The LC resonator and the bioresorbable planar coil were measured by inductively coupling them with a reader coil, whose real part of the impedance, or its phase was measured. The signal processing such as estimating the resonance frequencies from the measured data was performed as described in [18]. In this thesis, the resonance frequency f0 is used to refer to both fmax(Re) and fphase-dip from the original publications.
4.4.1 Measurement setups
The wireless readout of the LC devices was performed using three main setups.
Setup I consisted of a custom-made portable reader device [18] equipped with a reader coil (Figure 5a). The portable reader was smaller and capable of faster measurements compared to an impedance analyzer. Setup II involved a double-turn rectangular reader coil (Figure 5b) connected to an impedance analyzer (Agilent 4396B). Lastly, in Setup III the reader coil located inside an oven and was connected to the impedance analyzer with a cable (Figure 5c). All the pressure measurements
in this thesis were done through a glass bottle from a reading distance of approximately 6 mm. An external pressure calibrator was used to quantify the amount of applied pressure inside the bottle. Although Setup III was used in both Publication III and Publication IV, the oven was different in these studies.
Figure 5. The main measurement setups used in this thesis. (a) Setup I consisted of a custom-made reader device connected to a computer and a reader coil. (b) Setup II utilized a rectangular reader coil with two turns. The coil was connected to an impedance analyzer. (c) Setup III was used for the pressure measurements and utilized a reader coil (Ø = 33 mm) inside an oven. The coil was connected to the impedance analyzer with a cable. Furthermore, pressurized air was guided into a glass bottle from outside the oven.
Table 4 summarizes different setups used for measuring the LC devices in this thesis.
The reader coils used in Setups I and III were circular single-turn Cu coils with a diameter of 33 mm, except for the compression response measurements (Publication IV), where a single-turn Al coil with a diameter of 30 mm was used. The reader coil in Setup II was a rectangular double turn Cu coil on a printed circuit board (Figure 5b), where the diameter of the coil was approximately 20 mm. The self-resonance frequencies of the coils were not at the measured frequency ranges in any of the measurements.
Table 4. A summary of the setups used for different wireless measurements. Unless otherwise stated, the real part of the impedance Re (Z) was measured.
*Phase was measured instead of Re (Z)
**Unless otherwise stated in the results section, Setup II was used
***In oven
4.4.2 Wireless measurements in air
The reading distances of the LC devices were estimated using Setup II as a basis and adding microscopy slides (1 mm) stepwise between the reader coil and the LC device.
The measurements were performed after each microscopy slide addition until the device was not detected anymore. In this thesis, also the type 1 Mg pressure sensor (Publication III) was measured using Setup II. However, the effect of reading distance on the f0 was analyzed from the original Publication III data, where a similar measurement was performed using a larger reader coil.
The pressure response of the pressure sensors was tested from 0 to 200 mmHg and back at intervals of 20 mmHg in ambient conditions. The type 1 sensors were
Publication Resonator Subject of interest Setup
I Non-degradable LC
estimate and peak shape II III Type 1 bioresorbable
subjected to three measurement cycles from 0 to 200 to 0 mmHg, whereas only one cycle was conducted with the type 2 sensors.
The type 1 pressure sensors were also tested under stepwise static pressure from 0 to 100 mmHg and back to 0 mmHg at intervals of 50 mmHg. The pressure was kept constant for 1 hour in each step, during which 60 measurements were recorded.
Furthermore, 100 measurement samples were recorded and averaged immediately before and after each 1-hour step. The initial resonance frequency was chosen as the average of the first 100 measurements in ambient conditions. The pressure sensitivities obtained from the linear approximations of the pressure responses were used to translate the obtained resonance frequencies into pressure values.
The temperature response of the type 1 pressure sensors was tested by recording 100 measurement samples at set test points ranging from room temperature to 40 °C. In addition, the sensor response to room temperature changes was studied during a 40-hour period by recording a measurement sample every 5 minutes and comparing the data against temperature changes that were measured with an indoor weather station (Netatmo, France).
Complementary error source measurements performed with the type 1 pressure sensors involved placing a sensor into the middle of the reader coil, moving it 1 mm at a time and measuring it at each position. This effect of the displacement between the sensor and the reader coil was studied in X- and Y-directions (Figure S3a, Supporting Information in Publication III), as well as in Z-direction, or in other words as a function of reading distance.
The compression sensor (Publication IV) was measured using a modified setup I, where the reader coil (Ø = 30 mm) was attached to a vise with an electrically insulating Teflon block between the vise jaw and the reader coil. The sensors were axially compressed in the vise and the compression was measured with a digital caliper.
4.4.3 Wireless measurements under aqueous conditions
The immersion tests in Publication I were done by first attaching the polymer-encapsulated LC circuit onto the bottom of plastic containers. Then, 100 ml of pre-warmed Sörensen buffer solution was added into the containers. The sensors were wirelessly measured through the containers and stored in an incubator (+37 °C) between the measurements. The buffer was changed every two weeks. The
measurement period was 8 weeks except for the Parylene-coated sensors encapsulated with PLGA 80:20, which were measured for 120 days.
The performance of the immersed pressure sensors (Publications III and IV) was evaluated after first studying them in air at room temperature. The sensors were attached onto the bottom of a glass bottle from two corners using Kapton tape, after which pre-warmed Sörensen buffer (100 ml) was added. The f0 was recorded every 15 or 30 minutes and the pressure response was measured at pre-determined time points similarly to the measurements that were done in air. In Publication IV, one Mg pressure sensor was immersed in MEM solution to see if the different composition of the buffer solution affects the corrosion of the conductors. The pressure response of the sensor immersed in MEM was not tested in order to decrease the contamination risk.
The temperature response of the immersed non-degradable sensors (Publication I) was tested to evaluate temperature as a possible error source. The test was performed by first immersing a PLGA-encapsulated sensor into Sörensen buffer at room temperature for 18 hours to equilibrate the system. Thereafter, the sensor response was recorded at several test points from room temperature to 45 °C.
The bioactive glass-based LC resonators (n=2) were encapsulated with non-degradable Parylene C by chemical vapor deposition as described earlier in section 4.2.1. This was done to enable stable testing of the sensor characteristics under aqueous conditions. The sensor was placed on a Petri dish with a 1 mm thick bottom and immersed in various media. All the bioactive glass sensor tests were done at room temperature. The media included air, di-H2O, Sörensen buffer, DMEM with high glucose (Sigma Aldrich, Hampshire, UK) and 96 % ethanol (VWR Chemicals).
200 measurements were recorded in each condition. The sensor was rinsed with di-H2O and dried with tissue paper between each condition. In addition, the response of the sensor to an increasing ionic content was tested by immersing it in 9 ml of di-H2O and adding 9 % NaCl solution stepwise until a NaCl concentration of 4.5 % was reached. The measurement samples were recorded in di-H2O and then after each NaCl addition step.