Circuits for a cubic-millimeter energy-autonomous wireless intraocular pressure monitor

Circuit blocks for a 1.5 {\rm mm}³ microsystem enable continuous monitoring of intraocular pressure. Due to power and form-factor limitations, circuit blocks are designed at nanowatt power levels not completely explored before. The system includes a 75% efficient 90 nW DC-DC converter which is the most efficient reported sub-$\mu {\rm W} converter in literature. It also includes a novel 4.7 nJ/bit FSK radio that achieves 10 cm of transmission range at 10 ^{-6}~{\rm BER} which is also the lowest number reported for short-range through-tissue wireless links for biomedical implants. A MEMS capacitive sensor and \Sigma \Delta capacitance-to-digital converter measure IOP with 0.5 mmHg accuracy. A microcontroller processes and saves IOP data and stores it in a 2.4 fW/bitcell SRAM. The microsystem harvests a maximum power of 80 nW in sunlight with a light irradiance of 100 {\rm mW}/{\rm cm}² AM 1.5 from an integrated 0.07 {\rm mm}² solar cell to recharge a 1 {\rm mm} ² 1 \mu {\rm Ah} thin-film battery and power the load circuits. The design achieves zero-net-energy operation with 1.5 hours of sunlight or 10 hours of bright indoor lighting daily.