DEVELOPMENT AND CHARACTERIZATION OF PIEZOELECTRIC SILK FIBROIN NANOCOMPOSITE HYDROGELS

Abstract

This dissertation discusses the synthesis and characterization of a novel class of piezoelectric silk fibroin hydrogels containing ZnO, AlN, and LiNbO₃ nanoparticles that offer improved potential for applications in biomedicine and bioelectronics. Silk fibroin, which is a natural protein derived from the Bombyx mori silk cocoon, was chosen as the base polymer because of its high biocompatibility, biodegradability, flexibility, and suitability for forming hydrogels. Nonetheless, pure silk fibroin hydrogels face some limitations including poor ionic conductivity and poor mechanical strength. The addition of nanoparticles into the hydrogel improves the mechanical strength, ionic conductivity, piezoelectricity, and thermal stability. The hydrogels based on silk fibroin were synthesized using three approaches, namely the dialysis based technique, non-dialysis technique, and the soft freezing process. The effects of addition of nanoparticles and different approaches on the physicochemical properties of the hydrogels have been studied. Zinc oxide (ZnO) nanoparticles were used for conductivity, antibacterial activity, and increased mechanical stability of the obtained materials. Aluminum nitride (AlN) and lithium niobate (LiNbO₃) were added for increasing thermal conductivity, ionic conductivity, dielectric properties, piezoconductivity, and electroactivity. The synthesized hydrogels were characterized through the determination of swelling, rheology, conductivity, conductivity under heating, FTIR spectroscopy, cross-linking density, and electric current measurements. From the rheology studies conducted through viscosity, amplitude sweeps, frequency sweeps, and creep recoveries, an enhancement in viscoelastic properties and an increase in storage modulus (G′) values were observed. It was found that nanoparticle loading as well as different synthesis approaches significantly affected water absorption capacities and dimension stabilities of the hydrogels. From the conductivity tests, it was observed that the incorporation of ZnO and LiNbO₃ nanoparticles promoted both ionic and ionic conductivities, while AlN contributed to the effective transport of thermal energy. Temperature dependent conductivity analysis proved to show increased conductivity at higher temperatures due to an increase in ion mobility. From FTIR studies, it is clear that there is effective interaction of nanoparticles and silk fibroin chains along with β-sheet structure formation. Soft freezing technology used in the study provided porous and interlinked hydrogels without the use of any chemical cross-linking agents, and thus, were ideal materials for biomedical applications. The 12

fabricated silk fibroin nanocomposite hydrogels had improved mechanical, conductive, piezoelectric and thermal properties. The potential of these hydrogels for tissue engineering, regenerative medicine, wound healing applications and as biosensors, smart biomedical and bioelectronic systems was clearly evident.