# hydrogel

> network of polymer chains that are hydrophilic

**Wikidata**: [Q898925](https://www.wikidata.org/wiki/Q898925)  
**Wikipedia**: [English](https://en.wikipedia.org/wiki/Hydrogel)  
**Source**: https://4ort.xyz/entity/hydrogel

## Summary
A hydrogel is a network of polymer chains that are hydrophilic, classified as a specific type of gel. It functions as a substantially dilute cross-linked system that exhibits no flow when in a steady-state. These materials can range in consistency from soft and weak to hard and tough solid jelly-like substances.

## Key Facts
*   **Definition:** A hydrogel is defined as a network of hydrophilic polymer chains.
*   **Classification:** It is a subclass of "gel" and is categorized as a group or class of chemical substances.
*   **Physical Properties:** As a gel, it is a substantially dilute cross-linked system that does not flow in its steady state.
*   **Characteristics:** The primary characteristic of a hydrogel is hydrophilicity (affinity for water).
*   **Medical Identifiers:** The MeSH Descriptor ID for hydrogels is D020100, with tree codes D20.280.320.375 and D26.255.165.320.375.
*   **Variants:** Specific classes include PEGDMA hydrogel (used in biomedical applications) and cryogels (macroporous hydrogels produced at subzero temperatures).
*   **Composition:** Structurally, it consists of cross-linked hydrophilic macromolecules.
*   **Ontology IDs:** The entity has an IUPAC Gold Book ID (HT07519) and an Environment Ontology ID (01001562).

## FAQs
### Q: What is the chemical structure of a hydrogel?
A: A hydrogel consists of a network of polymer chains that are hydrophilic. It is defined as a substantially dilute cross-linked system.

### Q: How does a hydrogel behave physically?
A: While the specific properties can range from soft and weak to hard and tough, a hydrogel is fundamentally a solid jelly-like material that exhibits no flow when in a steady state.

### Q: What are the different types of hydrogels mentioned in knowledge bases?
A: Notable types include PEGDMA hydrogel, which is used in biomedical applications, and cryogels, which are macroporous hydrogels produced at subzero temperatures.

## Why It Matters
Hydrogels are significant in materials science and biomedicine due to their unique structure as a cross-linked network of hydrophilic macromolecules. Because they are a substantially dilute system that behaves as a solid rather than a liquid, they bridge the gap between fluid and solid states, making them distinct from other chemical substance groups. Their classification ranges from general chemical substances to specific biomedical tools, as evidenced by variants like the PEGDMA hydrogel. This versatility allows them to serve critical roles where a stable, water-loving structure is required, distinguishing them from standard polymers or simple fluids. Their inclusion in major ontologies like MeSH and the IUPAC Gold Book underscores their established importance in scientific taxonomy and application.

## Notable For
*   Being a **network of hydrophilic polymer chains**, distinguishing them from hydrophobic polymers.
*   Exhibiting **no flow in the steady-state**, despite being a dilute cross-linked system.
*   The ability to possess properties ranging from **soft and weak to hard and tough**.
*   The existence of specialized subtypes like **cryogels**, which require subzero temperatures for production.
*   Having a specific **IUPAC Gold Book ID** (HT07519), indicating standardized chemical recognition.

## Body

### Definition and Classification
A hydrogel is a group or class of chemical substances structurally defined as a network of polymer chains that are hydrophilic. It is a direct subclass of "gel," which is broadly defined as a solid jelly-like material. In the hierarchy of chemical substances, hydrogels are recognized as a distinct entity, evidenced by identifiers such as the GND ID 4160909-8 and the NALT ID 83075.

### Physical and Chemical Characteristics
The defining mechanical property of a hydrogel system is that it exhibits no flow when in a steady-state. These structures are substantially dilute cross-linked systems. Despite their dilute nature, they can achieve a wide spectrum of mechanical rigidity, ranging from properties that are soft and weak to those that are hard and tough. The primary chemical characteristic associated with hydrogels is hydrophilicity.

### Specific Variants
Knowledge bases identify several specific classes and related entities within the hydrogel family:
*   **PEGDMA Hydrogel:** A specific network of cross-linked hydrophilic macromolecules noted for its utility in biomedical applications.
*   **Cryogels:** A class of macroporous hydrogels distinguished by their production method, which occurs at subzero temperatures.
*   **Polymethylsiloxane Polyhydrate:** Identified as a related hydrogel polymer.

### Identifiers and Taxonomy
Hydrogels are extensively cataloged across scientific libraries and databases. Key identifiers include:
*   **MeSH Tree Codes:** D20.280.320.375 and D26.255.165.320.375 (under the qualifier 'gel').
*   **UMLS CUI:** C0600484.
*   **Environment Ontology (ENVO):** ID 01001562.
*   **Wikidata:** The entity has a Quora topic ID "Hydrogel" and a Google Knowledge Graph ID of /g/122jqy7b.

## References

1. Compendium of Chemical Terminology
2. YSO-Wikidata mapping project
3. [Environment Ontology](https://raw.githubusercontent.com/INCATools/environments2wikidata/master/matches/curated-high-confidence-envo.tsv)
4. UMLS 2023
5. GF WordNet