Mechanical characterizations of APTF hydrogels formed by varying Alg-NHS/PEGDA ratios. (a) Mechanical properties of APTF hydrogels with fixed total polymer concentration (22%) and varied treatment methods (40% TA/0%, 3%, 6% Fe3+). (i) Young's modulus, (ii) ultimate strength and (iii) toughness and (iv) ultimate strain of hydrogels treated with TA/Fe3+. Young's modulus, strain, and ultimate strength of APTF hydrogels formed with varied (b) Alg-NHS concentrations and (c) PEGDA concentrations. (d) Toughness of APTF hydrogels with varied Alg-NHS concentrations. (e) Representative stress-strain curve of the optimized engineered APTF hydrogel. (f) Stretching of the APTF hydrogel. (g) Mechanical parameters of optimized APTF hydrogel.