Highly conductive, flexible, bio-compatible poly-urethane based isotropic conductive adhesives for flexible electronics

Abstract

We demonstrated a novel approach to synthesize flexible isotropic conductive adhesives (ICAs) that can not only withstand a high deformation rate but also exhibit superior electrical conductivity and adhesion strength. The ICA is made of polyurethane (PU) filled with silver flakes. It can achieve resistivity as low as 1.1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> Ω.cm at 80 wt.% loadings, which is even better than most solders. The high electrical conductivity results from 1) large shrinkage of the PU matrix during cuing; 2) the in-situ reduction of the silver carboxylate layer present on the surface of silver flakes by the selected curing agent so that direct metallic contact can be formed between silver flakes; 3) the microphase separation that is unique to PU matrix providing more conduction paths. The combination of the three above effects leads to the superior electrical conductivity that can be rarely seen in other ICA materials at equivalent loading level. In terms of adhesion, lap shear test measurements show that the adhesion strength to Cu surfaces at room temperature can reach 0.12 kg/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 80 wt% loading, equivalent to some epoxy based ICAs reported before. In addition, the developed ICAs have also demonstrated other advantages such as a low curing temperature, which enable them to be printed on low cost and flexible substrates such as paper and fabrics; simple and cost-effective processing, eliminating the usage of Ag nanoparticles to achieve high electrical conductivity; and good bio-compatibility. These superior material properties combined with low cost and simple processing make it very promising for emerging flexible electronics. A wearable antenna fabricated by printing the PU based ICAs on flexible fabrics was also presented as a demonstration of such devices.