Fabrication of wireless readout electronics:

Designing the electronics layout with e-CAD software and cutting with a LPKF Protolaser U4 yielded a double-sided flexible electronics board cut in a 75 μm Cu / 18 μm PI / 75 μm Cu sheet (Pyralux, DuPont). Soldering of the following components then yielded a functional electronics readout platform (once connected magnetically to a Li-Polymer battery (12mAh to 45mAh)): nRF52832 (Nordics Semiconductor) Bluetooth Low Energy chip, 2.4Ghz ceramic antenna (Johanson Technology), ISL9016 (Renesas Electronics) LDO to reduce operating voltage to 3.3V, FDC1004 (Texas Instruments) capacitance-to-digital converter, as well as multiple passive components (402 and 201 sizes). High temperature neodymium magnets soldered to the board facilitated reversible connectivity between the modules: four 0.04” thick 0.1” diameter magnets (McMaster) for connection to the electrodes and shield, and two 0.06” thick 1/8” diameter magnets (McMaster) for connection to the battery. Magnets of opposite polarity soldered on the electrodes and on the battery allowed for a secure connection, with an alternation in the orientations of the magnets so that only one mounting position is possible. The flexible board and the battery were then encapsulated in thin low modulus silicone shell (Silbione 4420 RTV, Elkem Silicones, mixed with Silc-Pig silicone dye, Smooth-On) to yield in a soft platform without exposed electronic components or wiring. Validation of the performance of the electronics was performed using capacitors of known values. Measurements on the electronics readout part (C) with a power profile system show that the average current is 780 μA when connected to the BLE host at the operating frequency, which allows to run multiple exercising sessions with one charged battery.