Printing Next-Generation Flexible Electronics

Systems that precisely deposit tiny droplets of fluid can be used to print metals, organic electronic materials, graphene nanostructures, and other materials onto a wide range of substrates. Called Microplotters, these systems provide a unique approach to the rapid prototyping of next-generation flexible electronics, including electrode arrays, microfluidic applications, sensors, and graphene-based electronics.

Image Credit: Sonoplot

From bioelectronics to graphene devices, many emerging and developing electronics applications require the ability to pattern conductive, dielectric, and semi-conductive materials onto a variety of flexible substrates accurately.

For such applications, non-contact methods of fabrication generally offer lower costs and reduced material wastage compared to contact printing. However, the widely adopted approaches of screen-printing and ink-jet printing typically exhibit poor uniformity and low pattern resolution.1,2

Microplotters developed by SonoPlot offer an alternative route. These benchtop systems rely on controlled ultrasonics to deposit picoliter volumes of various inks, including suspensions of metallic nanoparticles, polymers, graphene nanostructures, and many other materials. This unique technology can produce features from 5-200 μm with deposited feature diameter variability as low as 10% in conjunction with a high-precision positioning system. The Microplotters offer significant advantages over screen-printing and ink-jet technologies in precision, flexibility, and cost.

Printing Electronics for Microfluidic Applications

Microfluidics is a well-established field that deals with the manipulation of small volumes of fluid. With the fundamental issues of microscale flow well addressed by the early 2000s, modern microfluidics research has focused on developing “integrated” devices that incorporate multiple fluidic, mechanical, and/or electronic components.3 Such devices are capable of incorporating many of the capabilities of room-sized laboratories into a chip-sized device. These devices, often categorized as “lab-on-a-chip” or micro-total analysis systems have broad applications, including biological and chemical analysis and rapid point-of-care diagnostics.

However, the marriage of electronics and microfluidics is far from straightforward. Microfluidic devices are typically fabricated from elastomeric materials such as plastics and silicone rubbers, which are well-suited to creating microfluidic channels and can be used to develop flexible and biocompatible devices. The staple materials of electronics, on the other hand, are typically stiff. Traditional semiconductors, dielectric insulators, and metal conductors are mechanically mismatched with microfluidics' flexible and deformable materials.4 This limits the mechanical deformity and reliability of integrated electronic/microfluidic devices.

To address this, a number of flexible polymer-based electronic materials such as conjugated polymers (CPs) and organic field-effect transistors (OFETs) have been developed and incorporated into microfluidic devices. Such materials exhibit increased mechanical compatibility with the essential microfluidics materials and are also compatible with non-contact printing technologies.5–7

Microplotters stands out as an ideal technology for the fabrication of integrated electronic/microfluidic devices, offering the required precision and uniformity, and compatibility with a huge range of materials – including those used in polymer-based electronics.8 Microplotters offer capabilities beyond that of standard inkjet printers: they allow for in situ modification of the polymer films during the deposition process. They can print truly continuous lines (as opposed to inkjet printers, which require the coalescence of multiple droplets).

Printing Microelectrode Arrays

Microelectrode arrays (MEAs) are small devices containing tens to thousands of microelectrodes. The high electrode density of MEAs renders them suitable for bioelectronics applications, specifically in developing neural implants where electrodes can read and/or stimulate neurons in vitro.9 Perhaps the most promising applications of such devices are biomedical implants in which an MEA can restore biological functionality. In 2016, such a device successfully alleviated gait deficiencies in primates with spinal injuries.10–13

Most previously-investigated MEA-based devices rely on silicon or polymer materials, which offer poor biocompatibility: their physical and mechanical properties can cause inflammatory responses in neurons or loss of functionality. To address this, much recent research has focused on the development of flexible MEAs with increased biocompatibility.14

Ink-jet printing has been successfully used to develop bioelectronic MEA interfaces by depositing carbon electrodes on PDMS and hydrogels.12 However, ink-jet technologies typically require high operational costs to achieve the level of uniformity and precision required in such applications. Microplotters typically exhibit higher performance at a much lower price, making them an ideal candidate technology for the fabrication of flexible MEAs and other biomedical devices. SonoPlot’s Microplotter systems can print any ink that a standard ink-jet can, plus a range of different inks.

Printing Next-Generation Electronics with Graphene

Researchers at Michigan State University demonstrated the Microplotter’s capabilities in the realm of printable, flexible electronics by using the technology to print graphene-based integrated circuits and thin-film transistors. The team used a SonoPlot Microplotter to deposit droplets of a carbon nanotube suspension on a polydimethylsiloxane (PDMS) substrate.7

Image Credit: Sonoplot

The approach laid the groundwork for the production of next-generation graphene-based and flexible electronics using an ultrasonic deposition. SonoPlot’s Microplotters can print with inks ranging from 0 to 450 centipoise and suspensions with particle size diameters ranging from nanometers up to 15 microns. This makes them suitable for printing organic and nanoparticle inks and polymers and graphene nanostructures for printed, electronic applications.

SonoPlot has developed two Microplotter systems: The entry-level Microplotter Proto and the ultra-precise Microplotter II. Both systems are designed to provide increased precision and flexibility to the microarray and polymer electronics market. To find out more about these Microplotter systems and the advantages they can offer over existing products, contact SonoPlot today.

References and Further Reading

  1. Han, S.-T. et al. An Overview of the Development of Flexible Sensors. Adv. Mater. 29, 1700375 (2017).
  2. Senthil Kumar, K., Chen, P.-Y. & Ren, H. A Review of Printable Flexible and Stretchable Tactile Sensors. Research 2019, 1–32 (2019).
  3. Erickson, D. & Li, D. Integrated microfluidic devices. Analytica Chimica Acta 507, 11–26 (2004).
  4. Cheng, S. & Wu, Z. Microfluidic electronics. Lab Chip 12, 2782 (2012).
  5.  Liu, C. et al. 3D Printing Technologies for Flexible Tactile Sensors toward Wearable Electronics and Electronic Skin. Polymers 10, 629 (2018).
  6. Khong Duc, C., Hoang, V.-P., Tien Nguyen, D. & Thanh Dao, T. A Low-Cost, Flexible Pressure Capacitor Sensor Using Polyurethane for Wireless Vehicle Detection. Polymers 11, 1247 (2019).
  7. Cai, L., Zhang, S., Miao, J., Yu, Z. & Wang, C. Fully Printed Stretchable Thin-Film Transistors and Integrated Logic Circuits ACS Nano doi:10.1021/acsnano.6b07190.
  8. Cheun, H. et al. Polymer light emitting diodes and poly(di-n-octylfluorene) thin films as fabricated with a microfluidics applicator. Journal of Applied Physics 100, 073510 (2006).
  9. Design and Implementation Challenges of Microelectrode Arrays: A Review. https://www.scirp.org/html/35354.html.
  10. Kessler, D. K. The Clarion® Multi-StrategyTM Cochlear Implant. Ann Otol Rhinol Laryngol 108, 8–16 (1999).
  11. Chapman, C. A. R., Goshi, N. & Seker, E. Multifunctional Neural Interfaces for Closed-Loop Control of Neural Activity. Advanced Functional Materials 28, 1703523 (2018).
  12.  Adly, N. et al. Printed microelectrode arrays on soft materials: from PDMS to hydrogels. npj Flexible Electronics 2, 1–9 (2018).
  13.  Capogrosso, M. et al. A brain–spine interface alleviating gait deficits after spinal cord injury in primates. Nature 539, 284–288 (2016).
  14.  Kim, R., Joo, S., Jung, H., Hong, N. & Nam, Y. Recent trends in microelectrode array technology for in vitro neural interface platform. Biomed. Eng. Lett. 4, 129–141 (2014).​​​​​​

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