Cleanability of Metal Surface Finishes Found in Medical Devices and the Environment of Care

Reusable medical devices are produced in various sizes and designs, which affects the choice of cleaning agents and the cleaning process. With increasing emphasis placed on proper cleaning, medical device manufacturers have the responsibility to provide clear and comprehensive cleaning instructions to support reusability claims.¹ 

The surface finish of reusable medical devices often is a consideration during design, with attention paid to utility, aesthetics, and cost. Studies have been performed in food industries to investigate the effect of surface roughness and/or surface defects on cleanability. Conclusions have been mixed, with some researchers indicating that surface roughness and topography may influence cleanability, while others have concluded that little difference exists for different surface finish types and surface finishes with average roughness (Ra) below 0.8 μm.^2–4 Ra is a quantification of surface roughness and is measured by the difference in height of the peaks and valleys from the average height. For stainless steel, an Ra of 0.8 μm has been reported to be a cut-off level, below which surfaces are assumed to be “hygienic.”² 

On the clinical side, Ali et al.⁵  observed that higher surface roughness and surfaces with irregularities may be more difficult to clean and disinfect in bed rails. Gonzalez et al.⁶  noted that rougher polymeric materials tend to retain more blood soil; however, surface roughness did not have a considerable effect on bacteria adhesion to the surface if no test soil was present.

However, cleanability of different surface finishes and roughness for medical devices and surfaces found in the environment of care remain relatively underexamined. In the current article, the ease of cleaning and removal of blood soil from metal surfaces of varying roughness and finish was investigated. For reference, reusable and single-use surgical scissors and needle holders have been reported to have Ra between 0.1 and 0.9 μm.⁷  The test soil is formulated to simulate patient-derived soils, modified from ASTM F3208-20:2020.⁸  Evaluation methods for cleanliness include visual inspection and adenosine triphosphate (ATP) testing. ATP testing was chosen because of its ease of use, real-time feedback, and quantitative evaluation.^9,10  This study sought to identify differences in cleanability of various surface textures and identify best practices. Therefore, ATP was chosen as the objective method of evaluation.

The main surface used in this study was a 22-swatch surface roughness comparator made of nickel alloy (S-22 Microfinish Comparator Surface Roughness Scale; GAR Electroforming, Danbury, CT). The soil used is bovine blood (Rockland Immunochemicals, Inc., Limerick, PA) mixed with fresh egg yolk in a 1:1 weight ratio.

A Gardner-scrub Abrasion Tester (Paul N. Gardner Company, Pompano Beach, FL) was used for the cleaning study. A customized holder made of stainless steel was used to hold the microcomparators in place while they were being wiped. Nylon Brush Testers (Paul N. Gardner Company) were put in each of the brush holders. The mass of each holder was set at 110 ± 1 g in order to ensure a consistent wiping force of approximately 1.08 N.

The ATP monitoring system used was a SystemSURE Plus (Hygiena, Camarillo, CA) luminometer. For the liquid system, AquaSnap Total (Hygiena) was used for measuring total ATP. For solid surfaces, UltraSnap (Hygiena) was used for measuring total ATP. All results were read by the luminometer within five seconds of activation. Absolute ATP readings are reported in relative light units (RLU).

CaviWipes 2.0 (Metrex Research, LLC, Orange, CA), which are commercially available wipes containing 17.2% isopropanol and 0.28% quaternary ammonium compounds, were used as cleaning agents in this study.

Because of the relatively quick degradation of blood, ATP measurement of pure blood diluted to 1% and 0.1% was measured using a commercially available ATP surface test swab. This measuring occurred at the beginning of the day before any runs were performed. Dilution was done with saline solution. The ATP swab has a maximum reading of 9,999. To ensure that blood is suitable for use in this experiment, the average ATP reading at 1% dilution must be greater than 9,000 RLU, with at least one of the readings at 9,999 RLU.

Fresh soil of bovine blood mixed with egg yolk was prepared every morning and kept in a 34.5°C water bath throughout the day. Immediately before every run, protamine sulfate was added to the soil at a 1:20 ratio and warmed for two minutes.

After warming for two minutes, 5 μL of the mixture was pipetted onto the tile of interest and left to dry and coagulate for 15 minutes. Figure 1 shows the setup with the soil on the microcomparators. One wipe (6 in × 6.75 in) then was folded in half along the long edge and put under the brushes before being drawn across the microcomparators. After every half cycle (one pass), the abrasion tester was manually paused and the wipe replaced with a new wipe. Every cycle consisted of one back-and-forth motion. The speed was kept constant at 10 cycles/minute. At the first wipe pass, the operator ensured that the droplet was aligned with the brush. If it was not aligned, the run was repeated from the beginning.

Eight passes were completed for every run. After every pass, a new wipe was used. This was done to prevent recontamination of the test spot, in order to reduce noise in the results. After the eight passes, the surface was left to air dry for two minutes before the soiled area was swabbed for ATP measurement. After swabbing the test area, the swab was placed in the luminometer for ATP reading. Visual inspection via the unaided eye was also performed to determine if any visible soil was left before swabbing. If the operator was unsure whether soil could be detected or if a discrepancy occurred between ATP reading and initial visual inspection, an optical microscope was used to confirm presence of soil.

To ensure the absence of cross contamination, only one tile on each of the three microcomparators was tested during each run.

Based on the technical bulletin published by Hygiena, the recommended passing ATP reading for operating rooms and intensive care units is less than 10 RLU.¹¹  As stated in the ATP measurement system guide, a fail limit of three times the pass limit was used.¹²  Therefore, the fail limit was considered to be more than 30 RLU. A caution level was established as the readings between the pass and fail limits. The grading criteria are shown in Table 1.

The surface finish and Ra values stated on the comparators are only nominal values. The microcomparators purchased were calibrated and are traceable via National Institute of Standards and Technology standards. The actual Ra values for the comparators are shown in Figure 2. The finishes in this study included lapped, ground, shape turned, milled, and profiled.

As shown in Figure 2, discrepancy exists between the expected (nominal) Ra and the measured, actual Ra. The biggest discrepancy is at 500ST, where the actual Ra is approximately 300 μin. It is also lower than 500M and 500P. This shows that different machining processes may result in different final Ra and may affect overall cleanability.

To ensure the wettability and sufficient adherence of the soil on all textures, a DSA100 Drop Shape Analyzer (KRÜSS Scientific, Hamburg, Germany) was used to visualize the effectiveness with which the soil spread on various surfaces after at least 15 minutes of drying. This is consistent with the time we allowed the soil to sit on the surface before wiping. We also measured the contact angle of the soil on each surface. Three measurements were performed on each surface of each comparator, for a total of nine measurements across three comparators.

Two wiping directions were investigated (Figure 3): wiping in the direction of the long edge of the microcomparator (horizontal; Figure 3A) and wiping in the direction perpendicular to the long edge (vertical; Figure 3B). For the horizontal direction, six repeats occurred (i.e., three repeats per run). For the vertical direction, six repeats occurred (i.e., one repeat for each run).

Because of the potential variability in ATP readings,^{13, 14}  we performed two types of data analysis: (1) determining percentages of readings in the pass, caution, and fail categories and (2) statistical analysis of the absolute ATP readings. These data were analyzed using JMP 15.2.0 (SAS Institute Inc., Cary, NC). The all-pairs, Tukey's honestly significant difference method was used to determine whether differences in absolute ATP readings were statistically significant (P < 0.05).

Figure 4 shows 500ST without (Figure 4A) and with (Figure 4B) soil. Figure 4B shows that the soil wets even the rougher surfaces instead of beading out, ruling out the lotus effect. Figure 5 shows the average contact angle for the various surfaces, with values ranging from 44.2° (250P) to 56.4° (500ST).

When the wipe direction is in the horizontal direction as microcomparator, rougher surfaces showed higher failure rates, indicating that those were more difficult to clean. Table 2 shows the absolute ATP values and the pass, caution, and fail rates of the surface textures tested.

Rougher surfaces greater than the nominal value of 250 μin appeared to be the least easy to clean given the same wiping conditions (Table 2). This is likely due to the entrapment of soil on rougher surfaces, which has also been observed in other studies.^6,15  There were higher failure rates of 83% at 500P and 100% at 500ST. The actual Ra of 500ST is lower than 500M and 500P, but it is less easy to clean. Optical microscopy images showed that deeper ridges were present in 500ST, which likely resulted in the contact of the wipe area being lower. As shown in Figure 6, most of the remaining soil was in these ridges.

These observations suggest that although a general trend of lower cleanability with increasing roughness was noted, Ra values alone may not be sufficient to determine cleanability. Surface finish should also be considered because of the presence of ridges that may reduce the soil's contact area with the wipe.

The surfaces were also wiped in the vertical direction. As in the horizontal direction, rougher surfaces showed higher failure rates, indicating lower cleanability. The ATP hygiene monitoring system routinely is used as a measure of cleanliness (i.e., the absence of soil). It is not designed for precise measurement but instead used to find RLU trends or for routine monitoring. Results typically are reported in pass, caution, and fail categories.¹⁶ Table 3 shows the results when wiping in a vertical direction for absolute ATP value and percentage in pass, caution, and fail categories.

Figure 7 shows the absolute ATP values after wiping in the horizontal and vertical directions for various surface textures. For wiping in the vertical direction, rougher surfaces also showed higher ATP values at the end of the test, similar to wiping in the horizontal direction. However, the absolute ATP values were lower for the vertical direction, especially for 500ST. This may be due to the deeper ridges being in the vertical direction, with the wipes better able to come into contact with the ridges, thus picking up more of the soil. Although the actual Ra values for 500ST were lower than those for 500M and 500P, the deeper ridges made it less easy to clean.

As indicated by Tables 2 and 3, 500ST showed 100% failure rates in both the horizontal and vertical wipe directions. Generally, horizontal and vertical wipe directions performed quite similarly based on the categorical results. The results demonstrated that horizontal and vertical wipe directions generally showed a similar percentage of passing and failure rates. In both cases, the rougher surfaces (500P and 500ST) showed failure rates between 80% and 100%, suggesting that rougher surfaces are more difficult to clean. In addition, the machining method used to achieve target Ra affects cleanability. For textures with an Ra of 500 μin, the milled surface had a higher passing rate than the profiled or shape-turned surfaces. This is likely due to the deeper ridges present in shape-turned surfaces, as seen in the microscope images in Figure 8A (milled) and Figure 8B (shape turned).

Statistical analysis using Tukey's honestly significant difference test on JMP 15.2 software was performed for the absolute ATP reading. The results for the horizontal wipe direction are shown in a connecting letters report (Table 4). In connecting letters reports, groups that do not share a letter are statistically significantly different. The readings for 500ST and 500P were significantly different from each other and from the others, confirming that rougher surfaces with shaped-turned or profiled finishes were more difficult to clean.

The results for the vertical wipe direction are shown in a connecting letters report (Table 5). Similar to the horizontal wipe direction, the readings for 500ST and 500P were significantly different from the others. The reading for 500M, on the other hand, was in the same grouping (group D) as some of the surfaces with lower nominal Ra values. There were more groupings in the vertical wipe direction than in the horizontal wipe direction.

Based on this study, to improve cleanability, it is recommended that surfaces in the environment of care be kept below the nominal Ra of 250 μin. In addition, wiping in multiple directions may allow the wipe to have better contact with soil, resulting in a cleaner surface.

Future research should focus on additional surfaces, including those made of different materials. Different soils with varying drying times may also be studied to confirm the role of roughness in cleanability of a surface. In addition, efficacy studies can also be included to determine the optimal roughness for cleanability and cost.

This study investigated the cleanability of various surface textures on nickel alloy. Rougher surfaces above a nominal Ra of 250 μin (actual Ra 230 μin) were found to have higher ATP readings when wiped in both horizontal and vertical directions, suggesting that surfaces above that Ra value may be more difficult to clean. In addition, different machining methods will have different cleanability despite similar nominal Ra.

We also found that different wipe directions imparted different final ATP readings after cleaning. We recommend that surfaces are smooth (Ra <250 μin for nickel alloy) in order to ensure better cleanability. Further, for optimal results, the cleaning of soil found on metal surface finishes of medical devices and within the environment of care should be performed in multiple directions instead of unidirectionally.

The authors thank James Chia (senior R&D director at Metrex Research, LLC), Ryan Lakey (formerly a chemist at Metrex Research, LLC), Tyler Hua (research associate at Metrex Research, LLC), and Harish Jani (research associate at Metrex Research, LLC) for their discussions and thoughts on the project.