IEEE Solid-States Circuits Magazine - Fall 2020 - 44

significantly smaller for the two
reflecting surfaces, as seen in Figure 14(d), which is an indicator that
this method is less susceptible to
skin color differences.
The performance of the readout
chip was tested on the human body by
placing a fingertip on the sensor head,
as demonstrated in Figure 15(a). The
output was sampled every 5 s. During the first 90 s, O2 values were
measured with normal blood circulation. After that, for the following
90 s, blood circulation was restricted
by wrapping a string around the
finger to create an occlusion. The
average lifetime during normal circulation was 51.7 μs, whereas it was
62.5 μs during the occlusion period.
The measured results are presented
in Figure 15(b).

Discussion

skin integrity is critical to the monitors' essential requirements of safety.
For any wearable/implantable technology, power consumption by the
communication system is the main
culprit. Different energy harvesting
schemes will need to be explored
to meet the power demands, as the
presence of a continuous and robust
power supply is the main requirement for constant monitoring.

As discussed in the previous section,
emerging fluorescent-based O2 monitors demonstrate promising results.
However, there are many challenges
associated with the design, develop--
ment, fabrication, and implementation
of such devices. These difficulties
are present in the form of software
and hardware constraints, form factors, weights, low power requirements, battery life, and wireless
operation. The biocompatibility of
encapsulation, water resistance, and
a device's shape and size all play
vital roles in users' experiences and
the equipment's effectiveness. These
emerging instruments should be
small and lightweight to prevent injuries to patients. For externally applied
devices, the effect of materials on

VBATT
VDD

POR

For any wearable/implantable device
to be considered and evaluated as a
medical-grade instrument, a certain
degree of accuracy is expected, reducing the rate of false positives and, even
worse, false negatives. According to the
U.S. Food and Drug Administration,

VBIASP

VDD

VREF

BGR

Beyond the Requirements of the
Food and Drug Administration

Bias

LDO

VPCAS
VBIASN
VBIAS1-4

X2
Power Management
EN
IIND

VISEN

Current
Sense

VLED

Pt-Porphyrin Film

Oxygen

VCO

VSUM
Clock
S

SAFE

RAMPH
RAMPL
+

Driver
VDRV

VRAMP

PWM

DRVCTL

VREFOSC
VREFCOMP

LED Driver
VDIODEP
IINP

RCTL
+

VDIODEN

- +

VAFEOP

VGA

TIA
IINN

VTIAOP

VTIAON

VAFEON

Analog FE

FIGURE 13: A block diagram of the readout IC designed by the Worcester Polytechnic Institute Integrated Circuits and Systems Lab [22]. VCO:
voltage-controlled oscillator; PWM: pulsewidth modulation; BGR: bandgap reference; VAFE: output voltage of the analog FE; REFOSC: oscillator
reference voltage; REFCOMP: comparator reference voltage; EN: enable signal.

FA L L 2 0 2 0

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IEEE Solid-States Circuits Magazine - Fall 2020

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