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Materials Science in Additive Manufacturing Biodegradable sustainable electronics

diodes (OLEDs) [125] , sensors [126-128] , transistors [129-132] , bio- that a gadget performs as intended for a particular amount
batteries [133] , and radiofrequency identification (RFID) of time, slow swelling polymers are preferred. Thus,
antennas [92,134,135] as substrate. CNFs were also used to metallic substrates serve as a good alternative option as
fabricate biodegradable and flexible devices using transfer they do not swell in biological fluids and, therefore, offer
printing method. Good thermal stability of the material dimensional stability. The dissolution rate of thin foils of
made direct printing possible on CNF papers [112,136] . Hsieh Fe, Mo, W, and Zn in PBS (pH 7.4 at 37°C) as substrates
et al. [137] successfully printed and annealed conductive silver for transient electronics was found to be 0.08, 0.02, 0.15,
lines on CNF papers, thus demonstrating its full potential and 3.5 µm/day , respectively.
[9]
for roll-to-roll manufacturing. Despite easy availability
of natural materials, such materials possess limitations 6.2. Encapsulating materials
for the wide applications due to high variation in quality Depending on the intended use, desired device operational
from batch to batch. These variations affect the working times may range from a few days to a few weeks or years.
of the electronic devices. The use of synthetic polymers Such time frames are crucial for application in healthcare
is one of the solutions to mitigate such issues. Synthetic or clinical settings. Most of the printed electronic devices
polymers can be chemically engineered for better control need to be protected through an encapsulation layer. Thus,
of physical and chemical properties. A combination of selection of the right material for the encapsulation is
PLA and PGA in different ratios gives bioresorbable paramount to achieve biodegradability. The electrically
polymer with tunable mechanical strength and controlled active components may quickly deteriorate in the presence
degradation time [138] . PVA [25,28,66,90,113] , PGS-PCL [139] , and of a high water permeation rate. When exposed to PBS at
sodium carboxymethyl cellulose (Na-CMC) [65,67,89] with room temperature, Mg thin film enclosed in 5 m PLGA
diverse mechanical and degradation capabilities have degrades within 10 min . Tuning the physical and
[15]
been employed due to the various needs for biodegradable chemical properties of biopolymers, such as composition,
healthcare devices. According to a study, numerous bio- thickness, crystallinity, and chemistry, can extend the
based polymer substrate types can be used for printed lifetime. A carefully formulated polyanhydride allows the
electronic applications. Compared to conventional PET intracranial pressure sensor to operate steadily for up to
film, screen printed silver on cellulose acetate propionate 3 days [146] and silk fibroin with high crystallinity can extend
(CAP) showed 18% lower resistance value and, hence, the Mg thin film’s lifetime to about 90 h . An alternative
[17]
better electrical properties [140] . Hao et al. [141] developed option is the use of dissolvable oxides, although care should
a multifunctional gelatin-alginate hydrogel-based be taken to not use single-layered oxides. Depending on the
soft sensor with improved sensing performance. The deposition conditions, single-layer oxide quickly dissolves
degradable sensor is able to sense very small changes in due to the presence of pinholes. In PBS, Mg with a 200 nm
strain, temperature, heart rate, and pH and has also been SiO encapsulation dissolves within 1 min, but Mg that has
2
explored for drug delivery application. Since the device is layers of alternate SiO and Si N can last up to 10 days.
4
3
2
both degradable and recyclable, it can be reconstructed According to the encapsulation studies for OLED devices,
with new functions (Figure 7C-E). A polymer of methyl combining oxide layers and biopolymers is projected to
1H-pyrrole-3-carboxylate monomer (“MPC polymer”) was significantly extend functional durations [147] . In addition,
explored as energy storage material for supercapacitors. recent research has demonstrated that using a mono-Si
This polymer, both as a planar electrode and as a composite thin film (1.5 mm) as the encapsulation layer can greatly
porous electrode with PLLA, demonstrated charge storage increase the device’s operating lifetime. Materials coated
ability that was comparable to that of the pseudocapacitive with Si NMs, such as Mg thin film, maintain their integrity
conducting polymer PPY. In aqueous environment after 60 days in PBS at 37°C .
[15]
(37°C, pH 8.2), its application in a supercapacitor with
an organic electrolyte revealed detectable evidence of 7. Biodegradable and transient electronics
deterioration in 8 h. [142] The field of biodegradable electronics falls under “green”
Stimuli-responsive polymeric materials offer electronics with the aim to develop electronic components
various transient modes in aqueous solutions and in and systems that have degradation and bioresorbability
ambient atmosphere with precise control over the start characteristics. Such systems should have degradation of
of degradation. These materials include temperature- over 80% in the presence of aqueous medium, temperature,
sensitive cyclododecane (CDD) and methanesulfonic humidity, oxygen, microorganisms, or radiations and should
acid/wax , moisture responsive polyanhydrides [143,144] , ideally convert to harmless substances. Bioresorbability is a
[11]
and photoacid generator/cyclic poly(phthalaldehyde) subclass of biodegradability that refers to the safe breakdown
(PAG/cPPA) that respond to ultraviolet light [145] . To ensure of the material in the human body after performing its

Volume 1 Issue 3 (2022) 13 https://doi.org/10.18063/msam.v1i3.15