FIGURE 6 (A) A
alternating-current nanogenerator based on a single piezoelectric fine
nanowire harvesting mechanical energy from the breath and heartbeat of a
live rat. Reproduced with permission.[104]Copyright 2010, Wiley-VCH GmbH. (B) A self-powered cardiac pacemaker by
biocompatible polymer-based PENG on the heart of an adult dog.
Reproduced with
permission.[29] Copyright 2021, Elsevier. (C) In
vivo simultaneously ECG signals recorded by flexible PENG attached to a
porcine heart harvesting energy from rhythmical cardiac contraction and
relaxation. Reproduced with permission.[106]Copyright 2017, Wiley-VCH GmbH. (D) Implantable thin film PENG powered
in vivo blood pressure monitoring. Reproduced with
permission.[107] Copyright 2016, Elsevier. (E)
Self-powered cardiac pacemaker with breath-driven implanted TENG in
vivo. Reproduced with permission.[30] Copyright
2014, Wiley-VCH GmbH. (F) VNS device for effective weight control
powered by TENG harvesting mechanical energy from gastric peristalsis.
Reproduced with permission.[112] Copyright 2018,
Springer Nature.
3.2.2. Biofuel cell
The source ingredients for biofuel cells are abundantly available in our
bodies including blood, sweat and tear. Due to the self-restorative body
fluid, the reactant glucose for biofuel cells is abundant enough for
long-term electricity generation. During the oxidation reaction, glucose
can generate 12 electrons per molecule and provide energy up to 16 kWh
g-1.[66] By inserting two
biofuel electrodes with enzymatic catalysts into the tissue, the biofuel
cell can convert chemical energy from glucose oxidization and dioxygen
reduction to electricity, and the product gluconolactone will be then
absorbed by organisms. It has been
reported that biofuel cells have been fabricated and implanted into
various organisms (snail, clam, lobster, slug, rat), and the power
densities generated by biofuel cells range from 2 μW to 97
μW.[102] Since 2012, sustainable generation of
electrical power from a snail was achieved for the first time, and it
has been proved that biofuel cells implanted in small animals can
operate for several months.[115] The reversible
decay in the electrical power generation can be observed
in Figure 7A , it was
reported that the local depletion of glucose at the electrode surface
led to the current decay, but the electrical output was restored after
feeding the snail followed by slow metabolic processes and glucose
diffusion. It is worthy of notice that small species have limited
amounts of glucose for biofuel mass transport, which is different from
the process in a mammal. To investigate the influence of the implanted
glucose biofuel cell on the habits and fitness of the host, researchers
investigated the weight of the rat and the food consumption when a
biofuel cell based on carbon nanotube/enzyme electrodes was
implanted.[116] It was implanted into the
abdominal cavity and generated an average open-circuit voltage of 0.57 V
and a power output of 38.7 μW, which is enough to power a light-emitting
diode, and as a result, no
immunological rejection happens in the rat after the implantation of the
biofuel cells for 110 days, as shown in Figure 7B . It shows a
promising prospect of the implantation of biofuel cells in a mammal’s
body fluids for the biomedical electronic power source. To further
decrease the dimension of the biofuel cell, a modified flexible carbon
fibre microelectrode as intravenous implantable biofuel cell was
developed and implanted in the thoracic region of living rats through a
catheter.[31] An open circuit voltage of
approximately 0.125 V was obtained and maximum output power of 95 μW
cm-2 at 80 mV in a living rat was achieved (Figure7C ). However, the significant limitations of biofuel cells lay
in the limited output voltage (less than 1 V) due to the limited
oxidation-reduction potential, which means the biofuel cells can only
drive electronics with a low voltage range. There are some improvement
methods for the limited output voltage of biofuel cells, such as using a
direct current to direct current (DC-DC)
converter and connecting several
biofuel cells in series.[117-118] But the DC-DC
converter will consume current while connecting several enzymatic
electrodes biofuel cells in series will bring more problems such as the
expanded volume and implantation difficulty. Also, inflammatory
reactions can also occur around the electrode surfaces and phagocytosis
of the enzyme coatings of the biofuel cell will shorten the lifespan of
the device, so developing a new biocompatible material to protect the
electrodes is necessary.[31] At the same time, the
oxidation reaction of glucose on the enzymatic electrodes will generate
by-products (hydrogen peroxide) which may cause harm to the living body
and therefore cause problems during long-term applications in
vivo.[102] In this case, the development of new
biocompatible materials to protect the enzyme coatings from phagocytosis
should be considered for the advancement of this novel long-term
applicable power electronics as a promising sustainable power source for
the miniaturised IMEs.