  A number of evidences suggests that thick-film resistors are close to a
metal-insulator transition and that tunneling processes between metallic grains
are the main source of resistance. We consider as a minimal model for
description of transport properties in thick-film resistors a percolative
resistor network, with conducting elements governed by tunneling. For both
oriented and randomly oriented networks, we show that the piezoresistive
response to an applied strain is model dependent when the system is far away
from the percolation thresold, while in the critical region it acquires
universal properties. In particular close to the metal-insulator transition,
the piezoresistive anisotropy show a power law behavior. Within this region,
there exists a simple and universal relation between the conductance and the
piezoresistive anisotropy, which could be experimentally tested by common
cantilever bar measurements of thick-film resistors.
