Fabrication of Fine Electrodes on the Tip of Hypodermic Needle Using Photoresist Spray Coating and Flexible Photomask for Biomedical Applications

The overall goal of this protocol is to fabricate fine interdigitated electrodes on the tip of a hypodermic needle with the diameter of less than one millimeter for biomedical applications. This method can achieve the dialects metal patterning on the rounded tip of the hypodermic needle by using spray coating of photoresist and photolithography process with a flexible film for a mask. The fabricated micro diode sensor can be widely used for biomedical applications, such as drug delivery, biopsy and even the surgery with high precision by cauterizing electrophotometer targetatious.

To begin this procedure, cut a three centimeter length of heat shrink tube adjusting the length if needed based on the penetration depth of the hypodermic needle. Insert the hypodermic needle into the cut heat shrink tube. Using a heat gun, set to 150 degrees Celsius and shrink the tube.

After this, separate the hypodermic needle from its hub. Clean the needle in a deionized water bath with ultrasonic agitation. Then, use a hot plate set to 105 degrees to dehydrate the insulated needle for 10 minutes.

Next, apply double-sided tape to a clean glass slide. Arrange the clean insulated needle side by side on exposed side of the double-sided tape. Using sputtering equipment, deposit chromium and gold onto the needles as outlined in the text protocol.

First, use double-sided tape to fix one of the gold sputtered needles onto a glass slide. Heat the chuck of the spray coater to 100 degrees Celsius. Place the slide on the heated chuck.

Allow the needle to heat up for two to three minutes. Then, spray the photoresist onto the gold sputtered needle while still heating using the condition shown here. After this, leave the glass slide on the chuck to heat at 100 degrees Celsius for three minutes for soft baking.

Using a microscope set to 100x magnification, inspect the needle to determine if the photoresist is uniformly coated. First, slowly lift a freely moveable sample holding plate until it fully contacts the fixed photomask holding plate. Using a pneumatic pump, fix the sample holding plate.

Next, place the photoresist coated needle onto the sample holding plate. Align two of the boundary lines from the projected image with two parallel alignment lines from the photomask using the CCD display monitor to monitor the process in real time. Slowly lift the needle to bring the needle into contact with the fixed flexible photomask.

Expose the needle to UV light for 30 seconds at an intensity of 15 millijoules per centimeter squared. Then carry out the developing process for three minutes. After this, use deionized water to rinse the developer out of the sample.

Using a microscope set to 200 times magnification, determine if the photoresist is clearly patterned. If the exposed photoresist is not perfectly removed, repeat the developing process at 30 second intervals. Using a pair of tweezers, detach the sample from the glass slide.

Immerse in gold wet etchant for one minute. Then, use deionized water to rinse the etchant out. Using a microscope set to 200x magnification, inspect the result.

If any gold to be removed still remains, repeat the etching process at 10 second intervals. Next, immerse the needle in chromium etchant for 30 seconds. Use deionized water to rinse the etchant out when done.

Immerse the etched needle into an acetone solution for one minute. After this, rinse with deionized water. Transfer the sample to a hot plate heated to 105 degrees Celsius to dehydrate for 10 minutes.

Then, cut the shrink tube so that it is two to three millimeters longer than the electrode. Using a heat gun, raise the temperature of the heat shrink tube to 150 degrees to tightly passivate the needle. In this study, interdigitated electrodes are fabricated on the tips of hypodermic needles using photoresist spray coating and a flexible photomask.

The electrodes are successfully fabricated with a diameter of 820 micrometers while the fabrication margin from the needle's tip is as small as 680 micrometers. The photomask's width and gap, which are 25 micrometers and 15 micrometers respectively, are designed to resolve UV dose imbalance which arise from imperfect contact between the outer portion of the photomask and the curved surface of the needle. These changes allow for both the width and gap of the electrodes to be successfully fabricated at 20 micrometers by optimizing UV exposure time.

The discrimination capability of the electrical impedance spectroscopy on a needle is evaluated using various concentrations of PBS. Because the PBS is diluted serially with deionized water, the electrical conductivity decreases as the concentration of PBS decreases. Thus, the magnitude of impedance increases as the concentration level of PBS decreases.

Based on this discrimination capability, depth profiling is performed at a frequency of one megahertz using four layered porcine tissue. According to the penetration depth of the EoN, the magnitude of the impedance and fat tissue, is clearly discriminated from that of muscle tissue. After watching this video, you should understand how to fabricate microelectrodes on a rounded surfaced tray to read a diameter of less than one millimeter.

Currently, we are better find the clinical applicability of the needles sensors for derogatory for the spinal anesthesia, thyroid biopsy, and even surgeries such as laparoscopic surgery and passion apronectomy.