13th International Symposium on Flexible Organic Electronics (ISFOE20)
6-9 July 2020, Thessaloniki, Greece
Soft, flexible and transparent graphene-based active spinal cord implants for
A. I. Velea1,2,3, S. Vollebregt2, V. Giagka1,3
1Bioelectronics Section and 2Electronic Components, Technology and Materials section,
Department of Microelectronics, Faculty of Electrical Engineering, Mathematics and Computer
Science, Delft University of Technology
Mekelweg 4, 2628 CD, Delft, The Netherlands
3Technologies for Bioelectronics Group, Department of System Integration and Interconnection
Technologies, Fraunhofer Institute for Reliability and Microintegration IZM
Gustav-Meyer-Allee 25, 13355, Berlin, Germany
e-mail: email@example.com; firstname.lastname@example.org
Patients affected by spinal cord injuries (SCI) are usually unable to perform trivial motor activities and thus,
for therapeutic purposes, epidural spinal cord stimulation (ESCS) is currently used. Moreover, more exploratory
research, using optogenetics, is being conducted in rodents for a better understanding of the mechanisms that
occur while delivering specific therapies. However, the availability of tailored neurotechnologies for such
experiments is limited.
This work reports the development and characterization of flexible, active spinal cord implants with
optogenetic compatibility1,2 (Fig.1). A scalable and reproducible microfabrication process has been developed,
using graphene3, a transparent, flexible and conductive material, to form the electrodes and interconnects of
the implant. Small and thin4 electronic chips were assembled via flip-chip bonding processes either on
graphene or on metal-on-graphene layers. Soft, polymeric encapsulation was employed to sustain the high
flexibility and transparency of the implant. The result is an active prototype consisting of a multi-layered
graphene structure between two polymeric-based encapsulation layers, with thin chips integrated on the
implant and test pads for interconnection to the outside world.
Raman spectroscopy and optical transmittance were employed for the characterization of the graphene
layer while cyclic voltammetry and electrochemical impedance spectroscopy were performed to benchmark the
electrical properties of the device. The assembly process of the chips was evaluated using four-point electrical
In this work, the first transparent, graphene-based active implants have been developed (Fig. 2 and Fig. 3).
The prototypes were extensively characterized and the results showed a transparency of ~80 % as well as no
deterioration over time when soaked in saline solution or when bent under various angles. The graphene
electrodes showed an impedance of ~8 kΩ at 1 kHz frequencies and the resistance after the bonding process
ranged from 10 mΩ up to 16 Ω for individual connections, depending on the substrate used.
Fig. 1. Envisioned structure of the proposed system
Fig. 2. Active prototypes developed
on a silicon wafer
Fig. 3. Soft, flexible,
1. A. I. Velea, S. Vollebregt, G. K. Wardhana, and V. Giagka, “Wafer-scale graphene-based soft implant with optogenetic
compatibility,” in Proc. IEEE MEMS 2020, Vancouver, Canada, Jan. 2020.
2. A. I. Velea, S. Vollebregt, Tim Hosman, A. Pak, and V. Giagka, “Towards a Microfabricated Flexible Graphene-Based Active
Implant for Tissue Monitoring During Optogenetic Spinal Cord Stimulation,” in Proc. IEEE Nanotech. Mater. Dev. Conf.
(NMDC) 2019, Stockholm, Sweden, Oct. 2019.
3. S. Vollebregt et al., “A transfer-free wafer-scale CVD graphene fabrication process for MEMS/NEMS sensors”, in Proc. IEEE
MEMS 2016, pp.17 – 20, Shanghai, China, Jan. 2016.
4. V. Giagka, N. Saeidi, A. Demosthenous, and N. Donaldson, “Controlled silicon IC thinning on individual die level for active
implant integration using a purely mechanical process,” in Proc. ECTC 2014, Orlando, FL, USA, May 2014, pp. 2213 – 2219.