New tissue-hugging implants using flexible electronics
Sure, LED tattoos might look cool, but now scientists have found an even better use for flexible silicon technology. In what represents the first use of such technology for a medical application a team of cardiologists, materials scientists, and bioengineers has created and tested a new type of implantable device for measuring the heart’s electrical output that the team says is a vast improvement over current devices and could also mark the beginning of a new wave of surgical electronics.
Several treatments are presently available for hearts that dance to their own tempo, ranging from pacemaker implants to cardiac ablation therapy, a process that selectively targets and destroys clusters of arrhythmic cells. Current techniques require multiple electrodes placed on the tissue in a time-consuming, point-by-point process to construct a patchwork cardiac map. In addition, the difficulty of connecting rigid, flat sensors to soft, curved tissue impedes the electrodes' ability to monitor and stimulate the heart.
New technology that is the result of a collaboration between researchers at Northwestern University and the Universities of Illinois at Urbana-Champaign and Pennsylvania allows the full power of silicon electronics to be directly applied to body tissue. Consisting of a flexible sensor array that can wrap around the heart to map large areas of tissue at once, the new device allows for measuring electrical activity with greater resolution in time and space. The new device can also operate when immersed in the body's salty fluids and can collect large amounts of data from the body at high speed.
The new device is 14.4mm x 12.8mm, roughly the size of a nickel. It is made of nanoscale, flexible ribbons of silicon embedded with 288 electrodes, forming a lattice-like array containing 2,016 silicon nanomembrane transistors, each monitoring electricity coursing through the heart. Standard clinical systems usually have only five to 10 electrode contact points. The patchwork grid of cardiac sensors adheres to the moist surfaces of the heart on its own, with no need for probes or adhesives, and lifts off easily.
By bringing electronic circuits right to the tissue, rather than having them located remotely, the device can process signals right at the source. This close contact allows the device to have a much higher number of electrodes for sensing or stimulation than is currently possible in medical devices. The researchers say the new device will be able to map the body's complicated electrical networks in much more detail, with more effective implantable medical devices and treatments likely to emerge.
The team tested the new devices on the hearts of live pigs, a common model for human hearts. They witnessed a high-resolution, real-time display of the pigs' pulsing cardiac tissues – something never before possible. They were able to demonstrate high-density maps of electrical activity on the heart recorded from the device, during natural and paced beats.
The current device is not wireless. The next big step in this new generation of implantable devices, say the researchers, will be to find a way to move the power source onto them. One solution could be to have the heart power the device.
The device is based on the flexible electronics developed by Yonggang Huang and his collaborator John A. Rogers in 2008. Any significant bending or stretching to circuits renders an electronic device useless, which is what limits conventional electronics for use on the body. Huang and Rogers jumped this sizeable hurdle by creating an array of tiny circuit elements connected by metal wire "pop-up bridges." When the array is bent or stretched, the wires - not the circuits - pop up. This approach allows circuits to be placed on curved surfaces.
The emerging technology holds promise for a new generation of flexible, implantable medical devices, for the treatment of abnormal heart rhythms or epilepsy, as well as new flexible sensors, transmitters, and photovoltaic and microfluidic devices.
Another focus of ongoing work is to develop similar types of devices that are not only flexible, like a sheet of plastic, but fully stretchable, like a rubber band. The ability to fully conform and wrap around large areas of curved tissues will require stretchability, as well as flexibility.
The team’s research appears in the paper "A Conformal, Bio-Interfaced Class of Silicon Electronics for Mapping Cardiac Electrophysiology," which is published in the March 24 issue of Science Transitional Medicine.