Several conditions and diseases are linked to the elevation or depression of internal pressures from a healthy, normal range, motivating the need for chronic implantable pressure sensors. A simple implantable pressure transduction system consists of a pressure-sensing element with a method to transmit the data to an external unit. The biological environment presents a host of engineering issues that must be considered for long term monitoring. Therefore, the design of such systems must carefully consider interactions between the implanted system and the body, including biocompatibility, surgical placement, and patient comfort. Here we review research developments on implantable sensors for chronic pressure monitoring within the body, focusing on general design requirements for implantable pressure sensors as well as specifications for different medical applications. We also discuss recent efforts to address biocompatibility, efficient telemetry, and drift management, and explore emerging trends.

A Parylene C neural probe with a three dimensional sheath structure was designed, fabricated, and characterized. Multiple platinum (Pt) electrodes for recording neural signals were fabricated on both inner and outer surfaces of the sheath structure. Thermoforming of Parylene was used to create the three dimensional sheath structures from flat surface micromachined microchannels using solid microwires as molds. Benchtop electrochemical characterization was performed on the thin film Pt electrodes using cyclic voltammetry and electrochemical impedance spectroscopy and showed that electrodes possessed low impedances suitable for neuronal recordings. A procedure for implantation of the neural probe was developed and successfully demonstrated in vitro into an agarose brain tissue model. The electrode-lined sheath will be decorated with eluting neurotrophic factors to promote in vivo neural tissue ingrowth post-implantation. These features will enhance tissue integration and improve recording quality towards realizing reliable chronic neural interfaces.

Recent advances in the micromachining of poly(p-xylylenes), commercially known as Parylenes, have enabled the development of novel structures and devices for microelectromechanical systems (MEMS). In particular, Parylene C (poly[chloro-p-xylylene]) has been explored extensively for biomedical applications of MEMS given its compatibility with micromachining processes, proven biocompatibility, and many advantageous properties including its chemical inertness, optical transparency, flexibility, and mechanical strength. Here we present a review of often used and recently developed micromachining process for Parylene C, as well as a high-level overview of state-of-the-art Parylene hybrid and free film devices for biomedical MEMS (bioMEMS) applications, including a discussion on its challenges and potential as a MEMS material.

The development of polymer micromachining technologies that complement traditional silicon approaches has enabled the broadening of microelectromechanical systems (MEMS) applications. Polymeric materials feature a diverse set of properties not present in traditional microfabrication materials. The investigation and development of these materials have opened the door to alternative and potentially more cost effective manufacturing options to produce highly flexible structures and substrates with tailorable bulk and surface properties. As a broad review of the progress of polymers within MEMS, major and recent developments in polymer micromachining are presented here, including deposition, removal, and release techniques for three widely used MEMS polymer materials, namely SU-8, polyimide, and Parylene C. The application of these techniques to create devices having flexible substrates and novel polymer structural elements for biomedical MEMS (bioMEMS) is also reviewed.

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