New method to assess the water vapour permeance of wound coverings

Newmethodto assess the water vapour permeanceof woundcoverings Marcel F. Jonkman,Iza&kMolenaarand Paul Nieuwenhuis Departments of Histology and Medical Electron Microscopy, University of Groningen, Oostersingel 69/2, The Netherlands

9713

EZ Groningen.

Peter Bruin and AlbertJ. Pennings Department

Of Polymer Chemistry, University of Groningen, Nijenborgh 16, 9747 AG Groningen, The Netherlands

(Received 9 April 1987; accepted 1 July 1987)

A new method for assessing the permeability to water vapour of wound coverings is presented, using the evaporimeter developed by Nilsson. This new method combines the water vapour transmission rate (WVTR) and the vapour pressure difference across a wound covering in one absolute measure: the water vepour permeance (WVP). The WVP of a wound covering is the steady flow (g) of water vapour per unit (m’) area of surface in unit (h) time induced by unit (kPa) vapour pressure difference, g . m-’ . h-’ . kPa_‘. Since the WVP of a wound covering is a more accurate measure for the permeability than the WVTR is, it facilitates the prediction of the water exchange of a wound covering in clinical situations. Keywords: Water vapour permeance,

water vapour transmission rate, evaporimeter, wound covering, wound dressing

An ideal wound covering should possess semipermeable characteristics: it should be impermeable to bacteria and should be permeable to water vapour. The optimal water vapour permeability of such a semipermeable wound covering, however, has not yet been clearly established’. On the one hand, a wound covering should limit excessive evaporative water loss, thus stimulating wound healing* and preventing wound desiccation and body heat loss. On the other hand, a wound covering should also allow the passage of enough wound exudate to prevent accumulation of the exudate underthe membrane, which might lead toseparation of the wound covering from the wound bed, tissue maceration or wound infection3. One of the reasons why the optimal water vapour permeability has not yet been clearly established, may be the variability of permeation test results, caused by the use of various methods under non-standard vapour pressures and the use of various definitions. Another reason may be the limited predictive value of these permeation results for the clinical situation because the methods used are suited for measuring water vapour exchange in vitro and not for measuring in vivo. In most studies on wound coverings the permeability to water vapour is defined as the water vapour transmission rate (WVTR): the steady flow (g) of water vapour per unit Correspondence to M.F. Jonkman. 0 1988

(m*) area of surface in unit (h) time at a specified humidity and temperature. The American Society for Testing Materials (ASTM) method E96-80 (Ref. 4). for example, formulates six procedures for gravimetrical determination of the WVTR of materials in sheet form, of which Procedures BW and D are most frequently used. The WVTR is calculated from the weight loss of a reservoir that is filled with water and covered by the experimental membrane. The WVTR is directly related to humidity and temperature, in this case the vapour pressure difference across the membrane. However, in most reports different vapour pressures are used and thus no real standard is available. Therefore, the WVTR has limited significance for comparing permeabilities to water vapour of a specific wound covering between studies. It would be more convenient to have a method that expresses the permeability, corrected for the water vapour pressure difference across the membrane. Therefore, we suggest water vapour permeance (WVP), which is the amount of watervapour permeating through a surfaceof one m* in one hour induced by a vapour pressure difference of one kilopascal (g . m-* . h-’ . kPa-‘). In other words, the WVP is the WVTR per kilopascal vapour pressure difference and is thus corrected for vapour pressure. It is the aim of this study to develop a standard method for WVP assessment of wound coverings, valid within a physiological range of conditions. The test samples used were an experimental polyetherurethane membrane5 (PEU),

Butterworth Et Co (Publishers) Ltd. 0142-9612/88/030263-05$03.00 Biomaterials

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Water vapourpermeance

of wound coverings: M.F. Jonkman et al.

Op-Site (Smith and Nephew Ltd, Hull, UK), Om~derm (Omikron Scientific Ltd, Israel) and split skin graft, and the control was a metal grid.

MATERIALS

AND METHODS

Evaporimeter

C

Wound coverings Five wound-covering membranes were tested for their capacity to allow the passage of water vapour:

(11 PEU:

an experimental semipermeable ~lyetherurethane wound covering synthesized in one of our laboratories5. PEU consists of a polymer matrix containing noncommunicating microscopic cell-like cavities. (2) Op-Site*: a commercially available polyetherurethane wound covering with an adhesive layer of vinyl ether applied to one side. Op-Site is considered to be occlusive3. (3) Omiderme: a commercially available polyetherurethane wound covering, which has been grafted with acrylamide. Omiderm is described as being permeable to water vapou?. (41 Skin graft: split skin samples harvested from human cadaver, dried by air exposure and stored at room temperature. The dry form was chosen to minimize interference with the evaporimetric measurements’. (5) Meshed metal grid: representing a maximal permeable membrane. The surface ratio of open area to total area was 64%.

Theory The water vapour transmission rate - WVTR (g - m-* - h-’ ) through a plane membrane, if in steady state, can be equated by Wroblinski’s modification of Fick’s first law of diffusions: WVTR = P(P, - P2)/L

(1)

where P (g . mm. rn-*. h-’ . kPa-‘) is the vapour permeability: the amount(g) of watervapourthrough unit (mm) membrane thickness per unit (m’) surface area per unit (h) time induced by unit (kPa) vapour pressure difference; P, - P2 (kPa) is the water vapour pressure difference across the membrane and L (mm) is the thickness of the membrane. The term P is a material property and can only be applied to flat, homogeneous (non-laminated), compact materials. P is not exactly constant since it can differ slightly with temperature and vapour pressure. The membrane thickness L is not necessarily constant either during the time of the experiment, since polymer membranes can swell at higher humidities; but at present no method is available to measure film thickness during evaporimetric measurementsg. The ratio of P to L is the water vapour permeance (WVP). We use WVP to express the water vapour permeability of complete wound coverings, instead of P, since P applies only to the permeabili~ of materials with a standard thickness. Obviously WVP is not necessarily constant over the range of vapour pressures. In our method, however, the small deviations of the VWP with temperature, vapour pressure and membrane thickness are probably within the error of measurement. For simplicity, we therefore assume WVP to be constant, with the reservation that this unit introduces some uncertainty as the WVTR is not necessarily linearly proportional to the vapour pressure difference. Thus, if thevapour pressures P, and P2 are known, the~Pcan be deduced from an observed value of WVTR as follows: WVP = wvTW(P,

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(2)

Figure 1

The experimental model for measuring water vapour permeance.

The experimental model (Figure 7) was built up in a closed cabinet (150 I) to prevent disturbance of the measurements due to air currents. A double-walled reservoir of glass, half filled (20 ml) with distilled water or a saturated salt solution, was used as the source of watervapour. The wound covering was stretched between two rings and secured with a screwed open cap on the reservoir. The position of the wound covering on the reservoir was similar to that when applied to wounds; for instance, Op-Site was positioned with its adhesive layer facing the moist air compartment in the reservoir. The vapour pressure under the wound covering was manipulated by warming the air compartment (20 ml) between water surface and wound covering to preselected temperatures, by means of circulating water with constant temperature between the double walls of the reservoir. Note that saturated air does not necessarily have a relative humidi~ of 100%. For instance, the relative humidi~ of saturated air above a saturated high-cohesion salt solution is lower than above pure water (Tab/e I). Because the air in the reservoirwas saturated with water vapour, we may assume:

PI = RHpHzO,~ where P, (kPa) is the vapour pressure and RH (%) is relative humidity of the air in the reservoir and PHzO,t is the saturated water vapour pressure (kPa) at temperature t of air above pure water” (Tab/e 7). In the cabinet, the average ambient air conditions were: P2 = 1.02 kPa at 32% RH and 25°C. Since the lowest P, was 1.68 kPa, we may assume:

Thus, water transfer was always in the upwards direction from the reservoir into the cabinet. For comparison, under extreme physiological conditions the vapour pressure difference is 5.2 kPa, in the case of a warm (36”C), humid (100% RH) skin in relatively cold

Table f Relative ~urn~d~jes and water vapour pressures of saturated air above saturated salt solutions and pure water at different temperatures Solution

Temperature (“C)

RH (%I

PI

Mg iNo,),

25 25 25 30 35 37

52.9 75.3 100 100 100 100

1.68 2.39 3.17 4.24 5.62 6.72

NaCl

“20 ‘9 “~0 ‘420

(@'a)

Water vapour permeance

0

1

2

3

4

Water vapour pressure difference

1

2

3

5 (kPa)

4

Water vapour pressure difference

6

5

6

(kPa)

of wound coverings: M.F. Jonkman et al.

(20X), dry (30% RH) ambient air close to the skin. In our system, P, - P, ranged from 0.73 to 5.83 kPa, broadly covering physiological conditions”. An evaporimeter (model EP-IC, Servo Med, Stockholm) was used for measuring WVTR. This apparatus, developed by Nillson’2, 13, digitally displays WVTR in g . me2 . hh' , RH in % and vapour pressure in mmHg (1 mmHg = 0.1333 kPa). The WVTR is computed from the estimation of the vapour pressure gradient of the air layer immediately above the surface of the membrane. Measurement must always be executed within 1 cm above a surface in a microclimate without air currents. The accuracy of the evaporimeter after calibration is: WVTR -t 15% + 2 g . me2 . h-’ and P2 + 6%, within 20-l 00% RH. The WVTR offset was adjusted by placing the probe of the evaporimeter above the opening, which was covered by an impermeable plastic shield. The display of the WWR was zeroed, using the offset-button of the evaporimeter. After removing the shield, the probe was placed directly above the membrane. The WVTR was continuously monitored on a potentiometric plotter (Tekman Ltd. UK) connected to the evaporimeter. Minor fluctuations of the WVfR output were smoothed by adjusting the electronic filter to average the WVTR over 30 s periods. The final WVTR was sampled when a steady state had been reached. The corresponding P2 above the membrane was measured immediately afterwards with the probe 10 cm from the reservoir. Twelve observations were performed with each type of membrane, i.e. two measurements under the six conditions listed in Table 1. Data were processed using a spreadsheet14 on a microcomputer. The WVP was calculated from the slope of linear regression line with zero-intercept of the WVTR as a function of the watervapour pressure difference. Differences between WNPs of membranes were analysed using the Student’s f-test. All data are presented as regression coefficient f 2.12 X standard deviation (95% confidence interval, n = 12).

RESULTS Fick’s law of diffusion was obeyed within the tested range of vapour pressure differences: the rate of water vapour transmission was linearly proportional to the vapour pressure difference (Figure Z), i.e. WP for a particular wound covering remained constant (Tab/e 2). The water permeation through PEU, however, was not quite Fickian. The rate of water vapour transmission showed some tendency to increase more with the vapour pressure difference than one would expect from Fick’s law (Figure 2). The deviation of the linear regression line with the real curve, however, was within the error of measurement. The 95% confidence borders of the regression Table 2 Srarisrics of water coverings

Water vapour pressure difference

(kPa)

The water vapour transmission rate as a function of the water Figure 2 vapoor pressure difference (PI - PZJ through (a) metal grid; (b) Omiderm; (c) PEU A, Op-Sire n and splir skin graft l. Each symbol represents one observation; linear regression lines with zero-intercept are drawn through the dara of each wound covering.

vapour permeance

assessment

of wound

Wound covering

WVPtSD

P

Pb

Split skin (dry) Op-Site PEU Omiderm Metal grid

0.28 + 0.03 1.96 k 0.05 6.69 f 0.15 7.63 + 0.25 8.15kO.16

0.84 0.98 0.99 0.98 0.99

< < < <

0.001 0.001 0.001 0.001

Toefficient of correlation. bLevel of significance with above membrane (one-sided).

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1988, Vol9 Ma/

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Water vapour permeance

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Jonkman et al.

Water v8pour tmnsmi$sion rates in vitro of v8rious wocmd coverings, test conditions and extrapolated water vapour permeances

Table 3

Wound covering

wiTR(Q.m-2.

Biobrane Biobrane Metalline Omiderm C&-Site Op-Site Op-Site Skin human (wet) Skin pig (wet]

58 154 53 208 33 21 30 15 9

h-‘)

WVP (g

‘ rn-‘.

h-’ kPa-‘)

23 55 10 83 5 8 11 4 2

coefficient (slope of line) differed less than rt2% from the mean (Table 3), showing a significant reproducibility. In all instances, a steady state of WVTR was reached within 20 minutes after placing the probe above the membrane. The split skin graft was the most occlusive wound covering, followed by Op-Site, which appeared to be the most occlusive synthetic wound covering tested. PEU was three times more permeable than Op-Site. Omiderm was almost four times as permeable as Op-Site (Tab/e 2) and just less permeable than the metal grid.

The results clearly demonstrate that WVP can indeed be used as a reproducible parameter for assessing the water vapour permeance of wound coverings. Moreover, the use of WVP allows us to compare results obtained under different in-vitro conditions and facilitates the extrapolation of these results to the evaporative water loss of a wound covering

in vivo. The term water vapour permeance is also mentioned in the ASTM (Ref. 4) method E96, where it is expressed in ‘perms’ (grains ft-’ h-l). The ASTM protocol defines the water vapour permeance (UP) as dependent on the vapour pressure difference. The limited validity of the ASTMdefined WVP could be the reason why this term is rarely used; the alternative term, WVTR, which is indeed dependent on the vapour pressure difference, qualifies the permeability of a membrane under that restriction equally well. In contrast, we observe a constant WVP over a wide range of vapour pressures, so that WVP becomes a more useful value than WVTR. For accurate WVP assessment it is not necessary to measure WVTR for a wide range of water vapour pressures as we did in this study. It would suffice to determine WVP under a water vapour pressure difference that models the physiol~ical condition. This can be done if distilled water is used as the vapour source and the cabinet, including the reservoir, is warmed up to 35°C with a relative humidity of 40%; the resulting vapour pressure difference across the membrane will be 3.4 kPa (Table 1). For statistical reasons at least four measurements should be made. We did not determine the diffusion constant of the wound covering material because this constant is only of importance for homogeneous compact materials. Many wound coverings, like Biobrane@, PEU and skin graft, have a porous structure. Therefore, the WVP is a more appropriate measure for such wound coverings, as it expresses both the effect of convection through the porous part, as well as the diffusion through the compact part of the wound covering’5. As mentioned before, WvTRs of wound coverings

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Temp (“C)

RH (%)

P, - P, (kPa)

Method

Author. reference

37 23 33 37 37 37 23 28 28

100-60 100-O 100-O 100-60 100-O 100-60 100-O 100-O 100-O

2.5 2.8 5.0 2.5 6.3 2.5 2.8 3.8 3.8

ASTM ASTM ASTM ASTM Payes ASTM ASTM ASTM ASTM

Behar, 6 Aiba. 16

E96 Proc 8W E96 Proc BW ES6 Proc D ES6 Proc BW Cup Method E96 Proc BW ES6 Proc BW ES6 Proc D E96 Proc D

Neuman. 17 Eehar, 6 S&N, 18 Behar, 6 Aibe, 16 Schwope. 19 Schwope, 19

reported in the literature vary considerably (Refs 6, 16-l 9). From these reports we calculated the WVP from the WVTR and the specified test conditions as reported using Equations (2) and (3) (Table 3). These calculated WVPs however, still differ considerably too, even if the same method of measurement has been used (see Table 3, Biobrane). The most important cause of this WVTR diversity is likely to be the use of different vapour pressures. But why do the calculated WVPs, which are corrected for vapour pressure, still vary? Newns’ has summarized possible errors in determining water vapour permeabili~ of laminae. We wifl now focus on two main sources of error for commonly used methods. The first main source of possible error is the hydrostatic pressure of the water column in the ASTM Procedure BW (Inverted Water-Method). Some investigators’5 favour this method because the static air layer, which is a diffusion barrier in the reservoir, is avoided. The cup is placed upside down and a water column presses on the membrane. However, the hydrostatic water pressure will increase the water loss, making this a non-physiological model. Moreover, any water leakage through the membrane will spoil the test. The second main source that may introduce errors is the disturbance of the micr~limate above the membrane. The ASTM protocol prescribes a continuous air current of ‘at least 500 feet per min’ over the exposed surface, to prevent accumulation of diffused water vapour above the membrane. However, the air current will influence the water loss to varying extents by forcing the evaporation. This drawback can be eliminated if a method is used which does not disturb the microclimate. Our new method of using the eva~rimeter for WUP assessment has several advantages. An absolute reproducible value for water vapour permeability is obtained. The microclimate above the membrane is not disturbed by air currents, so forced evaporation is eliminated. The measurements can be performed in 20 minutes, which is considerably faster than the 18 days needed for an accurate gravimetrical assessment4. Moreover, the additional advantage of the use of the evaporimeter is that it can also be used for measuring evaporative water loss in clinical situations. However, in a method like ours with an air compartment under the membrane, the WVPcan be underestimated. The water vapour in the reservoir can erroneously be assumed to be saturated when it is in fact not saturated, because of extreme water loss through very permeable barriers, like the control metal grid or hydrogel wound dressings. This error can be minimized by sampling WVTR only when a steady state has been reached. Fu~hermore, we foresee no problems of dehydration of hydrogel wound dressings by the empty space in the reservoir, since

Water vapour permeance of wound coverings: M.F. Jonkman et al.

measurements can be performed within a short period of 20 minutes. The WVTR measurement of the evaporimeter has an absolute error of 2 g . mm2 . hh’. This error limits the accuracy of the WVP calculation from low WVTR values, such as under a low vapour pressure difference or in case of very impermeable membranes, such as the dry skin graft in our study. If more observations are performed, an accurate WVP assessment can be obtained. If we moistened the skin the WVP increased from 0.3 to 3.0 g . m-* . h-’ . kPa-‘, which was probably partly due to the applied moisture itself. We conclude that our new method of using an evaporimeter is a valid method for determining the permeability to water vapour of wound coverings under a wide range of physiological conditions. Moreover, this method markedly simplifies the in-vitro measurement of optimal permeabilityforwatervapourof a wound covering.

Coverings, (Ed. D.L. Wise), CRC Press, Boca Raton, Florida, 1984, 4 5

6

7

8

9 10

ACKNOWLEDGEMENTS We wish to thank letse Stokroos and Dick Huizinga for the drawings, Dr. Berend van der Lei and Dr. Sjef Schakenraad for their critical comments on the manuscript and Ir. Gerard J. te Meerman for his advice on statistics. This work was supported in part bygrantsfrom the Groningen Science Park Foundation,and the Dutch Prevention Fund (no. 28-l 424).

11

12 13

14 15

REFERENCES 1

2

3

Qurnn, K.J.. Courtney, J.M., Evans, J.H.,Gaylor, J.D.S. and Read, W.H.. Principles of burn dressings, Biomaterials 1985, 6, 369-377 Winter, G.D., Formation of the scab and the rate of epithelialization of superficial wounds rn the skin of the young pig, Nature 1962, 193, 293-294 May, S.R.. Physiology, immunology, and clrnrcal efficacy of an adherent polyurethane wound dressing: Op-Site, in Burn Wound

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