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Background

Microresevoir Theory

Fabrication(1,2)

Microreservoir actuation

Integrated Microchip device

Advantages

References

Microreservoir actuation

Electrothermal microchip system

            A multimedia video of the electrothermal-based device can be seen through the hyperlink given below located at www.mchips.com , video rights belonging to MicroCHIPS, Inc. The video shows the nearly immediate degradation of the anode membrane when an electric potential is applied, and the subsequent exposure of the entrapped chemical within the reservoir.

MicroCHIPS, Inc. implantable microchip demo

             According to Maloney and Santini et al. [9], membrane failure can occur within 5 µs for the specified dimensions given and occurs within a current range of 0.3-5.6 A. By fabricating larger cross sectional areas for the traces than for the reservoir membranes and by incorporating different materials within the two, significantly increased the power efficiency. This is an important feature to the device since implantable electronic devices ideally should provide high power efficiency at very small sizes.
           The surrounding environment of the microchip can determine how much current is required to activate the membrane. In water, the thermal conductivity is 0.6 W/m K, and requires more current to activate the membrane than in air, which has a thermal conductivity of 0.026 W/m K.
            Light micrographs and scanning electron micrographs of membrane activation occurring for Au and Pt/ Ti/ Pt reservoir membranes at different nominal currents are shown below, respectively. Rather than using the instantaneous current that is variable with time, the nominal current was used and is calculated as the circuit resistance measured before activation divided by the applied voltage. The nominal current also represents the maximum possible current through the circuit since membrane resistance increases during activation.  

Figure: Membrane activation for Au membranes at different nominal currents. (Picture Rights belong to [9]) 

            For Pt/Ti/Pt membranes, less current was requires to activate the membrane as opposed to Au membranes. This is due to the higher resistivity Pt/Ti/Pt exhibits for any given current. The sides of the Pt/Ti/Pt membranes were found to remain intact after activation for currents less than 1.4 A. This is attributable to the larger thermal conductivity of Au. The overall results concluded Pt/Ti/Pt would produce a more favorable membrane as opposed to Au due to the lower current needed to open membranes.

Figure: Membrane Activation for Pt/ Ti/ Pt membranes under different nominal currents. (Picture Rights belong to [9]) 

            The electrothermal device is dependent on local resistive heating of the reservoir membrane material, and thus could produce some form of dissipated energy or heat exposure to the surrounding environment, and/or drugs or chemicals enclosed within the reservoir. The exposure of dissipated energy is distributed to both sides of the membrane and into the silicon substrate, and thus decreases some of the exposure to the surrounding environment or reservoir containments. The possibility of harmful thermal exposure due to membrane activation is assumed to be minimal since the current is applied in a very short time interval and the potentially affected volumes are very small.[9]