Wound dressing testing - Conformability 

Used for hydrocolloid, film and related dressings

Gavin Hughes, Laura Price, Louise Barry, Vikki Baines, Joanna Ford, Pete Phillips

November 2018

Introduction

Hydrocolloid and film dressings are designed to form a seal over the wound and to adhere to the patient's skin, forming a protective barrier. Film dressings sometimes act like 'second skins' and are mainly for protection, whereas hydrocolloid dressings contain gel-producing polymers which absorb wound exudate. Bordered hydrocolloid dressings stick directly to the patient's skin forming a border around the wound whereas non-bordered hydrocolloids require being fixed into place with tape or bandages. These types of hydrocolloid dressings are designed to protect the wound surface whilst promoting its healing. They should provide a barrier to external contamination, whilst maintaining a moist environment at the surface of the wound by reducing water vapour loss from the exposed tissue.

Hydrocolloid and film dressings are often applied to joints or to other areas of the body where movement and distortion of the dressing is likely. The dressing should remain in place and accommodate this movement without causing excessive pressure or shearing forces that could lead to skin trauma¹. This paper presents a test that was performed for the All Wales contract, where the 'conformability' of dressings was investigated. The test described here uses a modified version of the apparatus and methodology for waterproofness, from Pharmacopoeia 1988 XX² and European Standard EN 13726-3³.  The limits used in these tests were selected based on clinical opinion - products complying with these limits had no reports of adverse incidences such as skin trauma associated with them during use according to the Welsh clinical advisory group. Thus the limits reflect the expert clinical opinion of Welsh Tissue Viability Nurses.  

This conformability test measures the pressure required to distort the dressing. The lower the pressure required, the more conformable the dressing. High readings indicate stiffer, less conformable dressings.

Methods

• The apparatus for this test involves a length of tubing with a 60 ml syringe attached to one end and a test chamber at the other. The tubing is connected to a pressure transducer and indicator (as shown in Figure 1).

• Following this, a sample of the dressing (usually 10x10 mm) is placed on top of the test chamber, and fixed in position with a retaining ring, as shown in figure 1.

• A rigid marker is fixed 20 mm above the dressing. The system is then clamped into place and pressurized by forcing air into it via the syringe.

• The pressure of the system is increased until the dressing deforms sufficiently to touch the fixed marker 20 mm above it, whilst still being intact and fixed in place. The pressure that has been reached to achieve this (shown on the pressure indicator) is then recorded.

• This is carried out in triplicate and the mean pressure recording is calculated. A maximum pressure is stipulated, and dressings must not exceed this value in order to comply.  The particular value depends on the type of dressing being tested.

Figure 1. Diagram of experimental set up.

Results

Below are the conformity test results for different types of wound dressings. Results are displayed as the means of triplicate samples and the standard deviations are in brackets.

Table 1:  Bordered Hydrocolloid dressings-pressures must not exceed 240 mmHg in order to comply.

Table 2:  Hydrocolloid Non-bordered, Thin dressings-pressures must not exceed 120 mmHg in order to comply.

 Table 3: Semi-Permeable Adhesive Films Without absorbent pads - pressures must not exceed 90 mmHg in order to comply.

Results and Discussion

The pressure limits in this study were chosen based on historical data and recommendations from Welsh clinicians. Products meeting their requirements were known to have been clinically acceptable in previous contracts.  Setting limits is useful as it can help categorise dressings into groups that perform in a similar way and provide a cut-off point for what is deemed acceptable. Each of the tables above contain a specific class of wound dressing, and the products within the table should be comparable to each other.  However, results in terms of pressure measurements were wide ranging in some instances. All the adhesive film dressings complied with their limit, whereas some of the hydrocolliod (both bordered and non-bordered) did not.

Therefore, although hydrocolloid dressings are designed to perform the same role, some are more conformable than others. This means that some may be more effective than others when dressing difficult wounds in awkward locations, such as limb joints.

This test is one from a whole suite of tests which wound dressings can be assessed against,  in order to obtain data which can be used alongside clinician assessment to enable clinicians to make informed purchasing and dressing selection decisions.

Bibliography

1. Queen D, Evans J, Gaylor J, Courtney J and Reid W. An in vitro assessment of wound dressing conformability. 1987. Biomaterials;8:372-6

2. 'Surgical Dressings' BP Volume II, British Pharmacopoeia, (1988)

3. Test methods for primary wound dressings. Part 3;  Waterproofness' BS EN 13726-3, British Standards Institution (2003)