Shock waves are also used for disrupting samples, with ultrasonication being the best example. Shock is caused by a rapid change in pressure. Several tools are available that disrupt samples by using pressure differential.
Sonication: Sonication (also referred to as ultrasonication) is one process by which samples can be disrupted by pressure. In this case, the pressure is created by a probe that rapidly expands and contracts at high frequencies. The probe undergoes a high frequency oscillation due to the piezoelectric effect, a phenomena that occurs when oscillating current is applied to certain crystals, such as quartz and sodium potassium tartrate. (See link for further explanation on piezoelectric effect.) When a current is applied to these crystals, they contract while reversing the current causes them to expand. Rapid oscillation of the current causes tiny shock waves. Sonicators are designed with the crystals being attached to a metal probe so that the energy of the shock waves is focused to a small area.
The energy coming from the tip of a sonicator is extreme. Anyone misguided enough to touch the tip of a sonicator knows it is like touching red hot metal. Ultrasonic probes and baths oscillate up and down at 20,000 cycles/second, though the amplitude of the oscillation is very short. As liquids cannot flow as fast as crystals oscillate, small vacuum cavities are formed during the contraction. When the crystals expand, the cavities rapidly implode and create microscopic shock waves. This process, known as cavitation, is extremely powerful when the collective energy of all the imploding cavities is combined. The cavities are formed and collapse in microseconds.
Strengths – For cell suspensions and microorganisms, sonication is probably the most effective homogenization method. The extremely powerful forces generated by cavitation are capable of disrupting most cell samples in seconds.
Limitations – As effective as sonication can be, it is also has the greatest limitations. It is a powerful method if the sample contains small particles, i.e., either cells or homogenized tissue. For solid tissue, sonication is a very poor method (Fig. 15). Sonication also generates a tremendous amount of heat, which can denature many proteins. This can be offset by short bursts coupled with incubations on ice, but that becomes laborious and time consuming. Throughput is also an issue for sonication. If cross-contamination is not an issue, then a single probe sonicator can be used on multiple samples one at a time. There are multi-probe sonicator heads, but the sample to sample variability with this system is unknown.
Figure 15. Sonicator with micro probe tip yielded the lowest efficiency of all methods on muscle at 2.6% (see Fig. 20).