Self-healing nanocomposite dna hydrogels with high mechanical strength

DNA hydrogels are promising soft materials because of the distinctive properties of DNA such as biocompatibility, coil-globule transition, molecular recognition, and selective binding. However, such hydrogels synthesized so far suffer from poor mechanical properties and low dimensional stability, which limit their stress-bearing applications. What is more, advanced technological applications demand more sophisticated properties for materials such as self-healing ability and high recovery after applying a load or exposing an external damage. In this thesis study, the focus is to resolve these disadvantages and propose a novel concept for the preparation of high-strength double-stranded (ds) DNA hydrogels with temperature sensitivity and self-healing ability. The hydrogels were synthesized by free-radical polymerization of N-isopropylacrylamide (NIPA) or N'N-Dimethylacrylamide (DMAA) in aqueous nanoclay (Laponite) dispersions containing ds-DNA between 2-8 w/v % which is assumed to be in a highly entangled state since the critical overlap concentration c* of ds-DNA is around 0.043 w/v %. The nanoparticles of synthetic nanoclay (Laponite XLG® or Laponite XLS®) act as a dynamic cross-linker to form a three-dimensional physical network due to strong clay-polymer interactions. We observed that ds-DNA maintains its native structure and function during the formation of nanocomposite hydrogels. The hydrogels prepared at various Laponite contents display the characteristics of both the polymers poly(N-isopropylacrylamide) or poly(N'N dimethylacrylamide) and ds-DNA e.g., temperature sensitivity, self-healing ability, highly strechability, denaturation and renaturation of DNA strands, and they exhibit a high tensile fracture stress and elongation to break. A systematic series of experiments were conducted throughout the thesis study. At first, the gelation kinetics and formation of the network structure of hydrogels were monitored by rheometry in order to understand the internal dynamics of this novel gel network structure. Rheological measurements revealed the opposite effects of DNA and Laponite XLG on the viscoelastic properties of hydrogels. Within the constructed gel network, XLG acts like a chemical cross-linker and promotes the elastic behavior of the hydrogels by increasing the modulus and decreasing the loss factor. In contrast, DNA contributes to their viscoelasticity and produces hydrogels with a lower modulus and a higher loss factor, suggesting the weakening of clay-polymer interactions in the presence of DNA. Following the formation of 3D gel network structure, frequency sweep test were carried out by rheometry under isothermal conditions in order to gain a deeper understanding about the internal microstructure of hydrogels. Consecutive measurements showed that at low frequencies, nanocomposite DNA hydrogels (NC-DNA)s exhibit a lower modulus and a higher loss factor as compared to the classical nanocomposite hydrogels that is the hydrogels without DNA, revealing energy dissipation. This energy dissipation on long experimental time scales is attributed to the weak clay/DNA interactions. ds-DNA molecules in the hydrogel could follow the slow oscillation and slip on the surface of the nanoparticles by dissipating energy. At high frequencies, tan δ of NC-DNAs' is similar to that of NCs', likely because the semiflexible ds-DNA with a persistence length of 50 nm could not move with a high frequency in the gel network. Cyclic heating/cooling experiments conducted between below and above the melting temperature of ds-DNA revealed a drastic increase in the elastic modulus of nanocomposite DNA hydrogels due to the thermally induced denaturation and renaturation of DNA within the physical network. Unzipping of the helices of DNA by applying a thermal energy above its melting temperature, allow to form additional cross-linking during the cooling period by constructing H bonds irregularly.For testing the temperature sensitivity, The gel samples were heated from 20 to 50 oC at a rate of 3.25 oC min-1 during which the changes in the dynamic moduli were monitored as a function of temperature. The elastic modulus G' rapidly increases while the loss factor tan δ decreases at around 32 oC corresponding to the volume-phase transition temperature of PNIPAAM. Mechanical properties of NC-DNA hydrogels were investigated at 24 ± 1 °C by uniaxial elongation and compression tests. Because of the significant effect of the heating-cooling cycle between below and above the denaturation or the melting temperature (Tm) of ds-DNA on the hydrogel properties, the tests were carried out on the gel specimens before and after subjecting to the thermal cycles. As it is expected, a significant increase in Young's modulus and fracture stress was observed after the thermal cycle. Moreover, the stretchability of NC-DNA hydrogels is between 140 and 800% at or above 4 w/v % DNA, whereas it significantly increases with decreasing DNA content. Highly stretchable hydrogels exhibiting an elongation at break between 1100 and 1500% could be obtained at 1-4 w/v % DNA. Similar results were also obtained from uniaxial compression tests in which the same heating-cooling procedure was applied while this behavior is independent of the Laponite concentration. The quasi-reversible manner of dissociation of hydrogen bonds in NC-DNA hydrogels under stress suggests their self-healing abilities. When the two gel samples were put into contact and allowed to stand at 90 °C for 30 min followed by slowly cooling to room temperature, it was not possible to separate the two parts by stretching manually. It is interesting to note that both the Young's modulus E and the fracture stress σf of the hydrogel significantly increase after its healing at 90 °C. Enchanced mechanical properties after self-healing of NC-DNA hydrogels healed at 90 °C is due to the increase of the modulus E and fracture stress σf of DNA-containing hydrogels as observed during the heating-cooling cycles. This increment of mechanical properties as well as the self-healing behavior was not observed for the gel samples with or without DNA when the gels are healed at temperatures below 90 oC, suggesting that DNA itself faciliating the healing process by formation of bridges between the cut surfaces via the H bonds. Swelling profile of NC-DNA hydrogels also suggests that ds-DNA in the hydrogels partially releases during the initial swelling process. Thus, the hydrogel initially behaves as a polyelectrolyte, due to the osmotic pressure of phosphate counterions of DNA and attains a large swelling ratio. However, as the DNA is gradually released from the hydrogel, the ionic osmotic pressure reduces so that the hydrogel progressively changes into a less ionic hydrogel with a distinctly reduced swelling ratio. Increasing equilibrium swelling ratio mrel,eq with increasing DNA content indicates that a larger amount of DNA can be entrapped within the gel network in equilibrium with water as the DNA concentration at the gel preparation is increased. This DNA release behavior during swelling process may provide applications of NC-DNA hydrogels as DNA carriers in gene therapy methods.