Our previous work has shown that non-thermal plasma treatment of demineralized dentin significantly
(po0.05) improved adhesive/dentin bonding strength for dental composite restoration as compared
with the untreated controls. This study is to achieve mechanistic understanding of the plasma treatment
effects on dentin surface through investigating the plasma treated dentin surfaces and their interaction
with adhesive monomer, 2-hydroxyethyl methacrylate (HEMA). The plasma treated dentin surfaces from
human third molars were evaluated by water contact angle measurements and scanning electron
microscopy (SEM). It was found that plasma-treated dentin surface with subsequent HEMA immersion
(Plasma/HEMA Treated) had much lower water contact angle compared with only plasma-treated (Plasma
Treated) or only HEMA immersed (HEMA Treated) dentin surfaces. With prolong water droplet deposition
time, water droplets spread out completely on the Plasma/HEMA Treated dentin surfaces. SEM images of
Plasma/HEMA Treated dentin surfaces verified that dentin tubules were opened-up and filled with HEMA
monomers. Extracted type I collagen fibrils, which was used as simulation of the exposed dentinal
collagen fibrils after acid etching step, were plasma treated and analyzed with Fourier transform infrared
spectroscopy (FT-IR) and circular dichroism (CD) spectra. FT-IR spectra of the Plasma/HEMA Treated
collage fibrils showed broadened amide I peak at 1660 cm1 and amide II at 1550 cm1
, which indicate
secondary structure changes of the collagen fibrils. CD spectra indicated that 67.4% collagen helix
structures were denatured after plasma treatment. These experimental results demonstrate that nonthermal
argon plasma treatment was very effective in loosing collagen structure and enhancing adhesive
monomer penetration, which are beneficial to thicker hybrid layer and longer resin tag formation, and
consequently enhance adhesive/dentin interface bonding.