Soft conducting hydrogel materials

Soft conducting hydrogels present a solution to many current problems in softrobotics such as electrical and mechanical compliance matching. Gellan gum (GG), a gel-forming polysaccharide, was assessed in this project for its suitability at forming composite materials with electrically conducting carbon fillers such as nanotubes and nanofibres. This research shows that GG solutions are capable of efficiently dispersing carbon filler materials with the aid of probe sonication. Vapour-grown carbon nanofibres (VGCNFs), however, were shown to disperse more readily than either single- or multi-walled carbon nanotubes (SWNTs/MWNTs). Excessive sonication proved to be damaging to electrical and mechanical properties, with nanofibres breaking and being subsequently filled with the GG dispersant. Modified versions of the optimised VGCNF-GG dispersion were capable of being directly written onto a paper substrate with the aid of a commercially available fountain pen. Cross-linked composite GG hydrogels were electrically characterised through use of a custom-built impedance set-up. They exhibited the behaviour of a Warburg diffusion element in series with a resistor, which allowed for equivalent circuit modelling of impedance data to determine electrical properties. Using this characterisation method, conducting GG hydrogels containing combinations of MWNTs, VGCNFs and the conducting polymer Poly(3,4–ethylene-dioxythiophene)- poly(styrenesulfonate) (PEDOT:PSS) were prepared with optimum electrical properties. It was shown that composite PEDOT:PSS-VGCNF-GG hydrogels exhibited electrical conductivity values of 107 ± 6 mS/cm at a water content of 81% (swelling ratio = 9). The work described in this thesis serves to increase the knowledge of soft conducting materials and the development of a new method of electrical impedance characterisation. Recorded electrical and mechanical properties suggest that suitability for soft robotic applications can be achieved through the methods described in this research.