Wireless implantable passive strain sensor: design, fabrication and characterization

Abstract

This work presents a new passive sensor concept for monitoring the deformation of orthopedic implants. The novel sensing principle of the WIPSS (wireless implantable passive strain sensor) is based on a hydro-mechanical amplification effect. The WIPSS is entirely made from biocompatible PMMA and consists of a microchannel attached to a reservoir, which is filled with an incompressible fluid. As the reservoir is exposed to strain, its volume changes and consequently the fill level inside the microchannel varies. The wireless detection of the microchannel's strain-dependent fill level is based on ultrasound. The WIPSS' sensing principle is proved by finite-element simulations and the reservoir's design is optimized toward maximum volume change, in order to achieve high sensitivity. A fabrication process for WIPSS sensor devices entirely made from PMMA is presented. The obtained measurement results confirmed the sensor's functionality and showed very good agreement with the obtained results of the conducted FE simulations regarding the sensor's sensitivity. A strain resolution of 1.7 ± 0.2 × 10 −5 was achieved. Further, the determination of the cross-sensitivity to temperature and strains applied out of the sensing direction is presented. The response to dynamic inputs (0.1-5 Hz) has been measured and showed decreasing sensor output with increasing frequency. Test structures of the sensor device allow the application of a signal bandwidth up to 1 Hz. Therefore, the proposed sensor concept of the WIPSS presents a promising new sensor system for static in vivo strain monitoring of orthopedic implants. In combination with the developed ultrasound-based read-out method, this new sensor system offers the potential of wireless sensor read-out with medical ultrasound scanners, which are commercially available.

A Two-Dimensional Wireless and Passive Sensor for Stress Monitoring

“…A qualitative comparison between the proposed sensor in this paper and the existing implantable strain sensors is discussed. This novel sensor is similar to the implantable sensors proposed by F. Alfaro [25] and F. Umbrecht [26], respectively, in bone stress monitoring. Both of them detect stress in a wireless manner.…”

Advances in Sensing Technologies for Monitoring of Bone Health

“…The sensing principle of this sensor was based on amplification effect in hydromechanical systems. The strain resolution obtained was 1.70 ± 0.2 × 10 −5 with a dynamic input frequency range of 0.1-5 Hz, while this sensor works with a signal bandwidth up to 1 Hz, since increasing the input frequency range reduces the sensor output [109]. In 2009, Lin et al investigated a smart polymer hydrogel thin film used to convert tiny pressure sensors into chemomechanical sensors.…”

“…Schematic of the sensor concept of wireless implantable passive strain sensors (WIPSS) implant inside the body and sensor signal is detected by ultrasound waves (reproduced with permission from reference[109], copyright IOP).…”

Bio-compatible wireless inductive thin-film strain sensor for monitoring the growth and strain response of bone in osseointegrated prostheses

“…To power their solution, a vibration energy harvester is included in the hip prosthesis. While many more teams [17][18][19] have proposed a wide array of in vivo wireless sensing solutions for implants, the sensing solutions are designed in compact form factors that integrated into the implant design. This approach of integration in the implant drives costs high and may limit commercial viability.…”