Pain free Injections - Fabrication of Micro needles

Trypanophobia is fear of injection needles and this fear is quite real and prevalent. According to Wikipedia, over 10% of American adults suffer from fear of needles and I am sure this situation may be same in other parts of the world as well, if not worse than this. Research and innovations in medical field have led to many new methods of drug delivery that are transdermal (i.e. application of medicine through skin) and offer a good alternate to conventional injection needle method of drug delivery which is hypodermic (i.e. beneath the skin). Advantages of transdermal drug delivery are obvious like less or no pain, no gastric irritation, no skin rashes and improved patient compliance. One of the transdermal method of drug delivery is the use of micro needles. In this article we are going to learn about the fabrication process of micro needles.

Micro fabrication methods such as photolithography, silicon etching, laser micromachining, micro-molding and other MEMS techniques are used for the fabrication of micro needles. Among these methods, one method, photolithography is discussed here. Figure 1 shows the various processes involved in the fabrication of micro needles using photolithography. Polymer micro needles are fabricated using this process. Steps are as follows:-

Fig. 1 Fabrication of micro needles (Ref. [2])

• Silicon wafer cleaning is the first step which involves mixing of H₂SO₄ with H₂O₂ as the Piranha and then heat at 150 °C for removing the mist.
• This is followed by deposition of oxide of thickness 1µm on the wafer and coating with photoresist with thickness 4µm.
• Third step is the photolithographic process for developing a V-groove with a depth of 320µm on the wafer.
• Then removal of oxide and photoresist is done separately using BOE and Acetone respectively.
• Further, wet etching using KOH solution is done.
• A layer of negative photoresist, SU-8 is coated up to a thickness of 450 µm.
• Another photolithographic process is done to develop position of bottom of the micro needle.
• Lastly SU-8 is removed by putting the silicon wafer in MF-319, micro needles will move off from silicon wafer.

There are following four types of micro needles based on the type of application and type of material used:-

• Hollow Micro needles: These are mechanically strong and are manufactured in a way that these do not suffer with the problem of clogging during drug injection process and are made from silicon. Application is delivery of insulin.
• Coated Micro needles: These micro-needles are uniformly coated with drugs and are used for the delivery of peptides, proteins and DNA.
• Solid Micro needles: Diffusion via these micro needles is passive. They increase skin permeability by creating micro channels. A drug loaded patch is then used for drug delivery via these channels. These are made using stainless steel.
• Polymer Micro needles: These needles are made from polymers that can be dissolving, non dissolving or hydrogel forming types. Dissolving type is based on poke and release principle and use polymers like polysaccharides. Hydrogel forming micro needles do not contain drugs, they simply act as conduits or channel between dermal microcirculation and the attached patch-type reservoir.

By: Ms. Anshu Sharma - Assistant Professor (ECE), Chitkara University, H.P.

References

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4588187/
2. Yu-Tang Chen, Chin-Chun Hsu, Chieh-Hsiu Tsai, and Shung-Wen Kang, "Fabrication of microneedles", Journal of Marine Science and Technology, Vol. 18, No. 2, pp. 243-248 (2010)
3. https://www.omicsonline.org/microneedles-an-emerging-approach-for-active-transdermal-delivery-of-insulin-jbb.10000e24.php?aid=10754
4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5540964/