4D printing of pH-responsive bilayer with programmable shape-shifting behaviour

4D printing of smart materials has seen remarkable advancements in the domain of biomedical devices, with a particular focus on developing responsive and adaptive programmable structures. In this work, we report the 4D printing of two solvent-responsive hydrogels forming a bilayer that undergoes bidirectional actuation depending on the pH of the solvent. A strong interlayer adhesion between the hydrogels is formed without subjecting either of their surfaces to any chemical modification. These hydrogels have an interfacial toughness of 71.8 J/m² and undergoes no delamination during actuation inside a solvent. Conventionally, pH-responsive actuators are only limited to simple 2D films prepared by solvent casting. However, our work focuses on the design and fabrication of complex bilayer and patterned structures using Direct-Ink Writing (DIW) approach. These printed structures actuate upon immersion in a solvent medium, and the actuation is reversible in nature. The influence of programmable variables on the morphed structure was studied systematically by modifying the rheological properties (in the range of $10^2$–$10^5$ Pa s) and printing parameters (optimized at a printing speed of 5 mm/sec, with the extrusion pressure of 5–6 bar and nozzle diameter of 0.5 mm) of the 3D printed bilayer structure. The physicochemical properties of the printable hydrogel ink were tuned such that the same structure can respond to both acidic and basic pH (with non-morphing point at pH 7), by altering the directionality of actuation. We have demonstrated the applications of these pH-responsive actuators in smart valves.