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Abstract
The light-stimulated control of flow velocity in the hydrogel microchannel was demonstrated by utilizing a metal microstructure fabricated by multiphoton photoreduction. The metal microstructure was fabricated adjacent to the microchannel in a poly-N- isopropylacrylamide (PNIPAm) hydrogel. Owing to the photothermal conversion at the metal microstructure, local deformation of the microchannel and change in flow velocity was induced as a result of the volume phase transition of the supporting hydrogel around the metal microstructure. Wavelength-selective change in the flow velocity was realized by utilizing dissimilar metal microstructures, that exhibit different optical resonances. The results indicate that multiphoton photoreduction is a promising method for fabricating novel hydrogel devices having flow-controllable switches by arranging light-absorbing structures with high spatial selectivity.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Microfluidic devices enable the manipulation of small amounts of liquid and have been widely applied in the fields of chemistry and biomedicine for chemical synthesis [1], drug discovery [2], and cell culturing [3]. In addition to materials such as glass, ceramics, and conventional polymers, hydrogels have recently been actively investigated to be used as materials for microfluidic devices, owing to their biocompatibility and mechanical strength. Hydrogels are polymer materials that contain water in a three-dimensional network structure. By changing the molecular weight and water content, the mechanical strength can be adjusted comparable to that of various biological tissues [4]. Due to such properties, many studies have reported the application of hydrogels as lab-on-a-chip devices that mimics biological tissues, such as blood vessels and organs, by creating microchannels inside hydrogels [5,6]. Because the volume of a hydrogel changes according to the water content, hydrogels have also played an important role as materials for flow control in microfluidic devices. Specifically, stimuli-responsive hydrogels can be used as valves and pumps by blocking or deforming microchannels utilizing the volume phase transition induced by external stimuli, such as pH, temperature, or light [7,8]. Among those external stimuli, light stimulation allows for non-contact and spatially selective transition.
Nanoparticles, such as graphene oxide nanoparticles or metal nanoparticles, have been investigated to be used as light-absorbers within temperature-responsive hydrogels to fabricate light-responsive hydrogels. Sershen et al. fabricated wavelength-selective valves by dispersing gold colloids and gold nanoshells in a temperature-responsive hydrogel [9]. They showed that each nanoparticle significantly absorbs light of different wavelengths, which causes wavelength-selective shrinking of valves. It should be noted that most of the previous studies on flow control using light-responsive hydrogels are limited to the demonstrations of blocking the flow of liquid by stimulation. In addition, it is difficult to achieve flow control with spatial selectivity if the light-absorber nanoparticles are distributed throughout the hydrogel. Spatially selective flow control will be achieved if the light-absorber nanoparticles can be spatially selectively distributed.
Multiphoton photoreduction is a method that enables the fabrication of metal microstructure with high spatial resolution [10–12]. It has been demonstrated that the reduction of metal ions at the focal point can be induced by focusing femtosecond laser pulses into the hydrogel containing metal ions. The fabrication of metal microstructure in various hydrogels, such as gelatin or poly (ethylene glycol) diacrylate (PEGDA) hydrogels, using multiphoton photoreduction has been reported [13–16]. Furthermore, the bending of the temperature-responsive poly-N- isopropylacrylamide (PNIPAm) hydrogels has been demonstrated by illuminating light onto the metal microstructure fabricated inside the hydrogels using multiphoton photoreduction [17].
In this study, we have demonstrated the flow velocity control in the PNIPAm hydrogel microchannel by utilizing the metal microstructure fabricated by multiphoton photoreduction. PNIPAm is a temperature-responsive hydrogel that exhibits volume phase transition. The metal microstructure was fabricated adjacent to the microchannel in a PNIPAm hydrogel. Owing to the photothermal conversion at the metal microstructure, local deformation of the microchannel and change in flow velocity were induced with the volume phase transition of the supporting hydrogel around the metal microstructure. Wavelength-selective change in the flow velocity was also realized by utilizing dissimilar metal microstructures, that show different optical resonances.
2. Materials & methods
2.1 Material preparation
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. Then, the solution was placed in a mold (0.2 cm × 0.8 cm × 1.2 cm) with an inserted tungsten needle (tip diameter 35 µm, shank diameter 0.508 mm, length 1.25 cm). The sample was placed on a refrigerant and illuminated by 365-nm light from a UV lamp (LUV-16, AZ-1) for 20 min. the tungsten needle was then removed from the PNIPAm hydrogel to obtain the central microchannel.
2.2 Experimental design for the fabrication of microchannels and light-stimulation
First, a PNIPAm hydrogel with a central microchannel, in which a Luer lock needle was connected to perfuse the solution, was prepared (STEP 1). Then, a side microchannel branching from the central microchannel was fabricated by internal scribing with femtosecond laser pulses (STEP 2). After the fabrication of the side microchannel, the metal microstructure, which acted as a light absorber, was fabricated adjacent to the side microchannel by multiphoton photoreduction (STEP 3). Further reduction of metal ions was induced around the metal microstructure by utilizing a CW laser (STEP 4). Finally, the metal microstructure was stimulated during perfusion, and the flow velocity in the side microchannel was evaluated (STEP 5).
2.2.1 Fabrication of the side microchannel by irradiating femtosecond laser pulses (STEP 2)
The fabricated PNIPAm hydrogel was placed on a cover glass (145 ± 15 µm thick, Paul Marienfeld) which was set on an X-Y stage. Linearly polarized-high intensity laser pulses (central wavelength 800 nm, pulse width 100 fs, repetition rate 1 kHz) from a Ti:Sapphire femtosecond laser regenerative amplifier system (Libra, Coherent, USA) were focused using an objective lens (60x, aperture 0.7). A CMOS camera (DCC1645C, Thorlabs) was used for observation. The Z-axis of the stage was manipulated to set the focal point of laser pulses in the central microchannel inside the PNIPAm hydrogel. The side microchannel was fabricated by scanning focused femtosecond laser pulses at a speed of 100 µm/s. Reciprocating scan were performed in the Y direction, then the stage was moved in the X direction and then another reciprocating scan was performed in the Y direction. The distance moved in the X direction is defined as the line spacing. After the fabrication of the side microchannels, the PNIPAm hydrogel was immersed in fluorescein isothiocyanate dextran (FITC-dextran) (molecular weight 20000, Sigma-Aldrich) solution (0.1 mg/mL) for 1 hour. The fluorescence intensity of the obtained fluorescence microscope images was evaluated by using image analysis software, Image J. The channel width was measured from the full width at half maximum of the fluorescence intensity. The measurements were performed five times in the same microchannel, and all values are expressed as means standard error.
2.2.2. Fabrication of metal microstructures by multiphoton photoreduction (STEP 3)
The hydrogel was immersed in gold chloride solution (0.4 mg/mL, Sigma-Aldrich) or silver nitrate solution (1 mg/mL, Fujifilm Wako Pure Chemicals) for 10 min to allow metal ions to permeate inside the hydrogel. The PNIPAm hydrogel placed on a cover glass was set on the XYZ stage (OSMS60-10ZF, Sigma Koki). Then, femtosecond laser pulses (central wavelength 522 nm, pulse width 192 fs, repetition rate 63 MHz) from a femtosecond laser oscillator with a second harmonic generator unit (HighQ-2-SHG, Spectra-Physics, Inc.) were focused using an objective lens (60x, numerical aperture: 1.0). The femtosecond laser pulses with 522-nm wavelength induces efficient multiphoton photoreduction as previously reported [15]. The focal point of laser pulses was set to the Z-axis position, ∼500 µm from the surface of the hydrogel, where the side microchannel was fabricated. Metal microstructure with a line spacing of 5 µm was fabricated adjacent to the microchannel by scanning femtosecond laser pulses. For the fabrication of metal microstructures, the laser power and the scanning speed were fixed at 15 mW and 100 µm/s, respectively. After the fabrication of the metal microstructure, the PNIPAm hydrogel was immersed in pure water for one day to remove the residual metal ions in the PNIPAm hydrogel.
2.2.3 Additional reduction of metal ions around the metal microstructure (STEP 4)
The PNIPAm hydrogel with the fabricated metal microstructure was immersed in gold chloride or silver nitrate solution for 10 min. The PNIPAm hydrogel placed on a cover glass was set on the stage, then, CW light was illuminated onto the metal microstructure for 60 s. A laser diode with a wavelength of 520 nm (spot diameter of 200 µm) was used as a light source for the gold microstructure and 405 nm (spot diameter of 185 µm) for the silver microstructure. By utilizing a CW laser instead of a femtosecond laser, further reduction of metal ions can be realized in a comparably larger area at once, as well as limiting excessive increase in temperatures during additional reduction, which may induce volume phase transition during photoreduction. After the CW light illumination, the PNIPAm hydrogel was immersed in pure water for one day to remove the residual metal ions.
2.2.4 Evaluation of flow velocity in the side microchannel during light stimulation (STEP 5)
The PNIPAm hydrogel placed on a cover glass was set on the stage of an optical microscope, and a Luer lock needle (needle outer diameter 0.63 mm, needle inner diameter 0.32 mm, length 50 mm) was inserted into the central microchannel. Another end of the Luer lock needle was coupled to a tube that was connected to a syringe pump. Perfusion of a solution containing dispersed blue polystyrene particles (particle diameter of 1.0 µm (2.5%, Polyscience, part number 15712)) diluted to 2.5 g/mL, was performed at a flow rate of 10 µL/min using the syringe pump. Stimulation light was obliquely illuminated at 45 degrees at a power of 30 mW for 3 s. For the light source, a laser diode with a central wavelength of 520 nm (spot diameter of 200 µm) or 405 nm (spot diameter of 185 µm) was used. Movies were taken at a frame rate of 30 fps and a shutter speed of 1/60 sec. Two images with a time difference of 0.1 s were captured from the video to estimate the flow velocity. Five blue polystyrene particles in the images were evaluated to estimate the average flow velocity. For the velocity evaluation, the side microchannels were fabricated with a pulse energy of 10 µJ, a scanning speed of 100 µm/s, a line spacing of 6 µm, and a number of reciprocating scans in the Y direction of 8. The metal microstructure was fabricated at a power of 15 mW and a scanning speed of 100 µm/s. A gold chloride or silver nitrate solution with a concentration of 4.0 mg/mL was used. CW light was illuminated at a power of 30 mW for 60 s.
2.2 Optical absorbance spectra
The PNIPAm hydrogel was placed on a cover glass and the light from a halogen lamp (TQ8111, ADVANTEST) was illuminated to the PNIPAm hydrogel through the optical fiber. The intensity of the illuminated and transmitted light was obtained by a spectrometer (USB4000, Ocean Optics) to evaluate the absorbance of the PNIPAm hydrogel.
3 Results & discussion
Figure 2(A) shows fluorescence microscopy images of the side microchannel fabricated by irradiating femtosecond laser pulses. In addition to the formation of cavitation bubbles, as well as shock waves, focusing femtosecond laser pulses inside PNIPAM hydrogel may induce temperature-responsive phase transition, which may increase the roughness of the internal surface of the side microchannel. The influx of fluorescent molecules was confirmed by the fluorescence observed from inside of the side microchannels. The intensity of the irradiated femtosecond laser pulse is estimated to be ∼1.1×1016 W/cm2 even at the lowest pulse energy of 2 µJ, which is higher than the breakdown threshold of water [18]. Therefore, the irradiation of femtosecond laser pulses could induce optical breakdown in water and generates cavitation bubbles similar to the case reported by Vérit et al [19]. In the present experiment, the line spacing was defined as the distance moved in the X direction between the first and second reciprocating scan in the Y direction. Two separate microchannels were observed in the fluorescence microscopy images for the experimental conditions of larger pulse energy and the line spacing, such as the case with pulse energy of 6 µJ and line spacing of 15 µm. Figure 2(B) shows the dependence of channel width on line spacing distance at different pulse energies. The channel width monotonically increased with increasing line spacing, but the channel width did not change significantly when the line spacing was over 4 µm. In Fig. 2(B), the data, in which two side microchannels were observed, are not plotted.
Fig. 2. (A) Fluorescence microscope images of the fabricated side microchannels. Scale bars indicate 100 µm. (B) Channel widths of the side microchannels fabricated under the different experimental conditions of pulse energy and line spacing. In both (A) and (B), microchannels were fabricated at a scanning speed of 100 µm/s, and the number of reciprocating scans in the Y direction of 2.
Figure 3(A) and Fig. 3(C) show bright-field microscope images of gold and silver microstructures fabricated inside the PNIPAm hydrogel by multiphoton photoreduction. The metal microstructures showed red and yellow colors due to the localized surface plasmon resonance, suggesting that the metal microstructures were composed of metal nanoparticles. Figure 3(B) and Fig. 3(D) show bright-field microscope images of the metal microstructures fabricated by multiphoton photoreduction with additional illumination of CW light. Figure 3(E) and Fig. 3(F) show the absorbance spectra of the gold and silver microstructures before and after CW light illumination at different powers. The absorbance peaks of the metal microstructures increased by CW light illumination, suggesting that the number of metal nanoparticles generated in the PNIPAm hydrogel increased. CW light illumination at the plasmon resonance wavelength of gold and silver nanoparticles may induce local heat generation, as well as near-field enhancement, resulting in further reduction of the metal ions in the vicinity of the metal microstructures. In addition, the peak wavelengths of absorbance spectra of metal microstructures after illumination of CW light showed a red shift, suggesting that the average particle size of the metal nanoparticles increased by CW light illumination. Figure 3(G) and Fig. 3(H) show the absorbance spectra of gold and silver microstructures, respectively, before and after CW light illumination at 30 mW with different concentrations of gold chloride or silver nitrate solutions. The absorbance increased with the increase in the concentration of the metal ion solution, which can be attributed to the increase in the number of metal ions at the focal volume of laser pulses, and thus the number of generated metal nanoparticles increased.
Fig. 3. Bright-field microscope images of the fabricated (A),(B) gold microstructures and (C),(D) silver microstructures before and after CW light illumination: (A),(C) before illumination, (B) after 520 nm light illumination at 30 mW, and (D) after 405 nm light illumination at 30 mW. Scale bars indicate 100 µm. (E)-(H) Absorbance spectra of the (E),(G) gold and (F),(H) silver microstructures. Control indicates the spectra without CW light illumination. (E),(F) Dependence of absorbance spectra on the power of the CW light. The concentration of the metal ions solution was fixed to 0.4 mg/mL. (G),(H) Dependence of absorbance spectra on the concentration of the metal ions solution. The power of the CW was fixed to 30 mW.
Figures 4(A) and Figure 4(B) show bright-field microscope images of the side microchannel before and during light stimulation (1 s after the start of light stimulation), respectively. The geometric diagram of the structure is shown in Fig. 1. In this experiment, a side microchannel was fabricated in the PNIPAm hydrogel with the laser pulse energy of 10 µJ, line spacing of 6 µm, and the number of reciprocating scans in the Y direction of 8. The PNIPAm hydrogel around the gold microstructure shrunk and the side wall of the side microchannel was stretched toward the gold microstructure by light stimulation (Visualization1). The increase in flow velocity during light-stimulation can also be clearly confirmed from the increase in outflow of polystyrene particles from the hydrogel observed at the right side of the Visualization 1. Figure 4(C) shows the measured flow velocity in the side microchannel, which was measured at approximately at a position 200 µm downstream from the gold microstructure. The flow velocity before the light stimulation was approximately 70 µm/s, while the flow velocity during the light stimulation increased to around 100 µm/s. The increase in the flow velocity can be explained by the water pressure in the side microchannel. Fu et al. reported the increase of water pressure by increasing temperature due to photothermal conversion [20]. In addition, the water release from the PNIPAm hydrogel, or decrease in fluidic resistance of the microchannel by shrinkage in volume, may also contribute to the change in flow velocity. Note that the deformation of the microchannel recovers within 60 s after the light stimulation.
Fig. 4. Bright-field microscope images of the side microchannels before (A) and 1 s after the start of light stimulation (B). The white arrows and dotted lines in the magnified images indicate the position and the relative displacement of the particle, respectively, which was moved from the left to the right side of the images. (C) Evaluation of the flow velocity in the side microchannel stimulated by 520 nm light. The power of the light was 30 mW and the stimulation time was 3 seconds, which is represented by the green area in the figure. The video of during the shrinkage is available as a supporting material (Visualization 1).
Figure 5(A) shows a microscopic image of metal microstructures fabricated at different distances from the side microchannel. Figure 5(B) shows the flow velocities 1 s after the start of light stimulation at 30 mW for each of the metal microstructures fabricated in the same hydrogel with different distances. The side microchannel fabricated at a shorter distance from the side channel induced a larger change in flow velocity. Figure 6(A) shows microscopic images of metal microstructures fabricated at different sizes. The change in flow velocity was induced by light stimulation. Figure 6(B) shows the flow velocities at 1 s after the start of light stimulation at 30 mW. The metal microstructures were fabricated in the same hydrogel. The horizontal axis, the size of metal microstructures, indicates the length of one side of the quadrilateral metal microstructures. The spot diameter of the stimulated light was approximately 200 µm and 185 µm for central wavelengths of 520 nm and 405 nm, respectively. The metal microstructures were within the spot diameter of 520 nm and 405 nm light. Figure 6(B) shows that the change of flow velocity and the deformation are significantly large for the larger metal microstructures.
Fig. 5. (A) Bright-field microscope images of silver microstructures fabricated at different distances from the side microchannel. The metal microstructures were fabricated in a square area with a side length of 100 µm. The scale bar indicates 100 µm. (B) Dependence of flow velocity on the distance between the metal microstructures and the side microchannel.
Fig. 6. (A) Bright-field microscope images of silver microstructures fabricated of different sizes. All metal microstructures were fabricated at 40 µm distant from the side microchannel. The scale bar indicates 100 µm. (B) Dependence of flow velocity on the size of the metal microstructures.
Wavelength-selective change of the flow velocity by utilizing gold and silver microstructures was investigated. Two side microchannels were fabricated by irradiating femtosecond laser pulses in the PNIPAm hydrogel. The distance between the walls of the two side microchannels was set to be 280 µm. Then, gold and silver microstructures were fabricated between the two side microchannels as shown in Fig. 7(A). A laser diode with a central wavelength of 405 nm or 520 nm was used for light stimulation, and the flow velocity in the side microchannel was measured during illumination. Figure 7(B) and Fig. 7(C) show the flow velocity in the side microchannels when illuminated with stimulation light of 405 nm and 520 nm, respectively. The flow velocity in channel 1 adjacent to the silver microstructure increased significantly when illuminated by 405 nm light, and the flow velocity in channel 2 adjacent to the gold microstructure increased significantly when illuminated by 520 nm light. Note that neither volume phase transition nor flow velocity change was observed without the fabrication of a metal microstructure. These results indicate that wavelength-selective change in the flow velocity can be realized by utilizing dissimilar metal microstructures fabricated by multiphoton photoreduction.
Fig. 7. (A) Bright-field microscope images of the fabricated side microchannel and metal microstructures. The scale bar indicates 100 µm. Evaluation of flow velocity during light stimulation by 520 nm (B) and 405 nm (C) light. The power of the light stimulation was 30 mW. The illumination duration was 3 seconds.
4 Conclusion
In summary, we have demonstrated the flow velocity control in hydrogel microchannels by utilizing the metal microstructure fabricated by multiphoton photoreduction with a further reduction of metal ions by CW light illumination. The hydrogel around the metal microstructure shrunk and the side wall of the microchannel was stretched toward the metal microstructure by the light stimulation, which induced the change of flow velocity. Wavelength-selective change in the flow velocity by utilizing dissimilar metal microstructures was also demonstrated. Our results indicate that multiphoton photoreduction is a promising method for fabricating novel hydrogel microfluidic devices having flow-controlling switches by a high-spatial arrangement of light-absorbing structures.
Funding
Ministry of Education, Culture, Sports, Science and Technology (JP18H03551, JP22H03958).
Disclosures
The authors declare no conflicts of interest.
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. A. J. DeMello, “Control and detection of chemical reactions in microfluidic systems,” Nature 442(7101), 394–402 (2006). [CrossRef]
2. P. Neužil, S. Giselbrecht, K. Länge, T. J. Huang, and A. Manz, “Revisiting lab-on-a-chip technology for drug discovery,” Nat. Rev. Drug Discov. 11(8), 620–632 (2012). [CrossRef]
3. S. N. Bhatia and D. E. Ingber, “Microfluidic organs-on-chips,” Nat. Biotechnol. 32(8), 760–772 (2014). [CrossRef]
4. A. J. Engler, S. Sen, H. L. Sweeney, and D. E. Discher, “Matrix elasticity directs stem cell lineage specification,” Cell 126(4), 677–689 (2006). [CrossRef]
5. K. S. Lim, M. Baptista, S. Moon, T. B. F. Woodfield, and J. Rnjak-Kovacina, “Microchannels in development, survival, and vascularisation of tissue analogues for regenerative medicine,” Trends Biotechnol. 37(11), 1189–1201 (2019). [CrossRef]
6. R. Xie, W. Zheng, L. Guan, Y. Ai, and Q. Liang, “Engineering of hydrogel materials with perfusable microchannels for building vascularized tissues,” Small 16(15), 1902838 (2020). [CrossRef]
7. D. T. Eddington and D. J. Beebe, “Flow control with hydrogels,” Adv. Drug Deliv. Rev. 56(2), 199–210 (2004). [CrossRef]
8. C. Zhang, D. Xing, and Y. Li, “Micropumps, microvalves, and micromixers within PCR microfluidic chips: advances and trends,” Biotechnol. Adv. 25(5), 483–514 (2007). [CrossRef]
9. S. R. Sershen, G. A. Mensing, M. Ng, N. J. Halas, D. J. Beebe, and J. L. West, “Independent optical control of microfluidic valves formed from optomechanically responsive nanocomposite hydrogels,” Adv. Mater. 17(11), 1366–1368 (2005). [CrossRef]
10. Y. Cao, N. Takeyasu, T. Tanaka, X. Duan, and S. Kawata, “3D metallic nanostructure fabrication by surfactant-assisted multiphoton-induced reduction,” Small 5, NA (2009). [CrossRef]
11. T. Ikegami, R. Ozawa, M. P. Stocker, K. Monaco, J. T. Fourkas, and S. Maruo, “Development of optically-driven metallic microrotors using two-photon microfabrication,” J. Laser Micro/Nanoeng. 8(1), 6–10 (2013). [CrossRef]
12. A. Ishikawa, A. T. Tanaka, and S. Kawata, “Improvement in the reduction of silver ions in aqueous solution using two-photon sensitive dye,” Appl. Phys. Lett. 89(11), 113102 (2006). [CrossRef]
13. S. Kang, K. Vora, and E. Mazur, “One-step direct-laser metal writing of sub100 nm 3D silver nanostructures in a gelatin matrix,” Nanotechnology 26(12), 121001 (2015). [CrossRef]
14. M. Terakawa, M. L. Torres-Mapa, A. Takami, D. Heinemann, N. N. Nedyalkov, Y. Nakajima, A. Hördt, T. Ripken, and A. Heisterkamp, “Femtosecond laser direct writing of metal microstructure in a stretchable poly(ethylene glycol) diacrylate (PEGDA) hydrogel,” Opt. Lett. 41(7), 1392–1395 (2016). [CrossRef]
15. M. Machida, Y. Nakajima, M. L. Torres-Mapa, D. Heinemann, A. Heisterkamp, and M. Terakawa, “Shrinkable silver diffraction grating fabricated inside a hydrogel using 522-nm femtosecond laser,” Sci. Rep. 8(1), 187 (2018). [CrossRef]
16. M. Machida, T. Niidome, H. Onoe, A. Heisterkamp, and M. Terakawa, “Spatially-targeted laser fabrication of multi-metal microstructures inside a hydrogel,” Opt. Express 27(10), 14657–14666 (2019). [CrossRef]
17. K. Mizuguchi, Y. Nagano, H. Nishiyama, H. Onoe, and M. Terakawa, “Multiphoton photoreduction for dual-wavelength-light-driven shrinkage and actuation in hydrogel,” Opt. Mater. Express 10(8), 1931–1940 (2020). [CrossRef]
18. J. Noack and A. Vogel, “Laser-induced plasma formation in water at nanosecond to femtosecond time scales: calculation of thresholds, absorption coefficients, and energy density,” IEEE J. Quantum Electron. 35(8), 1156–1167 (1999). [CrossRef]
19. I. Vérit, L. Gemini, J. Fricain, R. Kling, and C. Rigothier, “Intra-volume processing of gelatine hydrogel by femtosecond laser-induced cavitation,” Lasers Med. Sci. 36(1), 197–206 (2021). [CrossRef]
20. G. Fu, W. Zhou, and X. Li, “Remotely tunable microfluidic platform driven by nanomaterial-mediated on-demand photothermal pumping,” Lab Chip 20(12), 2218–2227 (2020). [CrossRef]
