Hydrogel Wound Dressings: Where Do We Stand in 2003?
A new era in wound care is under way in which the old one-size-fits-all approach to dressing selection has given way to individualized treatment based upon the needs of the particular patient and wound assessment findings. Advanced wound care, as practiced in specialized wound management centers and by sophisticated providers, is based upon physiologic considerations and a comprehensive observation of the wound. With recent advances in the understanding of detailed mechanisms of wound repair, and in response to heightened interest in wound care, the wound dressing market has been flooded with new products. Hydrogels, hydrocolloids, films, gauzes, alginates, biologics, and foams are among the many classes of dressings available to the practitioner.1
The hydrogel, originally developed in the 1950s, comprises a wide variety of hydrated polymer dressings that regulate fluid exchange from the wound surface.2 Because of their excellent biocompatibility, hydrogels are used in a wide variety of products including medical coatings, biosensors, skin-friendly adhesives, contact lenses, and wound dressings.3 The US Food and Drug Administration (FDA) has designated hydrogels as Class I devices, requiring minimal regulation.4 Nearly every major dressing manufacturer has one or more hydrogels in its product line.
Hydrogel products are marketed in various physical forms: gels, sheets, and gels pre-applied (or impregnated) to ordinary cotton gauze pads.5 Hydrogel sheets are used as primary dressings for shallow, flat wounds; examples include NU-GEL™ (Johnson + Johnson Wound Management Worldwide, A Division of Ethicon, Somerville, NJ), Vigilon® (CR Bard, Inc., Covington, Ga.), Flexigel® (Smith + Nephew, Largo, Fla.) and Aquamatrix (Hydrogel Design Systems, Langhorne, Pa.). Some are provided with adhesive borders. Depending upon the degree to which a wound is draining, hydrogel sheets may remain in place up to 4 days. Most hydrogel sheets come packaged with a protective layer of plastic that is removed before application to the wound surface; an additional layer of backing plastic may be left in place or can be removed to facilitate diffusion of water through the hydrogel into the atmosphere. Manufacturers have created a wide variety of sizes and shapes to accommodate uses on awkward surfaces such as the perineum.
Amorphous hydrogel can be used to fill a deep wound with irregular contours and is held in place with a secondary dressing that is generally changed at least once daily. Examples include Curasol™ Gel Wound Dressing (HEALTHPOINT, Fort Worth, Tex.), Iamin® Hydrating Gel (ProCyte, Redmond, Wash.), DuoDerm® Hydroactive Gel (ConvaTec, A Bristol-Myers Squibb Company, Princeton, NJ) and Restore™ Hydrogel Dressing (Hollister, Libertyville, Ill.).6 Amorphous hydrogels are available in tubes, spray bottles, and foil packets.
Hydrogel-impregnated gauze is sold as Aquagauze™ (DeRoyal, Powell, Tenn.), MPM Gel Pad™ Hydrogel Impregnated Gauze Pad (MPM Medical, Inc,, Irving, Tex.), Curafil® Hydrogel Impregnated Gauze (Kendall, Mansfield, Mass.) and Derma Cool (Afassco, Carson City, Nev.) These products better satisfy the reimbursement rules set forth by certain insurers, who in some cases limit access to hydrogel. In addition, each hydrogel product has an individual reimbursement code; in the hospital setting, such products are covered under the surgical dressing benefit. Hydrogel-impregnated gauze is particularly useful in tunneled and undermined wounds because the dressing is able to fill in the dead space.
Chemistry of Hydrogels
Polymers are long, chain molecules made of regular repeating units/patterns of building blocks (monomers). Naturally occurring polymers are common in nature and have been used as wound treatments (eg, various forms of collagen). Many industrial polymers use a single monomer or combine two monomers into A-A-A or A-B-A structures, respectively. Purely synthetic hydrogels used in wound dressings are frequently made from polyvinyl pyrrolidone, polyacrylamide, or polyethylene oxide. The structure of polyethylene oxide, which is contained in Vigilon® (CR Bard, Covington, Ga.) is shown below:
Noncovalent interactions between the adjacent polymer molecules enable these strands to stick to each other, particularly if the monomers contain aromatic rings, and this effect can lend strength to devices constructed from the polymer. In order to impart further structural integrity to the polymer, polymer molecules are covalently cross-linked using free radical reactions to activate side chains that protrude from the monomers. While this cross-linking can be accomplished chemically, the least expensive and most uniform result is achieved by irradiating the uncrosslinked polymer with ultraviolet light or electron beam.
Hydrogels are polymers with hydrophilic side chains that bind up to three times their weight in water.7 Thus, hydrogels essentially comprise a three-dimensional network with water filling much of the interstices. This important feature provides the special advantages of hydrogels when compared with other dressings: fluid absorption, hydration of the wound bed, cooling of the wound surface, and pain control. The high water content of hydrogels also facilitates electrical conduction and, in fact, one of the biggest commercial uses for these compounds is as skin contacts for monitoring devices such as ECG pads. Depending upon the extent of crosslinking and the degree of hydration, hydrogels can be created in physical forms ranging from amorphous gels that can conform to the irregular depths of a wound bed to semi-stiff sheets that have enough structural integrity to function alone without a secondary dressing.
The interaction of hydrogel with a wound bed is variable. Depending on the relative state of hydration of the tissue and the dressing, the hydrogel can absorb or donate water to the wound environment.8 Hydrogel dressings are permeable to water vapor and oxygen, but do not leak liquid water.
Moist Wound Healing
Based on current knowledge of the cascade of biochemical and cellular events involved in wound healing, clinicians have come to accept and understand that use of dry dressings retards healing. The concept of moist wound healing was first proposed by Winter9 40 years ago, and subsequently an impressive number of studies has supported its value.10 The wound bed undergoes desiccation necrosis in the absence of adequate hydration, which can lead to a destructive cycle: The wound is debrided to healthy tissue, which then desiccates, creating more necrotic cellular debris to be removed. Subsequently, the wound may become deeper and larger. Various enzymes involved in autolytic debridement function only in an aqueous environment, and the various peptide growth factors and other molecules that mediate cellular repair can only be effective in solution.
Countless clinical studies attest to the benefit of moist wound healing.11 Gates and Holloway12 demonstrated that using hydrogel dressings led to quicker healing and reduced pain and cost savings when compared to saline wet-to-dry dressings in patients with open abdominal wounds. Mulder13 compared the effectiveness of mechanical debridement using wet-to-dry dressings with autolytic debridement using hydrogel dressings. He found that the time to 50% wound surface debridement was shorter in the hydrogel group, and his cost-effectiveness analysis showed that the higher cost of the hydrogel dressing was more than justified by this healing advantage. More recently, similar findings were noted by Trudgian.14
The ability to hydrate the wound surface and resist drying out has been among the most salient selling points of hydrogel dressings, making them excellent replacements for saline-moistened gauze. Similar to saline dressings, care must be taken (particularly with the high water content hydrogels) to avoid overlapping the dressing with the skin edges; over a short period of time, the periwound area may become macerated from constant exposure to moisture from the hydrogel.
Certain hydrogels such as Hydrosorb® (Paul Hartmann, Ltd., UK) also are capable of absorbing excess fluid created by some wounds. The polymer swells as it takes up water. Consequently, debris and bacteria are trapped in the polymer interstices, helping promote a reduced wound bioburden.
Wound Pain Relief
Hydrogel dressings, by virtue of their high water content, have a cooling influence on the wound that engenders an analgesic effect. This is particularly beneficial in partial-thickness burns. Some suggest that the backing often provided by the manufacturer should be removed to maximize this cooling effect.15 This effect lasts for up to 6 hours and can be increased by storing the dressings in a refrigerator before applying to the patient; however, this practice is discouraged because cooler temperatures may thin the gel, alter its viscosity, and change its ability to hydrate the wound bed. Reduced wound pain has been noted when hydrogel dressings are used in burns, venous ulcers, dermabrasion wounds, and in lactating women with de-epithelialized nipples and areolae.16 Dressing change discomfort is also minimized because hydrogels do not adhere to the wound surface.17 In addition, the wound base experiences less trauma due to non-adherence, maintaining cell viability.
Indications for Use
Many authors have enumerated the attributes of an "ideal" dressing.18 In addition to providing a moist wound environment, a dressing should promote healing, protect the wound from dirt and bacteria, have an attractive appearance, minimize pain, be cost effective, be easy to apply and remove, manage odor, and not adhere to the wound bed. Hydrogels have satisfied these requirements in a wide variety of clinical circumstances.
Perhaps the most frequent use of hydrogel dressings has been the management of pressure ulcers.19,20 A randomized trial comparing an aloe-derived amorphous hydrogel to saline-gauze for a variety of Stage I through Stage IV pressure ulcers demonstrated similar 10-week healing rates for the two dressings.21 Motta and colleagues22 compared the efficacy of a sheet hydrogel dressing and a common hydrocolloid for Stage II and Stage III wounds and found similar healing rates. However, because the hydrogel compared more favorably in terms of autolytic debridement and cost effectiveness, the authors concluded that the hydrogel was the superior choice for pressure ulcer management. Thomas et al23 found that a hydrogel dressing was more effective in absorbing fluid and controlling odor from pressure sores than a hydrocolloid dressing. A randomized study24 of 40 patients with Stage II and Stage III ulcers treated with hydrogel or hydrocolloid showed that the hydrogel was more user-friendly, although healing rates were similar in the two groups.
Hydrogels have been proven efficacious in many other circumstances in which moist wound healing is desired. Numerous authors have confirmed the utility of hydrogels in treating diabetic foot ulcers.25 Burn wounds have responded well to application of hydrogels; their cooling effect is both analgesic and also therapeutic, as it may limit the extent of thermal injury.26 Hydrogels have become popular among nurse midwives who use them on patients with sore, cracking nipples during lactation.27 Laser dermabraded facial wounds heal well with hydrogel dressings,28,29 as do meningococcal skin lesions.30
Drug Delivery Using Hydrogels
Another significant use for hydrogels is loading the dressings with putative therapeutic agents for topical delivery to the wound site.31,32 A complete summary of this literature is beyond the intended scope of the present review. Hydrogels have proven capable of holding and protecting a wide variety of chemical agents. The drug delivery kinetics often demonstrates "burst" release but may be adequate for many therapeutic agents. The largest issue in this field is not the delivery mechanism, but instead finding medications that can be topically absorbed, providing desirable therapeutic effects within wounds.
Is Hydrogel Product Differentiation Clinically Meaningful?
"The most prevalent challenge faced in moist wound dressing markets is product differentiation."33 Differences in fluid absorption among the various hydrogels caught the attention of clinical investigators and manufacturers. In 1996, Thomas and Hay compared the abilities of four hydrogel dressings to absorb water in vitro and found significant differences among the products. Sprung and colleagues, in a similar investigation, confirmed major differences in the absorption capacity of various hydrogels.34
Manufacturers stress the differences between various hydrogels. Johnson + Johnson (Somerville, NJ) states that its NU-GEL absorbs 50% more exudate than other hydrogel dressings.35 Coloplast compared Purilon to IntraSite Gel (Smith + Nephew, Largo, Fla.) in the treatment of venous ulcers and found that its own product was substantially more cost effective and was superior in debriding and reducing wound pain.36 Scant independent research has been conducted to examine wound outcomes by type of hydrogel used. One study noted that various hydrogels may be more (or less) able to transmit acoustic energy, which may be important and useful with patients receiving therapeutic ultrasound for the stimulation of wound healing.37 In some models, slight improvements were noted in the rate of epithelialization of wounds treated with some hydrogels but not others; however, these effects are modest.38
Many comparisons have failed to demonstrate important differences between the various brands of hydrogel dressings. Thomas and Hay,39 in a follow-up to their 1996 study, demonstrated that two common products - Aquaform (Medica, Dusseldorf, Germany) and Intrasite Gel (Smith + Nephew, Largo, Fla.) - were equal in their ability to absorb water. Bale and colleagues40 conducted a prospective, controlled, blinded comparison of two other amorphous hydrogels and found no differences in comfort, wound odor, surrounding skin condition, or time to debridement.
Many clinicians do not perceive major differences among hydrogel products in terms of efficacy, convenience, cost, or patient acceptance. Indeed, while the chemical nature of the polymer and the exact water content of hydrogels varies from product to product, little evidence supports the notion that any particular hydrogel product is associated with substantially better or worse clinical results. Various manufacturers have included zinc, vitamin E, aloe, and other therapeutic agents in an effort to differentiate their hydrogels from the competition.
Hydrogels are used extensively in medical coatings, biosensors, skin-friendly adhesives, contact lenses, and wound dressings. They have been shown to be highly effective for pressure ulcer treatment and are available in a variety of formats from major wound care manufactures. However, clinicians should remember that hydrogels are designated by the FDA as Class I devices, subsequently requiring minimal regulation. Therefore, it is the clinician's responsibility to use evidence-based research as the basis of product selection and to provide accurate and complete documentation of dressing need to receive reimbursement. - OWM
The author wishes to acknowledge the support of Broadreach Medical Resources, Inc. in writing this manuscript.
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