Silicon circuits for biomedical devices gain flex - implantable electronic circuits
A research group in South Korea has developed a range of flexible and large-scale integrated (LSI) devices for biomedical wireless communication. The team at the Korea Advanced Institute of Science and Technology in Daejeon, says the technology could be applied where full contact with the curvilinear surfaces of a human organ is needed on a long-term basis, for example as an artificial retina.
Silicon-based semiconductors are used in far-reaching applications from nerve stimulation to memory storage and implantable electronics. However, the bulk and rigidity of conventional LSI chips has hampered their use as bio-implants, and while flexible circuitry has been developed elsewhere, their chip-count has been low, limiting processing power. Also, many are not able to withstand the ingress of moisture for very long.
The team, led by Professor Keon Jae Lee, says its latest work overcomes these drawbacks by monolithically encapsulating silicon-based radio frequency integrated circuits (RFICs) in a biocompatible liquid crystal polymer (LCP).
The RFICs, which consist of about 1,000 nano-transistors, are fabricated on the top layer of a silicon-on-insulator (SOI) wafer using the conventional 0.18μm RF CMOS process. The wafer is sealed and the sacrificial bottom layer removed by boiling in solutions of potassium hydroxide or tetramethylammonium hydroxide. The chips are then transferred onto a flexible LCP substrate, a 50μm–thick monomer, and coated with silicone adhesive, then covered with another LCP layer, this time a 25μm-thick rigid monomer, and bonded in a thermal press. The final encapsulated chips are about 100μm thick.
Testing of the chips was divided into three strands – measuring their in vivo performance, calculating their expected in vivo lifetime and testing their mechanical robustness.
In the performance tests, a chip was implanted under a rat’s skin for several weeks and its electrical characteristics assessed. Over that time, the chip’s insertion loss was maintained at about 2.5dB and isolation was formed near 28dB. In addition, the wound to insert the chip recovered from its original suture without liquid build-up or abscess.
Calculating expected in vivo lifetime was carried out using accelerated in vitro soak tests, by soaking the encapsulated chips in phosphate-buffered saline (PBS) solutions at temperatures ranging from 75– 95°C. If the solutions seeped through, their salinity would destroy the transistors. Using these tests, and the Arrhenius equation, the team estimated a lifespan of two years.
For mechanical robustness, the chips were subjected to more than 1,000 bending cycles at a curvature radius of up to 10mm, after which only small changes in their threshold voltage were measured. Lee says large-scale production is now being pursued and notes the work could lead to fully flexible consumer electronics, such as mobile application processors for mobile operating systems.