Alginate, and other polysaccharide fibre dressings, also termed hydro-fibres, are primarily used in the management of exuding wounds, and are characterised based on their ability to absorb exudate and their physical properties following absorption.

Placed into the wound in a dry form, they absorb exudate whilst maintaining a moist wound interface. Once in contact with an exuding wound, an ion exchange reaction takes place between the calcium ions in the alginate dressing and the sodium ions in wound fluid. This results in the dressing fibres swelling and forming a gel. The degree of swelling/absorption will depend on the chemical composition of the alginate.

The SMTL are able to test the following characteristics of alginate and polysaccharide gelling fibre dressings:

British Pharmacopoeia 1993 Addendum 1995 - Alginate Wound Dressings
  • Absorbency
  • Acidity/Alkalinity
  • Dispersion Characteristics (UKAS accredited)
  • Weight Per Unit Area
  • Labelling
BS EN 13726-1 Test methods for primary wound dressings - Aspects of absorbency
  • Free-Swell Absorbency - Section 3.2 (UKAS accredited)

  • Gelling Characteristics of Fibrous Dressings - Section 3.5

  • Dispersion Characteristics - Section 3.6 (UKAS accredited)

SMTL in-house developed test methods
  • Modified Dispersion Characteristics - SMTL TM-112

  • Tensile Strength - SMTL TM-272
  • Absorbency Under Compression - SMTL TM-414

  • Free-Swell Absorbency & Fluid Retention - SMTL TM-404


Modified Dispersion Characteristics - SMTL TM-112

For some practitioners an important feature of alginate/hydrofibre dressings is their integrity during dressing removal from the wound. There are some recognised limitations with performing the BS EN 13726-1 dispersion characteristics method in that the dressings are simply swirled in artificial exudate for 30 seconds at ambient temperature. These conditions do not simulate the clinical environment and as such results from this test may not represent clinical use of the alginate dressing.

The SMTL have therefore developed a modified dispersion characteristics test in which the dressings are incubated with gentle agitation in artificial exudate over night at body temperature. Following incubation a visual and functional assessment of the dressing integrity is made. Images of the non-dispersion integral Kaltostat alginate dressing following incubation are presented below.


KaltostatA resize  KaltostatB resize


Tensile Strength Testing - SMTL TM-272

The integrity of gelling fibre dressings can also be assessed using a tensometer based tensile strength test. This test quantifies the strength of the dressings and can be performed in the dressings dry and wet state.

The images below highlights differences in the tensile strength between 2 different brands of gelling fibre dressings (click on images to see full size):


SMTL in-house absorbency methods - TM-404 and TM-414

As alginates/gelling fibre dressings are often used under compression garments, the SMTL have developed in-house methods to assess absorbency under different compression conditions.

Free-swell absorbency and the ability of the dressing to retain fluid following the application of pressure is measured in the SMTL TM-404 Free-Swell Absorbency & Fluid Retention.   This test is aimed to simulate a condition such as a wound in the sacral area in which pressure may be applied following absorption, for example if the patient rolls onto the affected area.

The Absorbency Under Compression - SMTL TM-414 method measures the ability of the dressing to absorb when placed under constant pressure.  This simulates use of the dressing under compression bandages, and as such a pressure in the range of  40mmHg is applied to the dressing throughout the testing period.


If you wish to commission testing of your alginate and polysaccharide gelling fibre dressings, then contact the SMTL:

Surgical Materials Testing Laboratory, Princess of Wales Hospital, Coity Road, Bridgend, UK, CF31 1RQ.

Tel: +44 (0) 1656 752820



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