A study of conductive hydrogel composites of pH-responsive microgels and carbon nanotubes

Abstract

Conductive gel composites are attracting considerable attention because of their interesting electrical and mechanical properties. Here, we report conductive gel composites constructed using only colloidal particles as building blocks. The composites were prepared from mixed dispersions of vinyl-functionalised pH-responsive microgel particles (MGs) and multi-walled carbon nanotubes (CNTs). MGs are crosslinked pH-responsive polymer colloid particles that swell when the pH approaches the pKa of the particles. Two MG systems were used which contained ethyl acrylate (EA) or methyl acrylate (MA) and around 30 mol% of methacrylic acid (MAA). The MA-based MG is a new pH-responsive system. The mixed MG/CNT dispersions formed thixotropic physical gels. Those gels were transformed into covalent interlinked electrically conducting doubly crosslinked microgel/CNT composites (DX MG/CNT) by free-radical reaction. The MGs provided the dual roles of dispersant for the CNTs and macro-crosslinker for the composite. TEM data showed evidence for strong attraction between the MG and the CNTs which facilitated CNT dispersion. An SEM study confirmed CNT dispersion throughout the composites. The mechanical properties of the composites were studied using dynamic rheology and uniaxial compression measurements. Surprisingly, both the ductility and the modulus of the gel composites increased with increasing CNT concentration used for their preparation. Human adipose-derived mesenchymal stem cells (AD-MSCs) exposed to DX MG/CNT maintained over 99% viability with metabolic activity retained over 7 days, which indicated non-cytotoxicity. The results of this study suggest that our approach could be used to prepare other DX MG/CNT gel composites and that these materials may lead to future injectable gels for advanced soft-tissue repair.

Rational design of microfabricated electroconductive hydrogels for biomedical applications

“…CNTs are nanostructures comprised of a cylindrical lattice of carbon atoms arranged either as single-walled or multi-walled nanotubes. CNTs have gained significant interest due to their unique properties, including high compressive and tensile strength, as well as high electrical conductivity [70,72,73,[101][102][103]. CNTs have been shown to be useful for engineering ECHs due to their ability to reduce brittleness and significantly increase electrical conductivity.…”

Functional Microgels: Recent Advances in Their Biomedical Applications

“…Cui et al designed conductive gel composites of vinyl-functionalized pH-responsive microgel particles and multiwalled carbon nanotubes (CNTs). [193] The mixed dispersions of microgels and CNTs formed thixotropic gels which showed increment in modulus and ductility with increasing CNT concentration. Exposure of human adipose-derived mesenchymal stem cells to the composites showed 99% viability exhibiting the potential of the developed system for injectable gels for advanced soft-tissue repair.…”

“…The microgels were found to be biocompatible and the mechanical properties could be tuned based on the amount of added microgels. Cui et al designed conductive gel composites of vinyl‐functionalized pH‐responsive microgel particles and multiwalled carbon nanotubes (CNTs) . The mixed dispersions of microgels and CNTs formed thixotropic gels which showed increment in modulus and ductility with increasing CNT concentration.…”

Drug release kinetics of pH‐responsive microgels of different glass‐transition temperatures

“…Most hydrogels are intrinsically brittle and to overcome this limitation, a number of toughened hydrogels, such as double network hydrogels, polyampholyte gels, and hybrid gels of polyacrylamide ionically crosslinked by hydrophobic moiety have been developed. In all these systems, a network of sacrificial crosslinks provides energy dissipation that opposes crack propagation, which reduces the brittleness of the hydrogels . In addition, it is possible to develop responsive gels, whose swelling characteristics and transitions can be manipulated by pH.…”

“…In all these systems, a network of sacrificial crosslinks provides energy dissipation that opposes crack propagation, which reduces the brittleness of the hydrogels. 8 In addition, it is possible to develop responsive gels, whose swelling characteristics and transitions can be manipulated by pH. One of the possible methods to control the pH responsive properties of the gels is to introduce hydrophobic segments that serve as physical crosslinks by copolymerizing a hydrophilic monomer, such as acrylic acid with hydrophobic alkyl acrylate monomer.…”