Different bioresorbable pressure sensors are presented in Figure 14 along with their resonance curves with increasing reading distance. Rescaled graphs of the Re (Z) spectra of type 2 Mg and Zn pressure sensors at higher reading distances are illustrated in Publication IV (Figure 4b and Figure 4d, respectively).
Both Mg-based sensor types showed a maximum reading distance of about 15 mm with only a slightly less attenuated resonance peak in the improved type 2 design (Publication IV). However, at smaller reading distances the height of the peak in these type 2 sensors was clearly larger compared to the type 1 sensor. The maximum reading distance of the Zn pressure sensors in air was approximately 10 mm.
Figure 14. Photographs of various bioresorbable wireless pressure sensors and their measured real part of the impedance spectra with increasing reading distances. (a) Type 1 Mg pressure sensor (Publication III). (b) Type 2 Mg pressure sensor where the conductor patterns differ from the type 1 sensors (Publication IV). (c) Type 2 Zn pressure sensor (Publication IV).
The pressure sensitivities of the different pressure sensor samples are illustrated in Table 7. The pressure responses of the type 1 Mg sensors (n=8) are plotted in Publication III (Figure 1d) and those of the type 2 Mg and Zn sensors (n=3) in Publication IV (Figure 4a and Figure 4c). In addition, the static pressure response of type 1 sensors is presented in Publication III (Figure 2a-b).
Table 7. Pressure sensitivities and initial resonance frequencies (f0) of the wireless pressure sensor samples
Sensor sample Initial resonance
frequency in air Pressure sensitivity Type 1 Mg sensor #1 91.50 MHz -6.5 kHz/mmHg Type 1 Mg sensor #2 81.85 MHz -6.7 kHz/mmHg Type 1 Mg sensor #3 83.98 MHz -3.3 kHz/mmHg Type 1 Mg sensor #4 76.96 MHz -5.7 kHz/mmHg Type 1 Mg sensor #5 89.04 MHz -5.8 kHz/mmHg Type 1 Mg sensor #6 94.20 MHz -6.0 kHz/mmHg Type 1 Mg sensor #7 92.28 MHz -5.3 kHz/mmHg Type 1 Mg sensor #8 84.49 MHz -5.9 kHz/mmHg Type 2 Mg sensor #1 106.57 MHz -7.2 kHz/mmHg Type 2 Mg sensor #2 99.57 MHz -6.7 kHz/mmHg Type 2 Mg sensor #3 98.67 MHz -5.4 kHz/mmHg Type 2 Zn sensor #1 84.58 MHz -5.1 kHz/mmHg Type 2 Zn sensor #2 95.79 MHz -7.2 kHz/mmHg Type 2 Zn sensor #3 88.32 MHz -9.4 kHz/mmHg
The baseline resonance frequency (f0) of the immersed Mg-based pressure sensors was noticed to change, as illustrated in Figure 15. The type 1 sensors showed fluctuating changes for the first 14-18 hours, after which the f0 of all the three sensors started to rapidly increase. The immersion tests concerning type 1 sensors were terminated after 24 hours. The resonance frequency of both type 2 sensors was similar for the first 12 hours, during which it decreased about 2 MHz from the initial value. Also the type 2 sensors showed increasing fmax(Re) values after about 14 hours, but the changes were not as steep compared to the type 1 sensors. In fact, the fmax(Re)
rise in the type 2 sensor #3 showed an increase of only about 2 MHz in MEM as compared to about 10 MHz regarding sensor #2 in Sörensen. After this, the sensor
#3 in MEM showed a gently descending fmax(Re) from day 1 to day 10, whereas the sensor #2 showed a steeper fmax(Re) decrease between 48 and 96 hours. The tests for the type 2 Mg sensor samples #2 and #3 were terminated after 10 and 12 days (or 240 and 288 hours), respectively, due to the severe attenuation of the resonance peaks. The Zn sensors were not reliably readable in these simulated physiological conditions.
Figure 15. Drifting of the resonance frequencies of different pressure sensor samples under immersion without any applied pressure.
The dimensions of the pressure-sensing cavities in the type 1 sensors were evaluated before and after 24 hours of immersion using micro-CT. The micro-CT images (Figure 5 in Publication III) revealed that the capacitor plates were further apart from each other after immersion, which is in agreement with the data showing increased f0 values. Moreover, the substrate was noticed to be partly detached from the spacer.
The photographs of the other LC resonators as well as their resonance peaks with different reading distances are shown in Figure 16. The f0 of the coil that was fabricated from the bioresorbable wire was successfully approximated from up to 7 mm. The coil was seemingly sensitive to changes in the reading distance. The bioactive glass-based LC resonator, whose dimensions were comparable to the bioresorbable coil, showed a similar reading distance of 8 mm. Interestingly, the LC resonator made using Mo wire (200 µm) showed somewhat comparable resonance characteristics compared with a reference device made using commercial Cu wire (180 µm). The Mo wire-based resonance sensor was readable up to 17 mm, which includes the 5 mm thick polymer screw tip that is not taken into account in the distances given in Figure 16c. The non-degradable PCB based sensor showed the highest reading distance of up to 23 mm.
Figure 16. LC resonators and their measured real part of the impedance spectra with increasing reading distances. (a) Planar coil fabricated from a bioresorbable conductive wire (b) Bioactive glass-based LC resonator (c) Molybdenum wire-based resonator on a PLDLA 96:4 screw after compression (d) Non-degradable complex permittivity sensor.
The responses of the LC circuit-based sensors to different parameters of interest are summarized in Figure 17. By immersing a Parylene-protected bioactive glass-based LC circuit in di-H2O and stepwise adding 9% NaCl solution, a non-linear decreasing resonance frequency was noticed along with an increasing ionic content (Figure 17a).
Correspondingly, various media were distinguished from each other by measuring the response of the immersed sensor in different immersion environments (Figure 17b). The response of the Mo wire based solenoidal resonator to compression was tested in a vice jaw, showing an fphase-dip decrease of -0.8 MHz and a further change of -0.5 MHz upon 0.7 % and 1.4 % axial compression of the polymer screw, respectively (Figure 17c).
Figure 17. Responses of various LC resonator sensors on different measurands. (a) The bioactive glass-based LC circuit was Parylene-coated and immersed in di-H2O, after which NaCl was added stepwise into the solution. (b) The same glass-based circuit was immersed in different solutions, which were detected based on the resonance frequency of the sensor. (c) The response of the compression sensor to various compressive strains. (d) The response of the non-degradable LC circuits (Publication I) onto the hydrolytic degradation of their polymer capsules.
The non-degradable sensors encapsulated in compression molded PLGA 80:20 and PDLGA 85:15 polymers showed decreasing resonance frequencies upon immersion of the samples. The resonance frequency of the sensors encapsulated in faster degrading PDLGA 85:15 decreased steeper during the first days of immersion, after which a more gently sloping period was noticed. Finally, around 40 days a steeper resonance frequency drop began and lasted until the end of the measurement period.
The other Parylene-coated sensor showed a steeper resonance frequency drop during the first days, but otherwise their behavior was similar to the non-coated sensors embedded in PDLGA 85:15. In the case of PLGA 80:20 encapsulations, the resonance frequency of the Parylene-coated sensors decreased in a gentler manner compared to the non-coated sensors. After 100 days, a steeper f0 drop was seen in both of the Parylene-coated sensors that were embedded into PLGA 80:20.