Saturday, April 1, 2023

Implantable Drug Delivery System

An implantable system-on-a-chip with integrated drug reservoirs for drug delivery has been developed recently. The reservoirs may be opened by wireless command, enabling the number of reservoirs opened to be precisely controlled. Here is what goes inside this drug-delivery system and how it works. -- Rathindra Nath Biswas

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The received command sequence is composed of start/stop bits, address code and a preamble code. The address code is used to specify which branch at the decoder output will be injected with current. The preamble code is used as an activation key sent to a comparator so that the comparator can check the preamble against a default examining code. If these two codes are identical, an enable bit is sent to the decoder and current injection to the designated reservoir initiated according to the address code.

POR circuit. A POR circuit is implemented in the system to reset the register so that the previous values stored in the MCU registers do not activate drug release by mistake.

Regulator. The OOK receiver (1.8V) is biased by a voltage regulator, whereas the other sub-units of the SoC are directly biased by the battery. Controlled by the MCU, the regulator alters its output voltage and switches the receiver between switching mode and quiet mode.

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The regulator consists of a startup circuit, a bandgap reference and an error amplifier. The function of the bandgap reference is to generate a temperature-insensitive voltage. A start-up circuit is implemented to prevent the reluctance of the bandgap reference circuit to turn on by the supply voltage. The error amplifier, as a output stage of the regulator, uses a negative feedback to lock its output DC voltage to the reference voltage generated by the bandgap reference, making the output voltage very stable.

At drug-release activation, a DC voltage of 3V is applied to the electrodes, which then conduct a 2mA current into the solution. After activation, microbubbles occur and diffuse from the rupture of the membrane. Only 1.5 mA is needed to open the reservoir after the opening command is given. A Li-ion nanowire battery with an energy density of 7 mA/mm3 and a button cell of 4.5mm diameter and 2mm height (32 cubic mm volume) can provide a total energy of about 403 mWh, enabling the device to power the system for 53 hours. The receiver is turned on for 1 μs every half second to conserve power.

Biocompatibility of the implant is very important. The surface of the device is made of silicon and silicon nitride, while for the packaging material titanium is used. These three materials are biocompatible.

The author retired from Metallurgical & Engineering Consultants (I) Ltd as deputy general manager, I/C



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