# Potential Effects of Photobiomodulation Therapy on Human Dental Pulp Stem Cells
## 4 Discussion  
There have been recent reports on the accumulating evidence for the use of photo-biomodulation (PBM) therapy in stimulating several biological responses in dental pulp stem cells to regenerate dentin [8; 18; 19; 20]. However, these reviews pointed to a major lack of consistency in the critical parameters defining the PBM dosing that has generated uncertainties and inconsistencies in clinical outcomes. Due to a lack of information regarding a well-defined set of parameters concerning laser PBM's effect on hDPSCs, the current study examined the key parameters such as the power output, time, irradiance (power density), and fluence (energy density). This study aimed to bridge the gap in knowledge by investigating the effects of PBM treatments on hDPSC proliferation, cell death, and cell cycle to outline optimal parameters for clinical regenerative protocols. We generated and characterized primary cell culture studies from human-extracted teeth and performed studies in varying serum conditions from normal (10% serum) versus stress-induced (0 to 5% serum) to mimic clinical scenarios of cells following pulp damage. The present study revealed that the 810 nm wavelength at 0.4 W for 10 s was more effective in stimulating hDPSC cell growth when compared to other treatment settings. These results were consistent in 5% and 10% FBS after 24, 48, and 72 h. Optimizing parameters in this preliminary study required various settings to standardize variables as indicated by [PERSON] and [PERSON]'s analysis and as stated by \"Arndt-Schultz Law\" [11; 12; 13]. For instance, although T6 and T9 yielded the same dose (0.3125 J/cm\({}^{2}\)), the biological response of the cells behaved differently; after 48h, DPSCs cultured in 5% FBS in T6 (0.3 \(\times\) 10 s) showed a significant increase in the proliferation rate of cells when cells compared to the control untreated group, while after 72 h, cells in the same group T6 showed a significant decrease in the proliferation rate compared to the control untreated group, and cells in group T9 (1 \(\times\) 3 s) showed a significant increase in the proliferation rate of the treated cells. These current findings, which demonstrate the non-reciprocity effect of laser photobiomodulation on cells, confirm previous reports that demonstrated the non-linear effect of laser photobiomodulation; even if the energy density is the same (inverse  
Figure 7: Statistical analysis of cell cycle phases (G2M, S, and G0G1) of DPSCs treated with different parameters of PBM under different serum conditions (**A**) 1% FBS, (**B**) 5% FBS, and (**C**) serum-free medium (SFM) for 48 h. A two-way ANOVA test was used for statistical analysis. (* \(p<0.05\)). linear relationship between irradiance and time), the changes in biological endpoint will not necessarily be equal [21]. Regarding wavelength, our findings are in line with previous studies, where an infrared 810 nm wavelength laser demonstrated the capability for cell biostimulation and proliferation on hDPSCs [2; 17; 18; 19]. Infrared light was chosen because its penetration through tissue is maximal in this wavelength range due to lower scattering and absorption by tissue chromophores [21]. The effectiveness of photobiomodulation on the target tissues is dependent on a combination of parameters such as wavelength, laser power, power density (irradiance), energy density (fluence), and exposure time [22]. The current study aimed to standardize the different variables to use them as a starting point for further studies. Laser power of 0.4 W for 10 s resulted in significant cell proliferation in comparison to other treated and control groups for cells cultured in 10% FBS. As demonstrated in Table 1, the power density was 0.5 W/cm\({}^{2}\) and the energy density was 0.416 J/cm\({}^{2}\).