25.07.2009, Hürriyet Daily News

Turk Researchers Make Techno Leap

Turkish researchers have developed arguably the world's first "bio-implantable passive sensors," meta-material-based strain detectors that are highly sensitive to mechanical deformation and help in the assessment of bone fractures. Rohat Melik, a doctoral student at Bilkent University, and university researchers Emre Ünal and Nihan Koşku Pekgöz carried out studies under the chairmanship of Assoc. Prof. Hilmi Volkan Demir, a lecturer in the Bilkent University Electrical and Electronic Engineering Department in Ankara.

A large U.S. firm has said it is interested in using the technology in its new-generation implants.

According to Demir, "One out of 10 bone fractures do not heal properly due to improper load distribution and strain profiles during the healing process." The professor said that in order to create bio-implantable tools that could assess such fractures, the team designed novel, passive, on-chip, RF-MEMS strain sensors that rely on the resonance frequency shifts that signal mechanical deformation.

"For this purpose, we modeled, fabricated and experimentally characterized two on-chip sensors with high-quality factors for in-vivo implantation," he said. Demir said one demonstrate that these novel sensors of the sensors has an area of 0.12 square millimeters with a quality factor of 60 and the other has an area of 0.07 square millimeters with a quality factor of 70.

In order to monitor mechanical deformation by measuring the change in resonance frequencies with the applied load, the team employed a controllable, point-load-applying experimental setup designed and constructed for in-vitro characterization. In the case of the sensor with the larger area, when the team applied a load of 3920 N, they obtained a frequency shift of 330 MHz and a quality factor of 76. With the smaller sensor, the frequency shift and quality factor were increased to 360 MHz and 95, respectively.

This data demonstrates that the sensor chips have the capacity to withstand relatively high physiologic loads and that the concomitant and very large resonant-frequency shift with the applied load was achieved while maintaining a high signal-quality factor.The experiments also have the capacity to produce highsensitivity strain readouts, even when the total device area is quite small. The team additionally demonstrated that bio-implantable passive sensors could deliver a telemetric, real-time readout of the strain on a chip.

By placing two more resonators on the sides of the sensor to serve as transmitter and receiver antennas, they achieved the transfer of contact-less power and read-out loads in the absence of direct wiring to the sensor. With this model, where telemetric measurements become simpler due to the fact that all sensor systems are built on the same chip, the team obtained a frequency shift of 190 MHz with an increase in the quality factor from 38 to 46 when a load of 3920 N was applied.

Therefore, as a first proof of concept, the team demonstrated the feasibility of the on-chip strain sensors for monitoring the mechanical deformation using telemetry-based systems. The research was published in the international magazine Applied Physics Letters.