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1 Introduction

Figure 1.50: The quartz crystal microbalance (QCM) consists of a quartz crystal sandwiched between two

electrodes. What is measured is the resonance frequency of the crystal, which changes depending on the

weight put upon the crystal.

The most common method to detect pressure on the micro- and nano-scale is piezoelec-

tricity. As already explained in Section 1.5, a piezo-electric material generates a current

when mechanically stressed. There are different piezo-electrical materials, and each

material can be calibrated, resulting in very sensitive pressure measurements for a wide

variety of pressures.

The most common output for sensors is a specific electrical voltage or current, or a

charge on a surface. Potentiometers of all sizes can be used to measure voltages. Static

charges can be measured by coulombmeters (Figure 1.52). Measuring the current of a

living cell, a cell membrane, or an ion channel is somewhat more complicated. For that,

the patch-clamp technique is used (Figure 1.53). Basically, an electrode in the form of a

pipet is attached to a membrane section, and the current of that electrode is compared

with the current of a reference electrode in the surrounding solution. There are some

electrodes that can also be used to measure currents inside a cell.