The properties of hydrogel dressings and their impact on wound healing
Karen Lay-Flurrie, RN, Staff Nurse.
Windsor Wing Day Hospital, Hemel Hempstead General Hospital, Hertfordshire, and student on the BSc (Hons) programme at the University of Hertfordshire
This paper examines the composition and properties of amorphous hydrogels. It looks at their application in clinical practice and evaluates their efficacy in wound healing, using a literature review as its basis (Box 1).
This paper examines the composition and properties of amorphous hydrogels. It looks at their application in clinical practice and evaluates their efficacy in wound healing, using a literature review as its basis (Box 1).
Wichterle and Lim (1960) developed the first synthetic hydrogel polymer by the copolymerisation of 2-hydroxyethyl methcrylate and ethylenedimethacrylate. Since then, hydrogels have been used in biological applications such as contact lenses, in coatings for surgical gloves, urinary catheters and surgical drainage systems, and in wound dressings (Wheeler et al, 1996).
Hydrogel dressings discussed in this paper are listed in Box 2. The structure, properties and action of hydrogels are described in Box 3.
Amorphous hydrogels are of the most benefit in the treatment of sloughy or necrotic wounds (Pudner, 2001). Depending on the composition of the gel and the level of hydration of the wound it is applied to, the nature of the gel enhances autolytic debridement (Thomas and Leigh, 1998; Pudner, 2001).
Autolysis is most effective in a moist environment (Tong, 1999). Hydrogels have the ability to donate water molecules to dehydrated tissue while allowing the passage of water vapour and oxygen to the wound surface (Jones and Milton, 2000). This helps to increase the phagocytic activity of leucocytes and enzymatic activity of damaged cells. This, in turn, removes devitalised tissue during the destructive phase of healing of a wound - autolysis (Thomas and Leigh, 1998; Tong, 1999; Pudner, 2001).
However, the lack of physiological evidence to support these theories seems to be a weakness in the literature. The studies and literature reviews appraised appear to accept that the main physiological effect of hydrogels is their ability to rehydrate devitalised tissue, promoting autolysis. While their fluid-handling capabilities have been assessed in vitro (Thomas and Hay, 1994; 1995), their action on leucocyte and enzymatic activity appears to be based on hypothesis, although widely cited. This is a major flaw and indicates a need for further in-vitro studies to establish the full range of action of hydrogels, rather than research relying simply on observation of their clinical effects.
Hydrogels may appear to be similar, but their fluid-handling properties are very different (Thomas and Hay, 1995). Thomas and Hay (1994; 1995) examined their fluid-handling properties in vitro. Gelatine and agar gel plates were used to simulate dry, necrotic wounds and moist-to-lightly exuding wounds respectively.
The gel and agar plates were stored at 20-22°C for 48 hours, with the test substrate applied. It is interesting to note that although the studies acknowledge that 30-37°C is closer to normal body temperature, they also state that early experimentation showed changes in the fluid and movement properties of the hydrogels above 20°C. It therefore seems appropriate to pose the question as to whether the same occurs in the gels in vivo at normal body temperature and how this might have affected the results of the study cited above.
The study, however, did demonstrate that the four gels tested - Carasyn Gel, Carasyn V, Duoderm Gel and SoloSite - were donators of fluid and more suited for use with dry, necrotic wounds to promote hydration and autolysis. Duoderm Gel had both fluid absorption and donation capabilities.
Previous testing on samples of original formulated starch-based and carboxymethylcellulosebased IntraSite Gel showed that the latter had better fluid absorbing properties, but that this was at the expense of the fluid-donating properties of the old formulation of starch-based IntraSite (Thomas and Hay, 1994).
Suitable wound types
Although in vitro testing can be indicative of the likely performance of a hydrogel in clinical practice, it is impossible to evaluate fully how such gels will perform, for example, with regard to patient comfort or adverse reactions. It is therefore important that the fluid-handling properties of hydrogels are tested in vivo to establish their potential uses in clinical practice.
The efficacy of one of the earliest hydrogels, IntraSite Gel, has been documented in case studies and clinical trials for treating leg ulcers and pressure ulcers (Thomas and Fear, 1993; Colin et al, 1996; Bale et al, 1998; Vernon, 2000a). It is also widely used for treating surgical and malignant wounds and burns, or in conjunction with sharp debridement (Vernon, 2000a; Pudner, 2001).
A multi-centre study carried out by Flanagan (1995) researched the efficacy, safety, comfort, ease of use and duration of wear of a modified formulation of IntraSite Gel. No restriction as to the aetiology of the wounds included in the study was imposed, although a 30% presence of non-viable tissue was recommended, as this was likely to impair wound healing.
A median value of a 75% reduction in non-viable tissue was recorded after 21 days, with leg ulcers achieving the poorest results, although this was attributed to the presence of other underlying pathologies.
The study also established a link between wound duration and level of debridement. For example, traumatic wounds with a duration of up to four weeks showed an 80% median reduction in area of non-viable tissue, compared with 52% for leg ulcers with a duration of 26 weeks or more.
While Flanagan’s study determined the effectiveness of hydrogels in a range of wound types, other studies have focused on their effectiveness in debridement compared with other dressings. Thomas and Fear (1993) and Colin et al (1996) compared the wound-cleansing properties of IntraSite Gel with Debrisan Paste in patients with sloughy pressure ulcers. IntraSite was found to perform better, showing statistically significant results with regard to the area of sloughy tissue debrided and the time taken to achieve this. In other variables tested, such as cost-effectiveness, patient comfort and ease of use, the gel was found to be superior to Debrisan Paste.
Debridement and gel efficacy
New additions to the range of available hydrogels are constantly being developed. Aquaform hydrogel has been tested in vitro alongside IntraSite Gel to assess their fluid-handling capabilities. Using a previously described method (Thomas and Hay, 1994; 1995), Thomas and Hay (1996) concluded that the two types of gel had similar fluid exchange properties. However, Aquaform absorbed more fluid under conditions that simulated moist wounds, indicating a clinical use more suitable for treating sloughy and necrotic wounds.
Subsequent pilot studies have indicated success in the debridement of sloughy pressure ulcers, leg ulcers, malignant wounds and eschar-covered haematoma (Thomas and Jones, 1996; Trudigan, 2000). Patients were also described as being satisfied with Aquaform, finding it comfortable and reporting less pain. Nurses found the gel easy to apply and were pleased with the outcomes achieved.
It must be stressed that these were very small-scale pilot studies, with sample sizes of five and 10 patients. While the results indicate the value of Aquaform in treating these types of wounds, there is a need for further controlled clinical trials to assess the effectiveness of this hydrogel in a variety of wound types in a range of care settings.
Bale et al (1998) undertook a study comparing the use of Sterigel with IntraSite Gel in 50 patients with pressure ulcers in both hospital and community settings. They assessed the overall performance of each gel, along with the extent of maceration of surrounding skin, wound odour, patient comfort and pain levels. They concluded that there was no significant difference in the performance of the gels, as each achieved debridement within a similar period; patients found the Sterigel to be comfortable and non-painful both at dressing changes and in use. IntraSite was found to be comfortable.
It is apparent from the studies cited above that amorphous hydrogels are of value in treating sloughy and necrotic wounds. They have been shown to be an effective treatment: they fulfil their basic function, provided the practitioner selects the appropriate hydrogel according to its fluid-handling properties. However, the outcomes of the studies raise questions that require investigation.
The studies cited have tended to focus on the application of hydrogels in patients with leg and pressure ulcers. They did not investigate hydrogels for patients with diabetes or those with other non-healing necrotic or sloughy wounds.
Although Flanagan’s (1995) study included such patients, they were grouped under ‘other’ wounds; the efficacy of these dressings in treating diabetes patients as a discrete group, for example, was not quoted. Further research on the use of hydrogels with these patient groups would, therefore, be advantageous.
This lack of evidence should be taken into consideration before applying a hydrogel to a diabetic foot wound. Jones (1999) suggested that the vascular supply to the sloughy or necrotic area should first be carefully assessed. Jones also stated that hydrogels should be avoided in patients with peripheral vascular disease in the presence of gangrenous tissue, because increasing the moist wound environment can promote the spread of infection.
Patients with diabetes are more prone to infection, as they have reduced leucocyte infiltration, decreased phagocytosis, poor bactericidal activity and build-up of debris (King, 2001). But the literature is unclear on whether hydrogels can be used safely on infected neuropathic or neuroischaemic foot wounds. However, the manufacturers’ guidelines offer advice: for example, Coloplast guidance included in the packaging of the product suggests that Purilon Gel may be used on non-infected diabetic wounds, which should be evaluated frequently depending on clinical condition.
It has been shown that application of hydrogels is effective in treating infected wounds, provided the patient is receiving systemic antibiotics and has daily dressing changes (Mentar and Mayet, 1992); however, studies have not extended to include patients with diabetes.
Other authors have recommended that hydrogels be avoided in treating infected wounds, as application may increase the risk of organisms spreading to the surrounding tissue (Jones and Milton, 2000). It should also be borne in mind that infected wounds may often be too wet, possibly contraindicating the use of a hydrogel. Nevertheless, it is important that the practitioner has access to sound evidence on which to base clinical judgement.
The use of an amorphous hydrogel requires application of a secondary dressing. Trudigan (2000) and Pudner (2001) advocated the use of a semi-permeable film for dry wounds and a foam-type dressing, such as Allevyn, as the necrosis becomes softer and the wound has more exudate. The film reduces the escape of water vapour from the primary dressing, preventing it from drying out, thus helping to maintain a moist wound-healing environment (Thomas et al, 1997a).
There appears to be a paucity of evidence-based literature to underpin the performance of these secondary dressings and the basis for selection of them. The use of a specific dressing type may in part depend on individual practitioner preferences and in part on their apparently successful use in clinical practice - this may mean that such choices remain unchallenged.
For example, trials to determine the efficacy of amorphous hydrogels have indicated that low-adherent or semi-permeable dressings were chosen apparently either simply because they were already routinely used for the purpose (Bale et al, 1998) or because it was in accordance with local wound-care guidelines (Flanagan, 1995).
Flanagan’s study also reported no difference in the rate of wound debridement between occlusive and non-occlusive secondary dressings. She did, however, concede that this was probably because of the small sample size and indicated that further randomised clinical trials using larger sample sizes would be advantageous.
Thomas et al (1997a) compared the use in the community of two film dressings - Duoderm Extra Thin and Tegaderm - as secondary occlusive dressings with various wound types. They found little difference in the effects of the primary dressing on wrinkling or maceration of skin surrounding the wound, although the sample size was too small to show statistically significant results.
Maceration - Maceration is acknowledged as a problem that can accompany the use of amorphous hydrogels. Bale et al (1998) recorded maceration surrounding pressure ulcers treated with Sterigel and IntraSite, although there was little difference in the number of cases reported for each gel type. Trudigan (2000) also noted that maceration occurred in two of the 10 patients in her study.
Maceration may occur if an excess amount of hydrogel has been applied to the wound (Jones and Milton, 2000; Pudner, 2001) or as a complication arising from the water-donating properties of hydrogels (Bale et al, 1998). Exudate leakage from secondary dressings has also been documented (Thomas and Fear, 1993; Flanagan, 1995). However, the problem may lie in the practitioner’s failure to reassess the wound and choose a product more suited to the level of exudate. It may also be due to the fact that the dressing is on a weight-bearing surface such as a heel. It is therefore important that the practitioner not only selects the most appropriate dressing for exudate management but also recognises the need for frequent dressing changes.
Given the plethora of dressings available, it is essential that nurses familiarise themselves with the product literature and manufacturers’ instructions for duration of hydrogel application, bearing in mind that some can be left in situ longer than others, and are also knowledgeable about the most appropriate dressing combinations. Using the expertise of specialist practitioners such as tissue viability nurses will also enable nurses to make clinical decisions based on sound rationale and prevent costly and ineffective dressing selection.
Further research is indicated, with larger sample sizes, to provide a clearer indication of the value of using occlusive versus non-occlusive dressings over hydrogels in wound debridement and to establish whether maceration occurs more with one dressing type than another.
It is also crucial to consider issues such as patient comfort and wound malodour and to address these (Box 4).
Selecting the appropriate dressing
Essentially, it is the practitioner’s choice as to when and how to use an amorphous hydrogel. One of the fundamental points is that the nurse should be able to identify sloughy tissue and avoid confusing it with fibrinous exudate or pus (Tong, 1999). Knowledge of the process of wound healing is therefore important for any practitioner involved in wound care. Another important issue is that of product selection and the factors that influence a practitioner’s choice. Further restrictions may arise due to limitation of product availability.
Pilot studies have indicated that successful product trials have resulted from inclusion of a particular hydrogel in a trust formulary on that basis (Trudigan, 2000). Further research into factors that affect nurses’ product choice is needed to establish whether they have the appropriate underlying knowledge about current products, how to select the appropriate one and who to consult for advice. This is particularly important in the light of developing new roles for nurses, such as prescribing.
Hydrogel application - Consideration should be given as to how hydrogels are used in clinical practice. Flanagan’s (1995) study found that in 66% of cases gels were applied directly to the wound, with 34% applied to the secondary dressing.
It may be a matter of debate as to which is the better method. Applying it to the dressing may make it more difficult to ensure the gel ends up in contact with the wound, rather than the surrounding skin when applied.
The positioning of the wound may also influence the way the gel is applied. For example, in some cases it may be more appropriate to use IntraSite Conformable, which is a gel-impregnated gauze version of the dressing. Further study to investigate a possible link between application method and level of maceration would be of value.
Hydrogels are single-use products (Pudner, 2001). However, in practice I have observed that this is not always the case: opened packs are sometimes sent for patient use where wound care is shared with the community nurse. This poses a risk of cross-infection and contamination. Another inappropriate practice is combining a hydrogel with an alginate (Young, 1997). Further research is recommended to establish the extent of the problem and why it happens.
Amorphous hydrogels have been demonstrated to be useful in treating sloughy and necrotic wounds (Thomas and Fear, 1993; Flanagan, 1995; Colin et al, 1996; Bale et al, 1998). Although they have the ability to rehydrate devitalised tissue, thus promoting autolysis, their exact method of action is unclear, which is an issue that needs to be addressed through further in vitro investigation.
There also appears to be some debate as to the appropriateness of using amorphous hydrogels with neuropathic, neuroischaemic foot wounds or infected wounds. It is clear that practitioners must not only use their clinical judgement wisely but must also consider manufacturers’ recommendations and seek the advice of expert practitioners, such as tissue viability nurses, to avoid making costly and inappropriate dressing choices.
This point extends to the issue of secondary dressing choice. Establishing a link between the type of secondary dressing used and rate of debridement and level of maceration would assist the practitioner to select the most appropriate dressing combinations from among the plethora of those available.
Nurses involved in wound management should have an underlying knowledge of the process of wound healing and be able to identify when a wound requires application of a hydrogel. It is essential that general nurses consider widening their knowledge base of tissue viability through ongoing education, not only in the clinical area but through in-service and postregistration study days. Highlighting inappropriate practices, such as the application of hydrogels to wet wounds or the use of hydrogels and alginates in combination, could help prevent such practices occurring. Maximising the role of tissue-viability link nurses to disseminate knowledge to colleagues regarding product use and developments could also prove beneficial.
The clinical indications for using amorphous hydrogels may extend beyond the accepted application with sloughy and necrotic wounds. They have, for example, been used in treating neonatal extravasation (Thomas et al, 1997b; Young, 1995) and excoriation (Vernon, 2000b). Hydrogels could also act as carriers for antimicrobial agents, growth factors or other biologically active molecules (Thomas and Hay, 1995).
Hydrogels have been shown have beneficial effects for treating, in particular, necrotic and sloughy wounds. However, studies involving greater patient samples are necessary. Further work also needs to be done to establish the best possible combination of secondary dressing to achieve optimum wound care. Research is also needed into the alternative uses of hydrogels as outlined above.
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