characterisation of poly vinyl alcohol

7/30/2019 Characterisation of Poly Vinyl Alcohol

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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 11, November 2012)

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Characterisation of Poly vinyl alcohol (PVA) / Silver nitratenanomembranes for their suitability in wound dressing

applicationsDr. K. P. Chellamani1, P. Sundaramoorthy2, T. Sureshram3

1,2,3The South India Textile Research Association (SITRA), Coimbatore641 014,

Tamil Nadu, India

Abstract - Special wound care product was developed using

Poly vinyl alcohol (PVA) blended with Silver nitrate.

Electrospinning technique was adopted to produce

PVA/Silver nitrate nanomembranes, which was used as

wound dressings. PVA/Silver nitrate wound dressings(PSNWD) have high moisture vapour transmission property

and good antimicrobial activity. PSNWD substrates have mild

cytotoxicity reactivity and have excellent odour absorbing

capability. PVA/Silver nitrate wound dressing do not cause

skin irritation even after 72 hours of contact with the wound

and the time taken for wound healing while using PSNWD is

just 50% of that in the case of an open wound.

Keywords- Cytotoxicity, Electrospinning, nanomembranes,

Poly vinyl alcohol (PVA), Silver nitrate and wound dressing.

I. INTRODUCTION Electrospinning [1] is a technique used to process

polymer fiber diameter in the range of micrometers tonanometers. Electrospinning process has increased in

recent years, and this technology has been exploited for a

wide range of applications. Among various nanomembrane

processing techniques like drawing, template synthesis,

phase separation and self assembly, electrospinning is the

only method capable of producing continuous polymer

nanofibres. Electrospun polymer nanofibers have very large

surface area to volume ratio and flexibility in surface; these

properties make the polymer nanofibers use in many

important applications. Electrospun nanofibers achieve

good result on properties required for wound dressings,

because of its nano-size structure.

Choice of wound dressings [2] is dependent upon the

severity and type of injury, as well as the stage of healing

and the presence or absence of infection. The essential

parameters require for wound dressing to heal wounds are

Absorptivity [3] and oxygen permeability [3]. Additionally,

ability to conform to the wound bed, provide appropriate

adherence/non-adherence to the healing tissue [4], provide

a barrier to bacteria, enhanced moisture management5,

bioactivity, resorbability [5] and occlusivity are highly

desirable and correlated with better outcomes.

Poly (vinyl alcohol) (PVA) has excellent chemical

resistance, physical properties, biodegradability and good

fiber forming capability in electrospinning techniques [6].

PVA polymer is suitable for blending various polymers and

additives in electrospinning process to achieve uniformnanofibers [7].

*Correspondence to the author:

P. Sundaramoorthy

E-Mail ID: [email protected]

Silver nitrate is a salt used to produce Silver

nanoparticles by insitu polymerisation with PVA polymer.

Silver nanoparticles have been widely used in various

biomedical fields like wound dressing materials, body wall

repairs, augmentation devices, tissue scaffolds,

antimicrobial filters, and so forth. In the present study, the

PVA polymer is blend with Silver nitrate in different blend

proportions and optimising the electrospinning parameters

using scanning electron microscope (SEM) analysis and

antimicrobial activity [8]. The optimised parameter is used

for further trials and the produced nanomembrane were

characterised for Fourier transform infrared spectroscopy

(FTIR) and transmission electron microscope (TEM)

analysis. The PSNWD is test for wound dressing properties

like Moisture vapour transmission rate (MVTR) [9],

cytotoxicity [10], odour control capability [9], Skin

irritation and wound healing ability

II. MATERIALS AND METHODSA. Materials

The polymers, chemicals and solvents used for this study

are given below

i. Poly Vinyl Alcohol (PVA) with a molecular weightof 1,24,000 g/mol

ii. Silver nitrate saltiii. Nitric acid andiv. Distilled water.


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B. Methods

The electrospinning technique was used to produce thePVA/Silver nitrate nanomembranes. In electrospinning

technique, there are two methods for the preparation of

polymer solution.

1. Method I:In this method, the polymer is dissolved in

the solvent to form polymer solution.

2. Method II:In this method, the polymer is heated upto

its melting point and thereby the polymer solution is

formed.

In the present study, Method I was adopted to prepare

the polymer solution.

C. Solution Preparation

The n gram of polymer is weighed with the help of

weighing balance and m milliliter of solvent is measured

with the help of micropipette.

The concentration of the polymer solution is determined

by the formula,

Concentration =n gram of polymer

X 100 (%)m milliliter of solvent

D. PVA/Silver Nitrate Blend Solution Preparation

The n gram of PVA and y gram of Silver nitrate salt

were dissolved in m millilitre of distilled water (pH of

distilled water - 4).

PVA/Silver nitrate blend solution of 0.5* %, 1 % & 2 %were prepared at 80 C.

* 0.5 % by weight of Silver nitrate in the blend solution of

PVA/Silver nitrate.

E. Electrospinning of Nanomembrane

The term nano represents the size of the material in

10-9

meter. The Polymeric nanofibers can be processed by a

number of techniques like Drawing, Template Synthesis,

Phase Separation, Self-Assembly and Electrospinning.

Among the various processing techniques, electrospinning

is the only method capable of producing continuous

polymer nanofibres. Electrospinning is a unique technology

that can produce non-woven fibrous articles with fiberdiameters ranging from tens of nanometers to microns, a

size range that is otherwise difficult to achieve by

conventional techniques.

A schematic representation of Electrospinning set up is

given in Figure 1. The basic setup for electrospinning

consists of three major components: a high-voltage power

supply, a spinneret (a metallic needle), and a collector (agrounded conductor).

A high electrical supply, typically 130 kV, is applied to

a polymer solution contained in a syringe.

Fig 1 Schematic Diagram of Electrospinning Setup

The electrospinning process is governed by manyparameters, classified broadly into solution parameters,

process parameters, and ambient parameters. Solution

parameters include viscosity, conductivity, molecular

weight, and surface tension. Process parameters include

applied electric field, tip to collector distance and feeding

or flow rate. Ambient parameters include temperature &

humidity maintained during electrospinning process. These

parameters significantly affect the fiber morphology

obtained. By proper manipulation of these parameters,

nanofibers of desired morphology and linear density can be

obtained.

III. PREPARATION AND OPTIMISATION OF PVA/SILVERNITRATE NANOMEMBRANE

PVA/Silver nitrate solution was prepared by In-situ

polymerisation method. In this method, the Silver nitrate

and PVA were mixed in distilled water with pH value at

4.0 (pH value of distilled water maintained by adding

Nitric acid with the same). Maintaining pH value of the

distilled water at 4.0 is mainly to control the ionisation

reaction of Silver ions. To produce PVA/Silver nitrate

nanomembrane, 4 different combinations of PVA/Silver

nitrate were taken. They are,

- 10% PVA/ 0.5*% Silver nitrate- 10% PVA/ 1.0% Silver nitrate- 10% PVA/ 1.5% Silver nitrate and- 10% PVA/ 2.0% Silver nitrate

* 0.5% by weight of Silver nitrate on the weight of PVA

solution with which Silver nitrate is added.

These 4 combinations of PVA/Silver nitrate were

electrospun to form nanomembranes.


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During electrospinning, process parameters like i)

Applied voltage, ii) Flow rate of solution and iii) Tip tocollector distance were optimised by adopting the same

method followed for optimisation of process parameters for

spinning PVA/Chitosan nanomembrane. A total numbers of

27 trials were conducted in the optimisation process.

Optimum values of electrospinning process parameters for

the 4 combinations of PVA/Silver nitrate are given in

Table 1.

Table 1

Optimum Values of Process Parameters* for the PVA / Silver NitrateNanomembranes

S.No.

Combinations

Process parameters

10%

PVA/

0.5%Silver

nitrate

10%

PVA/

1.0%Silver

nitrate

10%

PVA/

1.5%Silver

nitrate

10%

PVA/

2.0%Silver

nitrate

1.Applied voltage

(kV)18 15 18 18

2.Flow rate

(milliliter/hr)0.5 0.5 1.0 0.5

3.Tip to collector

distance (cm)13 8 10 8

*Optimum process parameter in electrospinning is one

where uniform fibres without any bead could be produced.

Figure 2, shows the electrospun PVA/Silver nitrate

nanomembrane obtained after 5 hours of process (21 cm

length).

Fig 2 Electrospun PVA/Silver Nitrate Nanomembrane (after 5 hours

of processing)

SEM picture of PVA/Silver nitrate nanomembrane

produced using different combinations of PVA/Silvernitrate are shown in Appendix I. PVA/Silver nitrate

nanomembrane produced using different blend proportions

were also tested for their antibacterial activity.

A. Antimicrobial Activity of PVA/Silver NitrateNanomembranes

The antimicrobial activity of the nanomembrane is very

important for wound dressing applications. Antimicrobial

test was conducted based on quantitative analysis by JIS L

1902 method. Bacteria used in this study were

staphylococcus aureus (ATCC 6538)*.

*S. Aureus is a Gram positive micro-organism.

Antimicrobial activity of the nanomembranes developedin various percentages of PVA/ silver nitrate viz 10/0.5,

10/1, 10/1.5 and 10/2 was tested as per the standard JIS L

1902. Briefly, 0.2 ml of the Staphylococcus aureus has

been inoculated on 0.5 g of each sterile test material and

incubated for 18 h at 37 C. Following incubation, ratio of

microbiostasis was calculated using the formula F=

Mb -Maand the result was expressed in log reduction. Here

F is growth value, Mb is average value of common

logarithm of number of living bacteria after 18 hr andMais

average of value of common logarithm of number of living

bacteria at 0th

hr. Antimicrobial activity of the PVA/ silver

nitrate nanomembranes and 100% PVA nanomembranes

are given in Table 2.The results confirm that amongvarious combinations of PVA/ silver nitrate, the

nanomembranes developed using 10% PVA/ 2% Silver

nitrate, completely arrested the bacterial growth (100 %)

when compared to control (0 %)*.Hence this combinationhas been chosen for further studies.

Table 2

Antimicrobial Activity of PVA/silver Nitrate Nanomembrane

S.No.

PVA/Silver nitrate

nanomembrane

combinations

% Bacteria reduction

after 18 h of treatment Remarks

Control Treated

1 10% / 0.5% 0 54Moderate

activity

2 10% / 1.0% 0 75Good

activity

3 10% / 1.5% 0 82Goodactivity

4 10% / 2.0% 0 100Very Good

activity

*Control refers to nanomembranes made out of 100% PVA.


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IV. CHARACTERISATION OF PVA/SILVER NITRATENANOMEMBRANE

A. Moisture Vapour Transmission Rate (MVTR) of PVA/Silver Nitrate Nanomembrane

The MVTR is an important criteria for an wound

dressing material. The liquid formed inside the wound layer

first changes to vapour state and then transported to

atmosphere. This moisture vapour transmission helps to

heal the wound; otherwise will be wound infection. The

PVA/Silver nitrate nanomembranes were tested for MVTR

as per BS EN 137262: 2002 [9].

A test sample of 40 mm diameter is taken and fixed over

a container of 35.7 mm inner diameter, containing 20 ml of

distilled water. The test sample container is weighed (W 1)

before the start of the test. Then the container is kept insidean incubator for 24 hours (conditions maintained inside the

incubator i) Temperature: 37 1 C and ii) RH 20%). After

24 hours, the container is taken out and again weighed

(W2).MVTR is calculated based on the formula,

X = (W1W2) 1000 x 24/T

Where,

X is MVTR (g/m2/24 hours)

W1 is the mass of the container, sample and liquid in

grams

W2 is the mass of the container, sample and liquid ingrams after the test duration

and

T is the test period in hours

MVTR values of PVA/ Silver nitrate nanomembrane

samples provided in this study are given in Table 3.

Table 3

MVTR Values of PVA/Silver Nitrate Nanomembranes

S. No.PVA/Silver nitrate

nanomembrane combinations

MVTR (g/m2/24

hours)

1. 10 %/2 % 3060

2. 10%/1.5 % 3070

3. 10%/1.0 % 31204. 10%/0.5 % 3180

With increase in the concentration of Silver nitrate in

PVA, MVTR values tend to increase. This is attribute to

the reduction in the diameter of the corresponding

nanomembrane substrates. For an infected skin, MVTR

value of 2000 to 2500 in good [11]. Hence, all the

PVA/Silver nitrate nanomembrane substrates made in this

study can be considered as suitable for infected skins.

B. Fourier-Transform Infrared Spectroscopy (FT-IR) forPVA/ Silver Nitrate Nanomembrane

Functional groups present in PVA/Silver nitrate

electrospun nanomembranes were studied using FTIR

spectroscopy.

The FTIR spectra of electrospun PVA/ Silver nitrate

nanomembrane (Combination of 10% / 2%) are shown in

Figure 3.

Fig 3 FT-IR Spectra of PVA/Silver Nitrate Nanomembrane

In FTIR spectra, X-axis represents wavenumbers and Y-

axis, the transmittance. It is discernable from the FTIRspectra, that these are peaks at i) Wavenumbers of 3336

cm-1

(peak 1) and ii) wavenumber of range below 500 cm-1

(peak 2). Peak 1 indicates the presence ofOH groups and

peak 2, the presence of silver groups. This inturn confirms

the presence of PVA polymer and nano Silver

materials in the substrates produced.

C. Cytotoxicity Studies on PVA/Silver NitrateNanomembrane

An in vitro cytotoxicity test was conducted for the

PVA/Silver nitrate substrates using Direct contact method

was performed using test sample PVA/Silver nitrate

nanomembrane as per ISO 10993-5 [10].

In this method, a monolayer culture of L-929 is used as

the medium. Test samples, negative and positive controls

were placed on the medium. After incubation at 37 1 C

for 24 hours, the monolayer medium is examined

microscopically for the response around the test specimen.

The reactivity levels observed are graded as 0,1,2,3 & 4

based on zone of lysis, vacuolization, detachment and

membrane disintegration as explained in Table 4.


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Table 4

Grading of Cytotoxicity

Reactivity Description of reactivity zone Grade

NoneNo detectable zone around or under

sample0

SlightlySome malformed or degenerated cells

under specimen1

Mild Zone limited to area under specimen 2

ModerateZone extending specimen size up to 0.33

cm3

SevereZone extended farther than 0.33 cm

beyond specimen4

As per ISO 10993 5, reactivity grade higher than 2 is

considered as cytotoxicity. The PVA/Silver nitratenanomembrane substrates produced in this study (10%/2%)

were found to have Mild reactivity and hence graded as

2 as far as cytotoxicity is concerned.

D. Odour Control Ability of PVA/Silver NitrateNanomembrane

The objective of the test is to assess the resistance of

wound dressing to penetration by odour. The sample is

tested as per BS EN 13726 6: 2003 method [9]. A

stainless steel container of 35.7mm inner diameter and

40mm height is used for carrying out the test. The test

consists of two parts.

1. Part I: Here, the container is heated for 1 hour at 105 C.Then, Nitrogen gas is purged inside the container. After

this, 0.5 l of pure Diethylamine is also passed inside the

container and the container is kept inside anincubator at 37C for 20 minutes. After 20 minutes, 250 l sample of

Nitrogen gas is removed from the container and injected

into GC (Gas Chromatography) instrument. The GC

instrument estimates the intensity ofDiethylamine presentin Nitrogen gas and provides the information in the form of

a graph (Figure 4 a). In the graph, the intensity of

diethylamine present in Nitrogen gas is plotted against

time.

2. Part II: The container is heated for 1 hour at 105 C.

After this, a 1.3% w/v solution of Diethylamine (0.26 gramof Diethylamine dissolved in 20 ml of distilled water) is

added into the container. Then, the test specimen is kept

inside the container, Nitrogen gas is purged inside the

container and the container is kept inside an incubator at 37

C for 30 minutes. After 30 minutes, 250 l sample of

Nitrogen gas is removed from the container and injected

into GC (Gas Chromatography) instrument for the analysis

of intensity of Diethylamine in Nitrogen gas. The

information is given in Figure 4 b.

Fig 4 a) Diethylamine Vs time in Control specimen

Fig 4 b) Diethylamine Vs time in PVA/Silver nitrate nanomembrane

In the Graph, the peak shown at 25 27 minutesrepresents the Diethylamine. The Diethylamine present in

PVA/Silver nitrate nanomembrane is just 5% of that

exhibited by control specimen. Hence, PVA/Silver nitrate

nanomembrane has excellent odour absorbing capability.

E. Skin Irritation Potential of PVA/Silver NitrateNanomembrane

PVA/Silver nitrate nanomembrane samples were

evaluated for potential skin irritation, when they are used

for covering the wound. The evaluation was as per ASTM

F 719-81 standards. The test method is explained in

Appendix II. Six healthy rabbit were selected for the study

and PVA/Silver nitrate wound dressings were applied on

the 6 rabbits. The study has shown that PVA/Silver nitrate

wound dressing do not causes any skin irritation even after

72 hours of contact with the wound.

F. Wound Healing Ability of PVA/Silver NitrateNanomembrane

The extent of wound healing provided by a given wound

dressing was evaluated using the method proposed by

Morton & Malone.

Time (minutes)

IntensityofDiethylamin

e

I

i

i

l

i

Time (minutes)


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As per this method, twelve healthy rats were employed

for the experimentation and they were separated into 2groups (Group I and Group II) each with 6 rats. Excision of

wounds was made on the rats as per the method suggested

by Morton & Malone [12].

The rats were anaesthetized with an aesthetic ether and

placed in operation table in their natural position. A squarewound of about 1.5 cm (width) x 0.2 cm (depth) was made

on depilated ethanol-sterilized dorsal thoracic region of

rats. Infection was made on wounds by staphylococci

aureus.

Table 5

Wound Healing in Open Wound and PVA/Silver Nitrate Nanomembrane Treated Wound

Type of woundsExtent of Wound healing

Day 0 Day 7 Day 14 Day 21 Day 28 Day 35

Open wound

PVA / Silver

nitrate

nanomembrane

Group I rats were left as they were withoutapplication of any wound dressing (open wound)

Group II rats were treated using PVA/Silver nitratenanomembrane wound dressing

The dressings were applied on the wounds of the rats

every day till the epithelialisation was complete. The extent

of wound contraction was studied by tracing the raw wound

area in a tracing paper on 0th

day, 7th

day, 14th

day, 21th

day,

28th

day and 35th

day.

The weights of the traced portions of the wounded area

of rats subjected to different treatments were measured

using electronic balance. Based on the difference in weight,

the superiority or otherwise of a particular wound dressing

is determined. Table 5 shows, the extent of wound healing

noticed while using different wound dressings on differentdays.

The weights of the traced portions of open wound and

PVA/ Silver nitrate nanomembrane treated wound ondifferent days are shown in Table It is clear from Table 6,

that there is a decrease in wound area with the application

of PVA/ Silver nitrate nanomembrane wound dressings.

Table 6

Weight Of The Traced Portions Of Wounds On Different Days

Type of

wound

Weight of traced portions of the wound (grams)

Day 0 Day 7Day

14

Day

21

Day

28

Day

35

Open

wound100 154.89

110.7

882.44 40.72

11.2

3

PSNWDtreatedwound

100 61.74 20.87 0


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G. Transmission Electron Microscope (TEM) Analysis ofPVA/Silver Nitrate Nanomembrane

The Transmission electron microscope (TEM) analysis

was used to study the size and distribution of Silver

nanoparticles inside the single PVA/Silver nitrate nanofiber

(Figure 5).

Fig 5 TEM Image of PVA/Silver Nitrate Nanofiber

In the Figure 5, the black dots represent the Silver

nanoparticles. It is clear that the distribution of Silver

nanoparticles on the nanofibre is quite uniform. The

average size of the Silver nanoparticles was found to

be 3.7 nm.

V. CONCLUSIONS SUMMARISING THE ACHIEVEMENTSAND INDICATION OF SCOPE FOR FURTHER WORK

SITRA has developed special type of wound dressingviz) PVA/Silver nitrate nanomembrane wound

dressing (PSNWD).

PSNWD are found to be ideally suited for infectedskins in view of their high moisture vapour

transmission property (MVTR).

PSNWD Substrates have good antimicrobial activity. PSNWD substrates have mild cytotoxicity reactivity

and exhibited excellent odour absorbing capability.

PVA/Silver nitrate substrates do not cause any skinirritation even after 72 hours of contact with thewound.

The time taken for wound healing while usingPSNWD is around 50% lower as compared to that in

the case of an open wound.

REFERENCES

[1] Zheng-Ming Huang et al., 2003. A review on polymer nanofibers byelectrospinning and their applications in nanocomposites. Compos.

Sci. Technol. 63: 2223-2253.

[2] Marian McCord, 2009. Challenges in advanced nanofiber wounddressings (Annual report, National Textile Center). Retrived from

http://ebookbrowse.com/f09-ns06-a9-pdf-d55286555.

[3] Joshua S. et al., 2008. Wound healing dressing and drug deliverysystems: A review. J. Pharm. Sci. 97 (8): 2892-2922.

[4] Zhong S. P. et al., 2010. Tissue scaffolds for skin wound healing anddermal reconstruction. WIREs Nanomedicine and Nanotechnology.

2:510-525.

[5] Mehrdad Kokabi et al., 2007. PVA-Clay nanocomposite hydrogelsfor wound dressing. Eur. Polym. J. 43: 773-781.

[6] Bin Ding, 2002. Preparation and characterisation of nanoscaledPoly(Vinyl alcohol) fibers via electrospinning. Fiber. Polym. 3(2):

73-79.

[7] Kim Soo-Hwan. et al., 2011. Antibacterial activity of Silver-nanoparticles against staphylococcus aureus and Escherichia coli.

Korean J. Microbiol. Biotechnol. 39 (1):77-85.

[8] JIS L 1902: 2008. Testing for antibacterial activity and efficacy ontextile products. Japanese Industrial Standard (JIS). ICS 07.100.99:59.080.01.

[9] BS EN 13726: 2002. Testing methods for primary wound dressings.British Standard. ICS 11.120.20.

[10] ISO 10993-5: 2009. Biological evaluation of medical devices Part5: Tests for in vitro cytotoxicity. International standard.

[11] Ching-Wen Lou, Ching-Wen Lin, Yueh-Sheng Chen, Chun-HsuYao, Zen-Shoung lin, Chieh-Yu Chao and Jia-Horng Lin. 2008.

Dressing properties evaluation of Tencel/Cotton nonwoven fabriccoated with Chitosan for wound. Text. Res. J. 78(3):248-253.

[12] Morton J. J. P. and Malone M. H., 1972. Evaluation of vulneraryactivity by an open wound procedure in rats. Arch. Int. Pharmacod.

T. 196:117-126.


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Appendix I

SEM pictures of PVA/Silver nitrate nanomembrane produced using different combinations of PVA/Silver nitrate

S.

No.

PVA / Silver nitrate

combination

SEM pictures of PVA/ Silver nitrate

nanomembrane

Nanofiber

diameter

(nm)

Results

1. 10% PVA/ 0.5% Silver

nitrate

242 56 Uniform fibers.

No bead

formation.

2. 10% PVA/ 1.0% Silver

nitrate


No bead

formation.

3. 10% PVA/ 1.5% Silver

nitrate


No bead

formation.

4. 10% PVA/ 2.0% Silver

nitrate

169 75 Non-uniform

fibers.

Bead formation


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Appendix II

Standard test method for assessing skin irritation

(ASTM F 719-81 standard)

Principle of Measurement

Exposure of skin to the test material is accomplished by

means of a patch test technique employing two intact sites

on the back of each of six albino rabbits. The skin is

clipped free of hair one day prior to testing. The test

substance is applied using 0.5 ml for liquids, 0.5 g for

solids or semisolids and a 2.5 by 2.5 cm square patch for

films. After application, each test site is covered with a 2.5by 2.5 cm gauze flat and entire trunk is occluded with a

polyethylene sleeve. After 24 hours the sleeve, flat and test

material are removed and test sites are evaluated for

erytheme and edema.

Scoring Method

Using the criteria given in Table 7, the test sites are

scored for Erythema (ER) and Edema (ED).

Table 7Skin Reactions Score For Erythema And Edema Formations

Reaction Description ScoreErythema (ER) Erythema and Eschar

No erythema

Very slight erythema (barely perceptible)

Well-defined erythema (pale red in colour)

Moderate to severe erythema (red and area well defined)

Severe erythema (beet redness to slight eschar formation)

0

1

2

3

4

Edema (ED) Edema formation

No edema

Very slight edema (barely perceptible)

Slight edema (edges of area well defined by definite raising)

Moderate edema (edges raising approximately 1 millimeter)

Severe edema (raised more than 1 mm and extending beyond the area of

exposure)

0

1

2

3

4

Test sites can also be scored for erythema and edema at

48 hours as well as 72 hours after removal (as per the usage

requirement) using the criteria given in Table 7.

characterisation of poly vinyl alcohol

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