## 4 Discussion
However, we are aware of the importance to test different energy densities in our experimental system to assess and verify the occurrence of a biphasic response with a peak dose response depicted by PBM that may account, likely together with the differences in the experimental settings and fibroblastic cell types used, for the discrepancy of our results with those reported in the literature [75; 76; 77]. This is a fundamental requisite to define the optimal PBM parameters able to elicit the best fibroblast responses with therapeutic effects and to appropriately translate this information into clinical protocols. A limitation of this study mainly correlates to the in vitro experimentation on cell monolayers. If on the one side, this basic model represents a very relevant tool for studying the impact of light on the cells in a simple and reproducible way, on the other side it eliminates may relevant factors including: i) the complex architecture of a tissue affecting light propagation; ii) many mechanisms possibly regulating in vivo paracrine/juxtacrine cell-cell interactions and cell functionality; iii) the microenvironmental mechanical forces such as ECM stiffness, known to affect cell behavior by instructive signals [78; 79; 80]. Hence, the translation of the findings obtained in vitro to in vivo situations must be, as always, considered with caution. However, this study may provide experimental groundwork to push clinicians towards the use of multiple combined photo-treatments for addressing wound healing [12; 14; 81]. It is tempting to speculate that red PBM treatment in combination with violet-blue or NIR PBM, if properly conducted respecting the sequence of events through which wound healing proceeds [82], could really elicit positive biological effects and provide better results than those achieved by the single treatments for the promotion and improvement of the healing process, thus holding a promising therapeutic potential. In particular red PBM may favor the proliferative stage of wound healing stimulating the essential cellular elements such as fibroblasts [32] from the wound margin to proliferate and migrate into the wound starting to form ECM (initially containing immature type-3 collagen), while modulating the first inflammatory response as reported [83; 84; 85]. On the other hand, PBM mediated by violet-blue or even more NIR light may promote the late matrix remodeling (maturation) stage of wound healing, likely stimulating the differentiation of fibroblasts towards myofibroblasts and the production and deposition of mature type-1 collagen by (myo)fibroblasts, thus leading to the formation of a transient contractile scar, which reduces the wound size and allows its final closure, in an attempt to rapidly restore tissue integrity. Experiments are ongoing in our lab to further investigate the proliferative, migratory, and secretive responses of fibrobl cells to different PBM treatments in wound models to corroborate our hypothesis. Finally, it is also worth saying that, from a clinical perspective, multi-photonic supportive treatment to wound healing appears to be a feasible therapeutic option. Being encompassed within the so-called low-energy phototherapies, it can be effectively performed by means of LED emitters, whose major advantages are the possibility to assemble different light sources at appropriate wavelengths in a single instrument, ease of use and low costs. On the other hand, photonic therapies usually require repeated applications until achievement of the therapeutic goals. To this end, the development of portable multi-LED instruments may be desirable. Such devices would allow for the delivery of efficacious photonic wound treatment protocols at the patients' home under medical supervision. Conceptualization, F.C., A.T., M.G., and C.S. ; formal analysis, F.C., A.T., and C.S. ; investigation, F.C., A.T., M.G., and C.S. ; resources, F.C., M.G., S.Z.-O., and C.S. ; data curation, F.C., A.T., and C.S. ; writing--original draft preparation, C.S.