The effect and safety of dressing composed by nylon threads covered with metallic silver in wound treatment

Abstract

Silver is used worldwide in dressings for wound management. Silver has demonstrated great efficacy against a broad range of microorganisms, but there is very little data about the systemic absorption and toxicity of silver in vivo. In this study, the antimicrobial effect of the silver‐coated dressing (SilverCoat®) was evaluated in vitro against the most common microorganisms found in wounds, including Pseudomonas aeruginosa, Candida albicans, Staphylococcus aureus, Methicillin‐resistant Staphylococcus aureus and Klebsiella pneumoniae. We also performed an excisional skin lesion assay in mice to evaluate wound healing after 14 days of treatment with a silver‐coated dressing, and we measured the amount of silver in the blood, the kidneys and the liver after treatment. Our data demonstrated that the nylon threads coated with metallic silver have a satisfactory antimicrobial effect in vitro, and the prolonged use of these threads did not lead to systemic silver absorption, did not induce toxicity in the kidneys and the liver and were not detrimental to the normal wound‐healing process.

Introduction

Infection can impair or delay the healing process of a wound (1). Silver has been used as an antimicrobial agent for a long time and has been applied to wounds to treat or prevent infections. Initially, silver nitrate and silver sulphadiazine were widely used for burn patients, and many silver dressings and formulations have since been used to control infections (2). A range of silver‐coated or silver‐impregnated dressings are now commercially available, and these dressings have been proven effective, even against antibiotic‐resistant bacteria. In addition, these silver dressings are less irritating and better tolerated than silver nitrate solution 3, 4.

There are many medical compounds for treating wounds that include silver in their structures. The most common technologies that contain silver are hydrofibres, alginates, polyurethane foams, activated charcoals, hydrocolloids and non‐adherent dressings with polyamide. The development of this broad range of silver‐containing technologies can be explained by a worldwide increase in interest in the wound‐management market. This market increase is mainly caused by the rapidly growing elderly population, the increase in patients with diabetes and other diseases or conditions that contribute to a higher incidence of wounds (5).

The silver concentration in wound dressings varies from 0·15% (e.g. activated charcoal dressings) to 15% (e.g. nanocrystalline dressings and non‐adherent dressings with polyamide). Despite the widespread use of silver dressings, there are few studies about the absorption of silver from topical use. Recently, Vlachou et al. demonstrated that silver was absorbed during the treatment of burn patients with Acticoat® (a three‐layer dressing impregnated with nanocrystalline silver, Smith & Nephew Corporation, London, UK), but no signs of haematological, biochemical toxicity or argyria were found, which indicated that these dressings are safe for use (6). Argyria is a blue‐grey discolouration of the skin caused by silver deposits and is a classic side effect of the use of silver dressings. The effect of silver against a broad spectrum of microorganisms is due to multiple mechanisms of action. Silver interferes with the components of the microbial electron transport system and also binds to DNA, which inhibits DNA replication 7, 8.

To successfully treat a wound, dressings should provide infection control, thermal isolation, maintenance of a moist environment and management of wound exudates. Moreover, wound dressings should relieve pain, present low allergenic potential and should not be toxic to the new tissue that is formed. Most silver dressings show an excellent ability to control infection; however, some authors have reported cytotoxic effects (9). The observed cytotoxicity is not related to the amount of silver present in the structure of the dressing but to the nature of the silver (metallic, bound or ionic). The homogeneous distribution of silver in the dressing is also important for its potential cytotoxic effect (9). The main role of silver dressings is to eliminate microorganisms in the wound site and allow for normal wound healing. Once the wound is free of microorganisms, the application of dressings that do not contain silver should be considered. Some detrimental effects of silver dressings to wound healing were demonstrated in both in vitro and in vivo studies 9, 10. Fredriksson et al. demonstrated that different types of silver dressings (Acticoat® and Flamazine®, Smith & Nephew Corporation; PolyMem Silver®, Ferris Mfg Corp., Fort Worth, TX; SilvaSorb®, Medline Industries, inc., Mundelein, IL; Silverlon®, Cura Surgical, Geneva, IL and Silver nitrate) caused a delay in re‐epithelialisation and different degrees of silver deposits (11). Innes et al. demonstrated a delay in re‐epithelialisation in human skin graft donor site wounds dressed with Acticoat® in comparison with those dressed with Allevyn, an occlusive, moist healing environment creating material. In this study, wounds dressed with Allevyn required 9·1 ± 1·6 days to achieve >90% of re‐epithelialisation while donor sites dressed with Acticoat® required 14·5 ± 6·7 days to achieve >90% re‐epithelialisation (P = 0·004) (12). However, further studies are necessary to elucidate the magnitude of the detrimental effects of silver dressings in vivo. This study aimed to evaluate the effects and safety aspects of the silver‐coated dressing from LM Farma Indústria e Comércio S/A.

Materials and methods

Reagents

The microorganisms used in the antimicrobial tests were purchased from the American Type Culture Collection (ATCC, Manassas, VA, USA): Klebsiella pneumoniae (ATCC 4352), Candida albicans (ATCC 10231), Methicillin‐resistant Staphylococcus aureus (MRSA – ATCC 33591), Staphylococcus aureus (ATCC 10145), Pseudomonas aeruginosa (ATCC 9027). The silver‐coated dressing was provided by LM Farma Indústria e Comércio S/A (SilverCoat®– LM Farma Indústria e Comércio S/A, SAO JOSE DOS CAMPOS, São Paulo, Brazil). This dressing is a fabric made with nylon threads that are coated with 15% metallic silver (XStatic® technologies, Noble Biomaterials Europe srl, Desenzano del Garda, Brescia, Italy).

Mice

Male Swiss mice that weighed 25–30 g were obtained from Fundação Oswaldo Cruz Animal Facility (Fiocruz, Rio de Janeiro, Brazil). These mice were used to perform the full‐thickness excisional wound model and to collect samples (blood, kidneys and liver) for silver quantification. The animals were kept at a constant temperature (25°C) under a 12‐hour light/dark cycle with free access to food and water. All experiments were conducted in accordance with the ethical guidelines of the Institutional Animal Care Committee (CEUA) in the Biomedical Institute of the Federal University of Rio de Janeiro (UFRJ), Rio de Janeiro, RJ, protocol code: DFBCICB028.

Full‐thickness excisional wound model

Full‐thickness excision wounds were created in normal mouse skin as described previously (12). Briefly, mice were anaesthetised with ketamine (112·5 mg/kg, i.p.) and xylazine (7·5 mg/kg, i.p.), their dorsal surfaces were shaved and cleaned with ethanol 70%, and a full‐thickness excision wound was made with a biopsy punch of 10 mm in diameter. The wounds were covered with 1 cm² of sterile silver‐coated dressing or sterile gauze pads only (control). Sterile gauze pads were used as a secondary dressing covering the silver‐coated dressing, and the dressings were fixed with hypoallergenic tape (3M™ Micropore™ Medical Tape, 3M Corporation, St. Paul, MN). The dressing was examined daily, and the dressings were changed when either the dressing fell off or on days 4, 7 and 10 after wounding. On day 14, the mice were euthanised and the kidneys (right and left), liver and blood were collected. Blood was collected from the orbital plexus. Immediately after blood collection, the animals were euthanised, and the organs (liver and kidneys) were harvested, weighed and stored at −80°C. The mice were weighed on day zero and again on day 14.

Wound areas were measured at days 0, 4, 7, and 14. Digital photos were obtained at a distance of 5 cm from the wounds, and the wound area was measured using ImageJ software (National Institutes of Health, Bethesda, MN). Areas obtained at day 0 were considered 100%, and the subsequent measurements were calculated as a percentage of the initial wound area. The graph represents the mean of all animals analysed in each group.

Testing of antimicrobial activity

The first test for antimicrobial activity was performed according to a protocol from the American Society for Testing of Material (ASTM G 21‐96). The test is based on the standard practice for determining the resistance of synthetic polymeric materials to fungi and bacteria contamination. Briefly, a suspension of 10⁸ colony forming units of P. aeruginosa (ATCC 9027) and S. aureus (ATCC 10145) were incubated separately in 10 ml of peptone water plus Tween 80 at 36°C for 30 minutes. A sample of 1 cm² of silver‐coated dressing was placed on Petri dishes (90 mm) containing tryptic soy agar and were incubated for 30 minutes. Then, 1 ml of the microorganism suspension was spread upon the culture medium. After 48 hours of incubation at 36°C, the halo that formed around the dressing was measured. The results were expressed in millimetres of halo formed due to inhibition of microorganism growth. The results are classified according to the inhibition zone formed as highly satisfactory (>6 mm), satisfactory (2–5·9 mm), acceptable (0–1·9 mm) and unacceptable (with growth upon or under the sample).

The second method used to evaluate antimicrobial activity was the ASTM E2149 test. This method was designed to measure the antimicrobial activity of non‐leaching (non‐water‐soluble) antibiotics when they are shaken in a liquid solution containing microorganisms. The microorganisms (K. pneumoniae, C. albicans, MRSA and P. aeruginosa) were grown in tryptic soy agar or Sabouraud dextrose agar with a known concentration (as indicated in Table 1) under constant agitation. After the incubation (30, 60 and 120 minutes), a count of viable cells was performed. The results show a reduction in microorganism viability after incubation with silver‐coated dressings.

Silver elution profile

A sample of silver‐coated dressing weighing 90 mg was incubated in 10 ml of sterile water at room temperature and was protected from light. At each specific time point (0·5, 8, 24, 48, 72, 96, 120, and 144 hours), the samples were moved to another flask containing 10 ml of fresh sterile water and were incubated until the next time point. After the last incubation period, the silver quantification was determined by graphite furnace atomic absorption spectrometry (GFAAS), as described below. The results represent the mean of triplicate experiments.

Silver quantification

Kidney and liver samples were collected on day 14 after wound treatment and were weighed at room temperature. Dry ashing and wet digestion methods were used to prepare samples for silver quantification (13).

Dry ashing

The samples were weighed and transferred to a muffle furnace that was heated to 600°C for 1 hour. The heat rate was 12°C/min with temperature plateaus of 30 minute at 100, 200, 300, 400 and 500°C. After digestion, 2 ml of HNO₃ P.A. (Merck®, Darmstadt, Germany) was added to the ashes. Then, the residues were transferred from crucibles to volumetric flasks and diluted to 25 ml with deionised water (Millipore, 18·2 MΩ/cm).

Wet digestion

The samples were weighed in glass beakers, 2 ml of H₂SO₄ P.A. (Merck®) was added to each sample and the samples were heated. The products of the digestion were transferred to volumetric flasks and diluted to 25 ml with deionised water.

The digestion of the blood samples before silver analysis was performed by transferring 1 ml of each blood sample to a 15 ml Teflon® (DuPont, Wilmington, DE) flask using a micropipette. A mixture of 3 ml of fluoridric acid and 1 ml HNO₃ P.A., both 20% v/v, was added to each sample. Then, this system was inserted into an aluminium case and digested at 110°C for 3 hours. After cooling, the treated samples in the Teflon® flasks were carefully transferred to a volumetric flask and diluted to 25 ml with deionised water.

Instrumentation

A spectrometer (PerkinElmer, LLC, Norwalk, CT; AAnalyst 800) equipped with flame and GFAAS modules was used for whole silver determination. These modules could be changed automatically with transversal heating to provide a uniform temperature. Background correction by the Zeeman effect was used for analytical determination. Stabilised temperature platform furnace conditions were applied throughout the analytical development test using electrothermic atomisation. Both analyte addition and recovery tests were used to assess the accuracy of the analytical method (14).

The analytical parameters employed by GFAAS were as follows: wavelength – 328·1 nm, slit width – 0·7 nm, signal – AA‐BG, measurement – peak area, matrix modifier – 3 µg Pd + 1 µg Mg(NO₃)₂, characteristic mass value – 7·6.

Temperature programme: evaporation step: 110°C, 1 second ramp holding 30 seconds, 250 l/second internal flow of Argonio; drying step: 130°C, 15 seconds ramp holding 30 seconds, 250 l/second internal flow of Argonio; pyrolysis step: 700°C, 10 second ramp holding 30 seconds, 250 l/second internal flow of Argonio; atomisation step: 1800°C, 5 seconds; 26 000°C, 1 second ramp holding 3 seconds, 250 l/second internal flow of Argonio.

Toxicological analyses

Quantification of glutamic‐pyruvic transaminases (GPT) and glutamic‐oxaloacetic transaminases (GOT) activities: the pyruvate and oxaloacetate plasmatic levels correspond, respectively, to the GPT and GOT activities and are measured by colorimetric assays (Analisa®, Belo Horizonte, Brazil) according to the manufacturer's instructions. Briefly, blood samples were centrifuged, and plasma aliquots, 50 µl for GPT and 100 µl for GOT, were transferred to test tubes containing 250 µl of the enzyme substrate. The samples were incubated for exactly 30 minutes at 37°C. After this period, 250 µl of the colorimetric reagent was added, and the tubes were incubated at room temperature for 20 minutes. Then, 2·5 ml of NaOH 0·4 Mol/l was added, and the tubes were incubated for 5 minutes at room temperature. The colorimetric products were measured at 505 nm using a cuvette spectrophotometer. The enzymatic activities were determined using standard curves provided by the manufacturer and were expressed as Reitman Frankel Unit/ml (RFU/ml).

Quantification of malondialdehyde

As an index of lipid peroxidation, we used the thiobarbituric acid reactive substances (TBARS) method for analysing malondialdehyde (MDA) products during an acid‐heating reaction, as previously described by Draper and coworkers (15). The MDA is a product of lipid peroxidation and is used as an indicator of free radical effects in organisms (16). Briefly, the samples of liver homogenate were mixed with 10% trichloroacetic acid (1:1) and centrifuged. The supernatant was mixed with 0·67% thiobarbituric acid (1:1). The samples were then heated in a boiling water bath for 1 hour. TBARS levels were determined by the absorbance at 532 nm and were expressed as MDA equivalents (nM/mg protein).

Statistical analysis

The Student's t‐test was applied for comparison between unpaired samples (control versus treated). The level of significance for significant difference between groups was set at P < 0·05 in all analyses.

Results

The efficacy of a silver‐coated dressing as an antimicrobial agent was evaluated in vitro against the most common microorganisms present in wound beds, such as P. aeruginosa, S. aureus, MRSA, K. pneumoniae and C. albicans. As shown in Figure 1, the halo formed by the inhibition of microbial growth was 5 mm for P. aeruginosa and 3 mm for S. aureus after 48 hours of incubation. According to the ASTM G21‐96 test, the silver‐coated dressing had a satisfactory effect as an antimicrobial agent. To further examine the antimicrobial effect, we evaluated the log reduction after the silver‐coated dressing was incubated with the most common microorganisms (bacterial and fungal) responsible for wounds infections. The log reductions in bacterial and fungal counts were as follows: 1·48 and 5·17 after 30 and 60 minutes, respectively, of incubation with K. pneumoniae; 0·11 and 1·30 after 30 and 60 minutes, respectively, of incubation with MRSA; 0·77, 2·38 and 3·22 after 30, 60 and 120 minutes, respectively, of incubation with C. albicans; and 0·11, 1·54 and 4·17 after 30, 60 and 120 minutes, respectively, of incubation with P. aeruginosa (Table 1). These results support the efficient and fast action of silver‐coated dressings as an antimicrobial agent.

Figure 1.

Effect of the silver‐coated dressing as antimicrobial agent. The inhibition zone (halo formed) 48 hours after incubation of the silver‐coated dressing with Staphylococcus aureus (ATCC 10145) and Pseudomonas aeruginosa (ATCC 6528) colonies was 5 mm and 3 mm, respectively. According to the ASTM G 21‐96 methodology these results are classified as satisfactory.

An ideal silver dressing should be able to steadily release silver for a long time, which reduces the number of dressing changes and results in better patient compliance with the treatment and lower costs. We evaluated the ability to maintain the silver release from the silver‐coated dressing into water, as shown in Figure 2. Our results showed that a detectable amount of silver was quickly released by the silver‐coated dressing after 30 minutes of incubation, and the silver release continued for 144 hours (6 days). The continued release of silver is a good feature that allows for a prolonged interval between dressing changes.

Figure 2.

Ability of the silver‐coated dressing to release silver. Samples of silver‐coated dressing weighing 0·09 g were incubated in 10 ml of sterile water and were transferred into new tubes with 10 ml of fresh sterile water in each time point. The silver was quantified in the water used for incubation in absence of the dressing by Graphite Furnace Atomic Absorption Spectrometry (GFAAS), as described in the Materials and Methods section. This assay was performed in triplicate. The silver concentration was expressed in mg/l as the mean ± SEM.

The antimicrobial activity of silver dressings is well established in the literature. However, the consequences of topical silver treatment for wound healing and the risks of systemic toxicity are not well understood. To evaluate toxicity and the implications for wound healing during a prolonged treatment with a silver‐coated dressing, we used a model of full‐thickness wounds in mice. We observed that the time course for wound healing did not change after 14 days of treatment with a silver‐coated dressing compared with control, as shown in Figure 3.

Figure 3.

The silver‐coated dressing does not affect normal wound healing. The excisional wound was performed in mice at time zero. The sizes of the wounds were measured at days 0, 4, 7 and 14. The results were expressed as a percentage of the initial size of the wounds (day 0). One square centimetre of silver‐coated dressing was used to cover the wounds of the treated group, and sterile gauze pads only were used to cover the wounds on the control group. The dressings were fixed by hypoallergenic tape (3M™ Micropore™ Medical Tape). No differences in wound healing were observed between the treated (n = 5) and control (n = 5) groups.

The next step was to evaluate the possible systemic silver absorption after topical treatment with a silver dressing. The accumulation of silver was measured in the blood, the kidneys and the liver after 14 days of treatment. After optimisation of the procedures, silver addition and recovery was tested with each digestion assay, validating the method used by the analyte recovery percentages. High recovery by the two silver digestion procedures was observed. The percentages of recovered analyte oscillated between 92·8% and 93·6% by dry ashing digestion and between 95·3% and 97·0% by wet digestion. Despite the variation observed between animals of the same group, we did not find significant differences in the amounts of silver in the blood, the kidneys and the liver between the treated and control groups (the values are expressed as median ± SD; blood: 0·01 ± 0·012 mg/l in treated group and 0·004 ± 0·0068 mg/l in control group; kidneys: 0·01 ± 0·58 mg/l in treated group and 0·01 ± 1·22 mg/l in control group; and liver: 0·01 ± 0·45 mg/l in treated group and 0·01 ± 0·0008 mg/l in control group) (Figure 4).

Figure 4.

Prolonged treatment with a silver‐coated dressing does not cause systemic silver absorption. After 14 days of silver‐coated dressing treatment, silver was quantified in (A) the blood (treated n = 12 and control n = 8), (B) the kidney (control n = 4 and treated n = 6) and (C) the liver (control n = 5 and treated n = 7). The quantification was performed by Graphite Furnace Atomic Absorption Spectrometry (GFAAS) as described in the Materials and Methods section, and the amount of silver was expressed in mg/l as median ± SD. The graph represents the pool of two independent experiments.

To rule out a possible systemic toxicity, we assessed the weight of the kidney and liver after wound treatment. It is known that in the case of systemic toxicity, the kidneys and the liver are the main organs affected because they play key roles in xenobiotic metabolism. In addition, the loss of body weight is a classic sign of detrimental health conditions. We did not observe body weight change or any sign of sickness in animals treated for 14 days (data not shown). Furthermore, the kidney and liver weights in the treated group (the values are media ± SD: kidney: 0·56 ± 0·03 g and liver: 2·31 ± 0·18 g) were normal and similar to the control group (the values are media ± SD: kidney: 0·55 ± 0·09 g and liver: 2·05 ± 0·23 g) (Figure 5A and B) without macroscopic signals of toxicity (data not shown).

Figure 5.

Absence of physical signs of toxicity after prolonged treatment with a silver‐coated dressing. After 14 days of silver‐coated dressing treatment, mice were euthanised and (A) kidneys (n = 4) and (B) livers (n = 4) were collected and weighed. The results were expressed in grams. No differences were found between the treated and the control groups.

Despite the fact that the toxicity was not evident by macroscopic analysis, we also evaluated biochemical markers of hepatic toxicity. GOT and GPT and hepatic lipid peroxidation (MDA) were measured in serum and liver homogenate, respectively, which were obtained from animals treated with silver‐coated dressings on day 14. GOT and GPT activities were similar between the treated (GOT: 72·1 ± 4·6 RFU/ml; GPT: 61·6 ± 2·6 RFU/ml) and the control groups (GOT: 70·7 ± 5·3 RFU/ml; GPT: 52·4 ± 5·8 RFU/ml) (Figure 6A and B). In addition, no difference was found in hepatic MDA in the treated (0·025 ± 0·007 nmol/mg of protein) and the control group (0·024 ± 0·01 nmol/mg of protein; Figure 6C).

Figure 6.

Absence of hepatic toxicity after prolonged treatment with a silver‐coated dressing. After wounding and 14 days of silver‐coated dressing treatment, mice were euthanised and biochemical analyses were performed in serum (GOT and GPT) or liver homogenate malondialdehyde (MDA) for hepatic toxicity. The levels of (A) glutamic‐pyruvic transaminases (GPT) (control n = 8 and treated n = 5), (B) glutamic‐oxaloacetic transaminases (GOT) (control n = 8 and treated n = 5), and (C) MDA (control n = 3 and treated n = 5) amount were analysed by colorimetric assays as described in the Materials and Methods section. The results were expressed as Reitman Frankel Unit/ml (RFU/ml) for GOT and GPT and as nMol/mg of protein for MDA. No differences were found between the treated and the control groups. These results are representative of two independent experiments.

Our results show the antimicrobial effect of silver‐coated dressings against micro‐organisms commonly involved in wound infections. In addition, the absence of systemic silver absorption and signs of toxicity indicate that silver‐coated dressings are safe.

Discussion

Wound infection is one of the main factors that compromises wound healing. Silver dressings are indicated for treating infected wounds and demonstrate a broad spectrum of bactericidal activities against gram‐positive and gram‐negative bacteria, aerobic and anaerobic, yeast, fungi and viruses. Studies have shown the ability of silver dressings to combat infection using different types of silver (ionic or nanocrystalline) 10, 17. Our study demonstrates the effect of a dressing composed of nylon threads that are coated with metallic silver (SilverCoat®, XStatic®technologies), against the most common microorganisms found in wounds (P. aeruginosa, S. aureus, MRSA, K. pneumoniae and C. albicans). We showed that this silver‐coated dressing was able to significantly and rapidly reduce the viability of the most prevalent microorganisms found in the wound bed. The effects of the antimicrobial activity were observed in 1 hour. Corroborating our data, Bowler et al. (18) demonstrated high antimicrobial activity of silver‐containing dressings against 10 different multidrug‐resistant organisms in a simulated wound fluid over 7 days and inhibitory and bactericidal effects against both free‐living and biofilm bacteria in simulated colonised wound surface models.

Beyond the fast antimicrobial effect, our study showed a persistent silver release over 6 days from the dressing. These results imply that fewer dressing changes are necessary, which would increase patient compliance to the treatment. Silverstein et al. (19) performed an open, parallel, randomised, comparative, multicentre study to evaluate the cost‐effectiveness, performance, tolerance and safety of a silver‐containing soft silicone foam dressing (Mepilex Ag) versus a silver sulphadiazine cream (control) in the treatment of partial‐thickness thermal burns. They reported that the soft silicone foam dressing is easy to use, painless at application, allows for fewer dressing changes and shows reduced adverse effects. Both treatments were well tolerated and effective. Nevertheless, the soft silicone foam dressing was found to be more cost‐effective. In addition, another study reported the in vivo effect of a sustained‐release silver sulphadiazine powder foam dressing on the bioburden and wound closure in infected venous leg ulcers. This study associated this dressing with a very high healing rate and a significant reduction in the bioburden after 8 weeks of treatment (20). These studies reinforce the advantages of silver‐impregnated dressings in comparison with sulphadiazine creams being the latest option commonly used because of its low cost.

Silver‐coated dressing treatment during a prolonged period did not impair normal wound healing in mice. However, we should consider that mouse skin differs from human skin in several ways. The main relevant difference between mouse and human skin that should be considered as a limitation for our model is that mouse skin heals preferentially by wound contraction, while human skin heals preferentially by re‐epithelialisation (21). This difference is caused by the panniculus carnosus, a thin muscle layer located under the skin of mice, which produces rapid wound contraction following an injury. In humans, this contraction is only found in the platysma muscle of the neck. We recognise the limitations for translational relevance of our mouse study; however, skin lesions in animal models are widely used, and these models provide relevant contributions to advances in wound care. Guthrie et al. (22) showed a significant antimicrobial effect of silver‐impregnated dressings using a wound infection model in mice with topical inoculation of S. aureus, indicating that silver‐impregnated dressings are effective in in vitro, as demonstrated in this study, and in vivo models.

Recently, Niazi et al. (23) demonstrated that silver clear nylon dressings are effective in reducing radiation‐induced dermatitis in patients with lower gastrointestinal cancer treated with combined chemotherapy and radiation treatment. We understand the necessity of studies involving wound healing in humans to provide more confident data about the safety and effectiveness of silver‐coated nylon thread dressings in humans. Nevertheless, because the antimicrobial effects of silver dressings are well described, we intended to address the benefits of silver dressings and their putative toxicity. In addition, we evaluated the safety of silver dressings in vivo, and our data have reinforced the beneficial effects of silver dressings for wound treatment. In addition, many silver dressings are used and recommended by health professionals to treat infected wounds without damaging the patient (6).

Argyria is the most common side effect reported with topical silver treatment. This blue‐grey discolouration of the skin is caused by silver deposition and is commonly observed after chronic exposure (17). Trop et al. (24) published a case study of a burn patient who was treated with Acticoat® (Smith & Nephew Corporation), a dressing impregnated with nanocrystalline silver, and presented argyria‐like symptoms and hepatotoxicity. In contrast, Vlachou et al. (6) demonstrated in a prospective, single centre, open‐label study of 30 patients with small burns in which the use of Acticoat® did not lead either to haematological or biochemical alterations. In accordance with the latter data, we analysed the percentage of leucocyte cells in the peripheral blood of mice treated with silver‐coated dressing for 14 days. We observed no signs of abnormality in the treated group (data not shown). In addition, we did not find systemic silver accumulation after 14 days of silver‐coated dressing treatment, and the mice presented normal organs (liver and kidneys) and body weight. In addition, the hepatic parameters, such as GOT and GPT in serum and MDA in the liver homogenate, were similar between the treated and control groups. Few studies are available about silver absorption and toxicity after topical use, and our data provides new information about this subject. In this study, we demonstrated the safety of a topically used silver‐coated dressing treatment showing no argyria or systemic silver accumulation (peripheral blood, kidneys and liver) in mice.

In all parameters evaluated, no sign of toxicity or systemic absorption of silver was observed, highlighting the safety of topical use of this dressing impregnated with metallic silver. Furthermore, the antimicrobial effect observed in our study was satisfactory.

Acknowledgements

This work was supported by LM Farma Industria e Comércio Ltda S/A (São José dos Campos, São Paulo, Brazil), Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). The fellowship was given from CAPES. We would like to thank Karla Maria Pereira Pires for the MDA methodology set up. The authors are grateful to Claudio Canetti for the careful reading of the manuscript.