Correlating The Structure of Hydrogels to Their Diffusion Dynamics & Design and Construction of Fluorescence Correlation Spectroscopy Setup

Abstract

The fundamental phenomenon responsible for the movement of agents in the body such as drug molecules, signaling molecules and essential nutrients is diffusion. The transport of the material by diffusion is of great importance in several biological and industrial applications. In applied sciences such as biochemical engineering and pharmaceutical industries, diffusion plays a crucial role to predict diffusant transport ex: drug release rates. Scientists in the field of physical chemistry, on the other hand often study diffusion to extract information on transport properties. The study of diffusion in real-life systems is often complex as many factors govern the diffusion phenomenon such as the inherent structural organizations or the presence of other molecules. Apart from the hindrance experienced due to all these factors, interactions also influence the diffusion process. All these environmental constraints present in the system makes it extremely difficult to figure out the exact parameters affecting the diffusion process. To circumvent this problem, “model systems” have been used in the past as an alternative to real-life systems to study the diffusion process. The main advantage of these model systems is the freedom to control and tune individual parameters and study the effects on the diffusion process. The studies on the model systems can be designed to vary the amount of crowding, interactions, or the presence of other molecules. This will help us to understand the diffusion processes which can then aid in deriving results for real-life systems. Among the various model systems that have been considered, hydrogels have been extensively used to study and understand the diffusion mechanism. Because of its unique mechanical properties and high water content, hydrogel resembles natural tissue more than any other material. Hydrogel is considered as an excellent vehicle for drug delivery and other applications due to its capability to constrain the diffusive movement of the solute molecule. The diffusion of solutes in hydrogels has found applications in a wide range of fields such as chromatography, prosthetic applications, cell encapsulation for fermentation and biomedical application among others. One of the significant parameters that affect the diffusion mechanism in hydrogels is the hydrogel network architecture. The hydrogel structure can be tuned by varying many parameters like polymer and crosslinker concentration, crosslinker types, pH, solvent, temperature, the addition of external moieties such as surfactants etc. It is therefore imperative to have a comprehending knowledge of the parameters governing solute diffusion in hydrogels as well as how the hydrogel network architecture affects the diffusion process. Taking into consideration all the above facts the research works presented in the current thesis have been framed under two main objectives. The first objective is to investigate how the diffusion dynamics are influenced by the variation in various parameters such as polymer concentration, crosslinker concentration, charge density etc. The second objective xxii is to try and correlate the observed diffusion dynamics to the change in the hydrogel network structure. The effect of the addition of external moieties such as surfactants of different head groups and concentrations on the structure and dynamics of gellan gum hydrogels were investigated. A strong interaction is seen between gellan gum and oppositely charged cationic surfactant, hexadecyltrimethylammonium bromide (CTAB) whereas rather weak or minimal interactions are observed when either anionic surfactant, sodium dodecyl sulfate (SDS), or non-ionic surfactant, Triton X-100 is added to the system. An interesting cross-over from stretched to compressed exponential was seen when CTAB was added beyond critical micellar concentration to the system, which was not evidenced for the other two surfactants. The attachment of CTAB clusters or micelles can lead to the distortion of the hydrogel network which then acts as stress releasing points resulting in the observed crossover. These results may have important repercussions for the use of polymer surfactant systems as potential products. The effects of mixed salts on the structure and dynamics of the physical gel of gellan gum were evaluated. It was seen that when mixed salts were present in the system, the storage modulus of the hydrogels decreased as compared to the pure hydrogels. The main reason for this type of decrease in the storage modulus as well as the relaxation time may be that the addition of mixed salts may not reduce the electrostatic repulsion between the gellan gum helices more effectively than the monovalent or the divalent salt alone. Thus there might be a hindrance to the formation of the strong networks, which leads to a decrease in the storage modulus values when two types of salts are present. The effect of monomer concentration and charge density on the structure and dynamics of a chemical hydrogel of acrylamide/sodium acrylate was investigated. With increasing total monomer concentration, the storage modulus of the hydrogel increased. However, when the charge density was varied, the storage modulus of the hydrogels first increased and then decreased monotonically. In the presence of Bisacrylamide crosslinker an interesting change in dynamics was seen which was dependent on both the monomer concentration as well the charge density of the system. The hydrogel changed from neutral polymer to charged polymer after the charges were introduced to the system. Owing to this charged nature an extra mode is observed in the relaxation dynamics. Moreover, the degree of spatial gel inhomogeneity was seen to decrease moderately with an increase in sodium acrylate concentration and decreased significantly with a decrease in total monomer concentration. The effects of varying the types of crosslinkers on the triple dynamics were investigated further. For this purpose, two multifunctional crosslinkers (divinyl sulfone, DVS and glutaraldehyde, GLU) were used. Firstly, the effect of varying the total monomer xxiii concentration was studied by rheology and it was seen that in both crosslinkers the storage modulus increased with increasing total monomer concentration. Also, it was evidenced that the DVS crosslinker produced the hydrogels with higher storage modulus as compared to the GLU crosslinker at all the total monomer concentration range studied. From dynamic light scattering measurements we evidenced an extra mode in both the crosslinker systems and for all the total monomer concentrations studied. It was seen that this extra mode was observed for all the hydrogels at and after 80% SA concentration. Neither the monomer concentration nor the type of crosslinker used had any effect on the emergence of the new relaxation mode as seen earlier. There was no particular trend in the onset of triple mode which was observed before while using the Bisacrylamide crosslinker. The observed difference may be due to the different crosslinking mechanisms of the crosslinkers studied. As part of the research work, we have designed and constructed a Fluorescence correlation spectroscopy (FCS) instrument in-house. This was used to probe the structure and dynamics of hydrogels under varied conditions. We strongly believe that these studies will support the fundamental understanding of real-life systems and their operation in complex environments. These studies can be utilized to design the structure of hydrogels that can be used for specific applications such as targeted drug delivery, although in a broader sense can be applied to a myriad of applications.