Shape and internal structure of parts of hydrogels. (a) Schematic illustration of the preparation of the peptide-agarose microgel scaffold for three-dimensional (3D) cell culture. Reprinted with permission from Ref. [18]. Copyright 2020, Yamada, Y. (b) Different forms of carrageenan composite hydrogel. Reprinted with permission from Ref. [23]. Copyright 2017, Liu, S. (c) Internal structure diagram of chondroitin sulfate composite hydrogel. Reprinted with permission from Ref. [27]. Copyright 2005, Dawlee, S. (d) Structure of dextran. Reprinted with permission from Ref. [28]. Copyright 2020, Chen, F.