Intimate integration of high quality flexible electronics onto medical devices for cardiac procedures is the focus for a team of scientists and electrical engineers at the University of Illinois and Northwestern University, USA.

Lead professor John Rogers at Illinois and his team have identified a possibility for integrating stretchable electronics onto thin, elastic membranes of conventional balloon catheters. These can be used in surgical procedures for patients who suffer from types of cardiac arrhythmias.

Current invasive arrhythmia procedures involve two separate and rigid catheter devices. One that maps the heart point by point as a tube is maneuvered in search for irregularities, and one with an electrode at the end that ablates spots identified as aberrant one at a time. The device formed by Rogers’ team would allow both functions to be formed over large areas of the heart simultaneously.

Rogers elaborates, ‘Conventional balloon catheters are passive devices, whose only function is mechanical, that is to deploy an intravascular stent, or to remove blockage in an artery for example. The electronics convert these same platforms into sophisticated tools for diagnosis, monitoring and surgery’.

To develop the device, the team used multifunctional sensors created from low modulus elastomers (silicone) impregnated with carbon for soft tactile sensors. Ultra-thin metals and polymers were also used to form interconnect, electrophysiological, flow and temperature sensors, and ablation electrodes, along with ultra-thin plates of gallium arsenide for the LEDs on the device.

A meshwork of tiny node sensors has been created to mount directly onto the catheter balloon, so that the large strains associated with inflation/deflation of the balloon substrate would not affect the devices’ operation. Using a release and transfer step, the systems were integrated onto the outer surface of a deflated balloon catheter.

Rogers explains, ‘The sensors and other devices on the surface of the balloon are softly and conformally pressed against the endocardial surface via pneumatic pressure applied remotely through the shaft of the catheter to the interior surface of the balloon. In this format, the balloon can establish non-invasive contact with the tissue, in a way that can follow the time dynamic curvature of the tissue as the heart is beating. In this contacting mode, the device can map the electrical activity associated with the beating of the heart, and it can also ablate away the aberrant tissue’.

The challenge, highlights Rogers, is ensuring the electrodes at all levels of the balloon inflation. Since the centre of the balloon stretches more than the ends, understanding the range of strain and making sure the balloon’s performance is consistent is essential. In partnership with Northwestern University, the problem was solved by mounting the sensors and electrodes on ‘tiny rigid islands’. The team also used spring-like interconnects between the sensors to handle the 100% distance between the islands when the balloon inflates.

Dr Christopher McLeod, Principal Research Fellow of Cardiov Instrumentation at Imperial College London, UK, observes, ‘It is a very interesting development. The potential functionality of minimally-invasive catheters and implants will be greatly enhanced as both sensing and actuating functions can be added with no size implications due to the increased integration, in this case, of hitherto incompatible mechanical and electronic features. There will be issues about the durability of such highly integrated devices, but as most procedures are acute, this should be solvable’.

The next step will involve improving the number and density of the sensors, to more precisely locate aberrant tissue and for advanced mapping and ablation procedures.