Novel Ultrasound Read-Out for a Wireless Implantable Passive Strain Sensor (WIPSS)

In the early 20th century the first metallic bone plates and bone screws were introduced in orthopedic surgery for the fixation of long bone fractures. When such an implant is loaded, dedicated in-vivo measuring techniques may be required in order to determine, for instance, strains or forces in the device. The measured information can be used to optimize the design of future implants, to prevent a possible overload of the implant and to asses the healing process of the fracture. The most common in-vivo strain sensors are based on strain gauges and electromagnetic signal transmission. They contain electronic circuits, which are relatively expensive and require a special read-out unit. Therefore, a completely new strain measuring concept consisting of a wireless implantable passive strain sensor (WIPSS) with an ultrasound read-out has been developed. Currently, the WIPSS is entirely made of PMMA. However, it is even envisioned to fabricate the WIPSS from a biodegradable material in order to use it on an implant made of such a material as well. The WIPSS transforms the measured strain into a varying amount of fluid inside a micro channel, whose fill level linearly depends on the strain. The micro channel has a width of 0.1 mm and is wound into a spiral with a diameter of 5 mm. The aim of this work is to find a suitable ultrasound based read-out method, which is able to determine the fill level of the micro channel through inhomogeneous, attenuating and scattering human tissue of up to 50 mm. Under this precondition the acquired ultrasound data cannot provide the necessary lateral resolution of 0.1 mm to detect the fill level of the micro channel directly. Therefore, a novel read-out method was developed (patent application submitted). It is based on two fundamental requirements, which state: a) The quantity resulting from the areal integration of the C-scan of the micro channel linearly depends on the fill level. b) The measurement is independent of the acoustic parameters of the materials covering the WIPSS, if it is calibrated with the echo signal from a reference reflector inside the WIPSS. We derived an analytical model, which describes the entire transmit-receive process of an ultrasound transducer performing the read-out. It accounts for various system parameters, such as transducer aperture (single element or phased array), excitation function, F-number, electronic noise, shape of the micro channel, fill level, insonification angle, attenuation and dispersion. Therefore, the model is a valuable