The
gelation of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS) has gained popularity for its potential applications in
three dimensions, while possessing tissue-like mechanical properties,
high conductivity, and biocompatibility. However, the fabrication
of arbitrary structures, especially via inkjet printing, is challenging
because of the inherent gel formation. Here, microreactive inkjet
printing (MRIJP) is utilized to pattern various 2D and 3D structures
of PEDOT:PSS/IL hydrogel by in-air coalescence of PEDOT:PSS and ionic
liquid (IL). By controlling the in-air position and Marangoni-driven
encapsulation, single droplets of the PEDOT:PSS/IL hydrogel as small
as a diameter of ≈260 μm are fabricated within ≈600
μs. Notably, this MRIJP-based PEDOT:PSS/IL has potential for
freeform patterning while maintaining identical performance to those
fabricated by the conventional spin-coating method. Through controlled
deposition achieved via MRIJP, PEDOT:PSS/IL can be transformed into
different 3D structures without the need for molding, potentially
leading to substantial progress in next-generation bioelectronics
devices.
Reactive inkjet printing holds great prospect as a multimaterial
fabrication process because of its unique advantages involving customization,
miniaturization, and precise control of droplets for patterning. For
inkjet printing of hydrogel structures, a hydrogel precursor (or cross-linker)
is printed onto a cross-linker (or precursor) bath or a substrate.
However, the progress of patterning and design of intricate hydrogel
structures using the inkjet printing technique is limited by the erratic
interplay between gelation and motion control. Accordingly, microreactive
inkjet printing (MRIJP) was applied to demonstrate a spontaneous 3D
printing of hydrogel microstructures by using alginate as the model
system. In addition, a printable window within the capillary number–Weber
number for the MRIJP technique demonstrated the importance of velocity
to realization of in-air binary droplet collision. Finally, systematic
analysis shows that the structure and diffusion coefficient of hydrogels
are important factors that affect the shape of printed hydrogels over
time. Based on such a fundamental understanding of MRIJP of hydrogels,
the fabrication process and the structure of hydrogels can be controlled
and adapt for 2D/3D microstructure printing of any low-viscosity (<40
cP) reactive inks, with a representative tissue-mimicking structure
of a ∼200 μm diameter hollow tube presented in this work.
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