TECHNICAL FIELD

In at least one aspect, the present invention is related to an apparatus for measuring electrochemical impedance.

BACKGROUND

Conductivity measurements are conventionally transmitted using a variety of techniques, with each making use of active circuitry such as microcontrollers, ASICs, or discrete components [1-6]. These components, along with an antenna or inductive coil, are typically soldered onto a printed circuit board (PCB). To mitigate cytotoxicity and the immune response, these electronics are typically encapsulated with biocompatible polymers such as Parylene C and PDMS [7-10]. However, the in vivo environment continues to pose a formidable challenge for the development of implantable microelectronic systems.

A variety of factors can induce sensor drift or failure. Water vapor and ion permeation create undesired paths for electric current and can alter critical bias voltages [11]. The inflammatory response to foreign bodies can subject the implant to a highly oxidative or acidic environment [12]. Blood flow and other involuntary movements can subject the implant to chronic and repeated mechanical strain. Such events can damage sensitive components or introduce errors in signal measurement and transmission.

The presence of the implant should also minimize harm to the host environment. The materials selected for the packaged device should be nontoxic and induce minimal immune response. The use of soft, flexible materials (i.e. those mechanically matching tissue properties) and elimination of sharp corners also serves to minimize damage to tissue [13]. Power requirements for electronics should be kept in check to minimize heating to safe levels.

To address these issues, an alternate approach would be the use of passive components. Currently, many methods exist to interrogate capacitive signals wirelessly, with notable examples of pressure, force, and strain measurement [13-15]. However, few attempts have been made to enable passive, wireless resistive measurement. One work incorporated a resistive strain gauge with a coil and capacitive element on a polyimide circuit board [16], but this was not intended for use in the wet, in vivo environment. Specifically, its coil and transduction elements were fabricated on a stiff substrate and the components are soldered together using non-biocompatible materials. A notable effort utilized eddy currents to passively and wirelessly determine conductivity and thickness of metallic layers [17]. This technique has a different purpose and is difficult to apply to in vivo measurements.

Accordingly, there is a need for improved method of making electrical conductivity and impedance measurements under wet and physiological conditions.

SUMMARY

The present invention solves one or more problems of the prior art, by providing in at least one embodiment a sensor for measuring electrochemical impedance of a medium. The sensor includes a primary circuit and a secondary circuit. The primary circuit includes a primary electrically conducting coil. The secondary circuit includes a first electrically insulating layer, a secondary electrically conducting coil disposed over and contacting the first electrically insulating layer, a first metal contact electrode, a second metal contact electrode, and a second electrically insulating layer disposed over the secondary electrically conducting coil. The secondary electrically conducting coil includes a first end and a second end that are attached respectively to the first metal contact electrode and the second metal contact electrode. The second electrically insulating layer defining openings that expose the first metal contact electrode and the second metal contact electrode so that the medium for which measurement are performed can be contacted. The primary electrically conducting coil is inductively coupled to the secondary electrically conducting coil. During operation, an alternating current source energizes (e.g., applies an alternating current to) the primary electrically conducting coil during impedance measurements.

In another embodiment, a method for measuring electrochemical impedance using the sensor set forth herein is provided. The method includes a step of contacting the first metal contact electrode and the second metal contact electrode of the secondary circuit with a liquid medium for which an impedance is to be measured. The primary electrically conducting coil is positioned proximate to the secondary electrically conducting coil such that the primary electrically conducting coil is inductively coupled to the secondary electrically conducting coil. An alternating current is applied to the primary electrically conducting coil to perform an impedance measurement.

In still another embodiment, a method for forming the sensor for measuring electrochemical impedance set forth herein is provided. The method includes a step of coating a support substrate with a first electrically insulating layer. An optional adhesion layer and a first metal layer is patterned onto the first electrically insulating layer with the optional adhesion layer if present interposed between the first electrically insulating layer and the first metal layer. The optional adhesion layer and the first metal layer are patterned into a secondary electrically conducting coil with a first contact electrode and a second contact electrode. A second electrically insulating layer is deposited over secondary electrically conducting coil. The first contact electrode and a second contact electrode are then exposed by removing a portion of the second electrically insulating layer. Finally, the support substrate is separated from the first electrically insulating layer to release a secondary circuit.

Advantageously, the noncontact method and apparatus for measuring electrochemical conductivity of the present invention can be used in wet, ionic environments such as in vivo settings. Chronic recording of physiological signals via the use of implanted sensors has been a long-standing goal for the medical and scientific community. Such sensors would enable continuous monitoring to facilitate greater understanding of disease progression as well as inform course of treatment. However, the in vivo environment poses a formidable challenge for the development of telemetry circuitry. The combination of the corrosive environment and constant movement can damage sensitive components or introduce errors in signal measurement and transmission. To minimize harm to the host environment, materials should be nontoxic and flexible. Power requirements should be kept in check to minimize heating to safe levels. Historically, wireless transmission of conductivity or resistivity has been limited to the use of active circuitry (i.e. microcontrollers, ASICs). This burdens system design by adding power constraints, thermal budget concerns, and bulk to the overall package. To address these concerns, the present invention introduces the use of reflected impedance across a pair of inductively coupled coils to achieve wireless resistive measurement. A variety of techniques based on electrochemical impedance measurement have been investigated (e.g. flow, pressure, or liquid volume sensing). This technique is widely applicable for resistive transduction and obviates the need for microcontrollers, discrete electronic components, and batteries. In addition, because of the thin film construction and simplicity, this method and sensing modality can be easily applied to a broad spectrum of applications.

