Stsinless for surgical tools.

I have a design job for a surgical tool that is basically a complex set of pliers. The client wants to be able to make a small run to proof out the design. So those would have to be machined. In production the would probability be forged. This is a hand operated device, but There are several sliding and turning contacts and there will be considerable bending forces on parts. All while having to be reasonable in weight and be autoclaved.
Question. What would be a good alloy for machining these and what heat treat spec?
Thanks for the help. I have designed lots of similar but non medical devices, so I am not experienced on the proper material.
Btw. I know that this will be crazy expensive to make. But it is what it is..

Also. I assume I can't use any lubricants. So planning on a plastic acme nut. Maybe bronze?

316 or 440 CRES seems to be popular with the medical parts we make. I think 316 would suit your purpose just fine, from what I'm imagining from your description.

No idea what the acme nut is for though. So it's hard to suggest anything. I don't know how much friction has to be negated, or if you're just trying to limit galling or something.

Can't really tell you anything about a heat treatment since you haven't given enough information to establish requirements. I thought you said you had the design

Thanks that is a start. Acme concern is to galling. I need some friction to self lok the screw .

I used to work for a major medical device/implant manufacturer. The go to material was 316L, even for implants.

Paul

A quick Google check yields 304, 316, 420, 430 and titanium.

Titanium Metal: Titanium Medical Metal of Choice – Supra Alloys

Surgical stainless steel - Wikipedia

Sorry...

I would ask what the customer wants and if he doesn't know, I would suggest he do the research. Liabilities can be high in the medical field.

Tom

I'll be a ditto head. 316 is a standard choice in this application, 440C if you need more strength or hardness after heat-treatment. 410 and 420 are also used for medical instruments. Ti6Al4V is another common material for medical stuff.

The 400 series alloys are not as corrosion-resistant as 316, however these are standard materials for medical instruments so the end user already should have material approvals, autoclaving procedures, etc. Hardened 440C is substantially more resistant to galling than most common stainless steels, which may be a plus for this application.

Some people have allergies to nickel, so using 400 series SS will reduce exposure risk. Greater hardness if heat-treatable, which can improve corrosion resistance of some 400 alloys.

There's a few post-treatments for hardness and lowering galling for SS, like that offered by this company: Surface hardening of stainless steel - - Improve wear, galling or corrosion - Expanite There's others that do similar nitrogen and carbon "packing", it's an interesting field.

Do *everything* you can to minimize parts count, sliding parts, or designs that create "trap areas" where pathogens can hide out. Design for sterilization, even if it requires extra cost. Learn about the common sterilizing methods and their failure modes, consider these when designing and machining.

Be sure any plastic you use is rated for sterilization, but better to avoid them if possible. Adds parts count and fits that can retain microbes and other baddies.

Almost everything I've done for medical instruments is 17-4 H900.
The working area is usually TiN coated.
I've only done one instrument out of 440C that got heat treated.
Most implants were 6Al4V, some 316.

Thanks all. I am a parts count reducing fein, but this device has 3 major mechanical functions so that means around 12 moving parts. Basicaly levers. Wherever possible structural parts wI'll be machined out vs built up but I am still stuck with at least 9 pivot pins. An acme screw a torsion spring and a nut or two.

I worked on a similar sounding set of pliers for the medical industry, material was 440C for the handles. After a few heat treat cycles they finished at 54-58 Rc.

Shouldn't a mechanical engineer figure out the design and materials? It's not like this device will ever be a real product without going through proper engineering.

Hi surplusjohn:
Don't forget one of my personal favourites: 17/4 PH stainless.
It's got decent corrosion resistance, it gets reasonably hard (44RC), it's not super great for gall and wear resistance but not bad either, it's easy to machine and harden. it's decently stable and lots of medical parts are done with it so getting FDA approval is less of a PITA.
Lots to love!!
Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
www.vancouverwireedm.com

Whoops...Booze Daily beat me to it!

John Welden said:

Shouldn't a mechanical engineer figure out the design and materials? It's most like this device will ever be a real product without going through proper engineering.

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Engineers don't necessarily come up with nifty ideas. I think it is good to have the customer take a stab at it if he has some good idea, then show it to the engineer and then show it to the machinist.

HuFlungDung said:

Engineers don't necessarily come up with nifty ideas. I think it is good to have the customer take a stab at it if he has some good idea, then show it to the engineer and then show it to the machinist.

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Sounds like he already has the idea. Now he needs the engineering.

I still say this is a chancy gig. You pick the material and process and design the mechanism and someone dies, you won't have enough liability insurance. I would only do something like this if a complete drawing with material specs are supplied and with a big company with deep pockets' professional engineer signs it.

Tom

17-4 Will corrode over time, especially (and relatively quickly) when exposed to saline. Even when precipitation hardened, it will still corrode (and isn't actually all that hard.)

316L is your best bet, but will not harden as much as 440C would, but is a better material choice in my book. One way to compensate for this is to use one of the 'new' 'plating' techniques out there, specifically for applications like this one.

Bodycote is one domestic supplier of this type of 'case hardening' (I'm using a lot of 'these' because the terminology isn't entirely correct. They're apt descriptions for the processes though.)

Couple of things to remember here: anyplace you have a sliding stainless on stainless contact, you're going to see galling. Exposure to saline will exacerbate this quite a bit. Hardening will help mitigate this, but 'hard' stainless is also a bit of a misnomer, and the tighter the fit, the faster you're going to see galling. Stay away from press-fit anything between stainless, unless you know it never needs to come apart. Remember to use never-seize on any threads, but remember the finished product will be assembled without it, no matter how many times you call it out on the assembly print with giant red letters (ask me how I know.)

I'm also a fan of electropolish for medical devices, as it removes toolmarks when done correctly - something medical device manufactures typically care about quite a bit in my experience.

I'm sure there are quite a few more things I'd remember if I were doing the design, but hopefully this helps.

If you get to a point where you want some more serious/in-depth help on this or any other designs, feel free to shoot me a message. I designed a fair number of medical devices over the years, and have done a ton of work in plants that are in the pharmaceutical world.

Good luck!

440C is the only off the shelf material heat treatable enough to make a plier type device which has a lot of leverage and needs to be strong, also used for springs.

316L for screws and acmes.

Since it is likely just proof of concept it will work for its intended purpose.

I used to repair and create surgical instruments and these are the only two that will work that you can buy off the shelf. I made all my screws out of 316L with a geometric die head.

Most surgical steels are proprietary and made in germany with custom alloying to get the properties required. Cheap disposable forceps like those used in the ER are simply vacuum cast 304.

BTW surgical instruments are typically lubricated during washing with a simple water soluble emulsion that every machinist knows as coolant, they only ones not lubricated are those such as eye instruments and dental that are exposed to mucosal membranes since its an irritant.

Make sure you get paid up front, 99.999% of all homemade surgical gadgets go no where unless purchased by a major distributor at which point all they want to buy is the idea and make it themselves and will be named after the doctor who helped develop it...you won't get credit for your idea.

Just saying, been there and done that.

4JawChuck said:

BTW surgical instruments are typically lubricated during washing with a simple water soluble emulsion that every machinist knows as coolant, ...

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I'm told the medical industry refers to soluble oil emulsion lubricant as "instrument milk" due to its appearance. I bet there are serious restrictions on which soluble oil, however, since this stuff is usually going to be poking around inside someone's flesh, thus much more serious bio-compatibility concerns than dermatitis.

Also, remember to passivate the machined parts. That may be covered by some of the treatment methods above, but autoclaving stainless exacerbates corrosion issues.

You may also need to discuss the alloys and post-machining treatment of these tools with the person in charge of instrument cleaning and autoclaving at the facility where they are to be used. I have found they often have some fairly stringent criteria they expect to be fulfilled. At a decent-sized hospital that person will likely be quite well educated in the pitfalls of corrosion and may even raise concerns of contamination of their other instruments if yours don't pass muster. (There are a number of reasons quality medical instruments are rediculously expensive).

Denis

Surgical Milk: Surgical Instrument Corrosion Control -ICT

From an MSDS for a surgical milk:
SECTION II – Hazardous Ingredients / Identity
COMPONENT % TOXICITY DATA
Polyoxyethylene Sorbitol Faty Acis Ester 1.6% LD50 > 31.6 g/kg
Polyoxyethylene Sorbitol Hexaoleate 2.2% LD50 > 31.6 g/kg
White Mineral Oil 11.8% LD50 > 5000 mg/kg
Sodium Benzoate 3.0% LD50 > 4100 mg/kg