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Materials Science in Additive Manufacturing Biodegradable sustainable electronics

them brittle and inflexible, which can be overcome values of mobility (1.7 × 10 cm /V.s) due to macroscopic
-2
2
by doping the conjugated polymers without affecting phase segregation.
their electrical properties [54-56] . PEDOT: PSS, a polymer
doped with poly(styrenesulfonate) (PSS), shows the best 3.3. Inks and pastes
known conductivity of up to 4.6 × 10 S/m in polymeric The emerging field of printed electronics, an additive
5
[57]
materials . This electrode was developed by Worfolk manufacturing technology, has opened doors to directly write
et al. to fabricate a sensor with a sensing range from electronic materials on desired substrates. The technology
[58]
30°C to 45°C (Figure 3B). requires the materials to be converted to functional inks
Another strategy to tune degradability is to blend and pastes to be printed through inkjet, aerosol jet, screen
conducting polymers with biodegradable polymers printing, pneumatic head, or laser-based techniques. Hence,
to achieve partial degradation. Partial degradation is many conducting metals have been dissolved in aqueous,
achieved, as these polymers do not break down completely polar, or non-polar solvents to convert to inks. Biodegradable
into monomers. Shi et al. used a composite of PPy inks differ in particle size, types of additives, and solvent.
nanoparticles (NPs) with poly(D,L-lactic acid) (PDLLA), In general, the biodegradable inks are made up of three
where NPs formed a conductive network within the PDLLA key ingredients, namely, nanoparticles, small quantity of
matrix for fibroblast growth. Adding even a small amount polymers as stabilizers, and suitable solvent. Mahajan et
[65]
of PPy leads to high conductivity (approximately 1 × 10 -3 al. prepared a stable conductive ink by dispersing Zn
S/cm), with the material being stable for about 1000 h . NPs (of average diameter 50 nm) with PVP particles in a
[59]
Similarly, Wang et al. used a combination of hyaluronic solvent mixture of methanol and butyl acetate. In another
[30]
acid (HA)-doped PEDOT (10% NP loading) with poly(L- work, Zn NPs were mixed with Ag NWs and polyethylene
lactic acid) (PLLA) to fabricate a conductive film. The oxide (PEO) to prepare the ink [26,66] . Addition of Ag NW also
degradation of PLLA was accelerated by PEDOT-HA by adds a stretchable character to the printed patterns. Most of
almost 10%. The enhanced degradation can be assigned to the prepared inks and pastes were processed using printed
increased water penetration in PLLA due to the presence techniques such as screen printing, inkjet, and aerosol
of hydrophilic HA. printing. To create a uniform dispersion of the biodegradable
material particles, polymers are used as additives. Most of the
In another work, a mixture of PEDOT: PSS with additives, however, are insulating in nature and could hinder
photosericin and Irgacure 2959 photoinitiator was electron transport. Thus, it is important to post-process the
synthesized to prepare a conducting material. The printed patterns by heat sintering to degrade the additives.
fabricated sensor showed a high sensitivity (−0.99% °C ) Huang et al. prepared a W paste with PEO particles as
-[1]
[67]
in the temperature range of 20 – 50°C and excellent stability additives in methanol. As stated earlier, most metal-based
on exposure to about 60% relative humidity. The sensor biodegradable materials are brittle at room temperature and,
was degradable in approximately 10 days under proteolytic thus, require some sort of post-processing to enhance the
conditions (Figure 3C) . Hybrid approaches have also mechanical properties. Li et al. improved the conductivity
[60]
[66]
been investigated to realize fully degradable materials of the Zn ink by an improved water sintering method.
wherein inorganic materials are mixed with organic This ink showed the capability to withstand 1500 repeated
materials. In one report, Si NPs were mixed with carbon bending test with a curvature of 3.8 mm. In another study,
black and PEDOT: PSS to make electrodes for lithium-ion an ink based on Zn and Ag NW composite exhibited a
battery application .
[61]
high conductivity with excellent mechanical using the same
Li et al. reported electrospun camphor sulfonic water sintering approach. The authors demonstrated that
[62]
acid (CPSA) doped with PANI and gelatin to obtain high the printed patterns were robust and could withstand 8000
-2
conductivity material (up to 2.1 × 10 S/cm). In another report, bending cycles .
[26]
CPSA/PANI composite was doped with poly(L-lactide-co-ɛ-
caprolactone) and showed high conductivities (up to 1.38 × 4. Biodegradable semiconducting materials
10 S/cm). Electrical stimulation through these conducting
-2
fibers showed increased adhesion and proliferation for 4.1. Inorganic semiconductors
fibroblasts and myoblasts cells . Hydrogels obtained by Semiconductors have a bandgap, and their electrical
[63]
grafting PANI with gelatin and doping with CPSA exhibited conductivity is between conductors and insulators.
conductivities around 10 S/cm . P3HT was electrospun They play a major role in fabrication of the electronic
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[31]
[40]
with polycaprolactone (PCL) and polylactic-glycolic acid devices. Present electronic industry is based on silicon
(PLGA) to introduce partial degradability. The resulting (Si) semiconductor, which is used in various forms
[64]
electrospun nanofibers with PCL, however, displayed lower to make devices, namely, monocrystalline silicon
Volume 1 Issue 3 (2022) 7 https://doi.org/10.18063/msam.v1i3.15