  We re-analyse earlier measurements of resistance R and piezoresistance K in
RuO2-based thick-film resistors. The percolating nature of transport in these
systems is well accounted by values of the transport exponent t larger than its
universal value t=2.0. Furthermore, we show that the RuO2 volume fraction
dependence of the piezoresistance data fit well with a logarithmically
divergence at the percolation thresold. We argue that the universality
breakdown and divergent piezoresistive response could be understood in the
framework of a tunneling-percolating model proposed a few years ago to apply in
carbon-black--polymer composites. We propose a new tunneling-percolating theory
based on the segregated microstructure common to many thick-film resistors, and
show that this model can in principle describe the observed universality
breakdown and the divergent piezoresistance.
