1. A history of safety
The base polymer materials used to make the most common hydrogels are known to be inert (non-reactive and non-toxic) and have been around for many years. They were first tested in simple forms as a safe medium for drug and cell delivery and scaffolds for tissue engineering.2. The protection factor
The hydrophilic nature of hydrogels permits drug delivery of therapeutic materials that would otherwise denature due to hydrophobic interactions, and the protective structure also prevents destruction of cells or proteins by host immune responses, since matrix pore size can be made small enough to prevent the entry of large immune cells and antibodies. An example of this is enhanced survival of encapsulated pancreatic islet cells for treatment of type I diabetes (Lin and Anseth, 2009).
Source: Lin, C. and Anseth, K. 2009. Glucagon-like peptide-1 functionalized PEG hydrogels promote survival and function of encapsulated pancreatic beta-cells. Biomacromolecules. doi:10.1021/bm900420f.
3. Control over the polymer properties
The properties of materials like poly(ethylene glycol) (PEG), one of the simplest polymer materials used to make hydrogels, can be controlled to optimize features like pore size, which is used to control rates of diffusion of the deliverable drugs or cells.4. Introduction of smart polymers
The design/synthesis of a basic hydrogel matrix can be modified to make them "smart" polymers. Changes to the polymer structure or added ligands that allow control over the release of deliverables using various biological triggers like changes in pH or temperature, or the presence/absence of biomolecules like glucose.5. In situ and ex situ control
More complex modifications like polymer cross-links or photodegradable linkages add to our ability to control the release of deliverables. Polymer cross-links targeted by specific enzymes are used to open up the gel matrix in situ (Aimetti et al., 2009). Photolysis of hydrogels with a photodegradable cross-linker enables external control through irradiation (Kloxin et al., 2009).
Source: Aimetti, A. et al. 2009. Poly(ethylene glycol) hydrogels formed by thiol-ene photopolymerization for enzyme-responsive protein delivery. Biomaterials (2009), doi:10.1016/j.biomaterials.2009.07.043.
Kloxin, A. et al. 2009. Photodegradable hydrogels for dynamic tuning of physical and chemical properties. Science 324:59-63.
