An Implantable Stimulator With Safety Sensors in Standard CMOS Process for Active Books

This paper presents a second-generation integrated circuit for the
Active Books
neural stimulation microsystem. It provides multi-channel stimulation with versatile control of stimulation profiles and reduced crosstalk from other stimulation channels. The new design features enhanced safety by monitoring the temperature and humidity inside the micropackage, and the peak electrode voltage at any stimulating electrode. The humidity sensor uses an interdigitated capacitor covered by a passivation layer and a polyimide covering. To boost sensitivity in the operating range of interest, the temperature sensor uses a temperature-insensitive current that is subtracted from a proportional-to-absolute-temperature current. A 3-b analog-to-digital converter is used to record the peak electrode voltage. All sensor data is sent to an implanted central hub using bidirectional connection with error checking. Both the stimulation electronics and sensors are integrated on a 6.2 mm
$\times $
4 mm silicon die using XFAB’s 0.6-
$\mu \text{m}$
CMOS high-voltage process. No post-processing steps are involved. The stimulator uses a five-wire cable to provide the power supply and bidirectional data signals. The chip operates from a 7.5–18 V power supply and can generate stimulation currents of 1 mA, 4 mA or 8 mA with a pulse duration of 2
$\mu \text{s}$
–1.07 ms. The humidity sensor output varies linearly with relative humidity (RH) with a normalized sensitivity of 0.04%/%RH over the range of 20–90%RH. The temperature sensor has a nonlinearity of 0.4% over the range of 20–90 °C and a resolution of 0.12 °C. The stimulator is the first of its kind to include integrated temperature and humidity sensors.