Unconventional direct ink writing of polyelectrolyte films

This article focuses on fundamentally analyzing the morphology of polyelectrolyte-based layer-by-layer (LbL) films, created using an unconventional direct ink writing (u-DIW) approach that allows printing low-viscosity inks via capillary action. With two oppositely charged weak polyelectrolytes as inks and using a range of characterization techniques, we have compared the films developed using this new approach against traditional dip-coating in terms of their key morphological features. Our results suggest that the extent of polyelectrolyte interdiffusion within the films is limited in the case of the u-DIW approach causing the formation of thinner films versus dip-coating. In comparison to traditional dip-coating, u-DIW has the advantage of enabling the formation of homogenous films with equivalent properties, but with significantly lower material usage. The fundamental analyses presented in this article will likely facilitate the application of LbL in various fields ranging from separations to biomedical drug delivery. Impact statement There is currently a burgeoning interest to develop the next generation of large-scale thin-film advanced manufacturing for a myriad of applications, including energy and biomedical fields. Direct ink writing (DIW) can be a transformative deposition method in that it does not necessitate the need for masking, offers large-scale manufacturing capabilities, and uses relatively little ink volume (e.g., compared to a dip-coating process) with no material waste. Although DIW is a well-known method in the additive manufacturing community, currently it is largely limited to highly viscous, non-Newtonian ink printing and 3D lattice fabrication, thus not allowing thin-film fabrication. Recently, through our work, the capability of low-viscosity inks to be directly written into thin films (with textured surfaces) without the use of external stimuli and/or pneumatic pressure has been demonstrated. This is possible by only using surface forces to deposit the solution-based ink onto a substrate in the form of a thin film. In this work, this unconventional DIW approach is used to deposit thin films comprising alternating layers of oppositely charged polyelectrolytes and compare them to samples made using a traditional dip-coating method. Using a range of characterization techniques, we demonstrate, for the first time, that our novel direct writing method can be used interchangeably with traditional manufacturing methods. We further sho