Page 353 - IJB-9-5

International Journal of Bioprinting 3D printing of tough and self-healing hydrogels

cystamine in PAA solution also imparted self-healing dropped at about 80°C. Due to the absence of TA, PVA/
properties. N, N′-bis(acryloyl)cystamine acted as a PAA ink, which could not form a reversible double bond,
crosslinking agent with a disulfide bond [31,32] . Disulfide caused breakage of crosslinking sites in it by heat, resulting
bonds, along with hydrogel bonds, contributed to self- in a sharp decrease in modulus. However, the PVA/TA/
healing properties as a type of reversible bond. Through PAA hydrogel exhibited overall similar results, with
the double network of the strong H-bonds with disulfide both G′ and G″ decreasing continuously with increasing
bond within the PAA chain and the weak H-bonds of temperature. The gap between G′ and G″ was small and
PVA/TA, desirable properties were obtained: the H-bond- sometimes reached equal values, indicating that the inks
based double network endowed toughness via energy were in the sol-gel state, which is suitable for printing to
dissipation mechanism that enabled maintaining strong effectively flow from the precise nozzle. These observations
H-bonds from physical damage by dissipating energy as (Figure 2C and D) suggest that PVA/TA /PAA hydrogel
1:1
weak H-bonds are broken, and the rapid breakage and ink is the most printable of all ink compositions. Because
reformation of weak H-bonds imparted stretchability and the ratio between PVA and TA affected the formation
self-healing ability . of a reversible double network and crosslinking density
[33]
in the PVA/TA/PAA hydrogel ink, too low or too high
In particular, the addition of TA was most influential in concentration of TA led to a decrease of the viscosity and
enabling the printing of the proposed hydrogel ink. Even if moduli. PVA/TA /PAA hydrogel ink displayed the highest
a high temperature (over 80°C) is applied to the hydrogel viscosity reaching approximately 100 Pa·s at 100 s , and
1:1
-1
ink for extruding from a precise nozzle and, consequently, the highest moduli, with both G′ and G″ reaching about
a weak bond is broken, reformation of the H-bond rapidly 10 kPa at 85°C, which indicates that the ratio of 1:1 is most
occurs again immediately after deposition, enabling the suitable for the rheological property. In addition, curves
ink to maintain its robust printed structure. To investigate of the temperature and the angular frequency-dependent
the optimal composition of high-performance hydrogel G′ and G″ of PVA/TA /PAA hydrogel ink showed that G′
1:1
inks, inks were prepared with different mass ratios of PVA was higher than G″ in all the temperature and frequency
and TA. As shown in Figure 2B, PVA/TA/PAA hydrogels regions. This suggests that the weak H-bonds can reform
with varying PVA and TA mass ratios (no TA, 1:0.5, 1:1, from external stimuli, preserving the double network and
and 1:2) were successfully formulated, and their material allowing the inks to maintain their structure after printing
characteristics were investigated. layer by layer without collapsing (Figure 2D; Figure S1 in

Supplementary File).
3.3. Rheological properties and swelling
behavior test The swelling tests also showed that the 1:1 ratio of
To ensure the functionality of bioelectronics through PVA and TA is optimal for the formation of a crosslinked
the manufacturing of hydrogels into specific shapes and network. The different ratios of PVA and TA were printed
patterns, the hydrogel inks used for printing must meet in the same structure, immersed in DI water for 960 min,
certain requirements, including shear-thinning properties and weighed at various time points. As shown in Figure 2E,
and shape fidelity [34,35] . To evaluate the suitability of the PVA/TA /PAA hydrogel showed almost 0% of swelling
1:1
hydrogel ink for printing, the viscosity of hydrogel was ratio with no increase in its weight. However, PVA/TA 1:0.5 /
tested at different mass ratios under varying shear rates PAA quickly expanded to a swelling ratio of 7.5% within
(Figure 2C). Under heat treatment at 85°C, all tested 240 min, since too low concentration of TA does not offer
hydrogel inks exhibited shear-thinning behavior with high sufficient crosslinking sites to form H-bond. Meanwhile,
viscosity at a low shear rate (10 s ) and low viscosity at a the weight of the PVA/TA /PAA hydrogel decreased to a
-1
-1
1:2
high shear rate (10 s ), as a result of weak H-bond breakage. swelling ratio of -3.7% within the first 100 min and rapidly
2 -1
To further assess the thermal response of the hydrogel inks, expanded up. This is because the concentration of TA was
the changes in the storage modulus (G′) and loss modulus too high to mix homogeneously in the hydrogel network,
(G″) were measured with temperature changes from 25°C which led the network inside PVA/TA /PAA hydrogel to
1:2
to 95°C (Figure 2D). Notably, the results showed that the G′ prevent the formation of dense crosslinking. As a result,
of the PVA/PAA hydrogel without TA was larger than G″ when the high temperature was applied to the hydrogel
within the tested temperature range, with G′ dominating and the weak bonds are broken, the reformation of the
even at high temperature. This indicates that the PVA/PAA H-bond and disulfide bond by the self-healing mechanism
hydrogel is in a gel state with elastic behavior regardless does not rapidly occur. Thus, by the breakage of the
of the temperature (within the tested range), which is hydrogel network after printing, the aggregated TA is
inappropriate for printing to extrude from the narrow and released and washed out with DI water. We also calculated
fine nozzle. In addition, it showed that G′ and G″ are quickly the water content of hydrogel by dividing the weight of the

Volume 9 Issue 5 (2023) 345 https://doi.org/10.18063/ijb.765