Development of flexible, durable and ionic materials based on poly(acrylamide) hydrogels for soft conducting and sensing applications

Soft ionic hydrogels have garnered significant interest for their applications in soft electronics and tissue engineering. However, further demands are still on the rise for developing these materials to possess flexibility, durability, low cost, non-toxic and reliable conductivity. In this work, a poly(acrylamide) (PAAm) hydrogel containing salt was utilised for its significant features such as high flexibility and excellent conductivity. Therefore, several hydrogels were prepared from the polymerisation reaction of the mononer acrylamide (AAm) to produce different polymers networks of PAAm hydrogels by the use of different crosslinking materials and methods aiming to optimise their mechanical and electrical characteristics, with the aim of applying these hydrogels in different applications such as soft sensing and conducting devices. Ionic-covalent entanglement hydrogels were prepared by mixing cross-linked gellan gum (GG) and CaCl2 ionically with PAAm and methylenbis(acrylamide) (MBAAm) covalently. The mechanical behaviour was modified by altering the ionic and the covalent polymers ratio. The electrical properties were investigated with varying hydrogel ratios which displayed optimised mechanical properties for use in conducting and sensing applications. It was observed that gels prepared with 0.1 M CaCl2 and 1.11 % (w/v) GG with PAAm consisting of 4.44 % (w/v) and AAm with 3 % (w/v) MBAAm exhibited optimum mechanical characteristics reporting 216±12 kPa (compressive stress to failure) for the compression test analysis and 264±5kPa (shear modulus) for the oscillatory rheology demonstration. The electrical conductivity and the water content for the optimised ICE gel displayed a noticeable increase from 3.3±0.5 mS.cm-1 to 127±15 mS.cm-1 and from 78 % to 85 %, respectively, after it was immersed in 2.7 M NaCl solution.