  The measurement of the biological tissue's electrical impedance is an active
research field that has attracted a lot of attention during the last decades.
Bio-impedances are closely related to a large variety of physiological
conditions; therefore, they are useful for diagnosis and monitoring in many
medical applications. Measuring living tissues, however, is a challenging task
that poses countless technical and practical problems, in particular if the
tissues need to be measured under the skin. This paper presents a bio-impedance
sensor ASIC targeting a battery-free, miniature size, implantable device, which
performs accurate 4-point complex impedance extraction in the frequency range
from 2 kHz to 2 MHz. The ASIC is fabricated in 150 nm CMOS, has a size of 1.22
mm x 1.22 mm and consumes 165 uA from a 1.8 V power supply. The ASIC is
embedded in a prototype which communicates with, and is powered by an external
reader device through inductive coupling. The prototype is validated by
measuring the impedances of different combinations of discrete components,
measuring the electrochemical impedance of physiological solution, and
performing ex vivo measurements on animal organs. The proposed ASIC is able to
extract complex impedances with around 1 Ohm resolution; therefore enabling
accurate wireless tissue measurements.
