Parylene C is a promising material for constructing flexible, biocompatible and corrosion-resistant microelectromechanical systems (MEMS) devices. Historically, Parylene C has been employed as an encapsulation material for medical implants, such as stents and pacemakers, due to its strong barrier properties and biocompatibility. In the past few decades, the adaptation of planar microfabrication processes to thin film Parylene C has encouraged its use as an insulator, structural and substrate material for MEMS and other microelectronic devices. However, Parylene C presents unique challenges during microfabrication and during use with liquids, especially for flexible, thin film electronic devices. In particular, the flexibility and low thermal budget of Parylene C require modification of the fabrication techniques inherited from silicon MEMS, and poor adhesion at Parylene-Parylene and Parylene-metal interfaces causes device failure under prolonged use in wet environments. Here, we discuss in detail the promises and challenges inherent to Parylene C and present our experience in developing thin-film Parylene MEMS devices.

A Parylene C polymer neural probe array with 64 electrodes purposefully positioned across 8 individual shanks to anatomically match specific regions of the hippocampus was designed, fabricated, characterized, and implemented in vivo for enabling recording in deep brain regions in freely moving rats. Thin film polymer arrays were fabricated using surface micromachining techniques and mechanically braced to prevent buckling during surgical implantation. Importantly, the mechanical bracing technique developed in this work involves a novel biodegradable polymer brace that temporarily reduces shank length and consequently, increases its stiffness during implantation, therefore enabling access to deeper brain regions while preserving a low original cross-sectional area of the shanks. The resulting mechanical properties of braced shanks were evaluated at the benchtop. Arrays were then implemented in vivo in freely moving rats, achieving both acute and chronic recordings from the pyramidal cells in the cornu ammonis (CA) 1 and CA3 regions of the hippocampus which are responsible for memory encoding. This work demonstrated the potential for minimally invasive polymer-based neural probe arrays for multi-region recording in deep brain structures. [2020-0018]

A novel Parylene C-based peripheral nerve interface that combines both electrodes and microfluidic channels in an adjustable cuff was designed, fabricated, and characterized. This minimally invasive interface incorporates a drug delivery system for targeted delivery of lysing agents and neurotrophic factors to the nerve surface to locally disrupt the epineurium and allow fascicular selectivity. Multiple platinum (Pt) electrodes were embedded in the microfluidic channels for neural stimulation and recording. The lyse-and-attract cuff electrode (LACE) uses a simple locking mechanism that is adjustable for close contact with nerves of varying diameters. Devices were fabricated using standard Parylene microfabrication techniques resulting in low variability and high device yield. A procedure for the implantation of the LACE was developed and successfully demonstrated in vivo around rat sciatic nerves. The adjustable locking mechanism demonstrated adequate holding strength and fit around the nerves. Benchtop electrochemical characterization of the thin-film Pt electrodes showed that the electrodes possessed high charge storage capacity (>1 mC/cm²) and low impedances (<2 kΩ at 1 kHz) suitable for neural stimulation and recording. As expected, embedded electrodes demonstrated higher impedance values. Acute neural recording from the rat sciatic nerve verified the capability of the LACE to record evoked neural activity (compound action potentials). Controlled and localized microfluidic infusions were achieved at low flow rates (<1 μL/min). Finally, infusion experiments in vivo demonstrated targeted drug delivery to the sciatic nerve fascicles. This multifunctional peripheral nerve interface has the potential to enhance implant-tissue integration in vivo and provide reliable chronic performance not available in the existing extraneural or intraneural interfaces. [2018-0192]

Parylene C is a thin-film polymer used as a biocompatible barrier layer in biomedical implants and implantable MEMS; free-film Parylene C may serve as both the substrate and insulation in polymer-based microdevices, a growing branch of biomedical technology. The adhesion of vapor deposited Parylene C, particularly when exposed to wet, in vivo environments, is a critical determinant of device lifetime for such polymer-based implants. This paper explores several novel strategies for improving the adhesion of multi-layer Parylene structures, including thermal annealing and the use of several chemical interposer layers. Interfacial adhesion of Parylene-Parylene and Parylene-platinum-Parylene films was examined using a standard T-peel test to quantify adhesion and measure film integrity under chronic exposure to saline up to two years. Improved adhesion and barrier properties in Parylene-Parylene films resulted from the inclusion of diamond-like carbon and ethylene glycol diacrylate layers. Thermal annealing improved Parylene film integrity in wet environments but was insufficient for improving the integrity of Parylene-platinum interfaces. A 100-fold increase in adhesive strength at such interfaces was achieved using a commercially available adhesion promoter, and the corresponding improvements in resistance to moisture driven delamination were observed. X-ray diffraction and X-ray photoelectron spectroscopy results are provided to highlight the role of film morphology and surface composition in adhesion integrity. [2018-0076]

We present for the first time the design, fabrication, and preliminary bench-top characterization of a high-density, polymer-based penetrating microelectrode array, developed for chronic, large-scale recording in the cortices and hippocampi of behaving rats. We present two architectures for these targeted brain regions, both featuring 512 Pt recording electrodes patterned front-and-back on micromachined eight-shank arrays of thin-film Parylene C. These devices represent an order of magnitude improvement in both number and density of recording electrodes compared with prior work on polymer-based microelectrode arrays. We present enabling advances in polymer micro-machining related to lithographic resolution and a new method for back-side patterning of electrodes. In vitro electrochemical data verifies suitable electrode function and surface properties. Finally, we describe next steps toward the implementation of these arrays in chronic, large-scale recording studies in free-moving animal models.