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. Author manuscript; available in PMC: 2023 Jan 15.
Published in final edited form as: Acta Biomater. 2021 Nov 4;138:1–20. doi: 10.1016/j.actbio.2021.10.056

Table 1.

Proposed hierarchical categorization of hydrogel-based therapies

0 Order Systems: Hydrogels that have no deliberate molecular orientation, macroscale design, or CBER/CDER-regulated biologics embedded (e.g., dermal fillers)
1st Order Systems:
1A: Hydrogel systems that include a single CBER/CDER-regulated biologic, either living cells or cell products, or a function-driven structure (alignment, orientation, etc.) (e.g., injectable HA gel with stem cells) 1B: Hydrogel systems from 1A that rely on a manufacturing method that requires further validation (e.g., 3D bioprinting)
2nd Order Systems:
2A: Hydrogel systems that employ multiple mechanisms of action, any combination of cells, cell components, and drugs, or one of those and a structural mode of action (orientation, channels, gradient, etc.) (e.g., allogeneic cellularized scaffold) 2B: Hydrogel systems from 2A that rely on a manufacturing method that requires further validation (e.g., 3D bioprinting)