1 Department of Dental Biomaterials Science, Dental Research Institute, School of Dentistry, Seoul National University, 101 Daehak-ro, Jongno-gu, Seoul 03080, Republic of Korea.
2 Department of Oral Health Sciences, Division of Restorative Dentistry, Faculty of Dentistry, The University of British Columbia, 2199 Wesbrook Mall, Vancouver, BC V6T 1Z3, Canada; Advancing Multifunctional Dental Biomaterials Research Excellence Cluster, The University of British Columbia, 2199 Wesbrook Mall, Vancouver, BC V6T 1Z3, Canada; Department of Chemical and Materials Engineering, Gina Cody School of Engineering and Computer Science, Concordia University, 1455 De Maisonneuve Blvd. W., Montreal, QC H3G 1M8, Canada.
3 Department of Oral Health Sciences, Division of Restorative Dentistry, Faculty of Dentistry, The University of British Columbia, 2199 Wesbrook Mall, Vancouver, BC V6T 1Z3, Canada; Advancing Multifunctional Dental Biomaterials Research Excellence Cluster, The University of British Columbia, 2199 Wesbrook Mall, Vancouver, BC V6T 1Z3, Canada. Electronic address: amanso@dentistry.ubc.ca.
4 Department of Dental Biomaterials Science, Dental Research Institute, School of Dentistry, Seoul National University, 101 Daehak-ro, Jongno-gu, Seoul 03080, Republic of Korea. Electronic address: den533@snu.ac.kr.

Objectives: This study evaluated the effects of delayed post-polymerization on color, degree of conversion (DC), flexural strength (FS), and cellular response of a 3D-printed dental interim resin.

Methods: Specimens were divided into six groups based on delays of 0 h, 2 h, 4 h, 6 h, 8 h, and 10 h before post-polymerization. Color was characterized using a spectrophotometer (n = 10), DC (n = 10) with FTIR-ATR, and FS (n = 10) with a universal testing machine. Cellular response (n = 30) was assessed through cell viability and pro-inflammatory gene expression of the gingival fibroblast cell line (L929) and human monocytic cell line (THP-1).

Results: Delayed post-polymerization significantly affected color (L* and b*; both P < 0.001,) and color difference (P = 0.016). L* values at 6 h, 8 h, and 10 h significantly decreased compared to 0 h (P < 0.001), while b* values showed a significant decrease in all groups. (P < 0.001). No significant difference was found in a* (P = 0.417) or translucency (P = 0.394). DC significantly decreased at 8 h and 10 h (P < 0.05). There was no significant difference in FS. Cytotoxicity and pro-inflammatory gene expression (IL-6 and TNF-a) were not significantly affected by time delays.

Significance: Within the limitations of this study, a delay of up to 6 h before post-polymerization did not compromise FS, FM, cell viability, or pro-inflammatory gene expression. However, notable changes in DC and color suggest that the appearance and polymerization characteristics may be adversely affected, while mechanical and biological properties remained stable.