Researchers develop a new way to 3D print smart electronic clothing

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Researchers from Zhejiang University, China, have developed a new method for 3D printing electronic circuits using liquid metal microgels.

The new approach involves encapsulating microdroplets of liquid metal (specifically gallium) in alginate-based microgel shells to create a 3D printable and recyclable liquid metal microgel (LMM) ink. According to the team, LMM ink solves many of the processability issues associated with raw liquid metals, enabling the printing of flexible electronics on a wide variety of surfaces.

Zhejiang researchers have already used their low-cost formulation to 3D print a near-field communication (NFC) tag on a t-shirt, paving the way for smart electronic clothing.

A T-shirt with a 3D printed NFC tag on the front. Photo via Zhejiang University.

Working with liquid metals

You may have heard the term ‘liquid metal’ before – it is used to describe metals with a melting point around room temperature, meaning they tend to be in a liquid state . Two of the most common liquid metals are mercury and gallium. Mercury has a melting point below -38.8°C, but it is far too toxic for use in many everyday applications. Gallium, on the other hand, has a melting point of 29.8°C but is safe enough to use around human beings.

Since gallium has excellent electrical conductivity, the liquid metal has reportedly seen renewed interest in recent years. The material is considered ideal for conductive components, enabling new applications in flexible electronic circuits, energy devices, wearable health monitoring devices, and even electronic skin.

Unfortunately, due to its high surface tension, gallium has difficulty forming continuous, intricate patterns and instead likes to form into low surface energy spheres. It is therefore difficult to model the liquid metal in its pure form in circuits and other high-precision devices.

Preparation of LMM ink using an aqueous solution of gallium and sodium alginate.  Image via Zhejiang University.
Preparation of LMM ink using an aqueous solution of gallium and sodium alginate. Image via Zhejiang University.

3D printing of liquid metal microgel ink

To solve the processability problem, the Zhejiang researchers simply developed their own 3D-printable LMM ink. They did this by stirring gallium with an aqueous solution of sodium alginate, which crosslinks Ga3+ with sodium alginate to form liquid metal droplets coated with microgel. Microgel shells improve the printability of liquid metal droplets, allowing them to be used in the direct-to-ink 3D printing process.

When initially printed, LMM circuits are not actually conductive. But they can be activated by stretching them slightly as this breaks the non-conductive alginate bonds in the ink. Conductivity can also be reactivated by freezing and pressing the 3D printed LMM circuits, which means they can be used in extreme environments such as outer space.

By testing the 3D-enabled circuit boards, the team discovered that they exhibited many characteristics essential to flexible electronics. This includes excellent conductivity, significant resistance response to strain with low hysteresis, and high durability against non-planar forces.

To test the performance of the 3D printed circuits on unconventional surfaces such as fabrics, they then 3D printed a flexible NFC tag on the front of a T-shirt. The beacon can communicate with smartphones to run a custom piece of code, in this case automatically opening a web page.

Ultimately, the team hopes that the combination of low-cost LMM ink and the personalization benefits of 3D printing can one day enable standardized applications of smart electronic wearables for personalized health monitoring, touch sensing and human-computer interaction.

Further details of the study can be found in the article titled “Liquid Metal Microgels for Three-Dimensional Printing of Smart Electronic Clothes”.

Enable 3D printed LMM circuits with a little stretch.  Image via Zhejiang University.
Enable 3D printed LMM circuits with a little stretch. Image via Zhejiang University.

Additively manufactured electronics is certainly not limited to the academic sphere, with more and more industrial organizations involved. Just last month, UK robotics company Q5D Technology secured $2.5 million in seed funding to fund the development of a new multifunctional wiring and electronics 3D printer. Known as the “CU500”, the 5-axis FFF system features an interchangeable tool head capable of rapidly depositing polymers, metallic pastes and wiring on large aerospace and automotive parts or electronic devices.

More recently, ETH Zurich spin-out Scrona AG raised $9.6 million to develop its own high-resolution method of 3D printing electronic devices. Unlike conventional inkjet, the company’s multi-nozzle electrostatic printing process uses a piezoelectric field generated just outside the tip of a printing nozzle to blast multiple materials onto substrates at a sub-micron level of precision. The company says its approach enables the creation of PCBs, semiconductors and displays at resolutions 100 times higher than before.

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Featured image shows a T-shirt with a 3D printed NFC tag on the front. Photo via Zhejiang University.

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