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Journal ArticleDOI

A polymer-based spiky microelectrode array for electrocorticography

01 Mar 2015-Microsystem Technologies-micro-and Nanosystems-information Storage and Processing Systems (Springer Berlin Heidelberg)-Vol. 21, Iss: 3, pp 619-624
TL;DR: In this article, a novel microelectromechanical array (MEA) with a polyimide (PI)-platinum (Pt)-SU-8 layer structure was presented for obtaining neural signals.
Abstract: The advanced technology of microelectromechanical systems (MEMS) makes possible precise and reproducible construction of various microelectrode arrays (MEAs) with patterns of high spatial density. Polymer-based MEMS devices are gaining increasing attention in the field of electrophysiology, since they can be used to form flexible, yet reliable electrical interfaces with the central and the peripheral nervous system. In this paper we present a novel MEA, designed for obtaining neural signals, with a polyimide (PI)--platinum (Pt)--SU-8 layer structure. Electrodes with special, arrow-like shapes were formed in a single row, enabling slight penetration into the tissue. The applied process flow allowed reproducible batch fabrication of the devices with high yield. In vitro characterization of the electrode arrays was performed with electrochemical impedance spectroscopy in lactated Ringer's solution. Functional tests were carried out by performing acute recordings on rat neocortex. The devices have proven to be convenient tools for acute in vivo electrocorticography.

Summary (1 min read)

Introduction

  • The advanced technology of microelectromechanical systems (MEMS) makes possible the batch fabrication of miniature custom-made devices with high precision, such as microfabricated electrode arrays, which are commonly utilized for both in vitro (Rousseau et al. 2009) and in vivo (Cheung 2007) electrophysiology.
  • Electrode array structure and fabrication Following this, the Pt layer is sputtered onto the front side as well, and the sacrificial Al layer is etched away, yielding the patterned conductive layer (Fig. 1b).

In vitro characterization

  • In vitro characterization of the electrodes was performed by electrochemical impedance spectroscopy (EIS), in physiological saline (0.9% w/v of NaCl), using an Ag/AgCl reference electrode (Radelkis Ltd., Hungary) and a counter electrode of platinum wire with relatively high surface area.
  • The probe signal was sinusoidal, with an RMS value of 25 mV.
  • A Reference 600 instrument (Gamry Instruments, PA, USA) was used as a potentiostat and Gamry Framework 6.02 and Echem Analyst 6.02 software were used for experimental control, data collection and analysis.
  • Experiments were performed in a Faraday cage.
  • Fig. 3 shows the obtained average impedance spectrum of a MEA, following the characteristics of platinum microelectrodes.

In vivo recordings

  • All procedures were in accordance with the European Council Directive of 24 November 1986 (86/609/EEC), the Hungarian Animal Act, 1998 and the Animal Care Regulations of the Research Centre for Natural Sciences of the Hungarian Academy of Sciences.
  • The same solution was used for maintaining anesthesia throughout the experiment, via intramuscular injections (0.1 ml / 30 min).
  • This procedure was performed at several locations in the craniotomy window.
  • Fig. 5 shows waveforms of a representative in vivo recording above the somatosensory cortex.
  • Using the thus obtained data, the RMS noise of the electrodes was estimated, which was 8.71 µV on the average.

Conclusions

  • Pt conductive and SU-8 top insulator layers and found that this structure of microfabricated layers allows a rapid and reliable technological process flow.
  • Acute in vivo functional tests on rat cortex were performed.
  • The 24 electrodes cover a 1.15 mm long linear section of the tissue, thus they make possible obtaining detailed electrophysiological recordings of a small area, containing neighboring cortical columns.
  • The polymer-based device is not so stiff and fragile as silicon-based neural probes, however, due to its relatively wide shaft, it is not suitable for implantation into deep areas.
  • On the other hand, this feature makes their device more invasive compared to them.

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A polymer-based spiky microelectrode array for
electrocorticography
Gergely Márton, Marcell Kiss, Gábor Orbán, Anita Pongrácz, István Ulbert
Abstract
The advanced technology of microelectromechanical systems (MEMS) makes possible precise and
reproducible construction of various microelectrode arrays (MEAs) with patterns of high spatial
density. Polymer-based MEMS devices are gaining increasing attention in the field of
electrophysiology, since they can be used to form flexible, yet reliable electrical interfaces with the
central and the peripheral nervous system. In this paper we present a novel MEA, designed for
obtaining neural signals, with a polyimide (PI) platinum (Pt) SU-8 layer structure. Electrodes with
special, arrow-like shapes were formed in a single row, enabling slight penetration into the tissue.
The applied process flow allowed reproducible batch fabrication of the devices with high yield. In
vitro characterization of the electrode arrays was performed with electrochemical impedance
spectroscopy (EIS) in lactated Ringer’s solution. Functional tests were carried out by performing
acute recordings on rat neocortex. The devices have proven to be convenient tools for acute in vivo
electrocorticography.
Introduction
The advanced technology of microelectromechanical systems (MEMS) makes possible the batch
fabrication of miniature custom-made devices with high precision, such as microfabricated electrode
arrays, which are commonly utilized for both in vitro (Rousseau et al. 2009) and in vivo (Cheung
2007) electrophysiology. Substrate materials include glass (Kibler et al. 2012) (Lin et al. 2009), silicon
(Kawano et al. 2010; Marton et al. 2013; Pongrácz et al. 2013) and titanium (McCarthy et al. 2011).
Furthermore, polymers have gained increasing attention in this field, since their production and
fabrication costs are relatively low, and their flexibility allows smoother coupling with the soft tissue
than rigid substrates (Hassler et al. 2011). Sufficient biocompatibility of the applied materials is also
necessary. Several types of polymers, such as SU-8 photoresist (Nemani et al.), Polyimide (PI)
(Seymour et al. 2011) and Parylene C (Chang et al. 2007; Rui et al. 2011) meet this criterion.
Polymer-based device components for neural interfacing show great heterogeneity in structure and
function. Intracranially implantable extracellular probes have been constructed with the use of SU-8
(Altuna et al. 2012), polyimide (PI) (Rousche et al. 2001; Patrick et al. 2006; Cheung et al. 2007) and
parylene (Seymour et al. 2011). Flexible cables for data transmission from chronically implanted
extracellular probes were developed (Cheng et al. 2013). In order to obtain electrical connection with
peripheral nerves, polymer-based sieve (Stieglitz et al. 1997), cuff (Rodriguez et al. 2000) and
transverse (Boretius et al. 2010) probes are available. Microfabricated polymer structures are
commonly applied for electrocorticography (ECoG) as well (Yeager et al. 2008; Myllymaa et al. 2009;
Rubehn et al. 2009; Ochoa et al. 2013), which is a widely used method for the localization of
epileptogenic zones (Keene et al. 2000). In this paper, a novel polymer-based microelectrode array is
presented, suitable for performing in vivo neuroscientific experiments, e.g. electrocorticography on
rats.
Electrode array structure and fabrication
The microfabricated component of the device is manufactured on an oxidized Si substrate wafer. It
consists of a bottom PI layer, a middle metal (TiO
x
+ Pt) layer and a top SU-8 layer. We have chosen

this polymer composition in order to exploit the preferential features of the materials: PI sufficiently
adheres to the substrate, yet it can be peeled off easily without the necessity of a sacrificial layer,
while SU-8 can be conveniently patterned by photolithography, which makes it more suitable for
forming the top layer. Two photolithographic masks with resolution of 1 µm define the layout, which
contains polymer components with lengths of 33.1 mm and widths of mm 8.4 mm and 24-channel
MEAs on their tip. The electrodes are arranged linearly in a single, 1.15 mm long row. Arrow-shaped
recording sites were designed on the edges of microscopic spikes. The sites can be approximated
with isosceles triangles with base lengths of 22 um and heights of 20 µm. The geometric surface area
of an electrode is approximately 220 µm
2
. This “spiky” geometry was chosen for two main reasons:
firstly, in order to allow the electrodes to slightly penetrate into the cortex during in vivo
experiments, and secondly to have them located apart from the bulk of the device.
Fig. 1 shows a 3D schematic of the fabrication process. First, a 4-inch double-side polished, (100)
oriented Si wafer is cleaned and a 1 µm thick thermal silicon-dioxide layer is grown on its sides. Then
the wafer is spin-coated by a 7 µm thick P84 polyimide layer (step a). The conductive layer, which
consists of 30 and 270 nm thick TiO
x
and Pt layers, respectively, is deposited on the PI layer and
patterned by lift-off technique. For this, a 500 nm thick sacrificial Al layer is deposited before the
TiOx/Pt layers by electron beam evaporation and patterned by lithography using a 1.8 µm-thick
Microposit 1818 photoresist layer and wet chemical etching. The TiO
x
is then deposited by reactive
sputtering, the purpose of this thin layer is to provide sufficient adhesion. Following this, the Pt layer
is sputtered onto the front side as well, and the sacrificial Al layer is etched away, yielding the
patterned conductive layer (Fig. 1b). Its different regions will be functioning as electrodes, bonding
pads and wiring between them. The process flow is carried on by spin-coating a 20 µm thick SU-8
upper insulator layer onto the front side, which is patterned with photolithography (step c).
Openings made on the SU-8 layer in this step define the electrodes, bonding pads and the contour
lines of the device. The contour line pattern is transferred onto the PI layer by reactive ion etching
(RIE) (step d). In this step, the SU-8 is thinned on the top as well, while Pt functions as a masking layer
at the electrodes and bonding pads. Hence, the bottom PI layer will be preserved below these
structures, providing sufficient mechanical stability for them. Finally, the whole wafer is submerged
into distilled water and the thus microfabricated device component can be peeled off from the
oxidized Si substrate (step e).

Fig. 1 Schematics of the fabrication process steps. Two photolithographic
masks are utilized, the first in step b for pattering the metal layer and the
second in step c, for patterning the top SU-8 layer.
The electrode array is electrically and mechanically connected to a custom-made, 1 mm thick, 10 mm
wide, 127 mm long printed circuit board (PCB), provided by Auter Ltd. (Budapest, Hungary). Metal
pins are soldered to through-hole vias of the PCB and to the bonding pads at the base of the
microfabricated component. The PCB is equipped with a Preci-Dip electrical connector for interfacing
with the preamplifier. The thus completed device had a width of 10 mm and a length of 134 mm.
Images of the MEA, prior to bonding to the PCB are shown in Fig. 2a and Fig. 2b. The microfabrication
process flow was carried out with high yield. Failures, such as imperfections in the pattern of the
conductive layer, or tearing the foils while peeling them off of the substrate occurred with a
prevalence of less than 10%.
Fig. 2 a The microfabricated component contains with electrodes,
corresponding bonding pads and lead wires. The bottom PI and top SU-8
isolating polymer layers are almost totally transparent. b Image of the spiky
electrode array at the tip. The center-to center distance of the sensors is 50
µm. c The microfabricated component mounted on a PCB. The MEMS
component is turned over, so the electrode surfaces are facing downwards.
In vitro characterization
In vitro characterization of the electrodes was performed by electrochemical impedance
spectroscopy (EIS), in physiological saline (0.9% w/v of NaCl), using an Ag/AgCl reference electrode
(Radelkis Ltd., Hungary) and a counter electrode of platinum wire with relatively high surface area.

The probe signal was sinusoidal, with an RMS value of 25 mV. A Reference 600 instrument (Gamry
Instruments, PA, USA) was used as a potentiostat and Gamry Framework 6.02 and Echem Analyst
6.02 software were used for experimental control, data collection and analysis. Experiments were
performed in a Faraday cage. Fig. 3 shows the obtained average impedance spectrum of a MEA,
following the characteristics of platinum microelectrodes.
Fig. 3 Bode plot of average electrochemical impedance spectrum of the microelectrodes on a
microelectrode array.
In vivo recordings
In vivo test of the device, functioning as an ECoG has been performed on rat neocortex. All
procedures were in accordance with the European Council Directive of 24 November 1986
(86/609/EEC), the Hungarian Animal Act, 1998 and the Animal Care Regulations of the Research
Centre for Natural Sciences of the Hungarian Academy of Sciences.
A 260 g Wistar rat has been anesthetized with 0.5 ml solution of 37.5 mg/ml ketamine and 5mg/ml
xylazine, injected intraperitoneally. The same solution was used for maintaining anesthesia
throughout the experiment, via intramuscular injections (0.1 ml / 30 min). After the initial anesthesia,
the rat was fastened in a frame, designed for stereotactic operations (David Kopf). Body temperature
was maintained at 37 °C. Craniotomy was performed at [-1.0 mm -4.0 mm] anterior-posterior (AP),
[+1.0 mm +5.0 mm] medial-lateral (ML) in reference to the bregma. The MEA was mounted on a
micromanipulator. The microdevice was adjusted into a transverse plane, its longitudinal axis was
perpendicular to the surface of the neocortex. The spiky tips, containing the electrodes were
submerged into the tissue and brain signals were recorded. This procedure was performed at several
locations in the craniotomy window. Figure 4 demonstrates the placement of the MEA during a
recording.

Fig. 4 a Coronal section of the rat brain -2.76 mm posterior to the bregma. Illustration is based on
Figure 56 of (Paxinos & Watson 2009). b Zoomed view of a somatosensory area and a proportional
representation of the MEA on the cortex. c Microscopic image the of vivo use of the device.
The signals were recorded using an amplifier and a data acquisition system with 23 sensor channels
and a 24
th
channel, allocated for providing trigger signals. The electrical activity was recorded with
20 kHz sampling rate, 16 bit resolution and a gain of 1000 (Ulbert et al. 2001). The thus obtained
waveforms were visualized and analyzed off-line with NeuroScan 4.5 software (Compumedics, El
Paso, TX).
Fig. 5 shows waveforms of a representative in vivo recording above the somatosensory cortex. An
oscillation of approximately 1.6 Hz is visible on all of the sensor channels. Such slow oscillation is the
result of ketamine-xylazine anesthesia (Steriade et al. 1993; Fontanini et al. 2003). Comparing them
to each other, a divergence is observable in the waveforms, showing the spatial variation of the LFP
in the cortex along the 1.2 mm length of the array. The experiment was performed as a proof of
concept for the MEA, further investigation on the physiological causes of the waveforms exceeds the
scope of this study. We have found that the microdevice tolerated these measurements without
noticeable deterioration.

Citations
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Journal ArticleDOI
TL;DR: A broad review of the progress of polymers within MEMS is presented in this paper, including deposition, removal, and release techniques for three widely used MEMS polymer materials, namely SU-8, polyimide, and Parylene C. The application of these techniques to create devices having flexible substrates and novel polymer structural elements for biomedical MEMS (bioMEMS).
Abstract: The development of polymer micromachining technologies that complement traditional silicon approaches has enabled the broadening of microelectromechanical systems (MEMS) applications. Polymeric materials feature a diverse set of properties not present in traditional microfabrication materials. The investigation and development of these materials have opened the door to alternative and potentially more cost effective manufacturing options to produce highly flexible structures and substrates with tailorable bulk and surface properties. As a broad review of the progress of polymers within MEMS, major and recent developments in polymer micromachining are presented here, including deposition, removal, and release techniques for three widely used MEMS polymer materials, namely SU-8, polyimide, and Parylene C. The application of these techniques to create devices having flexible substrates and novel polymer structural elements for biomedical MEMS (bioMEMS) is also reviewed.

112 citations


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  • ...One technique to improve metal adhesion is through the use of adhesive layers such as SiOx for Al [95] or Ti for Au [64] and Pt [270]....

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TL;DR: The in vivo performance of a recently developed electrophysiological recording system comprising a two-dimensional, multi-shank, high-density silicon probe with integrated complementary metal-oxide semiconductor electronics is demonstrated.
Abstract: Recording simultaneous activity of a large number of neurons in distributed neuronal networks is crucial to understand higher order brain functions. We demonstrate the in vivo performance of a recently developed electrophysiological recording system comprising a two-dimensional, multi-shank, high-density silicon probe with integrated complementary metal-oxide semiconductor electronics. The system implements the concept of electronic depth control (EDC), which enables the electronic selection of a limited number of recording sites on each of the probe shafts. This innovative feature of the system permits simultaneous recording of local field potentials (LFP) and single- and multiple-unit activity (SUA and MUA, respectively) from multiple brain sites with high quality and without the actual physical movement of the probe. To evaluate the in vivo recording capabilities of the EDC probe, we recorded LFP, MUA, and SUA in acute experiments from cortical and thalamic brain areas of anesthetized rats and mice. The advantages of large-scale recording with the EDC probe are illustrated by investigating the spatiotemporal dynamics of pharmacologically induced thalamocortical slow-wave activity in rats and by the two-dimensional tonotopic mapping of the auditory thalamus. In mice, spatial distribution of thalamic responses to optogenetic stimulation of the neocortex was examined. Utilizing the benefits of the EDC system may result in a higher yield of useful data from a single experiment compared with traditional passive multielectrode arrays, and thus in the reduction of animals needed for a research study.

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  • ...…et al. 1988; Du et al. 2009, 2011; Grand et al. 2011; Karmos et al. 1982; Khodagholy et al. 2015; Kipke et al. 2008; Kubie 1984; Lopez et al. 2014; Márton et al. 2015; McNaughton et al. 1983; Michon et al. 2016; Okeefe and Recce 1993; Ruther et al. 2010; Ruther and Paul 2015; Scholvin et al.…...

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18 Dec 2015-PLOS ONE
TL;DR: The microfabrication of a novel, all-flexible, polymer-based MEA, consisting of a three dimensional sensor configuration with an implantable depth electrode array and brain surface electrodes, allowing the recording of electrocorticographic signals with laminar ones, simultaneously is presented.
Abstract: Utilization of polymers as insulator and bulk materials of microelectrode arrays (MEAs) makes the realization of flexible, biocompatible sensors possible, which are suitable for various neurophysiological experiments such as in vivo detection of local field potential changes on the surface of the neocortex or unit activities within the brain tissue. In this paper the microfabrication of a novel, all-flexible, polymer-based MEA is presented. The device consists of a three dimensional sensor configuration with an implantable depth electrode array and brain surface electrodes, allowing the recording of electrocorticographic (ECoG) signals with laminar ones, simultaneously. In vivo recordings were performed in anesthetized rat brain to test the functionality of the device under both acute and chronic conditions. The ECoG electrodes recorded slow-wave thalamocortical oscillations, while the implanted component provided high quality depth recordings. The implants remained viable for detecting action potentials of individual neurons for at least 15 weeks.

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  • ...The rapid and cost-effective procedure had already been successfully employed for the construction of a linear array of electrodes, used as an ECoG [36]....

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TL;DR: The immunohistochemical study quantified the neuronal preservation and the severity of astrogliosis around SU-8 devices implanted in the neocortex of rats, after a 2 months survival, suggesting that neuronal survival is affected only in a very small area around the implant.

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TL;DR: A detailed review of the state-of-the-art ECoG electrodes and their relevant issues, such as electrode configuration and varying material choices, including the idea of neural signal classification and chronic implantation issues, are discussed in this paper.
Abstract: Implantable electrodes for neural signal recording have great potentials to provide various diagnostic options and curing methods in diverse neuroscience and biomedical fields. Electrocorticography (ECoG) electrodes are suitable for recording large-scale neural signals and overcoming rigid electrodes' limitations. There are excellent signs of progress achieved in micro/nanotechnologies and material science to solve many challenges, such as reducing the micromotion, alleviating the invasion, optimizing the shape and size, reducing the infection of the interface location, improving the biocompatibility characteristics, and integrating the sophisticated electronics which help to record neural signals for various applications. Reducing the issues of neural implantation are major research problems in neural engineering. ECoG electrode allows bi-directional communication between the human brain and external electronics. A detailed review of the state-of-the-art ECoG electrodes and their relevant issues, such as electrode configuration and varying material choices, including the idea of neural signal classification and chronic implantation issues, are discussed.

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"A polymer-based spiky microelectrod..." refers background in this paper

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TL;DR: In vivo and in vitro device characterization of the biological, electrical and mechanical properties of thin-film, polyimide-based, multichannel intracortical Bio-MEMS arrays suggest that these arrays could be a candidate device for long-term neural implants.
Abstract: The promise of advanced neuroprosthetic systems to significantly improve the quality of life for a segment of the deaf, blind, or paralyzed population hinges on the development of an efficacious, and safe, multichannel neural interface for the central nervous system. The candidate implantable device that is to provide such an interface must exceed a host of exacting design parameters. The authors present a thin-film, polyimide-based, multichannel intracortical Bio-MEMS interface manufactured with standard planar photo-lithographic CMOS-compatible techniques on 4-in silicon wafers. The use of polyimide provides a mechanically flexible substrate which can be manipulated into unique three-dimensional designs. Polyimide also provides an ideal surface for the selective attachment of various important bioactive species onto the device in order to encourage favorable long-term reactions at the tissue-electrode interface. Structures have an integrated polyimide cable providing efficient contact points for a high-density connector. This report details in vivo and in vitro device characterization of the biological, electrical and mechanical properties of these arrays. Results suggest that these arrays could be a candidate device for long-term neural implants.

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Frequently Asked Questions (1)
Q1. What contributions have the authors mentioned in the paper "A polymer-based spiky microelectrode array for electrocorticography" ?

In this paper the authors present a novel MEA, designed for obtaining neural signals, with a polyimide ( PI ) – platinum ( Pt ) – SU-8 layer structure.