Question

Which of the following gel/hydrogel is formed by a physical gelation mechanism?

• A. Polyester gel
• B. Gelatin
• C. CMC-g-acrylic acid
• D. Polydimethyl siloxane

Hint:

Remember the key difference between physical and chemical gelation: physical gels are formed by reversible non-covalent bonds (e.g., hydrogen bonds, hydrophobic interactions) and are often temperature-sensitive, while chemical gels involve irreversible covalent bonds. Gelatin's thermoreversible property is a strong indicator of its physical gelation mechanism.

Answer 1

The Correct Option is B

Definition of Gel/Hydrogel and Gelation Mechanisms

A gel or hydrogel is a three-dimensional network of polymer chains capable of holding a large amount of liquid, often water, while maintaining structural integrity. Hydrogels are widely used in biomaterials for applications like tissue engineering or drug delivery. Gelation mechanisms can be physical (non-covalent interactions like hydrogen bonding, hydrophobic interactions, or ionic bonds) or chemical (covalent cross-linking). Physical gelation is reversible and driven by environmental factors like temperature or pH, while chemical gelation forms permanent bonds. The question asks which gel/hydrogel is formed by a physical gelation mechanism, with options: polyester gel, gelatin, CMC-g-acrylic acid, and polydimethyl siloxane. Let’s evaluate each option.

1. Polyester Gel

Polyester gels are typically formed through chemical gelation, involving covalent cross-linking of polyester chains (e.g., polylactic acid or polycaprolactone). These gels are synthesized via reactions like polycondensation or ring-opening polymerization, creating permanent covalent bonds. Physical gelation is not common for polyester gels, as their structure relies on stable chemical cross-links rather than reversible physical interactions. Thus, polyester gel is unlikely to form via physical gelation.

2. Gelatin

Gelatin is a natural hydrogel derived from collagen, commonly used in biomedical applications like scaffolds or drug delivery. Gelatin forms a hydrogel through physical gelation when cooled. At higher temperatures, gelatin exists as a soluble coil in solution, but upon cooling, it undergoes a coil-to-helix transition, forming hydrogen bonds and hydrophobic interactions that create a reversible gel network. For example, gelatin desserts solidify when refrigerated due to this physical process. This makes gelatin a strong candidate for physical gelation.

3. CMC-g-Acrylic Acid

CMC-g-acrylic acid refers to carboxymethyl cellulose (CMC) grafted with acrylic acid, a synthetic hydrogel often used in absorbent or biomedical applications. The grafting process involves chemically bonding acrylic acid to the CMC backbone, typically through covalent cross-linking via free radical polymerization or other chemical reactions. This forms a stable, chemically cross-linked network, not a reversible physical one. Therefore, CMC-g-acrylic acid is formed by chemical gelation, not physical gelation.

4. Polydimethyl Siloxane

Polydimethyl siloxane (PDMS) is a silicone-based polymer commonly used in biomaterials, such as soft implants or microfluidic devices. PDMS gels are typically formed through chemical gelation, where siloxane chains are cross-linked using a curing agent (e.g., a platinum catalyst) to form covalent bonds. This creates a permanent, elastic network. While PDMS can exhibit some physical interactions (e.g., in uncured states), its gel form is predominantly chemically cross-linked, not physically gelled, making this option incorrect for physical gelation.

Why Gelatin is the Correct Answer

The correct answer is gelatin, as it forms a hydrogel through a physical gelation mechanism. Gelatin’s gelation occurs when a warm solution cools, triggering reversible hydrogen bonding and hydrophobic interactions that form a gel network without covalent bonds. This is distinct from the other options, which rely on chemical cross-linking:

• Polyester gel: Formed by covalent cross-linking, a chemical process.
• CMC-g-acrylic acid: Involves covalent grafting and cross-linking, also chemical.
• PDMS: Forms through chemical cross-linking with a curing agent.

For example, in tissue engineering, gelatin hydrogels are used because their physical gelation allows reversible properties, enabling applications like injectable gels that solidify at body temperature. The physical nature of gelatin’s gelation is driven by temperature-dependent molecular interactions, making it unique among the options.

Why Not the Other Options?

• Polyester Gel: Relies on chemical cross-linking, not reversible physical interactions.
• CMC-g-Acrylic Acid: Formed by chemical grafting and cross-linking, creating permanent bonds.
• Polydimethyl Siloxane: Chemically cross-linked to form a stable gel, not physically gelled.

A Simple Way to Remember

Think of gelation as making jelly:

• Gelatin: Like homemade jelly, it sets by cooling (physical gelation, reversible).
• Polyester Gel, CMC-g-Acrylic Acid, PDMS: Like epoxy glue, they harden through chemical reactions (permanent, covalent bonds).

Final Answer

The gel/hydrogel formed by a physical gelation mechanism is Gelatin.