Preparation and Physical Properties of Cavernous Calcium Alginate

Wound Dressings

Xinling Li1,2,a, Guangting Han1,2,b, Yuanming Zhang1,2,c,

Wei Jiang3,d, Yanzhi Xia4,e

1Laboratory of Fiber Materials and Modern Textile, the Growing Base for State Key Laboratory,

Qingdao University, Qingdao 266071, China

2Shandong Provincial Key Laboratory of Fiber Materials and Modern Textile,

Qingdao 266071, China

3College of Textiles, Donghua University, Shanghai 200051, China

4The Growing Base for State Key Laboratory, Qingdao University,

Qingdao 266071, China

aliawenqi@126.com, bkychgt@qdu.edu.cn, czhangyuanming001@163.com, djangwei1984@126.com, exyz@qdu.edu.cn

Keywords: Calcium alginate. Freeze-dry. Water vapor penetration. Water uptake capacity.

Mechanical properties.

Abstract. As a natural polymer, alginic acid is widely used in medical fields for its biodegradability,

low toxicity and immunogenicity. In this paper, four kinds of cavernous calcium alginate wound

dressings were prepared, and physical properties were tested, providing fundamental basis for

further study. The results indicate that glycerin is not suitable as a plasticizer for this cavernous

wound dressing; a relatively uniform structure of the calcium alginate wound dressing would be

obtained as the solution temperature increased; the mechanical properties decrease with the increase

of the pre-freezing time.

Introduction

Alginic acid, a naturally occurring polysaccharide found in all species of brown algae and some

species of bacteria, is a linear anionic block copolymer composed of α(1-4)-L-guluronic acid (G)

and β(1-4)-D-mannuronic acid (M) units in varying proportions and sequential arrangements[1].

Alginates are usually presented as the soluble sodium salt, which can be cross-linked with divalent

cations to form an insoluble alginate gelatin, which has been widely investigated for use in various

fields like food industry, medicine and biotechnological applications including cell encapsulation

and drug delivery, due to their biodegradability, low toxicity, immunogenicity [2-4].

The calcium alginate gelatin have been reported to produce the so-called ‘‘egg-box” structure in

a planar two-dimensional manner. And calcium alginate is the most widely used. Calcium alginate

dressings promote healing by maintaining a moist condition in wounds [5-7]. When applied to

wounds, calcium ions from the calcium alginate rapidly exchange with sodium ions in the wound

exudates. The release of free calcium ions provides one of the essential factors in the clotting

cascade. Therefore, calcium alginate has been used as a homeostatic agent for many years [8].

Advanced Materials Research Vols. 332-334 (2011) pp 1670-1675Online available since 2011/Sep/02 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.332-334.1670

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In this study, cavernous calcium alginate wound dressings were prepared by phase-separation

method, and physical properties were investigated, providing a basis for further study. In order to

study the effect of adding glycerin, sodium alginate solution temperature, pre-freezing time on

properties of calcium alginate dressings, four kinds of wound dressings were prepared and their

morphology, water vapor permeability, water uptake capacity, mechanical properties were

determined and analyzed.

Experimental

Materials. Sodium alginate (food-grade, 500mPa.s), was purchased from Qingdao (China). CaCl2,

NaCl, KCl, Na2HPO4·12H2O and KH2PO4 were all analytical reagent and obtained from Shanghai

(China).

Preparation of the wound dressing. To get an aqueous phase consisting of 2% w/w sodium

alginate, sodium alginate powder was dissolved with stirring in distilled water at room temperature.

The sodium alginate solution was placed at 4 ˚C for 24 hours to remove the air bubbles before

poured into Petri dishes (40 g each) with dimensions 10 × 10 × 1.5 cm. Then the Petri dishes

containing sodium alginate solution were pre-frozen at -70 ˚C for 24 hours and freeze-dried by

FA3002 freeze drier for 48 hours. After that the freeze-dried sodium alginate was immersed into an

aqueous solution of calcium chloride for 30 min to cross-link with Ca2+

forming calcium alginate

and then was washed with distilled water repeatedly to remove calcium chloride. At last, calcium

alginate wound dressings were obtained after freeze-dried again, marked as A. The calcium alginate

wound dressings obtained from the mother solution with not only 2% w/w sodium alginate but also

10% w/w glycerin, was marked as B. The calcium alginate wound dressings marked as C was

obtained just increasing the solution temperature to 45 ˚C. Changing the pre-freezing time from 24

hours to 48 hours, D was got.

Microscopy. The morphology of the calcium alginate wound dressing was observed on a

scanning electron microscope (SEM; JSM-6390LV) after gold coating. The thickness was obtained

by measuring five different positions of every calcium alginate wound dressing at random.

Water vapor penetration. Water vapor transmission rates (WVTR) of the calcium alginate

wound dressing were determined according to a modified ASTM standard E96-00 by monitoring

the amount of evaporated water through the test-sample dressing and by measuring the weight loss

from a water-filled permeability cup. Permeability cups (cylinder, 3 cm in inside diameter) were

filled with 25 ml of distilled water and the test-sample dressing was fixed onto its opening. Cups

were placed into an incubator of 35 % RH at 35 ◦C. A digital hygrometer with a continuous

percentage relative humidity (RH) and temperature was used to monitor the incubator conditions [9].

The water vapor transmission rate was calculated by:

( )= w

mass PWVTR p RH

area time L= × × ∆

×. (1)

Where P is the water vapor permeability, L the dressing thickness, pw the saturated water vapor

pressure at the experimental temperature, ∆RH the relative humidity difference across the dressing,

and P/L permeance of water vapor.

Water uptake capacity. The water uptake of the calcium alginate wound dressing was

determined gravimetrically. The wound dressings which were cut into 2 cm × 2 cm samples were

immersed into pH 7.4 phosphate buffer solution (PBS) in an incubator at 37 ◦C for 24 hours. The

water uptake rate (WUR) was calculated according to the follows:

Advanced Materials Research Vols. 332-334 1671

00

0

100W

WURW

= × . (2)

Where W0 is the dry weight of the wound dressing samples and W is the weight soaked in PBS

for 24 hours [10].

Mechanical properties. The tensile strength and breaking elongation of the calcium alginate

wound dressing were determined using a tensile test machine (Instron 3300, UK) at room

temperature. The initial gauge length was set at 50 mm, and the extension speed was 5 mm/min.

Results and discussion

Fig. 1 SEM images of the four kinds of calcium alginate dressings

Morphology. The pre-freezing is very important during the process of preparation, for it decides the

structure of cavernous calcium alginate wound dressings, such as the distribution and morphology

of the pores. While in the process of pre-freezing, the sodium alginate solution turned to solid from

liquid, and the water became ice nuclei, which gasified by freeze-drying. The space occupied by ice

nuclei formed pores, so the calcium alginate wound dressings were porous. The wound dressings

are crack-like on the surface, with layered structure of cross section, because of the direction of the

heat conduction. Then they were immersed into calcium chloride solution, chelating with calcium

ions, and the calcium alginate wound dressings were obtained after lyophilized again.

A, C, D are crack-like on the surface, and B shows lots of oval pores, as seen in Fig.1. That may

be attributed to the influence of the glycerin on the structure of the ice nuclei. The structure of C is

more uniform than A, which shows that the solution became more homogenous as the increase of

the solution temperature. The ice nuclei increase gradually in size with the increase of the

pre-freezing time, so the pores of D are bigger compared to A.

1672 Advanced Textile Materials

The thickness on different positions of one wound dressing vary, due to the directivity of the

thermal conduction during pre-freezing and chelation of the sodium alginate polymer chains with

calcium during cross-linking. The thickness of B decreases 22.23% compared to A, as it is seen in

Fig. 2a. It may be explained as follows: the glycerin results in a certain degree of the association of

polymer chains. The increase of the solution temperature gives rise to the increase of homogeneity

of the sodium alginate solution and the decrease of the molecular weight, and then the cross-linking

with calcium would be more adequate with further association of polymer chains, so the thickness

of C decreases. Compared A and D, no obvious changes of the thickness could be found with the

increase of the pre-freezing time.

a

1.9445

1.8439

1.9528

1.5123

0

0.5

1

1.5

2

2.5

3

3.5

A B C D

mm

b

2886.472595.51 2504.56

2810.04

0

1000

2000

3000

4000

A B C Dg

/m2

.day

c0.00E+00

5.00E-06

1.00E-05

1.50E-05

2.00E-05

2.50E-05

3.00E-05

A B C D

Kg

/m.P

a.s

-1.50E-09

1.00E-09

3.50E-09

6.00E-09

8.50E-09

1.10E-08

Kg

/m2

.Pa.

s

Permeability

Permeance

d

395.20

652.29

446.33461.12

0

200

400

600

800

A B C D

%

Fig. 2 Results of different properties of the four kinds of calcium alginate dressings,

a: thickness, b: WVTR, c: permeability and permeance, d: WUR.

Water uptake and permeation properties. The wound dressing should prevent a wound both

excessive dehydration as well as buildup of exudates, for which will affect the decrease of body

temperature and accelerate the rate of metabolism. The water vapor transmission rate for normal

skin is 204 g/m2 per day, while that for injured skin can range from 279 to 5138 g/m

2 per day. It

was recommended that a rate of 2500 g/m2 per day would provide an adequate level of moisture

without risking wound dehydration.

As shown in Fig. 2b, the water vapor transmission rates of the four kinds of the calcium alginate

wound dressings are ranged from 2504 to 2886 g/m2 per day. The WVTR, Permeance, and

Permeability of C are more or less the same with that of A (as shown in Fig. 2c), which

demonstrates that the water vapor transmission changed lightly with the increase of the solution

temperature. The WVTR, Permeance, and Permeability of D are better than A, attributing to the

bigger size of the pores as the result of extending the pre-freezing time. Similar results appeared in

the values of WVTR and Permeance of B, while the permeability of B is opposite, which may be

associated with the decrease of the thickness of B.

Advanced Materials Research Vols. 332-334 1673

With a porous structure and containing lots of carboxyl and hydroxyl of the macromolecular

chains, the water uptake capacity of the calcium alginate wound dressing is perfect. As shown in the

Fig. 2d, the higher of B must be attributed to the hydroxyl of glycerin. A and C are as almost the

same, showing water uptake capacity of the calcium alginate dressing is almost not affected by the

solution temperature. D has a lower WUR, which illustrates that addition of pre-frozen time results

in decrease of water uptake capacity of the calcium alginate wound dressing.

0 1 2 3 4 5 6 7 80

2

4

6

8

10

Tensile displacement (mm)

Bre

akin

g F

orc

e (N

)

A

B

C

D

Fig. 3 The load-displacement curves of the four kinds of the calcium

alginate wound dressings

Table. 1 Mechanical properties of the four kinds of

calcium alginate wound dressings

Breaking Force[N] Elongation [%] Mean Work [N .mm]

A 9.28068 0.91884 19.25176

B 2.32764 0.66706 1.26065

C 6.56663 1.95156 11.46971

D 4.39721 1.42890 18.03414

Mechanical properties. As seen in Fig. 3, the breaking force and mean work of A are the best.

The three indicators of mechanical properties of B are the worst, indicating that the strength and

tenacity of calcium alginate wound dressings containing glycerin are poor, so using glycerin as

plasticizer is not feasible. Compared A and C in Table.1, the breaking force and mean work of

calcium alginate wound dressings decrease but the elongation increases with the increase of

solution temperature. The degradation of sodium alginate accelerates and the molecular chains get

shorter at a higher solution temperature, causing the strength to reduce, while the uniformity

improves, so each part of the calcium alginate wound dressing shares the tension and the elongation

increases. The mean work of A and D shows little difference, and the breaking force decrease a lot

as the pre-freezing time extends.

1674 Advanced Textile Materials

Conclusion

The structure and properties of the cavernous calcium alginate wound dressing would be affected by

many factors during the preparation process. This paper studied the influence of the addition of

glycerin, solution temperature, and pre-freezing time on calcium alginate wound dressings. The

results are as follows:

With the glycerin, structures of the calcium alginate wound dressing are changed, and the

mechanical properties are poor though the water uptake and permeation properties are not bad. So,

using the glycerin as plasticizer is unworkable.

The increase of the solution temperature gives rise to the increase of homogeneity of the sodium

alginate solution and the decrease of the molecular weight, which made a more uniform structure, a

less breaking force and a greater elongation of the calcium alginate wound dressing. It can be

discovered that the elongation properties of calcium alginate dressings would be improved by

increasing the solution temperature.

Ice nuclei become bigger as extending the pre-freezing time, eventually resulting larger pores of

the calcium alginate wound dressing. The other properties of the dressing are excellent but the

breaking strength decreases 52.62 %. So extending pre-freezing time is not available.

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Advanced Materials Research Vols. 332-334 1675

Advanced Textile Materials 10.4028/www.scientific.net/AMR.332-334 Preparation and Physical Properties of Cavernous Calcium Alginate Wound Dressings 10.4028/www.scientific.net/AMR.332-334.1670

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