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A Method for Quantification of the Cleaning Performance in the Ultrasonic Bath M. Steinmann, U.B. Rosenberg*
U
ltrasonic baths (ultrasonic cleaners) are used extensively in hospitals for (pre-) cleaning complex instruments in the Central Sterile Supply Department (CSSD) as well as at decentralised locations, e. g. for cleaning ophthalmologic instruments and endoscopic ancillary instruments. Ultrasonic cleaning will also be the focus of a standardisation project to be carried out in the near future within the framework of EN ISO 15883. Not all detergents are suitable for use in an ultrasonic bath. But to date there has been no method available in the instrument decontamination setting for reproducible measurement of the cleaning performance using a process challenge device (PCD) and test soil. It was possible only to check the functional capability of the bath by means of an aluminium foil test (1), a «SonoCheck» test (2) or, at most, by means of a hydrophone (3) or a cavitation energy meter (4). However, there was no way of investigating the action generated by the cleaning process on a concrete contaminant (soil). Since ultrasonic baths are used in particular to clean complex instruments or poorly accessible surfaces, we set about finding an appropriate PCD. This is a 5-layer stainless steel mesh, in the form of discs with a 1 cm diameter and measuring 1.7 mm in thickness. Aliquots of 40 µl reactivated sheep blood were applied to such discs. The discs were cleaned individually in glass bottles with a diameter somewhat larger than that of the discs. The glass bottles were always placed in the same position in the ultrasonic bath, using a perforated plate to immobilize them and assure reproducibility. After cleaning residual soils were also extracted in the bottles in the ultrasonic bath. Thanks to the small extraction volumes, the sensitivity of the method is with-
in an ideal range. To quantify the residual contamination, the orthophthalaldehyde (OPA) method was used. With this newly developed test method it was possible to demonstrate the cleaning kinetics associated with different detergents. Under the chosen test conditions and on using detergents endowed with good activity, the cleaning course baseline (equilibrium) had been reached after around 5 min if no fixed blood was present. It was possible to demonstrate that the cleaning performance of a «biofilm cleaner» was inferior to that of an alkaline detergent and an enzymatic detergent. The alkaline detergent was most effective against heat-denatured blood. A non-fixing, aldehyde-free instrument disinfectant exhibited comparatively poor cleaning performance. Finally, it was demonstrated that a high ultrasonic energy level is not to be automatically equated with a good cleaning performance.
||Introduction
A good overview of the theoretical and practical application of ultrasound for cleaning purposes is given in the «Handbook for Critical Cleaning» (5, 6). There is also a very brief but succinct description (in German) in the abstract to a lecture by Jatzwauk (7). In a recent publication, Michels and Roth pointed out that manual cleaning of instruments with gap areas and joints did not produce reliable results if this was not backed up with ultrasound (8). That statement was based on the findings of a multicentre trial during which 20 Crile clamps that had been contaminated with sheep blood were cleaned, in each case, in nine CSSDs, in accordance with a standard op-
Key Words •• ultrasonic cleaning •• instrument decontamination •• detergent •• cleaning performance •• process challenge device •• test soil •• cavitation
erating procedure (SOP) specific to the respective CSSD. In a comprehensive review of the literature conducted some time ago, Fengler et al. reached the following conclusion: «Apparently, there are difficulties in generating reproducible effects that correlate with the clinical 'everyday contamination' found on surgical instruments. Ultrasound (US) continues to be a commonly used method, but whose action is difficult to quantify in terms of detachment of adherent surface contaminants» (9). There has been little change in that viewpoint during the past 14 years. A publication (in German) on «Improving decontamination of surgical instruments with ultrasound» by Reichl et al. presented the results of a newly developed measuring method for three-dimensional mapping of an ultrasonic cavitation field in an ultrasonic bath (10). While in this publica-
* Dr. Urs B. Rosenberg, Borer Chemie AG Gewerbestrasse 13, 4528 Zuchwil, Switzerland. E-mail:
[email protected]
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The Quality Task Group of the German Society of Sterile Supply (DGSV) has published recommendations on the use of ultrasonic baths for cleaning medical devices (13, 14). Among other information provided, they state that the filling level of the bath, temperature, type of load as well as positioning of the load play an important role. Fig. 1 shows how the ultrasonic action and chemical action are dependent on the temperature. This, in turn, demonstrates that ultrasonic action is highest at a temperature of around 70 °C, whereas the chemical action continues to increase when the temperature goes beyond 70 °C. The combined action, i. e. the cleaning performance, is greatest at around 90 °C. However, this relationship applies only for highly alkaline detergents as commonly used for industrial applications. In the case of mildly alkaline or neutral detergents, a temperature of 45 °C should generally not be exceeded so as to avoid protein denaturation (13).
That the position of the load is important is something that one can appreciate if one takes a look at the sound pressure or cavitation energy fields of ultrasonic cleaning baths (3, 10). However, their appearance and homogeneity are largely dependent on the design and quality of a device. Unlike in the hospital setting, for industrial applications devices are often used where the basket with the load moves to and fro during the cleaning process. This oscillating movement provides, on the one hand, for more uniform sonication and, on the other hand, expedites removal of partially dissolved soil particles from the surface. Another means of producing a more homogeneous sound field is by modulating or «wobbling» the frequency (5,15). As in a washer-disinfector (WD) process, the type of load is of paramount importance in the ultrasonic bath. Whereas in the WD, attention must be paid to avoidance of spray shadows, it is sound shadows that must be avoided in the ultrasonic bath. The importance of the US bath filling level referred to in the Quality Task Group's recommendation derives from the fact that sound waves are reflected at the water/air boundary surface and, as such, the sound field can change in accordance with the filling level (16). Degassing of the cleaning solution is another important factor in ultrasonic cleaning. The implosion frequency of the cavitation bubbles in a gaseous liquid is less than that in a degassed solution (5, 12). And the higher the temperature, the less the gas quantity that can be dissolved in the liquid. The choice of chemical detergents used will depend essentially on the items to be cleaned (material compatibility) and on the nature of the soils. But there is a general rule to help choose a chemical substance for the ultrasonic bath: lowering the sur-
Fig. 2: Comparative view of PCD size. The 5-layer Poremet discs have a diameter of 10 mm and thickness of 1.7 mm.
Fig. 3: Structure of PCD discs: a) protective layer, b) filtration layer, c) drainage mesh, d) support mesh
Ultrasonic action
Cleaning duration
Chemical action
Combined action
Temperature [°C] Fig. 1: Influence of temperature (according to Fuchs [5])
tion the authors alluded to investigation and quantification of the cleaning action as well as to optimisation of detergents, it was not possible to find any such references in the literature. In a recent study, Jung et al. determined the position-specific cleaning performance for titanium plates that had been contaminated with polishing paste and compared this with, likewise position-specific, cavitation sound level, measured with a hydrophone (11). Here the cleaning performance was quantified using a gravimetric technique and additionally by means of image analysis. In the latter case, photos were taken of the plates before and after cleaning, then converted to black-white images and finally the number of white pixels after cleaning was subtracted from the number of white pixels before cleaning. Thanks to these experiments the authors were able to establish a correlation between the cavitation noise level and the cleaning performance. A completely different approach was chosen by Strobel to demonstrate the ultrasonic cleaning performance. Using a galvanic technique, he applied a three-layer coating composed of nickel, tin and copper to a realistically constructed cleaning substrate (working from the inside towards the outside) and then, based on cavitation erosion of the copper, whose colour could be easily distinguished from tin and nickel, he was able to visualize the cleaning action, spatially resolved and directly, on the cleaning substrate and correlate it with the sound pressure amplitude (12).
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face tension of the cleaning solution by means of suitable, mainly foaming surfactants reduces the amount of energy needed to generate the cavitation bubbles (17). This can mean that a detergent that has an excellent cleaning performance in a WD process, will exhibit only an average or comparatively poor performance in the ultrasonic cleaner (5).
||Materials and Methods Process challenge devices (PCDs) The PCDs were made of «Poremet», which is a lead-like filter medium manufactured by G. Bopp + Co. AG in Zurich (see Fig. 2). Poremet consists of five different layers of steel mesh which are sintered together under the effects of pressure and heat (see Fig. 3). Since Poremet is primarily a filter medium, the manufacturer states that the wire cloth layers are put together so precisely that they assure an optimum combination of stability, filter fineness, flow rate and backwashing properties. For the experiments described below, Poremet Type 20 was used with a nominal filter fineness of 20 µm. The steel used was of the type DIN 1.4404 or AISI 316L. The PCD discs were cut to size with a laser, followed by reworking of the edges. Conditioning the PCDs: New as well as used PCDs were conditioned by cleaning them in the ultrasonic bath with the highly alkaline detergent combination 1.0 % deconex CIP POWER-x/0.5 % deconex 32 EMULGATOR (pH = 12.3). To that effect, the PCDs were individually sonicated in Wheaton glass bottles (Ref. WH-224885, Milian SA, Meyrin. See Fig. 4) with 2 ml cleaning solution at 60 °C for 5 minutes. After further 10-minute incubation without sonication, they were sonicated again for 5 minutes. Then the cleaning solution was poured away, the PCDs withdrawn with a forceps from the bottle and thoroughly rinsed with water. To remove fully the detergent residues, the PCDs were sonicated thrice for two minutes in each case with fresh demineralised water. The cleaned PCDs were then left to dry for one hour on open petri dishes in an oven at 60 °C . Contaminating and drying the PCDs: Aliquots of 40 µl undiluted sheep blood that had been reactivated with 1 % protamine sulphate (Acila Dr. Weidner GmbH, Weiterstadt, Order No. 2132005) were applied with a pipette to the protective layer side of the PCDs (see Fig. 5). To do this, the protective layer was touched with the tip of the pipette and the blood was slowly expelled and absorbed by the PCD. In general one portion of reactivated blood was enough to contaminate 20 PCDs, without any sign of coagulation occurring. The contaminated PCDs were dried for 24 hours in open petri dishes in an exsiccator at 30 °C in an oven. A saturated potassium carbonate solution provided for constant air humidity in the exsiccator (18). Before use, the dried PCDs were kept for up to one week in a petri dish sealed with parafilm at room temperature. For experiments where blood proteins were to be deliberately denatured, incubation took place in an oven for the first two hours at 70 °C. Then the temperature regulator was reduced to 30 °C and the PCDs withdrawn after a total of 24 hours.
Fig. 4: Elma Transsonic Digital 480/H-1 ultrasonic bath (f = 35 kHz, content = 1.9L). Stainless steel cover plate with 8 holes for accommodating the bottles with PCD discs. Bottle dimensions: external diameter = 19 mm, height = 68 mm, nominal content = 12 ml. The flat gasket (hard paper), used to rest the bottle on the stainless steel plate, is embedded between the lid and bottle neck.
Fig. 5: PCD disc under the microscope. Left: support mesh side, Right: protective layer side. The test soil was applied to the protective layer side.
Cleaning the PCDs: First, aliquots of 5 ml of the test cleaning solution (detergent concentrate diluted in demineralised water) were transferred to a glass bottle, suspended in the ultrasonic bath (see Fig. 4) and degassed while sonicating for 3 minutes at experimentation temperature. Then in each case one contaminated PCD, with the protective layer side facing upwards, was transferred upwards into a bottle and sonicated for the required time duration. On expiry of this time, the cleaning solution was poured away and replaced with 5 ml demineralised water for rinsing, and this in turn was poured away. The PCD disc was then withdrawn from the bottle with a forceps and its edges briefly wiped off with a paper towel. Then the PCDs were dried in the oven at 30 °C until extraction. In order to observe strictly the intervals needed for manipulations, for incubation times up to 6 minutes only one sample was processed in each case and this was always suspended in the same position in the ultrasonic bath. After incubating for 8 minutes, a second bottle was suspended in a second, recurrently similar, position. Evaluating the cleaning tests Extracting residual soils: PCD extraction was effected by sonicating in a glass bottle with 2 ml alkaline sodium dodecyl sulphate (SDS) solution (1.0 %, pH 12) for 20 minutes at 75 °C. This was very similar to the conditions under which Friedrich et al. had achieved an optimum
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Absorption Blutverdünnung / Blut extrahiert von PK Blood dilution absorption/blood extracted from PCD 0.3 Verdünnungsreihe Dilution series PCD extracts PK-Extrakte
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OPA-sensitive Aminogruppen vs. Hämoglobin 0.4
OPA-sensitive vs.haemoglobin Hämoglobin OPA-sensitive Aminogruppen amino groups vs. 550nm (Hämoglobin)
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Reinigungsdauer Cleaning duration[s][s] Fig. 7: Detection of OPA-sensitive amino groups compared with haemoglobin detection. For description of experiments, see text.
Water, alkaline detergent and enzymaticReiniger detergent Wasser, alkalischer und enzymatischer bei at 4545 °CºC 18 20 16 18 14 16 12 14 10 12 8 10 6 8 4 6 2 4 0 2 0
–– 0.002 cm-1 for the biofilm detergent –– 0.013 cm-1 for the enzymatic detergent –– 0.044 cm-1 for the aldehyde-free, non-fixing disinfectant Detergents The following detergents were used: –– Alkaline detergent, pH approx. 11.5 (1 % in demineralised water) Used between 20 – 60 °C –– Biofilm detergent, pH approx. 7 (1 % in demineralised water) Used at room temperature
0.15 0.2
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Determining the protein content: 50 µl or 400 µl (for a cleaning time > 2 minutes) of the extract was used for protein determination. Used here was the modified OPA method described by Köhnlein et al. (18). For each cleaning interval, at least three PCDs were cleaned and extracted. The values in the figures are average values and are given as either the absorption at 340 nm or blood quantity in µl. The standard deviations are also shown. The intrinsic extinction was determined from a solution of 50 µl sonicated SDS solution + 350 µl unsonicated SDS solution (and 400 µl sonicated SDS solution) + 600 µl OPA solution for each experiment and subtracted from the extinction of the samples. Furthermore, the extinction of eventual detergent residues on non-contaminated PCDs was measured after their extraction. Measurements were conducted in triplicate for each detergent used. The respective values were also subtracted from the samples. These were as follows: –– 0.000 cm-1 for the alkaline detergent
Fig. 6: Recovery rate: 6a) Absorption of blood dilutions and PCD extracts. 6b) Recovery rate calculated as the quotient of extract absorption/dilution absorption.
Restblut [uL] Restblut [uL] [µl] Residual blood
recovery rate of 100 % from flat test discs and 96.5 % recovery from arterial clamps (19). Since in our case the ultrasonic bath did not provide for a temperature of 80 °C, we had to limit ourselves to 75 °C.
–– Enzymatic detergent, pH approx. 8 (1 % in demineralised water) Used at 20 – 45 °C –– Aldehyde-free, non-fixing disinfectant based on alkylamine and a quaternary compound, pH approx. 11 (1 % in demineralised water) Used at room temperature The use concentration was 0.5 % in all cases except for the biofilm detergent where 1.0 % was used.
Wasser, alkalischer und enzymatischer Reiniger bei 45 °C VE-Wasser
Demineralised water Alkalischer Reiniger Alkaline detergent VE-Wasser Enzymatischer Reiniger Enzymatic detergent Alkalischer Reiniger
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Reinigungsdauer [s] and the enzymatic deterFig. 8: Kinetics of cleaning tests with the alkaline gent compared with demineralised water. For description of tests, see text.
||Results and Discussion Recovery rate Determination of the recovery rate is an indispensable precondition for evaluating the power of such cleaning tests. To that effect, the absorption values of a blood dilution series were compared with those of the extracts from PCDs that had been contaminated with different quantities of blood. The ratio of absorption of extracted blood to that of the dilutions yielded the recovery rate. To contaminate the PCDs the blood was not diluted, instead aliquots of 40, 10, 7, 5, 2 and 1 µl undiluted blood were pipetted onto the PCDs. This approach, on the one hand, assures a constant inter-
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Wasser, Biofilmreiniger und Desinfektionsmittel bei 25°C 20 18
VE-Wasser Wasser, und and Desinfektionsmittel bei Water,Biofilmreiniger biofilm detergent disinfectant at 25 25°C ºC Biofilmreiniger
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Restblut [uL] Restblut Residual blood [µl] [uL]
action with the stainless steel surface but, on the other hand, it results in different amounts of the contaminant being applied to the three-dimensional structure of the PCD. We do not know how important one or the other is. Fig. 6a shows the absorption values of dilutions and extracts, while Fig. 6b gives the recovery rates calculated from these. With one exception, these were above 90 %. On average for the entire series of tests, a 95 % recovery rate, as stipulated by EN ISO 15883-5, was not fully achieved. Nonetheless, we believe that the recovery rate achieved here is sufficient to permit conclusive results.
Desinfektionsmittel
VE-Wasser Demin. water Biofilmreiniger Biofilm detergent Disinfectant Desinfektionsmittel
18 14 16 12 14 10 128 106 84 62 40 2
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Cleaning tests with the alkaline detergent and the enzymatic detergent at 45 °C and comparison with demineralised water The results of these series of tests are summarised in Fig. 8. Whereas the cleaning results of both detergents after 1, 2, 4 and 6 minutes were markedly better than those obtained with water, the latter virtually matched the performance of the alkaline detergent after 8 minutes. However, here it must be pointed out that for this alkaline detergent 45 °C is not the optimum temperature. Besides, cleaning would never be carried out without a detergent, since if using water alone one would always have to contend with redeposition of the detached soils (redeposition effects). Overall, it can be stated that under the optimum US bath conditions employed here (absolutely no sound shadows), despite the complex structure of the PCDs, with suitable detergents the task was already completed after 4 – 6 minutes. Cleaning tests with the biofilm detergent and the aldehyde-free disinfectant at 25 °C and comparison with demineralised water As regards the biofilm detergent, we know that in the Flow Cell test it was not able to clean properly TOSI cleaning indicators that had been contaminated with artificial blood. (21).The results also showed that the disinfectant was not able to clean a TOSI-PCD in the immersion test. Conversely, such tests revealed that TOSIPCDs were properly cleaned without any mechanical support when, after incubation in this disinfectant, they were immersed in an enzymatic detergent. This means that the disinfectant does not have any protein-fixing properties. Since in an everyday set-
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Cleaning duration Reinigungsdauer [s][s] Fig. 9: Results of cleaning tests with the biofilm detergent and the aldehyde-free disinfectant compared with demineralised water. A description of the tests is given in the text. Prüfkörper 70°C getrocknet, Reinigung bei 45°C PCDs bei dried at 70 ºC, cleaned at 45 ºC 20 18
detergent Prüfkörper bei 70°C getrocknet, ReinigungAlkaline bei 45°C
Alkalischer Reiniger
Enzymatischer Reiniger Enzymatic detergent
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Restblut [uL]blood Restblut [uL][µl] Residual
Detection of OPA-sensitive amino groups compared with haemoglobin detection When using blood as a test soil, the haemoglobin quantity can also be determined as evidence of residual contamination. Within the visible light range, haemoglobin has a maximum absorption value of around 550 nm (20). To compare haemoglobin measurement with OPA protein detection, after one of the cleaning tests (neutral detergent at 45 °C) 1 ml of the PCD extracts was measured directly in the spectral photometer at 550 nm. In addition, 50 µl of each extract was used to determine the OPA-sensitive amino groups, as described under Materials and Methods (the 50 µl extract is present at the end in the 1 ml measurement solution). For the OPA method, spectral photometry measurement was performed at 340 nm. The absorption values of both measurements were combined for illustration in a graph (Fig. 7) and embarked on an almost perfectly parallel course. Since both curves were also very close to each other, this experiment suggests that the OPA method is around 20× more sensitive than haemoglobin determination.
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Fig. 10: Results of cleaning tests with PCDs dried at high temperature. For Reinigungsdauer [s] description, see text.
ting both the biofilm detergent and the disinfectant are used only at room temperature, the tests performed in the ultrasonic bath were carried out at 25 °C. Once again, demineralised water was used as a control. The results are illustrated in Fig. 9. Comparison with Fig. 8 demonstrates first of all that water at 25 °C had cleaned somewhat less well than at 45 °C, as evidenced by the somewhat less good soil detachment and slightly less cavitation (see Fig. 1). One also notes that the biofilm detergent initially cleaned better than water, but was eventually overtaken by the latter. As already noted in the case of the alkaline and the enzymatic detergent, it appears that whatever could be accomplished was done after 4 – 6 minutes, albeit, leaving behind a much greater quantity of residual blood. Noteworthy is the cleaning performance of the disinfectant, with its cleaning capacity exhausted after only two minutes, after which the blood quantity tended to rise again. It could be hypothesised that the disinfectant interacts with the blood such that the latter swells and clogs the porous structure of the PCD, even going on to intercept already dissolved soil particles, e. g. because of a certain stickiness. In any case this result clearly demonstrates that even a non-fixing instrument disinfectant is no substitute for a detergent and therefore, in particular, complex instruments should always be first thoroughly cleaned with a real detergent before disinfec-
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30 º C 70 °C drying, cleaning atbei 45 °C 30°C vs.vs. 70°C Trocknung, Reinigung 45°C Alkalischer Reiniger Alkaline detergent Enzymatischer Reiniger Enzymatic detergent 70°C -–Alkalischer Reiniger 70 °C Alkaline detergent 70°C -–Enzymatischer Reiniger 70 °C Enzymatic detergent
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Cleaning duration[s][s] Reinigungsdauer Fig. 11: Illustration of the cleaning results achieved with the alkaline detergent and the enzymatic detergent after drying the PCDs at 30 and 70 °C. Please note that the time axis is «displaced». For details of tests, see text.
Sonockeck Farbumschlag Sonocheck colour change
a Enzym. detergent45°C 45 °C Enzymreiniger Alkal.Reiniger detergent45°C 45 °C Alkal. Demin. water45°C 45 °C VE-Wasser Disinfectant25°C 25 °C Desin.mittel Biofilm detergent25°C 25 °C Biofilmreiniger
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Arbiträre ArbitraryEinheiten units
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-1 Proteinrückstand (Absorption [cm-1]) Protein residues (absorption [cm )])
Sonocheck Sonocheck
Fig. 12: a) Time until complete colour change in SonoCheck indicators. Average values of a total of four indicators based on the product of two tests. b) Protein residues on TOSI-PCDs after 15-minute incubation measured with the OPA method. Average values of, in each case, three PCDs. c) Summary of data from Fig. 12 a and b together with the results of ultrasonic cleaning tests for an 8-minute incubation time (protein residue).
tion. That finding concords with the experiences reported by Gebel with respect to investigations for validation of activities for manual instrument decontamination (22). Cleaning tests performed on PCDs with hot, dried blood Ultrasonic cleaning is resorted to when particularly stubborn soils are encountered. In one series of experiments in order to pose a greater challenge to the detergent, the blood-contaminated PCDs were dried at 70 rather than at 30 °C. It was expected that this would denature at least some of the proteins. But it is thought that fixation is not so pronounced here as it would be if one were to dip the PCDs into boiling water or glutaraldehyde. As soon as all the water has evaporated from the test soil during drying, denaturation will not be able to continue. The PCDs that had been dried at this high temperature were then cleaned with the alkaline and the enzymatic detergent at 45 °C. The results are given Fig. 10. As expected, it was more difficult to clean these PCDs than those harbouring non-denatured blood. Nor was it any surprise that the alkaline detergent was better able to clean this soil than its enzymatic counterpart. It is thought that when the cleaning temperature was increased to 60 °C, the former was able to further enhance its performance. Fig. 11 illustrates as a combined curve the cleaning results for the alkaline and the neutral detergent for both types of drying. With this form of depiction, and despite the fact that the time axis is «displaced», one can clearly see that for 30 °C drying the cleaning process baseline or equilibrium is reached, whereas for 70 °C drying, cleaning would still continue after 15 minutes – with the enzymatic detergent too. However, in everyday practice such prolonged cleaning times in the ultrasonic bath are not realistic. Nor should they be recommended. One must not forget that cavitation can attack not only soils but also material surfaces. Relation between ultrasound energy and cleaning performance Using SonoCheck indicators (2), the relative ultrasound energy was measured in the application solutions of the six detergents and disinfectants used. The measure of this was the time needed for a complete colour change in the indicator solution. In addition TOSI-PCDs were used to determine the purely chemical blood detachment of the product solutions in immersion tests. To that effect, the residual contamination was quantified after 15-minute incubation by means of SDS extraction and protein measurement with the OPA method. Both the SonoCheck and TOSI results are shown in Fig. 12 a and 12 b. Fig.12 c shows a summary of these results together with the ultrasonic cleaning results obtained for an exposure time of 8 minutes. The values on the Y axis are arbitrary units. The patterns of the results were compared, and hence the following conclusions could be drawn: –– Ultrasonic energy in water was greater at 25 °C than at 45 °C. This finding contradicts the «conventional wisdom» (Fig. 1). –– Ultrasonic energy in detergents and disinfectants was greater than in water. –– Despite having achieved greatest ultrasonic energy, the biofilm detergent and the disinfectant performed markedly less well in the ultrasonic bath than did water, while the alkaline and the enzymatic detergent performed much better than water. –– The general pattern of results obtained for the TOSI immersion tests was similar to that of the US cleaning tests: the biofilm
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detergent and disinfectant performed less well than water, while the alkaline detergent and the enzymatic detergent were better than water. However, a closer look revealed differences, which were particularly manifest in the case of the disinfectant compared with the biofilm detergent. The possible reasons for the disinfectant’s poor US cleaning performance have been discussed above. –– A faster colour change in the SonoCheck indicator or a high level of ultrasonic energy does not automatically imply a good cleaning result. This is evidenced most clearly on comparing the SonoCheck results with the US cleaning performance achieved by the disinfectant. –– The SonoCheck indicator as well as the cavitation measuring instruments can be used to measure the mechanical action or principal functional capability of an ultrasonic bath, but not the cleaning performance itself.
||Final remarks
With this study, using a suitable process challenge device consisting of sintered stainless steel layers and reactivated sheep blood as test soil, we have shown that it is possible to demonstrate the cleaning kinetics of an ultrasound process. We are aware that the tests described here were performed under ideal conditions. However, such conditions are not rare today in industrial component cleaning, but are hardly assured in the case of instrumentation decontamination in the CSSD. But the standard of ultrasonic cleaning could also be improved in hospitals if systems with basket movement and frequency modulation were used and if attention was also paid to assuring a proper, reproducible load. Not all aspects of our tests worked. For the tests using brain homogenate as test soil (dried at 30 °C) we were not able to depict any cleaning course on using an al-
kaline SDS solution, as had been the case for sheep blood. Already after 30-second incubation, the baseline for OPA-sensitive amino groups had been reached. Even with water, that was the case after 2 minutes. One could now speculate about the whereabouts of the fat (at least when cleaning with water alone), since fat cannot be detected with the OPA method. These results presumably indicate that a lattice structure like the fibrin scaffold of the blood is much more difficult to dissolve from a complex cleaning substrate such as the PCDs used here than is a nonlattice soil, also when the latter is bound more tightly to surfaces, as demonstrated in cleaning tests using less mechanical action in the flow cell model (21). ■
||References see p. 105
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