Regulating Fluid Transfer in Wound-Care Applications

Author: 
Bob Michaels
Controlled porosities in wound-care dressings promote proper fluid transport to optimize the healing process
Application-specific wound-care technologies from Adhesives Research contain hundreds of pores in a low-density, permeable structure.

The aging of America is placing myriad demands on the medical device industry, not least in the area of wound-care technologies. And in addition to the needs of an aging population, increasing obesity rates and the related rise of conditions such as diabetes are contributing to the expanding need for advanced wound-care treatments.

To satisfy these increasingly challenging wound-care needs, providers of gels, foams, and adhesives are offering customized, application-specific products for controlling the drainage of exudates from the wound site, the accumulation of which can hamper healing or lead to infection. In addition, manufacturers are meeting the need for products that adhere well to the skin but not to the wound and exhibit hydrophobicity or hydrophilicity depending on the application. To achieve these properties, providers of wound-care products are focusing on developing dressings with tunable porosities for managing fluid transport.

Ebbs and Flows
The key to effective wound care is facilitating the free flow of fluids from the wound site to dressings or bandages. This task is accomplished by altering the porosity and adhesive chemistry. “Porosity is important for fluid management in dressings that are expected to absorb excess exudates from a weeping wound,” remarks Scott Knorr, vice president and general manager of Adhesives Research (Glen Rock, PA).

A supplier of custom adhesives and dissolvable film components to wound-care companies, Adhesives Research offers pressure-sensitive adhesive (PSA) technologies that are formulated from acrylate-, silicone- and rubber-based polymers. Biocompatible, noncytotoxic, and nonirritating, these materials are often coated or laminated on films, specialty fabrics, or foam materials. The company’s products for treating chronic wounds are used in film- and foam-based dressings that are composed of elastic backings, nonwovens, and materials for IV and catheter sites. Porous adhesive technology is combined with other component materials to create breathable, occlusive, and hydrophobic or hydrophilic products that can be used for short- or long-term application to intact or damaged skin.

Porosity can affect fluid transfer in a couple of ways, Knorr explains. “The pores themselves provide channels or pathways for fluids to move directly from the wound through the adhesive to a microporous membrane and ultimately to an absorbent pad. But they also enable the free transport of oxygen to the wound to promote breathability for healing.”

Tailored to meet the specific needs of each application, the company’s porous PSA technologies are composed of hundreds of open pores—or cells—
in a low-density, permeable structure, according to Knorr. These pores range in diameter from approximately 200 to 500 µm and are distributed throughout the matrix, resulting in 30 to 50% porosity and a finished film thickness of 100 to
150 µm. Within the adhesive material, the pores
form isolated channels that enable the flow of aqueous-based fluids or gases from one substrate to the next through the z direction of the adhesive, while acting as a gasket seal in the x-y direction.

“The challenge can be balancing pore size and porosity,” Knorr says. “If the pores are too large, the adhesive may not have enough mass to adequately anchor the dressing to the skin. If they are too small, the free exchange of fluids or gases is restricted.”

In addition to pore size and distribution, the chemical nature of the adhesive controls how fluids are transferred, Knorr says. Thus, the adhesive can be formulated to be either hydrophilic or hydrophobic to control the rate of fluid passage from the wound. In the early stages of healing a severe, highly exudating wound, a hydrophilic adhesive is preferred for removing excess fluids quickly. As healing progresses, a dressing with a hydrophobic adhesive can be used to slow the rate of fluid removal, maintaining a moist healing environment and avoiding skin maceration.

“Porous technology can potentially be used in advanced wound-care applications for delivering oxygen, capillary-dilating nitric oxide, antimicrobial agents, growth factors, or other therapeutic agents,” Knorr states. It is also possible to incorporate oxygen- or nitric oxide–releasing materials into the adhesive for continuous release over time. Porous adhesives, he adds, can be combined with dissolvable films incorporating reagents or antimicrobial agents that are released over time as the film is exposed to fluids.

Breathing Easy
Fluid transfer is defined in terms of moisture vapor transmission rate (MVTR). To optimize fluid transfer in wound-care applications, Bluestar Silicones (East Brunswick, NJ) develops gel-based technologies featuring a variety of MVTRs. Its wound-care products—marketed under the Silbione brand—offer different tack levels for ensuring proper adhesion of the gel to the skin and to the dressing substrate.

Depending on whether they are applied to open wounds or scars, Bluestar Silicones’ gels feature different MVTR values.

The company classifies the porosity, or breathability, of its products according to the MVTR target, which differs between gels used to heal open wounds and those used to protect scar tissue, comments Umar Latif, a senior scientist and technical service specialist for healthcare products at Bluestar. In open wounds, the company’s gels are used as interface dressings between the wound and the absorbent, which consists of a layer on top of the gel or bandage. Because silicone gels are hydrophobic, they do not adhere to moist wounds. At the same time, while they are designed to adhere to surrounding healthy skin, they do not cause trauma when they are removed. “That’s how silicone gels are effective,” Latif says.

In contrast, the moisture associated with scars or healthy skin can result from everyday activity such as sweating or from different skin types, such as dry or oily skin, according to Latif. “There are two different MVTR targets because the moisture content in open wounds differs from that in scars or healthy skin.”

MVTR values depend on a gel’s thickness, or the coat weight at which it is applied, not on its cross-linked density, Latif explains. A thicker coat results in a lower MVTR, while a thinner coat results in a higher MVTR. But the thickness of the coat also affects the tack value of the dressing. “Thus, there is a compromise between the MVTR and the tack values for the final part—whether it’s a bandage or other product,” Latif remarks. “If you increase the thickness, the tack level increases.” An adhesive with a thicker coat will stay on the skin longer, but it will also have a lower MVTR, decreasing the rate of fluid transfer from the wound site. Conversely, accelerated fluid transfer accomplished by means of a thinner layer of gel results in a less-tacky adhesive. “To design the right gel for the right application, you have to find the fine balance between MVTR and adhesion,” Latif stresses.

Promoting Openness
“Porosity in wound-care products affects fluid transfer, MVTR, and breathability because it is a reflection of the openness of a material,” remarks Tiffany Overstreet, global market development manager of Filtrona Porous Technologies Corp. (Colonial Heights, VA; www.filtronaporoustechnologies.com), a supplier of medical-grade hydrophilic foams and bonded fibers. “In the case of foam, it is a reflection of the openness of the cells.” Higher-porosity wound-care foams absorb fluids at a faster rate than lower-porosity foams, she adds.

In wound-care materials, porosity is important because it relates directly to product performance, Overstreet notes. “Porosity determines the amount of fluid the material can absorb and the rate of absorption.” From an application standpoint, it affects how frequently a wound dressing must be changed, the type of wound a specific dressing should cover, and how a wound dressing affects healing. Reducing the frequency of dressing changes can also lower the potential risk of infection.

Suitable for a variety of general and specific applications, from conventional absorbent dressings to specialty foam media for negative-pressure and tunneling wounds, Filtrona’s custom-formulated hydrophilic foam products are designed for absorbency. With or without antimicrobials, pharmaceuticals, and controlled-release agents, they rapidly wick exudates to optimize the healing process. Made from an assortment of polymers, its MediSponge products, which are capable of absorbing up to 15 times their own weight in fluids, are available in cut-to-thickness and poured-to-thickness grades with thicknesses ranging from 0.2 to 0.6 cm.

Porosity and cell structure vary depending on the requirements of each application, according to Overstreet. For example, in negative-pressure wound therapy, in which a vacuum source is used to create subatmospheric pressure in the local wound environment, a higher-porosity material is required to manage fluid transport. In such environments, a porous foam with an open cell structure is desirable. However, in applications in which softness is preferred, a tighter cell structure is required. The chance of tissue migration into the foam dressing is less for lower-porosity materials than for higher-porosity materials. “There is no one-size-fits-all pore,” Overstreet adds. “Pore size and porosity are application specific and vary depending on the wound."