N-Isopropylacrylamide-based composite hydrogels containing carbon nanotubes: synthesis by two-step photopolymerization and characterization

Biocompatible and conductive polymer hydrogels are the subject of intensive research in the bioengineering field because of their use in bioelectronic devices and for the fabrication of electro-responsive tissues and drug delivery systems. In this study, we report the synthesis of conductive nanocomposite hydrogels consisting of a poly(N-isopropylacrylamide) (pNIPAM) matrix embedding carboxyl-functionalized multi-walled carbon nanotubes (MWCNTCOOH) using a novel two-step photopolymerization method. Functional hydrogels with controlled hydrophilicity and conductivity were prepared varying the carbon nanotube concentration in the range 0 - 3 wt. %. The thermo-responsive properties of the NIPAM-based nanocomposite hydrogels were measured by differential scanning calorimetry with both ultrapure water and PBS solution as swelling liquid. Results show that the endothermic peak associated with the temperature-dependent volume phase transition (VPT) shifts to higher temperatures upon increasing the concentration of the nanotubes, indicating that more energy is required to dissociate the hydrogen bonds of the polymer/filler network. In PBS solution, the swelling ratios and the VPT temperatures of the nanocomposite hydrogels are reduced as a result of salt-induced screening of the oppositely charged polymer/filler assembly, and the electrical resistivity decreases by a factor of 10 with respect to the water-swollen hydrogels.