Flexible polydimethylsiloxane-based in vivo neural interface with carbon-polymer composite electrodes
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1
Italian Institute of Technology (IIT), Neuroscience and Brain Technologies, United States
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2
Italian Institute of Technology (IIT), Nanochemistry, Italy
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3
Italian Institute of Technology (IIT), Neuroscience and Brain Technologies, Italy
1 Motivation
Most implantable microelectrode arrays suffer from a mechanical mismatch with the host tissue. This discrepancy can be largely overcome by resorting to stretchable and flexible substrates and electrode materials with a Young's modulus that is similar to that of cells. We report on prototype microchannel scaffolds with carbon-PEDOT:PSS-based composite electrodes, conductors and contact pads based on polydimethylsiloxane (PDMS). In a proof-of-concept mouse-model study, we validated their long-term implant stability and functional integrity over a period of 5 months. Both the electrical impedance spectra and the epicortical recordings of local field potentials confirm a functionally reliable probe-tissue interface.
2 Methods
2.1 Design
As described in a previous report, bi-level PDMS MEA microchannel designs were sketched with microelectromechanical systems (MEMS) CAD design software (Expert, Silvaco), photolithographically transferred through 4000 dpi mask transparencies (Repro srl) into high-aspect ratio SU-8 50 (MicroChem) photoresist and molded into PDMS using soft lithography [1]. As depicted in Fig. 1A, the polymer MEA (polyMEA) design featured 16 recording sites in two rectangular electrode matrix fields and two large counter electrodes at the two opposite ends of the probe. Electrode diameters ranged from 40 µm to 80 µm in 20 µm increments. Contact pads at the lower edge matched the pin arrangement of an Omnetics dual row nano strip connector (A79006-001).
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Fig. 1. Overall probe geometries and connectivity scheme of a particular in vivo polyMEA design. A) CAD pattern and resulting polyMEA; connector pad width: 414 µm. B-D) polyMEA squeeze-clamped between an Omnetics double-row connector (A79006-001) at 0.757 mm pitch (B front-, C back- and D side-view). E) Cross section views of the polyMEA electrodes (left), buried tracks (center) and contact pads (right).
2.2 Materials
Microchannel scaffolds were replica-molded in PDMS (Sylgard 184, Dow Corning) for its dielectric stability and comparatively low Young's modulus (< 1 MPa), thereby rendering the polyMEAs highly flexible and conformable. Medical-grade PDMS forms from a Pt-catalyzed two-component elastomer resin, which allows for the addition of fillers to alter its final properties. To render PDMS electrically conductive, a mixture of carbon, graphite and PEDOT: PSS (Clevios Pjet 700, 2.5 % DMSO and 2.5% sorbitol) were added in a 1:1:3 ratio to the uncured PDMS until the electrical resistance dropped below 10 kOhm per 10 mm.
2.3 Fabrication
The paste-like conductive polymer was filled into the PDMS scaffold cavities and cured. Thereafter, the backside of the probe was insulated with a thin layer of PDMS. The resulting polyMEA was folded along the shaft edge and inserted with its pads between the Omnetics connector pins. A folded laser transparency aided in pressing the pads against the pins to enhance contact. In vitro polyMEAs made from the same materials served for biocompatibility validation studies.
2.4 Characterization
The polyMEAs were characterized by impedance spectroscopy (Perstat 2273, Princeton Applied Research) and scanning electron microscopy (SEM) (JEOL JSM 6490LV) before implantation onto and after explanation from the dura mater of a mouse. Network morphology was characterized by immunostaining (DAPI, P-36931, Life Technologies; GFAP, G3893, Sigma; beta- III tubulin, T2200, Sigma).
3 Results
Biocompatibility was confirmed in vitro with cortical neurons (rat, E18) on flexible polyMEAs at 40 and 14 DIV (Fig. 2 and 3).
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Fig. 2. Absence of cytotoxicity as validated in vitro for a rat cortical network (40 DIV).
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Fig. 3. Immunostaining of cortical rat neurons (E18). Network architecture of neuronal cultures on composite-PDMS substrates at 14 DIV. Nuclei were stained with DAPI, glial cells with GFAP and neurons with beta-III tubulin.
An exemplary local field potential (LFP) recording from the dura mater of an anesthetized mouse 82 days after surgery is shown in Fig. 4.
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Fig. 4. Exemplary epicortical recording from a polyMEA implanted on the dura mater of a mouse at 82 days after surgery. The 50 Hz noise was removed by a band-stop resonator. The probe was grounded through the stereotactic frame. Stimulation on electrode 46 (see artifacts); ± 400 mV, 100 us.
Fig. 5 shows a slight increase in the electrode impedance after explantation.
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Fig. 5. polyMEA impedance characteristics before implantation and after explantation about 5 months later.
Fig. 6 illustrates the SEM image of the polyMEA recording sites at different magnifications before implantation and after extraction five months later.
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Fig. 6. SEM image of the probe with two electrode arrays pre-implantation (left) and post-explantation (right). Yellow and blue squares depict the cross-sections of the two electrodes at 700x magnification.
4 Conclusion
By using replica-molding microfabrication techniques, we designed, developed, characterized and validated a prototype of a very flexible microelectrode array with 16 recording sites that was implanted on the dura matter of a mouse brain. Impedance spectroscopy revealed electrode impedances of less than 1 MOhm at 1 kHz, which was sufficiently low to record epicortical signals in an in vivo mouse model over a period of 5 months.
6 References
[1] Blau, A., et al. (2011). Flexible all-polymer microelectrode arrays for the capture of cardiac and neuronal signals. Biomaterials, 32(7), 1778-86.
Acknowledgements
We thank Marina Nanni for her expert advice and assistance in the cell culture preparation. IIT intramural funds in support of this research are highly appreciated.
Keywords:
Biocompatibility,
PEDOT:PSS,
Replica molding,
Conductive polymer composite,
All-polymer microelectrode array (MEA),
epicortical recording,
polydimethylsiloxane PDMS
Conference:
MEA Meeting 2016 |
10th International Meeting on Substrate-Integrated Electrode Arrays, Reutlingen, Germany, 28 Jun - 1 Jul, 2016.
Presentation Type:
Poster Presentation
Topic:
MEA Meeting 2016
Citation:
Golabchi
A,
Trusel
M,
Scarpellini
A,
Habibey
R,
Tonini
R and
Blau
A
(2016). Flexible polydimethylsiloxane-based in vivo neural interface with carbon-polymer composite electrodes.
Front. Neurosci.
Conference Abstract:
MEA Meeting 2016 |
10th International Meeting on Substrate-Integrated Electrode Arrays.
doi: 10.3389/conf.fnins.2016.93.00118
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Received:
22 Jun 2016;
Published Online:
24 Jun 2016.
*
Correspondence:
Dr. Axel Blau, Italian Institute of Technology (IIT), Neuroscience and Brain Technologies, Genova, Italy, axel.blau@outlook.de