Hydrogels are cross-linked polymeric structures with dynamic swelling behavior in water. In particular, hydrogels are three-dimensional polymeric networks formed from hydrophilic homopolymers, copolymers, or macromers crosslinked to form insoluble polymer matrices, that can retain large amounts of water1.
Due to their unique biocompatibility, compliant elasticity, flexible methods of synthesis, range of constituents, and desirable physic-characteristics, hydrogels have been the material of choice for many biomedical applications2, 3, 4, 5. Injectable hydrogels can be administrated via minimally invasive procedures and appropriately fill irregular-shaped defects by acting as three-dimensional scaffolds. Injectable hydrogels have received much attention due to their potential biomedical and biological applications in the fields of imaging, biosensing, drug delivery tissue engineering, and regenerative medicine6, 7, 8, 9, 10, 11, 12, 13, 14, 15. Recently, there is an increasing demand for the development of biodegradable hydrogels endowed with fluorescent imaging moieties to further enhance the functions of the materials16, 17.
Conventionally, synthetic hydrogels with photoluminescent properties can be prepared by conjugating or doping hydrogel matrix with fluorescent moieties such as organic dye, fluorescent protein, colloidal semiconductor nanocrystal, metal-ligand complex and lanthanide ions19, 20, 21, 22, 23, 24. However, among them, organic dye and fluorescent protein are subjected to certain limitations such as photo-bleaching and cellular toxicity25, 26. Semiconductor nanocrystals also pose risks to human health and the environment under certain conditions27. Similarly, toxicity from the heavy metal contents of metal-ligand complex and lanthanide ion imaging probes evokes significant safety concern for their biomedical applications especially for their long-term use in vivo28.
Recently, the attempt to fabricate an injectable hydrogel by using silk protein sericin has been explored19. The gel is found to exhibit photoluminescence due to the intrinsic auto-fluorescence of sericin polypeptide. Nevertheless, the low quantum efficiency, untunable fluorescence property, eliciting immune response and the use of toxic glutaraldehyde as the cross-linker raise concerns for its biomedical applications.
Very recently, the development of a biodegradable polymer with potential biomedical application as an implanted elastomer and drug-loaded nanoparticle has been reported29, 30, 31. This newly developed biodegradable polymer displays superior biocompatibility both in vitro and in vivo, relative high quantum yields, photobleaching resistance, and tunable emission up to near infrared wavelengths and thus has potential biomedical applications, such as drug delivery nano-carriers and implanted scaffolds. However, the efforts to fabricate a hydrogel were unsuccessful due to the lack of functional cross-linking reactive moieties on the oligomers to form hydrogels.
Thus there still remains a need for a composition and method of preparing a hydrogel that contains both self-fluorescence and biodegradable characteristics without the above drawbacks. Furthermore there also remains a need in the art for a composition and method of preparing a hydrogel with the above properties that avoids eliciting an immune response and contributing to potential cytotoxicity and carcinogenesis.