The authors gratefully acknowledge support from the&#160;National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No.2022R1F1A1074266).

The stimuli-responsive soft gripper was composed of three different types of hydrogels: non-stimuli-responsive acrylamide (AAm)-based hydrogel, thermally responsive N-isopropyl acrylamide (NIPAM)-based hydrogel, and magnetic responsive ferrogel (Figure 1). The three hydrogel inks were prepared by modifying previously published methods29,30,31. The data presented in this study are available ...

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The authors declare no conflicts of interest.

In terms of material selection for the soft hybrid gripper, a multi-responsive material system composed of a non-stimuli-responsive AAm-based hydrogel, a thermally responsive NIPAM-based hydrogel, and a magnetic-responsive ferrogel was first prepared to allow the soft hybrid gripper to exhibit programmable locomotion and shape transformation. Owing to their thermally responsive swelling-deswelling properties, NIPAM-based hydrogels exhibit bending, folding, or wrinkling when fabricated as bilayer or bi-strip structures wi...

The development of stimuli-responsive soft robots is important from both technical and intellectual perspectives. The term stimuli-responsive soft robots generally refers to devices/systems composed of hydrogels, polymers, elastomers, or hybrids that exhibit shape changes in response to external cues, such as heat, pH, and light1,2,3,4. Among the many stimuli-responsive soft robots, N-isopropylacrylamide (NIPAM) hydrogel-based soft robots perform the desired tasks or interactions using spontaneous shape transformation5,6,7,8. Generally, the NIPAM-based hydrogels exhibit a low critical solution temperature (LCST), and swelling (hydrophilicity below the LCST) and deswelling (hydrophobicity above the LCST) property changes occur inside the hydrogel system near physiological temperatures between 32 &#176;C and 36 &#176;C9,10. This reversible swelling-deswelling mechanism near the sharp critical transition point of the LCST can generate the shape transformation of NIPAM-based hydrogel soft robots2. As a result, thermally responsive NIPAM-based hydrogel soft robots have improved operations, such as walking, gripping, crawling, and sensing, which are important in multifunctional manipulators, healthcare systems, and smart sensors2,3,4,11,12,13,14,15,16,17,18,19,20,21.
In the fabrication of stimuli-responsive soft robots, three-dimensional (3D) printing approaches have been widely employed using a direct layer-by-layer additive process22. A variety of materials, such as plastics and soft hydrogels, can be printed with 3D printing23,24. Recently, 4D printing has been extensively highlighted as an innovative technique for creating shape-programmable soft robots25,26,27,28. This 4D printing is based on 3D printing, and the key characteristic of 4D printing is that the 3D structures can change their shapes and properties over time.&#160;The combination of 4D printing and stimuli-responsive hydrogels has provided another innovative route to create smart 3D devices that change shape over time when exposed to appropriate external stimulus triggers, such as heat, pH, light, and magnetic and electric fields25,26,27,28. The development of this 4D printing technique using diverse stimuli-responsive hydrogels has provided an opportunity for the emergence of shape-transformable soft robots that display multifunctionality with improved response speeds and feedback sensitivity.
This study describes the creation of a 3D printing-driven thermally responsive soft gripper that displays shape transformation and locomotion. Notably, the specific procedure described can be utilized to fabricate various multifunctional soft robots with overall sizes ranging from the millimeter to centimeter length scales. Finally, it is expected that this protocol can be applied in several fields, including soft robots (e.g., smart actuators and locomotion robots), flexible electronics (e.g., optoelectrical sensors and lab-on-a-chip), and healthcare systems (e.g., drug delivery capsules, biopsy tools, and surgical devices).

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>2-Hydroxy-4&#39;-(2-hydroxyethoxy)-2-methylpropiophenone</td>
			<td>Sigma Aldrich</td>
			<td>410896-50G</td>
			<td>Irgacure 2959, photoinitiator</td>
		</tr>
		<tr>
			<td>3D WOX 2X</td>
			<td>sindoh</td>
			<td>n/a</td>
			<td>3D printer for fabricating a maze</td>
		</tr>
		<tr>
			<td>Acrylamide</td>
			<td>Sigma-Aldrich</td>
			<td>29-007</td>
			<td>&#8805;99%</td>
		</tr>
		<tr>
			<td>Airbrush compressor</td>
			<td>WilTec</td>
			<td>AF18-2</td>
			<td></td>
		</tr>
		<tr>
			<td>Ammonium persulfate</td>
			<td>Sigma Aldrich</td>
			<td>A4418</td>
			<td></td>
		</tr>
		<tr>
			<td>Auto CAD</td>
			<td>Autodesk</td>
			<td>n/a</td>
			<td>software for computer-aided-design file</td>
		</tr>
		<tr>
			<td>BLX UV crosslinker</td>
			<td>BIO-LINK</td>
			<td>U01-133-565</td>
			<td></td>
		</tr>
		<tr>
			<td>Cartridge</td>
			<td>CELLINK</td>
			<td>CSC010300102</td>
			<td></td>
		</tr>
		<tr>
			<td>Digital stirring Hot Plates</td>
			<td>Corning</td>
			<td>6798-420D</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorescein O-methacrylate</td>
			<td>Sigma Aldrich</td>
			<td>568864</td>
			<td>dye of AAm gel</td>
		</tr>
		<tr>
			<td>INKREDIBLE+ bioprinter</td>
			<td>CELLINK</td>
			<td>n/a</td>
			<td></td>
		</tr>
		<tr>
			<td>Iron(III) Oxide red</td>
			<td>DUKSAN general science</td>
			<td>I0231</td>
			<td></td>
		</tr>
		<tr>
			<td>Laponite RD</td>
			<td>BYK</td>
			<td>n/a</td>
			<td>nanoclay</td>
		</tr>
		<tr>
			<td>Microcentrifuge tube</td>
			<td>SPL</td>
			<td>60615</td>
			<td></td>
		</tr>
		<tr>
			<td>Micro stirrer bar</td>
			<td>Cowie</td>
			<td>27-00360-08</td>
			<td>&#934;3&#215;<img alt="Equation 1" src="/files/ftp_upload/64870/64870img01.png" />0&#160;</td>
		</tr>
		<tr>
			<td>N, N, N&#39;, N&#39;-tetramethylethylenediamine</td>
			<td>Sigma Aldrich</td>
			<td>T7024-100ML</td>
			<td></td>
		</tr>
		<tr>
			<td>N, N&#39;-methylenebisacrylamide</td>
			<td>Sigma Aldrich</td>
			<td>M7279</td>
			<td>&#8805;99.5%</td>
		</tr>
		<tr>
			<td>N-isopropylacrylamide</td>
			<td>Sigma-Aldrich</td>
			<td>415324-50G</td>
			<td></td>
		</tr>
		<tr>
			<td>Poly(N-isopropylacrylamide)</td>
			<td>Sigma-Aldrich</td>
			<td>535311</td>
			<td></td>
		</tr>
		<tr>
			<td>Rhodamine 6G</td>
			<td>Sigma Aldrich</td>
			<td>R4127</td>
			<td>dye of NIPAM gel</td>
		</tr>
		<tr>
			<td>Slic3r software (v1.2.9)</td>
			<td>Slic3r</td>
			<td>n/a</td>
			<td>open-source software to convert .stl file to gcode</td>
		</tr>
		<tr>
			<td>Sodium hydroxide beads</td>
			<td>Sigma Aldrich</td>
			<td>S5881</td>
			<td></td>
		</tr>
		<tr>
			<td>Sterile high-precision conical bioprinting nozzles</td>
			<td>CELLINK</td>
			<td>NZ3270005001</td>
			<td>22 G, 25 G</td>
		</tr>
		<tr>
			<td>Syringe</td>
			<td>Korea vaccine</td>
			<td>K07415389</td>
			<td>10 CC 21 G (1-1/4 INCH)</td>
		</tr>
		<tr>
			<td>Vortex mixer</td>
			<td>DAIHAN</td>
			<td>DH.WVM00030</td>
			<td></td>
		</tr>
	</tbody>
</table>

four-dimensional printing of stimuli-responsive hydrogel-based soft robots

The present protocol describes the creation of four-dimensional (4D), time-dependent, shape-changeable, stimuli-responsive soft robots using a three-dimensional (3D) bio-printing method. Recently, 4D printing techniques have been extensively proposed as innovative new methods for developing shape-transformable soft robots. In particular, 4D time-dependent shape transformation is an essential factor in soft robotics because it allows effective functions to occur at the right time and place when triggered by external cues, such as heat, pH, and light. In line with this perspective, stimuli-responsive materials, including hydrogels, polymers, and hybrids, can be printed to realize smart shape-transformable soft robotic systems. The current protocol can be used to fabricate thermally responsive soft grippers composed of N-isopropylacrylamide (NIPAM)-based hydrogels, with overall sizes ranging from millimeters to centimeters in length. It is expected that this study will provide new directions for realizing intelligent soft robotic systems for various applications in smart manipulators (e.g., grippers, actuators, and pick-and-place machines), healthcare systems (e.g., drug capsules, biopsy tools, and microsurgeries), and electronics (e.g., wearable sensors and fluidics).

The NIPAM-based hydrogel was primarily considered when designing the thermally responsive soft gripper owing to its sharp LCST, which causes it to exhibit significant swelling-deswelling properties9,10. In addition, the AAm-based hydrogel was considered as a non-stimuli-responsive system to maximize the shape transformation of the soft hybrid gripper while reducing the delamination of the interface during multiple heating and cooling processes. In addition, ferro...

Watch this Scientific Journal Video about Four-Dimensional Printing of Stimuli-Responsive Hydrogel-Based Soft Robots at JoVE.com

Four-Dimensional Printing of Stimuli-Responsive Hydrogel-Based Soft Robots

This manuscript describes a 4D printing strategy for fabricating intelligent stimuli-responsive soft robots. This approach can provide the groundwork to facilitate the realization of intelligent shape-transformable soft robotic systems, including smart manipulators, electronics, and healthcare systems.

The present protocol describes the creation of four-dimensional (4D), time-dependent, shape-changeable, stimuli-responsive soft robots using a ...

The presented protocol is expected to provide new directions for realizing intelligent, shape transformable, soft robotic systems for various applications. The 4D time-dependent printing process can create diverse stimuli-responsive soft robots, with a wide size range, from millimeters to centimeters. This 4D bioprinting technique can be extended towards targeted drug delivery, microsurgery, and less invasive biopsy in healthcare engineering.Text-only information can create ambiguity for the readers, so visual demonstrations are essential to help achieve the intended results. To prepare the non-stimuli-responsive acrylamide-based hydrogel inks, dilute the acrylamide, the cross linker, and the photo initiator in deionized water using a magnetic stirrer for 24 hours. To prepare the stimuli responsive N-isopropylacrylamide based hydrogel inks, dilute N-isopropylacrylamide, propyl N-isopropylacrylamide, and the photo initiator, in deionized water, using a magnetic stirrer for 24 hours.Then add dye to acrylamide gel, and N-isopropylacrylamide gel, and vortex the shearing agent, Laponite RD, at 1150 rotations per minute, or at least six hours, until they are completely diluted. Then follow the instructions provided in the text to prepare the hydrogel ink. For preparing the ferrogel inks, first, prepare A solution and B solution according to the protocol described in the text.To carry out polymerization, transfer 200 microliters of the A solution, and five microliters of the B solution, into a microcentrifuge tube, and vortex the mixture for 20 seconds. Using the Slicer software, generate a G code for each structure previously created by optimization of the gripper design. Assign a layer height of 0.4 millimeters.An infill density of 75%And a printing speed of 10 millimeters per second. Edit the G code file using dual print heads. Save the G code file on a secure digital or SD card, before connecting it to the 3D printer.After connecting the acrylamide-based and N-isopropylacrylamide-based hydrogel cartridges to the respective nozzles, check if the two print heads of the cartridges are at the same position on the Z-axis. Then calibrate the X and Y coordinates precisely, to avoid misalignments between the two nozzles. Now, set the printing pressure at 20 to 25 kilopascal for the acrylamide-based hydrogel, and at 10 to 15 kilopascal for the N-isopropylacrylamide-based hydrogel.Repeat the steps after each sample is completely printed, to print the next one. Before UV photocuring, inject the magnetic field-responsive ferrogel inks into the targeted thin hole area of the 3D printed soft gripper using a syringe. After the injection of the ferrogel, place the gripper structure inside a UV source chamber, having a wavelength of 365 nanometers, for six minutes.After UV photo curing, transfer the gripper structure to a deionized water bath, for at least 24 hours, until it reaches a fully swollen equilibrium state. The soft hybrid gripper performed a pick and place task via thermally-responsive actuation and magnetic locomotion. When the temperature increased above the lower critical solution temperature, or LCST, the N-isopropylacrylamide based hydrogel deswelled, and shrank, closing the gripper tip.In contrast, the gripper tip opened when the temperature decreased below the LCST, due to swelling of the N-isopropylacrylamide-based hydrogel. The gripper also demonstrated a pick and place task inside a 3D-printed sample maze filled with deionized water. The gripper, in its tip open state, was guided by an external magnet from its starting position to the target salmon roe.When the temperature reached 40 degrees Celsius, the tip of the gripper closed to grip the salmon roe. The gripper was guided out of the maze while holding the salmon roe, and it successfully released the intact salmon roe at the target area in a tip open state, at room temperature of 25 degrees Celsius. Experimental care must be taken while calibrating the coordinate points between the two nozzles.This process requires a lot of practice. This specific protocol provides the groundwork for further significant advances in the realization of precisely controllable, highly sensitive, and multi-functional smart-stimuli-responsive soft robots.

four-dimensional-printing-stimuli-responsive-hydrogel-based-soft

Research

JoVE Journal

Engineering

2.6K Views.  Sookmyung Women's University. The presented protocol is expected to provide new directions for realizing intelligent, shape transformable, soft robotic systems for various applications. The 4D time-dependent printing process can create diverse stimuli-responsive soft robots, with a wide size range, from millimeters to centimeters. This 4D bioprinting technique can be extended towards targeted drug delivery, microsurgery, and less invasive biopsy in healthcare engineering.Text-only information can create ambiguity f...