Antibacterial and Flow Rate Assessment of a Nonadherent Layer Interface on a Silver Alginate Dressing

The purpose of this study was to assess the potential differences in antibacterial activity and fluid flow rates of 2 commonly used silver dressings: an alginate-only dressing and an alginate dressing with a nonadherent contact layer.   Materials and Methods. The dressings’ antibacterial activities were tested against 2 major bacterial pathogens for skin wounds—Staphylococcus aureus and Pseudomonas aeruginosa—using 2 in vitro models of skin wounds. The fluid flow rate through the 2 dressings was also measured and compared. Results. While materially similar, the choice to include a nonadherent layer leads to reduced antibacterial performance and a reduced rate of fluid flow into and through the dressing. Conclusion. The use of the silver antimicrobial alginate dressing with a nonadherent layer provides a welcome feature in that it does not adhere to surfaces; however, the results demonstrate lower rate of fluid removal when compared to a silver antimicrobial alginate without the nonadherent layer.

  Modern science demonstrates that silver ions are relatively effective in killing otherwise resistant bacteria including methicillin-resistant Staphylococcus aureus (MRSA)¹ and vancomycin- resistant Enterococcus (VRE).^2-5 Silver has also been demonstrated to be capable of inactivating viruses and killing fungi⁶ and other parasites. An advantage of silver ions is that bacteria do not tend to easily develop resistance to its generic protein adduction-based mechanism of action. In wound care, silver is utilized in several formulations such as silver sulfadiazine, silver nitrate solutions, and in dressings that contain ionic silver. In addition to the various silver compounds used, there are also a variety of dressing materials used in conjunction with silver. These materials are further varied by the structure of the dressing or the inclusion of additional materials.

  While dressings that include silver and appear to be constructed using the same base material are chemically similar, differences in their physical construction may have an impact on desirable product features.⁶ Some of these physical differences are highlighted by the dressing’s manufacturer as features that differentiate their product from a materially similar dressing. What is sometimes overlooked are the trade-offs that may accompany these features. In the current study, the authors compare 2 clinically available silver alginate dressings. One dressing included a nonadherent layer and the other did not. The nonadherent layer is a feature that reduces the ability of the dressing to become attached to the wound surface. This feature allows for a less painful removal of the dressing while simultaneously reducing the likelihood of silver alginate fibers being left in the wound.

  While the benefits of a nonadherent layer are understood, the potential trade-offs are not. Hypothetically, materials placed between the wound and the antimicrobial agents in the dressing could serve as an obstacle; furthermore, dependent upon the composition of the materials, the intermediate materials might interfere with the rate of absorption of wound fluids by the dressing. To determine whether the use of a nonadherent layer could impact dressing performance, the authors tested 2 clinically available silver alginate dressings—one with a nonadherent layer (SilverCel, Systagenix, Gatwick, UK) and one without (Maxorb Extra AG, Medline Industries, Inc, Mundelein, IL)—in 2 in vitro microbiological assays to evaluate any differences in the antimicrobial activity of the dressings, and 1 in vitro fluid mechanics assay to assess if the nonadherent layer could impede the flow rate of fluids into or through the dressing.

  Zone of inhibition test. Standard stains of Staphylococcus aureus (ATCC35556) SA35556 and Pseudomonas aeruginosa (PA01) were obtained from frozen stocks and streaked on agar plates. The bacterial inhibition ring test method is a minor modification of the standard test used to assess the antibiotic sensitivity of bacteria samples as described previously by Gibson and colleagues.⁷

  Ex vivo pig skin explant model. Sterile pig skin explants 12 mm in diameter were prepared and used in the same manner as previously described by Gibson and coauthors⁷ to test the 2 dressings.

  Fluid flow rate assay. The initial rate at which 250 g of water passed through a sample was used to compare the relative barrier to fluid transport that each test sample possessed. Four conditions were tested: 1) the empty filter holder measured the amount of resistance due to the incorporated filter support (ie, frit) which represents the maximum flow rate possible with this system, 2) standard 12-ply cotton gauze sponge, 3) a silver alginate dressing, and 4) a nonadherent silver alginate dressing. Each test condition was replicated 5 times.

  For each dressing, a glass filter holder (Millipore, Darmstadt, Germany) with a built-in sintered glass frit was used to hold a cutout circle of each dressing big enough to cover the filter holder and to form a seal (ie, 47 mm). Parafilm was wrapped where the 2 components of the filter holder mated to ensure a good seal prior to applying the sealing clamp. The filter holder and dressing were suspended above a digital scale (GT 40000, Ohaus, Parsippany, NJ) attached to a custom digital data recorder using a ring stand. An empty plastic flask was placed on the scale, beneath the exit funnel of the filter holder, and the scale was zeroed. The data recorder was started and the mass was continuously recorded twice per second to a secure digital memory card as a comma-separated value file. Two-hundred and fifty grams of deionized water was rapidly poured into the upper reservoir of the filter holder and the water was allowed to flow through the dressing and accumulate for approximately 12 minutes. If the filter holder was leaking, or there was evidence of a blockage, the event was noted and the test was repeated with a new piece of dressing.

  The average initial rate of flow was calculated for the first 200 seconds for each condition and the mass flow rates were compared by Student’s t test to determine the statistical significance for any observed differences.

  Zone of inhibition. Both dressings created at least a 1 cm zone of inhibition (Figure 1A). The silver alginate had, on average, a greater zone of inhibition then the silver alginate with the nonadherent layer which was nearly significant against PA01 (Figure 1A, P = 0.06) and which was significant against SA3556 (P = 0.02).

  Ex vivo pig skin explant model. The ex vivo pig skin explant with the dressing possessing a nonadherent layer resulted in slightly more bacteria persisting on the skin (Figure 1B). The nonadherent layer-treated pig skin had ~0.44 log10 more PA01 than the normal silver alginate and ~ 2.5 log₁₀ more SA35556. The observed differences were highly variable leading to an inability for the differences to reach statistical significance (P = 0.48 and P = 0.37, respectively).

  Fluid flow rate assay. The inclusion of all dressings significantly slowed the rate of fluid flow (Figure 2 and Table 2, any of the dressings vs frit only, P ≤ 0.0003). The cotton gauze and silver alginate were nearly identical in terms of average rates and variance (P = 0.979). The silver alginate dressing with the nonadherent layer had the lowest flow rate and the highest variability. Some samples from the noadherent layer group had to be retested due to what initially appeared to be a blockage, but what ended up being air bubbles trapped under the dressing. In the instances of apparent blockage, pressing on the dressing with a blunt instrument led to the expression of air bubbles allowing the water to freely flow. No other dressing had issues with trapped air. Excluding the rates from those dressings with air bubble-based blockage, the nonadherent layer significantly reduced the rate of fluid transmission through the dressing (P = 0.013 for gauze, and P = 0.014 for silver alginate).

  The 2 silver alginate dressings assessed in this study share 2 key properties: antibacterial activity and the ability to maintain a moist wound healing environment. In this regard, both dressings demonstrated the capacity to kill bacteria and to absorb and transmit fluid. There were, however, differences in the quantitative performance of these functions. In all instances, the performance of the silver alginate dressing with a nonadherent layer was lower compared to a materially equivalent dressing without an obstructing interface. However, only in the zone of inhibition assay for S. aureus and in the fluid flow rate assay were the differences statistically significant. The clinical significance of the difference in antibacterial activity is not known, and would require additional clinical testing of the dressings. The behavior of the 2 dressings in terms of fluid transfer rate suggests that the use of a nonadherent layer may come at the cost of a reduced rate of fluid removal. However, the degree to which the size of the observed differences actually have any effect on the ability to manage wound hydration is not known.

  The most unexpected finding was in the qualitative observation of the nonadherent layer interfering with the removal of air bubbles trapped underneath the dressing. It is hypothesized that the hydrophobic, nonadherent mesh pattern does not permit the transmission of air bubbles into and through the dressing as easily, and that the reduced fluid flow rate results in the fluid having less momentum to push the trapped air out of the way. Additional studies are warranted to further pursue this observation.

  The use of the silver antimicrobial alginate dressing with a nonadherent layer does not adhere to surfaces, which is a welcome feature; however, the results demonstrate a lower rate of fluid removal when compared to a silver antimicrobial alginate without the nonadherent layer.

The authors are from the University of Florida, Gainesville, FL.

Address correspondence to:
Daniel J. Gibson, PhD
gibsondj@ufl.edu

  Disclosure: The authors disclose no financial or other conflicts of interest.