Two-part measurement system
Radio-frequency identification (RFID) generally targets applications like inventory control, security issues, and consumer-preference monitoring. Yet magnetic components specialist Premo and the Hamburg University of Technology (HUT) are developing implants to treat bone fractures.
Their objective is a system for monitoring the healing process of fractures treated by osteosynthesis. The system employs implants, a microcontroller (MCU), a strain gauge, and RFID components. It offers an alternative to X-rays and CT scans, accepted methods that have come under scrutiny due to radiation concerns. According to the researchers, the RFID system is highly sensitive and radiation free.
THE TREATMENT
Osteosynthesis, a routine clinical procedure, treats bone fractures using metallic implants. Doctors attach the implants, which look like plates and nails, to damaged bones where they apply mechanical pressure, moving the ends of the fractured bone close together. This enables the bone to fuse together over time.
To monitor the load on these implants and the healing process, strain-gauge measurement is applied to an angular-stable femoral waveplate (implant) via a two-part measurement system (see the figure). This system consists of a transponder module on the implant and an external wireless reader.
The transponder integrates a strain gauge, an MCU with an integrated programmable gain amplifier, an RFID resonant circuit, and a voltage limiter. The miniaturized transponder coil on the circuit comes from Premo. The tiny glass-tube coil allows for easy integration into the small circuit, allowing the system to fit onto the implant.
Essentially passive, the transponder hosts no power source, enabling it to withstand high temperatures such as those common to sterilization. It derives power by rectifying the 125-kHz carrier signal emanating from the reader unit. The input employs a series resonant transceiver circuit, and the output exploits the parallel resonant circuit of the transponder to maximize energy transfer.
Data transmissions between the transponder and reader is via load shift keying (LSK). In an LSK topology, load switching on the transponder incurs current changes in the transceiver. A signal processing chain then detects the data and converts it into a digital representation of the source signal.
Compared to systems using active transponders, the achievable range between the reader and transponder does not depend on the sensitivity of the detection and the transmission power of the transponder alone. The coupling between the energy consumption of the transponder circuit, the sensitivity of the detection, and the energy transmission between the reader and transponder define the maximum achievable distance. The detection range of the system is about 8 cm.
The researchers distinguish between two types of range. Energy range is the distance between the reader and transponder when the transponder is just operating, whereas detection limit is the maximum distance between the reader and transponder where the reader barely detects the transponder. The researchers say the detection limit must exceed the energy range to ensure the safe detection of transponder signals.
THE RESULTS
Deemed successful, the system monitors fracture healing over time and provides useful information about fracture loading during physical therapy. In turn, it delivers feedback on implant loading to patients during their daily routines. And, it does all of this without the expense and risks of X-rays and CT scans.
The researchers also conclude that it is possible to drive transponder modules within a range greater than 8 cm. Long-range monitoring with passive RFID modules is possible by better control of the magnetic field used for powering the transponder.
The system also uses easily obtainable surface-mount components. This enables easy configuration of the system for other applications such as temperature and strain measurements with sensors ranging from full-bridge strain gauge to quarter-bridge strain gauge. In other words, today a simple fracture, tomorrow, heart stents and implants. The sky’s the limit.