Hydrogels – a focus for visionary new biomaterials
Characterizing hydrogels is an area that we are seeing a rapid increase of interest, primarily due to their use in the development of novel biomaterials and in various biomedical applications. Hydrogels are a class of gels – materials with a three dimensional network that spans the volume of a liquid medium – in which water is the dispersion medium.
These soft, pliable materials have the ability to absorb significant quantities of water – as with naturally-occurring materials that play a vital part in all forms of life – making them compatible with most living tissue. Their viscoelastic nature permits implanting into a living host with minimal damage to surrounding tissue, and their ultimate mechanical properties can be engineered to closely match that of soft tissue. It’s no wonder that hydrogels will play a key part in progressing new strategies for regeneration and restoration of soft tissues and their associated bio-functionality.
A developing application where the use of hydrogels really could open eyes, is as replacement eye lenses for the treatment of cataracts. As the eye’s natural lens ages, optical defects can occur in it to varying degrees – from slight cloudiness through to near complete opacity. Cataracts can ultimately progress to significant vision loss, and in some cases if left untreated, even blindness. Cataract surgery typically involves removing the eye’s natural lens, and replacing it with a plastic substitute. Whilst this is an effective treatment, replacement plastic lenses are not a perfect fit in the capsular bag that holds the natural lens and can misalign, and they also have a limited focal range which may result in the need to wear glasses for either near or distance vision.
Researchers from Japan and Denmark have recently reported on the development of a nanocomposite gel that can be injected directly into the capsular bag following removal of the defective natural lens. The nanocomposite material then gels in the capsular bag to a tight fit, which accommodates best focus on the retina.
The hydrogel system used is a thermosensitive hydrophobically-modified poly (ethylene glycol) (HM-PEG) containing hydrophilized silica nanoparticles. The rheology of this nanocomposite hydrogel can be ‘tuned’ by varying component concentrations and temperature, such that regions exist in the phase diagram where the material is a transparent sol (to enable injection) and a transparent gel (to enable performance properties as a lens). These material property changes are reversible, without hysteresis, on changing temperature. The dispersed silica nanoparticles (2-5nm) in the gel matrix are important in engineering the required material properties – enabling a formulation which matches the modulus and the refractive index of the natural lens. Longer trials of this nanocomposite gel as an eye lens replacement are ongoing.
No longer just far sighted, the use of novel hydrogels and their highly tunable rheological, mechanical and biological properties is becoming truly visionary in opening up more and more applications in living tissue replacement and regeneration.