Piezoelectric Drop‐On‐Demand Inkjet Printing of High‐Viscosity Inks (original) (raw)

2021, Advanced Engineering Materials

Inkjet printing (IJP) has been adopted as a material deposition technology in different fields, such as, electronics, [1] biology, [2] and biomedicine, [3] being a fully recognized flexible, scalable, and cost-effective technique. [4,5] In contrast to more traditional manufacturing techniques, IJP is a solutionbased maskless additive technique, allowing minimum material waste combined with extreme precision in controlling the deposition of active material droplets in the picoliter range. [6-9] There are two main IJP modes of operation: the continuous inkjet printing (CIJ) and the drop-on-demand (DOD) system. [10] Despite CIJ being widely exploited in graphical applications, such as coding and marking, the great advantage of a DOD system over CIJ is the possibility to print smaller features (i.e. %20-50 μm vs %100 μm of CIJ) and that the ink droplet is ejected only when is needed, eliminating most of the complex structural parts present in CIJ. [11] Finally, CIJ systems involve a recycling system that can potentially cause the contamination of the ink itself. [12,13] Current DOD ink-printing technology works through various methods, such as thermal, electrostatic, piezoelectric (PZT), acoustic, and laser-assisted. [14-16] The active material is processed as a solution, i.e., ink, requiring specific physical properties to be framed in very restricted ranges of values to guarantee ink printability. [17] For example, the upper limit of viscosity for IJP is 20-40 mPa s. [18] This represents one of the major drawbacks of the DOD system, which shows severe limitations in ink printability. A possible solution to reduce the viscosity is to increase the temperature. Despite being a very intuitive and simple method, some issues may arise when dealing with temperature-sensitive inks as their properties can be degraded beyond a certain temperature value. The need for low-viscosity inks poses many limitations because inks generally require high dilution to be processed. This translates to a reduction of the functional material content, resulting in the necessity of printing more layers to reach a certain target thickness. However, printing many layers can affect and lower the final resolution of the printed pattern. In addition to this issue, the necessity of keeping low the concentration of the functional material is a potential limiting factor for the formulation of inks capable of limiting the well-known coffee ring effect (CRE). The CRE is an extremely common defect defined as the progressive accumulation of nonvolatile material toward the edges, with a consequent depletion from the inner