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General product and regulatory information only; this page is not medical advice and does not describe how to treat any wound.
Short answer
A hydrogel is a cross-linked network of hydrophilic polymer chains that holds a large share of water without dissolving in it. In US medical-device rules, a “hydrogel wound dressing” has a narrower meaning: a nonresorbable matrix of hydrophilic polymers combined with at least 50 percent water, under 21 CFR 878.4022.
Definition and scope
In the materials-science literature, a hydrogel retains a significant fraction of water within its structure but does not dissolve in water (Ahmed, Journal of Advanced Research, 2015). Two features explain this. Hydrophilic groups on the polymer backbone draw water in, and cross-links between chains stop the network from dissolving.
The same word therefore covers very different products. A research-grade gel, a contact-lens material and a wound dressing can all be hydrogels in the scientific sense, but only a dressing that meets the regulatory text is a “hydrogel wound dressing” in the US classification. Readers who want the wider field can start with what polymer science is.
What a hydrogel is made of
Every hydrogel has the same two ingredients: a hydrophilic polymer network and water. The polymer can be natural or synthetic, and the network is held together by chemical or physical cross-links. In 21 CFR 878.4022 the dressing version is described as “a nonresorbable matrix made of hydrophilic polymers or other material in combination with water (at least 50 percent) and capable of absorbing exudate.”
Types of hydrogel
The review by Ahmed (2015) sorts hydrogels along several independent axes, so one material can be placed on all of them at once. The same classification answers the common searches for “hydrogel types” and “hydrogel polymers”.
| Classification axis | Categories named in the review |
|---|---|
| Source | Natural or synthetic |
| Composition | Homopolymeric, copolymeric, multipolymer (interpenetrating or semi-interpenetrating network) |
| Configuration | Amorphous, semicrystalline, crystalline |
| Cross-linking | Chemical (permanent junctions) or physical (transient junctions from entanglements, ionic interactions, hydrogen bonds or hydrophobic interactions) |
| Physical form | Matrix, film, microsphere |
| Network charge | Nonionic, ionic, amphoteric, zwitterionic |
Hydrogel wound dressings under 21 CFR 878.4022
The US Code of Federal Regulations defines a hydrogel wound dressing as “a sterile or non-sterile device intended to cover a wound, to absorb wound exudate, to control bleeding or fluid loss, and to protect against abrasion, friction, desiccation, and contamination.” It is classified as Class I (general controls) and is exempt from premarket notification, subject to the limitations in 21 CFR 878.9.
The scope is deliberately narrow. The regulation excludes dressings with added drugs such as antimicrobial agents, added biologics such as growth factors, and materials derived from animal sources. For a facility reading a product description, this is the practical test: a dressing that contains a drug, a biologic or an animal-derived material is not described by 878.4022. The regulation says nothing here about how any excluded product is classified, so that question needs the product’s own labeling and FDA documentation. For stocking context, see the wound care clinic supply checklist and the burn care kit contents guide.
Materials-science definition compared with the US regulatory definition
| Question | Materials-science review (Ahmed, 2015) | 21 CFR 878.4022 |
|---|---|---|
| What is it? | A network that retains a significant fraction of water and does not dissolve | A device with a nonresorbable hydrophilic matrix combined with water |
| How much water? | “A significant fraction”; no fixed figure | At least 50 percent |
| Why does it not dissolve? | Cross-links between polymer chains | Not stated; the text specifies a nonresorbable matrix |
| Natural or synthetic? | Both are recognised | Animal-derived materials are excluded |
| Drugs or biologics added? | Not part of the definition | Excluded from this classification |
Common misunderstandings
- “A hydrogel dissolves in water.” It swells and holds water; cross-links are what keep it from dissolving.
- “Every product called a hydrogel dressing falls under 878.4022.” Only those matching the regulatory text do; the regulation excludes added drugs, added biologics and animal-derived materials.
- “A hydrogel is one material.” It is a structure type. Source, composition, cross-linking and charge vary independently, as the table above shows.
Frequently asked questions
What is a hydrogel?
A water-swollen, cross-linked network of hydrophilic polymers that does not dissolve in water.
What is a hydrogel made of?
Hydrophilic polymers, natural or synthetic, joined by cross-links, together with water. A regulated hydrogel wound dressing must contain at least 50 percent water.
What are the types of hydrogel?
They are classified by source, composition, configuration, cross-linking, physical form and network charge; see the table above.
Are hydrogel dressings regulated by the FDA?
Yes. 21 CFR 878.4022 defines the hydrogel wound dressing as a Class I device that is exempt from premarket notification, subject to the limitations in 878.9.
Sources
- eCFR, 21 CFR 878.4022, Hydrogel wound dressing and burn dressing (current version 2026-10-01), ecfr.gov. Checked 2026-10-30.
- Ahmed EM. “Hydrogel: Preparation, characterization, and applications: A review.” Journal of Advanced Research 2015;6(2):105-121, PubMed Central. Checked 2026-10-30.
