To regenerate large sections of bone, the GBR membrane needs to be malleable enough to easily conform to bone morphology, and have adequate mechanical strength to maintain its form until the new bone has formed. There is a correlation between mechanical strength and thickness—thicker membranes have higher strength [23
]. However, thick membranes are less flexible and formable, creating sharp edges when cutting, trimming, and bending them along the defect site. Thick membranes show less tissue adhesion, which permits penetration of soft tissue from the gap, preventing new bone formation. Thin membranes follow the bone morphology and do not create air pockets, which is advantageous for bone formation. However, with decreased mechanical strength, membranes can collapse into the defect cavity, decreasing the bone formation space and consequently the volume of new bone formed. There are reports that membranes of 100 to 200 μm thickness are suitable for healing large-scale bone defects [24
]. However, the lack of flexibility of membranes of this thickness gives them poor operability. An advantage of titanium membranes is that they maintain their mechanical strength even when thin. A thickness of 20 μm is most suitable for GBR treatment, and accordingly, commercially available and clinically applied titanium membranes are 20 μm thick. Our experiments were conducted with membranes of this thickness as well. As mentioned above in the methods section, titanium membranes were manufactured through the extension of a titanium metal mass by applying pressure through the gap between two rollers. Consequently, the surface topography of non-treated membranes had a roughened structure. In the study, we used acid and alkali treatments for surface modification. Titanium exerts corrosion resistance by forming an oxide layer on the surface; however, it is corroded by non-oxidizing acid solutions such as H2
and HCl [25
]. Van Gestel et al. [26
] reported that corrosion is mainly due to dissolution of the membrane material—especially in acid solutions with a pH < 3. This corrosion is considered to change the surface structure and thickness of the titanium membrane. The surface of the acid treatment showed a regularly rough surface with micro scale pore structures. Although a rough surface was created on the titanium surface, the irregular rough surface and grooves which were observed in the non-treated sample were not detected. This is attributed to the fact that corrosion by strong acid slightly changed and smoothed the topography rather than providing the irregularly roughed non-treatment membrane. The effect of acid treatment was also reflected in membrane thickness. Thickness and mechanical tensile strength of acid treatment were significantly decreased compared with non-treated membrane and alkali membrane. Also, the SEM image appeared different surface topography, according to the results of thickness and tensile strength measurements.
Several studies have reported that acid treatment modifies the wettability of titanium surfaces [12
]. In wettability test, the contact angle of acid treatment was significantly higher than non-treated and alkali treatment. Currently, we have no clear explanation for this discrepancy. However, these past studies were performed using finely polished, smoothly surfaced titanium disks as control samples. In our study, the control membrane had an irregular rough surface caused by the manufacturing process. Thus, acid treatment seems to have altered the titanium membrane from a rough surface to a smooth surface, and the acid membrane showed a high contact angle. These results indicated that acid treatment slightly changed the rough surface and reduced the mechanical strength of the titanium membrane.
Conversely, the surface of alkali-treated membranes created uniformly nanoscale pore structures on the rough surface, and this structure was consistent with that of previous reports [21
]. The alkali-treated membranes displayed enhanced hydrophilicity, which may be attributed to the nanoscale pore structure. Increased hydrophilicity promotes cell adhesion and nutrient supply, and is advantageous for bone regeneration [28
]. Previous studies have compared alkali-and acid-treated implants, and found that implants treated with alkali displayed enhanced mineralization of the implant surface [29
]. These results indicate that alkali treatment produces a hydrophilic topography with a nanoscale pore network. Additionally, titanium exerts corrosion resistance for alkali solution because of an oxygen layer on the surface. For these reasons, the influence of alkali treatment on the strength and thickness of titanium membranes was only slight.