Urea–Urease pH-Clock-Enabled Time-Lapse Thiol–Acrylate Adhesives: Two-Part Formulations and One-Pot Underwater Activation

This work presents time-lapse thiol-acrylate adhesive systems that use a urea-urease pH-clock reaction to enable programmable curing. Two complementary approaches are demonstrated: a two-part adhesive in which enzymatically generated ammonia provides a tunable induction period prior to rapid gelation, and a one-pot, water-activated formulation that could be used for underwater curing. In the two-part system, mixing a thiol component containing watermelon seed powder (WMSP) with an acrylate component containing an aqueous urea solution initiated a gradual pH increase that delayed the thiol-Michael polymerization, allowing programmable working time. This induction time is unlike other two-part systems that begin to react immediately upon mixing. The cured adhesive exhibited a lap shear strength of 7 MPa on polycarbonate. The one-pot formulation, stabilized with monomethyl ether hydroquinone (MEHQ) and phenylphosphonic acid, remained shelf-stable in the absence of water but cured rapidly upon wetting as the water dissolved the urea, which then was converted to ammonia by the WMSP, achieving lap shear strengths of ∼5 MPa. Together, these systems demonstrate the effectiveness of WMSP-derived enzymatic catalysis for achieving temporal control in thiol-acrylate adhesives.