Page 108 - IJB-10-6

International Journal of Bioprinting 3D-printed bioelectronic devices

3.2. Non-metallic materials Its electrical properties can be further enhanced by
incorporating conductive fillers such as graphene and
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3.2.1. Carbon-based materials CNT. By adding graphene nanoflakes to nonconductive
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Carbon-based materials such as graphene and carbon thermoplastic polyurethane, the electrical conductivity
nanotubes are electrically conductive nonmetallic increased to 0.59 × 10 S/m. Additionally, the
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materials. Graphene exhibits excellent electrical and stretchability of PEDOT:PSS can be improved by adding
thermal conductivities, high light transmittance, polymer like polyethylene oxide (PEO) or polyvinyl
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biocompatibility, and superior mechanical flexibility alcohol (PVA), which decreases the interaction between
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(Figure 2B). 89–91 These properties enable a broad range the polymer chains and increases the free volume between
of applications in bioelectronics. Since graphene has low PEDOT and PSS. The broad utilization of PEDOT:PSS
solubility in solvents, it is often modified to enhance its in bioelectronic devices such as biosensors, stretchable
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solubility while maintaining its functional properties. transistors, temperature sensors, and soft neural
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For example, graphene oxide (GO), its oxidized form, probes is achieved through tuning the aforementioned
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and reduced graphene oxide (rGO) are widely used in 3D parameters. Moreover, PEDOT:PSS can be used in diverse
printing because their solubility and electrical properties 3D printing methods including inkjet printing, DIW,
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can be easily controlled. 91 and SLA. 110
Carbon nanotubes (CNT), which are hollow cylinders PPy exhibits high electrical conductivity and excellent
rolled from graphene sheets, exhibit remarkable chemical stability under biological conditions. Due to its
mechanical, electrical, and chemical properties. 92,93 Their ability to enhance cell growth and differentiation, PPy
large surface area and extreme length-to-diameter ratio has been extensively used as a scaffold material for tissue
enable the construction of highly aligned structures, which engineering. Beyond tissue engineering, PPy has shown
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is advantageous for enhancing mechanical properties, great potential in electronic devices such as wearable
electrical performance and cell adhesion. CNT can flexible electronic sensors, cardiac patches, scaffolds,
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be 3D-printed using diverse methods, including inkjet and wearable storage devices. PANI is also a promising
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printing, SLA, and FDM. conductive polymer for biomedical electronics owing to its
Moreover, carbon-based materials are often used as ease of synthesis, low cost, and tunable conductivity.
fillers in polymers to improve the mechanical, electrical, 3.2.3. Conductive hydrogel
and chemical properties of other materials. For example, Conductive hydrogels have emerged as favorable options
GO-incorporated polylactic acid (PLA) scaffolds for biomedical electronic interfaces owing to their soft
have been synthesized to enhance the structural and mechanical properties, biocompatibility, and water content.
mechanical properties to promote bone cell attachment, Compared to conventional electrode materials, which
proliferation, and differentiation. In another study, the exhibit Young’s moduli several orders of magnitude higher
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addition of multi-walled carbon nanotubes (MWCNTs) than those of biological tissues, conductive hydrogels offer
to polycaprolactone scaffolds significantly increased the tunability to match the Young’s modulus of the targeted
electrical conductivity of the scaffold, allowing for the tissues. 116,117 Especially, ionic hydrogels containing mobile
application of electrical stimulation (ES) in bone tissue nanofillers exhibit both high conductivity and tissue-like
models. softness. Furthermore, their stretchability and flexibility
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3.2.2. Conductive polymers have led to their application in soft electronic materials.
Recently, conductive polymers have attracted significant Conductive hydrogel combined with 3D printing allows
interest due to their biocompatibility, tunability, and for the customization of electronic device; for instance,
flexibility. These polymers are composed of repetitive merging DLP printing technique with a conductive
monomer units designed to enhance their electrical hydrogel elastomer assembling process can be applied to
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conductivity. Their mechanical and electrical properties producing customized stretchable conductors.
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can be easily modified by incorporating other materials. Conductive hydrogels can also be made using a variety
The most extensively used conductive polymers include of polymers including PEDOT:PSS, PVA, PPy,
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poly(3,4-ethylenedioxythiophene) polystyrene sulfonate polyethylene glycol diacrylate (PEGDA), and PANI.
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(PEDOT:PSS), polypyrrole (PPy), polyaniline (PANI), and These hydrogels can be fabricated using various methods
polyacetylene. including inkjet printing, SLA, DLP, and DIW.
PEDOT:PSS exhibits high conductivity, However, challenges remain in obtaining the desired
biocompatibility, optical transparency, thermal and levels of electrical conductivity and mechanical toughness
chemical stability, and flexibility (Figure 2C). 102,103 while preserving their advantageous properties.
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Volume 10 Issue 6 (2024) 100 doi: 10.36922/ijb.4139