Plasma - the fourth state of matter - is composed of radicals, ions, and electrons. Low temperature plasma is widely researched to modify surfaces. It can be used to change the surface chemistry of a material by depositing a thin layer or activating the surface to make it more reactive. Silk fibroin (Bombyx mori) is an excellent candidate for plasma modification. Silk has potential for biomedical use as it is non-reactive, non-immunogenic, and biodegradable. However, silk films are relatively hydrophobic without modification, which can be problematic in vivo due to lack of interaction with biological species. Low-temperature nitrogen plasma is a potential strategy to make silk more hydrophilic. However, nitrogen plasma-modified polymers have been shown to exhibit hydrophobic recovery. In this process, modified polymer chains rearrange to a lower energy state and the surface reverts back to being hydrophobic. Temperature could potentially affect hydrophobic recovery as it is likely to impact polymer chain mobility.

In this study, the goal was to determine to what extent hydrophobic recovery occurred when silk films were subjected to high temperatures (60^○C) after nitrogen plasma treatment. Two variables were explored: 1) the effect of plasma treatment on surface wettability and, 2) the effect of temperature on hydrophobic recovery. Nitrogen treatments were conducted using optimized parameters: t = 30 s, p = 100 mTorr, P = 10 W. Nitrogen-treated and untreated control films were aged under both ambient and elevated temperatures. Samples were aged for up to six weeks, and sample wettability was analyzed at days 0, 7, 21, and 42. Wettability data (water contact angles, WCAs) were collected using a goniometer to determine how hydrophobic a surface is relative to another. Larger WCAs correspond to more hydrophobic surfaces, whereas lower WCAs correspond to more hydrophilic surfaces. WCAs of nitrogen-treated silk aged under ambient temperature remained constant, suggesting hydrophobic recovery did not occur. These results were contrasted with films aged under elevated temperatures.

Alexzandria Kubik

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