Smart materials, that react in a controllable and reversible way to external stimuli by varying a specific physical or chemical quantity, show great potential for the development of advanced biomedical strategies, including biosensing, tissue regeneration and repair, immuno- and cancer therapy.1 Among the different types of smart materials, piezoelectric ones, that convert mechanical solicitations into electrical potential variations and vice versa, represent suitable candidates to induce specific cell behaviors through electric cues, mainly by improving biomimicry of the cell microenvironment.2
Although the presence of piezoelectric properties in different tissues is known, their specific influence on cells is still unknown. Mesenchymal stem cells (MSCs) have been found to show increased formation of focal adhesions when cultured on negatively charged substrate.3 Triggering the piezoelectric effect, MSCs show increased differentiation on positively and negatively charged surfaces compared to those cultured on neutral substrates, exposing the importance of combined electrical and mechanical stimulation for bone-regeneration.4,5
One of the main obstacles for translating these materials into clinical practice is the lack of proper understanding of the mechanisms controlling the cellular response to them as well as to the fact that biomaterial-cell interactions are often mediated though proteins, fact that has not been properly addressed in the case of electroactive biomaterials. In fact, the behavior of cells is strongly influenced by conformation of extracellular-matrix proteins,6 and the electrostatic forces of the surfaces to which these proteins adhere, determine their conformation. Thus, we hypothesize that the impact of electrical stimulation on cells can be also tuned by the effect of the physicochemical properties of the biomaterials on the proteins that cover them. However, the influence of electrical stimulation on the material-protein interface remains largely undescribed.
In the present work, we explore the effect of the electric cues on the deposition of extracellular proteins on piezoelectric poly(vinylidene fluoride), PVDF, surfaces with distinct net surface charge, and how these differences affects MSC fate. Using microscopic, spectroscopic, and biochemical techniques, we have uncovered large differences in the deposition dynamics, surface coverage and supramolecular organization of collagen and fibronectin as a function of the electrical charge of the surface to which they adhere. Specifically, positively and negatively charged PVDF surfaces promote proteins adsorption, showing higher amount of protein immobilized on the surface compared to neutral PVDF. Regarding cell response, our semi-automatic analysis of fluorescently-stained MSCs, revealed significant differences not only in MSC spreading and nuclear area, but also in the focal adhesion density.
The presented results allow to regulate the structural features of the deposited extracellular proteins through the control of the surface charge, in order to guide cellular behavior and to obtain specific responses.
1.Langer, R. et al., Nature, 428, 487-492 (2004).
2. Kapat, K. et al., Adv. Funct. Mater., 30, 1909045-1909067 (2020).
3. Cavalcanti-Adam, E. A. et al., Biophys. J. 92, 2964–2974 (2007).
4. Sobreiro-Almeida, R. et al., Int. J. Mol. Sci. 18, 2391-2408 (2017).
5. Ribeiro, C. et al., J. Biomed. Mater. Res. A. 103, 2172–2175 (2015).
6. Trappmann, B. et al., Nat. Mat., 11, 642-649 (2012).