Christian A. Gutierrez

A Parylene Bellows Electrochemical Actuator

We present the first electrochemical actuator with Parylene bellows for large-deflection operation. The bellows diaphragm was fabricated using a polyethylene-glycol-based sacrificial molding technique followed by coating in Parylene C. Bellows were mechanically characterized and integrated with a pair of interdigitated electrodes to form an electrochemical actuator that is suitable for low-power pumping of fluids. Pump performance (gas generation rate and pump efficiency) was optimized through a careful examination of geometrical factors. Overall, a maximum pump efficiency of 90% was achieved in the case of electroplated electrodes, and a deflection of over 1.5 mm was demonstrated. Real-time wireless operation was achieved. The complete fabrication process and the materials used in this actuator are biocompatible, which makes it suitable for biological and medical applications.

3D Parylene sheath neural probe for chronic recordings

OBJECTIVE Reliable chronic recordings from implanted neural probes remain a significant challenge; current silicon-based and microwire technologies experience a wide range of biotic and abiotic failure modes contributing to loss of signal quality. APPROACH A multi-prong alternative strategy with potential to overcome these hurdles is introduced that combines a novel three dimensional (3D), polymer-based probe structure with coatings. Specifically, the Parylene C sheath-based neural probe is coated with neurotrophic and anti-inflammatory factors loaded onto a Matrigel carrier to encourage the ingrowth of neuronal processes for improved recording quality, reduce the immune response, and promote improved probe integration into brain tissue for reliable, long-term implementation compared to its rigid counterparts. MAIN RESULTS The 3D sheath structure of the probe was formed by thermal molding of a surface micromachined Parylene C microchannel, with electrode sites lining the interior and exterior regions of the lumen. Electrochemical characterization of the probes via cyclic voltammetry and electrochemical impedance spectroscopy was performed and indicated suitable electrode properties for neural recordings (1 kHz electrical impedance of ∼200 kΩ in vitro). A novel introducer tool for the insertion of the compliant polymer probe into neural tissue was developed and validated both in vitro using agarose gel and in vivo in the rat cerebral cortex. In vivo electrical functionality of the Parylene C-based 3D probes and their suitability for recording the neuronal activity over a 28-day period was demonstrated by maintaining the 1 kHz electrical impedance within a functional range (<400 kΩ) and achieving a reasonably high signal-to-noise ratio for detection of resolvable multi-unit neuronal activity on most recording sites in the probe. Immunohistochemical analysis of the implant site indicated strong correlations between the quality of recorded activity and the neuronal/astrocytic density around the probe. SIGNIFICANCE The provided electrophysiological and immunohistochemical data provide strong support to the viability of the developed probe technology. Furthermore, the obtained data provide insights into further optimization of the probe design, including tip geometry, use of neurotrophic and anti-inflammatory drugs in the Matrigel coating, and placement of the recording sites.

Low-cost carbon thick-film strain sensors for implantable applications

The suitability of low-cost carbon thick-film strain sensors embedded within a biomedical grade silicone rubber (Silastic® MDX4-4210) for implantable applications is investigated. These sensors address the need for robust cost-effective implantable strain sensing technology for the closed loop operation of function-restoring neural prosthetic systems. Design, fabrication and characterization of the sensors are discussed in the context of the application to strain/fullness measurements of the urinary bladder as part of the neuroprosthetic treatment of lower urinary tract dysfunction. The fabrication process, utilizing off-the-shelf screen-printing materials, is convenient and cost effective while achieving resolutions down to 75 µm. This method can also be extended to produce multilayer embedded devices by superposition of different screen-printable materials. Uniaxial loading performance, temperature dependence and long-term soak testing are used to validate suitability for implantation while proof-of-concept operation (up to 40% strain) is demonstrated on a bench-top latex balloon bladder model.

Epoxy-less packaging methods for electrical contact to parylene-based flat flexible cables

We present two methods for establishing rapid epoxy-less electrical connectivity to Parylene-based flat flexible cables (FFC). The first utilizes commercially available zero-insertion-force (ZIF) connector technology and the second utilizes a custom-fabricated connector with an acrylic/polydimethylsiloxane (PDMS) interface featuring screen-printed contacts. A contact pitch of 0.5 mm was achieved using both connector methods. These techniques are simple to implement, reversible, scalable and unlike epoxy-based approaches, do not require manual intervention to secure individual contacts.

A subnanowatt microbubble pressure sensor based on electrochemical impedance transduction in a flexible all-Parylene package

We present the first all-Parylene microbubble pressure transducer (µBPT) harnessing the ability of a microbubble (µB) to respond instantaneously to external pressure variations. A µB is electrolytically generated and physically trapped within a Parylene microchamber such that pressure-induced bubble size variation is detected by electrochemical impedance (EI) measurement. Real-time hydrostatic pressure measurement (−11.8 O/psi, ±0.1 psi) of negative and positive pressures (−2 – 4psi) is demonstrated. The open-package device design leverages the ambient liquid environment obviating the need for hermetic packaging techniques. μBPTs are biocompatible, flexible, ultra-miniature (200 µm diameter, 10 µm thick), and can be operated at very low power (≤ nW) making them especially attractive for wet, in vivo pressure measurement.

Impedance-Based Force Transduction Within Fluid-Filled Parylene Microstructures

We report on the use of electrochemical impedance (EI) as the basis for force transduction in Parylene-based microdevices. Electrolyte-filled microstructures were realized for extremely sensitive contact-force detection (10 mN range, ±0.023 mN resolution) enabled by EI-based transduction and are a promising platform for next-generation biomedical sensing technology. The design, fabrication, and characterization of Parylene-based electrochemical-MEMS (EC-MEMS) devices capable of microNewton contact-mode force measurement are presented and discussed.

Improved self-sealing liquid encapsulation in Parylene structures by integrated stackable annular-plate stiction valve

We report on an improved self-sealing structure for Parylene-based liquid encapsulation. The specific improvements are an integrated stiction valve which reduces overall device footprint by nearly 50% over in-plane designs and the use of an annular-plate design in a stackable multi-layered configuration for successful longterm liquid encapsulation. We achieve automatic wafer-level liquid entrapment without using adhesives or processing at elevated pressures or temperatures. We also demonstrate the use of electrochemical impedance measurements as a means for tracking internal liquid volume.

Liquid Encapsulation in Parylene Microstructures Using Integrated Annular-Plate Stiction Valves

We report the design, fabrication and characterization of micromachined Parylene structures for self-sealing liquid encapsulation applications. Automatic sealing is enabled through the use of an integrated annular-plate stiction valve which greatly reduces device footprint over in-plane configurations. We achieve automatic wafer-level liquid entrapment without using adhesives or processing at elevated pressures or temperatures. The ability to track changes to the internal liquid volume through the use of electrochemical impedance measurements is also presented.

A dual function Parylene-based biomimetic tactile sensor and actuator for next generation mechanically responsive microelectrode arrays

We present the first multimodal Parylene-based biomimetic platform with the ability to measure tactile forces while simultaneously enabling active stimulation/recording of neural tissue via movable microelectrodes. A liquid-filled microchamber encapsulating interdigitated microelectrodes performs the dual functions of actuation and tactile sensing. The electrodes are impedance-based tactile sensing elements and sites for electrolytic gas generation for pneumatic control of microelectrode position. Impedance sensitivity of up to 1.7 %/µm and out-of-plane positioning up to 8 µm were demonstrated. We also report on the mechanotransduction of biomimetic channel structures.

Parylene-based electrochemical-MEMS force sensor array for assessing neural probe insertion mechanics

We present the first use of a Parylene-based electrochemical-MEMS (EC-MEMS) sensor array for instrumentation of ceramic-based neural electrode probes. The sensor array consists of a liquid-filled Parylene-based microchannel and an array of enclosed electrodes that monitor local variations in impedance during mechanical deformation of the channel. The array provides real time measurement of out-of-plane interfacial forces produced directly on the electrode shank surface (<;5 mm2) during insertion of the probe. We demonstrate the ability to examine the relative force distribution of interfacial forces produced on the shank surface during insertion, thereby providing a clearer understanding of probe insertion mechanics. Our approach enables, for the first time, robust mechanical instrumentation of electrode shanks providing a means for assessing the poorly understood interfacial mechanics between neural probes and tissue.