Hydrogels from biomimetic resources: synthesis, characterization and applications

Abstract

The objective of the thesis was to undertake design and synthesis of mechanically robust hydrogels from biomimetic resources such as polysaccharides, by employing the double network (DN) strategy. Hydrogels are water-swollen three-dimensional network of hydrophilic polymers crosslinked by chemical or physical interactions. Hydrogels show numerous applications, particularly in medical and pharmaceutical fields, such as ophthalmological devices, biosensors, bio-membranes, substitutes for organs such as artificial skin, carriers of drug delivery devices, etc. The major strategy employed in regenerative medicine for reconstructing organs is the replacement of degraded organ part(s) with natural extracellular matrix (ECM), which is a complex and dynamic three-dimensional structure surrounding cells in an organ. Biocompatible and hydrophilic hydrogels can provide many of the normal signals and interactions to cells by the ECM in tissues. This unique property of hydrogels as synthetic ECM analogs is primarily due to their ability to retain large amounts of water along with their soft and rubbery consistence which closely resembles the living tissues. Two major drawbacks of hydrogels generally encountered in biomedical applications are their low mechanical strength and toxic degradation bye products. Therefore, major focus is now given on developing mechanically strong and tough hydrogels from biomimetic resources such as polysaccharides with non-toxic bye products.

In this context, three polysaccharides were selected for the preparation of biomimetic hydrogels, which are carboxymethyl xyloglucan (CMX), carboxymethyl cellulose (CMC) and alginate (Alg). Michael-type addition reaction between the primary hydroxyl group in the polymer backbone and divinyl sulphone (DVS) was utilized to crosslink CMX and CMC polysaccharides. These crosslinked structures were swollen in water to form transparent hydrogels. Alginate hydrogels were prepared by the physical crosslinking method, where divalent cations such as Ca2+ were used to form an ionic bond between the polymer chains.

CMX was used to design neural interface by in-situ crosslinking of CMX with DVS by Michael-addition reaction, inside the hollow carbon nano-fibers (CNF). Neural interfaces are “communication bridges” between the artificial electronic prosthesis and the central nervous system of the physically challenged personnel. Resultant microelectrodes do possess dual characteristics of electrical as well as conductivity by virtue of the inherent properties of CMX and CNF. The concentration of CMX plays a crucial role in the formation of a homogeneous hydrogel network inside the hollow core of CNF. Electrochemical studies show that these microelectrodes possess higher charge density and active surface area than pristine CNF, which enhances and promotes ion conducting channels that are ideal for developing neural interfaces. Cytotoxicity studies revealed the biocompatible characteristics of these microelectrodes and their cyto compatible nature. The results indicate the potential of these microelectrodes for their applications in designing new generation neural interfaces.

CMX, CMC and Alg were used for the preparation of mechanically robust double network (DN) hydrogels, along with another synthetic polymer, namely Poly (hydroxyethyl acrylate) (PHEA) and/or Poly (hydroxyethyl acrylate-co-stearyl methacrylate (PHEA-co-SM), by a sequential process. Initially, hydrogels were prepared using CMX, CMC and Alg, called the first network. These hydrogels were washed and immersed in aqueous solution of hydroxyethyl acrylate (HEA) and/or stearyl methacrylate (SM) containing 0.1 Wt. % photoinitiator for seven days. Equilibrium swollen hydrogels were exposed to UV radiation to affect the free-radical photo-polymerization of HEA and/or SM to form a second hydrogel network of PHEA and/or PHEA-co-SM, inside the first network. Gelation, swelling and mechanical and/or rheological properties of these DN hydrogels were studied.

Microgels (MGs) were prepared by the co-polymerization of SM and HEA using oil-in-water (O/W) emulsion method, where concentration of HEA was 5, 10 and 15 mol % as that of SM. Average size and range of these MGs were measured with micro-CT. Curcumin as a model drug is loaded onto these MGs and embedded inside DN hydrogels prepared from CMX and PHEA. Release of curcumin in PBS (pH 7.4 and 37 Deg C) from these MGs embedded in the DN hydrogels showed release pattern dependent on the concentration of HEA. These drug loaded DN hydrogels showed promise for applications in developing rate pre-programmed release systems for addressing chronic wound healing process.

During the preparation of CMC DN hydrogels, concentration of SM (co-monomer in second network) was varied from 2, 4 and 6 mol % of HEA, to study the effect of hydrophobic co-monomer (SM) in the network structure of DN hydrogels. Hydrophobic co-monomer (SM) is incorporated into the second network by micelle-assisted free-radical co-polymerization method and the role of SM in energy dissipation mechanism of these DN hydrogels were studied by cyclic strain, step strain and stress relaxation measurements. Compressive strength of the CMC DN hydrogel was 280 times more than that of CMC hydrogel and with increase in SM concentration DN hydrogels showed better recovery after deformation. Microstructure of porous xerogels prepared from CMC DN hydrogels was studied in detail by micro-CT (Micro-computed Tomography), to explore more information about their porosity and pore-characteristics. Cell viability studies showed the advantage of DN hydrogels over it single network counterparts, where biocompatibility of DN hydrogels was enhanced due to a better response of cells towards hydrogel network structure with faster relaxation rates and also due to the presence of bioactive polysaccharide units in them.

During the preparation of Alg DN hydrogels, the concentration of SM (co-monomer in the second network) was increased up to 10 Wt. % of HEA, to impart self-healing property to the hydrogel network. Thermo-reversible behaviour of Alg hydrogel (first network) was studied by rheometry as a function of temperature. Step strain rheological measurements explored self-healing characteristics of the Alg DN hydrogels. These DN hydrogels were also examined to study the in-situ bio-mineralization of HAP, confined in the DN hydrogel network.

DN strategy was proven to be a unique route for designing polysaccharide based hydrogels with exceptional mechanical behaviour. These hydrogels exhibited great potential in drug delivery, regenerative medicine, implant design etc.