## 3 Conception of the Bio-Implantable Customized Multi-Sensor
### Results  
A bipolar power scheme able to supply a regulated differential voltage of \(\pm\)1.2 V and a maximum current of \(\pm\)1.5 mA has been implemented. The Texas Instrument(r) TRF7960 is used as external reader with a maximum emission power of 200 mW at 13.56 MHz. The desired on-chip regulated voltages of \(\pm\)1.2 V are obtained for a distance up to 20 mm on air between coupling antennae. This analysis has been carried out in terms of the distance (Z-axis), between the external antenna and the coil designed in the PCB which defines the full implantable, that is, between the reader and the implantable. However, it is also necessary to have an approach to the misalignments between both antennas in the XY plane. Figure 9 depicts the rectified voltage (V\({}_{\mathrm{rec}}\)) distribution in function of the XY misalignment for Z distances of 10, 15, and 20 mm. It can be noticed that the further the antenna is placed from the centre the lower the rectified voltage is. A more accurate study with human tissue is beyond the scope of this study. Figure 9: Distribution of the Vrec voltage in the XY plane for three different distances: 10, 15 and 20 mm (Reproduced from [60] with kind permission from Springer Science + Bussiness Media B.V). A suitable solution for the detection of threshold values is based on the use of comparators, in terms of silicon area and power consumption, to detect one or several threshold values with medical interest. In the implemented approach, some comparators capable of detecting three different threshold voltages (V\({}_{\mathrm{th1}}\), V\({}_{\mathrm{th2}}\) and V\({}_{\mathrm{th3}}\)), generated on-chip, have been implemented. These values are used to define a simple AM modulation protocol. The signal is always a high level \"1\" but when a threshold value is achieved then a \"0\" level is generated. This functionality is based on the use of the comparators, monostables flip-flops and a very simple digital circuitry. As soon as there is enough voltage, the Power-On-Reset module generates a signal that activates the circuitry and the antenna starts to transmit continuously a series of \"1\". When the first threshold level is achieved, the system transmits one zero (T\({}_{\mathrm{th1}}\)). If the second is reached, two zeros are transmitted (T\({}_{\mathrm{th2}}\)), and when the third is achieved a series of three zeros are sent (T\({}_{\mathrm{th3}}\)). A zero time slot interval is defined as 250 ms (T\({}_{\mathrm{th1}}\) = 250 ms). In this way, the external reader can be quickly advised every time the desired substance exceeds the programmed threshold level or levels. The instrumentation and the communication protocol were validated using several concentrations of K\({}_{4}\)[Fe(CN)]\({}_{6}\) in PBS. In this case, a commercial sensor was used [60]. Several cyclic voltammetries (CVs) were carried out in order to verify the performance of the Control and conditioning modules. These measurements were compared with those obtained with a commercial potentiostat amplifier, the CH 1232A from CHInstruments(r). These measurements were also used in order to calibrate the setup, and check the obtained values of the measured current peaks for the oxidation peak (around 240 mV), and the reduction peak (around 170 mV), for each concentration of K\({}_{4}\)[Fe(CN)]\({}_{6}\) in PBS tested, from 1-5 mM. Also, this setup was used to validate the measured CV shapes obtained by the commercial equipment and the full-custom implementation. After the CV characterization some amperometric tests were done for different concentrations of K\({}_{4}\)[Fe(CN)]\({}_{6}\) in PBS: 1 mM, 2 mM, 3 mM, 4 mM and finally 5 mM. Then, an experiment was carried out where the concentration was changed from 1 to 5 mM in time, with a fixed voltage of 500 mV in the sensor. This voltage is defined not at the oxidation peak. For this value of voltage applied in the sensor, the current varies from an average current of 3 \(\mu\)A (1 mM), to 16 \(\mu\)A (3 mM), up to 28 \(\mu\)A (5 mM).