Hydrogel

Hydrogels are biomaterials that can be engineered to respond sensitively to multiple parameters such as temperature, pH, magnetic field, electricity field, and light like, and are frequently used for various purposes. They are applied to the body either invasively, by injection into tissues, or non-invasively, through external contact.

Structure and Properties of Hydrogels

Hydrogels are defined as three-dimensional hydrophilic networks. Their most fundamental characteristic is the ability to retain water within their structure without deformation. Although they are known to be resistant to Organic solvents, in an appropriate solvent environment they can exhibit swelling performance up to nearly 1000 times their original physical dimensions, with a minimum water content of 10–20%. Since hydrogels interact with biological systems, the most critical requirements during their development are that they be biodegradable and biocompatible. Another essential criterion is that they must be non-cytotoxic, meaning they should not produce toxic effects when applied to the body. In particular, hydrogels intended for skin contact must be produced in a porous structure with antimicrobial properties.

Composition of Hydrogels

The primary components of hydrogels are Polymer, water, or another biological fluid, along with a crosslinking agent that binds them together. The composition of the developed hydrogel is enriched according to its intended region and purpose. A single polymer may be used, or multiple polymers can be combined to leverage their complementary properties. While water was traditionally the primary liquid component, modern hydrogels developed for drug delivery now incorporate drugs directly as the liquid phase.

An image illustrating the polymeric structure of a hydrogel

Polymer Selection

Selecting a polymer suitable for the intended purpose of the hydrogel is one of the most critical stages in its production. Both natural and synthetic polymers can be chosen. For example, for wound healing, chitosan derived from crustacean shells or fungi, or synthetic polymers such as polyvinyl alcohol may be selected. However, the two polymers differ significantly in their target region, the effects they produce, and numerous other factors such as compatibility with other materials. Beyond these two examples, numerous other polymer options exist, including natural polymers such as keratin, collagen, cellulose, gelatin, and alginate, or synthetic polymers such as poly(ethylene glycol) (PEG), poly(acrylic acid) (PAA), and poly(lactic-co-glycolic acid) (PLGA). The most fundamental principle in hydrogel production is selecting the most appropriate polymer for the target application, combining different polymers to achieve desired properties, and then integrating them with compatible materials using optimal methods to maximize functionality.

Other Components

To enhance swelling and water absorption capacity, crosslinkers such as glutaraldehyde or calcium are used. Stabilizers and preservatives are added to extend shelf life. Fillers are incorporated to achieve target mechanical properties. In systems designed for use with magnetic or electric fields—recent studies have shown that specific frequencies at controlled doses can promote resolution of certain physiological issues—nanocomposite and nanoparticle materials are added to improve conductivity and antimicrobial properties.

An image of a hydrogel administered in liquid form via injection

Usage Forms and Applications

Hydrogels can be applied in more liquid forms by injection into the body, depending on their intended use. Alternatively, after all components are prepared according to requirements, they can be dried under appropriate conditions or fabricated using 3D printing techniques to form solid gel structures.

An image of a hydrogel used as a wound dressing

Tissue engineering employs hydrogels as building scaffolds, bone tissue engineering utilizes them for bone repair and prosthetics, surgical applications use them to seal cavities and prevent air leaks, and wound healing commonly employs hydrogels as dressings or in liquid form. Additionally, hydrogels are widely used in controlled drug delivery and cell transport, tissue regeneration, artificial organ production, contact lenses, and many other fields. Generally, these multifunctional biomaterials are developed by materials and biomedical engineers who combine chemistry to design material composition and medical and biological approaches to ensure biocompatibility, all focused on addressing specific biological challenges.