  Inkjet printing technology achieves the precise deposition of liquid-phase
materials via the digitally controlled formation of picoliter-sized droplets.
Beyond graphical printing, inkjet printing has been employed for the deposition
of separated drops on surfaces or the formation of continuous layers, which
allows to construct materials gradients or periodic features that provide
enhanced functionalities. Here, we explore the use of multinozzle,
drop-on-demand piezoelectric inkjet technology for the manufacturing of
mechanochromic materials, i.e., materials that change their color or
fluorescence in response to mechanical deformation. To accomplish this,
suitable polyurethane polymers of differing hardness grades were tested with a
range of organic solvents to formulate low-viscosity, inkjet-printable
solutions. Following their rheological characterization, two solutions
comprised of "soft" and "hard" polyurethanes were selected for in-depth study.
The solutions were imbibed with a mechanochromic additive to yield fluorescent
inks, which were either dropcast onto polymeric substrates or printed to form
checkerboard patterns of alternating hardness using a lab-built, multimaterial
inkjet platform. Fluorescence imaging and spectroscopy were used to identify
different hardness grades in the dropcast and printed materials, as well as to
monitor the responses of these gradient materials to mechanical deformation.
The insights gained in this study are expected to facilitate the development of
inkjet-printable, mechanochromic polymer materials for a wide range of
applications.
