Subnanowatt Microbubble Pressure Transducer

We present the first all-Parylene microbubble pressure transducer (μBPT). The μBPT utilizes an electrolytically generated microbubble (μB), trapped in a Parylene microchamber, for pressure sensing. Pressure-induced bubble size variation is detected by electrochemical impedance measurement. Real-time hydrostatic pressure measurement with excellent sensitivity (-10.7 Ω/psi, ±0.1 psi) up to 1.8 psi and tracking of internal pump pressure in a MEMS-based implantable drug delivery system were achieved. The transduction method and device format uniquely leverage the surrounding liquid environment, obviating the need for hermetic or special packaging techniques for sensing in wet environments. Arrayed μBPTs are biocompatible, fabricated on flexible substrates, ultra-miniature (200 μm diameter, 10 μm thick), and can be operated at very low power (≤ nW) making them especially attractive for in vivo pressure measurement applications. INTRODUCTION It is known that gas bubbles respond to external pressure [1] but few efforts exploit this phenomena for sensing. At frequencies below resonance (55 kHz for a bubble of 50 μm radius [2]), microbubbles respond instantaneously to pressure variations. Using microelectromechanical systems (MEMS) technology, miniature devices which can precisely produce, localize, and measure bubble response at very small scales can be fabricated. Polymer-based MEMS fabrication technology in combination with microbubble dynamics are leveraged here to introduce a new class of pressure transducer with unique capabilities. State-of-the-art ultra-miniature pressure sensing technologies generally occupy 400-500 μm diameter footprints. Our unique electrochemical-based pressure sensing technique does not utilize conventional diaphragm-based pressure transduction, enabling significant reductions in overall footprint, materials processing costs, and power consumption. Electrochemical impedance-based sensing can be accomplished at nanowatt power levels, making this attractive for wireless and implantable applications. Previously, pressure measurements within a microfluidic channel were demonstrated using μB-based transduction [3], however this approach utilized rigid substrate materials and thus was not amenable to wide application (e.g. sensing small pressure variations in vivo). Here, Parylene C is featured as both the flexible substrate and structural material; this combination enables a novel sensor configuration capable of controlling bubble generation, localization, and transduction in a completely released and portable device compatible with diverse liquid environments. In particular, these features address the unmet need for robust in vivo pressure sensors. Our μB pressure sensing approach eliminates the need for hermetic packaging (for operation in wet environments) and the exclusive use of polymer materials reduces cost, adds mechanical flexibility, and facilities integration for medical applications. THEORY Microbubble Dynamics The response of suspended bubbles to changes in ambient pressure is well studied with regard to bubble dynamics and mass diffusion processes across the gas-liquid interface. The fundamental dynamics of a suspended bubble fixed in an unbounded incompressible viscous liquid are governed by the Rayleigh-Plesset equation [4]: ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − − − = + ∞ r r r p p r r r g & & & & μ σ