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

Summary

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.

Abstract

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).

Introduction

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 light^1,2,3,4. Among the many stimuli-responsive soft robots, N-isopropylacrylamide (NIPAM) hydrogel-based soft robots perform the desired tasks or interactions using spontaneous shape transformation^5,6,7,8. 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 °C and 36 °C^9,10. 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 sensors^{2,3,4,11,12,13,14,15,16,17,18,19,20,21}.

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 printing^23,24. Recently, 4D printing has been extensively highlighted as an innovative technique for creating shape-programmable soft robots^25,26,27,28. 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. 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 fields^25,26,27,28. 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).

Protocol

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 methods^29,30,31. The data presented in this study are available on request from the corresponding author.

1. Preparation of hydrogel inks

1. Non-stimuli-responsive AAm-based hydrogel inks (Figure 1A)
   1. Dilute the acrylamide (AAm), the crosslinker N, N'-methylenebisacrylamide (BIS) (see Table of Materials), and the photoinitiator 2-Hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (see Table of Materials) in distilled (DI) water using a magnetic stirrer for 24 h.
   2. 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.
   3. 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.
   4. After total dilution, transfer the AAm-based hydrogel ink into an empty 3D printing cartridge (see Table of Materials) using a syringe.
2. Stimuli-responsive NIPAM-based hydrogel inks (Figure 1B)
   1. 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.
   2. 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.
   3. 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.
   4. After complete dilution, transfer the NIPAM-based hydrogel ink into an empty 3D printing cartridge using a syringe.
3. Ferrogel inks (Figure 1C)
   1. Prepare the A-solution: Dilute acrylamide (AAm) and crosslinker, N, N'-methylenebisacrylamide (BIS), ferric oxide (Fe₂O₃), and N, N, N', N'-tetramethylethylenediamine (TMEDA) (see Table of Materials) in DI water.
   2. 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 µL of TMEDA accelerator.
   3. Prepare the B-solution: Dilute 0.8 g of ammonium persulphate (APS, see Table of Materials) in 10 mL of DI water.
   4. For polymerization, transfer 200 µL of the A-solution and 5 µL of the B-solution into a microcentrifuge tube.
   5. Vortex the microcentrifuge tube for 20 s.

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).

1. Two-dimensional AAm-based hydrogel layer design
   1. Draw an elliptical shape with a vertical axis of 24 mm and a horizontal axis of 20 mm at the outermost part.
   2. 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.
   3. Draw a three-point arc passing through the points (−8.24, 2), (0, 6), and (8.24, 2) away from the center point of the ellipse.
   4. Trim the small upper part of the eclipse divided by the arc.
2. Two-dimensional NIPAM-based hydrogel layer design
   1. 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.
   2. 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.
   3. Draw a three-point arc passing through the points (−7.86, 1.83), (0, 5.6), and (7.86, 1.83) away from the center point of the ellipse.
   4. Draw a three-point arc passing through the points (−5.47, 1.64), (0, 3.18), and (5.47, 1.64) away from the center point of the ellipse.
   5. Trimthe small upper part of the ellipses divided by the arcs.
   6. To make a pedestal, draw an arc with two points away from the center point at (−4.75, −2.71) and (4.75, −2.71) as both endpoints and one point away from the center point at (0, -4.59).
3. Two-dimensional ferrogel layer design
   1. Draw a three-point arc passing through the points (−7, 4.92), (0, 9.2), and (7, 4.92) away from the center point of the ellipse.
   2. Draw a three-point arc passing through the points (−7, 4.92), (0, 7.6), and (7, 4.92) away from the center point of the ellipse.
4. Two-dimensional gripper tips design
   1. 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.
5. Three-dimensional hybrid gripper design
   1. 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.

3. Three-dimensional printing of the soft hybrid gripper

1. 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⁻¹ printing speed, and an infill density of 75%. Edit the G-code file using dual print heads.
2. 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.
3. Connect an air pump pressure control to the 3D printer.
4. Choose nozzle tips with diameters of 0.25 mm and 0.41 mm for the NIPAM-based hydrogel and AAm-based hydrogel, respectively.
5. Connect the AAm-based hydrogel cartridge to nozzle 1 and the NIPAM-based hydrogel cartridge to nozzle 2.
6. Check if the two print heads of the cartridges are at the same position on the z-axis.
7. Calibrate the X and Y coordinates precisely to avoid misalignments between the two nozzles.
8. Set the printing pressure at 20-25 KPa for the AAm-based hydrogel and at 10-15 KPa for the NIPAM-based hydrogel.
9. Repeat steps 3.5-3.8 when each sample is completely printed (Figure 2A).

4. UV photocuring of the soft hybrid gripper

1. 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.
2. 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.
3. 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).

Results

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 properties^9,10. 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...

Discussion

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...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
2-Hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone	Sigma Aldrich	410896-50G	Irgacure 2959, photoinitiator
3D WOX 2X	sindoh	n/a	3D printer for fabricating a maze
Acrylamide	Sigma-Aldrich	29-007	≥99%
Airbrush compressor	WilTec	AF18-2
Ammonium persulfate	Sigma Aldrich	A4418
Auto CAD	Autodesk	n/a	software for computer-aided-design file
BLX UV crosslinker	BIO-LINK	U01-133-565
Cartridge	CELLINK	CSC010300102
Digital stirring Hot Plates	Corning	6798-420D
Fluorescein O-methacrylate	Sigma Aldrich	568864	dye of AAm gel
INKREDIBLE+ bioprinter	CELLINK	n/a
Iron(III) Oxide red	DUKSAN general science	I0231
Laponite RD	BYK	n/a	nanoclay
Microcentrifuge tube	SPL	60615
Micro stirrer bar	Cowie	27-00360-08
N, N, N', N'-tetramethylethylenediamine	Sigma Aldrich	T7024-100ML
N, N'-methylenebisacrylamide	Sigma Aldrich	M7279	≥99.5%
N-isopropylacrylamide	Sigma-Aldrich	415324-50G
Poly(N-isopropylacrylamide)	Sigma-Aldrich	535311
Rhodamine 6G	Sigma Aldrich	R4127	dye of NIPAM gel
Slic3r software (v1.2.9)	Slic3r	n/a	open-source software to convert .stl file to gcode
Sodium hydroxide beads	Sigma Aldrich	S5881
Sterile high-precision conical bioprinting nozzles	CELLINK	NZ3270005001	22 G, 25 G
Syringe	Korea vaccine	K07415389	10 CC 21 G (1-1/4 INCH)
Vortex mixer	DAIHAN	DH.WVM00030