4142 | Soft Matter, 2016, 12, 4142--4153 This journal is © The Royal Society of Chemistry 2016 Cite this: Soft Matter, 2016, 12, 4142 A study of conductive hydrogel composites of pH-responsive microgels and carbon nanotubes† Zhengxing Cui, a Mi Zhou, b Paula J. Greensmith, a Wenkai Wang, a Judith A. Hoyland, bc Ian A. Kinloch, a Tony Freemont bc and Brian R. Saunders* a 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 pK a 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. Introduction The interest in hydrogels has been growing rapidly, 1,2 due to remarkable improvements in gel modulus, 3 ductility, 4 swelling ratios 5 and toughness 6 which have great importance for soft matter science, tissue engineering and regenerative medicine. 7 Moreover, the range of properties available increases greatly for hydrogel composites. 8 However, the overwhelming majority of hydrogel-based research has used small molecules (monomers and crosslinkers) for construction of the gel component which contrasts to the approach used in the present study which involved microgel (MG) particles. Most hydrogels are intrinsically brittle. To overcome this limitation a number of toughened hydrogels have been established which include double network hydrogels, 9 polyampholyte gels 10 and hybrid gels of polyacrylamide and ionically crosslinked alginate. 11,12 In each case a network of sacrificial crosslinks provided energy dissipation which opposed crack propagation. Indeed, the development of toughened hydrogels has enabled a key problem that plagued hydrogel composites to be addressed; that is, the tendency of high modulus particulates to cut through soft hydrogel networks when strained. 13 Toughened hydrogels have also enabled new fabric-reinforced hydrogel composites to be prepared that have potential for use as prosthetics. 14 Here, we construct a new family of gel composites that have improved mechanical properties when carbon nanotubes are present without the use of a special toughening methodology. MG particles are crosslinked polymer particles that swell when the pH approaches the pK a of the particles. 15 Here, these MGs are termed singly crosslinked microgels (SX MGs) because they are internally crosslinked. Vinyl-functionalised SX MGs a School of Materials, The University of Manchester, MSS Tower, Manchester, M13 9PL, UK. E-mail: Brian.Saunders@manthester.ac.uk b Centre for Tissue Injury and Repair, Institute for Inflammation and Repair, Faculty of Medical and Human Sciences, University of Manchester, Oxford Road, Manchester, M13 9PT, UK c NIHR Manchester Musculoskeletal Biomedical Research Unit, Manchester Academic Health Science Centre, Manchester, UK † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6sm00223d Received 27th January 2016, Accepted 6th April 2016 DOI: 10.1039/c6sm00223d www.rsc.org/softmatter Soft Matter PAPER Open Access Article. Published on 06 April 2016. Downloaded on 6/6/2019 10:50:09 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue