Blood Glucose Meter tear down: Contour Next One

A recent study by the Diabetes Technology on a cohort of 1000 people found that of 18 popular glucometers in the US, only 6 met the FDA standard for accuracy:

Investigation of the Accuracy of 18 Marketed Blood Glucose Monitors. David C. Klonoff, Joan Lee Parkes, Boris P. Kovatchev, David Kerr, Wendy C. Bevier, Ronald L. Brazg, Mark Christiansen, Timothy S. Bailey, James H. Nichols and Michael A. Kohn. Diabetes Care 2018 Aug; 41(8): 1681-1688.

and another good study on accuracy here: Performance Evaluation of Three Blood Glucose Monitoring Systems Using ISO 15197

I was surprised by 2 things:

  1. Of course that only 6 BGMs met the standard….
  2. The FDA standard itself which specifies accuracy of only +- 15mg/dL at or below 100mg/dl and +-15% above that number. If your fasting blood glucose was measured at say 95mg/dl, that could mean to a 95% confidence level you are anywhere from 80mg/dl (relatively close to hypoglycemia) to 110mg/dl (pre-diabetic).

I decided to take a look inside the best scoring BGM: Contour Next One. Basically it looks like this:

 PCB top side

Top side features a simple connector housing for the strip insert, a Toshiba custom analogue front end and an MCU.

A decap of the MCU reveals that it is from Renesas and is likely a R5F51135ADLJ (128KB ROM flash, 32KB RAM, 8KB data flash). This is based on Renesas’ proprietary RX CPU architecture which is a big change from the last generation of Bayer BGM which used an STM32F103 ARM-based MCU. Someone did a complete code re-write to port the code over from the ARM code of the last generation. Given that the peripheral set, performance and power consumption of the 2 MCUs are not massively different, there must have been a compelling price reason for doing so. Alternatively, perhaps enough of the code needed to be rewritten to support real-time Bluetooth comms for the first time that it was worth revisiting the whole code set?

 Renesas decapped die shot

Contour test strips use an enzyme glucose dehydrogenase that catalyzes the oxidation of blood glucose. The reaction produces a measurable current, however given the tiny 0.6uL blood sample size, that current is only in the single digit nA range. The analogue front end therefore needs low leakage, low drift, low noise trans-impedance amplifiers as well as stable voltage reference <50ppm/degC. Also, since the rate of the enzyme reaction is temperature dependent, temperature measurement accurate to 1-2 degC is needed. Likely 12bit ADC would be used to set the strip reference electrode voltage and amplifier bias voltage. Finally a 14bit ADC for the working electrode.

The Toshiba device is the same one as prior generations of the BGM and probably features low noise current amplifiers and ADCs. It seems to be a custom or ASSP since the device is not in their public catalog. However, the NJU9101 Japan Radio analog front end is probably quite similar since it was designed for the same application:

In their FDA 510(k) filing here, Bayer talk about their “MultiPulse” algorithm current is measured at several points in time allowing for mathematical compensation against hematocrit and temperature effects.

The ST memory is a 32Kb serial EEPROM probably for storing the blood glucose log.

 PCB bottom side

Back side, the device of note is the TI CC2540 which is a Bluetooth Low Energy (BLE) controller. This device has an 8051-based CPU and a USB controller, but I think in this design is being used only as a standalone Bluetooth modem. The 2.4Ghz antenna is a PCB strip that runs alongside the glucose strip connector. And of course the buzzer.

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