure became turbid, as shown in Fig. 2(a). Excessive heating during the direct laser writing may have induced irreversible modification, such as cleavage and carbonization, which changed the property of the volume phase transition. The swelling ratio of gold microstructures decreased with increasing line spacing, which is attributable to the lower photothermal effect in the microstructure by the lower optical absorption to gold microstructures. Light stimulation of the PNIPAm hydrogel without metal microstructures induced no shrinkage under comparable light illumination parameters (data not shown). Figures 3(c)–3(f) show the typical shrinkage behavior of the metal microstructures. Spherical patterns were observed at the ends of the lines during light stimulation [Fig. 3(d)], which may be due to localized heating by higher optical absorption by metal dots at the ends. The size of the metal microstructures gradually swells after stopping the light stimulation, indicating the reversible size change of the fabricated metal microstructures. The shrinkage of the metal microstructure with light stimulation is also shown as a video in Visualization 1.
The temporal size profiles of the metal microstructures with light stimulation are shown in Fig. 4. Wavelengths of 405 nm [Fig. 4(a)] and 520 nm [Fig. 4(b)] from LDs were used. The swelling ratios decreased during the 10 s of light stimulation, and the size of the metal microstructures recovered gradually over more than 60 s after stopping the light stimulation. Since the recovery velocity of soft materials is highly dependent on the relaxation property of the materials, 60 s was not sufficient to recover to the initial value, although swelling ratios recovered to approximately 97% of the initial value in the cases of silver with 520 nm and gold with both 405 nm and 520 nm. It should be noted that the comparative degrees of change in swelling ratios were opposite for the silver and gold microstructures with light stimulations of 405 nm and 520 nm. The swelling ratios of the metal microstructures are dependent on the optical absorption. Therefore, the swelling ratio of the silver microstructure with 405-nm light illumination was larger than that of the gold microstructure because of the resonant wavelength of the silver microstructure, and vice versa with 520-nm light illumination because of the resonant wavelength of the gold microstructure. The results shown in the figure suggest that controllable shrinking properties can be obtained using a combination of metals and wavelengths. The 10 s of light stimulation was repeated 10 times at 60-s intervals [Fig. 4(c)]. The wavelengths of the light stimulation were selected because of the respective resonant wavelengths of the metal microstructures. Note that the amplitude of the swelling ratio showed no significant change over 10 cycles of light stimulation, and reproducible shrinking was demonstrated.
We fabricated neighboring silver and gold grating microstructures in the PNIPAm hydrogel, in which both microstructures have lengths of 100 µm and widths of 60 µm, as illustrated in Fig. 5(a). Two metal microstructures were fabricated in a same hydrogel by stepwise manner, which was described in our previous paper . Figures 5(b)–5(d) show the microstructures illuminated at 405 nm, while Figs. 5(e)–5(g) show the structures illuminated at 520 nm. The laser powers for the light stimulation were 8.7 mW and 11.6 mW for 405 nm and 520 nm, respectively. The center of the laser beams for the light stimulation was aligned to the boundary of the center between silver and gold microstructures. The silver microstructure showed a relatively large shrinkage at 405 nm, while the shrinkage of gold microstructures was predominant at 520 nm. The results show that, thanks to the multiphoton photoreduction for the fabrication of multi-metal microstructures in the same hydrogel, selective dominant shrinkage of microstructures is possible only by changing the wavelength of the light stimulation. Since the resonant wavelengths are tunable by changing laser irradiation conditions and geometry of the microstructures , further optimization of the absorption spectra for the efficient stimulation would be possible.
We attempted to control the actuation direction of the hydrogel by taking advantage of the spatially targeted fabrication of multi-metal microstructures inside the same hydrogel. Silver and gold microstructures were fabricated 50 µm from the opposite surfaces of the PNIPAm hydrogel. Fabrication of the microstructures inside the hydrogel prevent the detachment of the microstructure from the hydrogel. The line spacing was 10 µm. The obliquely incident laser beam, 45° to the top surface of the hydrogel, was loosely focused to the middle of the silver and gold microstructures for the light stimulation as shown in Fig. 6(a). The actuation of bending was observed using an optical microscope with light stimulation of either 405 nm or 520 nm. Figure 6(b) shows the temporal profiles of the bending angles with the respective wavelengths of light stimulation. The bending behavior is also shown as a video in Visualization 2. The PNIPAm hydrogel bent in a clockwise direction with light stimulation at 405 nm, consistent with the position of the silver microstructures. Conversely, light stimulation at 520 nm induced bending in the anticlockwise direction, corresponding to the position of the gold microstructures. The bending direction is therefore attributed to the dominant absorption of the stimulating light by the metal microstructures, which induces a spatially selective volume phase transition. It is noteworthy that the amplitude of the bending angle at 520 nm, which is governed by the gold microstructure, was larger than that at 405 nm by the silver microstructure. This result is contrary to the case shown in Fig. 4. The opposite results are explained by the difference in optical absorbance at the respective wavelengths, in addition to the initial slightly curved shape of the hydrogel. The light stimulation illuminating the top side of the hydrogel was absorbed by both metal microstructures. Although one metal showed dominant optical absorbance, the other metal also showed a certain level of absorbance, even when the wavelength was off-resonant. The difference in the absorbance of silver and gold microstructures at 405 nm was comparably smaller than that at 520 nm, as shown in Fig. 2(i). Tensile forces were generated at both sides of the hydrogel with light stimulation, resulting in a decrease in the bending angle.
Figure 7 shows the result of the experiment demonstrating the rapid recovery of the bending actuation using the same hydrogel used for the experiment shown in Fig. 6. Stimulating light at 520 nm was illuminated for 10 s, followed by illumination at 405 nm for another 10 s. A time period longer than 60 s was required for bending recovery without the illumination at 405 nm. With illumination at 405 nm, the bending was recovered to the initial angle within 10 s. The illumination of 405 nm induced a tensile force on the opposite side of the hydrogel, resulting in the force of the bending in the clockwise direction. The recovery velocity in the actuation of soft materials is highly dependent on the relaxation property of the materials in general. Although further improvement in response velocity is necessary, our results demonstrate the proof-of-concept of recovery acceleration because of the selective fabrication of multi-metal microstructures with micrometer resolution in the same PNIPAm hydrogel, which was realized by multiphoton photoreduction.
We have demonstrated the fabrication of multi-metal structures in a PNIPAm hydrogel. The metal microstructures showed shrinkage upon light stimulation concurrently with the volume phase transition of the supporting hydrogel. The change in the size of the metal microstructures was reversible and repeatable over 10 cycles of light stimulation. By taking advantage of the different optical absorbances attributable to the properties of silver and gold microstructures, wavelength-dependent shrinkage has been demonstrated. The results indicate the potential of multiphoton photoreduction for future applications in optical devices with soft materials that are actively controlled by different wavelengths. Furthermore, bending actuation of the hydrogel was performed by fabricating silver and gold microstructures on opposite sides inside the same hydrogel. The bending direction of the hydrogel was controlled by switching the wavelengths of light stimulation because of the different resonant wavelengths for silver and gold microstructures. Note that rapid recovery of bending was also demonstrated using the two metals and wavelengths. Although further improvement is necessary, for example, the response velocity, our experiments demonstrate proof-of-concept results for future wavelength-controlled light-driven soft robots by taking advantage of multiphoton photoreduction.
Ministry of Education, Culture, Sports, Science and Technology (18K18958).
The authors declare no conflicts of interest.
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