Real-time monitoring of cell adhesion on to a soft substrate by a graphene impedance biosensor

Soft substrates are interesting for a range of applications from mimicking cellular micro-environment to implants. Conductive electrodes on soft substrates open a broad spectrum of possibilities such as electrical and electrochemical sensing coupled with the flexibility, elasticity and transparency of the underlying substrate. Single layer graphene on a soft substrate as a candidate for such flexible electrodes brings the additional advantage that the active area of the sensor is transparent and conformal to the underlying substrate. Here, we overcome several challenges facing the routine realization of graphene cell sensors on a canonical soft substrate namely poly(dimethylsiloxane) (PDMS). Specifically, we have systematically studied the effect of surface energy before, during and after the transfer of graphene. Based on this, we have identified a suitable support polymer, optimal substrate (pre-) treatment and an appropriate solvent for the removal of the support. Using this procedure, we can reproducibly obtain stable and intact graphene sensors in millimeter-scale on PDMS, which can withstand continuous measurements in cell culture media for several days. From local nanomechanical measurements with an AFM, we infer that the softness of the substrate is slightly affected after graphene transfer. However, we can modulate the stiffness using PDMS of differing composition. Finally, we show that graphene sensors on PDMS can be successfully used as electrodes for real-time monitoring of cell adhesion kinetics on a soft substrate. The routine availability of single layer graphene electrodes on a flexible soft substrate with tunable stiffness will open a new avenue for a range of studies, where the PDMS-liquid interface is made conducting with minimal alteration of the intrinsic material properties such as softness, flexibility, elasticity and transparency.