Hydrogel

A hydrogel is a polymer network swollen with water — most of its mass is water, held in place by chains that are cross-linked chemically (covalent bonds), physically (entanglement, hydrogen bonds) or ionically (bridging by ions such as calcium). The result is a soft solid that can be moulded, injected before it sets, or cast into a shape.

In bone grafting a hydrogel can play two roles. As a carrier, it binds particulate graft into a cohesive mass for handling. As a scaffold, it occupies the defect as a three-dimensional matrix that cells can enter and that degrades as bone forms. The 2013 review in Acta Biomaterialia set out the difference between the two, and the reviews of alginate hydrogels describe how the same polymer has been used for both.

Hydrogels used in this field include collagen and gelatin, hyaluronic acid, chitosan, fibrin, alginate, and synthetic polymers such as poly(ethylene glycol) and poloxamer. They differ in origin (animal, plant, algal, synthetic), in how they set, in how long they persist, and in whether cells can attach to them.

On this site

The composition under development uses an ionically cross-linked alginate hydrogel as the carrier for particulate graft; see how the composition is designed.