A Shared Resource for Building Polymer-Based Microelectrode Arrays as Neural Interfaces

Chronic functionality of neural interfaces (NI) is hampered by the physiological response to foreign objects, in part due to the mismatch of mechanical properties between soft neural tissue 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 specific research missions; many researchers do not have access to polymer-based NIs technology and among those who do there is a severe lack of standardization in material, construction, packaging, and testing, leading to a lack of repeatability among datasets. Here we present the Polymer Implantable Electrode (PIE) Foundry, a shared-resource for fabricating and disseminating standardized polymer-based microelectrode arrays for 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 microelectrode arrays 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, spinal paddle electrodes, surface arrays for electroencephalography, 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 ex vivo and in vivo electrode performance, and key results from the first three years of Foundry operation.

I. Introduction

Chronically viable neuroelectronic interfaces are required for neuromodulation therapy and fundamental neuroscience research, but most available tools have short functional 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 with polymers 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.