Fabrication & finishing of hypodermic needles & cannulas

A. There are some tricky ways to electropolish tips without loosing sharpness. I cannot say for sure if it works until we test your samples. My experience tells me that acid-free electrolyte might work.

A. J,
I'm stepping a little bit outside my wheelhouse here, but I have my doubts that electropolishing will be a sufficient deburring process for you. You may just end up with polished burrs. Of course you'll have to try it out to see. A more traditional mechanical deburring process like tumbling or vibratory bowls might be the better option in the end, though. Either way, most likely the easier path will be to do your deburring before, rather than after, sharpening the tip.

A. Hi Carol,
I believe that the only way to do this is to firstly make a much larger tube (using stainless for needles) and to FILL that tube with a material that can be easily, so-to-speak, removed. I think it was copper.
Why? Because if you cold extrude or cold size, and pass unfilled tubing through a range of ever decreasing dies to get very, very small tube diameters, you'll get ovality if not complete distortion and flattening.

In the rolling mill industry the super fine rolling mill design is called a Zenzimir (or a word sounding very much like that) but I don't think that by itself it can make hypo needles. But the initial tubing? Most probably.

The next problem is to try to remove the 'filler' material. Thousands and thousands of needles are placed in a nitric acid tank ... and not days, not weeks BUT months later the acid eventually leaches out all the copper. Presto, super fine holes in a super thin material. Ergo, you have your fine needle. OK? Aren't you lucky you are growing up in a world where, compared to Ye Olden Days, injections used to hurt!

Now you can go tell all your pharmacists and doctors HOW TO DO IT!

A. Having been in the medical needle business for over 20 years, I can tell you how this is done. A flat, ( I believe it is about 1 1/2" in width) strip of 304 stainless steel is roll formed and welded at the joint into a tube. The tube is then annealed and drawn down through a series of dies with a floating mandrel on the inside of the tube that keeps the I.D. consistent. Where ever needed, additional annealing is performed to keep the desired physical properties and draw characteristics of the stainless steel. If you look at a relatively large needle under magnification, you can find the weld line! This is the beginning for every needle manufactured even down to a 30 gage.

For more information you can contact the tubing manufacturer K-Tube Corporation in San Diego California. I hope this helps!

Best regards,

I would like to thank Frank for his accurate response to this posting. He is correct. These days, tubing is made on a tube mill by first forming then welding, either with a laser or TIG welding operation.

A. You can use an electropolishing method, to clean ID of 304 stainless steel tubings. As we manufacture reusable hypodermic needles, we are following this method to clear ID of 304 s.s. tubes, and it gives satisfactory results. Moreover, ultrasonic cleaning method is also very much useful (media should be T.C.)

A. Amrut,

My first suggestion would be for you to go to a local 'pipe bending Company' for some advice ... although you can be l00% sure they would have never 'bent' something so small !

Bend radius! If you cold bend your needle, either it will break or it will flatten out. The absolute min. bend radius I would consider is 6 x diameter and that's using heat to form over(i.e., around) a mandrel. Maybe start off @ 8. The larger the radius, the better (quality) the bend.

You state that the needle's end(s)are closed. In the metal pipe bending field in order to make perfect bends, they use a 'rosin' which is heated up to well under 100 °C but, upon cooling, becomes solid. This allows for excellent bends to be made in certain materials (copper) without heat.

Somehow that sure doesn't seem feasible for you. Summary... use heat (to red hot), use a large bend radius.... and cross your fingers!

A. I understand your drawing to show a section of the tube, with the end closed. I see nothing in your query or sketch about bending. I think Freeman has mis-read the situation.

Assuming you need a gas/liquid-tight closure, you'll probably be able to get it done by a company that manufactures thermocouples. The usual way of closing the end of the metal sheath of mineral-insulated thermocouples is by TIG welding. This is on tubing down to a couple of mm inside diameter. It's a small TIG-welding setup! For a tube as small diameter as yours, you wouldn't need any filler metal.

Probably, electron-beam welding would also do the job OK, but might be a problem accommodating the tube in the vacuum chamber. I'd go for the TIG.

A. Sanjeev!

I would have thought that the 'polishing', as you call it, would really be due to the extruding process using the multi roller Zendsimir (Zensimere? ... spelling?) used for ultra pressure and ultra small diameters.

I've seen hypodermic needles by the mega thousands all resting in a nice nitric solution to leach out the copper cores ... a very slow process indeed!

A. Point Technology reports super polishing of W (tungsten) needles with a Ra below 10 nm.

A. I assume you are holding your parts on a fixture in a straight line. If this is the case the peripheral parts on your jig will always pull more current than the center parts. Thus your edge parts are possibly over polishing or rounded.

There are methods of improving the capability of your process i.e. equal current distribution:
1) round Jig and round cathode
2) individual rectifier to each part
3) curved cathode using a straight jig

I would be interested to see if there are other solutions to providing an equal current distribution across all parts.

A. Placing cathode above the needle end will cause the high current density on the end of the needle, thus making round the tip.

In order to reduce CD, place the needle inside the rounded cathode, tip of the needle must be 10-15 mm above the cathode.

The proper cathode configuration will improve the results, however in order to guarantee sharp deburred needle tip, you need to select proper electrolyte and polishing regimes.

adv. We see good results in our Universal electrolyte or in our Ultrapolishing electrolyte. Presently electropolishing is developed for titanium, nitinol and stainless steel I.V. needles.

A. Hi Aimran; Hi Roozbeh; Hi Kinh. Electropolishing smooths parts and passivates them. It can brighten them if they are dull. However, 'deburring' is a different operation than electropolishing as far as I know (but I have no hypodermic needle experience).
Secondly, a problem to bear in mind is that electropolishing doesn't "know" that you consider "this lump" a design feature but "that lump" a burr. We can't expect the electropolishing solution to know to not dissolve this lump but to continue to dissolve that lump :-) It might be that mechanical finishing (tumbling, vibratory finishing, etc.) is better suited to your particular burrs and should be used before electropolishing. My experience is limited, but I not aware of electropolishing being used to sharpen a point, and it sounds unlikely to me.

Luck & Regards,

A. Samut,
It all has to do with how electric current flows off the microscopic features of the piece while immersed in an electrolyte bath. You are correct in that the current flows more off the "peaks", but the tricky part is learning how that works when you have a macroscopic feature like the needle tip. As you said, the tip gets thinner rather than being solely dissolved down from the point.

I'm not well versed in this, other than to know it's very complicated. In many respects, electropolishing is as much an art as a science, being able to craft the electrode arrangement and such in order to control current density as required for the shape of the piece.

A. It depends on what surface finish you want on the markings. Maybe you cannot use paints. So, you can mask with 9 mm wide vinyl tape leaving 1 mm openings and sand blast them. Another way would be to grind them very lightly with a 1 mm thick cutting disk. If the needles are rotated, both methods will leave a circular mark. A small suitable turning device can be purchased for a few dollars.

A. Our company has a US patent on the application of a bis-silane to stainless steel for medical devices. The silane is a silicon based chemical, not SiH4 but bis-tri-ethoxysilyl ethane that is both corrosion resistant and imparts a non-stick characteristic to the part.

A. Siliconisation is required for reducing the coefficient of friction on the needle. DOW makes a solution which is used for siliconisation. SOP I do not know but one can ask Dow.

A. Siliconisation of needles/cannulas can be done by Dow Corning's MDX4-4159, which is a 50% Medical Grade Silicone Dispersion.