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Fabrication of Convex PDMS-Parylene Microstructures for Conformal Contact of Planar Micro-Electrode Array

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Fabrication of Convex PDMS-Parylene Microstructures for Conformal Contact of Planar Micro-Electrode Array

Woo Ram Lee et al. Polymers (Basel).

Abstract

Polymer-based micro-electrode arrays (MEAs) are gaining attention as an essential technology to understand brain connectivity and function in the field of neuroscience. However, polymer based MEAs may have several challenges such as difficulty in performing the etching process, difficulty of micro-pattern generation through the photolithography process, weak metal adhesion due to low surface energy, and air pocket entrapment over the electrode site. In order to compensate for the challenges, this paper proposes a novel MEA fabrication process that is performed sequentially with (1) silicon mold preparation; (2) PDMS replica molding, and (3) metal patterning and parylene insulation. The MEA fabricated through this process possesses four arms with electrode sites on the convex microstructures protruding about 20 μm from the outermost layer surface. The validity of the convex microstructure implementation is demonstrated through theoretical background. The electrochemical impedance magnitude is 204.4 ± 68.1 kΩ at 1 kHz. The feasibility of the MEA with convex microstructures was confirmed by identifying the oscillation in the beta frequency band (13-30 Hz) in the electrocorticography signal of a rat olfactory bulb during respiration. These results suggest that the MEA with convex microstructures is promising for applying to various neural recording and stimulation studies.

Keywords: beta frequency band; conformal contact; convex microstructure; electrocorticography (ECoG); hybrid microstructure; micro-electrode array (MEA); molding fabrication process; polymer; rat olfactory bulb; respiration.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Schematic of the micro-electrode arrays (MEA) with ten convex electrode sites in four arms for in vivo Electrocorticography (ECoG) recording from rat olfactory bulb. (a) Experimental setup. (b) The overall dimensions of the MEA. The unit of length is micrometer (μm).
Figure 2
Figure 2
Schematic of the fabrication process flow for an MEA with replicated convex microstructures.
Figure 3
Figure 3
SEM images of fabricated convex structures. (a) Top view of a PDMS convex structure. (b) Side view of the PDMS convex structures. (c) Top view of an electrode site having a PDMS/parylene/metal hybrid convex structure. (d) Top view with different magnification of the electrode sites. Some images were adapted and modified from a doctoral dissertation [25,26].
Figure 4
Figure 4
Impedance spectra of electrode sites with a diameter of 100 μm in a phosphate-buffered saline solution. The error bars are standard error of mean.
Figure 5
Figure 5
Comparison of fabrication process results with PDMS layer (the left of the red dotted line) and PDMS–parylene hybrid layer (the right of the red dotted line). (a) PDMS residues after dry etching; (b) PDMS dissolution; (c) Well-patterned parylene layer after dry etching. The red arrow shows the edge of etched parylene layer; (d) The surface of metal deposited on the PDMS surface damaged by excessive oxygen plasma treatment; (e) Metal film wrinkles by thermal expansion on PDMS layer; (f) Metal layer on PDMS–parylene hybrid layer after thermal treatment; (g) Cracks of the metal pattern and the photoresist layer due to the mismatch of the coefficient of thermal expansion; (h) Cracks of photoresist film on PDMS layer by thermal expansion; (i) Well-patterned metal layer on PDMS–parylene hybrid layer; (j) Exfoliation of metal layer deposited on the non- plasma- treated PDMS surface and (k) on the plasma-treated PDMS surface; (l) Preservation of metal layer deposited on the plasma-treated parylene surface on the PDMS layer. Some images were adapted and modified from a doctoral dissertation [25,26].
Figure 6
Figure 6
Magnification of the stiffness according to the thickness of parylene. The stiffness magnification was obtained by dividing the total stiffness by the stiffness of each PDMS (Dtot/DPDMS). The red asterisk refers to the thickness of PDMS and parylene layer used in this paper.
Figure 7
Figure 7
The experiment of air pocket entrapment. (a) Schematic of air pocket entrapment and variable descriptions (R: water radius, θ: contact angle, w: trench width, h: trench depth). (b) Experimental results of air pocket entrapment according to depth of concave structure. Some images were adapted and modified from a doctoral dissertation [25,26].
Figure 8
Figure 8
Frequency components of ECoG signals recorded from a rat olfactory bulb via 10 channels of MEA during respiration. (a) Averaged power spectral density estimates derived from 20 epochs. Shaded areas indicate the standard deviation; (b) Representative time-frequency distributions extracted from a single epoch; (c) Time courses of spectral power in the beta frequency band during a single epoch.

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