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

Summary

The fabrication method for fine interdigitated electrodes (gap and width: 20 µm) at the tip of a hypodermic needle (diameter: 720 µm) is demonstrated using a spray coating and flexible film photomask in the photolithography process.

Abstract

We have introduced a fabrication method for electrical impedance spectroscopy (EIS)-on-a-needle (EoN: EIS-on-a-needle) to locate target tissues in the body by measuring and analyzing differences in the electrical impedance between dissimilar biotissues. This paper describes the fabrication method of fine interdigitated electrodes (IDEs) at the tip of a hypodermic needle using a photoresist spray coating and flexible film photomask in the photolithography process. A polyethylene terephthalate (PET) heat shrink tube (HST) with a wall thickness of 25 µm is employed as the insulation and passivation layer. The PET HST shows a higher mechanical durability compared with poly(p-xylylene) polymers, which have been widely used as a dielectric coating material. Furthermore, the HST shows good chemical resistance to most acids and bases, which is advantageous for limiting chemical damage to the EoN. The use of the EoN is especially preferred for the characterization of chemicals/biomaterials or fabrication using acidic/basic chemicals. The fabricated gap and width of the IDEs are as small as 20 µm, and the overall width and length of the IDEs are 400 µm and 860 µm, respectively. The fabrication margin from the tip (distance between the tip of hypodermic needle and starting point of the IDEs) of the hypodermic needle is as small as 680 µm, which indicates that unnecessarily excessive invasion into biotissues can be avoided during the electrical impedance measurement. The EoN has a high potential for clinical use, such as for thyroid biopsies and anesthesia drug delivery in a spinal space. Further, even in surgery that involves the partial resection of tumors, the EoN can be employed to preserve as much normal tissue as possible by detecting the surgical margin (normal tissue that is removed with the surgical excision of a tumor) between the normal and lesion tissues.

Introduction

Hypodermic needles are widely utilized in hospitals for biopsies and drug delivery because they are inexpensive and easy to use. They also have excellent mechanical properties despite their thin diameter and a sharp-edged structure suitable for invasion. During a biopsy, the target tissues are sampled in the hollow of the hypodermic needle with ultrasonography guidance¹. Although ultrasonography is free of radiation, safe for fetuses and pregnant women, and provides real-time imaging, it is difficult to see organs that are deep within the body, especially in the case of obese patients because ultrasonic waves cannot penetrate air or fat tissues². In addition, a surgeon cannot acquire depth information from the two-dimensional ultrasonography that is conventionally utilized in the majority of hospitals, resulting in the need for multiple biopsies if physicians lack skill or experience. In drug delivery for spinal anesthesia, physicians determine that the needle has reached the spinal space if the cerebrospinal fluid (CSF) flows backward into the syringe while carefully inserting the needle into the patient's back. After confirming the reflux of CSF, the anesthesia drug is injected into the spinal space³. However, physicians risk penetrating or cutting off nerve fibers in the spinal space, causing severe pain to patients and even paraplegia^4,5. Thus, this procedure also requires a skillful physician. One solution to overcome and mitigate the aforementioned difficulties is to add a navigation function to the hypodermic needle so that objective information on the needle's position can be provided. This would help a physician readily perform a biopsy, drug delivery, and even a surgery without relying on their empirical judgment only.

In order to electrically localize the target tissues in the body, a hypodermic needle incorporating an electrical impedance spectroscopy (EIS) sensor has been introduced as EIS-on-a-needle (EoN)⁶. The EIS sensor is currently utilized in the field of biomedical engineering for applications such as DNA detection^7,8,9, bacteria/virus detection^10,11,12, and analysis on cells/tissues^{13,14,15,16,17,18,19,20,21,22}. The EoN can discriminate between dissimilar materials in a frequency domain based on their electrical conductivity and permittivity. The discrimination capability of the EoN was verified for various concentration levels of phosphate buffered saline (PBS)²³, porcine fat/muscle tissues^6,23, and even human renal normal/cancer tissues^24,25. This capability of the EoN is expected to considerably increase the biopsy accuracy by locating the target tissues based on the differences in electrical impedance between the target lesion tissues and the neighboring normal tissues. In a similar manner, investigating differences in the electrical impedance between the drug injection space (spinal or epidural space) and surrounding tissues can help physicians deliver an anesthesia drug at the exact target location. Furthermore, the EoN can be utilized to electrically stimulate the brain/muscle as well as to determine an optimal surgical margin during surgeries that involve the partial resection of a tumor, such as partial nephrectomy, to preserve as much normal tissue as possible.

One of the biggest challenges in the realization of the EoN is the fabrication of electrodes on the curved surface of a hypodermic needle having a small radius of curvature. Direct metal patterning using a conventional photolithography process has been regarded as unsuitable for the fabrication of micro-sized electrodes on a curved substrate with a diameter of several millimeters or less. So far, various methods, including conformal printing²⁶, flexible dry film photoresist²⁷, the microfluidic method²⁸, nanoimprint lithography²⁹, and substrate-rotating lithography³⁰, have been introduced to fabricate metal/polymer patterns on a curved surface. However, there are still limitations due to the EoN requirements, such as the required substrate with a diameter of less than 1 mm, total electrode length of 20 mm or more, width and gap of electrodes ranging in tens of micrometers, and high volume production.

In the present study, direct metal patterning by employing photoresist spray coating and a flexible film photomask is proposed to realize micro-sized electrodes on the curved surface of a hypodermic needle. The diameter of the needle is as small as 720 µm (22-gauge), which is widely used for biopsies and drug delivery in hospitals. The production yield of the proposed fabrication method is also evaluated to determine the feasibility of bulk production at an affordable price.

Protocol

1. Electrical Insulation of Hypodermic Needle

NOTE: A transparent heat shrink tube (HST) is employed for the electrical insulation of a hypodermic needle that is 720 µm in diameter and 32 mm in length. The HST is made of polyethylene terephthalate (PET), which shows good chemical resistance to most acids and bases, excellent mechanical durability, and biocompatibility. The initial inner diameter and wall thickness of the HST are 840 µm and 25 µm, respectively. The diameter of the HST tends to be reduced by more than 50% at a temperature of 100 °C, with even greater reduction at higher temperatures up to 190 °C. Note that PET HST is a thermosetting material that has the property of becoming permanently hard and rigid when cured. The size of the hypodermic needle and shrink tube can be adjusted depending on the research purpose and applications. The overall fabrication process is graphically summarized in Figure 1.

1. Cut the HST to a length of 3 cm. Adjust the length of the tube depending on the penetration depth of the hypodermic needle.
2. Insert the hypodermic needle into the cut HST.
3. Shrink the tube using a heat gun at a temperature of 150 °C, which is set to prevent unwanted additional contraction when dehydration is carried out at 105 °C in the cleaning process (in step 1.6).
4. Separate the hypodermic needle from its hub.
5. Clean the hypodermic needle insulated by the HST in a deionized (DI) water bath (20 °C) with ultrasonic agitation at 30 kHz and 350 W power.
6. Dehydrate the hypodermic needle insulated by the HST on a hotplate at 105 °C for 10 min.

2. Au Deposition Using Sputtering

NOTE: In this study, the sputtering process that is available is used to deposit an Au layer for electrodes, although an e-beam evaporation process can be an alternative method. It has been confirmed that the temperature rise induced in the sputtering process rarely causes additional shrinkage of the HST. However, a process that continues for more than several minutes might heat the HST above the initial shrinkage temperature. This can cause additional shrinkage of the HST, resulting in an increase in the fabrication margin from the tip.

1. Arrange the cleaned hypodermic needles insulated by the HST side by side on a glass slide using double-sided tape for Cr/Au deposition.
2. Using sputtering equipment, deposit Cr/Au on the cleaned hypodermic needles insulated by the HST.
   NOTE: In this case, the thicknesses of Cr and Au were 10 nm and 100 nm, respectively (Cr was used for the adhesion layer between the HST and the Au layer).
   1. Arrange as many needles as possible in order to reduce production cost and production time. Use the sputtering conditions below to deposit 10 nm Cr and 100 nm Au.
   2. For Cr sputtering, set Cr target diameter: 4 inch, RF power: 300 W, argon pressure: 5 mTorr, and shutter open time: 20 s (10 nm).
   3. For Au sputtering, use Au target diameter: 4 inch, DC power: 300 W, argon pressure: 10 mTorr, and shutter open time: 80 s (100 nm).

3. Spray Coating

NOTE: A low-viscosity (14 cp) photoresist is used in the spray coating process to increase spray efficiency. The photoresist can be easily coated on the Au-sputtered needle only when the needle is heated.

1. Fix one of the Au-sputtered hypodermic needles on a glass slide using double-sided tape.
2. Place the slide glass on a chuck of the spray coater that is being heated at 100 °C. Wait 2 - 3 min until the needle is sufficiently heated.
3. Spray the photoresist on the Au-sputtered needle while heating the needle at 100 °C. Perform the spray-coating process using the following conditions. Set nozzle diameter: 400 µm, nozzle moving speed: 70 mm/s, spray pressure: 500 kPa, and distance between chuck and nozzle: 13.5 cm.
4. After spray coating is finished, leave the glass slide on the chuck at 100 °C for 3 min to perform a soft baking process.
5. Inspect the result using a microscope set to 100X magnification to determine whether the photoresist is uniformly coated on the Au-sputtered needle.

4. UV Exposure and Developing

NOTE: In general, prior to UV exposure, a flexible film photomask is attached to a flat transparent plate to remove the air gap between the photomask and the sample to be exposed to UV light. However, in this study, the photomask is used without the flat transparent plate to realize direct metal patterning on the curved surface of the hypodermic needle. The photomask can conformably bent along the curve of the hypodermic needle to achieve the best patterning resolution feasible with the contact aligner. The bending allows the flexible photomask to keep the contact area between the photomask and the curved surface of the hypodermic needle as large as possible. Taking a wet etching process (not a lift-off process) for metal patterning into consideration, the use of a positive photoresist is more advantageous than the use of a negative photoresist. This is because the entire area except the electrode pattern is transparent, thereby providing a wide field of view to readily align the electrode pattern with the center of the needle.

1. To minimize wedge error, slowly lift a freely movable sample-holding plate until it fully contacts the fixed photomask-holding plate. Then, fix the sample-holding plate using a pneumatic pump.
   1. Carry out this process to possibly avoid undesirable patterns, which may be formed by the scattering of UV light in the air gap, and caused by the incomplete contact between the sample and photomask.
      NOTE: In addition, the minimization of wedge error ensures that the photoresist-coated hypodermic needle does not move when it contacts a film photomask in the next alignment step, even though the contact surface of the hypodermic needle has a round shape.
2. Place the photoresist-coated hypodermic needle on the sample-holding plate of the aligner.
3. Align the projected image of the photoresist-coated hypodermic needle with the alignment pattern of the film photomask.
   NOTE: In this case, the alignment pattern of the film photomask was designed as two parallel lines at a distance of 800 µm, considering the thickness of the HST and coated photoresist.
   1. Align two boundary lines of the projected image with two parallel alignment lines of the photomask (Figure 1e); thus, the photoresist-coated hypodermic needle can be positioned at the center of two parallel alignment lines, with an alignment error of 10 µm or less.
   2. Monitor the alignment process in real-time through the display monitor connected to the charge-coupled device (CCD) camera and microscope.
4. Bring the photoresist-coated hypodermic needle into contact with the fixed flexible photomask by slowly lifting the needle towards the photomask.
5. Carry out UV exposure for 30 s (UV intensity: 15 mJ/cm²) and follow this by the developing process for 3 min.
6. Rinse the developer out of the sample using DI water.
7. Inspect the result through a microscope set to 200X magnification to determine whether the photoresist is clearly patterned on the Au-sputtered hypodermic needle. If the exposed photoresist is not perfectly removed after the developing process, repeat the developing process at 30 s intervals.

5. Cr/Au Wet Etching

CAUTION: Avoid skin/eye contact with the Cr and Au wet etchants.

1. Use a tweezer to detach the sample (photoresist-patterned hypodermic needle) fixed on the glass slide.
2. Immerse the sample into the Au wet etchant for 1 min.
3. Rinse the Au etchant out of the sample using DI water.
4. Inspect the result through a microscope set to 200X magnification. If the gold to be removed still remains, repeat the wet etching process at 10 s intervals. Excessively long wet etching time makes the interdigitated electrode (IDE) thinner.
5. Immerse the sample into the Cr etchant for 30 s.
6. Rinse the Cr etchant out of the sample using DI water.

6. Removal of Residual Photoresist and Passivation

1. Immerse the sample (metal-patterned hypodermic needle) into an acetone solution for 1 min.
2. Rinse the sample with DI water and dehydrate it on a hot plate at 105 °C for 10 min.
3. For electrical passivation of the connection lines, cut the shrink tube so that it is 2 - 3 mm longer than the electrode (20 mm, the maximum depth to penetrate), as shown in Figure 2, because the length of the HST will be reduced after the HST shrinks.
4. After positioning the HST as far as possible from the end of the IDE, raise the temperature of the HST using a heat gun at 150 °C to tightly passivate the needle.

Results

The interdigitated electrodes (IDEs), as shown in Figure 2, result in a larger effective sensing area on a limited surface compared to other shapes of electrodes. The overall length of the IDEs is designed to be 860 µm to detect and analyze the impedance changes at less than 1 mm intervals in the biotissues, which will provide a high locating accuracy in biopsy and drug delivery procedures. The total width of the IDEs is 400 µm, which is a geometric...

Discussion

We demonstrated that photolithography using spray coating and a film photomask is a feasible method to fabricate fine IDEs on the curved surface of a hypodermic needle with a small diameter of less than 1 mm. Both the width and the gap of the IDEs are as low as 20 µm, and the fabrication margin from the tip is as small as 680 µm. Within the protocol, the alignment process, including wedge error removal, is a critical step. The production yield was over 90% when the EoN was manufactured individually through a ri...

Materials

Name	Company	Catalog Number	Comments
Heat shrink tube	VENTION MEDICAL, Inc.	103-0655
Hypodermic needle (22G)	HWAJIN MEDICAL co. ltd	-	http://www.hwajinmedical.com
Heat gun	Weller	WHA600	http://www.weller-tools.com/en/Home.html
Ultrasonic cleaner	HWASHIN INSTRUMENT CO, LTD.	POWERSONIC 620-	http://www.hwashin.net
Hotplate	AS ONE Corporation	006560
Sputtering	A-Tech System. Ltd.	ATS/SPT/0208F	http://www.atechsystem.co.kr
Glass slide	Paul Marienfeld GmbH & Co. KG	1000412
Spray coater	LITHOTEK	LSC-200
Photoresist	AZ electronic materials	GXR 601	http://www.merck-performance-materials.com/en/index.html
Developer (solution)	AZ electronic materials	MIF 300	http://www.merck-performance-materials.com/en/index.html
Aligner	MIDAS SYSTEM CO.,Ltd.	MDA-400M	http://www.midas-system.com
Microscope	NIKON Corporation	L200	http://www.nikonmetrology.com
Au wet etchant	TRANSENE COMPANY, Inc.	Au etchant type TFA	http://transene.com
Cr wet etchant	KMG Electronic. Chemicals, Inc.	CR-7	http://kmgchemicals.com
Au target	Thin films and Fine Materials	-	http://www.thifine.co.kr
Cr target	Thin films and Fine Materials	-	http://www.thifine.co.kr
Argon gas (99.999%)	SINIL Gas Co.Ltd	-	http://www.sigas.kr
Acetone solution	OCI Company Ltd	-	http://www.ocicorp.co.kr/company/index.asp
Impedance analyzer	Gamry Instruments Inc	Reference 600	https://www.gamry.com
Height Controller	Mitutoyo Corporation	192-613
Phosphate buffered saline	Life Technologies Corporation	10010023