Ultra-elastic conductive silicone rubber composite foams for durable piezoresistive sensors via direct ink writing three-dimensional printing

Conductive nanomaterial/silicone composite foam with stable electrical conductivity, high porosity and ultra elasticity is an ideal flexible material in sensor field. High porosity of composite foams has been achieved through direct ink writing (DIW) three-dimensional (3D) printing. However, low thixotropic properties of printed inks hinder the realization of complex, high-resolution 3D porous structures. On the other hand, the distribution of nanofillers in composite foams make it hard to simultaneously obtain stable electrical conductivity and outstanding elasticity. Herein, ultra-elastic multi-walled carbon nanotube (MWCNT) / silicone rubber foams with stable electrical conductivity and high hierarchical porosity were fabricated through DIW 3D printing. Complex shaped and high-resolution 3D printed porous scaffold structures were achieved through a high-performance printing ink which was a water-in-oil Pickering emulsion fabricated from the emulsification of MWCNT aqueous dispersion in a silicone precursor through a solid emulsifier amphiphilic SiO2 nanoparticles. Combining highly hierarchical porosity with unique distribution of MWCNTs, the 3D-architectured MWCNT/silicone rubber foams exhibit excellent stretchability (156 % strain), ultra-low compression modulus of 0.73 KPa and outstanding compressibility/cycling endurance (near-zero stress/strain loss under 1000 compression cycles). Excellent piezoresistive performance, including rapid response time (180 ms) and high linear sensitivity (3.32 KPa-1) over a broad working range (27-900KP), is demonstrated for such foams, together with prominent durability (18000 compression cycles at 200 KPa). A wearable piezoresistive sensor assembled from the as-prepared MWCNT/silicone rubber foam could capture various movements from wrist bending to small deformation resulted from human pulse.