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 on request from the corresponding author.
1. Preparation of hydrogel inks
<ol>
	<li>Non-stimuli-responsive AAm-based hydrogel inks (Figure 1A)
	<ol>
		<li>Dilute the acrylamide (AAm), the crosslinker N, N&#39;-methylenebisacrylamide (BIS) (see Table of Materials), and the photoinitiator 2-Hydroxy-4&#39;-(2-hydroxyethoxy)-2-methylpropiophenone (see Table of Materials) in distilled (DI) water using a magnetic stirrer for 24 h.</li>
		<li>Vortex the shear-thinning agent, laponite RD nanoclay, and fluorescein O-methacrylate dye (see Table of Materials) at 1,150 rpm for at least 6 h until they completely dilute.</li>
		<li>Prepare specific weights of AAm-based hydrogel ink per total 20 mL of solution base: 1.576 g of AAm, 0.332 g of BIS, 1.328 g of laponite RD, 0.166 g of photoinitiator, 0.1 mg of NaOH, 0.1 mg of fluorescein O-methacrylate (see Table of Materials), and 16.594 g of DI water.</li>
		<li>After total dilution, transfer the AAm-based hydrogel ink into an empty 3D printing cartridge (see Table of Materials) using a syringe.</li>
	</ol>
	</li>
	<li>Stimuli-responsive NIPAM-based hydrogel inks (Figure 1B)
	<ol>
		<li>Dilute N-isopropyl acrylamide (NIPAM), poly N-isopropyl acrylamide (PNIPAM), and the photoinitiator (see Table of Materials) in DI water using a magnetic stirrer for 24 h.</li>
		<li>Vortex the shear-thinning agent, laponite RD nanoclay, and fluorescein rhodamine 6G dye at 1,150 rpm for at least 6 h until they completely dilute.</li>
		<li>Prepare specific weights of NIPAM-based hydrogel ink per total 20 mL of solution base: 1.692 g of NIPAM, 0.02 g of pNIPAM, 1.354 g of laponite RD, 0.034 g of photoinitiator, 0.1 mg of rhodamine 6G (see Table of Materials), and 16.92 g of DI water.</li>
		<li>After complete dilution, transfer the NIPAM-based hydrogel ink into an empty 3D printing cartridge using a syringe.</li>
	</ol>
	</li>
	<li>Ferrogel inks (Figure 1C)
	<ol>
		<li>Prepare the A-solution: Dilute acrylamide (AAm) and crosslinker, N, N&#39;-methylenebisacrylamide (BIS), ferric oxide (Fe2O3), and N, N, N&#39;, N&#39;-tetramethylethylenediamine (TMEDA) (see Table of Materials) in DI water.</li>
		<li>Consider the specific weight percent (wt%) of the materials: 71% AAm, 3.5% BIS, and 25.5% Fe2O3 in 1.2 mL of DI water with 10 &#181;L of TMEDA accelerator.</li>
		<li>Prepare the B-solution: Dilute 0.8 g of ammonium persulphate (APS, see Table of Materials) in 10 mL of DI water.</li>
		<li>For polymerization, transfer 200 &#181;L of the A-solution and 5 &#181;L of the B-solution into a microcentrifuge tube.</li>
		<li>Vortex the microcentrifuge tube for 20 s.</li>
	</ol>
	</li>
</ol>
2. Optimization of the soft hybrid gripper design
NOTE: The elliptical soft hybrid gripper is composed of an AAm-based hydrogel outer layer, a NIPAM-based hydrogel inner layer, and a ferrogel upper layer (Figure 1D). The overall elliptical soft hybrid gripper was created using the AutoCAD software (see Table of Materials).
<ol>
	<li>Two-dimensional AAm-based hydrogel layer design
	<ol>
		<li>Draw an elliptical shape with a vertical axis of 24 mm and a horizontal axis of 20 mm at the outermost part.</li>
		<li>Draw another elliptical shape with a vertical axis of 20.8 mm and a horizontal axis of 16.8 mm with the same center point as the shape drawn in step 2.1.1.</li>
		<li>Draw a three-point arc passing through the points (&#8722;8.24, 2), (0, 6), and (8.24, 2) away from the center point of the ellipse.</li>
		<li>Trim the small upper part of the eclipse divided by the arc.</li>
	</ol>
	</li>
	<li>Two-dimensional NIPAM-based hydrogel layer design
	<ol>
		<li>Draw an oval with a vertical axis of 20.2 mm and a horizontal axis of 16.4 mm with the same center point as the shape drawn in step 2.1.1.</li>
		<li>Draw an ellipse with a vertical axis of 16.16 mm and a horizontal axis of 13.12 mm with the same center point as the shape drawn in step 2.1.1.</li>
		<li>Draw a three-point arc passing through the points (&#8722;7.86, 1.83), (0, 5.6), and (7.86, 1.83) away from the center point of the ellipse.</li>
		<li>Draw a three-point arc passing through the points (&#8722;5.47, 1.64), (0, 3.18), and (5.47, 1.64) away from the center point of the ellipse.</li>
		<li>Trimthe small upper part of the ellipses divided by the arcs.</li>
		<li>To make a pedestal, draw an arc with two points away from the center point at (&#8722;4.75, &#8722;2.71) and (4.75, &#8722;2.71) as both endpoints and one point away from the center point at (0, -4.59).</li>
	</ol>
	</li>
	<li>Two-dimensional ferrogel layer design
	<ol>
		<li>Draw a three-point arc passing through the points (&#8722;7, 4.92), (0, 9.2), and (7, 4.92) away from the center point of the ellipse.</li>
		<li>Draw a three-point arc passing through the points (&#8722;7, 4.92), (0, 7.6), and (7, 4.92) away from the center point of the ellipse.</li>
	</ol>
	</li>
	<li>Two-dimensional gripper tips design
	<ol>
		<li>To make the grasping part of the gripper, cut 0.8 mm from each side from the center line at the bottom of the ellipse.</li>
	</ol>
	</li>
	<li>Three-dimensional hybrid gripper design
	<ol>
		<li>To turn the overall 2D hybrid gripper design into 3D, extrude the pedestal of the responsive gel by 0.8 mm, and extrude the non-responsive gel, the cut oval of the responsive gel, and the ferrogel by 2.5 mm.</li>
	</ol>
	</li>
</ol>
3. Three-dimensional printing of the soft hybrid gripper
<ol>
	<li>Generate a G-code30 for each structure created in step 2 using Slic3r software (see Table of Materials) with a 0.4 mm layer height, a 10 mms&#8722;1 printing speed, and an infill density of 75%. Edit the G-code file using dual print heads.</li>
	<li>Save the G-code file on a secure digital (SD) card, and connect it to the 3D printer (see Table of Materials) to generate the printing paths of the soft gripper.</li>
	<li>Connect an air pump pressure control to the 3D printer.</li>
	<li>Choose nozzle tips with diameters of 0.25 mm and 0.41 mm for the NIPAM-based hydrogel and AAm-based hydrogel, respectively.</li>
	<li>Connect the AAm-based hydrogel cartridge to nozzle 1 and the NIPAM-based hydrogel cartridge to nozzle 2.</li>
	<li>Check if the two print heads of the cartridges are at the same position on the z-axis.</li>
	<li>Calibrate the X and Y coordinates precisely to avoid misalignments between the two nozzles.</li>
	<li>Set the printing pressure at 20-25 KPa for the AAm-based hydrogel and at 10-15 KPa for the NIPAM-based hydrogel.</li>
	<li>Repeat steps 3.5-3.8 when each sample is completely printed (Figure 2A).</li>
</ol>
4. UV photocuring of the soft hybrid gripper
<ol>
	<li>Before UV photocuring, inject the magnetic field-responsive ferrogel inks (prepared in step 1.3) into the targeted thin-hole area of the 3D-printed soft gripper using a syringe.</li>
	<li>After the injection of the ferrogel, place the gripper structure inside a UV source chamber with a 365 nm wavelength for 6 min. Fix the intensity of the UV light at 4.9 mJ/s.</li>
	<li>After UV photocuring, transfer the gripper structure to a DI water bath for at least 24 h until it reaches a fully swollen equilibrium state (Figure 2B-D).</li>
</ol>

<ol>
	<li>Gracias, D. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stimuli+responsive+self-folding+using+thin+polymer+films.">Stimuli responsive self-folding using thin polymer films.</a> Current Opinion in Chemical Engineering. 2 (1), 112-119 (2013).</li><li>Zhang, Y. S., Khademhosseini, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Advances+in+engineering+hydrogels.">Advances in engineering hydrogels.</a> Science. 356 (6337), (2017).</li><li>Erol, O., Pantula, A., Liu, W., Gracias, D. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transformer+hydrogels:+A+review.">Transformer hydrogels: A review.</a> Advanced Materials Technologies. 4 (4), 1900043 (2019).</li><li>Liu, X., Liu, J., Lin, S., Zhao, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hydrogel+machines.">Hydrogel machines.</a> Materials Today. 36, 102-124 (2020).</li><li>Hu, Z., Zhang, X., Li, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synthesis+and+application+of+modulated+polymer+gels.">Synthesis and application of modulated polymer gels.</a> Science. 269 (5223), 525-527 (1995).</li><li>Klein, Y., Efrati, E., Sharon, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Shaping+of+elastic+sheets+by+prescription+of+non-Euclidean+metrics.">Shaping of elastic sheets by prescription of non-Euclidean metrics.</a> Science. 315 (5815), 1116-1120 (2007).</li><li>Kim, J., Hanna, J. A., Byun, M., Santangelo, C. D., Hayward, R. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Design+responsive+buckled+surfaces+by+halftone+gel+lithography.">Design responsive buckled surfaces by halftone gel lithography.</a> Science. 335 (6073), 1201-1205 (2012).</li><li>Breger, J. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Self-folding+thermo-magnetically+responsive+soft+microgrippers.">Self-folding thermo-magnetically responsive soft microgrippers.</a> ACS Applied Materials &amp; Interfaces. 7 (5), 3398-3405 (2015).</li><li>Schild, H. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Poly+(N-isopropylacrylamide):+Experiment,+theory+and+application.">Poly (N-isopropylacrylamide): Experiment, theory and application.</a> Progress in Polymer Science. 17 (2), 163-249 (1992).</li><li>Ahn, S., Kasi, R. M., Kim, S. -. C., Sharma, N., Zhou, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stimuli-responsive+polymer+gels.">Stimuli-responsive polymer gels.</a> Soft Matter. 4, 1151-1157 (2008).</li><li>Stuart, M. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Emerging+applications+of+stimuli-responsive+polymer+materials.">Emerging applications of stimuli-responsive polymer materials.</a> Nature Materials. 9, 101-113 (2010).</li><li>Ionov, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biomimetic+hydrogel-based+actuating+systems.">Biomimetic hydrogel-based actuating systems.</a> Advanced Functional Materials. 23 (36), 4555-4570 (2013).</li><li>Ghosh, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stimuli-responsive+soft+untethered+grippers+for+drug+delivery+and+robotic+surgery.">Stimuli-responsive soft untethered grippers for drug delivery and robotic surgery.</a> Frontiers in Mechanical Engineering. 3, 7 (2017).</li><li>Kirillova, A., Ionov, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Shape-changing+polymers+for+biomedical+applications.">Shape-changing polymers for biomedical applications.</a> Journal of Materials Chemistry B. 7, 1597-1624 (2019).</li><li>Le, X., Lu, W., Zhang, J., Chen, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recent+progress+in+biomimetic+anisotropic+hydrogel+actuators.">Recent progress in biomimetic anisotropic hydrogel actuators.</a> Advanced Science. 6 (5), 1801584 (2019).</li><li>Xu, W., Gracias, D. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Soft+three-dimensional+robots+with+hard+two-dimensional+materials.">Soft three-dimensional robots with hard two-dimensional materials.</a> ACS Nano. 13 (5), 4883-4892 (2019).</li><li>Yoon, C. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Advances+in+biomimetic+stimuli+responsive+soft+grippers.">Advances in biomimetic stimuli responsive soft grippers.</a> Nano Convergence. 6, 20 (2019).</li><li>Lee, Y., Song, W. J., Sun, J. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hydrogel+soft+robotics.">Hydrogel soft robotics.</a> Materials Today Physics. 15, 100258 (2020).</li><li>Shen, Z., Chen, F., Zhu, X., Yong, K. T., Gu, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stimuli-responsive+functional+materials+for+soft+robotics.">Stimuli-responsive functional materials for soft robotics.</a> Journal of Materials Chemistry B. 8, 8972-8991 (2020).</li><li>Kim, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Shape+morphing+smart+3D+actuator+materials+for+micro+soft+robot.">Shape morphing smart 3D actuator materials for micro soft robot.</a> Materials Today. 41, 243-269 (2020).</li><li>Ding, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multifunctional+soft+machines+based+on+stimuli-responsive+hydrogels:+From+freestanding+hydrogels+to+smart+integrated+systems.">Multifunctional soft machines based on stimuli-responsive hydrogels: From freestanding hydrogels to smart integrated systems.</a> Materials Today Advances. 8, 100088 (2020).</li><li>Wang, X., Jiang, M., Zhou, Z., Gou, J., Hui, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=3D+printing+of+polymer+matrix+composites:+A+review+and+prospective.">3D printing of polymer matrix composites: A review and prospective.</a> Composites Part B: Engineering. 110, 442-458 (2017).</li><li>Bartlett, N. W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+3D-printed,+functionally+graded+soft+robot+powered+by+combustion.">A 3D-printed, functionally graded soft robot powered by combustion.</a> Science. 349 (6244), 161-165 (2015).</li><li>Wehner, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+integrated+design+and+fabrication+strategy+for+entirely+soft,+autonomous+robots.">An integrated design and fabrication strategy for entirely soft, autonomous robots.</a> Nature. 536, 451-455 (2016).</li><li>Tibbits, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=4D+printing:+Multi-material+shape+change.">4D printing: Multi-material shape change.</a> Architectural Design. 84 (1), 116-121 (2014).</li><li>Gladman, A. S., Matsumoto, E. A., Nuzzo, R. G., Mahadevan, L., Lewis, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biomimetic+4D+printing.">Biomimetic 4D printing.</a> Nature Materials. 15, 413-418 (2016).</li><li>Momeni, F., Hassani, S. M., Liu, X., Ni, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+review+of+4D+printing.">A review of 4D printing.</a> Materials &amp; Design. 125, 42-79 (2017).</li><li>Ionov, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=4D+biofabrication:+Materials,+methods,+and+applications.">4D biofabrication: Materials, methods, and applications.</a> Advanced Healthcare Materials. 7 (17), 1800412 (2018).</li><li>Liu, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dual-gel+4D+printing+of+bioinspired+tubes.">Dual-gel 4D printing of bioinspired tubes.</a> ACS Applied Materials &amp; Interfaces. 11 (8), 8492-8498 (2019).</li><li>Son, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Untethered+actuation+of+hybrid+hydrogel+gripper+via+ultrasound.">Untethered actuation of hybrid hydrogel gripper via ultrasound.</a> ACS Macro Letters. 9 (12), 1766-1772 (2020).</li><li>Ding, Z., Salim, A., Ziaie, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Squeeze-film+hydrogel+deposition+and+dry+micropatterning.">Squeeze-film hydrogel deposition and dry micropatterning.</a> Analytical Chemistry. 82 (8), 3377-3382 (2010).</li><li>Ongaro, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Autonomous+planning+and+control+of+soft+untethered+grippers+in+unstructured+environments.">Autonomous planning and control of soft untethered grippers in unstructured environments.</a> Journal of Micro-Bio Robotics. 12, 45-52 (2017).</li><li>Scheggi, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Magnetic+motion+control+and+planning+of+untethered+soft+grippers+using+ultrasound+image+feedback.">Magnetic motion control and planning of untethered soft grippers using ultrasound image feedback.</a> 2017 IEEE International Conference on Robotics and Automation (ICRA). IEEE. , 6156-6161 (2017).</li></ol>

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 ...

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

Universidad San Sebastian

Research

JoVE Journal

Engineering

2.8K Views.  Sookmyung Women's University. 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. 

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

Micro 3D Printing Using a Digital Projector and its Application in the Study of Soft Materials Mechanics

Buckling is a classical topic in mechanics. While buckling has long been studied as one of the major structural failure modes1, it has recently drawn new attention as a unique mechanism for pattern transformation. Nature is full of such examples where a wealth of exotic patterns are formed through mechanical instability2-5. Inspired by this elegant mechanism, many studies have demonstrated creation and transformation of patterns using soft materials such as elastomers and hydrogels6-11. Swelling gels are of particular interest because they can spontaneously trigger mechanical instability to create various patterns without the need of external force6-10. Recently, we have reported demonstration of full control over buckling pattern of micro-scaled tubular gels using projection micro-stereolithography (P&mu;SL), a three-dimensional (3D) manufacturing technology capable of rapidly converting computer generated 3D models into physical objects at high resolution12,13. Here we present a simple method to build up a simplified P&mu;SL system using a commercially available digital data projector to study swelling-induced buckling instability for controlled pattern transformation.
A simple desktop 3D printer is built using an off-the-shelf digital data projector and simple optical components such as a convex lens and a mirror14. Cross-sectional images extracted from a 3D solid model is projected on the photosensitive resin surface in sequence, polymerizing liquid resin into a desired 3D solid structure in a layer-by-layer fashion. Even with this simple configuration and easy process, arbitrary 3D objects can be readily fabricated with sub-100 &mu;m resolution.
This desktop 3D printer holds potential in the study of soft material mechanics by offering a great opportunity to explore various 3D geometries. We use this system to fabricate tubular shaped hydrogel structure with different dimensions. Fixed on the bottom to the substrate, the tubular gel develops inhomogeneous stress during swelling, which gives rise to buckling instability. Various wavy patterns appear along the circumference of the tube when the gel structures undergo buckling. Experiment shows that circumferential buckling of desired mode can be created in a controlled manner. Pattern transformation of three-dimensionally structured tubular gels has significant implication not only in mechanics and material science, but also in many other emerging fields such as tunable matamaterials.

We demonstrate controlled pattern transformation of swelling gel tubes by elastic instability. A simple projection micro stereo-lithography setup is built using an off-the-shelf digital data projector to fabricate three-dimensional polymeric structures in a layer-by-layer fashion. Swelling hydrogel tubes under mechanical constraint display various circumferential buckling modes depending on dimension.

Buckling is a classical topic in mechanics. While buckling has long been studied as one of the major structural failure modes1, it has recently drawn new ...

SSVEP-based Experimental Procedure for Brain-Robot Interaction with Humanoid Robots

Brain-Robot Interaction (BRI), which provides an innovative communication pathway between human and a robotic device via brain signals, is prospective in helping the disabled in their daily lives. The overall goal of our method is to establish an SSVEP-based experimental procedure by integrating multiple software programs, such as OpenViBE, Choregraph, and Central software as well as user developed programs written in C++ and MATLAB, to enable the study of brain-robot interaction with humanoid robots.
This is achieved by first placing EEG electrodes on a human subject to measure the brain responses through an EEG data acquisition system. A user interface is used to elicit SSVEP responses and to display video feedback in the closed-loop control experiments. The second step is to record the EEG signals of first-time subjects, to analyze their SSVEP features offline, and to train the classifier for each subject. Next, the Online Signal Processor and the Robot Controller are configured for the online control of a humanoid robot. As the final step, the subject completes three specific closed-loop control experiments within different environments to evaluate the brain-robot interaction performance.
The advantage of this approach is its reliability and flexibility because it is developed by integrating multiple software programs. The results show that using this approach, the subject is capable of interacting with the humanoid robot via brain signals. This allows the mind-controlled humanoid robot to perform typical tasks that are popular in robotic research and are helpful in assisting the disabled.

The overall goal of this method is to establish an SSVEP-based experimental procedure by integrating multiple software programs to enable the study of brain-robot interaction with humanoid robots, which is prospective in assisting the sick and elderly as well as performing unsanitary or dangerous jobs.

Brain-Robot Interaction (BRI), which provides an innovative communication pathway between human and a robotic device via brain signals, is prospective in ...

3D Printing of Biomolecular Models for Research and Pedagogy

The construction of physical three-dimensional (3D) models of biomolecules can uniquely contribute to the study of the structure-function relationship. 3D structures are most often perceived using the two-dimensional and exclusively visual medium of the computer screen. Converting digital 3D molecular data into real objects enables information to be perceived through an expanded range of human senses, including direct stereoscopic vision, touch, and interaction. Such tangible models facilitate new insights, enable hypothesis testing, and serve as psychological or sensory anchors for conceptual information about the functions of biomolecules. Recent advances in consumer 3D printing technology enable, for the first time, the cost-effective fabrication of high-quality and scientifically accurate models of biomolecules in a variety of molecular representations. However, the optimization of the virtual model and its printing parameters is difficult and time consuming without detailed guidance. Here, we provide a guide on the digital design and physical fabrication of biomolecule models for research and pedagogy using open source or low-cost software and low-cost 3D printers that use fused filament fabrication technology.

Physical models of biomolecules can facilitate an understanding of their structure-function for the researcher, aid in communication between researchers, and serve as an educational tool in pedagogical endeavors. Here, we provide detailed guidance for the 3D printing of accurate models of biomolecules using fused filament fabrication desktop 3D printers.

The construction of physical three-dimensional (3D) models of biomolecules can uniquely contribute to the study of the structure-function relationship. 3D ...

Fabrication of Soft Pneumatic Network Actuators with Oblique Chambers

Soft pneumatic network actuators have become one of the most promising actuation devices in soft robotics which benefits from their large bending deformation and low input. However, their monotonous bending motion form in two-dimensional (2-D) space keeps them away from wide applications. This paper presents a detailed fabrication method of soft pneumatic network actuators with oblique chambers, to explore their motions in three-dimensional (3-D) space. The design of oblique chambers enables actuators with tunable coupled bending and twisting capabilities, which gives them the possibility to move dexterously in flexible manipulators, to become biologically inspired robots and medical devices. The fabrication process is based on the molding method, including the silicone elastomer preparation, chamber and base parts fabrication, actuator assembly, tubing connections, checks for leaks, and actuator repair. The fabrication method guarantees the rapid manufacturing of a series of actuators with only a few modifications in the molds. The test results show the high quality of the actuators and their prominent bending and twisting capabilities. Experiments of the gripper demonstrate the advantages of the development in adapting to objects with different diameters and providing sufficient friction.

Here we present a fabrication method of soft pneumatic network actuators with oblique chambers. The actuators are capable of generating coupled bending and twisting motions, which broadens their application in soft robotics.

Soft pneumatic network actuators have become one of the most promising actuation devices in soft robotics which benefits from their large bending ...

Scalable Stamp Printing and Fabrication of Hemiwicking Surfaces

Hemiwicking is a process where a fluid wets a patterned surface beyond its normal wetting length due to a combination of capillary action and imbibition. This wetting phenomenon is important in many technical fields ranging from physiology to aerospace engineering. Currently, several different techniques exist for fabricating hemiwicking structures. These conventional methods, however, are often time consuming and are difficult to scale-up for large areas or are difficult to customize for specific, nonhomogeneous patterning geometries. The presented protocol provides researchers with a simple, scalable, and cost-effective method for fabricating micro-patterned hemiwicking surfaces. The method fabricates wicking structures through the use of stamp printing, polydimethylsiloxane (PDMS) molding, and thin-film surface coatings. The protocol is demonstrated for hemiwicking with ethanol on PDMS micropillar arrays coated with a 70 nm thick aluminum thin-film.

A simple protocol is provided for the fabrication of hemiwicking structures of varying sizes, shapes, and materials. The protocol uses a combination of physical stamping, PDMS molding, and thin-film surface modifications via common materials deposition techniques.

Hemiwicking is a process where a fluid wets a patterned surface beyond its normal wetting length due to a combination of capillary action and imbibition. ...

Hybrid Printing for the Fabrication of Smart Sensors

A method to combine additively manufactured substrates or foils and multilayer inkjet printing for the fabrication of sensor devices is presented. First, three substrates (acrylate, ceramics, and copper) are prepared. To determine the resulting material properties of these substrates, profilometer, contact angle, scanning electron microscope (SEM), and focused ion beam (FIB) measurements are done. The achievable printing resolution and suitable drop volume for each substrate are, then, found through the drop size tests. Then, layers of insulating and conductive ink are inkjet printed alternately to fabricate the target sensor structures. After each printing step, the respective layers are individually treated by photonic curing. The parameters used for the curing of each layer are adapted depending on the printed ink, as well as on the surface properties of the respective substrate. To confirm the resulting conductivity and to determine the quality of the printed surface, four-point probe and profilometer measurements are done. Finally, a measurement set-up and results achieved by such an all-printed sensor system are shown to demonstrate the achievable quality.

Here we present a protocol for the fabrication of inkjet-printed multilayer sensor structures on additively manufactured substrates and foil.

A method to combine additively manufactured substrates or foils and multilayer inkjet printing for the fabrication of sensor devices is presented. First, ...

Studying Large Amplitude Oscillatory Shear Response of Soft Materials

We investigate the sequence of physical processes exhibited during large amplitude oscillatory shearing (LAOS) of polyethylene oxide (PEO) in dimethyl sulfoxide (DMSO) and xanthan gum in water &#8212; two concentrated polymer solutions used as viscosifiers in foods, enhanced oil recovery, and soil remediation. Understanding the nonlinear rheological behavior of soft materials is important in the design and controlled manufacturing of many consumer products. It is shown how the response to LAOS of these polymer solutions can be interpreted in terms of a clear transition from linear viscoelasticity to viscoplastic deformation and back again during a period. The LAOS results are analyzed via the fully quantitative Sequence of Physical Processes (SPP) technique, using free MATLAB-based software. A detailed protocol of performing a LAOS measurement with a commercial rheometer, analyzing nonlinear stress responses with the freeware, and interpreting physical processes under LAOS is presented. It is further shown that, within the SPP framework, a LAOS response contains information regarding the linear viscoelasticity, the transient flow curves, and the critical strain responsible for the onset of nonlinearity.

We present a detailed protocol outlining how to perform nonlinear oscillatory shear rheology on soft materials, and how to run the SPP-LAOS analysis to understand the responses as a sequence of physical processes.

We investigate the sequence of physical processes exhibited during large amplitude oscillatory shearing (LAOS) of polyethylene oxide (PEO) in dimethyl ...

Multimodal 3D Printing of Phantoms to Simulate Biological Tissue

Biomedical optical imaging is playing an important role in diagnosis and treatment of various diseases. However, the accuracy and the reproducibility of an optical imaging device are greatly affected by the performance characteristics of its components, the test environment, and the operations. Therefore, it is necessary to calibrate these devices by traceable phantom standards. However, most of the currently available phantoms are homogeneous phantoms that cannot simulate multimodal and dynamic characteristics of biological tissue. Here, we show the fabrication of heterogeneous tissue-simulating phantoms using a production line integrating a spin coating module, a polyjet module, a fused deposition modeling (FDM) module, and an automatic control framework. The structural information and the optical parameters of a &#34;digital optical phantom&#34; are defined in a prototype file, imported to the production line, and fabricated layer-by-layer with sequential switch between different printing modalities. Technical capability of such a production line is exemplified by the automatic printing of skin-simulating phantoms that comprise the epidermis, dermis, subcutaneous tissue, and an embedded tumor.

Spin coating, polyjet printing, and fused deposition modeling are integrated to produce multilayered heterogeneous phantoms that simulate structural and functional properties of biological tissue.

Biomedical optical imaging is playing an important role in diagnosis and treatment of various diseases. However, the accuracy and the reproducibility of ...

Rapid Manufacturing of Thin Soft Pneumatic Actuators and Robots

This protocol describes a method for rapid manufacturing of soft pneumatic actuators and robots with an ultrathin form factor using a heat press and a laser cutter machine. The method starts with the lamination of thermoplastic polyurethane (TPU) sheets using a heat press for 10 min at the temperature of ~93 &#176;C. Next, the parameters of the laser cutter machine are optimized to produce a rectangular balloon with maximum burst pressure. Using the optimized parameters, the soft actuators are laser cut/welded three times sequentially. Next, a dispensing needle is attached to the actuator, allowing it to be inflated. The effect of geometrical parameters on the deflection of the actuator are studied systematically by varying the channel width and length. Finally, the performance of the actuator is characterized using an optical camera and a fluid dispenser. Conventional fabrication methods of soft pneumatic actuators based on silicone molding are time consuming (several hours). They also result in strong but bulky actuators, which limits the actuator&#8217;s applications. Moreover, microfabrication of thin pneumatic actuators is both time-consuming and expensive. The proposed manufacturing method in the current work resolves these issues by introducing a fast, simple, and cost-effective fabrication method of ultrathin pneumatic actuators.

This protocol describes a method for rapid manufacturing of soft pneumatic actuators and robots with a thin form factor. The fabrication method starts with lamination of thermoplastic polyurethane (TPU) sheets followed by laser cutting/welding of a two-dimensional pattern to form actuators and robots.

This protocol describes a method for rapid manufacturing of soft pneumatic actuators and robots with an ultrathin form factor using a heat press and a ...

Fabrication of Carbon-Based Ionic Electromechanically Active Soft Actuators

Ionic electromechanically active capacitive laminates are a type of smart material that move in response to electrical stimulation. Due to the soft, compliant and biomimetic nature of this deformation, actuators made of the laminate have received increasing interest in soft robotics and (bio)medical applications. However, methods to easily fabricate the active material in large (even industrial) quantities and with a high batch-to-batch and within-batch repeatability are needed to transfer the knowledge from laboratory to industry. This protocol describes a simple, industrially scalable and reproducible method for the fabrication of ionic carbon-based electromechanically active capacitive laminates and the preparation of actuators made thereof. The inclusion of a passive and chemically inert (insoluble) middle layer (e.g., a textile-reinforced polymer network or microporous Teflon) distinguishes the method from others. The protocol is divided into five steps: membrane preparation, electrode preparation, current collector attachment, cutting and shaping, and actuation. Following the protocol results in an active material that can, for example, compliantly grasp and hold a randomly shaped object as demonstrated in the article.

This article describes a fast and simple manufacturing process of ionic electromechanically active composite materials for actuators in biomedical, biomimetic and soft robotics applications. The key fabrication steps, their importance for the actuators&#39; final properties, and some of the main characterization techniques are described in detail.

Ionic electromechanically active capacitive laminates are a type of smart material that move in response to electrical stimulation. Due to the soft, ...