effect of hydrogel grafting, water and surfactant wetting on the adherence of pet wound dressings

JBUR-4260; No. of Pages 8

Effect of hydrogel grafting, water and surfactantwetting on the adherence of PET wound dressings

Chenxi (Coco) Ning a, Sarvesh Logsetty b, Shivkumar Ghughare a,Song Liu a,c,d,e,*aDepartment of Textile Sciences, Faculty of Human Ecology, University of Manitoba, Winnipeg, MB, CanadabManitoba Firefighters Burn Unit, Department of Surgery, Faculty of Medicine, University of Manitoba, Winnipeg,

MB, CanadacDepartment of Chemistry, University of Manitoba, Winnipeg, MB, CanadadDepartment of Medical Microbiology, University of Manitoba, Winnipeg, MB, CanadaeDepartment of Biosystems Engineering, University of Manitoba, Winnipeg, MB, Canada

b u r n s x x x ( 2 0 1 4 ) x x x – x x x

a r t i c l e i n f o

Article history:

Accepted 31 December 2013

Keywords:

Burn wound care dressing

Dressing adherence

In vitro dressing adherence test

Hydrogel grafting

a b s t r a c t

Traditional wound dressings, including cotton gauze, absorbent pads and bandages, can

cause trauma and pain to wounds during dressing changes, leading to a variety of physical

and psychosocial sequelae. The aim of this study was to adapt an in vitro model of adherence

to evaluate the effects of various methods to theoretically reduce the adherence of wound

dressings. Gelatin in liquid form was cast onto poly(ethylene terephthalate) (PET) fabric and

allowed to solidify and progressively dry to simulate wound desiccation in the clinical

setting. A 1808 peel test of PET from the gelatin slab yielded adherence data of peeling

energy. The peeling energy of PET increased with the drying time. It was possible to reduce

the force by drying at 75% relative humidity (RH). After drying for 24 h, either 500 mL of water

or surfactant solution was added onto the PET surface (16 � 60 mm2). The peeling energy

decreased dramatically with wetting and there was no significant difference between water

and surfactant. As a long-term strategy for decreasing adherence, a thin layer of poly-

acrylamide (PAM) hydrogel was deposited onto PET fabric via UV irradiation. This resulted in

a much lower peeling energy without severely compromising fabric flexibility. This hydrogel

layer could also serve as a reservoir for bioactive and antimicrobial agents which could be

sustainably released to create a microbe-free microenvironment for optimized wound

healing.

# 2014 Elsevier Ltd and ISBI. All rights reserved.

Available online at www.sciencedirect.com

ScienceDirect

journal homepage: www.elsevier.com/locate/burns

1. Introduction

Minimal trauma upon removal has been one of the most

important requirements on modern burn wound dressings.

According to a worldwide online survey among burn care

specialists, out of total 121 participants, 105 identified

* Corresponding author at: University of Manitoba, Department of TextiMB, Canada. Tel.: +1 204 474 7592/+1 204 474 9616; fax: +1 204 474 759

E-mail address: [email protected] (S. Liu).

Please cite this article in press as: Ning C, et al. Effect of hydrogel grafdressings. Burns (2014), http://dx.doi.org/10.1016/j.burns.2013.12.024

0305-4179/$36.00 # 2014 Elsevier Ltd and ISBI. All rights reserved.http://dx.doi.org/10.1016/j.burns.2013.12.024

non-adhesion as either an essential or a desirable property

of an ‘‘ideal’’ dressing [1]. However, many commercially

available wound dressings cause second trauma and sig-

nificant pain upon removal because of adhesion to the wound

bed [2]. These dressings can adhere to wounds as exudate

dries, and capillary loops and granulation tissue can grow

through the dressing fabrics. Pain-induced stress can delay

le Sciences, 190 Dysart Road, W581 Duff Roblin Building, Winnipeg,2/+1 20 447 47593.

ting, water and surfactant wetting on the adherence of PET wound

http://dx.doi.org/10.1016/j.burns.2013.12.024

mailto:[email protected]


http://www.sciencedirect.com/science/journal/03054179

www.elsevier.com/locate/burns


b u r n s x x x ( 2 0 1 4 ) x x x – x x x2


wound healing and adversely affect patients’ quality of life. A

recent study has shown that burn related pain is associated

with severe depressive and posttraumatic stress symptoms

among patients, increasing incidence of anxiety, depression,

and suicidal ideation [3,4].

Traditionally adherence testing has been done with cell

adhesion test or in vivo testing. Human skin cells such as

keratinocyte and fibroblast were seeded onto the dressings,

and the morphology of cells adhered were then observed

with a phase contrast microscope after incubating at 37 8C

for 1, 2, and 3 days. The number of cells can be quantified

using various methods such as MTT cell proliferation assay

[5–7]. However, cells are not the only component in wounds.

There are a variety of substances including water, protein,

electrolytes, enzymes and waste products that also exist in

the wounds. In the in vitro cell adhesion test the cell/sample

complexes are not allowed to dry as would happen in

wound desiccation. Therefore, it might not appropriately

reflect the real situation of wound healing. In vivo evaluation

of wound dressing performance on animal models or

human beings is more direct but not without drawbacks

[8–10]. Using animals in research for screening purposed is

of ethical concerns. It’s difficult to obtain a reproducible

measurement of adherence forces in the clinical setting as

wounds of patients are quite variable and hard to sample.

Furthermore, both animal and human models require a

substantial commitment of time, effort, and are expensive.

In recognition of this problem, we adapted a previously

reported in vitro gelatin model to simulate in vivo dressing

contact with wounds [11]. Gelatin is a strongly hydrogen-

bonded proteinaceous material which can adhere to dres-

sing materials similarly to wound exudates. It can be

allowed to dry progressively through the gel state into a

rigid solid to mimic the process of wound dessication, and

different clinical situations can be simulated by allowing the

gelatin to dry to different extents.

The necessity of a low-adherent dressing drove the current

study to explore different strategies for decreasing adherence

of common burn wound dressing materials. The short-term

strategy was to wet adherent dressings with water or

surfactant before the peeling and the long-term strategy to

deposit hydrogel onto commonly used dressing materials to

reduce adherence. It has been reported that hydrogel dres-

sings can decrease adherence as long as they remain hydrated

[12–14]. Poly(ethylene terephthalate) (PET) woven fabric was

chosen as the representative dressing material in view of its

popularity as dressing base materials in burn dressings

(ActicoatTM and Atrauman1) for its readily availability, good

biocompatibility and excellent mechanical strength. We

present our evaluation of the effect of water and surfactant

wetting and hydrogel grafting on the adhesiveness of PET

dressings using an in vitro model.

2. Materials and methods

2.1. Materials

Acrylamide (AM) and N,N0-methylene bisacrylamide (MBA,

crosslinker) were purchased from Sigma-Aldrich (Oakville,


ON). Gelatin, silver nitrate (AgNO3) and sodium citrate dehy-

drate (Na3C6H5O2�H2O) were purchased from Fisher Scientific

(Ottawa, ON). Sodium borohydride (NaBH4) came from Acros

(New Jersey, USA). PET plain woven fabric (no. 777H) was

purchased from Testfabrics, Inc. (West Pittiston, PA).

2.2. Preparation of hydrogel coated PET

PET plain woven fabric (6 � 14 cm) was first extracted with

distilled (DI) Water for 24 h to remove impurities before any

treatment. The extracted PET fabric was treated with O2

plasma (flow rate of O2: 2–4 standard cubic centimeter per

minute) for 20 min to produce peroxide functional groups on

the surface of the fabric. 3 mL monomer solution containing

9.8% (w/v) AM and 0.2% (w/v) MBA was uniformly placed onto

the plasma treated PET fabric. Then the fabric was sandwiched

by two glass plates prior to UV irradiation (365 nm, 3000 mw/

cm2). Crosslinked polyacrylamide (PAM) hydrogel was grown

from the surface of PET after UV irradiation (as shown in

Scheme 1).

After the radical grafting crosslinking copolymerization,

the PET sample was extracted with DI water in a 65 8C water

bath at the shaking speed of 150 RPM for 16–24 h to remove

ungrafted monomers and polymers, dried and stored in a

desiccator to reach constant weight. The resultant PET fabric is

referred to as ‘‘PET-PAM’’. The amount of hydrogel deposited

was calculated by the weight increment of the PET fabrics after

the polymerization as follows.

Weight increment ¼ ðWt � W0ÞW0

� 100%

where W0 is the weight of untreated PET and Wt is the weight

of sample after t min of UV irradiation, t = 30, 50 and 70 min.

2.2.1. Swelling ratio testBefore the peeling force test, fabric samples were immersed in

DI water for 5 min and centrifuged for 30 s at 2800 rpm to

remove the excess water adsorbed on the surface. The

swelling process was confirmed by change in weight: the

ability for swelling is called ‘‘swelling ratio’’.

Swelling ratio ¼ ðM1 � M0ÞM0

� 100%

where M0 is recorded as the weight of dry sample, and M1 is the

weight of swollen sample.

2.2.2. Flexibility testA Cantilever Bending Tester was used to test the flexibility of

fabric samples according to ASTM standard D 1388-96 [15].

Briefly, a fabric sample was immersed in DI water for 5 min

and centrifuged for 30 s at 2800 rpm to remove the excess

water adsorbed on the surface. Then the swollen sample

(2 � 12 cm2) was put on a horizontal platform and slid

gradually in a direction parallel to its long dimension. The

length overhang was measured at which the tip of the

specimen was depressed under its own mass to the point

where the line joining the top to the edge of the platform made

a 41.58 angle with the horizontal. Samples were tested at both

ends and the two readings were averaged. The bending length

c and flexural rigidity were then calculated:



Scheme 1 – Surface grafting with polyacrylamide (PAM) hydrogel onto PET Dressing via plasma treatment and

photopolymerization.

c ¼ o2; where c ¼ bending lengthðcmÞ; o ¼ length of overhangðcmÞ;

G ¼ W � c3; where G ¼ flexural rigidityðmg cmÞ; W ¼ fabric mass per unit areaðmg=cm2Þ:

b u r n s x x x ( 2 0 1 4 ) x x x – x x x 3


2.3. Peeling force test

An in vitro model was chosen to mimic the environment

between human skin and wound dressing: Briefly, a poly-

tetrafluoroethylene (PTFE) window frame with an open area

16 � 60 mm2 for gelatin casting wad created. All the fabric

samples (3 � 13 cm2) were soaked in DI water for 5 min,

centrifuged for 30 s and then spread on a clean bench; one

frame was placed onto each piece of fabric. 40 wt% gelatin

solution was prepared in 70 8C DI water and then poured into

the window frame. To simulate the wound desiccation

process, the gelatin/fabric module was dried in the incubator

at the skin temperature 32 8C for different time durations

while maintaining a dry (<25% RH) or humid (75% RH)

environment. Sulfuric acid (H2SO4) was used to maintain a

constant humidity as per previously published protocols [16].

After a period of time (4, 8, 16, 24 h), the PTFE window was

removed from the specimen and an Instron 5956 machine

(Instron, MA, USA) was used to peel the gelatin off the sample

at a constant rate of 100 mm/min with 1808 peeling angle. The

five highest peaks were chosen to obtain the average force.

Peeling energy per unit area (J/m2) was expressed in this thesis

as: u = 2P/b, where P was the average peeling force and b was

the width of the gelatin strip (1.6 cm).

2.4. Loading of AgNPs onto hydrogel coated PET

A beaker containing 200 mL Milli-Q water was put in an ice

bath for 5 min. Then, 38 mg AgNO3 and 65.33 mg sodium

citrate dehydrate were added into the solution under vigorous


stirring at 1500 RPM in ice bath. To this solution, 10 mL of

NaBH4 solution (42.22 mg) was added dropwise and the color

of solution turned dark brown, indicating the reduction of Ag+

to silver nanoparticles (AgNPs). Following 3 h of additional

stirring in the water bath, the pH of AgNP solution was

adjusted to 5 and then the PET-PAM fabric was immersed in

the solution. After 3 h shaking, the fabric was taken out,

washed with copious amount of DI water, and dried at room

temperature.

2.5. Statistical analysis

The values are presented as mean � standard deviation. A

student’s t-test was conducted to compare means. P � 0.05

was considered significant.

3. Results

3.1. Evaluation of the adherence of PET dressings

Fig. 1 plots the peeling energies of untreated PET as a function

of drying time (4, 6, 8, 16 and 24 h) at two relative humidities

(RHs). Initially <25% RH was used to follow the previously

reported procedure [4]. As the gelatin dried out at this lower

humidity, the peeling energy kept increasing rapidly with the

continuous loss of water in gelatin, reaching as high as

4491.3 � 418.8 J/m2 after 16 h drying. After drying for 24 h, as

the gelatin became more desiccated and more hardened failure

occurred within the gelatin instead of at the gelatin-dressing



0500

100015002000250030003500400045005000

0 5 10 15 20 25

Peel

ing

ener

gy (J

/m2)

Drying �me (h)

< 25 % RH75% RH

Fig. 1 – Peeling energy of untreated PET drying at different

humidity.

Table 1 – Hydrogel deposition on PET as a function of UVirradiation time.

Sample denotation PET-PAM-I PET-PAM-II PET-PAM-III

UV irradiation time 30 min 50 min 70 min

Weight increment 0 5.81 � 1.56% 10.21 � 0.71%

b u r n s x x x ( 2 0 1 4 ) x x x – x x x4


interface. The peeling energy results were unrealistically

high and did not well simulate the clinical situation. So, we

slightly adapted the reported gelatin model by drying

gelatin-fabric module at a higher relative humidity. It is

reported that 75% RH promotes the healing of deep burns, so

we chose to set the RH in the incubator at 75% RH [17]. We

found that the peeling energy also increased with the drying

time at 75% RH, but the slope of increase was much lower

compared with drying at <25% RH. After drying for 4 h at

75% RH, the peeling energy arrived at the same level

(436.3 � 32.5 J/m2) as that at <25% RH (421.3 � 52.5 J/m2)

(P > 0.05), but the adhesion of PET dressing increased slowly

to 808.8 � 78.8 J/m2 after 16 h drying, only one fifth that of

drying at <25% RH.

3.2. Effect of water/surfactant wetting on the adherenceof PET dressings

Thomas [18] suggested that the peeling energy exceeding 300–

400 J/m2 is not suitable on a drying wound. Even at a moist

drying environment (75% RH), the peeling energy of PET after

24 h drying (2231.3 � 296.3 J/m2, Fig. 1) is still more than 4

times higher than the above limit. In clinical situations, water

or surfactant solution might be used to moisten wound

dressings prior to detachment. After drying for 24 h at 75% RH,

500 mL DI water or foaming surfactant (2% chlorhexidine

gluconate solution, Hutington) was added onto the surface of

PET and allowed to penetrate into the interface between

gelatin and the fabric for 1, 5 and 10 min before the peeling

force test. The peeling energy dropped drastically from

0

500

1000

1500

2000

2500

0 2 4 6 8 10 12

Peel

ing

ener

gy (J

/m2)

Water/sur factant tr eatme nt �me (min)

DI waterSurfac tant solu�on

Fig. 2 – Peeling energy of untreated PET after water/

surfactant wetting (after 24 h drying).


2231.3 � 296.3 to 481.3 � 87.5 J/m2 and 446.0 � 43.8 J/m2 after

1 min of water and surfactant treatment, respectively (Fig. 2).

However, there is no further decrease with the extended

water/surfactant treatment time before the peeling force test.

And no significant difference is notified between the effects of

water and surfactant solution (P > 0.05), even though surfac-

tant is known for promoting water permeation.

3.3. Effect of hydrogel on the adherence of PET dressings

3.3.1. Characterization of hydrogel deposited on PETdressingsO2 plasma treatment was used generate peroxides on the

surface of PET fabric, which degrade upon UV irradiation to

generate radicals allowing surface grafting polymerization.

Upon UV irradiation, a thin layer of polyacrylamide (PAM)

hydrogel was formed on the surface of the O2 plasma treated

PET fabric. Three irradiation durations were used and the

corresponding sample denotation and weight increment are

listed in Table 1. The amount of deposited hydrogel increased

with the increase of UV irradiation time. 30 min UV irradiation

was not long enough to form well-crosslinked hydrogel on PET

fabrics. PET-PAM-II and PET-PAM-III were further character-

ized in terms of their hydrophilicity, water swelling capacity

and adherence.

Hydrophilicity of PET fabrics changed after PAM hydrogel

deposition. As shown in Fig. 3, water contact angle for

untreated PET fabric was about 1278 due to its hydrophobicity,

and it decreased to 778 and 568 for PET-PAM-II and PET-PAM-

III, respectively. After 5 min, the water contact angle for PET

decreased by 48, whereas the water contact angle for PET-

PAM-II and PET-PAM-III turns to 308 and 218, respectively, and

further decreases to 08 after 20 min for the PET-PAM-III. The

swelling capacity of hydrogel coated PET is relevant to its

ability to maintain a moist interfacial layer while being dried

with gelatine slab. The swelling ratios of PET-PAM-II and PET-

PAM-III were determined as 77.2% � 2.1% and 155.6% � 9.9%,

respectively, significantly higher than that of untreated PET

(11.1% � 0.7%) (Fig. 4).

Typically, the frequency of dressing changes should be at

least 1 day. So 40 wt% gelatin solution was casted onto PET-

PAM-II and PET-PAM-III, and the fabric/gelatin modules were

kept at 32 8C, 75% RH for 24 h before the peeling test. After the

deposition of hydrogel, the peeling energy of PET fabric

dropped from 2231.3 � 296.3 J/m2 to lower than 300 J/m2 as

presented in Fig. 5. There is NO significant difference between

the peeling energy of PET-PAM-II and PET-PAM-III (P > 0.05)

even though PET-PAM-III has a higher water swelling capacity.

3.3.2. The flexibility test of PET dressings after the PAMhydrogel depositionBioactive agents are often loaded onto wound dressings for

their later release into the wound to assist healing. So, the



Table 2 – The flexural rigidities of wet fabric samples: untreated PET, PET-PAM-II and PET-PAM-III.

s PET PET-PAM-II PET-PAM-III

Overhang length (o) 1.55 � 0.02 cm 1.70 � 0.00 cm 1.56 � 0.00 cm

Bending length (c) 0.78 cm 0.85 cm 0.78 cm

Fabric mass per unit area (W) 15.00 mg/cm2 27.71 mg/cm2 27.79 mg/cm2

Flexural rigidity (G) 7.40 mg cm 17.02 mg cm 13.19 mg cm

b u r n s x x x ( 2 0 1 4 ) x x x – x x x 5


flexibility and conformability of dressing materials are also

essential for maximum dressing contact with the wound to

ensure the optimal delivery of bioactive agents, especially

applied to the bumpy wound surfaces. Flexibility was tested

by measuring the overhang length recommended by ASTM

standard D 1388-96 [15] as the sample gravitationally

bended under its own weight. Flexural rigidity is defined

as the resistance to bending, which is a measure of the

interaction between the sample weight and its bending

stiffness. Table 2 presents the flexural rigidities of the

following wet fabric samples: untreated PET, PET-PAM-II

and PET-PAM-III. PET is well-known for its excellent

flexibility and conformability as the representative material

of wound dressings, and the flexural rigidity of wet

untreated PET fabric is merely 7.40 mg cm. After the

deposition of hydrogel, the flexural rigidity increased to

17.02 and 13.19 mg cm for wet PET-PAM-II and PET-PAM-III,

respectively. It has been reported that the flexural rigidity

of a strong yet drapeable non-woven fabric fabric with a

very attractive handle was 40 mg cm, and fabrics with the

flexural rigidity of 154 mg cm could be used as bed sheets,

underwear and a substrate for a coated fabric for rainwear

Fig. 3 – Water contact angle of PET after (a) 0 min and (b) 5 min

PET-PAm-III after (e) 0 min and (f) 5 min.


[19]. Although the flexural rigidity of the untreated PET

fabric slightly increased after the deposition of hydrogel,

PET-PAM samples can still well conform to body contours

(Fig. 6).

3.4. Incorporation of silver nanoparticles (AgNPs) ontoPET-PAM

The hydrogel layer can not only maintain a moist environment

to help ease the peeling of dressings, but also serve as a

reservoir for bioactive agents such as biocides to prevent the

infection during wound healing. There is growing interest in

utilizing AgNPs as an antibacterial in wound dressings. Ag+ is

continuously released from AgNPs to the wound surface for a

long-term antibacterial activity (up to 7 days) in a controlled

fashion. In Fig. 7, AgNPs are uniformly distributed on the

surface of PET-PAM-III, with some white spots ascribed to the

shining of silver. While for untreated PET, the color was much

lighter and splotchy, indicating fewer AgNPs were deposited

onto the surface and distributed unevenly. The antibacterial

activity of samples incorporated with AgNPs needs to be

further investigated.

; and of PET-PAm-II after (c) 0 min and (d) 5 min; and of



11.11

77.18

155.58

0

20

40

60

80

100

120

140

160

180

PET PET-PAM-II PET-PAM-III

Swelling ra�o (%)

Fig. 4 – Swelling ratios of untreated PET, PET-PAM-II and

PET-PAM-III.

b u r n s x x x ( 2 0 1 4 ) x x x – x x x6


4. Discussion

4.1. The evaluation of the adherence of PET dressings

As a proteinaceous material, gelatin can be used as an in vitro

simulant of the protein-rich wound surface. Gelatin dissolves

in hot water and passes from solution to gel again when

cooled. This thermoreversible property makes it possible to

solidify hot concentrated gelatin solution onto a fabric strip in

a uniform manner, hence enabling subsequent peel force

testing. The gelatin slab was completely hydrated at the

outset, surrounded by water molecules either at a dry or moist

environment. During the drying process, more and more

hydrogen bonding sites of gelatin were available to the PET

fabric due to the evaporation of water, so that the peeling

energy tended to increase with the drying time. The water loss

rate of gelatin was much lower at 75% RH than at <25% RH, and

the gelatin dried at 75% RH remained hydrated for longer time.

Therefore, the peeling energy went up only a little in the first

16 h at 75% RH, when compared with a substantial increase at

<25% RH. Meanwhile, gelatin can also penetrate into the pores

of PET fabric and become incorporated into fabric structure

when it’s solidified which is similar to the clinical situation

where capillary loops and granulation tissue grow through a

wound dressing and become part of the dressing. This

mechanical interlocking is another factor influencing the

adherence of dressing materials and varies with the degree of

dryness of gelatin. When the adhesion caused by mechanical

2231.5

258.75 273.75

0

500

1000

1500

2000

2500

PET PET-PAM-II PET-PAM-III

Peeling energy (J/m2)

Fig. 5 – Peeling energies of untreated PET, PET-PAM-II and

PET-PAM-III.


interlocking overpasses the strength of gelatin, failure

occurred in gelatin itself while peeling as in the case of

untreated PET fabric/gelatin module after 24 h drying at < 25%

RH.

A moisture balance of the skin tissue is achieved by

constant supply from the blood circulation and drainage

through the lymphatic system. In a wound, fluid leaks out of

the blood vessels and white blood cells escape from the

capillaries into body tissues, making up wound exudates in the

early stage of the healing process. And exudate production

generally reduces over time during the wound healing and

regeneration. The significance of this analysis is that wound

dressings adhere to a drying wound with exudates may

behave in a similar manner to the gelatin dried in a controlled

moist environment of 75% RH.

4.2. Effect of water/surfactant wetting on the adherenceof PET dressings

Water has been identified as the principal element in

debonding adhesives and substrates. Water molecules and

the surface of substrate tend to compete for the hydrogen

bond sites in the dried extrudates (gelatin in this in vitro

model). After being added onto the wound dressing, water can

permeate the dressing material, migrate to the interfacial

region and even diffuse into gelatin, leading to a reduction in

the bonding strength of the wound-dressing interface through

a displacement mechanism [20,21]. Gelatin lost most of water

after being dried in the incubator for 24 h, and added water

molecules re-formed hydrogen bonding with dehydrated

gelatin and superseded the bonding between gelatin and

PET, leading to a lower peeling energy. Surfactants are well

known in decreasing the surface tension of water to allow

easier wetting of surfaces and surfactant in the form of soap is

used clinically to help release dressings, but there was no

significant difference between water and surfactant treat-

ments in the reduction of adherence. It indicates that 500 mL of

water can fully swell interface gelatin in the area of

16 � 60 mm2 within 1 min for maximal interface debonding.

4.3. Effect of hydrogel on the adherence of PET dressings

The most common types of hydrogel dressing used in burn

wound care are amorphous gels, sheets, and hydrogel

impregnated gauze. However, these hydrogels generally need

secondary dressings, can be difficult to handle due to their low

mechanical strength, and leave a lot of debris in the wound

[22]. The ability of hydrogels to form an intimate contact with

wounds might also be weakened after absorbing exudates,

especially for the thick hydrogel sheets which are not

conformable to contours or movement. In recognition of

these problems, a very thin layer of hydrogel is grown from the

substrate radical graft polymerization. Plasma treatment with

gases such as Ar, N2, O2 and NH3 has been used extensively to

insert or substitute chemically reactive functionalities onto a

non-reactive substrate [23]. In this study, O2 plasma creates

peroxides which can be degraded into radicals upon UV

irradiation for surface initiated crosslinking copolymerization

of AM and MBA. This 2-step process successfully deposited

PAM hydrogel onto PET surface and two glass plates were



Fig. 6 – Images of PET-PAM applied to different body parts.

Fig. 7 – Images of AgNP incorporated PET (left) and PET-PAM-III (right).

b u r n s x x x ( 2 0 1 4 ) x x x – x x x 7


utilized to sandwich the PET fabric to control the thickness of

formed hydrogel.

PAM polymers, long parallel chains of molecules, can

create a three-dimensional network of polymeric chains

when crosslinked. Water is brought into the network through

the process of osmosis and quickly journeys into the central

part of the polymer network. Then hydrogel reserves water by

forming hydrogen bonding with water molecules and appears

as the gel. In DI water, hydrogels can absorb 10–1000 times

their original weight and become up to 99.9% liquid [12,13].

When the surroundings begin to dry out, the crosslinked

hydrogel can gradually dispense nearly 100% of the stored

water [24]. This process of rehydration and dehydration of

hydrogel can be repeated multiple times to maintain an

optimal moisture balance. Before the peeling force test, PET-

PAM was immersed in water for 5 min to reach equilibrium

swelling of the deposited hydrogel. During the drying process,

gelatin gradually becomes hard and desiccated owing to the

continual water loss. The application of the hydrogel dressing

can donate water to keep gelatin hydrated at the interface,

ensuring that the hydrogen bonding sites on gelatin are

surrounded by water molecules and thus unavailable for

bonding to the fabric surface, producing a much lower peeling

energy even after drying for 24 h. In addition, the swollen

hydrogel coated PET fabric becomes less porous, minimizing

the penetration of gelatine solution into fabric structure and

mechanical interlocking. Therefore, the grafted hydrogel

layer radically reduces the adherence of PET dressings using

this gelatin model. 5.81% hydrogel grafting was enough to

decrease the peeling energy of 24 h dried fabric/gelatine

module to below the threshold of 300 J/m2. Even though more

hydrogel grafting did not yield further decrease of the peeling

energy, it is expected that the higher amount of hydrogel


could maintain a hydrated fabric/gelatine interface for

longer.

4.4. Dressing flexibility

Flexibility is an important performance parameter of wound

dressings, influencing their ability to conform to the body

shape and form an intimate contact with the wound. If the

dressings are very poorly conformable, they may quickly

detach when body movement occurs. Sometimes it is also

difficult to apply wound dressings to the body parts with some

irregular contours and shapes. The flexibility and conform-

ability of wound dressing is requisite to ensure its effective-

ness for wound healing in different manners. It can help

maintain a moist environment at the wound-dressing inter-

face by absorbing and retaining the wound fluid without

spreading to the periwound skin and reducing the possibility

of maceration [25]. It contributes to preventing blisters and

improving patient comfort by minimizing the shear forces of

wound dressings and allowing greater mobility for patients

[26]. It is also conductive to avoid ‘‘dead space’’ where bacteria

can accumulate and proliferate and ensure the highly efficient

delivery of the bactericides incorporated in dressings [27]. It

has been proved that the thin layer of PAM hydrogel grown

from the PET substrate in this study via radical graft

polymerization does not significantly compromise the flex-

ibility of the PET fabric.

5. Conclusions

This in vitro gelatin model is much easier and cheaper for the

adherence evaluation of dressings, when compared with the



b u r n s x x x ( 2 0 1 4 ) x x x – x x x8


cell adhesion test or in vivo testing. Basing on the results of

peeling force test, drying of fabric/gelatin complexes at 32 8C,

75% RH could better simulate clinical conditions. Water and

surfactant solution do help ease the peeling of dressings from

the gelatin model. By applying 500 mL of water onto 16 � 60 mm2

sized gelatin adhered dressing, maximal interface debonding

can be achieved within 1 min soaking. This provides practical

guideline regarding how to effectively decrease the adherence

of current commercially available wound dressings. Hydrogel

grown from the PET fabrics is low-adherent and conforms to the

body shape ideally. It does not stick to the drying secretions of

the wound, thereby resulting in less pain and trauma on

removal. This thin layer of hydrogel also enables the deposition

of antibacterial agents such as AgNPs.

Conflict of interest statement

All the authors have no conflicts of interest.

Acknowledgements

This work was supported by the Collaborative Health Research

Projects (CHRP) operating grant (Grant no.: CHRP 413713-2012),

the Natural Sciences and Engineering Research Council of

Canada (NSERC) Discovery grant (Grant no.: RGPIN/372048-

2009), and the Firefighters Burn Fund Inc. (Manitoba).

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effect of hydrogel grafting, water and surfactant wetting on the adherence of pet wound dressings

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