2023 11th International IEEE/EMBS Conference on Neural Engineering (NER) | 978-1-6654-6292-1/23/$31.00 ©2023 IEEE | DOI: 10.1109/NER52421.2023.10123883

A Shared Resource for Building Polymer-Based
Microelectrode Arrays as Neural Interfaces
Kee Scholten
Department of Biomedical
Engineering
University of Southern California
Los Angeles, United Stateskscholte@usc.edu

Huijing Xu
Center for Neural Engineering
University of Southern California
Los Angeles, United Stateshuijingx@usc.edu

Abstract— Chronic functionality of neural interfaces (NI) ishampered by the physiological response to foreign objects, in partdue to the mismatch of mechanical properties between soft neuraltissue and the rigid materials used in interface construction.
Polymer-based NIs have emerged as a key new technology in the
pursuit of chronically stable neural recording and stimulation, but
most polymer NIs are bespoke devices developed as part of specificresearch missions; many researchers do not have access topolymer-based NIs technology and among those who do there is asevere lack of standardization in material, construction,
packaging, and testing, leading to a lack of repeatability amongdatasets. Here we present the Polymer Implantable Electrode(PIE) Foundry, a shared-resource for fabricating anddisseminating standardized polymer-based microelectrode arraysfor use in NIs. The model is based on the successful shared
prototyping concept developed for the field of semiconductor
research. Professional staff, supported by the BRAIN Initiative
funding and operating in cleanroom space provided by the
University of Southern California, offer design, fabrication,
packaging, and testing of polymer-based microelectrode arrays as
a free service to academic and non-profit research groups. The
core enabling technology is a standardized set of micromachining
protocols applied to the biocompatible, thin-film polymer
Parylene C. By leveraging this method, we produce microelectrodearrays of varied size, shape, channel count, and application,disseminating hundreds of arrays to 18+ research groups in our
first three years of operation. By standardizing materials,
fabrication, and packaging, we create repeatable and comparable
devices and have built a library of shareable designs. Channel
counts range from 2 to 64, electrode sizes range from 15 µm
diameter to 1 mm, designs include penetrating neural probes,
spinalpaddleelectrodes,
surfacearraysforelectroencephalography, and peripheral nerve cuffs for recording
and stimulation, animal models include songbird, mouse, rat, cat,and sheep. Here we present details of our organizational structure,fabrication and packaging methods, representative examples of exvivo and in vivo electrode performance, and key results from thefirst three years of Foundry operation.
Keywords—electrodes,
microelectrodes, flexible

polymer,neuralrecording,

Dong SongCenter for Neural Engineering
University of Southern California
Los Angeles, United Statesdsong@usc.edu

Ellis Meng
Department of Biomedical
Engineering
University of Southern California
Los Angeles, United Statesellis.meng@usc.edu

I. INTRODUCTION
Chronically viable neuroelectronic interfaces are
required for neuromodulation therapy and fundamental
neuroscience research, but most available tools have shortfunctional lifetimes due in part to the body’s immune response[1], [2]. Soft, flexible microelectrode arrays (MEA) are an
emerging technology that offer a means to mitigate the immune
response by replacing silicon and metal-based electrodes withpolymers as a softer alternative [3]–[8]. However, there are few
polymer MEAs (pMEA) available to researchers, and fewer still
which meet the range of needs of current neuroscience research.
PMEAs customized to different species and applications
have been developed and demonstrated. These are commonly
produced via sponsored research involving collaboration
between neuroscientists and select academic laboratories withboth
microfabrication
expertiseandaccesstomicro/nanofabrication facilities. Since such collaborations
usually target a specific region in a specific species, this one-byone approach produces few custom-designed devices suitablefor initial demonstration purposes yet does not scale to support
research studies requiring large numbers of devices. This is inpart attributed to the use of research trainees to design, fabricate,and package devices as an intermediate step in their careerdevelopment and the absence of affordable and convenient
contract research services to rapidly develop and produce
custom pMEAs. Highly trained research staff can support rapid
prototyping, scale up of custom device production for specificneuroscience users, and disseminate generic designs to a wider
user base. Therefore, to address the needs of the researchcommunity, a flexible process was devised, which could
produce pMEA devices of arbitrary design, size, and channel
count, and incorporate form-factors including surface arrays,
penetrating probes, and peripheral nerve cuffs (Fig. 1).
We launched the Polymer Implantable Electrode (PIE)
Foundry as a shared-resource center to develop and fabricate
polymer MEAs for neuroscience researchers.

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Fig. 1. Representative examples of polymer microelectrode arrays of different archetypes: (A) A 64 channel penetrating array for (sub)cortical recording withmagnified view of electrodes in inset; (B) A 32 channel surface recording electrode array for electrocorticography; (C) An 8 channel extrafasicular peripheral
nerve interface with inset showing the pMEA in a curled configuration.

II. MATERIALS & METHODS
A. Shared-Resource Model
The PIE Foundry is supported as a research resource grantthrough the National Institutions of Health under the BRAIN
Initiative [9] and is based in the labs of Dong Song and Ellis
Meng and core research facilities at the University of Southern
California. The Foundry is operated by full-time professional
staff who offer design, fabrication, and testing of pMEAs. Users
can also access training services that include multielectrodearray design, fabrication protocols, and implementation ofdevices in animal studies. PIE Foundry services are provided at
no cost to research community for the duration of the award andat cost to commercial users.
The PIE Foundry offers fabrication of user-submitted
designs using a multi-project wafer (MPW) model, in which
designs adhering to a standard set of guidelines are processed
simultaneously, with multiple user designs sharing space on thesame carrier wafer. Users who require assistance designing theirelectrodes, or who require non-standard materials or design
features as part of their project, can request custom fabrication
services. Under this model PIE Foundry staff design, fabricate,package, and provide basic characterization of the users. Due tothe added effort and cost, custom projects require the submissionof a short proposal, which is reviewed by an external scientificsteering group. In addition, genericized “off-the-shelf” electrodearrays are available for users who do not need custom solutions.
This includes a 64 channel Parylene C ‘standard’ recording array(Fig. 1A) for use in rats and small mammals, described
previously [20].
Users submit digital design files (.gds, .dxf, .cif) or a
proposal through email or the online portal [10].
Microfabrication services typically take 1-2 months dependingon the current workload, while custom projects can take 3-4
months following approval, depending on complexity of the
project. Designs include brain-machine interfaces, peripheral
nerve cuffs, spinal cord arrays, and retinal arrays among others.

the electrodes, traces, and contact pads, and an insulating layerof Parylene, which is etched to uncover the electrodes andcontacts. Parylene C was chosen due to its proven history inchronic biomedical implants, strong insulating properties, andthermoplastic nature, which allows planar arrays to be
thermoformed into cuffs, coils, and other shapes. PMEAs arefabricated using a combination of photolithography, chemicaland physical vapor deposition, and plasma etching, with allprocessing steps performed in a class 1000 (ISO 6) cleanroom.
The typical fabrication process is summarized here. First,
100 mm diameter silicon carrier wafers are conformally vaporcoated in approximately 10 µm of Parylene C to form the baseof the pMEA. Next, a single thin-film metal layer, containing all
traces, contact pads and electrodes, is fabricated using a
combination of image-reversal photolithography, metal
evaporation, and solvent lift-off. Wafers are treated with
AZ5214 photoresist, 1.1 µm thick, and patterned in image
reversal mode. Following a brief O2 plasma treatment to clean
the Parylene C surface, wafers are coated with a metal layerusing e-beam evaporation. Typical processing entails either asingle 200 nm Pt layer or a Ti/Au/Pt stack (20/155/25 nm). Liftoff is performed in a bath of N-methyl-2-pyrrolidone (NMP) at
60 °C; Pt coated wafers are treated with mild agitation while
Ti/Au/Pt coated wafers require an ultrasonic bath.
A second Parylene C layer, also approximately 10 µm thick,is deposited, forming the insulation and top surface of the
pMEA. Wafers are first treated with A174 silane, an adhesion
promotor, then coated in Parylene C. Total thickness of themulti-layer stack is approximately 20 µm
To remove the insulation above the electrodes and contact
pads, and to cut-out the shape of the polymer MEA, wafers are
etched with O2 plasma using two separate photoresist masks.
Wafers are treated with P4620 photoresist, 15 µm thick, thenpatterned using contact UV lithography. Then Parylene C isetched to a depth of 10 µm using an O2 reactive ion etch (RIE)

B. Polymer Microelectrode Array Fabrication
Foundry pMEAs consist of a simple multi-layer design,comprising a base layer of Parylene C, a metal layer containing
Research supported by NIH U24NS113647-01.

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process (150 mT, 150 W, 50 sccm O2, Oxford RIE80), yielding
an approximate etch rate of 0.21 µm/minute. A switchedchemistry Bosch-like process can also be used in a deep reactive
ion etch tool, and details have been reported previously [11].
Following the first etch step, the photoresist mask is stripped,
the second etch mask is applied in the same manner, and theetching process is repeated. The second etch mask is thenstripped, and the devices are released by submerging the wafer
in water and peeling devices off the wafer using tweezers.
To improve adhesion between the Parylene C layers pMEAs
are annealed for 48 hours under vacuum at 200 °C, sandwiched
between ceramic alumina plates (0.635 mm thick). Finally,
electrodes are cleaned with a brief exposure to O2 plasma (100W, 100 mT, 300 seconds).
C. Packaging
The majority of devices manufactured by the Foundry areattached to printed circuit boards (PCBs) for connection toexternal electronics and headstages using a direct ultrasonic
weld referred to as polymer-ultrasonic on bump (PUB) bond[14]. Contact pads on PCBs (200×350 µm, 50+ µm thick, softgold finish) are first ‘bumped’ using a ball bonding tool
(Hybond 626). A gold ball with an approximate diameter of 100µm is ultrasonically bonded to the center of each pad, thenflattened into a disk using a wire bonding tool fitted with a tapeautomated bonding (TAB) attachment (7045-Ti, Smart
Precision Tools). This step flattens and levels the gold bump,while imprinting a waffle pattern into the surface. Using thesame tool, force and ultrasonic energy is applied to the back of
the pMEA, welding the metal of the pMEA contact pad to thegold bump. The process typically takes 2 seconds perconnection. Afterwards the bonded connection is underfilledwith epoxy (301-3M, EpoTek) for mechanical support.
When required, PUB bonding can be replaced withalternative packaging scheme, including the use of a zeroinsertion force (ZIF) connector. ZIF connectors offer reversible
connection between flat flexible cables and PCBs using amechanical clip. ZIF compatible contact pads are incorporated
into the pMEA design, oversized by 2.5% to account forpolymer shrinkage during annealing steps; pMEAs are thenmounted onto PEEK films (25.4 µm thick) using epoxy (3013M). The film provides the necessary thickness and mechanicalsupport to mate with the ZIF connector.
D. Characterization
Representative electrodes are characterized withelectrochemical impedance spectroscopy (EIS) and cyclic
voltammetry (CV) using a Gamry Reference 600 potentiostat(Fig. 2). CV is performed in 0.05 M H2SO4 in the potential rangeof −0.2–1.2 V versus an Ag/AgCl reference and large Pt counterelectrode. The solution is purged with N2 prior to and during theCV. Scan rate was 250 mV/s for 30 cycles. EIS is performed in
1× phosphate-buffered saline (PBS) following CV cycling, alsousing Ag/AgCl and Pt as the reference and counter, respectively.
Impedance is measured in the range of 1 Hz–1 MHz. To assess
end-to-end continuity, single-point electrochemical impedance,at either 1 or 10 kHz in 1× PBS is performed on electrode sites.
Full electrochemical characterization is typically performed
only upon request, with results provided to the user.

Fig. 2. (A) Cyclic voltammetry profile of a representative 30 µm Pt electrodeon a PIE Foundry standard array. (B) Electrode impedance of Pt coated circularelectrodes of different sizes in 1× PBS at 1 KHz against a Pt counter.

E. Implantation
Penetrating brain probes built from pMEAs require mechanicalassistance to implant. PIE Foundry users have used a variety ofmethods, including metal wire shuttles, dissolvable braces, andtemporary stiffening agents. The most common method,developed by Foundry staff, is to dip-coat arrays in molten
polyethylene glycol (PEG) with 8 kDa molecular weight.
Cortical and subcortical implantation entails removing both duramater and pia mater before inserting arrays using a stereotaxic
controller. PEG dissolves upon contact with physiological fluid.
Stainless steel anchor screws are placed in the skull of the animalmodel, and dental cement is used to fix the pMEA interfaceboard to the skull, using the anchor screws as an attachment
point. In-house testing of the 64-channel ‘standard’ arrayentailed implantation of Parylene C shanks 5 mm deep in adultrats, targeting either the somatosenory cortex or CA1/CA3
hippocampal layers. Both acute and chronic tests erre performed;
typical chronic tests involved recording from free-moving rats
over a period of 6-12 weeks. A more detailed account was
published previously [12]. Surface and peripheral nerve pMEAs
are typically implanted based on user developed surgicalmethods.
III. RESULTS
Since launching in November of 2019, the PIE Foundry hasdelivered more than 400 individual pMEAs to 18 different
research groups at 12 different institutions, totaling many
thousands of individual electrode sites. Our fabrication methods
have been successfully used to develop a wide range of devices,

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exploring an open field. Individual channels show stable
recording over a period beyond a month (Fig. 3).
IV. DISCUSSION

Fig. 3. In vivo recording obtained with the standard pMEA. (A) Unitaryactivities recorded from the hippocampus of an anesthetized rat. (B) Unitaryactivities recorded from the cortex of an anesthetized rat. (C) Units recorded by
two different channels (orange and green) over a period of 34 days.

including surface arrays, peripheral nerve interfaces, andcortical and sub-cortical penetrating arrays for animal models
including songbird, mouse, rat, cat, and sheep. Electrode counts
range from as low as 2 per device to 64 per device, andconstructed layouts include stereotrode, tetrode, linear arrays,
and anatomically mapped custom arrays. pMEA size ranges
from surface electrode arrays 25 mm2 in area, to penetratingshanks just 100 µm wide. While these arrays are designed
principally for recording, several users have reported using
paddle and cuff designs for stimulation of peripheral nerves.
Our fabrication technique has proven flexible and robust;
hundreds of pMEAs are now regularly produced in batchprocessing, with multiple user designs mixed into each MPW
run. Penetrating arrays, surface arrays and peripheral nerve cuffs
can all be fabricated on the same wafer as a result of thegeneralizable fabrication process, which enables fast turnaround times for users and reduces cost for the Foundry. The
lithographic methods allow micromachines features, including
metal traces, with critical size and pitch as small as 2 µm,
minimizing implant size while maximizing the number ofelectrode sites. The PUB bonding method has proven the mosteffective packaging method, more scalable and more robust thanthe use of ZIF connectors, and faster and more reliable thanmanual alternatives such as conductive epoxy. A 64 channel
pMEA can be PUB bonded to a PCB in just a few minutes, in aspace 1/3 the size of the smallest available ZIF connector, andthe technique easily scales to hundreds of channels, far morethan the largest available ZIF. Across more than 100 tests, wehave yet to observe a PUB bond fail after packaging. In contrast,
ZIF connectors can damage the thin film metal contacts ofpMEAs.
Fig. 2 presents representative datasets showing electrode CV
response and electrochemical impedance. Electrode impedances
scale inversely with electrode size. While the PIE Foundry does
not offer electrochemical coatings, several users have used PIE
Foundry devices with commercial or personally-developed
systems for coating porous gold, PEDOT, and other surface
finishes, and have achieved significant decreases in impedance.
In vivo testing of pMEAs have proven chronic stability ofpolymer interfaces in free-moving animals. In-house testing of
the 64-channel standard array entails 5 mm deep implantations
in male rats, targeting hippocampal layers, with unitary
recording from both cortex and hippocampi with animals

Use of pMEAs for brain-machine interfaces is likely to expand,as researchers increasingly adopt the technology for developing
longer-lasting devices. As polymer microfabrication is in
general much less mature than traditional semiconductor
micromachining, and as there remain few commercial options
for pMEAs, the shared-resource model may be the best
available option for most neuroscientists to gain access to thisimportant technology. Over the next few years, we expect tosignificantly expand the number of Foundry users, make key
improvements in our pMEA technology, including increasing
channel count, increasing the library of standardized devices,
and incorporating multiplexing, and we expect to disseminate
fabrication and packaging protocols to other institutions.
ACKNOWLEDGMENT
This work was funded through the BRAIN Initiative and the
National Institute of Health under award U24NS113647.
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