Sodium polyacrylate is a superabsorbent polymer, capable of absorbing hundreds of times its own mass in water. This property, and the swelling behaviour during absorption, has seen application in diverse areas, including as anti-flood measures, water-retentive soil modifiers and recently soft actuators. However, this material is typically used in granular form or in simple cast shapes, which limits the scope of application. The ability to 3D print this material would widen its applicability, but previous demonstrations of sodium polyacrylate printing have shown a significantly reduced degree of swelling when compared to the typical material.

In this work, we demonstrate material extrusion of a sodium polyacrylate-based ink, with a total swelling factor of over 200 being retained in the printed material. The rheological properties of this ink are modified with Carbopol, an acrylic acid based polymer, and are determined to show that the ink is strongly shear-thinning (with flow behaviour index under 0.14) and indicating desirable response for this type of printing. We further demonstrate the printability through a range of empirical tests.

This new approach to sodium polyacrylate printing significantly widens the scope of application of this reactive hydrogel, while introducing the potential for application in novel soft actuators which harness the high degree of swelling and the shape changing/4D behaviour of the material as it swells.

References:

[1] DANIEL GOSDEN  Jonathan R  Matthew Studley. Material extrusion of sodium polyacrylate superabsorbent polymer[J]. Additive manufacturing, 2023, 78: Article 103886. DOI:10.1016/j.addma.2023.103886.