IEEE Solid-States Circuits Magazine - Fall 2021 - 31

depths only [25]-[27], where the surrounding
tissues have less impact
on harvesting efficiency. The recent
works for deep tissue operation either
isolate the sensor in an air-filled tube
before implanting it [28] or do not provide
energy-harvesting functionality
at all [29]. Additionally, these recent
works mainly focused on the energyharvesting
efficiency at the expense of
limited data rates (<1 Mb/s) trading off
bandwidth with sensitivity.
The work in [30] describes μmedIC,
shown in Figure 9(a), the first batteryless
microimplanted system capable
of self-reconfiguration for energy
harvesting and backscatter communication
inside tissues. The system
employs a reconfigurable architecture
with programmable antennas, harvesting
circuits, and backscatter throughput,
as illustrated in Figure 9(b). The
design also introduces a rate-and-resonance-adaptation
protocol for wireless
microimplants. This article also
presents a prototype implementation
with an IC on a flexible antenna substrate
and an evaluation of different
tissues (see Figures 9 and 10).
Cross-Tissue Reconfigurable
Rectenna
To adapt to different environments,
the rectifier and antenna optimized
structure, rectenna, employs dual
6-b, binary-weighted capacitor tuning
for both antenna resonance as well as
harvester matching reprogrammability.
The antenna utilizes a two-loop
coupled structure, where the first tunable
capacitor bank acts as a differential
load to the outer-coupled loop, as
shown in Figure 9(a). The harvester
tuning employs the second capacitor
bank in parallel with the rectifier to
tune the input impedance of the chip
(Figure 11) and achieves matching
with the adaptive antenna to guarantee
maintaining a high energy-harvesting
efficiency for powering up.
Figure 12(a) shows the harvested
voltage of a rigid design, i.e., when
the tuning capacitor (of the antenna)
and the matching capacitor are fixed.
The figure plots the harvested voltage
as a function of the frequency
of the signal transmitted by the
reader. The same measurement was
repeated across five tissue environments:
oil solution, saline-water solution,
lean meat tissue, fatty meat
tissue, and a meat tissue mixed with
bones. The figure also plots a solid
black line around 0.65 V, which indicates
the minimum voltage required
to turn on the low-dropout voltage
regulator. The figure shows that for
this fixed configuration, the microimplant
can be powered up in the
Industry, Science, Medicine (ISM)
band only when it is placed in the
oil-based tissue (red curve), where
the harvested voltage is above the
minimum threshold. For all other
tissues, the peak is either shifted
outside the ISM band (the shaded
gray region), or the rigid design is
completely unable to power up due
to low efficiency.
Channel-Aware Rate Adaptation
Bringing bit-rate adaptation to inbody
microimplants is desirable for
two main reasons. First, as the person
moves (or as the microimplant moves
inside the body), the wireless channel
changes and the bit rate must adapt
to it. A second and equally important
reason arises from the relationship between
power consumption and backscatter
bit rate. Specifically, higher bit
rates consume more power because the
oscillator needs to be driven at a higher
frequency. (The overall power consumption
P is directly proportional to bit rate
f according to
Pf ,CV2
=
where C is the
capacitance and V is the voltage.) Thus,
the bit rate also needs to be adapted to
the harvested energy to ensure that the
microimplant does not consume all of
its energy and die off.
In summary, this work presents
a platform for cross-tissue implants
through the use of reprogrammable
structures in both the antenna elements
as well as the on-chip matching
components. Rate adaptation is incorporated
to adapt to different channel
conditions, providing the most optimal
availability and throughput.
Prolonged Energy Harvesting
for Ingestible Devices
Thanks to recent advances in ingestible
electronics, it is now possible
to perform video capture, electronically
controlled drug release, pH,
temperature and pressure recording,
and heart rate and respiration monitoring
from within electronic, pill-like
capsules placed in the GI tract. Recent
progress in energy harvesting and
wireless power transfer is offering
new options to power these devices,
but many are not well suited to ingestible
capsules. For example, traditional
harvesting sources such as thermal
and vibration energy harvesting are
IFG
BIAS
2.1 mm
SENSE
V-to-I
SAR
ADC
Comp
DAC
DIG
EXC
2 mm
FIGURE 8: The copackaged chip-on-board prototype of an IFG sensor and readout circuitry,
which can be cointegrated on silicon, as shown in [15], [16] and [23]. DIG: digital; EXC: excitation;
SAR ADC: successive approximation register analog-to-digital converter.
IEEE SOLID-STATE CIRCUITS MAGAZINE
FALL 2021
31

IEEE Solid-States Circuits Magazine - Fall 2021

Table of Contents for the Digital Edition of IEEE Solid-States Circuits Magazine - Fall 2021