INVESTIGATION OF DNA SEQUENCE-DIRECTED REVERSIBLE SWELLING OF DNA-CROSSLINKED HYDROGEL

Stimulus-induced shape change of hydrogels is a technology critical to the development of many soft microscale robots. As a result, the study of stimulus-induced hydrogel shape change has attracted substantial interest from researchers in diverse fields, especially in biomedical research. The shape change induced by DNA molecules with specific sequences, unlike the shape change induced by nonspecific stimuli (UV light, pH, and temperature), can be highly selectively controlled, allowing control of hydrogel swelling akin to the control achieved over electronically controlled robots. DNA-crosslinked hydrogels can recognize and respond to specific these sequences of nucleotides and can be copatterned to assemble complex devices; this technology shows great potential for building smart machines. In this thesis, a novel DNA-controlled reversible swelling hydrogel system was investigated. Hairpin-shaped DNA single strands are inserted into DNA crosslinks through strand displacement reactions sequentially to elongate the crosslinks, and the size of the hydrogel increases after this addition. The inserted hairpins can hybridize with reversals to be removed from the crosslinks to realize contraction in size. To better understand the swelling process, we designed mass balance experiments to measure the amount of DNA hairpins added to a hydrogel during swelling and removed during hydrogel contraction. This measurement was performed by measuring the concentration of the hydrogels remaining in the solution at different points during swelling and then calculating the hydrogel's hairpin intake as the decrease in the concentration remaining in solution over time. Comparing the rates of hairpin intake and the rates of hydrogel swelling allowed us to conclude that 1) the concentration of DNA crosslinks in the gel have an enormous influence on the speed of hairpin uptake and the equilibrium extent of swelling, 2) higher DNA hairpin concentrations lead to a larger final hydrogel size, and 3) the osmotic pressure has an impact on the strand displacement reaction. We also applied the Maxwell model of polymers with the goal of understanding why 1) the degree of swelling declined slightly with the number of reaction cycles, 2) more significant swelling results in a less elastic polymer with the same system and concentrations of DNA crosslinks, and 3) the time lag between hairpin intake and hydrogel size change. These ideas will help us better understand the process of reversible swelling of DNA-crosslinked hydrogels and suggest means to improve its speed and efficiency during further studies