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Polyelectrolyte Block Copolymers for Thermally Gellable and Electrically Conductive Hydrogels
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Abstract
The ability to respond to environmental stimuli with a change in behavior is omnipresent in nature, and essential to the functioning of biological systems. A widely used method of incorporating this stimuli-responsive behavior in synthetic materials is the use of stimuli-responsive polymers, which respond to changes in their biomolecular environment (pH, temperature, light, etc.) with a change in properties such as color, physical state, or mechanical properties. The addition of stimuli-response to electrically conducting polymers can offer exciting possibilities towards the development of a new class of soft electronics which display intrinsic mechanical tunability. In Chapter 1, we discuss recent developments in stimuli-responsive and conductive polymers, with an emphasis on molecular design and synthetic strategies. We compare different types of stimuli and response mechanisms to induce tunable mechanical properties in conductive polymers. Finally, we highlight the applications of stimuli-responsive and conductive polymers in drug delivery, biosensing and bioelectronics. Starting at Chapter 2, this thesis describes our approach to achieving sol-gel transitions in thermo-responsive and conductive polymers. In particular, we induced thermo-responsive behavior in poly (3, 4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), a conductive and water-dispersible polyelectrolyte complex. The key to this research is the synthesis of polyelectrolyte block copolymers (BCPs) of PSS with a thermo-responsive polymer, poly(N-isopropylacrylamide) (PNIPAM). PNIPAM displays a reversible hydrophilic-hydrophobic transition at 31-35 °C and can be used to induce temperature-controlled self-assembly in hydrophilic polymers such as PSS. When complexed with PEDOT, the resulting PEDOT:PSS-b-PNIPAM polyelectrolyte complex displays a thermal gelation around physiological temperature which can be leveraged for advanced bioelectronics applications.
Chapter 2 describes the synthesis of PSS-b-PNIPAM with varying molecular weights and block ratios by reversible addition fragmentation chain transfer (RAFT) polymerization. We explored various RAFT agents and initiators, and reaction conditions such as solvent system, concentration, and mechanisms of initiation to find the optimal method for the synthesis of this block copolymer. The self-assembly of PSS-b-PNIPAM was systematically investigated as a function of block ratio and temperature using scattering techniques. Through a collaboration with the Dhong lab, we are studying the potential utility of these polymers for the preventative treatment of osteoarthritis.
Chapter 3 describes the synthesis of PEDOT:PSS-b-PNIPAM, a novel conductive and thermo-responsive polyelectrolyte complex. The thermo-response of PEDOT:PSS-b-PNIPAM was tuned by varying the mass ratio of PSS:PNIPAM from ~1: 0.2 to ~1:2. Using this method, we developed fully thermo-reversible and conductive polyelectrolyte complexes (TR-CP) which displayed a sol-gel transition close to body temperature. We found that the modulus and electrochemical behavior of the TR-CPs was highly tunable, and could be controlled by changing the EDOT loading, concentration, pH, and/or the presence of aqueous salts, without significantly affecting their reversible sol-gel transition. Further, the mechanism of gelation was studied by characterizing the microstructure before and after the thermal transition. The formation of highly stable, conductive and reversible gels was attributed to controlled self-assembly from fibrils/ core shell structures to a 3-D network, which was only possible with a block copolymer morphology.
In Chapter 4, we describe the processing and stability of these reversible gels for bioelectronics applications. We showed that TR-CPs are self-healing, and can be patterned into tissue-like substrates using a fine needle with negligible spreading. We demonstrated their use in bioelectronics by incorporating them as reusable electrodes into a surface electromyography set-up. Finally, the TR-CPs were combined with alginate using a simple blending approach to form thermo-responsive and shear thinning composite materials.
Overall, in this thesis, we leveraged the controlled self-assembly of dynamic materials using temperature as a stimulus to develop multifunctional thermo-responsive polyelectrolytes and conductive hydrogels. These studies offer detailed insights into the self-assembly of charged macromolecules, in addition to the development of functional polymeric materials for potential applications as injectable therapeutics, shape-conformable electronics, injectable and patternable conductive interfaces, and scaffolds for neural tissue engineering.