A novel pressure sensor is proposed exhibiting generative properties from
displacement-induced ionic charge separation in gel electrolytes. A
mechano-ionic or ‘piezo-ionic’ effect, in analogy to the
well-known piezoelectric effect, is hypothesized to originate from a difference
in mobilities between cationic and anionic species causing a localized ionic
charge gradient upon application of pressure. This gradient can be detected as a
voltage or current by using copper electrodes placed at the sides or at regular
intervals along a surface of the gel. The voltage generated may be a result of
the local concentration gradient induced by the deformation of the gel or
perhaps is the result of some ions moving faster through the porous gel than
others. In this work, ionic polymer gels based on Poly(vinylidene
fluoride-hexafluoropropylene) (PVDF-HFP) co-polymer were synthesized in
situ to incorporate an organic electrolyte consisting of
bis(trifluoromethane)sulfonimide lithium salt in propylene carbonate. With two
electrodes placed under the gel, the samples were subjected to a sinusoidal
mechanical force while open circuit voltage was measured to determine the
relationship between electrical signal and mechanical input. The voltages
generated are 10’s of mV in magnitude at 1 kPa. Results suggest a
maximum sensitivity of 25 µV/Pa at 10 mHz, comparable to the voltages
expected in piezoelectric polymers such as PVDF (44 µV/Pa for similar
dimensions). The non-aqueous, solid-state ionic gels presented in this work
provide improved stability and is less prone to evaporation than its aqueous,
hydrogel based counterpart. The new mechanism of sensing provides an alternative
to the more rigid and less stretchable piezoelectric sensors.