Bioresorbable sensors that are ultimately metabolized by the body are a promising technology for many applications where the monitoring need is only temporary. As these materials are cleared from the body without a removal surgery, they hinder the complication risks related to long-term implantation of non-degradable devices or their removal.
This thesis addresses bioresorbable materials, their performance and fabrication methods related to orthopedic inductor-capacitor circuit-based wireless sensors. The sensing method was chosen due to the wireless readout and simple structure of the sensors. Due to the delicate nature of bioresorbable materials, the project was started by studying sensors made from conventional non-degradable materials. Thereafter, bioresorbable inductor coils were fabricated. Finally, similar fabrication principles were applied to build functional fully bioresorbable sensors.
The conductors were mainly made by evaporating magnesium (Mg) films onto polymeric or glass substrates, but also sputtered zinc (Zn) films and commercial molybdenum (Mo) wire were used. No significant differences in the resistances of the Mg and Zn films of similar thicknesses were noticed. Thus, Zn was estimated to offer a similarly conducting but slower degrading alternative for commonly used Mg. In this study, the thickness of the sputtered Zn conductors was limited due to excessive heating of the polymer substrates.
The sensor substrates used in the study included conventional printed circuit boards, bioresorbable polymeric screws, bioresorbable metallized polymer fibers and sheets, as well as bioactive glass discs. The fabricated bioresorbable sensors were wirelessly readable up to distances of about 15 mm, as compared to the non- degradable sensor with 23 mm.
Different measurands included pressure, compression of the screws and complex permittivity of the sensor environment. The polymer-based pressure sensors were most rigorously studied, and their performance was uniform in ambient conditions. One of the pressure sensors was wirelessly readable and responsive to pressure (0-200 mmHg) for 10 days in simulated physiological conditions, but its stability should be improved for practical applications.
The results indicate that the deterioration of the sensor performance was caused by water, which diffused into the sensor substrates and thus corroded the metal conductors, causing dimensional changes to the sensor structure.
In summary, the fabricated devices included simple sensor architectures that could be assembled with only few processing steps. It was shown that depending on the sensor design, different measurements and thus various orthopedic applications could be possible.
However, especially the relatively poor stability of the bioresorbable sensors requires attention in the future. In addition, the short-range reading distances may limit potential clinical applications. Nevertheless, the presented results provide a good reference point for choosing the right bioresorbable materials for future studies.
The doctoral dissertation of Master of Science (Technology) Aleksi Palmroth in the field of biotechnology titled Bioresorbable Wireless Resonance Sensors: Materials and Processes will be publicly examined at the Faculty of Medicine and Health Technology of Tampere University at 12 o'clock on Friday 19 November, 2021. The venue is Sähkötalo building auditorium S2, address: Korkeakoulunkatu 3. Professor Reijo Lappalainen from University of Eastern Finland will be the opponent while Professor Minna Kellomäki will act as the custos.
The event van be followed also via remote connection
The dissertation is available online at
http://urn.fi/URN:ISBN:978-952-03-2162-8
Photo: Esa Hallinen