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Materials Science in Additive Manufacturing Biodegradable sustainable electronics

prepared by introducing reversible imine linkages between 5. Biodegradable dielectrics
diketopyrrolopyrrole (DPP) and p-phenylenediamine. The
conjugation along the backbone of this polymeric film When an electric field is present, dielectric polymers,
allows for hole conduction. The degradation of the material which are insulators, can become polarized. The dielectric
constant (κ), which can be high or low depending on
takes place through breaking of imine bonds under acidic the application, determines polarization. High-κ fillers
conditions to give aldehyde and amine precursors as the can be added to a degradable polymer matrix to produce
by-products (Figure 4B) . Xu et al. reported a polyurethane- biodegradable dielectrics (Table 3). Common high-κ metal
[78]
based conductive elastomer (DCPU) based on PCL, which oxides include aluminum oxide (Al O , κ = 9), silicon oxide
3
2
used dopant as dimethyl propionic acid and aniline trimer (SiO , κ = 3.9), and hafnium oxide (HfO , κ = 25). Al O
3
linkers. The conductivity of the synthesized polymer could was combined with cellulose acetate (CA), and Figure 5A
2
2
2
vary between 10 and 10 S/cm in the dry state. Depending shows that this combination produced a higher κ value of
-8
-5
on the concentration of dopant present in the polymer, 27.57 at a low frequency of 50 Hz . Besides metal oxides,
[93]
conductivity could be enhanced further on soaking in PBS. carbon nanotubes also improved the κ of biodegradable
Due to the presence of hydrophilic carboxylic functional paper made from cellulose nanofibers (CNFs) from ~ 0 to
groups, with the increase in DMPA content, degradation 3198 at 1 kHz .
[94]
rate increased in aqueous PBS solution. The polymer
degraded to ~75% of its weight in 14 days in PBS and in the Plant-based fibers such as cotton, bamboo, jute, and
presence of lipase without any decline in conductivity during banana fibers also possess dielectric properties, as shown
[95-100]
degradation period (Figure 4C). The chemical linkage in the in Figure 5B . This is because there are free hydroxyl
matrix helps to stabilize the dopant and leads to improved functional groups present, which add polarity and provide
electronic performance . Natural pigments [80,81] , conjugated high κ values. Cotton exhibits a dielectric constant of 17
[79]
[101]
molecules such as Indigofera tinctoria and Isatis tinctoria , between frequency range of 60 and 1000 Hz . Banana,
[82]
melanin, and β-carotene [83,84] have also been explored for bamboo, and jute fibers were also used as fillers into
the preparation of biodegradable electronics. Some natural dielectric composites. The dielectric constant of these
semiconductor materials include indigo with a band gap of composites was found to be increasing with increase in
[102]
1.7 eV and decent carrier mobilities. Eumelanin, a subclass fiber content . Natural sugars also behave as dielectrics.
of melanin, also exhibits electronic behavior. Conductivity High breakdown voltages of 1.5 MV/cm and 4.5 MV/cm,
low loss tangents on the order of 10 at 100 mHz, and
−2
of melanin depends on temperature, its hydration state, and dielectric constants of 6.35 and 6.55 at 1 kHz, respectively,
physical form. Mostert et al. demonstrated that eumelanin are all characteristics shown by glucose and lactose .
[80]
[84]
conducts free electrons and protons for electronic and
ionic conduction, respectively, after absorption of water Besides natural materials, synthetic materials also exhibit
molecules, as shown in Figure 4D. The material was used as biodegradability. An example of synthetic biodegradable
a regenerative medical scaffold, which could resorb in about dielectric elastomer is poly(glycerol sebacate) (PGS). Such
8 weeks . elastic materials are useful for capacitive sensors since they
[81]
can withstand compression more effectively and thus can
Fully biodegradable conducting polymers can be a useful alternative to viscoelastic polymers. Boutry
be prepared by interrupting the conjugation with et al. [103] reported a degradable capacitive pressure sensor
the introduction of flexible non-conjugated linkers fabricated using PGS as the dielectric sandwiched between
in the polymer backbone. This process makes the biocompatible Mg and Fe metal electrodes. Fabricated
polymers flexible and processing easy, but decreases sensor showed excellent time response while detecting small
the conductivity as compared to partially degradable weights of single grain of salt weighing only 5 mg (Figure 5C).
conductive polymers [31,79,85,86] . Due to the intrinsic It is important to note that most of the dielectrics mentioned
flexibility of the material, these polymers are used in in this review were investigated at frequencies lower than
sensors as interconnects where low conductivity is not few kHz. For practical use of these biodegradable dielectrics
an issue. Electrical stimulation to promote cell growth in complex electronic devices, their optimization is required
and tissue regeneration of scaffolds and muscle tissues for high-frequency performance.
is another application of low conducting polymers [86-88] .
There is a need to develop new chemistries and 6. Biodegradable insulators
increase the conductivity of polymers to fabricate high-
performance degradable electronics. Conductivities of 6.1. Substrates
various biodegradable conducting materials are given in Substrates typically constitute most of the weight and
Table 2. volume of an electronic device. Therefore, overall

Volume 1 Issue 3 (2022) 9 https://doi.org/10.18063/msam.v1i3.15