Thin Film Encapsulation for LCP-Based Flexible Bioelectronic Implants: Comparison of Different Coating Materials Using Test Methodologies for Life-Time Estimation

Abstract

Liquid crystal polymer (LCP) has gained wide interest in the electronics industry largely due to its flexibility, stable insulation and dielectric properties and chip integration capabilities. Recently, LCP has also been investigated as a biocompatible substrate for the fabrication of multielectrode arrays. Realizing a fully implantable LCP-based bioelectronic device, however, still necessitates a low form factor packaging solution to protect the electronics in the body. In this work, we investigate two promising encapsulation coatings based on thin-film technology as the main packaging for LCP-based electronics. Specifically, a HfO2–based nanolaminate ceramic (TFE1) deposited via atomic layer deposition (ALD), and a hybrid Parylene C-ALD multilayer stack (TFE2), both with a silicone finish, were investigated and compared to a reference LCP coating. T-peel, water-vapour transmission rate (WVTR) and long-term electrochemical impedance spectrometry (EIS) tests were performed to evaluate adhesion, barrier properties and overall encapsulation performance of the coatings. Both TFE materials showed stable impedance characteristics while submerged in 60 °C saline, with TFE1-silicone lasting more than 16 months under a continuous 14V DC bias (experiment is ongoing). The results presented in this work show that WVTR is not the main factor in determining lifetime, but the adhesion of the coating to the substrate materials plays a key role in maintaining a stable interface and thus longer lifetimes.

Lifetime engineering of bioelectronic implants with mechanically reliable thin film encapsulations

“…However, they still need further development in their fabrication methods, as the typical method of LCP device fabrication involves thermal lamination of relatively thick (∼25 µm) films, which must be further thinned via additional steps such as laser thinning in order to comply with the recent trend of ultra-thin and flexible devices, and LCPs may still only be applied to planar devices (i.e. they cannot be dip coated or deposited in a gas phase) [158,246]. Though relatively well-performing standalone polymer encapsulations have been developed such as LCPs, the incorporation of impermeable inorganic material domains will still likely be necessary to achieve truly long-lasting encapsulation strategies.…”

“…The recent in vitro study of electrode degradation by Oldroyd et al [292] is exemplary in this regard. Awareness of this need has been increasing in recent years [187,246], but simple thermal accelerated aging tests remain the norm at present, most likely due to their simplicity and convenience compared to more comprehensive simulations.…”

Longevity of Implantable Silicon-ICs for Emerging Neural Applications: Evaluation of Bare Die and PDMS-Coated ICs After Accelerated Aging and Implantation Studies

“…Driven by the need for miniaturization, these emerging applications are moving away from the hermetic metal enclosures, traditionally utilized for IC protection in the body, and are opting for newly engineered thin organic and inorganic coatings 6,[17][18][19][20][21][22] . This shift, however, introduces reliability risks by bringing the chip closer to the corrosive environment of the body.…”

Conformal Low‐Temperature Molecular Layer Deposited Coatings as Interface Modifications to Support Ceramic Protective Films on Macroscopic Flexible Devices

“…A showcase of different encapsulation methods with list of pros and cons associated with each respective method. Image of titanium casing reused without alteration from Winkler et al [ 55 ] under CC BY 4.0 licence and TEM images of ALD nanolaminate coated substrate adapted (figure annotations removed) from Pak et al [ 12 ] under CC BY 4.0 licence.…”