FIGURE 5 (A) A
biocompatible supercapacitor based on body fluids as electrolyte
combined with solar cells for energy harvesting and storage. Reproduced
with permission.[98] Copyright 2017, Elsevier. (B)
An miniature implantable supercapacitor based on a biocompatible aligned
carbon nanotube fibre electrode in PBS. Reproduced with
permission.[99] Copyright 2017, Elsevier. (C) A
carbon-based flexible micro-supercapacitor-like system for various types
of bioelectronic interfaces (scale bar on the top right, 5 mm).
Reproduced with permission.[100] Copyright 2020,
Springer Nature. (D) Solid-state supercapacitors powering the flexible
smart contact lenses wirelessly. Reproduced with
permission.[101] Copyright 2019, Science. (E) A
biomolecule-based fibre-type implantable supercapacitor with micro
diameter for in vivo energy storage. Reproduced with
permission.[27] Copyright 2018, Elsevier.
3.2. Human Body Energy Harvester
Many forms of energy contained in the living organism, including
chemical energy from the reaction of organic molecules
such as glucose oxidation and
mechanical energy from such as heart beating and respiration, have been
widely studied for energy harvesting in vivo from the human body. The
commonly studied internal energy-harvesting devices include the
nanogenerator (NG) converting mechanical energy (cardiovascular,
respiratory and gastrointestinal) into electrical energy by
piezoelectric and triboelectric effects, and another kind of harvester
is biofuel cells generating energy from glucose oxidation. With these
permanent energy sources in the human body, the implanted energy
harvesters can be implanted once for all and work for a long period of
time enough to support the electronics to finish the missions during the
lifetime of the hosts, which is a promising method to reduce the pain
and incision caused by a repeated second surgery.
3.2.1. Nanogenerators (NGs)
Recently, various NGs have been implanted in organisms for energy
harvesting, sensing, and stimulating nerves and
muscles.[102-103] Two kinds of energy-harvesting
devices are included: PENG and TENG. Due to the piezoelectric effect,
abundant mechanical energy can be harvested by PENG from human bodies
including essential motions such as heartbeat, gastrointestinal moving,
daily walking and breathing. Since 1880, the piezoelectric effect was
discovered by the Curie brothers, many piezoelectric materials were
studied. There are several common piezoelectric materials such as zinc
oxide (ZnO), lead zirconate titanate (PZT), barium titanate
(BaTiO3), polyvinyl chloride (PVC), poly (lactic acid)
and polyvinylidene fluoride (PVDF). However, inorganic piezoelectric
materials such as ZnO and PZT have good performance but are usually
rigid and brittle, which causes difficulty to match well with soft
tissues. The flexible ZnO nanowire fabricated by Li et al. improved the
flexibility of PENG for real-time rat heart rate monitor, the
two-ends-bonded piezoelectric nanowire generator can drive the electron
motion by mechanical deformation.[104]The output voltage and current are
usually less than 50 mV and 500 pA,
the robustness of the nanowire need to be improved with flexible polymer
to isolate it from the biological surrounding. The average output
voltage and current from the heartbeat of a live rat were about 3 mV and
30 pA (Figure 6A). While ZnO/rGO can reinforce the mechanical stress
with increased local deformation, the enhanced piezoelectric output can
be generated. According to Azimi et al., the optimized composite of PVDF
nanofibres with ZnO and rGO as hybrid filler generated an output voltage
which is nearly 10-fold increase than that of the pristine PVDF
nanofibres.[29] The optimized PENG was sutured on
the heart of an adult dog and every heartbeat can generate electrical
energy as high as 0.487 μJ which is enough to power commercial
pacemakers as demonstrated in Figure 6B. To further improve the output
current, Kim et al. fabricated a soft PENG with Mn-doped PMN-PZT
((1−x)Pb(Mg1/3Nb2/3)O3−(x)Pb(Zr,
Ti)O3) thin film on a PET substrate
(Figure 6C). The PMN-PZT-based PENG
can provide an open-circuit voltage of 17.8 V and a short-circuit
current of 1.74 µA from porcine rhythmical cardiac contraction and
relaxation which is higher than the previous
research.[105-106] The PENG system successfully
enabled ECG signal recording, controlled a
light bulb wirelessly and
transferred signals. Hypertension (high-blood pressure) is another
health risk factor for heart disease that need long-term intensive
real-time monitor but traditional air-filled cuffs for blood pressure
test can only provide interrupted
results.[107-108] To realize real-time and
accurate monitoring of blood pressure, a flexible continuous
battery-free electronic system was developed for in vivo blood pressure
monitoring with a miniaturised thin-film PENG wrapped on the aorta as
illustrated in Figure 6D. The devices were encapsulated by the
biocompatible flexible polyimide film sandwiched in a 200 μm
piezoelectric polarized PVDF thin film and thus can produce current
continuously with the periodical expansion and retraction of the aorta.
The long-term stability and possibility of long-time in vivo use enable
the PENG to be a real-time continuous power source for the visualized
blood pressure monitoring system. However, even though PENGs have
advantages including high output voltage and good scalability as energy
supply, the most widely used material PZT of PENG is toxic and risky to
be implanted inside the human body even though it can be sealed by other
nontoxic materials such as PDMS and polyimide encapsulation layer, while
other biocompatible materials such as ZnO and PVDF can only provide
millivolts (mV) and nano-amperes (nA) output lower than PZT-based PENG
devices.[16]
Similarly, the TENG relying on the
triboelectric effect at the interface between two different dielectrics
also provided a promising way to harvest energy from body movements.
Generally, the triboelectric effect of TENG occurs between inorganic and
organic films with different frictional electric sequences. Various
materials in nature such as silk, metal, wood and polymer have such
effects. At present, TENG has been successfully applied in biomechanical
energy collection for biomedical signal sensing such as
respiratory,[109] cardiovascular and digestive
systems.[110-111] For example, Zheng et al.
fabricated a self-powered cardiac pacemaker with an energy supply of
TENG harvesting energy from respiration (Figure
6E).[30] The size of the TENG device inserted into
the left chest skin of a rat is about 1.2 cm wide and 0.2 cm thick. The
output voltage and current generated by the TENG device are 3.73 V and
0.14 μA, which was directly applied to power a cardiac pacemaker for
heartbeats regulation. Also, the TENG can harvest mechanical energy from
gastric peristalsis and can be converted to biphasic electric pulses
through contact and separation of the triboelectric
layers.[112] The generated electric signals were
delivered to the fibres of the vagus nerve stimulation (VNS) device and
decreased the food intake, achieving the final purpose of weight control
as illustrated in Figure 6F. Comparatively, TENGs have the advantages
such as low-cost, selective materials and high
energy conversion
efficiency.[113-114] However, both TENGs and PENGs
as biomechanical energy harvesters face unique challenges such as the
stability of continuous energy supply, adaptability to the biological
surroundings with different humidity and temperature, long-term safety
inside the body and the limitation for long-term
application.[16]