Fluid handling by foam wound dressings: From engineering theory to advanced laboratory performance evaluations

Abstract

This article describes the contemporary bioengineering theory and practice of evaluating the fluid handling performance of foam-based dressings, with focus on the important and clinically relevant engineering structure-function relationships and on advanced laboratory testing methods for pre-clinical quantitative assessments of this common type of wound dressings. The effects of key wound dressing material-related and treatment-related physical factors on the absorbency and overall fluid handling of foam-based dressings are thoroughly and quantitively analysed. Discussions include exudate viscosity and temperature, action of mechanical forces and the dressing microstructure and associated interactions. Based on this comprehensive review, we propose a newly developed testing method, experimental metrics and clinical benchmarks that are clinically relevant and can set the standard for robust fluid handling performance evaluations. The purpose of this evaluative framework is to translate the physical characteristics and performance determinants of a foam dressing into achievable best clinical outcomes. These guiding principles are key to distinguishing desirable properties of a dressing that contribute to optimal performance in clinical settings.

Keywords: absorbency and retention; adhesion of adhesive dressings; leakage and failure of wound care; testing methods and standards; treatment.

FIGURE 1

Wound care clinical outcome‐related and cost‐effectiveness‐related implications of the performance of wound dressings (including outside wound care) depending on how specifically a dressing may fail to manage exudate in clinical practice: (A) Scheme of the factors involved in dressing failure scenarios. (B) An illustrative wound care case demonstrating the failure of a dressing involving a number of factors listed in panel (A), both at the wound‐facing surface of the dressing and at the outer dressing surface. The case shown in panel (B) is of a 58 years‐old male with a mixed wound aetiology (more venous than arterial). There are right medial and anterior superficial wounds, which were treated by a dressing and compression therapy. This patient could only tolerate low compression levels of approximately 20 mmHg and had returned to the clinic of author TS with the wound and exudate noted in the photos. The patient was visited by a district nurse twice a week but on the day when the photos were taken, the dressing has leaked massively. Visual inspection of the exudate type suggested that this patient might had local infection colonized with Pseudomonas and it was only after cleansing that the wound care clinician could determine that it was not a spreading infection. The dressing was supposed to be highly absorptive, however, due to gravity and possibly how the patient sleeps that dressing was clearly ineffective in capturing the wound fluid. The exudate gathered at the edges and was held by the film of the dressing, pooling at the edges and causing inflammation and maceration of the peri‐wound. To treat the condition documented in panel (B), the wound and peri‐wound were cleansed with antiseptic solutions and then mechanically debrided. A topical antimicrobial fibre dressing was applied, and a non‐adherent absorbing secondary dressing was used along with the compression. The frequency of dressing changes was at least twice a week but may have increased if the level of exudation warranted it. Importantly, the leakage of wound fluid into the sides of the dressing caused substantial damage to the peri‐wound. There are also multiple pressure points from the dressing, which likely exacerbated the inflammation of the peri‐wound. In addition, it is noteworthy that although the dressing capacity to absorb more into the dressing was visible, the dressing did not have the capability to wick that fluid laterally to be absorbed vertically. Of note, the dressing failure case presented in panel (B and C) is a representation of a well‐recognized problem that is associated with different wound care dressings manufactured by various companies. (C) Clinically documented cases of failure of different dressing products, related to the dressing selection interacting with the practice of treatment (courtesy of author TS).

FIGURE 2

Patient‐related and wound‐related factors as well as dressing design‐related and manufacturing‐related factors that may associate with the failure of a dressing to handle wound fluids in a clinical setting (of note, there may be additional factors involved that were not considered here, but the many factors listed demonstrate the complexity of the relevant physico‐chemical interactions). Failure of the dressing to handle the fluid is often a result of the exudate production rate (EPR) exceeding the sum of the absorbed or retained fluid content (FC) that can be locked‐in within the dressing and the moisture vapour loss (MVL) per unit time. The occurrence of EPR > FC + MVL over a critical time period may result in overflow of the applied dressing. This then leads to pooling of exudate in the wound bed under the dressing or spillover/leakage of exudate outside the dressing, which may compromise the wound bed and peri‐wound skin and/or stain the clothing and bedsheets and create malodors, or affect medical devices (e.g., secondary or compression bandaging) in the vicinity. The clinical and cost‐effectiveness implications of such events were depicted in Figure 1A.

FIGURE 3

The FLUHTE (acronym for ‘fluid handling test equipment’) wound leg simulator with its heating plate and computer‐controlled syringe pump (A). Examples of fluid handling after simulated use for 24 h at a flow rate of 0.75 mL/h is shown, demonstrating leakage of simulated wound fluid (SWF) to the secondary bandage by dressing F (B), pooling of SWF (0.34 g) on the leg model after removal of dressing E (C) and a retention test of dressing post simulated use at 40 mmHg (D).

FIGURE 4

Fluid handling properties of six wound dressings marked ‘A’ to ‘F’, measured by means of the FLUHTE (acronym for ‘fluid handling test equipment’) wound leg simulator after simulated use for 24 h at a testing (‘wound surface’) temperature of 30°C. The flow rates were either 0.5 mL/h (panels A, B; n = 4 specimens per dressing type) or 0.75 mL/h (panel C, D; n = 3). Grey bars indicate fluid reflux; black bars indicate fluid pooling; white bars indicate moisture vapour loss (MVL); and hatched bars indicate absorbed or retained fluid content (FC). Error bars show the standard deviations from the mean values. One‐way analysis of variance followed by Tukey pairwise comparisons was conducted to compare the fluid handling performance metrics of the tested dressings A–F in the FLUHTE apparatus. Horizontal brackets indicate that the dressing products underneath the bracket are significantly different. ** indicates significant differences in fluid reflux (p ≤ 0.01); ## indicates significant differences in pooling (p ≤ 0.01); *** indicates significant differences in both MVL and FC data (p ≤ 0.001); n/a indicates dressing failure in fluid handling under the FLUHTE test due to leakage from the tested dressing product (for product F).

FIGURE 5

Fluid handling properties of six wound dressings marked ‘A’ to ‘F’ after simulated use for 24 h on the FLUHTE (acronym for ‘fluid handling test equipment’) wound leg simulator, at a ‘wound surface’ temperature of 30°C and flow rates of either 0.5 mL/h (left column; n = 4 specimens per dressing type) or 0.75 mL/h (right column; n = 3). The area of each bubble in the bubble plots is proportional to the total amount of fluid that was not absorbed or retained by the relevant dressing (A–F) with the position indicating moisture vapour loss on the y‐axis and absorbed or retained fluid content on the x‐axis. One‐way analysis of variance followed by Tukey pairwise comparisons was conducted to compare the mean areas. ### indicates significant differences between product A versus C to F (p ≤ 0.01); *** indicates significant differences between product E versus A to D (p ≤ 0.001); n/a indicates dressing failure in fluid handling under the FLUHTE test due to leakage from the tested dressing product (F, for the 0.75 mL/h flow rate). SWF, simulated wound fluid.