Advancing Flexible Electronics: High-Resolution Microplotter Printing Unveils New Horizons

Inkjet printing, a prevalent method in the advanced manufacturing sector, plays an essential role in the creation of electronic devices. However, a notable challenge that accompanies this technology is the “coffee ring effect,” a phenomenon observed when a droplet containing suspended particles evaporates on a surface, leaving behind a ring-like stain. This effect, mirroring the pattern of a dried coffee spill, results in uneven material deposition, presenting a significant hurdle in achieving high-quality prints, especially in applications demanding precision and uniformity such as in printed electronics. The crux of this issue lies in the capillary flow within the droplet: as it dries, the liquid’s surface tension draws particles towards the edges, leading to a more concentrated deposition at the perimeter than at the center. This uneven distribution can compromise the quality and functionality of the printed material, particularly in flexible electronics where uniformity is vital for device performance. Increasing the viscosity of the deposition fluid is one method, among others, that has been shown to be efficient in suppressing the “coffee ring effect”. However, a typical inkjet printer requires ink with a low viscosity, usually not exceeding 20-40 cP, depending on the ink’s density and surface tension.

Addressing these challenges, a new study published in ACS Applied Electronic Materials by Dr. Yunnan Fang, Dr. Haiyang Zou, Song Peng, Dr. Gaoya Dong, and Professor Manos Tentzeris from the School of Electrical and Computer Engineering at the Georgia Institute of Technology, introduced a novel approach in fabricating flexible electronic devices using microplotter printing technology. This technique, distinct from conventional inkjet printing, is adept at handling high-viscosity (e.g., 450 cP) inks, thus mitigating the “coffee ring effect” and its associated drawbacks like material nonuniformity and the occurrence of defects such as grooves, pits, and cracks. The research team employed a microplotter printer, and was able to utilize a concentrated (7 wt %) and correspondingly viscous PVDF ink  to produce nanogenerators and chemiresistive gas sensors with remarkable efficiency and quality. They found that unlike inkjet-printed patches that displayed significant nonuniformity and defects even after numerous passes, microplotter-printed PVDF patches exhibited morphological homogeneity and flawlessness, demonstrating the superiority of this printing technique in maintaining material integrity and uniformity.

The in-situ fabrication process introduced by the researchers simplifies the production workflow by integrating all necessary steps including printing, ink changing, printhead cleaning, and post-printing treatments directly on the printer platform. This innovation not only streamlines the fabrication process, reducing the labor and time involved, but also ensures that the workpiece remains undisturbed throughout, thus preserving the integrity and uniformity of the printed layers. Moreover, the PVDF-based nanogenerators produced through the new method showcased the promising piezoelectric performance, generating twin pulses of short-circuit current in response to mechanical bending and unbending, with a current density reaching up to approximately 0.4 μA/cm². This reported performance is indicative of the high quality and uniformity of the printed PVDF patches, which were found to be homogeneous and free from the typical defects associated with inkjet printing. Furthermore, the compatibility of the microplotter printing technology with low-viscosity inks was demonstrated through the successful printing of a fine-featured flexible carbon nanotube-based chemiresistive gas sensor. The inks used in this process had a viscosity of only around 0.9 cP. This highlights the potential of microplotter printing as a robust and flexible tool for the fabrication of a wide range of electronic devices, and offers a viable solution to the current limitations of conventional inkjet printing.

According to the authors, the new in-situ fabrication process minimizes and streamlines nonprinting operations, significantly reducing the complexity and time involved in the production of electronic devices. The approach enhances the performance and reliability of the devices as well as offers a scalable and cost-effective method for their production, which is important for the widespread adoption of flexible electronics in various industrial applications. Overall, the study by Dr. Yunnan Fang and colleagues contributes to advancing the field of flexible electronics by providing an alternative fabrication method that addresses several limitations of conventional printing techniques. This could accelerate the development and deployment of flexible electronic devices in various applications, including wearable technology, healthcare monitoring, and energy harvesting.