The structures of all the wireless LC circuits used in this study are shown in Figure 4.
In addition to these devices, planar three-turn coils fabricated from the bioresorbable conductive wire (Publication II) were also wirelessly measured. Photographs showing the actual sizes of the pressure sensors (introduced in Figure 4c-d) and the other devices are presented later in Figure 14 and Figure 16, respectively.
Figure 4. Illustration of the structures of the LC circuits used in this study. (a-b) A schematic cross-sectional view and photographs of the polymer-encapsulated non-degradable sensor (Publication I) used to monitor the degradation of their shell. (c) Schematic structure of the bioresorbable pressure sensor studied in Publication III. These are called type 1 pressure sensors in this thesis. (d) A modified bioresorbable pressure sensor version (Publication IV) with slightly thicker PCL adhesive layers as well as tuned conductor patterns. These devices are termed type 2 pressure sensors in this thesis. (e) The bioactive glass S53P4-based LC resonators (Publication IV) with a diameter of 14 mm. (f) A photograph illustrating the structure of the insulated Mo wire-based compression sensors fabricated onto bioresorbable PLDLA 96:4 screws (Publication IV). In addition to the indicated through hole via, another one can be found between the coil and the capacitor.
4.2.1 Non-degradable LC circuit (Publication I)
A four-layer circuit board design was used for the non-degradable LC circuits. These wireless sensors were commercially manufactured (Prinel Piirilevy Oy, Tuusula, Finland) upon specification and comprised an interdigitated capacitor as a primary sensing component, a parallel-plate capacitor and a coil as shown in Figure 4a.
The number of finger electrodes in the interdigitated capacitor was 24. The thickness of the copper conductors was 35 µm with an approximately 30-40 µm thick electrically insulating solder resist layer on top. In addition to these untreated sensors, four sensors were coated with an approximately 14 µm thick layer of Parylene C.
This was done to test the effect of the coating on the consistency of the measurements.
The 20 mm by 20 mm by 1.60 mm sensors were encapsulated in either PLGA 80:20 or PDLGA 85:15 to study the degradation of these copolymers in aqueous conditions. The encapsulation layers were fabricated by compression molding an LC circuit between two 32 mm by 32 mm by 2 mm copolymer sheets to form a single piece.
4.2.2 Bioresorbable conductive wire and planar coil (Publication II)
PDTEC powder was dried in vacuum for 4 days and extruded under nitrogen atmosphere using a microextruder (Gimac, Gastronno, Italy). The resulting monofilament fiber was collected at a rate of 4 m/min using a conveyer. The curvature-containing flat fibers were cleaned with an isopropanol-dampened cloth and metallized by e-beam evaporating Mg pellets to obtain a conductive Mg surface (7.5 µm). Thinner Cu and Mg layers (0.5 µm) were evaporated onto similar fibers as reference materials. During the process, the fibers were attached onto a holder with Kapton tape, resulting in discontinuous metal films on continuous PDTEC fibers.
The continuous lengths of the thicker Mg surfaces varied from 9 to 14 cm, whereas those of the control films were 25 cm long. The fibers with thicker Mg layers (7.5 µm) were extrusion coated with PCL using a crosshead die with a nozzle diameter of 2 mm. The coated wire was collected with the conveyer at a rate of 3 m/min.
Three similar planar coils with three turns were formed by coiling bioresorbable conductor wires around a template pillar (Ø = 7 mm). The insulating PCL layer was melted with a soldering iron and cooled to adhere the coil turns into each other. In the original study, three reference coils were made from commercial insulated Cu wire (Flexi-E 0.10, Stäubli Group, Switzerland) using the same template and super glue to attach the coil turns.
4.2.3 Bioresorbable pressure sensors (Publications III and IV)
The LC circuit-based pressure sensors were fabricated by first extruding PDTEC rods with a twin-screw extruder (Mini ZE 20*11.5 D, Neste Oy, Finland) and compression molding (ZB110, NIKE Hydraulics Ab, Eskilstuna, Sweden) about 430 µm thick substrates from the rod pieces. The compression was done against glass to enable low surface roughness for the substrates. Conductor patterns were then formed either by e-beam evaporating Mg (7.0-7.5 µm) or by DC magnetron sputtering Zn (~4 µm). The Mg-based pressure sensors presented in Publication III are termed type 1 pressure sensors in this thesis, whereas type 2 pressure sensor indicates the slightly modified sensors that were studied in Publication IV. The differences in the sensors are illustrated in Figure 4c-d.
The conductor patterns in the type 1 sensors were defined by 3D printed plastic masks, as opposed to laser-cut metal sheets in the type 2 devices. The sensors were assembled by attaching two substrates onto a PDTEC spacer with two laser-cut holes in it. The attachment was done by melting spin coated (Type 1; Publication III) or compression molded (Type 2; Publication IV) PCL adhesive layers between the spacer and the substrates.
The adhesive PCL films were first placed on both sides of the spacer, molten and then cooled back to room temperature. Thereafter, excessive PCL was removed from the spacer holes. The substrate-spacer-substrate sandwich was then heated to 80 °C under gentle pressure to melt the PCL adhesive again and to thereby adhere the substrates onto the spacer. Finally, the sides of the sensor were heat sealed (Hawo HPL ISZ, Obrigheim, Germany) and trimmed with scissors to yield approximately 35 mm by 25 mm by 1 mm sized sensors.
4.2.4 Other resonance circuits (Publication IV)
A bioactive glass S53P4-based LC circuit was fabricated by first preparing the S53P4 substrate discs. After melting the glass, cutting discs and polishing them, an e-beam evaporated Mg coil (7.5 µm) was deposited through 3D printed PLA masks. A subsequent dielectric layer was fabricated by spin coating 12 % PDTEC in cyclohexanone solvent at 3000 rpm. Thereafter, a further Mg layer (7.5 µm) was deposited to complete the resonator as illustrated in Figure 4e. To protect the Mg conductors in aqueous conditions, the bioactive glass resonators were coated with Parylene C (~13 µm). A similar method with spin coated dielectric layers on an
e-beam evaporated Mg spiral was experimented with PDTEC and PLDLA 96:4 substrates, but the conductors were noticed to crack during the solvent drying phase.
Wireless compression sensors on threaded PLDLA 96:4 screws were fabricated using biodegradable Mo wire (200 µm) as the conductor. The wire was insulated with Parylene C (~10 µm) to avoid short-circuits in the structure. The coil was formed by winding Mo wire around the PLDLA screw. The capacitor consisted of a double strand of the insulated wire in each screw thread (Figure 4f). The coil and the capacitor were connected by drilled through hole vias on the PLDLA screw.