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Electrodes for Implants 3-D Printed on Soft Materials

Electrodes on gummy bears

Microelectronic circuits printed on gummy bear candy gelatin (Nouran Aldy, Technical University of Munich)

22 June 2018. An engineering team produced with an ink-jet printer working electrodes like those used in medical implants on bio-friendly soft materials rather than metal or silicon. Researchers from Technical University of Munich in Germany describe their process in a recent issue of the journal NPJ Flexible Electronics.

A team from the neuroelectronics lab led by bioengineering professor Bernhard Wolfrum is seeking better materials for implants with electrodes, used in a range of medical applications. These microelectronic arrays, note the authors, can monitor electrical signals in cardiac and nervous system networks, restore sensory functions, and deliver therapeutic electronic pulses. Most of these electrode collections today are fabricated on silicon or some metals, which while immediately functional, do not match the structure of living tissue, and can also cause adverse reactions such as inflammation in recipients.

Wolfrum and colleagues are also seeking faster and more economical methods to produce these circuits. While other labs are also working with electrodes on soft materials, say the authors, they’re using conventional fabrication methods designed for rigid materials like silicon. As a result, most of today’s methods with these materials are slow and require access to specialized — and expensive — production facilities.

The Munich team instead chose to use ink-jet printing, an old technology updated for three-dimensional output. One advantage of ink-jet printing is the ability to quickly produce and test different solutions, something that’s difficult to do with conventional facilities. “If you instead print the electrodes,” says Wolfrum in a university statement, “you can produce a prototype relatively quickly and cheaply.  The same applies if you need to rework it. Rapid prototyping of this kind enables us to work in entirely new ways.”

The researchers produced and tested electrodes in several soft materials, using a commercially available ink-jet printer. In all of the materials, the electrodes were printed with carbon inks, adding a neutral protective layer between the circuits and the surrounding substances. One material tested is agarose, a common substance in labs that forms into inert gels.

Another material is polydimethylsiloxane, or PDMS, a widely used biocompatible polymer found in a number of products and used more recently in fabricating biomedical micro-electromechanical systems. The PDMS circuits in this case require a preliminary treatment so the carbon ink would stick. In January 2018, Science & Enterprise reported on hydration sensors made from PDMS, built into wearable devices. A third material is candy gelatin, popularly known as gummy bears, that the team melted down and fed into the ink-jet printer.

In proof-of-concept tests, the team evaluated the ink-jet printed circuits in lab cultures with chemically-stimulated cardiac muscle cells. The results show the electrodes printed with carbon ink on all three soft materials recorded electrical signals from the cardiac cells as well as carbon ink on gold circuits. The tests also show the small size of the soft material circuits — about 30 micrometers wide, on average — can record signals from small numbers of cells, and even individual cells.

The researchers are now advancing the technology to print more complex 3-D electrode arrays, and investigating printed sensors for detecting chemical changes as well as electrical signals.

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