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

Yeonjae Lee; Jiho Choi; Youngeun Choi; Sue Min Park; ChangKyu Yoon

doi:10.3791/64870

Engineering

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

Published: January 13, 2023 doi: 10.3791/64870

Yeonjae Lee¹, Jiho Choi¹, Youngeun Choi², Sue Min Park¹, ChangKyu Yoon^1,3

¹Department of Mechanical Systems Engineering, Sookmyung Women's University, ²Department of Biological Sciences, Sookmyung Women's University, ³Institute of Advanced Materials and Systems, Sookmyung Women's University

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 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 °C and 36 °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. 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).

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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 methods²⁹^,³⁰^,³¹. The data presented in this study are available on request from the corresponding author.

1. Preparation of hydrogel inks

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.
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.
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).

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.
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).
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.
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.
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

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

4. UV photocuring of the soft hybrid gripper

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.
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.
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).

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Representative 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 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, ferrogel was integrated into this hybrid system to create a magnetic field-responsive soft hybrid gripper for the untethered control of magnetic field-driven locomotion. In particular, the ferrogel ink injection must be conducted before polymerization to avoid separation from the NIPAM-based hydrogel structure.

The actuation of thermally responsive opening and closing was primarily considered to determine the optimal geometry of the hybrid gripper. Initially, the swelling and deswelling of the NIPAM-based and AAm-based hydrogels were assessed by measuring the diameter changes from room temperature to 60 °C. Based on this verification of the swelling power, the AAm-based hydrogel was placed in the outer part of the structural layer, and the NIPAM-based hydrogel was placed inside the responsive layer. This work verified the gripping function of several different structures of the hybrid gripper, such as circular and elliptical geometries. Specifically, an overall elliptical shape with a flat NIPAM-based plate inside was chosen to increase the swelling-deswelling power to allow the device to grip well and to hold targets safely without dropping them during pick-and-place tasks. In addition, a symmetric crescent-shaped ferrogel area was designed on top of the elliptical structure to integrate the precise magnetic-responsive locomotion of the hybrid gripper.

The hybrid gripper was fabricated using a path-oriented additive 3D-printing method (Figure 3). First, the AAm-based hydrogel was printed on the exterior of the gripper as a structure-supporting layer (Figure 3A), and then the NIPAM-based hydrogel was printed in the interior as a stimuli-responsive layer (Figure 3B). Subsequently, ferrogel was injected into the well at the top of the hybrid gripper (Figure 3C). For the first step of the dual 3D-printing and injection processes, the synthesized AAm-based and NIPAM-based hydrogels were carefully transferred to an empty 3D cartridge so as not to let air inside. The injection of the ferrogel to precisely connect with the AAm-based structural hydrogel layer had to be carefully conducted to avoid bubbles.

A variety of printing parameters, such as the printing pressure, speed, nozzle diameter, and ink composition, were verified to determine the optimal 3D-printing conditions. We observed that the viscoelastic properties of the inks were the most important parameters for obtaining precise printing and UV-curing processes. The viscoelastic properties are mainly determined by the weight ratio of the sheer thinning agent (e.g., laponite RD). To identify the appropriate rheological features of the ink solutions, it is essential to adjust the shear-thinning agent for precise printing and quick solidification after printing and before the UV-curing process. In addition, the AAm-based and NIPAM-based hydrogel layers had to be precisely connected without overlap or gaps between them during the 3D-printing process. A small misalignment in the X-Y directions and an offset in the Z direction during the dual 3D printing process can result in significant errors in the final structure. If any misalignment is observed, the preset positioning of the X and Y directions with an offset in the Z direction in the G-code must be aligned again at every printing step until the dual print heads are perfectly aligned. To achieve precisely aligned gripper structures with no errors, small cube-shaped calibration markers were inserted at the four corners to preserve the center of each structure.

The soft hybrid gripper performed a pick-and-place task via thermally responsive actuation and magnetic locomotion. Initially, thermally responsive actuation of the soft hybrid gripper was observed. When the temperature increased above the lower critical solution temperature (LCST), the NIPAM-based gel shrank, and the gripper tip closed, owing to the deswelling of the NIPAM-based hydrogel. In contrast, the gripper tip of the soft hybrid gripper opened when the temperature decreased below the LCST, owing to the swelling of the NIPAM-based hydrogel (Figure 4A). In addition, we verified that the incorporation of ferrogel did not affect the folding of the soft hybrid gripper during temperature changes.

A simple maze using a 3D printer was fabricated, filled with DI water, and placed on a hot plate. The fully swollen soft hybrid gripper was then placed at the starting position of the maze in a tip-open state, and salmon roe was placed in the target area. The soft hybrid gripper was guided using an external magnet until it reached the salmon roe. Then, the tip of the soft hybrid gripper closed to grip the salmon roe when the temperature reached 40 °C. Finally, the soft hybrid gripper was moved out of the maze while holding the salmon roe, and it then released the salmon roe at the target area in a tip-open state at a room temperature of 25 °C (Figure 4B). The salmon roe maintained its shape without any damage during the entire pick-and-place task. In addition, neodymium magnets were used to guide the soft hybrid gripper during the magnetic-responsive locomotion.

Figure 1: Hydrogel preparation and the soft hybrid gripper design. (A) AAm-based hydrogel. (B) NIPAM-based hydrogel. (C) Ferrogel inks. (D) The soft hybrid gripper design made using AutoCAD and Slic3r software. Please click here to view a larger version of this figure.

Figure 2: Fabrication process for the 3D printing of the soft hybrid gripper. (A) Dual printing modes with AAm-based hydrogel and NIPAM-based hydrogel inks. (B) Ferrogel layer. (C) UV photocuring. (D) Equilibrium state in DI water. Please click here to view a larger version of this figure.

Figure 3. Fabrication of the soft hybrid gripper. (A) Exterior non-stimuli-responsive Aam-based hydrogel layer. (B) Interior stimuli-responsive NIPAM-based hydrogel layer. (C) Ferrogel layer. Please click here to view a larger version of this figure.

Figure 4. Actuation and locomotion of the soft hybrid gripper. (A) Thermally responsive actuation of the soft hybrid gripper. (B) Demonstration of pick-and-place tasks with the soft hybrid gripper. Please click here to view a larger version of this figure.

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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 with hydrogels with different swelling properties, such as AAm-based hydrogels¹. In addition, hydrogels can be designed to be magnetically responsive by embedding iron oxide (Fe₂O₃) nanoparticles. Importantly, this Fe₂O₃-incorporated acrylamide-based ferrogel can play an important role in enabling magnetic responsiveness to facilitate soft robot magnetic field-driven locomotion. In particular, magnetically responsive hydrogels have been proposed to be used in untethered hydrogel-based soft robotic systems, which would provide less invasive approaches in dynamically cluttered environments³².

Importantly, the soft hybrid gripper required good adhesion among the three hydrogels. When the adhesion is poor, the interface between the hydrogels will be delaminated during repeated swelling and deswelling in response to external triggers. In particular, acrylamide-based hydrogels were introduced to ensure good adhesion under the repeated thermally and magnetically responsive manipulation and locomotion of the soft hybrid gripper. In addition, the swelling and deswelling of thermally responsive NIPAM-based and non-stimuli-responsive AAm-based hydrogels were verified to anticipate the degree of bending of the soft hybrid gripper. It should be noted that a simulation model based on the thermodynamics framework with hydrogel swelling (e.g., the Flory-Huggins model) and mechanics (e.g., the Neo-Hookean model) can aid in determining the extent of bending as a function of the swelling and temperature⁸. Based on these experimental and theoretical characterizations of the gripper folding, a thermally responsive NIPAM-based hydrogel layer was chosen for the inner part, and a non-stimuli-responsive AAm-based hydrogel layer was chosen for the outer part to allow for the bending of the gripping tips into the center with increasing temperatures.

In terms of the fabrication of the soft hybrid gripper, our four-dimensional (4D) time-dependent printing process can be used to create diverse stimuli-responsive soft robots with a wide size range from millimeters to centimeters. Recently, the combination of 4D printing and stimuli-responsive smart materials has provided a new route for developing intelligent 3D structures that are shape-transformable when exposed to an appropriate stimulus source. Along with the 4D-printing technique using a programmable stimuli-responsive hydrogel, diverse 3D-printing paths of stimuli-responsive materials can present different final swollen geometries that display varying curved, rolled, folded, or helical structures²⁶. The development of this innovative 4D-printing strategy has attracted significant attention owing to its significant scalability and manufacturability for creating intelligent stimuli-responsive soft robots.

However, the 4D printing of diverse hydrogels requires several challenges to be overcome. First, the response time for the shape-changeable actuation of 4D-printed hydrogels is rather slow. Further fine-tuning of the hydrogel composition integrated with functional materials (e.g., nanoparticles, low-dimensional materials, liquid crystals, and even biological DNAs) is needed to improve the response time. In addition, the positioning calibration of the Z direction and the alignment of the X-Y directions have to be double-checked at every step during the dual printing process. To obtain a continuous printing process without any misalignment, the preset values in the X, Y, and Z directions in the G-code files have to be double-checked and repeated multiple times until the print heads are perfectly aligned.

From an application perspective, this paper introduces thermally and magnetically responsive soft hybrid grippers that actively perform pick-and-place tasks. The sequential process of safely gripping and securely holding an object is critical in soft robotics. The stimuli-responsive soft gripper has shown the possibility for developing an intelligent manipulating system that can grip and release objects precisely in a less invasive or non-invasive manner according to the external stimuli on-off process³². More recently, to achieve the automated locomotion of a soft gripper for accurate pick-and-place tasks, ultrasound image feedback-coupled gradient magnetic field systems have been developed in parallel³³. Although still at the conceptual level, we expect that this specific protocol for the 4D printing of a soft stimuli-responsive hybrid gripper will provide a basis for further significant advances in the development of precisely controllable, highly sensitive, and multifunctional smart stimuli-responsive soft robots.

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Disclosures

The authors declare no conflicts of interest.

Acknowledgments

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

Materials

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

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