An injectable hydrogel is a 3D polymer network that can absorb and contain a high amount of water or biological fluid. Applications of hydrogels include tissue engineering, soft contact lenses, wound-healing, sensors, mucoadhesives and therapeutic delivery systems. Hydrogels can be loaded with and provide controlled release of proteins and drugs.
Hydrogels are used as scaffolds in tissue engineering and for regenerative medicine. In bone and cartilage engineering, hydrogels support the differentiation of chondrocytes and osteoprogenitor cells to osteoblasts. Injectable hydrogels are useful because they can replace implantation surgery with a less invasive injection method that can form any shape or match irregular defects. Injectable hydrogels are used in spinal cord regeneration after spinal cord injury to fill in small spaces between spinal cord tissue and transected parts. The liquid hydrogel converts to a gel form that provides a scaffold for regerating tissue.
Hydrogels have structural similarity to the extracellular matrix (ECM) which found in connective tissues. They provide a porous framework that allows cell transplantation and proliferation. The structure of hydrogels consist of three-dimensional (3D) cross-linked networks formed by hydrophilic homopolymers, copolymers or macromers. These networks swell in aqueous solution and provide a microenvironment similar to the ECM, that facilitates cell migration, adhesion, proliferation and differentiation. The ECM delivers nutrients and growth factors.
For bone and cartilage tissue engineering applications, injectable hydrogels have been made using a variety of natural and synthetic biomaterials including chitosan, collagen or gelatin, alginate, hyaluronic acid, heparin, chondroitin sulfate, poly(ethylene glycol) (PEG), and poly(vinyl alcohol). Both physical and chemical methods are used for cross-linking individual polymer chains.
The following are classifications of injectable hydrogels:
- enzymatically cross-linked hydrogels
- photo-cross-linked hydrogels
- Schiff base cross-linked hydrogels
- Michael addition-mediated hydrogels
- click chemistry-mediated hydrogels
- ion-sensitive hydrogels
- pH-sensitive hydrogels
- temperature-sensitive hydrogels
