FIGURE 4 (A) Injectable fibre sodium-ion battery for all-region
power supply in vivo. Reproduced with
permission.[26] Copyright 2021, Royal Society of
Chemistry. (B) Biodegradable and rechargeable sodium-ion batteries with
fibre configuration. Reproduced with
permission.[25] Copyright 2021, Royal Society of
Chemistry. (C) A tissue-like soft all-hydrogel battery powering a
hydrogel strain sensor for the heartbeat monitor. Reproduced with
permission.[90] Copyright 2022, Wiley-VCH GmbH.
(D) Biodegradable rechargeable zinc ion battery with controlled
degradation and stable electrochemical performance. Reproduced with
permission.[91] Copyright 2022,
Wiley-VCH GmbH.
Combined with bioresorbable
features, the syringe-assisted injectable bioresorbable fibre battery
can provide a better solution for the non-invasive implantation of
electronics with low health risks. For example, Mei et al. demonstrated
a biocompatible aqueous sodium-ion battery that can be injected into the
body without immune responses and degraded after completing the
mission.[25] They utilised biodegradable materials
to construct the fibre battery, the polydopamine/polypyrrole composite
as anode and MnO2 as the cathode was coated on the
conducting fibre and the
biodegradable chitosan was applied as the separator. After twisting the
electrodes fibre together and injecting it into the abdominal subcutis
of an experimental mouse, with the body fluid act as the electrolyte,
the fibre battery showed a specific capacity of 25.6 mAh
g-1, and a good cycling stability with retention of
69.1% after 200 cycles. As demonstrated as a power source for the
biosensor, the fibre battery succeeds in detecting the pressure changes
in the implanted area and is biodegraded as designed through hydrolysis
and enzymolysis. The flexible battery also showed excellent stability
during the pressing, bending and stretching as can be observed in Figure
4B, the biodegradable fibre sodium-ion battery was finally fully
biodegraded after several weeks avoiding the need for surgery for
removal. The minimised injectable batteries are very promising choices
for IMEs due to their better biocompatibility, longer service time and
better stability inside the human body.
With a coplanar structure, the
ultrasoft rechargeable batteries with low Young’s moduli based on
lithium ion and zinc ion were exploited. Ye et al. proposed tissue-like
ultra-soft batteries based on an all-hydrogel design based on the
lithium-ion battery and zinc-ion battery
system.[90] With the interfacial dry crosslinking
strategy, superb electrical conductivity and high interface charge
transfer efficiency were achieved. Benefiting from this strategy, the
lithium-ion battery-based all-hydrogel battery achieved high specific
capacities of 82 mAh g-1 and the all-hydrogel zinc-ion
batteries represented 370 mAh g-1 at a current density
of 0.5 A g-1. While integrated with a hydrogel strain
sensor, the all-hydrogel battery can supply a stable power output for
the heartbeat monitor while precisely detecting the strain change
(Figure 4C). In addition, a
biodegradable rechargeable zinc ion battery with controlled degradation
and stable electrochemical performance was
reported.[91] In this battery, a biodegradable
cellulose aerogel-gelatin solid electrolyte was designed, a
biodegradable highly flexible silk protein film was used as passivation,
and the zinc thin-film battery
achieved a stable output voltage of up to 1.6 V. After implantation, the
degradation in subcutaneous area of rats can be observed in Figure 4D.
The complete degradation of this thin-film battery in vivo demonstrated
a non-toxic and harmless implantation procedure to the hosts (rats) and
an advanced power source for clinical electronics.
3.1.3. Supercapacitors
With fast charging and discharging rates, high power density
(>10 kW kg-1), and splendid cycling
lifespan (>100,000 times), supercapacitor bridged the gap
between electrolytic capacitors and rechargeable batteries. Such kind of
energy storage devices can act well as an energy reservoir for
electronics when unexpected interruptions in the power source occur,
which showed exceptional rate capability and good
stability[92-93]. Depending on different
mechanisms, supercapacitors can be divided into two types. One is
electrical double-layer capacitance (EDLC) relying on ionic
electrochemical adsorption/desorption at the electrode/electrolyte
interfaces, another is pseudo-capacitors depending on pseudo-capacitance
from rapid redox reactions occurring on (or near) electrode surfaces.
Due to the uncertainty of the pathological characteristics of most
biocompatible materials, and the discontinuity of electrode elements
under the complex and variable surface strain, new degradable energy
devices often show unpredictable performance and huge safety risks in
the diagnosis process. Therefore, with minimised
dimension,[94] good flexibility and strong
selectivity of non-toxic electrode materials, microscale supercapacitors
have a bright application prospect in the area of medical auxiliary
energy sources.[95] At present, wearable devices
usually use micro-batteries and micro-supercapacitors as energy supply
and have the potential to be integrated with electronic devices in a
small size. For microelectronics requiring long-term service and
high-power density, micro supercapacitors would be a good choice and
therefore have drawn wide attention. In the field of biomedical
electronics and sensors, micro-batteries as energy supply inevitably
suffer from frequent replacement due to the short cycling life of about
several thousand times. Comparatively, micro supercapacitors represent
excellent cycling stability with negligible capacitance decay, which
effectively avoids the frequent replacement issue. Also, unlike micro
batteries, micro supercapacitors with higher power density with small
volumes are more suitable for flexible IMEs requiring high power density
than batteries requiring additional integration to achieve high power
density.[96-97] However, conventional
supercapacitors also suffer from rigid and heavy structures, unstable
electrolytes and rigid sealing before implantation, which is unfit for
IMEs. Some researchers reported strategies that body fluid was used as
electrolytes without encapsulation. According to Chae et al., a
conceptual system adopts solar cells as energy supply and supercapacitor
as energy reservoir, MnO2 and carbon were used as
electrodes inserted into the subcutaneous area of a rat, and biofluids
were used as electrolytes (Figure5A ).[98] The working voltage window of
the implanted supercapacitor is from 0.2 to 1.0 V and good cycling
stability was achieved with over 1000 cycles. Similarly, as shown in
Figure 5B , with biocompatible carbon nanotube fibres as
electrode material and biofluid as the electrolyte, He et al. fabricated
supercapacitors without additional encapsulation, a specific capacitance
of 10.4 F cm-3 and long cycling stability (98.3 %
retention after 10000 cycles in PBS) were
achieved.[99] Furthermore, for the electrical
modulation of tissues and organs, bio-interfaces at the macroscopic
level are in demand. Fang et al. fabricated a flexible micro
supercapacitor-like system with micelle-enabled self-assembly approach
for various types of bioelectronic interfaces, as shown
in Figure 5C . With
interdigitated electrode design, the micro supercapacitors show a
smaller size and are more suitable for subcellular interfaces. With this
flexible micro supercapacitor-like electronic system, multifunction was
achieved including modulation of cardiomyocytes in vitro, excitation of
isolated heart and retinal tissues ex vivo, stimulation of sciatic
nerves in vivo, and bioelectronic cardiac
recording.[100] More than that, micro
supercapacitors have also been applied to assist wireless power transfer
for the continuous operation of electronic devices such as smart contact
lenses due to the long cycling lifespan and high-power density. With a
wireless charging antenna continuously receiving external inductive
power, the integrated micro supercapacitors can store energy and enable
independent operation of the smart electronic device inside the human
body. Park J et al. reported a soft smart contact lens integrated with
an antenna for wireless recharging and a solid-state supercapacitor for
energy storage.[101] The monolithically integrated
solid-state supercapacitors showed excellent cycling stability up to
10,000 cycles and a stable temperature was maintained during wireless
operation ensured the safety of the wearer’s eyes as illustrated in
Figure 5D . Furthermore, fibre supercapacitor with high aspect
ratio structure and micro diameter was constructed and applied for
surgical suture for wound stitching and implantation in the blood
vessel.[27] The highly flexible and conductive
electrode fibres was fabricated with biocompatible poly
(3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS)/
ferritin nanoclusters trapped multiwalled carbon nanotube (MWNT) sheets,
which enabled the fibre supercapacitor operate well in a mouse with
excellent biocompatibility. An areal capacitance of 32.9 mF
cm-2 in PBS solution was achieved and above 90% of
capacitance was maintained in the mouse after 8 days implantation as
illustrated in Figure5E .
Though the supercapacitor suffers from low energy density, good cycling
stability and high power density are highly desirable as an assistant
energy storage device. It is promising that the miniaturised micro
supercapacitors in collaboration with the wireless charging technologies
will support the sustainable long-term service of the IMEs system.