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Background

Microresevoir Theory

Fabrication (1,2)

Microreservoir actuation

Integrated Microchip device

Advantages

References

     As of 2006, there are two forms of the microreservoir chip system. The first was created around 1999 by Santini et.al. [6] and utilizes an electrochemical reaction to release drugs. The second form of the device was proposed around 2005 by Maloney and Santini et.al. [9] and is developed from the original theory. The device however, utilizes an electrothermal reaction to release drugs. The first microreservoir system will be discussed in order to provide a basic foundation of the theory behind the device, and the second will be discussed to show the advancement and development that the device underwent.

Theory of Operation: Electrochemical microreservoir system

            The microchip is a device that consists of an array of reservoirs that extend through an electrolyte impermeable substrate. Reservoirs are sealed on one end by a thin membrane of material that acts an anode in an electrochemical reaction that dissolved when an electrical potential is applied to it within an electrolyte solution. Another electrode is placed on the device to act as a cathode that assists in the electrochemical reaction. The cathode electrode is made of the same conductive material as the anode, and can be designed in any particular shape or size as desired for the specific application. The reservoirs are filled through the open end with the desired drug or chemical. The open end of the reservoir is then sealed with a water proof material that acts as a barrier membrane keeping the drug or chemical enclosed within the reservoir until the top or surface anode membrane is dissolved. The device must be submerged within an electrolyte containing ions in order to allow the formation of a soluble complex with the anode materials in its ionic form. When an electric potential is applied to the anode membrane, the membrane material oxidizes and forms a soluble complex with the electrolyte ions. The soluble complex dissolves in the electrolyte and the membrane vanishes. The now opened reservoir exposes the chemical or drug contained within to the outside electrolyte. Exposure allows the drug to diffuse out of the reservoir and dissolve into the electrolyte surroundings.[1]

Figure: Electrochemical chip system. Picture rights belong to [1].

Theory of Operation: Electrothermal microreservoir system

            The main components of the electrothermal system include the reservoirs, the membrane covering the reservoirs, and traces that act as a pathway for current to reach the reservoir membrane. Although initial prototype devices utilized electrochemical corrosion of the anode membrane that depends on the surrounding medium to release drugs, this latter development by MicroCHIPS, Inc. utilizes electrothermal methods. The device is similar to the electrochemical based device and employs a metal membrane over each reservoir; however an electrothermal method removes the reservoir membrane by local resistive heating of an applied current through traces and thus is independent of the chemistry of the surrounding medium. [8] Since local resistive heating occurs by applying current through traces leading to the reservoir membrane as opposed to applying an electric potential, there is no need for a cathode membrane in the electrothermal microreservoir system.

            Figure: Cross sectional diagram of microchip device and reservoir for an electrothermal chip system. (Picture rights belong to [9])