Construction of a Wireless-Enabled Endoscopically Implantable Sensor for pH Monitoring with Zero-Bias Schottky Diode-based Receiver

The protocol shows the fabrication of wireless implantable pH sensor that transmit data to a fully passive receiver, enabling very power efficient delivery of data from one implantable device to another. The sensor can be manufactured by hand and by commonly available techniques. Next, the method of data transmission can be directly reused in the development of different implantable devices.

It is critical to thoroughly inspect the wireless pH sensor after every step. The components used for its fabrication are very small and short circuits or misplacement during the fabrication can occur. To begin with, place the ISFET pH sensor mounted on a PCB on a flat surface and solder a 15 millimeter section of fluorinated ethylene propylene-coated cable to the solderable electrodes of the pH sensor without contaminating dye and PCB with flux.

Mix a minimum of two milliliters of two-part epoxy to encapsulate the soldered electrodes and then use black opaque epoxy for later inspection. Observe the easy visibility of sensor parts exposed to the environment without opaque epoxy. Transfer the mixed epoxy to a one milliliter syringe with a 0.5 millimeter flat-end needle and coat the soldering area of pH sensors with epoxy, ensuring coating of the whole area of PCB electrodes and the exposed wire.

Inspect the coated area under a microscope for any uncoated exposed metal parts. After repeating the coating process of uncoated metal, trim the wires to the length and angle, then coat the ends with solder to avoid fraying. After placing PCB with components in a side up position, apply solder paste to all the exposed gold-plated pads.

Next, place passive and active components using tweezers. After soldering and cooling to room temperature, inspect the PCB under the microscope to verify the placement of the components and shorts. Program firmware to the micro controller and set up the programming software as described in the text.

Next, set the programmer to power the device with a voltage of approximately 2.5 volts. Then de-solder the five wires after programming. Solder two battery holders to the opposite part of the PCB.

Next, solder the pH sensor assembly to the terminals of the PCB, then insert two AG1 batteries into the battery holders. After preparing epoxy, encapsulate the device with the epoxy as previously described. Let the epoxy cure at room temperature.

Attach the wire hook to the device with a drop of fast curing epoxy. After curing at room temperature, pH sensor could be located on the bottom left side of the implantable device. Solder the components using the soldering gun.

To manufacture the rectenna receiver again or proceed without receiver matching, use the values of the components provided here and skip the recording of S11 Smith chart in the next step. Otherwise, solder the SMA connector to the PCB. Attach a vector network analyzer input to the SMA connector.

Record the S11 Smith chart of the rectenna. Observe the response and record the impedance. Use an impedance matching calculator software to determine the values of matching components.

Solder the impedance matching components and inspect under a microscope for short circuits and component placement. After measuring with spectrum analyzer again, confirm that the voltage standing wave ratio is under three between 300 to 500 megahertz. Otherwise, repeat with different matching components.

After the device is activated post 24 hour of the battery's insertion, inspect the sensor's output by observing the signal shown in the oscilloscope. Prepare 2%hydrochloric acid solution and 100 millimolar buffer solution of pH four, pH seven, and pH 10. Submerge the capsule in every beaker and record at least three samples.

Measure the period between the second and third pulse. And after filling it in the provided spreadsheet, determine the calibration coefficients for the pH sensor using the spreadsheet. After preparing an ex vivo endoscopic porcine model of the stomach and a long segment of the esophagus, grasp the sensor externally with a hemostatic clip.

Insert the endoscope with the sensor in the clip in the standard way into the model. After placing the clip with the sensor close to the lower esophageal sphincter, rotate the endoscope against the esophageal wall, open the clip and then push towards the esophageal wall. Close and release the clip.

While the sensor remains attached to the esophageal wall at the desired location, extract the endoscope. Place the receiver within 10 centimeters of the implanted sensor. Inject 50 milliliters of the solutions with various pH values into the esophagus and observe the changes in the sensor's response.

Retract the endoscope after every injection and read the value no earlier than 30 seconds after injection and use the spreadsheet to calculate the pH measured by the sensor. Wash the esophagus with 100 milliliters of deionized water between injecting solutions with different pH. When the passive rectenna receiver was put into proximity of the pH sensing device, clear voltage spikes were observed when the device was transmitting.

The first two short pulses were synchronization pulses. The time between second and third pulse linearly translates to the pH of the environment that the sensor was subjected to. Based on a simple two-point calibration with the buffers of pH four and pH 10, the sensor returned stable and repeatable pH value readings.

The mean and standard deviation of errors were 0.25 and 0.30 when tested in solutions and beakers and were 0.31 and 0.36 in an ex vivo model. The effect of a mobile phone antenna with an active GSM call had only a minor negative effect on receiving the data from the sensor as demonstrated for 20 centimeters, 10 centimeters, and five centimeters distance between the edge of the phone and the receiver. The encapsulation of the sensor must be done and inspected properly to prolong the lifetime of the device when implanted.

Next, correct placement of the hemostatic clip is critical. This research paves the way for the development of wireless networks of implantable devices by creating an energy efficient way to transfer data especially from the receiver point of view.