Page 301 - IJB-9-1

International Journal of Bioprinting 3D printable conductive composite inks for biocompatible electrodes

distribution and percolation threshold concentration. rapidly increased with the GO concentration >2 vol% to
Pidcock et al. dispersed MWCNT into gellan gum (GG) , the printable range and showed shear-thinning behavior.
[52]
which is a linear, anionic polysaccharide and has been The research team hypothesized that some GO flakes
approved by both the US Food and Drug Administration roll up at high concentrations, enhancing their elastic
and the European Union for medical usage . The properties by forming strong network. The GO ink showed
[62]
biocompatible property of GG allows the use of wearable or conductivity of around 100 S/m with and without additional
transplantable strain gauges. The research team dispersed components such as F127. Similarly, Shi et al. formulated
the ink at the ratio of 1:10-1–5:10-1 (indicating 1 mg/mL aqueous ink with electrochemically derived GO (EGO)
of MWNT and 0.1–0.5 mg/mL of GG) and 2:10-1–2:10-15 and polydimethylsiloxane (PDMS) submicrobeads . The
[66]
(indicating 1–15 mg/mL of MWNT and one-fifth of GG) PDMS/EGO ink showed shear-thinning behavior, proven
with ultrasonication and analyzed the viscosity and the by viscosity decrease with shear rate, and complex modulus.
conductivity. The viscosity of 2:10-1 is comparable to that Due to its thixotropic and viscoelastic properties, ink can be
of water (~1 mPa∙s) at all shear rates. The viscosity of printed with a small inner nozzle diameter (50 µm). After
2:10-15 at a low shear rate is 1000 mPa∙s, which is more the annealing process, EGO deoxygenated and covered
than twice higher than that of 4 mg/mL GG. However, the the PDMS submicrobeads, increasing the conductivity
viscosity decreases to around 200 mPa∙s at a high shear rate, (0.06 S/m for 0.83 vol%).
similar to that of 4 mg/mL GG. This observation supports
the enhancement of shear-thinning behavior with the 4.3. MXene-based printing ink
introduction of filler. The strain gauge with a gauge factor After the introduction of graphene, the potential of 2D
value of around 15 can be fabricated using the ink. materials has caught the attention of research community.
Pedrotty et al. mixed SWCNTs in nanofibrillated A novel 2D material called MXene is fabricated from
cellulose with the surfactant Pluronic F127 at a the MAX phase, composed of Mn+1AXn, where M is a
concentration of 1.9 wt% . The complex modulus NFC/ transition metal, A is an A group (mostly IIIA and IVA
[63]
[74]
SWCNT ink implied elastic behavior (tan δ < 1) below group) element, X is C and/or N, and n = 1,2,3 . The A layer
100 Pa and viscous behavior (tan δ > 1) over 100 Pa, proving can be etched out using an acid such as hydrofluoric acid,
shear-thinning behavior for extrusion-based printing. The leaving the Mn+1Xn layer. MXenes show extraordinary
decrease in viscosity in response to the shear rate verifies mechanical and electrical properties. Among various
this property. The conductivity of the NFC/SWCNT types of MXenes, Ti C Tx, where Tx represents the surface
2
3
was around 43 S/m. The NFC/SWCNT ink is printed on terminators (e.g. OH, O , or F), is known to be non-
2
[75]
biocompatible BNC film and attached to the ventricle toxic . Boularaoui et al. used low-concentration GelMA
incision; the conductive patch proved the ability to restore hydrogels as a polymer, and Ti C Tx MXene as a filler
2
3
[67]
the conductive velocity of the heart. to create a conductive composite . The ink containing
0.05 mg/mL MXene showed high cell viability after day 7,
4.2.2. Graphene-based printing ink although the cell viability of 0.1 mg/mL MXene dropped
Graphene is a single layer of carbon sheet with high on day 7. The ink showed shear-thinning behavior with a
conductivity and mechanical properties. Jakus et al. slight increase in viscosity at a low shear rate as expected.
fabricated a 3D printable graphene (3DG) composite by The conductivity of the ink was around 0.65 S/m and
combining 75 wt% graphene flakes with biocompatible 0.94 S/m at 0.05 mg/mL and 0.1 mg/mL, respectively,
and biodegradable polylactide-co-glycolide (PLG) . which is comparable to the conductivity of the electroactive
[64]
The solvent was evaporated in a sonicating bath until the tissues (0.4 – 0.9 S/m). Orangi et al. dispersed Ti C Tx
3
2
viscosity reached 30 Pa∙s. The 3DG showed high fidelity MXene and super absorbent polymer beads with DI water
[73]
that could stack >700 hundred layers and conductivity and 3D-printed micro-supercapacitors . The ink showed
of >800 S/m after annealing. However, the hydrophobic shear-thinning behavior and the storage modulus larger
property of graphene hinders its biomedical and than the loss modulus (tan δ < 1), making the ink printable
implantation applications. Therefore, graphene oxide and sustaining the structure without additive at room
(GO), oxidized graphene that includes oxygen functional temperature. The printed structure exhibited outstanding
groups, or reduced graphene oxide (rGO), which is redoxed areal capacitances and maximum energy density due to the
GO for its desired properties is used instead. García- high electrical conductivity of MXene.
Tuñón et al. generated a water-based paste with GO and 5. Electrode integrated cell culture platform
various materials (e.g., polymers, ceramics, and steel) .
[65]
GO suspension showed shear-thinning behavior when The extrusion-based printing technology facilitates the
diluted in water at 0.1 – 0.6 vol%. The storage modulus embedding of the electrode inside the base polymer

Volume 9 Issue 1 (2023) 293 https://doi.org/10.18063/ijb.v9i1.643