4D Printing of Multi-Hydrogels Using Direct Ink Writing in a Supporting Viscous Liquid

Abstract

We propose a method to print four-dimensional (4D) stimuli-responsive hydrogel structures with internal gaps. Our 4D structures are fabricated by printing an N-isopropylacrylamide-based stimuli-responsive pre-gel solution (NIPAM-based ink) and an acrylamide-based non-responsive pre-gel solution (AAM-based ink) in a supporting viscous liquid (carboxymethyl cellulose solution) and by polymerizing the printed structures using ultraviolet (UV) light irradiation. First, the printed ink position and width were investigated by varying various parameters. The position of the printed ink changed according to physical characteristics of the ink and supporting liquid and printing conditions including the flow rates of the ink and the nozzle diameter, position, and speed. The width of the printed ink was mainly influenced by the ink flow rate and the nozzle speed. Next, we confirmed the polymerization of the printed ink in the supporting viscous liquid, as well as its responsivity to thermal stimulation. The degree of polymerization became smaller, as the interval time was longer after printing. The polymerized ink shrunk or swelled repeatedly according to thermal stimulation. In addition, printing multi-hydrogels was demonstrated by using a nozzle attached to a Y shape connector, and the responsivity of the multi-hydrogels to thermal-stimulation was investigated. The pattern of the multi-hydrogels structure and its responsivity to thermal-stimulation were controlled by the flow ratio of the inks. Finally, various 4D structures including a rounded pattern, a spiral shape pattern, a cross point, and a multi-hydrogel pattern were fabricated, and their deformations in response to the stimuli were demonstrated.

Keywords: 3D printing; 4D printing; stimuli-responsive hydrogel.

Figure 1

Concept of our proposed method for fabricating 4D structure with internal gaps. (a) Pre-gel monomer ink is ejected into supporting viscous liquid to print 3D ink pattern. (b) The printed ink is exposed to UV and is polymerized to a obtain 3D hydrogel structure. (c) After replaced the supporting liquid into water, the polymerized 3D hydrogel structure deforms in response to stimulation.

Figure 2

Schematic of setup. A nozzle is fixed on a z stage, and a container of carboxymethyl cellulose aq (CMC aq) is fixed on xy stages. By moving the xyz stages, the pre-gel ink is ejected via the nozzle into CMC aq by a syringe pump.

Figure 3

Measurement of the position of the printed ink in the supporting viscous liquid. (a) Schematic of z_top and z_bottom for the printed ink and parameters. (b) Table of the standard conditions. (c) Images of the printed ink with various concentrations of CMC aq. The z_top and z_bottom increased when the concentration of CMC aq increased. (d) The z_top and z_bottom for the printed ink under various conditions. The dotted line shows the position of the tip of the nozzle.

Figure 4

Measurement of the width of the printed ink in the supporting viscous liquid. (a) w_z is defined as the z-axis width of the printed ink at 10 mm away from the nozzle. (b) w_y is defined as the y-axis width of printed ink at 10 mm away from the nozzle. (c) Images of the printed ink with various stage speeds. w_z and w_y decreased as the stage speed increased. (d) w_z and w_y under various conditions. w_z and w_y can be mainly controlled by the flow rate and stage speed.

Figure 5

Evaluation of printed ink pattern. (a) The error area is defined as the dragged area of the patterned ink at the corner. (b) Images of the printed corners with different angles θ. The patterned ink (v = 1.0 mm/s, Q = 1.0 µL/s) was distorted depending on the angle of the corner. (c) Relationship between the error area and θ with different Q and v values. The error area increased as the angle increased, reaching the maximum when the angle was 120° for all three nozzle speeds. For all angles, the slower the nozzle speed, the lower the error area.

Figure 6

Polymerization of the printed ink in CMC aq. (a) Gelation ratio is defined as the diameter of the polymerized ink after UV irradiation divided by the diameter of the printed ink pattern before UV irradiation. (b) Fluorescence images of the polymerized printed ink. (c) Gelation ratio for different interval times. The gelation ratio decreased as interval time increased.

Figure 7

Responsivity of the polymerized hydrogel to thermal stimuli. (a) Definition of the shrinking ratio d_n/d₀. Shrinking ratio is obtained by the diameter of the polymerized printed ink after being heated or cooled divided by the initial diameter of the polymerized printed ink before stimuli. (b) The repeatability of shrinking/swelling behavior of the polymerized printed ink. The polymerized printed ink shrunk or expanded according to heating or cooling.

Figure 8

Printing of multi-hydrogel structures. (a) A schematic of the setup to print multi-hydrogel inks. The N-isopropylacrylamide-based stimuli-responsive pre-gel solution (NIPAM)-based pre-gel ink and the acrylamide-based non-responsive pre-gel solution (AAM)-based pre-gel ink were ejected into CMC aq via the nozzle attached to a Y shape connector. (b) The definition of the patterned ratios P_N and P_A which is obtained by the width of poly-N-isopropylacrylamide (pNIPAM) gel or polyacrylamide (pAAM) gel divided by the total width of the polymerized inks, respectively. (c) Results for the patterned ratios P_N and P_A with different flow rates Q. The scale bars are 500 µm. The P_N increased and the P_A decreased when the flow rate of the NIPAM ink increased and that of the AAM ink decreased. (d) The definition of the curvature radius of a heated multi-hydrogels structure. (e) Results for the curvature radius of heated multi-hydrogels structures with different flow rates. The curvature increased when the flow rate of the NIPAM-based ink increased. (f) Images of the deformed multi-hydrogels structures.

Figure 9

A demonstration of 4D printing. (a) Schematic illustration and images of printed structures and heated structures with a rounded pattern. (b) Schematic illustration and images of a fabricated T-shaped structure and a heated T-shaped structure with a cross point. (c) Schematic illustration and images of a fabricated spring structure and heated spring structure with an internal gap. (d) Schematic illustrations of printing a C-shaped structure with multi-hydrogels (d.1) and its 3D deformation from the C-shaped structure to the spring-shaped structure. (d.2) Fluorescence images of the fabricated C-shaped structure (d.3) and the transformed spring structure obtained by heating. (d.4) Time-lapse images of the C-shaped structure.