LONG-TERM EVALUATION OF A NON-HERMETIC MICROPACKAGE TECHNOLOGY FOR MEMS-BASED, IMPLANTABLE PRESSURE SENSORS.

This paper reports long-term evaluation of a micropackage technology for an implantable MEMS pressure sensor. The all-polymer micropackage survived 160 days when subjected to accelerated lifetime testing at 85 °C in a 1% wt. saline solution. The package shows minimum effect on sensors' sensitivity and nonlinearity, which deviated by less than 5% and 0.3%, respectively. A 6-month in vivo evaluation of 16 MEMS-based pressure sensors demonstrated that the proposed micropackage has good biocompatibility and can protect the MEMS pressure sensor. To the best of our knowledge, these results establish new lifetime records for devices packaged using an all-polymer micropackaging approach.

Development of an integrated surface stimulation device for systematic evaluation of wound electrotherapy.

Ideally, all chronic wounds would be prevented as they can become life threatening complications. The concept that a wound produces a 'current of injury' due to the discontinuity in the electrical field of intact skin provides the basis for the concept that electrical stimulation (ES) may provide an effective treatment for chronic wounds. The optimal stimulation waveform parameters are unknown, limiting the reliability of achieving a successful clinical therapeutic outcome. In order to gain a more thorough understanding of ES for chronic wound therapy, systematic evaluation using a valid in vivo model is required. The focus of the current paper is development of the flexible modular surface stimulation (MSS) device by our group. This device can be programed to deliver a variety of clinically relevant stimulation paradigms and is essential to facilitate systematic in vivo studies. The MSS version 2.0 for small animal use provides all components of a single-channel, programmable current-controlled ES system within a lightweight, flexible, independently-powered portable device. Benchtop testing and validation indicates that custom electronics and control algorithms support the generation of high-voltage, low duty-cycle current pulses in a power-efficient manner, extending battery life and allowing ES therapy to be delivered for up to 7 days without needing to replace or disturb the wound dressing.

Multi-layered poly-dimethylsiloxane as a non-hermetic packaging material for medical MEMS.

Poly-dimethylsiloxane (PDMS) is an attractive material for packaging implantable biomedical microdevices owing to its biocompatibility, ease in application, and bio-friendly mechanical properties. Unfortunately, devices encapsulated solely by PDMS lack the longevity for use in chronic implant applications due to defect-related moisture penetration through the packaging layer caused by conventional deposition processes such as spin coating. This paper describes an effort to improve the performance of PDMS as a packaging material by constructing the encapsulant from multiple, thin roller casted layers of PDMS as a part of a polymeric multi-material package.

In vivo deployment of mechanically adaptive nanocomposites for intracortical microelectrodes.

We recently introduced a series of stimuli-responsive, mechanically adaptive polymer nanocomposites. Here, we report the first application of these bio-inspired materials as substrates for intracortical microelectrodes. Our hypothesis is that the ideal electrode should be initially stiff to facilitate minimal trauma during insertion into the cortex, yet become mechanically compliant to match the stiffness of the brain tissue and minimize forces exerted on the tissue, attenuating inflammation. Microprobes created from mechanically reinforced nanocomposites demonstrated a significant advantage compared to model microprobes composed of neat polymer only. The nanocomposite microprobes exhibit a higher storage modulus (E' = ~5 GPa) than the neat polymer microprobes (E' = ~2 GPa) and can sustain higher loads (~12 mN), facilitating penetration through the pia mater and insertion into the cerebral cortex of a rat. In contrast, the neat polymer microprobes mechanically failed under lower loads (~7 mN) before they were capable of insertion into cortical tissue. Further, we demonstrated the material's ability to morph while in the rat cortex to more closely match the mechanical properties of the cortical tissue. Nanocomposite microprobes that were implanted into the rat cortex for up to eight weeks demonstrated increased cell density at the microelectrode-tissue interface and a lack of tissue necrosis or excessive gliosis. This body of work introduces our nanocomposite-based microprobes as adaptive substrates for intracortical microelectrodes and potentially for other biomedical applications.

Low voltage nanoelectromechanical switches based on silicon carbide nanowires.

We report experimental demonstrations of electrostatically actuated, contact-mode nanoelectromechanical switches based on very thin silicon carbide (SiC) nanowires (NWs). These NWs are lithographically patterned from a 50 nm thick SiC layer heteroepitaxially grown on single-crystal silicon (Si). Several generic designs of in-plane electrostatic SiC NW switches have been realized, with NW widths as small as approximately 20 nm and lateral switching gaps as narrow as approximately 10 nm. Very low switch-on voltages are obtained, from a few volts down to approximately 1 V level. Two-terminal, contact-mode "hot" switching with high on/off ratios (>10(2) or 10(3)) has been demonstrated repeatedly for many devices. We find enhanced switching performance in bare SiC NWs, with lifetimes exceeding those based on metallized SiC NWs.

Effects of biomedical sterilization processes on performance characteristics of MEMS pressure sensors.

The effects of steam and gamma sterilization on the performance of bulk-micromachined pressure sensors were investigated using a variable pressure setup. Commercially available piezoresistive MEMS (microelectromechanical systems) pressure sensor die were characterized prior and subsequent to sterilization over a 0-500 Torr pressure range. The effects of sterilization were examined as changes in sensor output voltage (DeltaV) at various applied pressures. For steam sterilization, DeltaV decreased with applied pressure ranging from +0.27 mV at 100 Torr to -0.14 mV at 500 Torr. In contrast, the corresponding values for gamma-sterilized sensors were lower, decreasing from +0.01 mV 100 Torr to -0.06 mV at 500 Torr. The increased variation in DeltaV for the steam-sterilized sensors was attributed to the formation of an oxide film, which was confirmed using energy dispersive X-ray (EDX) spectroscopy. Statistical analysis revealed that the effect of both sterilization procedures on sensor performance was insignificant.