Alternatives and preferences for materials in use for pressure ulcer prevention: An experiment-reinforced literature review

Abstract

Alleviation of localised, sustained tissue loads and microclimate management are the most critical performance criteria for materials in use for pressure ulcer prevention, such as in prophylactic dressings, padding or cushioning. These material performance criteria can be evaluated by calculating the extents of matching between the material stiffness (elastic modulus) and the thermal conductivity of the protective dressing, padding or cushioning with the corresponding properties of native skin, separately or in combination. Based on these bioengineering performance criteria, hydrocolloids, which are commonly used for prophylaxis of medical device-related pressure ulcers, exhibit poor stiffness matching with skin. In addition, there is remarkable variability in the modulus and thermal conductivity matching levels of different material types used for pressure ulcer prevention, however, it appears that among the materials tested, hydrogels provide the optimal matching with skin, followed by gels and silicone foams. The stiffness matching for hydrocolloids appears to be inferior even to that of gauze. This article provides quantitative performance criteria and metrics for these evaluations, and grades commonly used material types to biomechanically guide clinicians and industry with regards to the selection of dressings for pressure ulcer prevention, both due to bodyweight forces and as a result of applied medical devices.

Keywords: biomechanical properties; laboratory testing; padding and cushioning; pressure injury; prophylactic dressings.

FIGURE 1

The structure–function‐property principle in material science (A) and its key implications concerning the performance of dressings used for pressure ulcer prophylaxis: (B) the material stiffness (elastic modulus) determines the extent of deformation of a dressing when it is subjected to the forces of the bodyweight or a medical device, and thereby, whether the dressing will deform under the device, hence alleviating skin deformations, or be displaced with little self‐deformation onto the skin, to imprint it. (C) The thermal conductivity of the dressing determines the level of heat transfer from the skin surface to the environment. A dressing with low thermal conductivity, that is, an insulator, will cause accumulation of heat at the skin‐dressing interface and hinder heat release to the surface of the dressing and from there, to the environment, such as through convection. These illustrations demonstrate how the physical and engineering characteristics (“properties”) of a dressing material (“structure”) define its biomechanical protective efficacy (“function”) (A) against pressure ulcers, both of the bodyweight‐induced type and medical device‐related injuries

FIGURE 2

Examples of compressive stress–strain curves and elastic moduli of different commercial wound dressing products that are commonly used in the clinical practice of pressure ulcer prophylaxis, for preventing both bodyweight‐induced and medical device‐related pressure ulcers, and which were measured historically at the author's laboratory following a protocol modified from the ASTM D3574‐11 test standard, as described in the body of the text: (A) Example stress–strain curve for a single‐layer foam dressing #1 (SLF1); and an additional example (B) of a stress–strain curve for gel padding (GP) material. Note the stress softening behaviour of the foam (where the slope of its stress–strain curve decreases at the higher strains), in contrast to the stress stiffening behaviour of the gel (where the corresponding slope increases). (C) Elastic moduli obtained by linearisation of the stress–strain curves for protective materials over the 30%–50% strain domain; and (D) the corresponding tangent elastic moduli at the 5% and 20% strain levels, to capture stiffness variations associated with potential non‐linear stress–strain (stress stiffening) behaviours of the tested dressing products. Both bar graphs demonstrate that the hydrocolloid dressing product is remarkably stiffer than the range of adult skin stiffnesses under compression (which is up to 200 kPa; see the literature reviewed in the text), whereas the foam‐based and the gel‐based products generally fall within that range for skin. N = 3 specimens per product type; the standard deviations around the mean values were negligible

FIGURE 3

The elastic modulus (A) and thermal conductivity (B) properties of the tested hydrogel‐based dressing (HydroTac Transparent by Paul Hartmann AG), which both fall within the respective ranges of skin properties. These measurements were acquired for dry and moist dressing conditions, as described by Grigatti and Gefen and the thermal conductivity data were collected by means of two different methods, namely, a heat‐flow meter and an infrared thermography (IRT) based system

FIGURE 4

Scatter plots to compare the resemblance of stiffness and thermal conductivity properties of materials that are commonly used in dressings for pressure ulcer prophylaxis, to the corresponding properties of adult human skin. The extent of the matching between the dressing material properties and the corresponding skin properties is represented by the level of the overlap (if any) between property ranges over the elastic modulus and thermal conductivity domains (A); the ideal property range, which is the property range reported for native, living human skin in the literature reviewed here, is depicted in grey shadow with black dash line borders. In addition, the extent of property matching between the investigated materials and human skin is plotted as the distance of the (normalised) property ratios from the unity (1,1) coordinates on a logarithmic scale (B). Please see the text for a detailed description of the calculation method used for generating the results shown in panel (B). Ideally, a dressing for prophylactic use would have an elastic modulus that is identical to the mid‐range stiffness of skin (ie, E _skin = ~100 kPa) and thermal conductivity that equals the mid‐range thermal conductivity of skin (ie, k _skin = ~0.4 W/mK). Accordingly, the nearer the property ratio coordinates of a certain dressing material (E _dressing/E _skin, k _dressing/k _skin) to the unity coordinates, the better is the matching between the dressing material and skin, and hence, the greater is the biomechanical protective efficacy of that dressing. Using this bioengineering grading system of distances from the unity coordinates, the grading of biomechanical protective efficacies of common dressing materials, from best to worst, is: (a) hydrogel‐based dressing; (b) gels; (c) silicones; (d) foams; (e) gauze; and (f) hydrocolloids. Note that the hydrocolloids are distant by two orders of magnitude from the unity coordinates with respect to the other material types, and, hence, from the modulus matching perspective, are inferior even to gauze. The mid‐range properties of skin were taken as E _skin = 101 kPa and k _skin = 0.4 W/mK for the purpose of the above grading calculations