0.1 Design of an in-vivo radiation measurement scheme using a  (Page 7/7)

With these filtering and analysis techniques, a good measure of the On and Off times of the signal was observed. This sequence is followed for the data of all input signals collected from the LabVIEW program for all the combinations of frequencies and radiation dosages.

7.3 Calibration Algorithm:

The On time and Off time values for different radiation dosages (100CGy, 1000CGy, 3000CGy and 5000CGy) are plotted in Table 3, for different signal frequencies (50Hz, 80Hz and 100Hz).

These plots illustrate the linear change in timing values with respect to radiation dosages. All the timing values for these combinations are stored in the file,‘calibration.xls’. The measured On and Off time values are illustrated in the below table. We can clearly observe a decreasing trend in On times and an increasing trend in Off times. This data can be used to estimate the unknown dosage of an irradiated sensor (CMOS inverter). Using Matlab, the calibration data is read from the spreadsheet file. For a specific input frequency, if the measured timing values (On and Off times) lie between the corresponding values of two consecutive dosages, the unknown radiation dosage is calculated by linearly interpolating these known dosages.

Table 3: Change in On and Off time values of sensor output with radiation

The overall setup is tested with few intermediate dose values (2000CGy and 4000CGy) and obtained the accurate results. As the range of difference in timing values is very less, it is very difficult to estimate the dosages exactly with smaller resolutions.

8.1 Design for Powering up the Sensor:

The Implantable Micro Devices (IMDs) need continuous power from the external world. This can be achieved by connecting a power and data cable, breaching the skin, to the IMDs. This causes the sensitive body tissues to be exposed to the environment resulting in infections. Another method is to power the IMDs using batteries which need to be replaced after a certain period of time. This requires periodic surgery, further increasing risk as well as financial burden on the part of the user.

An alternative to these solutions is wireless transmission of power. This can be achieved by an inductive link between two magnetically-coupled coils. It is now one of the most common methods to wirelessly transmit power from the external world to IMDs. One of the primary requirements in such a system is that the inductive power transmission should be very efficient to minimize the size of the external portable battery, and eliminate overheating of the surrounding tissue by surpassing the exposure limit to electromagnetic field.

8.2 Power Transfer:

Transmitter -

The transmitting power through inductively coupled coils requires an AC excitation which in turn makes the power driver to be a DC to AC inverter. Furthermore, a high voltage level is usually required by the primary coil (approx. 80-100V).

For this, a Class-E power amplifier should be chosen to drive the transmitter coil with high efficiency (i.e., 90–95%). Class-E power amplifier drives the primary coil L1, which is coupled to the implantable coil L2. In this amplifier, a choke inductor provides constant dc current with a high-speed switching power MOSFET operating around at 500-kHz frequency, driven by the Class-E driver.

Receiver -

On the receiver side, Cr forms a resonant tank with the receiver coil L2. A bridge rectifier converts the AC signal on the receiver-tank circuit to provide a DC voltage. A low drop out regulator supplies a constant 5V supply from the output of the rectifier. This is the VDD to all the other circuitry on the implantable device side.

9. Bio Compatibility;

The use of biocompatible materials is important in determining the longevity of chronically implantable sensors. Protein adsorption is the initial event that takes place once an implant is in the body. These adsorbed proteins are then a significant influence on the subsequent adsorption of further proteins and cells. So, they dictate the interfacial reactions and ultimately biocompatibility of the material. Parylene-c and Polydimethylsiloxane (PDMS), Polypyrrole (PPY) are few materials which are biologically inert and can be used in preventing protein adsorption, a characteristic required for any implantable sensor. The device should be coated with these biocompatible passive materials which provide electrical isolation to the active areas of each array element and protect it from corrosive environments. The coating must also provide good mechanical characteristics. The observed lifetimes of the devices coated with these materials are sufficient for long-term use inside the human body. Apart from testing in air, these coated devices should also be tested in underwater using pulse excitation.