Medical Design Briefs - January 2023 - 30

safety mechanisms and interior packaging
to protect them from the ambient
environment and electrical conditions
(overvoltage and overcurrent). Solidstate
lithium microbatteries don't need
these structures, and further improve
VED using an ultra-thin (10-µm) stainless
steel substrate and ultra-compact
stacking and packaging of the battery's
energy-producing cells. The higher VED
enables designers to, for instance, create
smaller hearing aids that integrate more
features, and health tracking rings that
monitor more sensor input over longer
periods before recharging.
The same anode-less architecture offers
faster charging speed than Li-ion and
Higher VED enables designers to create smaller hearing aids that integrate more features.
n Wearable Device Requirements
Many wearable requirements are driven
by the photoplethysmography (PPG) technology
used to measure the volumetric
variations of blood circulation in health
and fitness monitoring applications. Because
PPG sensors measure how light
shined into the body is scattered from
blood flow, they work better in some body
locations than others, depending on the
application. Heart rate can be monitored
well using PPG sensors in nearly any body
location, but blood pressure monitoring
may not be as accurate as necessary from
locations like the wrist or ankle because of
their bone, muscle and tendon physiology
and the effects of arm and leg movements.
Because of these dynamics, batteries
must support many different
wearable
types, sizes and shapes. Smart-ring manufacturers
have begun exploring alternative
to smart rings based on Li-ion microbatteries
because they that fit better into these
wearables' curved shape. Lithium- polymer
(Li-poly) batteries also meet this need, but
at the cost of energy density, which is half or
less that of Li-ion. Manufacturers need battery
alternatives, but they also need greater
energy density, not less.
Users have their own needs. Smaller is
better, but this reduces energy density.
They also want new features that require
batteries to do more in a smaller package so
that wearables can support, besides PPG,
electrocardiogram (ECG), accelerometer
and gyroscope sensors and more, along
with their associated communications requirements.
Users also want a comfortable
fit in many different situations at rest and
during sleep, exercise, or daily activities.
30
These and other challenges can be
solved with solid-state lithium microbattery
technology that have been adapted to
wearable needs.
n Bringing Solid-State Lithium
Technology to Wearables
One of the biggest obstacles to using
solid- state lithium technology in wearables
has been the lithium metal anode. The anode
was deposited on top of the battery
electrolyte during manufacturing, requiring
a zero- humidity and argon (AR)-based
environment.
When an anode-less solid-state microbattery
is manufactured, no anode layer is
deposited on the electrolyte. Instead, a layer
of lithium is formed between the anode
current collector and the solid-state electrolyte
after manufacturing when it is first
charged. This layer becomes the anode in
an anode-less solid-state microbattery.
Benefits range from increased energy
density and faster charging to greater
form-factor freedom and new, simpler design
and manufacturing options.
Energy density and faster charging are
among the biggest benefits. The former is
generally calculated as storage per unit
volume, or volumetric energy density
(VED). If two batteries have the same capacity
and one is twice the size, then the
smaller one has twice the VED. Tests show
that the best-performing anode-less solid-state
micro battery technology delivers
up to twice the VED of Li-ion alternatives
(see Figure 1).
VED gains come from freeing up
space inside the microbattery. Li-ion
micro batteries require space-consuming
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higher current discharge pulses for wireless
communication. Since Li-ion batteries
can only supply up to twice their rated
current, wearable designers generally use
higher-capacity barriers than needed just
to support their high pulse current discharge
requirement. Solid-state lithium
microbatteries deliver up to 10 times
their rated current. They also have lower
leakage and, therefore, longer shelf life
than Li-ion, taking up to four years or
more to fall to half their fully charged
state while a typical Li-ion microbattery
might lose its entire charge in no more
than three months.
Just as important as these performance
improvements
is
form-factor
freedom. Rather than forcing a cylindrical
shape that has been the easiest for
Li-ion battery manufacturers to build,
solid- state lithium technology enable
customizable rectangular form factors
to suit the end-products ergonomically
desired shape. Proven high-volume rollto-roll
manufacturing techniques are
used to deposit the microbattery's cathode
and solid electrolyte, after which
these unit cells are cut from the roll at
customized rectangular lengths and
widths and then stacked to the desired
height based on capacity and other requirements
(see Figure 2).
Solid-state lithium technology also offers
new ways to look at old wearable design
challenges. For the first time, developers
can optimize their wearables for either
longer battery life or expanded feature
sets, or a balance depending on the application.
They will no longer need the protection
and charging circuitry of Li-ion
batteries, since solid-state lithium microbatteries
require only a simple, low-cost
constant voltage charging solution.
Medical Design Briefs, January 2023
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Medical Design Briefs - January 2023

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