Tunable resistive pulse-sensing is a technique to measure the size of small particles when they flow through a nano-size pore. If the medium in which the particles are suspended has ions, it is possible to induce (and measure) a current through the hole. As particles flow through it, they would block the ionic exchange and a drop in voltage can be observed.
The technique is equivalent to a Coulter Counter but at smaller scales.
The "Tunable" aspect of it, is that the nanopore can be stretched depending on the pressure that is applied to drive the flow. Which is also one of the main limitations, since different pore sizes induce different peak-height measurements, there is a continuous need for calibration. Pore-to-pore variability and degradation over time limit the ability of the technique to provide absolute numbers when measuring size of nanoparticles.
Two companies commercialise the same technology: iZon and Spectradyne.
These are the other notes that link to this one.