Investigation of the viscoelastic properties of smart hydrogel materiasls using microrheology method

This thesis investigates the viscoelastic properties of smart material hydrogels using the passive video microrheology method. Microrheology is an innovative technique that analyzes the random motion of microscopic colloidal particles to determine the properties of viscoelastic environments. Compared to conventional macroscopic techniques for rheological measurements, this method provides the opportunity to evaluate the local mechanical properties of the material at a microscopic scale. Since the characteristics of a viscoelastic medium influence the Brownian motion of colloidal particles, this technique enables a precise analysis of the elastic and viscous behavior of the system. In this study, two temperature-responsive hydrogel types, PNIPAM (Poly(N-isopropylacrylamide)) and PEGMA (Poly(ethylene glycol methyl ether methacrylate)), were used. The changes in the viscoelastic moduli of these hydrogels as a function of temperature were examined. The high biocompatibility and elastic properties of PEGMA, along with the lower critical solution temperature (LCST) behavior of PNIPAM, provided a strong basis for investigating the mechanical responses of these materials to temperature variations. The video microrheology method enables precise measurement of the viscoelastic moduli of the medium by utilizing the random movements of embedded colloidal particles. Experimental studies were conducted over different temperature ranges, and the variations in the elastic and viscous properties of PEGMA and PNIPAM hydrogels were thoroughly analyzed and evaluated. Additionally, video microscopy and image processing techniques were employed to track and analyze the motion of colloidal particles. This thesis highlights the potential of the microrheology method in understanding the temperature-dependent changes in viscoelastic properties and aims to address a significant gap in the literature. The microrheological measurements of PEGMA hydrogels have demonstrated a distinct sol-gel transition in response to temperature. The analyses revealed significant changes in the viscoelastic response of PEGMA within the frequency range of 0.1 rad/s to 10 rad/s. At low frequencies (ω < 0.1 rad/s), the loss modulus (G'') was greater than the storage modulus (G'), indicating that the hydrogel predominantly exhibited fluid-like properties. However, with increasing temperature, a gradual transition process was observed in PEGMA samples, and at approximately 36°C, the G' modulus exceeded the G'' modulus, marking the onset of network stiffening. In the evaluation of this phase transition process, the movement of 1 μm-diameter aminated and carboxylated fluorescent particles embedded in the hydrogel matrix was analyzed, and the effect of particle surface chemistry on the viscoelastic properties of the environment was investigated. Aminated particles exhibited stronger interactions with the hydrogel network, leading to more restricted mobility even at lower temperatures. This resulted in the caged motion regime beginning at earlier temperatures. In contrast, carboxylated particles exhibited weaker interactions with the hydrogel matrix and showed higher mobility at lower temperatures, remaining in the free diffusion regime for a longer duration. Mean squared displacement (MSD) analyses clearly demonstrated the influence of different surface chemistries on the viscoelastic transition process in PEGMA hydrogels, indicating that aminated particles exhibited more restricted motion within PEGMA and that the viscoelastic moduli reached significantly higher values. The findings of this thesis confirm that PEGMA undergoes a gradual viscoelastic transition at approximately 36°C, with progressive changes observed in the viscoelastic moduli. The fact that PEGMA exhibits a phase transition at a higher temperature suggests a significant advantage for biomedical applications, particularly for temperature-sensitive systems that respond to body temperature. The analyses conducted with aminated and carboxylated particles indicate that different surface chemistries can influence interactions with the hydrogel network, thereby altering the viscoelastic transition mechanism. These results also demonstrate that microrheological measurements can precisely reveal these effects, contributing to a deeper understanding of the thermoresponsive behavior of PEGMA hydrogels.