The pill is a dome-ended cylinder 32mm long and 12mm in diameter. From its central section, four VGA cameras, each with four illumination LEDs, face outward to provide a 360° sideways view at 4frame/s, while a gold plated hemispherical electrode at each end provide an electrode pair that can couple signals into local body tissue, whose feint traces can be picked up at the body surface where a vest with eight skin electrodes receive them – a scheme dubbed ‘body channel communication (BCC). Power comes from a pair of 1.55V 55mAh cells.
Key to operation is a sophisticated transceiver scheme implemented in two custom chips, developed between KAIST and Kangwon National University, one in the pill and one per electrode in the vest.
Pill-to-skin image up-link transmission, after compression through a DPCM encoder, is over two bands: QPSK modulation (DAC-based) at 40MHz and BPSK at 160MHz. Control codes travel in the other direction separately through an OOK-modulated low-speed data link.
The four image sensors are enabled sequentially with 2% duty cycle onto the up-link, consuming <1mW from 1V to send all the images, then the chips at both ends switch into location mode for part of the remaining cycle.
In this mode impedance is initially used as a proxy for distance, allowing the four closest skin electrodes to be adaptively selected for more accurate location which finally uses a Levenberg-Marquardt algorithm on cost functions to extract actual position from the various impedance values.
For this scheme to work, the vagaries of electrode-to-skin resistance are reduced using a further live measurement scheme. Up to 10kΩ of contact impedance variation is measured by injecting a 1.25MHz current.
“With contact impedance compensation, average localization error decreases from 4.89cm to 0.98cm,” said the team at the International Solid State Circuit Conference (ISSCC) in San Francisco, where the pill was revealed.
The 4x4mm pill chip was fabricated in 65nm mixed-signal CMOS process and consumes 0.8-1.7mW, equating to an efficiency of 0.022nJ/bit. The received chips, also 4 x 4mm, consume 8mW. ISSCC paper 17.2 4 ‘Camera VGA-resolution capsule endoscope with 80mb/s body-channel communication transceiver and sub-cm range capsule localization’.
Photo from ISSCC 2018 digest of papers, with thanks to the IEEE.