Conference PaperPDF Available

Wafer-Scale Graphene-Based Soft Electrode Array with Optogenetic Compatibility

Authors:

Abstract and Figures

This paper reports on the characterization of a microfabricated wafer-scale, graphene-based, soft implant for spinal cord applications. Graphene is used because of its high transparency and good conductivity, making it suitable for optogenetic applications. Moreover it has a high mechanical strength and is potentially biocompatible. The implant consists of multi-layered chemical vapor deposited graphene, in the form of electrodes and tracks, encapsulated between 2 layers of silicone. Methods such as Raman spectroscopy, optical transmittance, and electrical measurements combined with bending tests and in-vitro experiments, using phosphate-buffered saline (PBS) solution, were employed to characterize the device. The results have shown high bendability and no critical damage of the graphene after immersing the device in PBS solution up to 7 days. To the authors' best knowledge, this is the first work that presents a soft and fully scalable optogenetics-compatible graphene-based spinal cord electrode array.
Content may be subject to copyright.
WAFER-SCALE GRAPHENE-BASED SOFT ELECTRODE ARRAY WITH
OPTOGENETIC COMPATIBILITY
Andrada I. Velea,
1,2
, Sten Vollebregt
2
, Gandhika K. Wardhana
1,2
and Vasiliki Giagka
1,3
1
Bioelectronics Section, Department of Microelectronics, TU Delft, Delft, THE NETHERLANDS,
2
Electronic Components, Technology and Materials Section, Department of Microelectronics, TU
Delft, Delft, THE NETHERLANDS and
3
Technologies for Bioelectronics Group, Department of System Integration and Interconnection
Technologies, Fraunhofer IZM, Berlin, GERMANY
ABSTRACT
This paper reports on the characterization of a
microfabricated wafer-scale, graphene-based, soft implant
for spinal cord applications. Graphene is used because of
its high transparency and good conductivity, making it
suitable for optogenetic applications. Moreover it has a
high mechanical strength and is potentially biocompatible.
The implant consists of multi-layered chemical vapor
deposited graphene, in the form of electrodes and tracks,
encapsulated between 2 layers of silicone. Methods such
as Raman spectroscopy, optical transmittance, and
electrical measurements combined with bending tests and
in-vitro experiments, using phosphate-buffered saline
(PBS) solution, were employed to characterize the device.
The results have shown high bendability and no critical
damage of the graphene after immersing the device in
PBS solution up to 7 days. To the authors best
knowledge, this is the first work that presents a soft and
fully scalable optogenetics-compatible graphene-based
spinal cord electrode array.
KEYWORDS
Wafer-scale implant, graphene, silicone, optogenetic
compatibility.
INTRODUCTION
Spinal cord stimulation studies are an important
research topic, mostly for pain relief but also for restoring
locomotion after spinal cord injuries. Apart from clinical
trials, more exploratory research is mostly conducted in
rodents. However, the availability of suitable
neurotechnologies tailored to small animals is limited. For
instance, the spinal cord shows significant mechanical
mismatch with rigid neural implants. Even flexible
implants encapsulated in stiffer polymers such as
polyimide can cause significant damage and compression
to the spinal cord [1].
Therefore, effort has been put into the design and
fabrication of reliable soft spinal cord implants, based on
silicone elastomers.
Existing spinal cord microelectrode arrays (MEAs)
feature opaque electrodes [1, 2] which are used to
electrically activate the tissue or record evoked electrical
activity. Alternatively, tissue activation by means of light,
i.e. optogenetics, is promising to increase the resolution of
activation. This is due to the fact that it activates the tissue
in a more type-specific manner. Only neurons that are
genetically modified to respond to light will be activated,
as long as they are in the vicinity of the light source and
the intensity of it is enough to trigger action potentials [3].
To quantify the effect of this new technology there is
great need to monitor the electrical response of cells at the
site of optogenetic stimulation. To this end, MEAs with
transparent electrodes are a necessary tool. In such a
system, the transparency will guarantee that light, coming
from an external source, can pass through the electrode
site, activating the target location. Moreover, the
conductivity of MEAs will allow for in situ recording of
the evoked electrical response. This temporally and
spatially concurrent recording during stimulation is not
possible with conventional electrodes, as these systems
require both the activating and recorded signals to be in
the electrical domain. Transparent parylene-based
graphene electrode arrays, with metal tracks, have
previously been developed to allow for electrophysiology,
in-vivo imaging and optogenetics in the brain [4].
However, parylene, with a Young’s modulus of ~2 GPa
[5] is still very stiff for the spinal cord.
This work proposes a wafer-scale fabrication process
for a silicone-sandwiched graphene-based implant. The
materials used ensure the required transparency and
conductivity but also provide the necessary softness for
interacting with the spinal cord. The design of the
proposed implants ensures that any metal layers, if
present, only appear at areas which are not under
mechanical strain as they have been proven to have poor
adhesion to graphene, thus causing delamination [6].
MATERIALS AND METHODS
The proposed solution was implemented by
employing a scalable, 2-mask microfabrication process,
inspired by the flex-to-rigid” (F2R) approach [7],
together with a transfer-free CVD process for graphene
growth [8] as shown in Fig.1.
Figure 1: Microfabrication process flow. SiO2 deposition
(I). Graphene growth on the molybdenum (Mo) catalyst
(II). First layer of silicone spin-coated and cured on top
of the structure (III). Release of the complete area using
DRIE for Si and wet etching steps to remove the oxide
and Mo layers (IV, V).
As the developed prototypes are intended to be used
for in-vivo experiments in rats, the MEAs had to be
designed accordingly. A total of 12 electrodes were
distributed as in Fig. 2, each of them being connected to
individual test pads. This number allows for a large
coverage of the spinal cord but also fits into the limited
space available for implantation in rats [9].
For the microfabrication process, 6 µm of plasma-
enhanced chemical vapor deposited (PECVD) oxide were
deposited on both sides of a double-side polished (DSP)
silicon (Si) wafer. Then, on the frontside, 50 nm of
molybdenum (Mo) were sputtered and patterned. This
serves as a catalyst for graphene growth. Next, graphene
was grown using a CVD process [8] to ensure uniformity
of the layer, reproducibility and scalability of the
manufacturing.
Before encapsulating the structures in silicone,
Raman spectroscopy and two-point measurements were
employed to evaluate the graphene layer. Next, 50 µ m of
Sylgard 184 silicone were spin coated on top of the
structures and cured for 1 h at 90 °C. Finally, for the
backside of the wafer, a deep reactive ion etching (DRIE)
process, was employed only over the areas containing the
implants, thus suspending them completely (Fig. 3). This
was followed by wet etching of the oxide and Mo layers.
Before spin-coating the final encapsulation layer on the
backside of the wafer, the structure was again evaluated
by means of Raman spectroscopy and 2-point
measurements to ensure that no critical damage was
present after the microfabrication process. Moreover,
optical transmittance measurements were performed to
approximate the number of graphene layers and the
transparency of the released graphene-on-silicone
structure. In addition, in-vitro experiments were
conducted using PBS saline solution for 24 h and 7 days
to quantify how much graphene degrades over time.
The structures were then cut out from the wafers and
bending tests were performed by placing the implant
around rods with different diameters to emulate handling
and deformation expected at the site of implantation. For
the actual implant, the final layer of silicone encapsulation
can be applied and openings for the electrodes and test
pads can be created before removing the samples from the
Si wafer. The robust process achieves high yield and the
authors are employing a similar microfabrication process
as a basis for the first graphene-based active implants
(details can be found in [10]).
RESULTS AND DISCUSSION
Raman Spectroscopy
After developing the graphene MEAs on the Si wafer,
Raman spectroscopy was employed to determine the
presence of graphene and to qualitatively evaluate it. The
results in Fig. 3 demonstrate that a multi-layer graphene is
present, judging from I
2D
/I
G
< 1 [11]. To monitor
graphene, Raman spectroscopy was employed after
suspending the implants and removing the oxide and Mo
layers (the structures are shown in Fig. 4). Also, after
conducting the in-vitro experiments, in saline solution, to
emulate the biological environment, the graphene layer
had to be investigated to evaluate if the graphene is
damaged by the PBS solution.
The results in Fig. 5, illustrate that there was no
critical damage present on the graphene layer after the
microfabrication process. After immersing the structures
in PBS, the D peak (indicating the number of defects
present) slightly increases. This could be caused by
surface contamination of the graphene layer with different
particles from the solution. The additional peaks seen on
the Raman spectra represent the influence of the silicone
Raman signal over the measurement.
Figure 4: Photograph of the 8 implants fabricated on a
single 10 cm wafer.
Figure 2: Masks used to develop and suspend the
graphene-based passive implants.
Figure 3: Raman spectroscopy evaluation (using a 633
nm laser) after the CVD process. The I2D/IG ratio, indicate
that a multi-layer graphene has been grown [11].
Two-Point Measurements
Apart from Raman spectroscopy, the electrical
properties of the graphene were investigated. To this end,
two-point measurements were employed after graphene
growth (Fig. 6), after suspension of the passive implants
(Fig. 6 and Table 1), as well as after conducting the in-
vitro experiments (Table 1). All measurements were
performed over a graphene line of 70 µ m in width and 1
mm in length.
A significant difference in resistance can be seen
from Fig. 6. This originates from the fact that after
graphene growth, the measured value is the resistance of
both Mo and graphene, whereas for the suspended
implants, only the graphene layer was probed as the Mo
catalyst had been removed. From this it can be concluded
that the resistance of Mo is significantly lower than that of
graphene. Moreover, for the suspended implants, the
probes were landed directly on graphene which was
resting on a 50 µm soft silicone substrate, without any
metal interface between the measured layer and the two-
point probes. Therefore, the accuracy of the measurement
was significantly reduced due to the less controllable
contact resistance. Ideally, the graphene track should be
measured using a four-point measurement setup to reduce
the contact resistance and, on top of that, a metal layer is
needed for landing the probes and not damaging the
graphene layer. Such designs were not included in the
current layout but will be included in future work.
Having the implants suspended, in-vitro experiments
using PBS solution were conducted for periods of 24 h
and 7 days respectively. After each period of time, the
samples were electrically evaluated, and the results
synthesized in Table 1 demonstrate that graphene does not
deteriorate in-vitro. The observed differences likely
originate from the inaccuracy of the two-point
measurements.
Bending Tests
As the implantation site for the described structures is
subjected to different types and degrees of movements,
mechanical evaluation is also needed. To this end,
bending tests using metal rods with diameters ranging
from 8 mm down to 3 mm, followed by resistive
measurements were employed. In Fig. 7, a passive
implant on a 3 mm metal rod is shown while Table 2
synthesizes the electrical measurements performed after
bending the samples on different rods. The measured
resistance is ~2 M, which originates from the large
length-to-width (L/W) ratio of the implant and the high
contact resistance. Since the probing setup was not
tailored for such small prototypes, longer tracks had to be
measured such that the probes could properly be landed
on the sample under test. However, the results in Table 2,
all in the same range, demonstrate that the passive
structures can be bent down to 3 mm without damage.
Figure 5: Raman Spectroscopy results DRIE process and
after immersing the structure in PBS for 24 h and 7 days,
respectively. The grey area represents the silicone Raman
signal.
Table 1. Two-point electrical measurement results
Sample type Voltage
range
Measured
resistance
After DRIE -5 V to 5 V 200 k
After DRIE and
24 h in PBS -5 V to 5 V 160 k
After DRIE and
7 days in PBS -5 V to 5 V 130 k
Figure 7: Implant released from the wafer and placed on
top of a 3 mm bending rod.
Figure 6: Two-point electrical evaluation of graphene
after the CVD process. The results show a resistance
value of ~250 for graphene on Mo, and ~200 k for
graphene on silicone. The contact resistances from the
probes also contribute to the overall resistance.
Optical Transmittance
As these MEAs are intended to be used in
optogenetics, optical transmittance measurements had to
be conducted to evaluate the degree of transparency for
the final implant. From the optical transmittance result in
Fig. 8 it was determined that the implant consists of ~11
graphene layers and has a transmittance of 72-77% [12].
CONCLUSIONS
This paper describes the methods used to evaluate
graphene-based passive implants for spinal cord
applications. Graphene quality (using Raman), electrical
and mechanical characterizations of the final prototypes
were performed. Although there were no dedicated
structures to evaluate the electrical conductivity of the
implants, it has been proven that graphene does not
deteriorate and moreover, still conducts after immersing
the implants in PBS up to 7 days, as well as after bending
them over different rod diameters, down to 3 mm.
This paper demonstrates the potential of soft,
graphene-based passive implants, to enable unique,
currently not available, spinal cord tissue monitoring
during optogenetic activation, which is paramount to map
the neuronal activation at this level.
ACKNOWLEDGMENTS
We acknowledge the staff of Else Kooi Laboratory
(EKL) and the Bioelectronics group from Delft University
of Technology for their support throughout the project.
REFERENCES
[1] I. R. Minev et al., “Electronic dura mater for long-
term multimodal neural interfaces”, Science, vol. 347, no.
6218, pp. 159-163, 2015.
[2] V. Giagka, A.Demosthenous and N. Donaldson,
“Flexible active electrode arrays with ASICs that fit
inside the rat’s spinal canal”, Biomed. Microdev., vol. 17,
no. 6, pp. 106-118, 2015.
[3] K. L. Montgomery et al., “Beyond the brain:
Optogenetic control in the spinal cord and peripheral
nervous system”, Science Transl. Med., vol. 8, no.
337rv5, pp. 1-12, 2016.
[4] D. W. Park et al., “Fabrication and utility of a
transparent graphene neural electrode array for
electrophysiology, in vivo imaging and optogenetics”,
Nat. Protoc., vol. 11, no. 11, pp. 2201-2222, 2016.
[5] K. Scholten and E. Meng, “Materials for
microfabricated implantable devices: a review”, Lab on a
Chip, vol. 15, no. 22, pp, 4256-4272, 2015.
[6] J. Robinson et al., “Contacting graphene”, Appl. Phys.
Lett., vol 98, no. 5, 2011.
[7] B. Mimoun et al., “Flex-to-rigid (F2R): A generic
platform for the fabrication and assembly of flexible
sensors for minimally invasive instruments”, IEEE
Sensors, vol. 13, no. 10, pp. 3873-3882, 2013.
[8] S.Vollebregt et al., “A transfer-free wafer-scale CVD
graphene fabrication process for MEMS/NEMS sensors”,
in Proc. IEEE MEMS 2016, Sanghai, China, 2016, pp. 17-
20.
[9] V. Giagka et al., “Flexible platinum electrode arrays
for epidural spinal cord stimulation in paralyzed rats: An
in vivo and in vitro evaluation”, in Proc. 3
rd
Annual Conf.
IFESSUKI 2012, Birmingham, UK, 2012, pp. 52-53.
[10] A. I. Velea, S. Vollebregt, T. 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
NMDC 2019, Stockholm, Sweden, 2019.
[11] Z. Jian et al., “Irradiation effects of graphene and thin
layer graphite induced by swift heavy ions”, Chinese
Phys. B, vol. 24, no. 8, 2015.
[12] K. F. Mak et al., “Measurement of the optical
conductivity of graphene”, Phys. Rev. Lett, vol. 101, no.
19, pp. 2-5, 2008.
CONTACT
*A. I. Velea; A.I.Velea@student.tudelft.nl;
*S. Vollebregt; S.Vollebregt@tudelft.nl;
*G. K. Wardhana; G.K.Wardhana@tudelft.nl;
*V. Giagka; V.Giagka@tudelft.nl.
Table 2. Bending test results (using two-point
electrical measurements)
Sample type Voltage
range
Measured
resistance
No bending -5 V to 5 V 1.2 M
Bending (8 mm rod) -5 V to 5 V 1.8 M
Bending (5 mm rod) -5 V to 5 V 2.5 M
Bending (3 mm rod) -5 V to 5 V 2.4 M
Note: In case the application requires, this resistance
could be lowered (Fig. 5) by leaving the Mo on some
parts of the graphene tracks. Using Mo instead of
another metal has the advantage that the graphene-Mo
bonds are strong, originating from the CVD process.
Figure 8: Optical transmittance results for the graphene-
based implant. On the X-axis, the light wavelength range
(visible spectrum). On the Y-axis, the percentage of the
light transmitted through the sample.
... We have previously pioneered a transfer-free approach to fabricate multilayer CVD graphene on silicon (Si) substrates [8]. We have utilised this technique to fabricate optically transparent and MRI-compatible graphene neural electrodes on flexible substrates with excellent stimulating properties [1], [3], [9]. Furthermore, we have recently shown that further electrochemical enhancement can be achieved by locally printing Pt nanoparticles using spark ablation, without compromising the optical transparency, which is essential for optogenetic compatibility [3]. ...
... These ratios reflect the quality of graphene and were 0.52 for quartz and 0.3 for sapphire. This criterion was paired with the requirement for minimal noise in the Raman spectra [1], [9], [14], [15]. ...
Conference Paper
Full-text available
This study explores the application of a novel transfer-free method for the synthesis of multilayer Chemical Vapour Deposition (CVD) graphene directly on transparent sub-strates, specifically to create transparent Microelectrode Arrays (MEAs) for optogenetic studies. Traditional methods typically involve a graphene transfer step that can compromise the material's integrity and electrical properties. By eliminating this step, our approach simplifies the fabrication process. The developed MEAs were characterised by Raman spectroscopy, optical transmittance, and electrochemical impedance spectroscopy. We also assessed the stability and recording capabilities of the fabricated MEAs, alongside a comparative assessment with a commercial MEA. Turbostratic graphene grown directly on quartz and sapphire was successfully achieved. Our transfer-free MEAs exhibit promising signal detection capabilities, despite a relatively high baseline noise of ∼ 23 µV , and a significantly large impedance at 1 kHz (3.2 to 9.89 M Ω) surpassing values in other studies. The devices exhibited low stability after exposure to liquid media during the soaking and ageing tests, causing large variations in the electrochemical measurements post-exposure. This was due to the permeability of the encapsulation layer and the biodegradability of the molybdenum structures, which led to significant structural and chemical changes in the devices. While further work is required to prevent the failure mechanisms of the device, this study demonstrates the feasibility of transparent MEA fabrication by means of a transfer-free approach directly on quartz substrates.
... The aim of the current study is to use CVD multilayer graphene to create fully-transparent and MRI-compatible neural electrodes with better electrochemical performance. To prevent the presence of polymer residues caused by the transfer process, but also, to make the process more compatible with conventional wafer-scale fabrication and post-processing technologies, we have adapted the process reported in 37 , which uses a transferfree method to grow graphene on a Molybdenum (Mo) catalyst 38 , to create the neural electrodes. This method enables the fabrication of a multilayer graphene electrode without any transfer involved. ...
Article
Full-text available
Multimodal platforms combining electrical neural recording and stimulation, optogenetics, optical imaging, and magnetic resonance (MRI) imaging are emerging as a promising platform to enhance the depth of characterization in neuroscientific research. Electrically conductive, optically transparent, and MRI-compatible electrodes can optimally combine all modalities. Graphene as a suitable electrode candidate material can be grown via chemical vapor deposition (CVD) processes and sandwiched between transparent biocompatible polymers. However, due to the high graphene growth temperature (≥ 900°C) and the presence of polymers, fabrication is commonly based on a manual transfer process of pre-grown graphene sheets, which causes reliability issues. In this paper, we present CVD-based multilayer graphene electrodes fabricated using a wafer-scale transfer-free process for use in optically transparent and MRI-compatible neural interfaces. Our fabricated electrodes feature very low impedances which are comparable to those of noble metal electrodes of the same size and geometry. They also exhibit the highest charge storage capacity (CSC) reported to date among all previously fabricated CVD graphene electrodes. Our graphene electrodes did not reveal any photo-induced artifact during 10-Hz light pulse illumination. Additionally, we show here, for the first time, that CVD graphene electrodes do not cause any image artifact in a 3T MRI scanner. These results demonstrate that multilayer graphene electrodes are excellent candidates for the next generation of neural interfaces and can substitute the standard conventional metal electrodes. Our fabricated graphene electrodes enable multimodal neural recording, electrical and optogenetic stimulation, while allowing for optical imaging, as well as, artifact-free MRI studies.
... Our proposed method allows for a gradual adjustment of the mechanical properties of the encapsulation, from a relatively rigid (Parylene C) layer to a softer one (PDMS), with properties similar to those of soft tissues, while yielding a fully transparent encapsulation stack over a broad wavelength spectrum. Therefore, the proposed solution can be used for the conformal protection of a variety of flexible devices and even be combined with transparent conductors [17], making the device suitable for optical imaging such as calcium imaging and optogenetic applications. ...
Conference Paper
Full-text available
In this paper, we investigate the long-term adhesion strength and barrier property of our recently proposed encapsulation stack that includes PDMS-Parylene C and PECVD interlayers (SiO2 and SiC) for adhesion improvement. To evaluate the adhesion strength of our proposed stack, the sample preparation consisted in depositing approximately 25 nm of SiC and 25 nm of SiO2 on half wafers, previously coated with Parylene C. Next, 50 µm PDMS was spin-coated on top. Finally, the samples were detached from the Si wafer and soaked in a PBS solution at 67 °C to accelerate the aging process. Two samples were also implanted, subcutaneously, on the left and right subscapular regions of a rat. The optical inspection and peel tests performed after two months confirmed our preliminary findings and showed a significant improvement of the adhesion in our proposed encapsulation stack compared to the case of PDMS on Parylene C alone. In addition, the X-ray photoelectron spectroscopy (XPS) analysis at the interface between SiC and Parylene C showed different peaks for the interface compared to the reference spectra, which could be an indication of a chemical bond. Finally, water vapor transmission rate (WVTR) tests were performed to investigate the barrier property of our proposed encapsulation stack against water vapor transmission. The results demonstrated that the proposed stack acts as a significantly (two orders of magnitude) higher barrier against moisture compared to only Parylene C and PDMS encapsulation layers. The proposed method yields a fully transparent encapsulation stack over a broad wavelength spectrum that can be used for the conformal encapsulation of flexible devices and thus, making them compatible with techniques such as optical imaging and optogenetics.
... How can we ensure autonomy under the above restrictions [3]? Eventually, how can we make our medicine more precise, i.e. increase the specificity at which we interact with the tissue [4,5]? And if we achieve all these, how will the pill of the future look like? ...
Conference Paper
Full-text available
In a world where medicine is becoming more personalised the promise of Bioelectronic Medicine is that tiny implants will deliver energy in the form of electrical impulses, replacing pharmaceuticals, their conventional chemical counterparts. But how can we develop such tiny smart and autonomous implants that (need to) seamlessly interact with the tissue and live in the body for decades [1]? How can we protect all the components in such an implant while still maintaining the small form factor and essential flexibility? How can we design electronics such that they remain better protected in such a harsh environment [2]? How can we ensure autonomy under the above restrictions [3]? Eventually, how can we make our medicine more precise, i.e. increase the specificity at which we interact with the tissue [4, 5]? And if we achieve all these, how will the pill of the future look like? References [1] V. Giagka, and W. Serdijn, "Realizing flexible bioelectronic medicines for accessing the peripheral nerves-technology considerations,"
... How can we ensure autonomy under the above restrictions [5]? Eventually, how can we make our medicine more precise, i.e. increase the specificity at which we interact with the tissue [6,7]? This talk will aim to address these questions and present an overview of how to engineer long-lasting and spatially selective active neural interfaces. ...
Conference Paper
Full-text available
In a world where medicine is becoming more personalised the promise of Bioelectronic Medicine is that tiny implants will deliver energy in the form of electrical impulses, replacing pharmaceuticals, their conventional chemical counterparts. But how can we develop such tiny smart and autonomous implants that (need to) seamlessly interact with the tissue and live in the body for decades [1]? How can we protect all the components in such an implant while still maintaining the small form factor and essential flexibility [2]? How can we design electronics such that they remain better protected in such a harsh environment [3, 4]? How can we ensure autonomy under the above restrictions [5]? Eventually, how can we make our medicine more precise, i.e. increase the specificity at which we interact with the tissue [6, 7]? This talk will aim to address these questions and present an overview of how to engineer long-lasting and spatially selective active neural interfaces.
Conference Paper
Full-text available
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 compatibility 1,2 (Fig.1). A scalable and reproducible microfabrication process has been developed, using graphene 3 , a transparent, flexible and conductive material, to form the electrodes and interconnects of the implant. Small and thin 4 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 measurements. 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.
Preprint
Full-text available
Patients affected by spinal cord injuries (SCIs) are most of the time unable to perform motor activities that are trivial for healthy people. Currently, for therapeutic purposes, epidural spinal cord stimulation (ESCS) is widely used. Apart from this, 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 as well as the characterization of flexible, active spinal cord implants with optogenetic compatibility 1,2. To this end, a scalable and reproducible microfabrication process has been developed. Graphene 3 , which combines transparency, flexibility and conductivity, was used to form the electrodes and interconnects of the microelectrode arrays. Small and thin 4 electronic chips were assembled via flip-chip bonding processes either on graphene or on metal-on-graphene layers while soft, polymeric encapsulation was employed to sustain the high flexibility and transparency of the implant. The end 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 metal test pads for interconnection to the outside world. The prototypes were extensively characterised using Raman spectroscopy and optical transmittance for the graphene layer. Cyclic voltammetry and electrochemical impedance spectroscopy were performed to benchmark the electrical properties of the device while the assembly process of the chips on the graphene or metal-on-graphene structures was evaluated using four-point electrical measurements. Graphene showed a transparency of ~78 for the developed prototypes as well as no deterioration over time when soaked in saline solution for several days or when bent over rods down to 3 mm in diameter. For the graphene electrodes, impedance values of ~8 kΩ at 1 kHz frequencies were measured and the resistance after the bonding process ranged from 10 mΩ up to 16 Ω for individual connections, depending on the substrate used. In this work the first fully transparent, graphene-based active spinal cord implants have been developed. The results obtained from their characterization illustrate that the process is stable and the performance of the devices is promising.
Conference Paper
Full-text available
This work aims to develop a smart neural interface with transparent electrodes to allow for electrical monitoring of the site of interest during optogenetic stimulation of the spinal cord. In this paper, a microfabrication process for the wafer-level development of such a compact, active, transparent and flexible implant is presented. Graphene has been employed to form the transparent array of electrodes and tracks, on top of which chips have been bonded using flip-chip bonding techniques. To provide high flexibility, soft encapsulation, using polydimethylsiloxane (PDMS) has been used. Making use of the "Flex-to-Rigid" (F2R) technique, cm-size graphene-on-PDMS structures have been suspended and characterized using Raman spectroscopy to qualitatively evaluate the graphene layer, together with 2-point measurements to ensure the conductivity of the structure. In parallel, flip-chip bonding processes of chips on graphene structures were employed and the 2-point electrical measurement results have shown resistance values in the range of kΩ for the combined tracks and ball-bonds.
Conference Paper
Full-text available
Our aim is to develop a smart neural interface with transparent electrodes to allow for electrical monitoring of the site of interest during optogenetic stimulation of the spinal cord. In this work, we present the microfabrication process for the wafer-level development of such a compact, active, transparent and flexible implant. The transparent, passive array of electrodes and tracks have been developed using graphene, on top of which chips have been bonded using flip-chip bonding techniques. To provide high flexibility, soft encapsulation, using polydimethylsiloxane (PDMS) has been used. Preliminary measurements after the bonding process have shown resistance values in the range of kΩ for the combined tracks and ball-bonds.
Article
Full-text available
Transparent graphene-based neural electrode arrays provide unique opportunities for simultaneous investigation of electrophysiology, various neural imaging modalities, and optogenetics. Graphene electrodes have previously demonstrated greater broad-wavelength transmittance (â 1/490%) than other transparent materials such as indium tin oxide (â 1/480%) and ultrathin metals (â 1/460%). This protocol describes how to fabricate and implant a graphene-based microelectrocorticography (μECoG) electrode array and subsequently use this alongside electrophysiology, fluorescence microscopy, optical coherence tomography (OCT), and optogenetics. Further applications, such as transparent penetrating electrode arrays, multi-electrode electroretinography, and electromyography, are also viable with this technology. The procedures described herein, from the material characterization methods to the optogenetic experiments, can be completed within 3-4 weeks by an experienced graduate student. These protocols should help to expand the boundaries of neurophysiological experimentation, enabling analytical methods that were previously unachievable using opaque metal-based electrode arrays.
Conference Paper
Full-text available
In this paper we report a novel transfer-free graphene fabrication process, which does not damage the graphene layer. Uniform graphene layers on 4" silicon wafers were deposited by chemical vapor deposition using the CMOS compatible Mo catalyst. Removal of the Mo layer after graphene deposition results in a transfer-free and controlled placement of the graphene on the underlying SiO2. Moreover, pre-patterning the Mo layer allows customizable graphene geometries to be directly obtained, something that has never been achieved before. This process is extremely suitable for the large-scale fabrication of MEMS/NEMS sensors, especially those benefitting from specific properties of graphene, such as gas sensing.
Article
Full-text available
Epidural spinal cord electrical stimulation (ESCS) has been used as a means to facilitate locomotor recovery in spinal cord injured humans. Electrode arrays, instead of conventional pairs of electrodes, are necessary to investigate the effect of ESCS at different sites. These usually require a large number of implanted wires, which could lead to infections. This paper presents the design, fabrication and evaluation of a novel flexible active array for ESCS in rats. Three small (1.7 mm 2) and thin (100 μm) application specific integrated circuits (ASICs) are embedded in the polydimethylsiloxane-based implant. This arrangement limits the number of communication tracks to three, while ensuring maximum testing versatility by providing independent access to all 12 electrodes in any configuration. Laser-patterned platinum-iridium foil forms the im-plant's conductive tracks and electrodes. Double rivet bonds were employed for the dice microassembly. The active electrode array can deliver current pulses (up to 1 mA, 100 pulses per second) and supports interleaved stimulation with independent control of the stimulus parameters for each pulse. The stimulation timing and pulse duration are very versatile. The array was electrically characterized through impedance spec-troscopy and voltage transient recordings. A prototype was tested for long term mechanical reliability when subjected to continuous bending. The results revealed no track or bond failure. To the best of the authors' knowledge, this is the first time that flexible active electrode arrays with embedded electronics suitable for implantation inside the rat's spinal canal have been proposed, developed and tested in vitro.
Article
Full-text available
The application of microfabrication to the development of biomedical implants has produced a new generation of miniaturized technology for assisting treatment and research. Microfabricated implantable devices (μID) are an increasingly important tool, and the development of new μIDs is a rapidly growing field that requires new microtechnologies able to safely and accurately function in vivo. Here, we present a review of μID research that examines the critical role of material choice in design and fabrication. Materials commonly used for μID production are identified and presented along with their relevant physical properties and a survey of the state-of-the-art in μID development. The consequence of material choice as it pertains to microfabrication and biocompatibility is discussed in detail with a particular focus on the divide between hard, rigid materials and soft, pliable polymers.
Article
Full-text available
Graphene and thin graphite films deposited on SiO2/Si are irradiated by swift heavy ions (209Bi, 9.5 MeV/u) with the fluences in a range of 1011 ions/cm2–1012 ions/cm2 at room temperature. Both pristine and irradiated samples are investigated by Raman spectroscopy. For pristine graphite films, the “blue shift” of 2D bond and the “red shift” of G bond with the decrease of thickness are found in the Raman spectra. For both irradiated graphene and thin graphite films, the disorder-induced D peak and D′ peak are detected at the fluence above a threshold Φ th. The thinner the film, the lower the Φ th is. In this work, the graphite films thicker than 60 nm reveal defect free via the absence of a D bond signal under the swift heavy ion irradiation till the fluence of 2.6 × 1012 ions/cm2. For graphite films thinner than 6 nm, the area ratios between D peak and G peak increase sharply with reducing film thickness. It concludes that it is much easier to induce defects in thinner films than in thicker ones by swift heavy ions. The intensities of the D peak and D′ peak increase with increasing ion fluence, which predicts the continuous impacting of swift heavy ions can lead to the increasing of defects in samples. Different defect types are detected in graphite films of different thickness values. The main defect types are discussed via the various intensity ratios between the D peak and D′ peak (HD/H D′).
Article
Full-text available
The mechanical mismatch between soft neural tissues and stiff neural implants hinders the long-term performance of implantable neuroprostheses. Here, we designed and fabricated soft neural implants with the shape and elasticity of dura mater, the protective membrane of the brain and spinal cord. The electronic dura mater, which we call e-dura, embeds interconnects, electrodes, and chemotrodes that sustain millions of mechanical stretch cycles, electrical stimulation pulses, and chemical injections. These integrated modalities enable multiple neuroprosthetic applications. The soft implants extracted cortical states in freely behaving animals for brain-machine interface and delivered electrochemical spinal neuromodulation that restored locomotion after paralyzing spinal cord injury. Copyright © 2015, American Association for the Advancement of Science.
Article
Optogenetics offers promise for dissecting the complex neural circuits of the spinal cord and peripheral nervous system and has therapeutic potential for addressing unmet clinical needs. Much progress has been made to enable optogenetic control in normal and disease states, both in proof-of-concept and mechanistic studies in rodent models. In this Review, we discuss challenges in using optogenetics to study the mammalian spinal cord and peripheral nervous system, synthesize common features that unite the work done thus far, and describe a route forward for the successful application of optogenetics to translational research beyond the brain.