A Superabsorbent Sodium Polyacrylate Printing Resin as Actuator Material in 4D Printing

Superabsorbent polymers are materials that exhibit a high swelling behavior in liquids and can hold the absorbed liquid even against externally applied pressure. They are commercially used, for example, in baby diapers, fake snow, or swellable children's toys. Most commercially available superabsorbent polymers are based on polymerized and crosslinked sodium acrylate. Here, a material formulation to create 3D objects using stereolithographic printing of sodium acrylate is demonstrated. The material shows typical superabsorbent properties that cannot be reached with conventional 3D printing materials. The printed structures swell strongly (up to 20 times in weight) in aqueous environments and still show 65% of the swelling under an external load of 100 kPa. This swelling can be used for 3D printed parts that can automatically change their size or shape when exposed to water. To show the versatility of this approach, selected structures are 3D printed, including a ship and a medical stent. Also the applicability of actuation by printing a structure is demonstrated, which deforms to a self-closing container upon exposure to water.