Using a facile two-step method, the researchers synthesized a pH-responsive double-network (DN) hydrogel with high strength and toughness, consisting of a B(OH)(4)(-) linked hydroxypropyl guar gum (HPG) network and a covalently cross-linked polyacrylamide (PAAm) network. The mechanical strength of HPG/PAAm DN gel was investigated as a function of synthesis conditions, HPG content, sodium borate concentration and soaking time. The resulting DN gel exhibited unique pH-responsive properties and excellent mechanical properties, with a fracture stress of 843 kPa, and an excellent extensibility of 1400% (12-14 times its initial length). The relationship between mechanical property and the synergy of HPG and PAAm networks was discussed systematically. The DN gel possessed self-recovery under small deformation, but displayed self-reinforcement under large deformation. (C) 2016 Elsevier B.V. All rights reserved.