Conductive hydrogel preparation and evaluation
Single-walled carbon nanotubes (CNTs; 76.9 mg, P3, 3-6-wt% R-COOH; Carbon Solutions Inc., Riverside, CA, USA) with a bundle diameter of ~4–5 nm and a bundle length of ~1 µm were purified with nitric acid and added to an aqueous solution of a nonionic, noncytotoxic surfactant Pluronic F127 BioReagent (10 mL, 0.1-wt%; Sigma-Aldrich, Merck KGaA, Darmstadt, Germany). After heating for 8 hours at 70°C, the mixture was sonicated for 16 hours creating a 0.77-wt% dispersion of CNTs. Nanofibrillated cellulose (NFC; 2 g≈2 ml, 3-wt%, carboxymethylated nanocellulose, charge density ~515 μeq/g; Innventia AB, Stockholm, Sweden) was Electron Beam sterilized and then combined with 2 ml of the dispersion using a SpeedMixerTM (FlackTek Inc., Landrum, SC, USA) at 2000 rpm twice for 2 minutes each. The result was a homogeneous hydrogel with dry matter of 20-wt% and NFC:CNT dry weight ratio of 4:1. Our method for assessing conductivity of the hydrogel has been described previously(12).