Stainless steel (SS) 316l constitutes a popular biomaterial with various applications as implants in cardiovascular and orthopedic surgery, as well as in dentistry. Nevertheless, its cytocompatibility against neuronal cells has not been investigated, a feature that is important for the construction of implants that require contact with neurons, e.g., neuronal electrodes. In addition, most cytocompatibility studies have focused on decorated or surface-modified SS 316l. On the other hand, SH-SY5Y cells are an established cellular model for cytocompatibility studies of potential biomaterials given their ability to differentiate into neuron-like cells. Here, we used retinoic-acid-differentiated SH-SY5Y cells and SH-SY5Y controls to investigate the cytocompatibility and biomimetics of uncoated SS 316l. The assessment of cytocompatibility was based on the determination of differentiation markers by immunofluorescence, RT-qPCR, and the neurite growth of these cells attached on SS 316l and standard tissue culture polystyrene (TCP) surfaces. Even though the neurite length was shorter in differentiated SH-SY5Y cells grown on SS 316l, no other significant changes were found. In conclusion, our results suggest that the uncoated SS 316l mimics a natural bio-surface and allows the adhesion, growth, and differentiation of SH-SY5Y cells. Therefore, this alloy can be directly applied in the emerging field of biomimetics, especially for the development of implants or devices that contact neurons.

The NerveRepack project is a European initiative that aims to develop biomimetic exoskeletons/exoprostheses for amputated or paralyzed leg patients that will receive and transmit signals to enable movements and sensations for the patient. To implement the project, it is fundamental to develop implantable neuronal electrodes that will allow bidirectional signaling between the sensors placed on the exoskeletons/exoprostheses and the nervous system. In this direction, two electrodes, plug and cuff, have been designed as integral parts of the final implantable device. The electrodes should comply with strict regulations to ensure their safe implantation in patients. The purpose of this study was to support the compliance of the implant platforms of certain key components with the ISO and ASTM standards that would be required for clinical applications. We have used an indirect method to assess the biocompatibility of the developed electrodes against neuronal cells, fibroblasts, and keratinocytes. Also, we assessed hemocompatibility, i.e., the potential of implantable electrodes to induce hemolysis or complement activation. Finally, the mutagenic/genotoxic potential was tested against the internationally recommended CHO cells. Both representative plug and cuff electrode components were found non-cytotoxic, non-mutagenic, and unable to induce hemolysis. Therefore, from the point of early evaluation of in vitro material and process biocompatibility, the selected implant platforms for the electrodes could be implanted in preclinical models to delineate their potential in vivo applications as neuronal interface with the biomimetic exoskeleton/exoprostheses.