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Abstract
Photothermal microactuators are often used as microswitches or microgrippers in micro-electromechanical systems, whereas it is difficult to fabricate three-dimensional microactuators with a high aspect ratio, since the gravity may lead to undesired deformations during printing processes. In this work, we reported a 3D printing / UV curing process flow in the support of a hydrogel to obtain a photothermal microactuator with a high-aspect-ratio polyline waveguiding structure. The waveguiding structure also served as the driving arm. The temperature parameter was investigated by the Finite Element Method while the experiment was carried out to study the temperature and displacement during the laser actuation. A demonstration showed the driving arm achieves a free-end displacement of 133.2 µm driven by 90 mW laser (46.1°C). This study helps obtain waveguiding photothermal microactuators with integrated and more complex multi-dimensional structures.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photothermal microactuators are often used as microswitches or microgrippers in micro-electromechanical systems (MEMS) or biomedical sciences [1–3]. The light-driven strategies enable clean, safe, and remote control of actuators in a noncontact manner without complex coupled instruments [4]. A variety of photothermal microactuators with different structures have been developed through LIGA [5], PolyMUMPs [6], and soft-lithography [7] techniques. However, the manufacture of microactuators with complex three-dimensional (3D) structures remains challenging. 3D printing has been considered an attractive way to flexibly fabricate multi-dimensional architectures [8]. 3D printed photothermal microactuators have been reported, demonstrating the merits such as good material compatibility, rapid prototyping, and simplified post-processing. On the other hand, it is still difficult to fabricate 3D microactuators with a high aspect ratio, since the gravity may lead to undesired deformations during printing processes.
Recently, hydrogels have been induced to the 3D printing of materials in the liquid phase [9–20]. The hydrogels provide robust supports for printing, minimizing the detrimental deformations. The printing of polystyrene microspheres [9], biological cells [9], polydimethylsiloxane (PDMS) [12] and liquid metal crafts [13] in hydrogel have been reported. Therefore, this protocol is possible to freely fabricate microactuators with complex 3D structures.
From the photothermal actuator prospective, hydrogels have been used as the structural materials to enable photothermal actuations [21–23] in recent decades, showing large response to the control light. Light-responsive materials such as rare-earth-oxide particles [24], Au nanoparticles [23], graphene oxide [22,24], single-walled carbon nanotubes [25], multi-walled carbon nanotubes [26] are also applied to the photothermal actuators to enhance deformations. However, to our best knowledge, no photothermal actuator was fabricated using 3D printing in the support of hydrogel yet.
Herein, we developed a 3D printing / UV curing process flow in a hydrogel to fabricate a photothermal microactuator with a 3D-waveguiding structure. The process parameters of the printing process are given in detail. As a demonstration, a square-spiral waveguiding structure was fabricated. The multi-dimensional-structure microactuator was excited by an IR laser (980 nm) in the form of a waveguide. The temperature distributions were investigated by both simulation and experiment. The displacement in respect of the laser power is also discussed. Compared with traditional photothermal actuators which rely on an external laser beam to irradiate and heat the driving arms [27], waveguiding photothermal actuating modules are more compact, without using discrete lens systems. Also, the waveguide structures demonstrate superiorities on the compatibility with complex actuating arms, since the external laser beams can be blocked by the other parts of the devices. The process flow we demonstrated may provide alternative methods for the fabrication of photothermal actuators.
2. Printing waveguiding structure in hydrogel
Figure 1 shows the general fabrication process flow for the actuator. A self-made direct writing equipment with a program-controlled 3-axis platform was used for the printing [28]. A syringe was installed in the syringe infusion pump and injected the UV-curable resin into the hydrogel [Fig. 1(a)]. Note that, the as-printed waveguiding structure without UV-curing was still in a liquid phase, whereas the waveguiding structure maintained its geometric feature due to the support of the hydrogel. After printing in the hydrogel, a UV-curing post-treatment was conducted via UV light irradiation (365 nm, 200 mW/cm2) for 6 min to solidify the resin actuating arm, as shown in Fig. 1(b). The actuating arm was taken out from the hydrogel, washed in DI water, and dried naturally. Then, monodispersed Ag nanoplates ink with a concentration of ∼20% (w/w) was carefully coated onto the actuating arm as the photothermal conversion (PTC) material. The detail of the Ag nanoplates ink can be found in the lecture [29]. The silver nanoplate ink was dried on a hotplate at 60°C as shown in Fig. 1(c). Finally, adhering the actuating arm onto a glass bracing structure completed the fabrication.
Fig. 1. The fabrication process flow for the square-spiral actuating arm, including (a) 3D printing in a hydrogel, (b) (UV) curing, and (c) spread a coating of the PTC material.
The fabrication procedure starts with the preparation of the hydrogel. The hydrogel acts as the supporting material to provide robust support to the as-printed driving arm structure. In preparing the hydrogel, 2.25 g Carbopol 940 powder (The Lubrizol Corporation) was added to 300 ml deionized (DI) water with a string at 200 rpm for 20 min. Then, a 0.2 ml NaOH aqueous solution (10 M) was added to the mixture to neutralize the Carbopol 940 gel [12]. Then the mixture was stirred at 200 r/min for 30 min until the mixture was fully swollen. And the hydrogel was formed. A commercial UV-curable resin, NOA61 (Norland Products Inc.) was used as the actuating arm material. The UV-curable resin was diluted by cyclopentanone with a resin concentration of 87.5% (w/w). The solution was stirred for 20 min and stored in a syringe before printing.
An investigation of the influence of processing parameters was conducted to provide an optimum design guideline. The basic parameters are marked in Fig. 2(a). The inset optical microscope image shows a printed line structure. Generally, the soft, solid hydrogel with an extremely low yield stress and elastic shear modulus can flow under its own body forces. The hydrogel locally transitions between solid and fluidized states under the stress applied by the moving needle. This enables the liquid resin to be injected and trapped in hydrogel without relying on the solidification of the resin [10]. Hence, one key parameter for the printing is the rheological property of the hydrogel [9,14]. Figure 2(b) plots the shear viscosity and the shear stress of the as-prepared hydrogel as a function of the shear rate. Typically, a higher shear rate, which corresponding to a faster-moving speed of the nozzle in practice, leads to larger shear stress and smaller shear viscosity. A too low viscosity cannot provide stable support, while an excess high viscosity prevents the hydrogel from recovering rapidly, leading to an enlarged and irregularly shaped structure as the inset image show in Fig. 2(b). Therefore, a proper shear viscosity with a shear rate is needed.
Fig. 2. Characterization of hydrogel and printed lines. (a) The mechanism of printing resin into the hydrogel. The inset shows the optical microscope images of a printed sample. (b) Rheological and viscosity of Carbopol 940 hydrogel. The scale bar indicates 500 µm. (c) Mean diameters of the printed lines versus moving speeds of the needle of 200 µm in inner diameter. (d) Mean diameters versus extrusion rates. (e) Mean diameters versus inner diameters of the needles with extrusion rates of 19.2 µL/min and 1.2 µL/min. (Error bars represent 95% confidence intervals. No error bar indicates less error than the size of the marker.)
Here we directly investigate the influence of the printing parameters to the dimensions of the obtained samples, providing an optimum guideline. The main diameter of the printed lines as a function of moving speed of the needle, vn, is plotted in Fig. 2(c) with a constant extrusion rate of the resin (E=15.8 µL/min). Though the moving speed determines the shear rate, only slightly fluctuates of the main diameters are observed with a varying low moving speed. Hence, the moving speed can be freely varied in a range at least from 0.167 mm/s to 0.583 mm/s. However, an extremely high moving speed, for example, 1 m/s [10], may generate an air gap at the needle, leading to a distorted shape. On the contrary, an increasing extrusion rate leads to an enlarged mean diameter of the printed line as shown in Fig. 2(d) (vn=0.5 mm/s). Therefore, the extrusion rate is the dominant parameter to control the diameter of the printed line in our case.
Additionally, the diameter of the needle should be taken in consideration. Figure 2(e) plots the mean diameters of the printed line versus inner diameters of the needles. Two typical extrusion rates of 19.2 µL/min and 1.2 µL/min compared at a constant moving speed of 0.5 mm/s. At a low extrusion rate of 1.2 µL/min, only slightly decrease of the main diameter was observed at a smaller inner diameter (ID = 150 µm). Note that, at a high extrusion rate of 19.2 µL/min, the samples fabricated by the needle with moderate diameters of 200 µm and 250 µm shows the similar feature as the ones printed at the low extrusion rate of 1.2 µL/min. However, significant increases and deviations of the main diameters of the printed lines exist for both thick (ID = 400 µm) and thin (ID = 150 µm) needles. Therefore, needles of 200 µm or 250 µm in inner diameter is more preferred for printing thinner line structures.
3. Demonstration of the waveguiding photothermal actuator
Since 3D printing in the hydrogel is possible to construct multi-dimensional structures freely, we demonstrate a photothermal microactuator consist a square-spiral actuating arm [shown in Fig. 3(a)] inspired by spiral stairs, which is more difficult to fabricate via other techniques [5–7]. One end of the actuating arm is anchored by a glass bracing structure and the free-end provides displacements. In terms of mechanism, the laser is guided from the anchor end and propagates along the actuating arm. PTC material (silver nanoplates), which is attached to a certain area of the actuating arm, converting laser power into heat, leading to a thermal extension of the driving arm with a displacement at the free-end. Figure 3(b) shows the geometric dimension of the driving arm. The length l and the diameter d of each side of the square-spiral actuating arm are set as 6.18 mm and 200 µm, respectively. The incline angle θ is 14° respected to the horizontal.
Fig. 3. The structure of the square-spiral actuator. (a) The schematic of the photothermal microactuator, and (b) the geometric parameters of each arm.
Figure 4(a) shows the printed photothermal microactuator. Two square-spiral actuating arms were printed simultaneously [Fig. 4(b)], and were cut apart by a stainless steel blade. The microactuator consists of an actuating arm assembled on a glass bracing structure [Fig. 4(c)] and a dark-grayish photothermal conversion area [Fig. 4(d)]. The optical fiber is end-coupled to the anchor-end of the actuating arm [29,30] to guide the laser into the actuator.
Fig. 4. The square-spiral photothermal microactuator. Photo image of (a) 3D printing in the hydrogel, (b) an as prepared driving arm, (c) photothermal microactuator with glass support, (d) Microscope images of the driving arm under external light illumination from a white LED, (e) Laser (638 nm, 0.1 mW) propagation through the driving arm with an output light spot at the free end of the cantilever beam. The scale bars in (a) - (c) indicate 20 mm, and in (e), (f) indicate 300 µm.
Note that, the sharp bends of the square-spiral structure cause larger bending losses. However, due to a high refractive index contrast (Δn=1.546) between the polymer core and air cladding of the waveguide, the leaky modes at the bends have been restrained [31] Also, the bending corners of the fabricated samples are rounded with a radius about 0.3 mm, which further reduced the bending losses [29], compared with a sharp right bend. The image in Fig. 4(e) demonstrates a 638 nm laser propagation along the optical waveguiding microactuator. Low scattering light can be abserved along the whole waveguide, whereas the relatively bright out-put spot proving the effective guiding of the laser.
The PTC material (silver nanoplates) in our case exhibits the maximum light absorption at a wavelength range from 700 nm to 1100 nm [32]. Therefore, an infrared (IR) laser (980 nm) was used for the actuation.
One factor that should be first considered for the thermal extension is the temperature rising of the actuating arm. We monitored the temperature distribution via capturing the infrared (IR) images by a thermal imager (222s, Fotric, Inc.) during the laser actuation. Figure (5) illustrates the temperature with respect to the increasing laser power. The white dashed line in Fig. 5(a) outlines the actuating arm in the IR images. The introduction of the laser leads to a temperature rising of the actuating arm. It should be mentioned that, due to the low thermal conductivity of the NOA61 resin (0.16 W/(m·K)), the heat was localized around the thermal conversion area with an appropriately 22°C discrepancy (90 mW) to the other area of the actuating arm. As a contrast, no obvious temperature rising was observed for the actuating arm without PTC material [Fig. 5(d)] at a laser power of 90 mW, which demonstrates the functionality of the PTC material.
Fig. 5. The temperature distributions of the photothermal microactuator in the cases (a) laser off (0 mW) with the PTC material, (b) laser on (45 mW) with the PTC material, (c) laser on (90 mW) with the PTC material, (d) laser on (90 mW) without the PTC material, and (e) the temperature variations as a function of the laser powers. The black solid line marked the experiment results and the red dash-dotted line plots the simulation results.
Finite Element Method (FEM) was used to further demonstrate the temperature distributions of actuating arm [32]. The material parameters and structural dimensions for the simulation are listed in Table 1. The calculated temperature distribution of the actuating arm is illustrated in the inset of Fig. 5(e), which is consistent with the experimental results.
Table 1. Material parameters and structural dimensions of the microactuator
Figure 5(e) also plots the maximum temperature through the actuating arm at certain laser powers. With an increasing laser power from 0 mW to 90 mW, a quasi-linear rising temperature from 23.7°C to 46.1°C was observed in the experiment, showing a similar regularity as the calculation results. The diversity of the laser power between experimental results and calculation results is attributed to the limited thermal conversion efficiency of the silver nanoplates, coupling loss between the fiber and the actuating arm, and the propagation loss [32,33], since a 100% efficiency was assumed in the simulation.
The motion of the free-end was captured by video microscopes as shown in Fig. (6). A significant displacement with respect to the initial position (the white dashed cycle in Fig. 6(b) was observed at two states, laser off (0 mW) and laser on (90 mW). The displacement of the free-end can be decomposed into three orthogonal directions as shown in Fig. 7(a), which are all quasi-proportional to the laser power. The X and Y directions illustrate approximately doubled displacements to that at Z direction. The maximum displacements of 87.5, 95, and 32.5 µm are obtained at the maximum laser power of 90 mW, respectively. The total displacement was plotted in Fig. 7(b) with a maximum displacement of 133.2 µm at the laser power of 90 mW and the corresponding temperature of 46.1°C. As a reference, without PTC material, only slight displacements of the free-end were observed as the red dash-dotted line shown in Fig. 7(b), due to the low light absorption by UV resin at 980 nm wavelength. This confirmed the functionality of the waveguiding photothermal actuator.
Fig. 6. The motion of the microactuator when actuated by an IR laser (980 nm). (a) Initial state (0 mW), (b) actuation state (90 mW). The scale bars indicate 300 µm.
Fig. 7. The free-end displacement of the actuator. (a) The displacement in three directions. (b) The total displacement of the actuator with/without PTC material.
4. Conclusions
In this study, we reported a 3D printing/UV curing process flow in a hydrogel to fabricate photothermal microactuators with 3D-waveguiding structures. The process parameters of the printing process are investigated, showing that both the nozzle inner diameter and extrusion rate affects the diameter of the waveguiding structures. As a demonstration, a square-spiral waveguiding structure was fabricated. A total displacement of 133.2 µm was achieved with a 90 mW laser power (46.1°C). This study helps obtain waveguiding photothermal microactuators with integrated and more complex multi-dimensional structures.
Funding
National Natural Science Foundation of China (11904177, 61704090); Natural Science Foundation of Jiangsu Province (BK20170903, BK20170908); National and Local Joint Engineering Laboratory of RF Integration and Micro-Assembly Technology (KFJJ20180201).
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.
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