Printed lines of the water-soluble, conductive paste that consists of a polymeric binder (PBTPA) and Mo micro/nanoparticles, for applications in bio/ecoresorbable devices.
Mo/PBTPA paste used as an interconnect with an RF coil of Mg. Here, a Cu wire electrically joined by Mo/PBTPA to the start and end points of the coil closes the loop. A primary coil transfers radio frequency power to the secondary coils (Mg RF coils), thereby activating an LED.Transient electronics technology is an emerging field that requires materials and devices to be capable of degrading with minimal or non-traceable remains over a period. Electronics able to degrade naturally without hazardous waste streams, biomedical implants that could dissolve in biofluids and eco-resorbable antennas are not science fiction but rather a science fact. Although this concept has only a few years of development, this emerging technology has already gained researchers attention.
One area of growing interest is in conductive materials that could be printed at sufficient thickness to enable three-dimensional interconnects and additional features for circuit boards. So far, composites of hydrophilic polymers that could yield water-soluble, printable conductors have been reported, however, the main disadvantages are the rapid degradation upon immersing in water and their cytotoxicity. Recently, an interdisciplinary research group from Northwestern University, USA, engineered a transient conductive paste which is biocompatible and proves to offer stable operation over extended periods of immersion in aqueous environments [Rogers et al., Materials Today (2018), doi: 10.1016/j.mattod.2017.12.005].
This highly conductive ink derived from an in situ polymerisation yields a polyanhydride, a hydrophobic polymer, containing a dispersion of molybdenum (Mo) composites. The resulting material can be used as a printable conductor for water-soluble interconnects, antennas, resistive pressure sensors and strain gauges.
“Our bio/ecoresorbable conductive inks can be printed easily, so they can serve as a printable conductor for electrical interconnects, antennas, strain gauges, and resistive pressure sensors,” explains Seungae Lee, first author of the study.
The main advantage of this paste formulation is its electrical continuity. The group examined that microparticles of Mo are a better choice for other transient metals (Fe, Zn, W) because they are conductive, resistant to shaping thick oxides and economical. During the in situ polymerisation, the polymer homogeneously blends with the Mo microparticles, yielding uniform dispersions with enhanced interfacial adhesion between Mo and the polymer. This compact conductive network also prevents the permeation of water through the interfaces. Thanks to these features, the current persists for over 9 days of immersion in deionized water at 37 °C, thereby greatly exceeding the lifetime of other paste formulations that dissolve in less than few hours. This indicates that electronic devices embedded with these inks could operate solidly over prolonged periods of immersion in aqueous environments.
As proof of concept, the research team demonstrated the suitability of these conductive inks through patterning conductive traces of the Mo/anhydride ink and incorporating them into biodegradable electronic devices. The paste was successfully integrated into strain and pressure sensors and applied as an interconnection in coils. Tests in animal models verifies the biocompatibility of the paste and its degradation products, paving the way to an extensive potential for biomedical devices.
“We believe this work promises the practical application of bio/ecoresorbable devices," Lee says.