Is polydopamine beneficial for cells on the modified surface?

Since the pioneering work of Messersmith's group discovering that polydopamine (PDA) can serve to adhere to many types of materials, the PDA coating has, as a biomimetic approach, been widely used to enhance cell adhesion by surface modification to bind biologically active substances to a bioinert substrate. Nevertheless, it is unclear whether or not the PDA itself is beneficial for cells. Herein, we report that a PDA coating decreases viability of cells under normal culture and observation conditions. Such an inhibition effect was not caused by the free PDA or any inherent cytotoxicity of this chemical substance but a contact-dependent phenomenon. Human bone marrow mesenchymal stem cells were employed as the default cell type and tissue culture plates were used as the default substrate, although some other cell types and substrates were also examined to confirm the universality of such an 'abnormal' phenomenon of a superstar molecule. The viability of cells on the PDA coating exhibited time dependence, and the decreased cell viability during the normal observation time was found to come from the decrease of cell number instead of the decrease of average viability per cell. The PDA coating led to less cell global migration yet more local motility of cells. Based on the concept of 'background adhesion' of cells on a surface without significant motifs of specific cell adhesion, we supposed that cells adhered to the PDA coating better, which influenced mobility and eventually proliferation. Hence, the cell behaviors on the PDA coating are reasonable, albeit a bit complicated.