Femtosecond laser surface roughening and pulsed plasma polymerization duplex treatment on medical-grade stainless steel substrates for orthodontic purpose

This study developed a surface modification technique to obtain nearly superamphiphobic films on AISI 304 stainless steel substrates, which are usually made into dental appliances in orthodontics for malocclusion. Direct fabrication of micro- and nanoscale roughened surfaces on stainless steel substrates were treated using femtosecond laser (FL) processing. Subsequently, pulsed plasma polymerization (P3) was adopted using hexamethyldisiloxane (C6H18OSi2) and octafluorocyclobutane (C4F8) as the precursors to deposit siloxane and fluorocarbon stacking films. The surface characteristics of the deposited films were examined, and in vitro cytotoxicity and antimicrobial tests were performed. Experimental results showed that the surfaces treated by FL and P3 processes exhibited nearly superamphiphobicity with water apparent contact angle (CA) 160°, advancing CA 161°, receding CA160°, contact angle hysteresis (CAH) 1°; sunflower oil apparent contact angle 146°, advancing CA 141°, receding CA137°, CAH 4°; and hexadecane oil apparent contact angle 124°, advancing CA 118°, receding CA113°, contact angle hysteresis CAH 5°, respectively. The nearly superamphiphobicity are attributed to the synergistic effects of pre-roughening by the FL process and post-flourinating with siloxane and fluorocarbon coating by the P3 process, which were consistent with the results of scanning electron microscopy, energy-dispersive X-ray spectroscopy and Fourier transform infrared analyses. In addition, the steel wool abrasion test exhibited the film's surface wear resistance, and in vitro tests revealed the film's non-cytotoxicity and effective inhibition of bacterial growth. Thus, the nearly superamphiphobic coating may be beneficial for applications in orthodontics.