Diabetes is a widespread metabolic disorder, and having it puts people at increased risk for heart disease and stroke. There are two types of diabetes patients: type 1 patients for whom the pancreas is not producing any insulin, and type 2 patients for whom the body cells have a resistance against insulin. Patients with type 1 need continuous monitoring of their glucose value and an insulin pump that injects insulin into the body via a catheter in the blood stream. The usual procedure is that the patient measures his or her glucose value using a drop of blood, a test strip, and a monitor. Then, the patient adjusts the insulin pump, according to the value. There are several approved continuous glucose measurement (subcutaneous measurement) devices available, but they usually are used for only a few days time.
Developing the Microchip Technology
Senseonics, Inc., Germantown, MD, developed a sensor based on fluorescent technology to be used in an implantable continuous glucose measurement device. Zentrum Mikroelektronik Dresden AG (ZMDI), Dresden, Germany, partnered with Senseonics and developed a new microchip for use in that device. ZMDI’s design specifications for this application-specific integrated circuit (ASIC), which is implemented as a system- on-a-chip (SoC) for control and analysis had to meet the following main requirements: LED driver, measurement and analysis of reflected light, data pre-processing, memory, wireless interface for data transfer, no battery due to extremely low power and low voltage requirements, medical certification, and special form factor.
The ZMDI engineers selected a semiconductor technology that is usually used for hearing aids. This technology can use an operation voltage as low as 0.85V and has very low leakage currents. Two additional important criteria were the integration of electrically erasable programmable read-only memory (EEPROM), which is used to store small amounts of data that must be saved when power is removed, and on-chip photodiodes using this technology. (See Figure 1) Based on the selected technology, the following architecture has been defined:
ISO15693 (RFID) Front-End: For energy harvesting and for communication to an external reader, a wireless radio-frequency identification (RFID) interface based on the ISO15963 standard was implemented. During the development of the circuit, extreme care was taken to ensure that changes in the power supply did not cause unwanted backscatter feedback in the wireless communication. The wireless interface protocol between the chip and the reader was implemented via a “hard-wired” logic designed with a focus on minimum power consumption.
Measurement-Control State Machine: The ASIC’s measurement- control block controls the analog interface unit. It also provides communication between the analog front-end and the ISO controller. The unit was designed with hard-wired logic in order to reach an optimum size vs. performance.
Analog Interface: The ASIC’s analog interface block contains the LED driver, which is controlled by a 3-bit digital-to-analog converter (DAC). The DAC allows switching on the LED smoothly so that it does not cause abrupt voltage drops. The sensor signals from the integrated temperature sensor, the integrated photodiode, the optional external photodiodes, and the actual field strength of the supplying magnetic field are measured via the input multiplexer, transimpedance amplifier, and 11-bit analog-to-digital converter (ADC). The resulting data is transferred by the measurement-control state machine to the RFID interface and transmitted to an external reader.
A primary focus of the ASIC development has been power management for the chip. Because the design of the complete system is very size-limited and the receiver antenna in the implant is very small, the efficiency of the energy transfer from the reader to the device is very low, which causes only a limited amount of energy to be available for the device. Therefore, extreme care was taken to reduce power consumption in the chip as much as possible. It is also very important not to disturb the wireless communication due to abrupt voltage drops when the analog interface and, especially, the LED driver turn on. The system is designed so that blocks are only powered up if their functionality is needed.