  Conductive elastomers present desirable qualities for sensing pressure
in-vivo, such as high piezoresistance in tiny volumes, conformability and,
biocompatibility. Many PDMS-based electrically conductive nanocomposites
however, are susceptible to electrical drift following repeated stress cycles
and chemical aging. Innovative approaches are needed to stabilize their
percolation network against deformation to improve reproducibility and
facilitate sensor calibration. One approach we propose here is to decouple the
tunnelling-percolation network of HOPG nanoparticles from the incomplete
viscoelastic recovery of the PDMS matrix by inserting minute amounts of
insulating SiO$_2$ nanospheres. SiO$_2$ nanospheres effectively reduce the
number of nearest neighbours at each percolation node switching off the
parallel electrical pathways that might become activated by incomplete
viscoelastic relaxation. We varied the size of SiO$_2$ nanospheres and their
filling fraction to demonstrate nearly complete piezoresistance recovery when
SiO$_2$ and HOPG nanoparticles have equal diameters ($\approx$400nm) and
SiO$_2$ and HOPG volume fractions are 1% and 29.5% respectively. We demonstrate
an in-vivo blood pressure sensor based on this bi-filler composite.
