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

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.