a, High-concentration acrylamide (AAm) solution forms an entangled hydrogel in the absence of cross-linker, N, N′-methylenebisacrylamide (MBAA). b, A compression test on 40% (wt/vol) pAAm hydrogels with (upper panels) and without (lower panels) 0.5% (wt/vol) MBAA. The gels were prepared with 0.005% (wt/vol) VA-044 and nitrogen gas-bubbling. Gels with 9-mm thickness were compressed to 1 mm. λ, compression strain. c, A demonstration of stretching a 36% (wt/vol) entangled pAAm gel (no MBAA and 0.005% (wt/vol) VA-044 with nitrogen gas-bubbling). Representative images are shown from experiments repeated two times (a and b) and three times (c) with similar results.