Ultrasound is a well-known cleaning method which has stood the test of time and
is used in precision mechanical workshops and also for surgical
instruments. Indeed in some Anglo-Saxon countries surgical
instruments are cleaned in an ultrasonic bath followed by manual or
Ultrasound waves are mechanical waves that are emitted in liquid media and whose
intensity decreases in accordance with the square of the distance.
They are produced by
ULTRASONIC TRANSDUCERS operating with either magnetic or electrical energy
(piezoelectric effect). The ultrasound waves are broken on solid
objects and walls, thus giving rise to ultrasonic shadowing. The
ultrasonic baths customarily used in medical technology operate in
the frequency range between 32 - 50 kHz.
The ultrasonic effect is based on what is know as cavitation, occurring
mainly at the boundary surfaces between water and a solid object.
Here cavities are formed containing gas at a negative pressure. When
these cavities collide with each other
SUCTION PRESSURES of up to 100 bar are generated and result in
the removal of soil particles from solid surfaces. This effect cannot
be generated in the case of soft objects, e.g. materials made of
rubber, latex and silicone rubber. Recent investigations have
demonstrated that distribution of the energy in a bath can vary
greatly, thus producing different cleaning results.
Damage can occur during ultrasonic treatment. Therefore the general
guidelines must be observed. To avoid an adverse effect on metallic
parts, the frequency range should be complied with. The sonication
period should be restricted to around 5 min, but this depends on the
energy input. For example for ultrasonic cleaning of biopsy forceps,
30 min are recommended. Under no circumstances should optics, fibre
optic cables and multiply coated instruments be cleaned with
ultrasound. In the latter case there would be a risk of the coating
being detached. In addition, the
MANUFACTURER'S INSTRUCTIONS must be observed.
To reduce the surface tension and promote gas removal, a
SPECIAL CLEANING AGENT must be added. It is important that the
gas bubbles should emerge from the liquid. The cleaning agent should
produce little foam, especially if cleaning is followed by automated
processing. Otherwise, the item to be sterilised must be rinsed after
removal from the ultrasonic bath. Surfactant cleaning agents with or
without enzymes should be used preferably. But there are also
suitably acidic or alkaline agents that can be used in the ultrasonic
bath. On using these agents, their efficacy and the suitability of
the material must be checked.
Under no circumstances should chlorine-based cleaning agents be used. It
must be borne in mind that normal domestic cleaning agents may
contain common salt so as to ensure viscosity. Hence chlorine may be
released in the ultrasonic bath, thus damaging instruments and
stainless steel baths. Therefore domestic cleaning agents, such as
e.g. wash-up agents, should not be used either.
Ultrasound per se is not endowed with a
DISINFECTANT EFFECT Instrument disinfectants recommended for
immersion baths are often unsuitable for use in an ultrasonic bath.
The increase in temperature expedites coagulation processes between
proteins, aldehydes and quaternary ammonium compounds. This can
result in adherence of solid residues to instruments in which
microorganisms can survive, while the bath solutions show only low
microbial counts. Appearing on the list of the
German Society for Hygiene and Microbiology
(DGHM) are only disinfectants intended for use in immersion
procedures at 20°C, and which have
been tested under the conditions applicable to the immersion bath.
There are no certified instrument disinfectants or combined cleaning
and disinfectant agents that have been tested as per the DGKH methods
under the conditions applicable to the ultrasonic bath. But there are
combined cleaning and disinfectant agents specially formulated for
use in the ultrasound bath.
To protect personnel it must also be possible to inactivate the
hepatitis B virus within the relatively short exposure times. But as
a rule this cannot be guaranteed. Therefore, despite the use of a
MEASURES FOR PERSONNEL PROTECTION must be taken such as avoidance of
unnecessary handling, gloves, protection against splashing, etc. In
the case of ultrasonic baths integrated in washer-disinfectors, e.g.
in tunnel washers, the addition of suitable cleaning agents is
sufficient in any case because this step is followed by thermal
disinfection without the sterile items being handled. If
aldehyde-based products are used the ultrasonic bath should be fitted
with a lid to reduce escape of aerosols into the ambient air and
1 This article focuses primarily on the basic principles for using ultrasound. Loading instructions and experience from the practical setting will be given in the next issue of Central Service.