Signaling Skin pH with Indicator Dyes

In wound care, dressings protect the skin while it heals. To effectively protect against infection, however, the dressing must be strongly affixed to the skin, which can result in an injurious and painful removal for the patient. This experience, generalized by the expression “ripping off the bandage,” could become a thing of the past, though, as Gerhard Mohr, PhD, head of the sensor materials group at The Fraunhofer Research Institution for Modular Solid State Technologies (EMFT), has developed dyes for wound dressings that change color when they detect infection. Interestingly, initial research for this wound-healing application originated in the water treatment field.

Beginnings in pH Detection

According to Mohr, research to develop his indicator dyes began in aquaculture.

“[My team] had a project to simultaneously detect ammonia and pH in water,” he said, explaining their device continuously tested water for ammonia and pH. “Fish in these environments make their water basic [when they expel ammonia], so the pH changes from neutral to alkaline, which is not good for the fish; if the pH gets too high [i.e. basic], the fish die.”

For this device, Mohr’s team used indicator dyes that change color with the change in pH. Like litmus paper, the dyes showed a marked change; however, it was important to create indicator dyes that were sensitive across a range of pH values.

“The problem with indicator dyes is that they are highly sensitive [within] a limited region, pH 6– pH 9. You can measure precisely in this range but not outside of it.”

Mohr and his team therefore designed dyes to be more precise than a pH electrode by mixing them.

“By combining [dyes], we cover the whole pH range with high sensitivity,” he said.

Although Mohr could not indicate which dyes his team uses, he noted they have a similar structure to food dyes and are currently being investigated for safety. After developing the dyes, the team considered other applications for the technology, at which point they began investigating wound dressings.

Indicating Infection

In addition to sensitivity across a broad pH range, for wound dressings, the dyes also needed to exhibit perceptible color changes and remain stable when immobilized in polymer material. “The chromophore, or color-carrying structure, must absorb in the visible spectral range,” Mohr explained.

Also, similar to their use in water treatment, the dyes needed to change color at a specific pH range. According to the researchers, a pH of 6.5–8.5 indicates infection; if the skin beneath the dressing is healthy, its pH typically is below 5.

Mohr’s team developed the dyes in several steps to introduce various functionalities into both the dyes and the dressing. They also had to ensure the dyes were immobilized in the material in which they were bound.

“The dye must have specific chemical functions that allow it to bind to the polymer and the dressing material. It must not leach out,” Mohr explained.

The process of binding the polymer to the dye differs based on the polymer and dye used, which varies with application. This immobilization process, while proprietary, is a large part of Mohr’s current research.

“With synthesis, functions were introduced into the dye ... and dressing material to achieve stable linkage between the materials,” he said.

Mohr further stated that the indicator dyes are not designed for use in dressings for bleeding wounds because blood can stain the dyes. Rather, the dyes are designed for chronic wounds.

Future Research

The next step for Mohr and his team is to conduct clinical studies as well as additional fundamental research on the correlation between pH and bacterial growth.

“Dermatologists need to know more about the correlation between pH and bacterial growth because testing for bacteria in a wound requires dipping a pH electrode into the wound. This is [obviously] not pleasant for the patient, so there are not many studies on the pH of wounds,” he noted.

In addition to better understanding the processes that take place in wounds and healing, the team plans to integrate optical sensor modules into dressings to measure the pH value and indicate results on a reader unit. Further, they also are considering the use of indicator dyes to measure parameters other than pH.

Mohr concluded, “We have experience in developing indicators for different materials. We develop the dye based on the application.”

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