Integrated Bio-Electronics
December 10, 2012, International Electron Devices Meeting, San Francisco—John Rogers from the University of Illinois at Urbana-Champaign described the bifurcating directions of bio-electronics. The field is splitting into integrating biological functions with electronics, and transient electronics.
The electronics revolution has transformed dimensions for biological interfaces from room-sized, to hand-held, and now towards embedded and integrated systems. At the same time, uses are moving from highly specialized to now cover all aspects of life. These trends are possibly complementary and only need changes in substrates. For integrated bio-electronics, one needs to map to existing biological structures, which happen to be curvy and stretchy. Integrated bio-electronics cannot follow the rigid, planar form factors in most electronics.
For example, brain interfaces are incompatible with normal silicon, which comes in wafer form. The planar substrates don’t mate well with the lumpy brain structures, and previous attempts have put multiple probes on a substrate. The problem is that the contacts to the brain from the reverse pin cushion degrades rapidly, somewhat like glass shards in jello.
When the silicon is in non-wafer form, in a nano-membrane, it is much more flexible. The stiffness is a cubic function of the thickness, so a very thin silicon membrane can be used for implanted sensors. Most other sensors need a flexible platform, because the silicon sensors are thick and brittle, so other materials are necessary. Very thin silicon, on the other hand, can be bonded to almost anything.
Bio-sensors need stretch in addition to flexibility, so a wavy material is needed. If you apply silicon to a pre-strained substrate, the silicon forms an “accordion” structure when the strain is released. Additional strain is possible by applying the silicon in “loopy” structures.
The ideal characteristics for an electronic skin patch are ultra thin, stretchable, air and water permeable but waterproof. These characteristics require a mix of mechanical, chemical, and electronic engineering and the result is the ability to integrate electronics onto the skin on a silicon rubber substrate. It turns out the design implementation is fairly easy, since the adhesion is due to van der Waals forces, and not any special adhesives.
It turns out that the substrates already exist, they are used in temporary transfer tattoos. These are silicon rubber material with a water soluble PVA backing. Adding sensors and RF circuitry is just a matter of detaching the circuit from the pre-stretched precursor and laying it on the rubber.
The temporary tattoo sensors can remain on the skin for about 2 weeks, but then fall off due to skin exfoliation. The on-board circuitry can be scaled up to higher densities to measure physiological parameters like eeg, ecg, emg, temperature, pressure, etc. The patches can apply stimulus as well as measure signals.
One application they tried was a neck emg and eeg combination sensor that included a small sense vocabulary. The output controlled some parts of a video game. Other controls and functions are obviously possible with more sensor types and software. Applications in healthcare are interesting because the patches with RF links can eliminate the wires normally associated with hospital monitoring equipment. For neonatal and intensive care, the removal of the wires makes it easier to perform other procedures and is more comfortable.
These small devices can benefit many users and the advanced mechanisms enable novel applications. The challenges are in high reliability functions and long operating life. Unfortunately, biology tends to fight everything not native.
An alternative for long-term monitoring is to use re-sorbable or transient electronics. These devices are designed to disappear over time at a programmed rate or with some trigger event. The result is an ability to implant electronic diagnostics, sense environments, and use a zero electronic waste device. The silicon is water soluble at physiological pH and temperatures and becomes SiOH4, which is eliminated in 2-3 weeks.
The chemical engineering for this type of sensor is very important. A thin sheet of silicon, about 35 nm thick, will last about 10 days in a solution. The substrate for the metallization is silk and the conductors are magnesium and magnesium oxide. All of these materials are bio-compatible and exist within the body. The quantities are orders of magnitude lower than the USDA recommended daily allowance, so the devices are eatable.
Future topics for investigation include greater degrees of bio-integration which will need more efforts on the mechanics, shapes, and structures of the devices, and other functions in transient electronics that will require much more chemistry and biological research. The unusual manufacturing practices will require multi-disciplinary efforts to implement products.
