P2 - Direct Colour Printing on Zirconia by UV-C Photons

Abstract

<p><strong>Purpose / Aim:</strong> This study aims to investigate the direct photon colour printing ability and its optical, physical, mechanical and biological effects on zirconia (3Y-TZP) using 222 nm UV-C.</p><p><strong>Materials & Methods:</strong> 222 nm UV-C (Irradiance: 1.870 mW/cm²) was employed to photofunctionalise white-coloured 3Y-TZP (Upcera ST, China. Diameter: 6.00 mm for biological tests and 15.50 mm for other tests; Thickness: 1.20 mm) for different durations ranging from 15 minutes to 24 hours. The change and stability of <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/materials-science/optical-property" title="Learn more about optical properties from ScienceDirect's AI-generated Topic Pages">optical properties</a> (colour difference (ΔE*) and translucency parameter (TP)) were evaluated using a <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/engineering/colorimeter" title="Learn more about colourimeter from ScienceDirect's AI-generated Topic Pages">colourimeter</a>. The physical properties were investigated: the crystalline structure was determined using <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/materials-science/x-ray-diffraction" title="Learn more about XRD from ScienceDirect's AI-generated Topic Pages">XRD</a>, <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/engineering/surface-morphology" title="Learn more about surface morphology from ScienceDirect's AI-generated Topic Pages">surface morphology</a> was observed by SEM, and S_a was tested using <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/chemistry/atomic-force-microscopy" title="Learn more about AFM from ScienceDirect's AI-generated Topic Pages">AFM</a>. Biaxial <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/physics-and-astronomy/flexural-strength" title="Learn more about flexural strength from ScienceDirect's AI-generated Topic Pages">flexural strength</a> (BFS) was investigated according to ISO6872: 2008 standard. <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/engineering/water-contact-angle" title="Learn more about Water contact angle from ScienceDirect's AI-generated Topic Pages">Water contact angle</a> (WCA) and surface <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/chemical-engineering/gibbs-free-energy" title="Learn more about free energy from ScienceDirect's AI-generated Topic Pages">free energy</a> (SFE) were measured using static sessile drop and OWRK methods, respectively. Lastly, the biological properties, including <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/agricultural-and-biological-sciences/cell-adhesion" title="Learn more about cell adhesion from ScienceDirect's AI-generated Topic Pages">cell adhesion</a>, proliferation and morphology were evaluated by CCK8 and CLSM, and antibacterial ability was tested by CFU and CLSM. SPSS 28.0 was used for statistical analysis (α=0.05).</p><p><strong>Results:</strong> 222 nm UV-C irradiation could shade the white 3Y-TZP into tooth colors. The longer the irradiation duration, the higher the ΔE*, such that the 24-h group achieved the highest ΔE* (9.43 ± 0.23), and the lowest was obtained for 15-min group (7.65 ± 0.10). With the increase of ΔE*, the TP values decreased (Fig 1), and 24-h treatment induced a more obvious reduction than 15-min group. Despite the induced optical changes being prone to recover, inversely proportional relationships were revealed with ΔE*, TP and the testing time points, i.e. the altered color would retain its stability after a certain period, and the final level was logarithmically related to the irradiation <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/earth-and-planetary-sciences/fluence" title="Learn more about fluence from ScienceDirect's AI-generated Topic Pages">fluence</a>. Differently, TP returned to the baseline level after a certain time, with the fading rates inversely related to the irradiation <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/physics-and-astronomy/fluence" title="Learn more about fluence from ScienceDirect's AI-generated Topic Pages">fluence</a>. ΔE* and TP values of different groups with fitted equations derived for ΔE* and TP in relation to testing points were illustrated in Figure 1. Prolonged UV-C treatment (≥ 1 hour) resulted in increased <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/engineering/water-contact-angle" title="Learn more about WCA from ScienceDirect's AI-generated Topic Pages">WCA</a> and <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/chemical-engineering/gibbs-free-energy" title="Learn more about SFE from ScienceDirect's AI-generated Topic Pages">SFE</a>, and those of 4-h and 24-h were higher than 1-h group both immediately after irradiation and 14 days later. Whereas a partial recovery of WCA and SFE was also noticed after 14 days. Other properties, including the crystalline structures, <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/engineering/surface-morphology" title="Learn more about surface morphology from ScienceDirect's AI-generated Topic Pages">surface morphology</a> and roughness, <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/physics-and-astronomy/flexural-strength" title="Learn more about BFS from ScienceDirect's AI-generated Topic Pages">BFS</a>, and even biological properties were not altered by the UV-C treatment.</p><p><strong>Conclusions:</strong> The current study illustrated for the first time that direct color printing using 222 nm UV-C photon on 3Y-TZP is feasible due to the alteration of <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/chemistry/color-center" title="Learn more about color centre from ScienceDirect's AI-generated Topic Pages">color centre</a> and band energy (E_g) of zirconia without compromising physical, mechanical and biological properties. The UV-C-induced optical changes were stabilised after a certain time and were highly predictable.</p>