Inspired by Coffee Rings, Cambridge Researchers Create Ink for Next-Gen Printed Electronics

August 17, 2020 by Antonio Anzaldua Jr.

When inkjet droplets dry in a "coffee stain" pattern, printed electronic devices can malfunction. A new formula spreads evenly to eliminate the issue.

A morning cup of coffee is one of the best ways to start a workday—unless you spill it over all your desk, leaving a coffee ring on all your papers. 

But in the case of researchers at the University of Cambridge, spilled coffee rings actually inspired a new method to optimize industrial-scale inkjet printing for printable electronic devices.


The Coffee Ring Effect

As coffee evaporates, it leaves behind a ring of liquid known as the coffee ring effect. In the past, this effect—which extends beyond coffee—has been a stumbling block in functional inks that include 2D materials, graphene, and nanoparticles. In these inks, the coffee ring effect (or uneven distribution of particles in inkjet droplets) inhibits the normal function of electronic devices in industrial applications. 


Coffee stain-inspired inkjet solution

The researchers claim this new solution could streamline time to market for batteries, solar cells, functional coating, and other electronic and photonic devices. Image used courtesy of the University of Cambridge


Inks behave similarly to coffee particles, accumulating around the edges and creating irregular shapes and uneven surfaces. This is especially true when printing on hard surfaces like silicon wafers or plastics.


Inkjet Printing for 2D-Crystal Electronics 

In recent years, researchers have devoted remarkable efforts to adapt 2D crystals for functional printing—the goal being scalable and low-cost device fabrication.

To this end, inkjet printing is one option to produce a range of printed electronic devices, such as solar panels, sensors, and transistors. But inkjet printing is sometimes overlooked because of the coffee ring effect; that is, it often fails to produce uniformly deposited functional structures. 


This technique allowed Hasan's team to print 4,500 nearly carbon-copy devices atop a silicon wafer/plastic substrate combo

This technique allowed Hasan's team to print 4,500 nearly carbon-copy devices atop a silicon wafer/plastic substrate combo. Image used courtesy of Science Advances


There are three critical parameters behind this phenomenon:

  1. Suboptimal droplet jetting
  2. Poor control over substrate wetting
  3. Uneven drying of the ink droplets

The University of Cambridge paper, published in Science Advances, explains that nonuniform printing can result in poor device performance by decreasing the material’s conductivity. 

This study allowed researchers to enhance their control of the droplet dimensions to benefit inkjet-printed 2D crystal devices.


Uniform, Pancake-Like Dried Ink

Since 2013, a team of researchers led by Dr. Tawfique Hasan has been observing the coffee ring effect to improve printable electronics manufacturing.

The solution they created is a mixture of isopropyl alcohol and 2-butanol used to modify how ink droplets dry—mitigating the coffee ring effect altogether. In contrast to traditional inkjet solutions, the particles in the new solution spread uniformly across the droplet, forming even thickness in a pancake-like shape.


This formula for inkjet printing of 2D crystals enables scalable device fabrication with consistent properties

This formula for inkjet printing of 2D crystals enables scalable device fabrication with consistent properties. Image used courtesy of Science Advances


According to the press release, this study could enable large-scale, high-speed additive manufacturing for all-printed sensors and systems.

The new inks do not contain polymers or commercial additives traditionally used to tackle the coffee ring effect either. These additives can sometimes interfere with the electronic properties of graphene and other 2D materials.

One of the researchers Guohua Hu explains, “Understanding this fundamental behavior of ink droplets has allowed us to find this ideal solution for inkjet printing all kinds of two-dimensional crystals."


Spilled Coffee Opens Doors for Electronic Printing

These researchers say their ink formulations can also print pure nanoparticles and organic molecules. The variety of materials could boost the manufacturing of electronic and photonic devices. 

Printed sensors and photodetectors were their first proofs of concept, which have shown promising results in terms of consistency. Dr. Hasan has high hopes for what this discovery will bring for him and his team: “Our technology could speed up the adoption of inexpensive, low-power, ultra-connected sensors for the internet of things.” 

Hu adds, "Our formulation can be easily scaled up to print new electronic devices on silicon wafers, or plastics, and even in spray painting and wearables, already matching or exceeding the manufacturability requirements for printed devices.