Bend me, shape me

Flexibility has always been one of the promises of hybrid electronics approaches that add an organic structural element to the conventional inorganic semiconductors. In recent years, circuitry that is not only bendable but often stretchable is beginning to emerge from research laboratories to open up entirely new form factors for the military, for consumer gadgets and for medical applications. Benjamin Leever of the Air Force Research Laboratory (AFRL) at Wright-Patterson Air Force Base, near Dayton, Ohio, USA, and colleagues are at the forefront of such efforts, which they refer to as flexible hybrid electronics..

Leever recently offered insights into the developments taking place in the AFRL to delegates at the 250th National Meeting & Exposition of the American Chemical Society (ACS), which took place in Boston, Massachusetts, during August, 2015.

"Basically, we are using a hybrid technology that mixes traditional electronics with flexible, high-performance electronics and new 3D printing technologies," explains Leever. "In some cases, we incorporate 'inks,' which are based on metals, polymers and organic materials, to tie the system together electronically. With our technology, we can take a razor-thin silicon integrated circuit, a few hundred nanometers thick, and place it on a flexible, bendable or even foldable, plastic-like substrate material," he adds.

The Wright-Patterson team turned to liquid gallium alloys as an interconnect material for their hybrid circuitry. "While these liquid alloys typically oxidize within minutes and become essentially useless," Leever says, "the team has been able to dramatically reduce the effects of the oxidation through the use of ionic species confined to the walls of microvascular channels within the flexible substrates." The resulting materials are thin and foldable so could be used to pack circuitry into tight spaces or squeeze it through gaps into voids wherein it unfurls, such as within a complex curved volume like an aircraft wing, for instance.

Such circuits might be used to measure stresses and strains on wings. Similarly, flexible devices could be incorporated into a flightsuit to monitor aircrew health without risk of body movements cracking a circuit board. Similar technology could be incorporated into civil engineering structures such as bridges and skyscrapers and in the tracksuits and footwear of athletes and for in-home, wearable tele-healthcare devices for the sick and infirm.

"Overall, the military has the advantage of being able to move ahead with potentially higher risk research," Leever points out. "Commercial investors want a clear demonstration before making an investment. The military can pursue possibly transformational applications at earlier stages if we see a promising approach to realize and advance a technology's revolutionary potential. When we are successful, the commercial sector directly benefits."

"In the near-term we are investigating approaches to transition from single-layer circuits to multi-layered circuits with printed vias. electrical interconnects between the layers of a multi-layered circuit board, which will maximize the functionality we can add to a small space," Leever told Materials Today. "The key challenge here is adapting printing processes to carefully print different materials in quick succession. For the community overall, the most important next step is developing and maturing the manufacturing technology that will enable us to advance from demonstrations in the lab to actual products for military, medical, and consumer use."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".