SubscribeWhen you are a latecomer to an area you have the opportunity for a fresh appraisal - a new look at what has gone before - and so it has been with the Avery Dennison Hydrocolloid Programme. Avery has been manufacturing hydrocolloids at the Specialty Tape Division in Turnhout, Belgium for nearly 5 years.
Hydrocolloid adhesives are unique in that they are inherently pressure sensitive adhesive and also absorbent of body fluids. This important duality had led these materials to become key components in a number of medical device fields, in particular ostomy care and wound care. In ostomy care, hydrocolloids replaced karaya gum barriers as protectants of peristomal skin in the mid-1970’s, and hydrocolloid dressings, introduced in 1983, revolutionised the management of chronic wounds, and allowed the idea of occlusive wound healing to become a broad reality.
The first hydrocolloid dressing (ConvaTec’s DuoDERM) had an impressively benign and gentle performance on intact skin. But it was not integrated, which means that it forms a gel in the presence of body fluids, particularly wound exudate. This is a drawback for wound care, although not for certain aspects of ostomy care.
The hydrocolloid is still in demand for ostomy care, and Avery manufactures a similar product, H2430. Integrated products were later introduced to solve the wound care problem. The Avery H2410, a moderately integrated product formulated to be especially skin friendly, is broadly comparable in its properties to other, mostly captively produced, integrated commercial products.
Let us now explain the thinking behind the new Avery Advanced Hydrocolloids (AHC’s), and show you how these products are different, and why we believe these new products have important advantages, especially for wound care.
Hydrocolloid adhesives are multi-phase systems. They consist of a rubbery adhesive, the continuous phase, into which is dispersed a particulate absorbent medium, the discontinuous phase. The original non-integrated DuoDERM dressing from ConvaTec had a continuous phase of low molecular weight polyisobutylene, into which was dispersed a discontinuous phase made up of a 1:1:1 blend of pectin, gelatine and sodium CMC.
When imbibed with wound fluid, this hydrocolloid becomes a soft amorphous gel, which remains in the wound after dressing removal. Because of the nursing problems and the patient pain problems that this gel creates as it is irrigated from the wound with each dressing change, a product modification was crucial. Much effort was expended by wound care OEM’s in the 80’s to solve this integrity problem.
In the 1980’s it was shown with the original non-integrated hydrocolloid dressings that polyisobutylene could become incorporated into giant cells during healing. While this finding appeared to have little clinical significance, it demonstrated the lack of permanence of the early compositions. The second generation products generally comprise a continuous phase of thermoplastic elastomers (TPE’s) with tackifiers and polyisobutylene.
The objective is to lock all the ingredients into the cross-linked polymeric network that forms the continuous phase. But in these so-called improved integrated products, leaching of materials into the wound can still occur, and in fact a larger number of chemical species contacts wound tissues than with the original non-integrated composition. It was discovered later that there is indeed with some of these integrated dressings a reduction in epithelial migration during the acute secondary healing phase. Inflammation from absorbents leaching into the wound was also reported.
Wound healing studies with these products, although showing satisfactory overall performance, raised questions about the effect of leaching ingredients on healing wounds. Finally, anecdotal reports came to light of increased skin irritation of integrated hydrocolloids compared to the original DuoDERM product.
So, for the improved integrity of second generation products a price was paid. The product was no longer as gentle as was original DuoDERM to intact skin, and other chemicals were introduced into the formulation and hence, potentially, to the healing wound environment. It was time for a re-think. Is there a better way of integrating hydrocolloid compositions while at the same time reducing the possibility of chemical interference in the healing process?
We set ourselves the goal of a completely integrated hydrocolloid that introduced no foreign materials in the wound. We wanted the same skin friendliness as the original DuoDERM product. These were absolute musts. Improved absorption was a desirable goal, too. The currently available integrated products absorb far less wound fluid at a far lower rate than did the first DuoDERM wound care product. Another desirable attribute was transparency, or at least translucency, to visualise the healing wound.
We needed a new chemical approach that would eliminate tackifiers and complex stabilizer systems. We eventually explored a new plasticizer system that recently became available for the formulation of hot melt adhesives. This low molecular weight styrene-isoprene copolymer plasticizer was ideal in terms of the goals we had set. In order better to understand our approach, let us now take a look at the polymer chemistry of these systems.
In the conventionally integrated hydrocolloid, a styrene-isoprene-styrene block copolymer provides a network shown in schematic form in Figure 1. The black dots represent the glassy polystyrene domains present in the solid state. These domains form when the rubber cools from the melt. Each polystyrene domain links polyisoprene chains into a three dimensional structure which give the rubber its elasticity.
Thus, the rubber behaves as if it is covalently cross-linked, with the glassy domains acting as the cross-link junctions. To give the rubber its pressure sensitive tack properties, a tackifier and/or plasticiser must be added. Stabilizers, often comprising organo-phosphates in part, are also necessary to prevent degradation during processing of the initial hot melt pressure sensitive adhesive, which acts as the continuous phase. The thick lines in Figure 1 represent these plasticising and stabilising entities which, since they are not bound into the structure, are free to migrate.
Figure 1
Representation of Integrated Continuous Phase of Existing Hydrocolloid Systems
This is the key point. This chemically complex matrix (the continuous phase) then has the absorbent hydrocolloids (the discontinuous phase) dispersed within it.
Compare Figure 1 with the structure of the Avery AHC formulations, shown in Figure 2. We have exactly the same TPE, a styrene-isoprene-styrene block copolymer with its pseudo cross-links of glassy polystyrene domains, but instead of a migratory plasticizer we have a compatible plasticizer that is chemically identical with the rubber itself. One end of this styrene-isoprene plasticizer becomes part of the glassy domains of the TPE, locking it within the network. The other end of the plasticizer chain is free, and softens the rubber to give it adhesive properties. Because conventional tackifier materials are absent, simpler processing stabilizer systems can be used. As before, the plasticised rubbery continuous phase has the absorbent discontinuous phase dispersed within it.
Figure 2
Representation of Integrated Continuous Phase of Avery AHC System
The chemistry of our new hydrocolloid adhesive is, we believe, the basis for its superior performance and unique clinical potential. The new hydrocolloid is more integrated than any other on the market as measured by our in-house test.
Our system has wide formulation latitude and hydrocolloids can be formulated to give a very high absorption capacity with low cold flow and high shear strength.
We have looked broadly at two versions of our new AHC system. One of them contains only styrene-isoprene copolymers, stabilizer and absorbent filler. This constitutes an elegantly simple formulation, analogous in fact to the original DuoDERM product in its simplicity and paucity of ingredients.
We call this product H2460. Another version is similar, except that low molecular weight polyisobutylene is added. The polyisobutylene improves tack and particularly skin adhesion. This composition has the product code H2440. Both compositions pass the battery of tests for compliance to ISO10993 for direct wound contact.
Further tests assessed the potential of the new system in relation to skin friendliness that was an important goal of our work. A superficially abraded rabbit skin wound was chosen for a study in which dressings were applied and removed daily for 5 days. The results here showed a strikingly reduced propensity for irritation compared to the current market leading hydrocolloid, which was used as a control.
We believe that this finding is relevant to the question of how a hydrocolloid dressing performs on intact skin. The intact skin surrounding chronic wounds is often irritated and friable, and can be easily damaged by aggressive pressure sensitive adhesives.
Encouraged by these early findings, we approached the Department of Dermatology, University of Miami, the group that performed the original wound healing studies on the first DuoDERM product in 1981. Employing the same partial thickness pig wound model that was developed in the late 1970’s, preliminary wound healing data on our hydrocolloids were generated that are exciting. Both versions of our AHC were evaluated against the market leading hydrocolloid product.
First, the healing data show re-epithelialisation to commence on day 3 post-wounding, but it is on day 4 that things really start to happen. Then the H2460 product, without polyisobutylene, is already 80% healed compared to 40% for the leading hydrocolloid and the Avery polyisobutylene containing product H2440. But remember that even the H2440 achieves its healing with greatly reduced irritation propensity, as we have seen. This trend continues, with both of the Avery AHC’s showing 100% healing after day 5, with the leading hydrocolloid product showing 100% healing on day 6.
Looking at the infiltration of white blood cells into the healing wound, we see a clear advantage for the Avery H2460 product, containing no polyisobutylene. The data therefore suggest that this wound model is able to differentiate between polyisobutylene containing hydrocolloids and hydrocolloids without polyisobutylene, since this is the only difference between the two Avery samples. This is of interest given the above-mentioned studies on the abnormal giant cell formation in the presence of polyisobutylene.
Measurements of scale/crust formation show our new system to have a reduction when compared to the leading hydrocolloid. In fact, the differences between the three test dressings become magnified as the healing period progresses. In particular, H2460 hydrocolloid without polyisobutylene shows very little scale/crust at the end of the healing period. But the H2440 product also comes out ahead of the market leading hydrocolloid. These data indicate that our new hydrocolloid, as well as in chronic wound applications, may also perform particularly well on certain acute wounds.
The data suggest that our approach to hydrocolloid integration may have eliminated many of the deleterious effects caused by leaching of ingredients into healing wounds.
To summarise, we have a new hydrocolloid system, a highly integrated product, which reduces or eliminates leaching of chemicals into the wound. Our system is extremely gentle to intact skin for fixation of dressings, and we have indications of superior rates of wound epithelialisation and reduced crusting compared to other hydrocolloids. The Avery AHC’s provide a technical platform on which to build a wide variety of consumer and professional skin and wound care products. * Copyright? 2002 Avery Dennison Specialty Tape Division, Used with permission