The photo-cross-linkable ink formulation comprised 30% (w/v) polyethylene glycol diacrylate (PEGDA, Mn = 700, Sigma), 70% deionized (DI) water, and 1% (w/v) photoinitiator Irgacure-819 (Ciba). It was mixed thoroughly by sonification for an hour before DLP printing.
The 4D SMP ink was formulated using bisphenol A diglycidyl ether (monomer), poly(propylene glycol) bis(2-aminopropyl) ether (cross-linker), and decylamine (cross-linking modulator), which were all purchased from Sigma. Graphene nanoplatelets (6–8 nm thick × 5 μm wide) were obtained from Strem Chemicals Inc. The material was added into an open-mouthed container and mixed manually at the weight ratio of 3.4:0.69:0.66 corresponding to monomer:cross-linker:cross-linking modulator (to obtain a transformation temperature of above 37 °C).44 The ink was stored at −80 °C and was placed in a 75 °C oven for 5 min before use to both defrost the solution and lower its viscosity. For the fabrication of 4D material, the ink was extruded at room temperature and then defoamed in a vacuum chamber for 2 h. After this, the printed 4D material was cured in an oven at 100 °C for at least 1 h.
The 4D SMP ink was formulated using bisphenol A diglycidyl ether (monomer), poly(propylene glycol) bis(2-aminopropyl) ether (cross-linker), and decylamine (cross-linking modulator), which were all purchased from Sigma. Graphene nanoplatelets (6–8 nm thick × 5 μm wide) were obtained from Strem Chemicals Inc. The material was added into an open-mouthed container and mixed manually at the weight ratio of 3.4:0.69:0.66 corresponding to monomer:cross-linker:cross-linking modulator (to obtain a transformation temperature of above 37 °C).44 The ink was stored at −80 °C and was placed in a 75 °C oven for 5 min before use to both defrost the solution and lower its viscosity. For the fabrication of 4D material, the ink was extruded at room temperature and then defoamed in a vacuum chamber for 2 h. After this, the printed 4D material was cured in an oven at 100 °C for at least 1 h.