Polymer integration for packaging of implantable sensors

Inexpensive, easy-to-process, light-weight polymer-based materials that are biocompatible, mechanically flexible, and optically transparent have emerged as alternatives to metals and ceramics in the packaging of implantable sensors. These materials have been used to package components such as microelectrode arrays, telemetric coils and structural membranes. Polymers are also being used for the encapsulations and coatings of the implants. The devices and packages require fine-pitch, low-loss, and highly-conductive paths on mechanically and chemically reliable polymer films. In this review, several polymers used for implantation and related integration technologies are identified. We give an overview of novel applications of polymers in implantable sensor packages, and identify future directions for their application. Polymers exhibit high moisture absorption rate, high-frequency electrical loss, and low mechanical stability. These properties are aggravated when polymers are used for in vivo applications. Also, the integration of polymers with polymers/metals at high bonding temperatures and pressures may degrade their properties and interfaces. Furthermore, adhesive bonding and physical/chemical deposition methods for the integration may introduce non-hermetic, permeable, optically opaque, and poorly conductive interfaces. Thus, creating polymer-based high-density and small-dimension structures are critical for packaging. To address these issues, polymers with improved characteristics as well as integration techniques using low bonding temperature and pressure are indispensable. Liquid crystal polymer (LCP) and surface activated bonding (SAB) technologies meet these requirements. SAB technologies enable nanoscaled polymer-polymer/metal bonding to realize reliable, miniaturized, and high-performance packages for implantable sensors. This article is meant to serve as a reference for future research in the emerging field of implantable sensors by critically assessing the specific merits and drawbacks of several material-process combinations. (C) 2014 Elsevier B.V. All rights reserved.