Hydrogels have been of interest to biopharmaceutical scientists since the 60's, when Wichterle and Lim published their work on crosslinked HEMA hydrogels. Since then, common applications of hydrogels include sustained release delivery systems, biosensors and scaffolds in tissue engineering. Recently, hydrogel research has been revitalized by novel approaches in hydrogel design. Prof. Langer's group at the Massachusetts Institute of Technology has reviewed the use of microengineered hydrogels for tissue engineering applications.

Hydrogels are hydrophilic polymer networks, which may be chemically stable or they may degrade, disintegrate or dissolve. Depending on the networks, hydrogels are either named chemical or physical. Chemical gels are covalently-crosslinked networks, while the networks of physical gels are held together by ionic, H-bonding or hydrophobic forces.

Traditionally, hydrogel matrices have been fabricated from a variety of natural and synthetic polymers, such as dextran sulfate, chitosan, PEG-PLGA-PEG, PHB, etc. Conventional methods of hydrogel synthesis include a number of copolymerization techniques. Nevertheless, traditional hydrogels exhibited limitations for biomedical applications due to poor mechanical properties or lack of structure control.

Tissue engineering is the use of the principles of engineering and physical and biomedical sciences to improve or replace biological functions. According to the authors, microengineered hydrogels are a powerful tool to overcome the barriers that prevent the generation of transplantable tissues. These barriers include difficulties in generating a vascular network, which would mimick the functions of biological tissues.

Microengineered hydrogels are hydrogels with dimensions as small as a few tens of nanometers. A variety of techniques have been developed to generate them, such as emulsification, photolithography, microfluidic synthesis and micromolding. Applications of microengineered hydrogels may be classified into two categories: "top-down tissue engineering" and "bottom-up tissue engineering". Top-down tissue engineering approach controls the microscale features of large pieces of hydrogels, whereas bottom-up tissue engineering approach generates scaffolds by the assembly of small functional units.

The latest research on microengineered hydrogels has shown that they may have a smart future. In the opinion of Langer and co-workers, it is foreseen that scientist's ability to microfabricate hydrogels has the potential to overcome many of the current challenges in the application of hydrogels in tissue engineering.

Source: Khademhosseini A, Langer R. Biomaterials 28 (2007) 5807-5092