Halochromic dyes for pH-sensitive textiles production

Some synthetic and natural dyes, named halochromic dyes, can change color in response to certain external stimuli, like pH variations of the surrounding environment. It is reported that dermatological diseases are correlated with the alteration of some physiological parameters, first of all the pH of the skin. In particular, a shift towards alkalinity in typical skin pH values has been observed, the normal range of which is approximately 4.2-5.6. The pH value of the skin plays also an important role in the wound healing process. In fact, small pH variations could compromise the correct functioning of the matrix metalloproteinases, which are responsible for morphogenesis, wound healing, tissue repair, and remodeling in response to injury. Furthermore, a wound pH increase helps bacterial growth and affects the antibiotic response. Therefore, the control of the skin physiological pH becomes strategic for both preventive and curative purposes. In the past, halochromic dyes were rarely used in the textile field because of their color instability. However, textiles dyed with halochromic dyes take on pH-sensitive properties becoming useful materials for application in both local and large-area sensor systems. In this work, a natural pH-sensitive dye, Curcumin, and a synthetic pH-sensitive dye, Bromothymol blue, were successfully applied on cotton/polyamide 6 medical dressings. The performances of the dyed medical dressing were evaluated regarding washing fastness, dye fastness to perspiration, halochromic properties, hemolytic activity, cytotoxicity and permeation of the dye through the skin. The dyed samples evidenced pH sensitivity in a pH range from 5,5 to 8.5, which is the range of interest to monitor skin diseases. The biocompatibility tests showed that the samples have no cytotoxicity on human keratinocytes and that there is no permeation through the skin barrier. Curcumin and Bromothymol blue were found in traces only in the most superficial skin layers. On the basis of their properties, textiles dyed with halochromic dyes can be considered pHsensitive materials usable as potential pH biomedical sensors in the control of skin diseases.