Fig. 1. BA hydrogel. (A) Illustration of BA hydrogels formation; (B) phase diagram of BA–Na₂B₄O₇ mixture (the BA, NaOH and inorganic borate salt in deionized water were mixed, heated, and cooled down to observe the sample state). (C) SEM photograph of BA hydrogel (BA : B = 100 mM : 200 mM).