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222 WOUNDS: A Compendium of Clinical Research and Practice

ritical colonization and infection ofwounds present a dual problem for clini-cians. First, there is the possibility of

delayed wound healing, particularly in the pres-ence of a compromised immune system or wherethe wound is grossly contaminated or poorly per-f u s e d .1 Second, colonized and infected woundsare a potential source for cross-infection—a par-

ticular concern as the spread of antibiotic-resis-tant species continues. For patients, an infectedwound can have additional consequences includ-ing increased pain and discomfort, a delay inreturn to normal activities, and the possibility of alife-threatening illness. For healthcare providers,there are increases in treatment costs and nursingtime to consider.1,2

Original Research

Silver Antimicrobial Dressings in WoundManagement: A Comparison of Antibacterial,

Physical, and Chemical Characteristics

David Parsons, PhD; Philip G. Bowler, MPhil; Viv Myles, MSc; Samantha Jones, BSc

From ConvaTec Ltd, Flintshire, United Kingdom

WOUNDS 2005;17(8):222–232

Address correspondence to:David Parsons, PhD, ConvaTec Wound Therapeutics Global Development Centre

Deeside, Flintshire CH5 2NU United KingdomPhone: 44 1244 584349; E-mail: [email protected]

A b s t r a c t : Silver-containing dressings are widely used to assist with management of infected woundsand those at risk of infection. However, such dressings have varied responses in clinical use due to techno-logical differences in the nature of their silver content and release and in properties of the dressings them-selves. This study examines the relationship between silver content, rate of silver release, and antibacterialactivity in a simulated wound fluid model against Staphylococcus aureus and Pseudomonas aeruginosa.The study also looks at other important measures for the clinical performance of dressings including fluid-handling properties and dressing pH. Seven proprietary silver-containing dressings (AQUACEL® A g[ H y d r o f i b e r®; “nonwoven A”], Acticoat™ Absorbent [alginate; “nonwoven B”], SILVERCEL™ [ a l g i n a t e - c a r-boxymethylcellulose nylon blended fibers; “nonwoven C”], Contreet® Foam [nonadhesive; “foam A”], Poly-M e m® Silver [“foam B”], Urgotul® S.Ag [“gauze”], and SilvaSorb® [“hydrogel”]) were assessed. No directcorrelation between silver content, silver release, and antibacterial activity was found. Dressings with thehighest silver content were nonwoven B and nonwoven C, while the lowest levels were found in nonwovenA and hydrogel. Nonwoven A, gauze, and nonwoven B were most effective against S. aureus and P .a e r u g i n o s a; however, their silver release rates differed widely. Free fluid absorption was greatest for the 2foam dressings and least for gauze. However, nonwoven A and nonwoven B showed the best fluid reten-tion under conditions of compression, while nonwoven A demonstrated the lowest level of capillary wick-ing. Dressing choice is a vital part of the successful management of infected wounds and those wounds atrisk for infection. This study suggests that dressing selection should be based on the overall properties ofthe dressing clinically relevant to the wound type and condition.

Disclosure: All authors are paid employees of ConvaTec Ltd, Flintshire, United Kingdom.

Until recently, local wound infection has beena challenge with few management options. How-ever, the advent of advanced wound dressingscontaining topical antimicrobial agents, such assilver, has provided a new approach to the con-trol of wound pathogens.3,4

Silver has proven antimicrobial activity thatincludes antibiotic-resistant bacteria, such as methi-cillin-resistant Staphylococcus aureus (MRSA) andvancomycin-resistant enterococci (VRE).4 Its role asan antimicrobial agent is particularly attractive, as ithas a broad spectrum of antimicrobial activity5 , 6

with minimal toxicity toward mammalian cells atlow concentrations7 and has a less likely tendencythan antibiotics to induce resistance due to its activi-ty at multiple bacterial target sites.8

Topical creams or solutions containing silver (eg,silver sulfadiazine) have long been used as a main-stay of wound management in burn patients whoare especially susceptible to infection.1 H o w e v e r ,disadvantages to their use include staining the skinand toxicity.3 In addition, the need for frequentremoval and reapplication of silver sulfadiazine dueto the development of pseudoeschar is both timeconsuming for professionals and painful forp a t i e n t s .3 , 9 A range of antimicrobial dressings con-taining silver either incorporated within or appliedto the dressing are now available for clinical use.1 0

This new class of dressings is designed to providethe antimicrobial activity of topical silver in a moreconvenient application. However, the dressingsthemselves differ considerably in the nature of theirsilver content and in their physical and chemicalp r o p e r t i e s .

This study compares the i n - v i t r o a n t i b a c t e r i a lactivity of 7 such dressings against 2 commonwound pathogens, Staphylococcus aureus a n dPseudomonas aeruginosa. The correlation betweensilver content and/or silver release from eachdressing and its antibacterial effect is examined,and factors relating to the provision of an optimalenvironment for wound healing are compared toprovide a basis for an overall assessment of theclinically valuable properties of each dressing.

Methods

This study assessed the characteristics of 7 pro-prietary silver-containing antimicrobial dressings: 3

fibrous dressings—AQUACEL® Ag (ConvaTec,Skillman, NJ, USA; referred to throughout this arti-cle as nonwoven A), Acticoat™ Absorbent (Smith &Nephew, London, UK; referred to throughout thisarticle as nonwoven B), and SILVERCEL™ ( J o h n s o n& Johnson Wound Management, Somerville, NJ,USA; referred to throughout this article as nonwo-ven C); 2 foam dressings—Contreet® Foam (Colo-plast, Holtedam, Denmark; referred to throughoutthis article as foam A) and PolyMem® Silver (Ferris,Burr Ridge, Ill, USA; referred to throughout thisarticle as foam B); a gauze dressing—Urgotul® S . A g(Laboratoires Urgo, Chenôve, France; referred tothroughout this article as gauze); and a nonadhe-sive polymer hydrogel sheet—SilvaSorb®

(AcryMed/Medline, Mundelein, Ill, USA; referredto throughout this article as hydrogel).

While all 7 dressings contain silver, they varyin their components and structures (Table 1). Thedressings ranged in weight from 1.05 g to 6.93 gfor a 10 cm x 10 cm piece.

B a c t e r i a . The dressings were tested against 2common wound pathogens, namely S t a p h y l o-coccus aureus NCIMB 9518 (gram positive) andPseudomonas aeruginosa NCIMB 8626 (gram neg-a t i v e ) .

Measurement of antibacterial activity.Antibacterial activity was assessed in repeat-chal-lenge assays over a period of 7 days for each ofthe 7 silver-containing dressings (SCD) and for anonsilver-containing control dressing (NSCD,AQUACEL®, ConvaTec).

To best reproduce the clinical conditions inwhich these dressings are used while providing aconsistent and reproducible environment acrossall the dressings tested, a simulated wound fluidwas prepared consisting of 50% fetal calf serum(First Link [UK] Ltd, mycoplasma tested) and50% maximum recovery diluent (MRD, LABM,UK; 0.1% w/v peptone [beef protein extract] and0.9% w/v sodium chloride).

Bacteria were inoculated into simulated woundfluid (SWF) such that the final volume was 10 mL,and the population density was approximately 1 x1 06 cfu/mL. Due to the higher free swell absorptioncapacity of the foam dressings, a 20 mL volume wasused to enable serial samples to be taken withoutdistorting the dressings; however, to provide anequivalent bacterial challenge, the bacterial popula-

Vol. 17, No. 8 August 2005 223

PA R S O N S, ET AL.

tion density was halved. A 5 cm x 5 cm square ofSCD or NSCD control was transferred to the inocu-lum, and tubes were incubated at 35˚C. Samples(100 µl) were removed for total viable counts at 4,24, 48, 72, and 96 hours and on Day 7. At the 48-hour time point, each test sample was re-inoculatedwith approximately 1 x 106 cfu/mL of the originalchallenge organism. Each test was performed on 4separate occasions.

Chemical assays. Measurement of total silvercontent. Samples were digested by heating in amixture of concentrated sulfuric and nitric acidsto break down the dressing matrix and to releaseand dissolve all of the silver present. The digestwas then filtered and diluted with deionizedwater as required to enable quantification of thesilver by atomic absorption spectrophotometry.Determinations were performed in triplicate.

Measurement of dressing pH. Three samples fromeach dressing were suspended in deionized water ata ratio of 1:100 (w/v) and were roller mixed at roomtemperature for 3 hours to ensure the samples hadreached equilibrium. The pH was measured using apH meter with a combination pH electrode, calibrat-ed at pH 4 and 7 or pH 7 and 10, as appropriate tothe pH of the sample being measured.

Measurement of silver release into water over time.A weighed portion (in duplicate) from each dress-ing was suspended in deionized water at a ratio

of 1:100 (w/v), and the samples were placed intoa temperature-controlled environment (37 ± 3˚C)for 7 days. During this period, aliquots wereremoved at timed intervals, and the liquid wasreplaced to maintain a constant volume. Sampleswere filtered, diluted as appropriate, and ana-lyzed by atomic absorption spectrometry.

Absorption testing (fluid handling proper-t i e s ) . Measurement of fluid absorption under variousapplied pressures. A 5 cm x 5 cm square sample ofeach dressing was weighed (W1), placed onto aperforated stainless steel plate, and covered with aflat Perspex plate slightly larger than the dressing.The desired compressive pressure was applied byplacing weights on top of the Perspex plate. Theentire arrangement was then immersed into a trayof solution A (sodium chloride and calcium chlo-ride solution, 0.142 mol l- 1 and 0.0025 mol l- 1,respectively) at a temperature of 20˚C for 20 sec-onds so that the dressing material was completelycovered. The sample was removed and placedonto a double layer of absorbent paper towel toremove freely draining fluid then reweighed (W2) .The weight of fluid absorbed and retained pergram was calculated by (W2 – W1) / W1.

Measurement of vertical wicking. Vertical wickingdistance was measured for the fibrous dressingsonly, as this method is unsuitable for freely drain-ing foams and gauzes and solid hydrogel products.



Table 1. Typical weight of a 10 cm x 10 cm dressingDressing

Nonwoven A (AQUACEL® Ag)Nonwoven B (Acticoat™ Absorbent)Nonwoven C (SILVERCEL™)

Foam A (Contreet® Foam)Foam B (PolyMem® Silver)

Gauze (Urgotul® S.Ag)

Hydrogel (SilvaSorb®)

Dressing Type

Fibrous nonwoven feltHydrofiber®22 with 1.2% w/w ionic silverMetallic nanocrystalline silver-coated calcium alginateCalcium alginate-carboxymethylcellulose fiber blendedwith metallic silver-coated nylon fiber

FoamSilver-impregnated polyurethane foam Silver-impregnated polyurethane foam

GauzeGauze dressing coated in a Vaseline™ paste containinghydrocolloid and silver sulfadiazine particles

HydrogelMicrolattice synthetic hydrogel matrix containing silverchloride

Weight (g)

1.051.511.65

6.936.48

1.72

6.67

A strip of dressing 15 mm wide and 100 mm longwas lowered vertically into a bath of solution Acontaining a red dye (0.25 g/L eosin) until 10 mmof the length was submerged. After 60 seconds, thevertical liquid movement (in mm) up the dressingabove the surface of the liquid was measured.

Measurement of dehydration rate. A 5 cm x 5 cmsquare sample of each dressing was weighed andthen submerged in an excess volume of solution Aat 37˚C for 30 minutes. Samples were thenremoved, suspended by a corner for 30 seconds toremove freely draining liquid, and then reweighed.The hydrated samples were laid on dry Petri disheswithout lids and placed in an incubator at 37˚C.The weight loss of each sample was measuredhourly and the rate of weight loss calculated.

Results

Antibacterial activity. The antibacterial activi-

ty of the 7 SCDs against S. aureus (Figure 1) andP. aeruginosa are as shown (Figure 2).

Nonwoven A, nonwoven B, nonwoven C, andgauze demonstrated the greatest overall antibac-terial activity, reducing bacterial counts for bothS. aureus and P. aeruginosa from more than 1 mil-lion colony-forming units per mL fluid (cfu/mL)to less than 500 cfu/mL within 48 hours. Nonwo-ven B reduced the S. aureus count below the limitof detection (less than 10 cfu/mL) by 24 hours.Nonwoven A and nonwoven B were both highlyeffective against P. aeruginosa, reducing the viablemicrobial count below the limit of detection by 24hours. Upon rechallenge with a high concentra-tion of bacteria 48 hours after the start of the test,both nonwoven A and nonwoven B remainedhighly effective against both test organisms.Gauze (which contains silver sulfadiazine) andnonwoven C also showed continuing efficacyagainst both organisms but were less effective

Vol. 17, No. 8 August 2005 225


Figure 1. The antibacterial efficacy of silver-containing dressings against S. aureus over a 7-day period (re-inoculation occurred at 48 hours). Simulated wound fluid 10 mL volume (re-inoculation at approximately 2.1 x 106

cfu/mL [R1]). For foam A and foam B, a 20 mL volume was used (re-inoculation at approximately 1.4 x 106 cfu/mL).

against P. aeruginosa following rechallenge. FoamA, foam B, and hydrogel showed only limitedantibacterial activity against these organisms.

Silver content and silver release. The mea-sured total silver content of the dressings rangedfrom 6 mg to 113 mg for a 10 cm x 10 cm sample.Content was greatest for nonwoven B and non-woven C and least for nonwoven A and hydrogel(Table 2). The amount of silver released into puri-fied water also varied extensively ranging from17 to 111 µg/10 cm x 10 cm of dressing for themajority of dressings after 48 hours and rising toa little over 3,000 µg/10 cm x 10 cm for nonwo-ven B (1 mg = 1,000 µg). There was no correlationbetween silver release and silver content (Figure3). For example, nonwoven B and nonwoven Chave very similar total silver content, but theamount of silver released after 48 hours wasapproximately 50-fold greater for nonwoven Bcompared with nonwoven C.

Comparison of the antibacterial activity for thedifferent dressings also revealed no correlationbetween antibacterial effect (as measured in aSWF model) and either silver content of thedressings (Figure 4) or total silver released intowater (Figure 5). In particular, silver content wasfound not to be a predictor of antibacterial activi-ty. For example, there was approximately a 10-fold difference in silver content between nonwo-ven A and nonwoven B, 2 dressings with verysimilar antibacterial effects. Conversely, while thesilver content of nonwoven A, gauze, and hydro-gel was broadly similar, antibacterial activity dif-fered significantly between the dressings (Figure4). It is important to note, however, that the tech-nique used in this study measures the totalamount of silver in solution and cannot differen-tiate between antibacterially active forms of solu-ble silver (silver ions, Ag+) and inactive forms,such as metallic silver (Ag0).



Figure 2. The antibacterial efficacy of silver-containing dressings against P. aeruginosa over a 7-day period (re-inoculation occurred at 48 hours). Simulated wound fluid 10 mL volume (re-inoculation at approximately 1.3 x 106

cfu/mL [R1]). For foam A and foam B, a 20 mL volume was used (re-inoculation at approximately 8.6 x 105 cfu/mL).

Nonwoven B showed themost rapid release of largeamounts of silver into water(all values relate to a 10 cm x10 cm dressing; 3,011 µg by 48hours, 3,116 µg by 7 days) andhad good antibacterial activi-ty. Nonwoven A showed amuch lower release of silver(17 µg by 48 hours, 27 µg by 7days); however, this was asso-ciated with equivalent activityagainst P. aeruginosa and onlymarginally reduced activityagainst S. aureus . Gauze,which was marginally lesseffective against P. aeruginosathan nonwoven A and nonwoven B,had a slightly greater rate of silverrelease than nonwoven A (49 µg by 48hours, 79 µg by 7 days). Hydrogelshowed a more rapid rate of silverrelease than gauze (111 µg by 48 hours)and reached a level of 179 µg by day 7.Hydrogel showed less antibacterialactivity than nonwoven A, nonwovenB, or gauze. A summary of silver con-tent, rate of silver release, and antibac-terial activity for all dressings is shownin Table 2.

Fluid-handling properties. F l u i dabsorption. The free swell fluid absorp-tion (when no compression wasapplied to the dressing) ranged from0.2 to 66.8 (all values in g per 10 cm x10 cm) and was greatest for the 2 foamdressings and least for the gauze. Freeswell fluid absorption for nonwoven A wasalmost as great as the absorption for foam B butwas greater than the absorption for the other non-foam dressings.

When this experiment was repeated with adressing compression of 40 mmHg (typical of theforce applied by compression bandaging), fluidabsorption remained greatest for foam A (32.9)but was greater for nonwoven A (11.4) than forfoam B (Table 3). Fluid absorption for foam B andthe other dressings ranged between 0.1 and 8.1.

The difference in fluid absorption between

these 2 experiments was used to indicate howmuch fluid might be squeezed out of the dressingif pressure was applied (the fluid retention of thedressing). The percent fluid loss was approxi-mately 20% for nonwoven A and nonwoven B,compared to approximately 50% for the otherdressings (Figure 6).

Vertical wicking distance. Vertical wicking dis-tance was determined for the 3 fibrous dressings.For nonwoven A and nonwoven C, the wickingdistances were 12.5 and 17.8 mm, respectively,using the standard test procedure. Using this pro-

Vol. 17, No. 8 August 2005 227


Figure 3. Correlation between silver content and release.

Table 2. A comparison of silver content, rate of silver release, andantibacterial activity

Nonwoven ANonwoven BNonwoven CFoam AFoam BGauzeHydrogel

Ag Content (mg/10cm x 10 cm)

91041137713146

After 3 h

3.25550.074.6

0.000.535.4

Excellent +++Good (consistent bacterial reduction) ++Positive behavior (or bacterial control) +Ineffective –

Silver Release intoDeionized Water

(µg/10 cm x 10 cm)

AntibacterialActivity in SWF

48 h AfterRechallenge

++++++++++

+++

After 48 h

173,011

641027049

111

cedure, fluid did not appear to be absorbed bynonwoven B but remained on the surface of thedressing, suggesting that there is likely to be adelay before fluid absorption occurs with thisdressing. The test period was then extended fornonwoven B until absorption occurred and thewicking distance was allowed to equilibrate.Under these conditions, the vertical wicking dis-tance for nonwoven B was 33 mm.

Dehydration. The rate of dehydrationwas assessed for 6 dressings. (Nonwo-ven B was excluded, since it was not pos-sible to reproducibly hydrate this dress-ing.) The rate of dehydration rangedfrom 0.0116 g/min for nonwoven A to0.0251 g/min for foam A (Figure 7).Most dressings dried out within approxi-mately 23 hours; however, for gauze,complete dehydration occurred afterapproximately 40 minutes. The assaywas discontinued at this point.

Dressing pH. The pH of each of thedressings in water was measured overthe course of 1 day. After 3 hours, pHvalues ranged from 5.4 for nonwoven Ato 9.5 for nonwoven B (Table 4). After 24hours, the pH range was reduced: thelower values remained constant at 5.4(nonwoven A), but the higher valuesreduced to 7.7 (nonwoven B, foam B).

Discussion

As expected, each SCD examined inthis study showed a degree of antibacte-rial activity against the woundpathogens tested, with the exception offoam B, which was ineffective against P.a e r u g i n o s a and only marginally effectiveagainst S. aureus. Nonwoven B reducedthe S. aureus count to below the limit ofdetection by 24 hours. However, nonwo-ven A, nonwoven B, gauze, and nonwo-ven C all remained effective after rechal-lenge with S. aureus. Nonwoven A andnonwoven B were both highly effectiveagainst P. aeruginosa, reducing the bacte-rial count to undetectable levels within24 hours. Gauze and nonwoven C were

also effective against the initial challenge butwere less effective against rechallenge with P .aeruginosa. These data demonstrating the antibac-terial activity of silver-containing dressings aresimilar to those reported previously for nonwo-ven A,5 , 6 nonwoven B (in alternative forms),1 1

hydrogel,10 and foam A.10

Comparison of the silver content of the 7 dress-ings revealed a more than 10-fold difference



Figure 4. Correlation between silver release and activity.

Figure 5. Plot of silver release against antibacterial activity at 48hours for S. aureus and P. aeruginosa.

between nonwoven C and nonwoven B(dressings with the highest silver con-tent) and nonwoven A and hydrogel(dressings with the lowest). There was aneven greater difference (180-fold) in theamount of silver released into water after48 hours between nonwoven B (showingthe greatest release) and nonwoven A(showing the lowest release of silver).These differences, however, do not corre-late with the antibacterial activity seen.

It is important to remember that allpublished tests of aqueous silver concen-trations (including this study) fail to dis-tinguish between active ionic silver (Ag+)and inactive silver in solution (eg, Ag0)—that is, they measure total silver only. Thefindings of this study show, however,that a greater amount of silver (in anyform) released by a dressing does notnecessarily lead to a greater rate ordegree of antimicrobial activity.

In conjunction with a measured or cal-culated total aqueous silver concentration,a widely reported test that has been usedto predict the potential antimicrobial effi-cacy of dressings is the minimum inhibito-ry concentration (MIC). In these laborato-ry tests, ionic silver is added to a test cul-ture in the form of a simple solution, andit is assumed that all the silver addedremains active. Under these conditions,the MIC for silver is typically found to bein the range of 5–40 µg/mL.1 2 This value isless than the concentration of silver thathas been shown to be released by nonwo-ven B in this and other studies,1 2 w h i c hwould support the case for use of MICs indressing selection. However, other dress-ings (eg, nonwoven A) have shown verysimilar antimicrobial activity to nonwovenB but with a far lower level of silverrelease (17 µg compared to 3,011 µg per 10cm x 10 cm dressing over 48 hours) andwith a measured total silver concentrationin solution of as little as 1 µg/mL.13 T h i swould suggest that using MIC data inselection of a SCD might be flawed and,hence, inappropriate.

Vol. 17, No. 8 August 2005 229


Figure 6. Percentage fluid retained under pressure for a range ofsilver-containing dressings.

Table 3. Fluid absorption of silver-containing dressings


Zero Compression

14.14.99.9

66.817.50.22.9

40 mmHg Compression

11.43.95.232.98.10.11.2

Fluid Absorption (g/10 cm x 10 cm)

Figure 7. Comparative dehydration rates for silver-containingdressings.

In the case of SCDs, the assumptions made forMIC testing may not be valid. For example, a sim-ple solution delivered as a bolus cannot be usedto represent a complex slow release formulation.Product promotional literature and company-sponsored research indicate that many of theproducts tested have a low tendency to dosedump silver and provide an extended and/orcontrolled availability of silver.14 Similarly, silvercannot be equated to the active forms of silver,and as demonstrated in this study, there appearsto be no correlation between total silver in solu-tion and antimicrobial efficacy.

A potential explanation for why SCDs behavein this way is the oligodynamic nature of ionic sil-ver.4 Exposure to low levels of constantly replen-ished ionic silver over an extended period of timecauses selective accumulation of silver ions with-in the bacterial cell and subsequent death. Theconcentration of ionic silver is kept low due to thelow solubility of silver ions in wound fluids.Optimal activity is therefore observed for dress-ings that can produce and maintain the highestconcentration of ionic silver permitted by thetotal wound environment.

Since it is difficult to assess each of these prop-erties of a dressing accurately by simple chemicalmeasurements, it is likely that a direct measure ofantibacterial activity in a simulated wound envi-ronment (as used in this study) is a more accuratemeasure of potential clinical antimicrobial activi-ty than measures of silver content or release intoan unrealistic solution, such as water, or mea-sures of MIC data.

Some commentators have suggested that therelease of large amounts of silver into the woundmay have detrimental effects on healing,1 5 a n dthere have been some reports of systemic toxiceffects, such as renal dysfunction.1 6 B u r r e l l1 2 h a s

commented on the fact that treatments, such assilver sulfadiazine (SSD), which compensate forthe inactivation of silver ions by providing a largeexcess of active silver agent, have created prob-lems for healthcare providers and patients. Dur-ing the course of the present study, it was notedthat the deionized water into which silver wasbeing released turned yellow after use with non-woven B and with nonwoven C. This suggeststhat in cases where the silver is initially presentedin a metallic and not an ionic form and where theconcentrations of silver in the dressing are partic-ularly high, a reaction occurs between the dress-ings and the silver contained within them. Thewound may be exposed to the resulting yellowcompound or complex, the effects of whichremain to be determined. Clinical experience withvarious forms of nonwoven B has shown that itcan lead to the deposition of silver in the woundand subsequent staining.1 7 Three i n - v i t r o s t u d i e shave also shown that the release of nanocrys-talline silver from dressings is toxic to ker-atinocytes and fibroblasts.1 8 – 2 0 Further investiga-tion is needed to clarify the effects of this onwound healing.

As Lansdown2 1 has highlighted, the physicalcomponents of SCDs are also important for therole they play in enhancing the wound environ-ment and promoting favorable conditions forreepithelization and repair. Of the propertiesexamined in this study, fluid handling is of par-ticular importance for dressing choice. Ideally, adressing should have the ability to rapidlyabsorb exudate, have a high absorptive capacity,and also not release fluid when compressed (eg,when a patient turns in bed). Comparison of thefluid-handling properties of the 7 SCDs demon-strated a variety of effects. Nonwoven C and the2 foams showed a high absorptive capacity;however, a much lower retention capacity sug-gests that as much as 50% of the fluid absorbedmight be lost under conditions of compression.Nonwoven A showed a high level of absorptionbut also demonstrated superior fluid retentionwith a drop of only around 20% under compres-sion. This was combined with a low degree ofcapillary wicking. In contrast, gauze demon-strated poor fluid-handling properties, having avery low absorptive capacity. Initially, the sur-



Table 4. Dressing pH in deionized water


pH After 3 Hours5.49.57.28.08.08.26.6

pH After 24 Hours5.47.76.87.27.77.46.5

face of nonwoven B dressing appeared to behydrophobic, resisting the absorption of anyfluid. When absorption did occur, it was lessthan that expected for an alginate-type dressing.Nonwoven B also showed a tendency to wickfluid, a physical property that may lead to leak-age, maceration, and possible tissue damage tothe periwound area.

Dehydration is a measure of how well fluid isbound into the dressing and may be an indicationof the dressing’s ability to maintain a moistwound environment for optimal wound healing.The dehydration rates in this study were mea-sured without the presence of a secondary coverdressing and are an indication of the relativeproperties of the dressings themselves. For afixed area of dressing, nonwoven A and nonwo-ven C had the lowest dehydration rates. Gauze,the foams, and the hydrogel showed significantlyhigher rates of dehydration.

Dressing pH was measured to provide an indi-cation of how the surface of a dressing changeswhen wet. It has been suggested that dressingswith a slightly acidic pH (similar to that of healthyskin; pH of 5.5) may be most comfortable to wear.There have been reports, however, of some dress-ings causing irritation or stinging after absorbingexudate, suggesting that a change in dressing pHmay be occurring. Dressings such as nonwoven Band gauze showed an alkaline pH (greater thanpH 7), which gradually adapted to a more neutralpH (pH 7) by the 24-hour timepoint, indicatingthat some form of chemical reaction may be takingplace. In contrast, nonwoven A and hydrogelremained stable throughout, with slightly acidicpH values of 5.4 and 6.6/6.5, respectively.

Table 5 summarizes the physical, chemical,

and antibacterial properties of the proprietarySCDs studied. This demonstrates the range ofproperties of the dressings examined, which mayhave implications for their clinical use. The mix-ture of antibacterial and fluid-handling propertiesshown in these studies suggests that individualsilver-containing antibacterial dressings have dif-ferent characteristics that make them more or lesssuitable for different types of wounds. This studysuggests that antibacterial dressing selectionshould be based on an assessment of the overallproperties of the dressing clinically relevant tothe wound type and condition rather than on sil-ver content or deposition alone.

Conclusions

Dressing selection is a vital part of the success-ful management of infected wounds and those atrisk of infection. Choice of an appropriateantibacterial dressing should be based on thewound type and condition and on clinicallyapplicable measures, such as antibacterial, heal-ing, and exudate handling effects, and not on anysingle laboratory parameter.

References

1. White RJ, Cooper R, Kingsley A. Wound colo-nization and infection: the role of topical antimi-crobials. Br J Nurs. 2001;10(9):563–578.

2. Bowler PG. Progression toward healing: woundinfection and the role of an advanced silver-con-taining Hydrofiber dressing. Ostomy Wound Man-age. 2003;49(Suppl 8A):2–5.

3. Cutting KF. A dedicated follower of fashion?Topical medications and wounds. Br J Nurs. The

Vol. 17, No. 8 August 2005 231


Table 5. A summary of physical, chemical, and antibacterial properties of silver-containing dressings

Antibacterial activity(after 7 days)

Fluid handling properties

Moisture retention

Skin-friendly pH

Nonwoven A

+++

+++

+++

+++

Foam B

+

++

++

+

Nonwoven B

+++

++

+++

+

Hydrogel

+

++

++

+++

Nonwoven C

++

++

++

++

Foam A

+

+++

+

++

Gauze

++

+

++

++

Silver Supplement. 2001;10:9–16.4. Lansdown AB. Silver. I: its antibacterial proper-

ties and mechanism of action. J Wound Care.2002;11(4):125–130.

5. Bowler PG, Jones SA, Walker M, Parsons D.Microbicidal properties of a silver-containinghydrofiber dressing against a variety of burnwound pathogens. J Burn Care Rehabil .2004;25(2):192–196.

6. Jones SA, Bowler PG, Walker M, Parsons D. Con-trolling wound bioburden with a novel silver-containing Hydrofiber dressing. Wound RepairRegen. 2004;12(3):288–294.

7. Demling RH, DeSanti L. Effects of silver onwound management. WOUNDS. 2001;13(1 SupplA):5–14.

8. Percival SL, Bowler PG, Russell D. Bacterial resis-tance to silver in wound care. J Hosp Infect.2005;60(1):1–7.

9. Wright JB, Lam K, Burrell RE. Wound manage-ment in an era of increasing bacterial antibioticresistance: a role for topical silver treatment. AmJ Infect Control. 1998;26(6):572–577.

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