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Introducing FlexiSens, one of the smallest open source wearable devices for electrochemical sensing.
With only 30 mm of diameter and 10 mm of height that device has up to 7 days of autonomy (estimated, depending on the number of measurements).
How did we make it ?
FlexiSens main features :
- A two PCB design with minimal clearance to minimize the volume of the electronics while separating the MCU PCB and the Sensor PCB. It increases the reliability and resilience of the device by enabling the change of only one of the PCBs, also reducing extra development costs.
- Use of the low power Nordic Semiconductor nRF52840 MCU for BLE and NFC and control over the sensor and ICs.
- AD5940 electrochemical analog front end to perform accurate measurements of extra low (nA orders of magnitude) currents through three-electrode setups. Coupled with LDOs and ferrite beads to minimize power supply noise.
- FFC/FPC connector for electrodes to make the removal and change of an electrochemical cartridge easy.
- High energy density Lipo battery CS-SMR950SH, 3.88VDC 1.01Wh (charged at 4.35V).
- External NFC antenna (for BLE authentication and to send encryption keys) from MOLEX (1462362151).
- 0.8 mm thick PCBs made by JLCPCB (with ENIG treatment for chemical inertia and a lead-free finish).
Under CERN-OHL-S license.
The programming is done by SWD with an nRF52840-DK and can then be done by USB with MCUboot + DFU.
The USB permits sending data to a computer and performing practical measurements like standalone electrochemical lab equipment. Measurement gain is set by RTIA (internal selectable, or external for higher accuracy), and the AFE self-calibrates against an external precision reference (RCAL).
It was successfully used during the SensUs2026 competition by the ParisBioTek team. Its known issues are the following :
- Major overheating on the battery charger caused by a too high current input drawn on the USB supply. It looks like because of the small package and LDO it dissipates too much heat during battery charge. (to be solved)
- Power enable resistor shorted (already replaced from the PCB design with a 0ohm one) (solved)
Needed components to build that Wearable not found on JLCPCB :
- NFC antenna : MOLEX1462362151
- Lipo Battery : CS-SMR950SH
- Development board for SWD : NRF52840-DK
- Wire Find schematics here : https://oshwlab.com/anonymousse_1/project_ztutahwx
Device in use during SensUs2026 :
PCBs :
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