Protecting wounds with liquid coatings.
Although coatings are not typically considered when wound dressing materials are discussed, they have many attributes that could make them suitable for this application. Traditional coatings serve to protect the surfaces on which they are applied. In fact, liquid applied barrier wound dressings have the advantage of flowing over the wound and covering the entire surface regardless of variations. Furthermore, if they have adequate flexibility, they can move with the skin as it is extended and relaxed.
Jan Gooch, currently a professor at Georgia Institute of Technology, recognized the potential for use of liquid coatings as barrier dressings. While working as a National Research Council Associate with the United States Army Institute of Surgical Research from 2001 to 2004, he developed a liquid applied barrier coating that the Army has used in the field during campaigns in Iraq and Afghanistan.
Prior to that time, Gooch was involved in research on miniemulsions for coating applications. Unlike the conventional emulsion polymerization process that is used to manufacture most latex paints (where ingredients are continuously fed into the reactor), miniemulsions are prepared by reacting all of the monomers at once.
"Traditional emulsion polymerization is somewhat limited because only monomers that can be fed into a micelle can be used," he notes. That is not the case, however, with miniemulsions, which opens up more possible resin chemistries.
Miniemulsion polymerization is a batch type reaction where a water insoluble hydrophobe material containing mixed or dissolved reactants and a polymerization initiator is activated at a higher than ambient temperature. The resultant mixture polymerizes completely over a short period with a high exotherm. Emulsion polymerization consists of surfactant micelles and initiatiors that suspend continuously polymerized controlled feed monomers until the reaction is terminated.
Miniemulsion particles are usually smaller than emulsion polymerized particles (specifically those found in latexes). Differences in properties range widely because of the polymerization kinetics and hydrophobe content. A miniemulsion particle can contain a greater range of materials including antimicrobial agents that an emulsion polymerized particle cannot. The film characteristics depend on the nature (tensile strength, surface energy, etc.) of the main chain polymer that is designed for specific purposes. Both types, however, do form latex films.
During his research on miniemulsions, Gooch developed a coating that seemed to meet many of the requirements for a wound barrier dressing. Others were interested too, and he thus began to develop the coating system with support from the United States Army Institute of Surgical Research.
"The formulation was carefully designed to meet several key criteria for an effective barrier dressing," explains Gooch. "Firstly, all of the ingredients that comprise the coating are biocompatible materials for topical applications. Secondly, the coating can be applied over a wet surface. Thirdly, it is applied rapidly (via pump spray or brush), dries quickly (within five minutes), and adheres to the wound for up to 48 hours."
Once the liquid coating is spray applied on the wound, resin particles in the miniemulsion coalesce and form a relatively continuous film (there are nanometer sized voids) as the water evaporates. Most notably, Gooch designed the coating to adhere to the skin through hydrogen bonding of the resin particles with the amino acids in the peptides and proteins present in the skin cells.
Due to the nature of this bonding and the composition of the polymer, the dressing is very flexible, hydrophilic, and slightly permeable to oxygen and water vapor. It also protects the wound from exposure to bacterial agents and is water and abrasion resistant. Also as a result of the type of adhesion, the liquid applied combat bandage will be sloughed off after 24-48 hours as cells are renewed and the wetness of the wound increases. Alternatively, the dressing can be removed with pressure sensitive tape using a special rolling machine designed by Gooch.
The miniemulsion on which the liquid coating formulation produces a polymeric acrylic gel is prepared from randomly sequenced monomers including vinyl acetate (67.7% w/w), 2-ethyl hexylacrylate (19.6% w/w), and dioctyl maleate (15.7%). "These particular monomers were chosen for the characteristics they contribute to the overall polymer," Gooch remarks.
For instance, polydioctyl maleate (PDM) and poly(2-ethyl hexylacrylate) "Gooch designed the coating to adhere to the skin through hydrogen bonding of the resin particles with the amino acids in the peptides and proteins present in the skin cells." (PEHA) are both flexible and exhibit moderate hydrogen bonding capability. PDM is also internally plasticizing, while PEHA is hydrophilic. Polyvinyl acetate (PVAC), which is the largest component in the resin, is semi-permeable to water and oxygen, contributes moderate strength, and is capable of forming strong hydrogen bonds.
In addition to the three monomers, the miniemulsion formulation for the liquid applied wound barrier dressing includes polyvinyl alcohol as a viscosity aid and wetting agent, alkylaryl poly-ether alcohol and nonlylphenoxpoly (ethyleneoxy) alcohol as surfactants and emulsifiers, t-butyl hydroperoxide as the initator, benzoate esters as hydrophobes and plasticizers, and very minor amounts of different reagents that act as pH adjusters, defoamers, and reducing agents.
Polyvinyl acetate is particularly important for coating performance. It not only exhibits strong hydrogen bonding and provides permeability for oxygen and water vapor, it also absorbs up to 10% water, which plasticizes the film structure and helps "wet" the moist wound surface, according to Gooch.
Physical property analysis of the liquid applied coating confirmed the performance attributes of the combat bandage. It was shown to be impermeable to nutrients necessary for supporting the growth of microorganisms and semi-impenetrable to microorganisms yet still exhibit a standard water vapor transmission rate (63.3 g/m2/hr). The modulus (measure of flexibility) was determined to be 0.2-0.4, while adhesion (according to ASTM D 1000) was found to be 2.1 lb/in. Furthermore, the coating is shelf stable for up to 12 months if stored between -40[degrees] and 50[degrees]C.
The coating, in fact, met many challenging requirements of the Army and has been evaluated for protection of combat wounds since 2004. Gooch has also developed a version of the coating system that is based on ethanol. "Water in the original emulsion can freeze, and thus use of the original liquid coating system is limited to use when temperatures are too low. The ethanol-based formulation addresses this issue."
In addition, ethanol evaporates much more quickly than water, so the coating forms even more rapidly. "It is not difficult to imagine that in a combat situation even a few minutes or seconds can mean the difference in saving a life," Gooch states.
He concludes: "It is extremely rewarding to be able to take my experience with miniemulsions and apply it to the development of a product that has such an important use. The potential applications of coating technology extend well beyond what is traditionally considered, and this liquid applied barrier wound dressing is just one example. There are many more possible applications that are just waiting to be discovered."
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|Title Annotation:||COATINGS XPERIENCE|
|Date:||Aug 1, 2011|
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