Many device applications using stainless steel tubing now require improved surface finish on the ID of the tubing. 

• This creates smoother inner surfaces that reduce friction for mating parts or parts that must travel in and/or out of the tubing during the surgical procedure.

There are also an increasing number of fluid dispensing applications where improved surface finish is critical to success.  Blood sampling and testing equipment uses very expensive chemical reagents to mix with blood during the testing cycle.  This type of carryover reduction can be supported through careful attention to the surface finish in the injection probe and transfer tubing.  In these types of applications it is critical to reduce carryover from one sample to another. 

• This reduces total cost of the testing (due to a more efficient use of reagents) and improves the accuracy of test results (with more precise amounts of chemicals used in the tests). 

The 3 steps for improving surface finish on the ID of tubing are:

1. Define the desired outcome, measurement methods and measurement units.  This will assure a clear definition of what improved performance results are needed.

          A note on measurement units -> The finish on hypodermic tubing is usually measured in Ra, Rq, or Rms.

          Rq = Rms: The Root-Mean-Square

•              Rq is the same as RMS is a statistical measurement of the square root of the average of the squares of the measurement. The Rq or Rms value is generally 11% higher than the Ra value for the same surface roughness.  Keep in mind that the ~11% difference represents measurements from a normal test patch with a pure sine-wave profile. However, there is actually no mathematical relationship between the two parameters and, depending on the actual manufacturing process and the resulting surface profile, the ratio between Rq and Ra can vary by as much as 200%.

           Ra: The arithmetic mean

•               The average roughness, Ra, is expressed in units of height. In the Imperial (English) system, Ra typically expressed in "millionths" of an inch. This is also referred to as "microinches" or sometimes just as "micro" (however the latter is just slang). In the metric system, Ra is typically expressed as "millionths of a meter" also called "micrometers" or "microns".

Cadence can use any of the above units of measurement to generate the desired results…

2. Generate variations of the product with better finish.  This can be done via numerous options, such as:

• Micro-Polishing and/or passivation
• Bore enhancement of tubing via use of a “bright draw” process.
• Coatings, such as Silicone, Teflon or PSX Coatings.  These may also provide greater lubricity.
• Abrasive Flow Machining or Extrude Hone Process.  This process is distinctive in that it effectively provides precision deburring, polishing and edge rounding of internal and external surfaces. Anywhere the unique abrasive laden polymer — or, media — can flow, precision finishing occurs. This process is fairly expensive with the ultimate ID finish improvement a function of the tube ID size and the tube length.

3. Test the variations in the surface finish to correlate actual finish improvement with desired outcome improvement.

Repeat and refine steps #1 through #3 as needed to optimize the final product design!
 

According to ASTM A 380 – 96, the term passivation is commonly applied to several distinctly different operations or processes relating to stainless steels.  Common meanings include the following:

• Passivation is the process by which a stainless steel will spontaneously form a chemically inactive surface when exposed to air or other oxygen-containing environments.

• Passivation is removal of exogenous iron or iron compounds from the surface of a stainless steel by means of a chemical dissolution, most typically by a treatment with an acid solution that will remove the surface contamination but will not significantly affect the stainlees steel itself.

• Passivation is the chemical treatment of a stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of enhancing the spontaneous formation of the protective passive film.

There are three (3) main methods for passivation that have been used in the medical device and diagnostics markets for years:

Nitric Acid Passivation: Nitric acid passivation is one of the most well known and long standing passivation methods. The major advantage of using Nitric acid for passivating stainless steel is the fact that Nitric acid not only removes free iron from the surface, but it also contributes as surface oxidizer by contributing to form an extra oxide layer that is often 10-20 angstroms thick. Another advantage is that surface analytical methods also reveal that the Chromium oxide layer can be up to 25% thicker when using Nitric acid passivation. There are major issues related to using Nitric acid as a passivation chemistry due to safety, handling and environmental concerns.

Citric Acid Passivation:  Recent developments in green technologies led to a very common chemistry that is now utilized for passivation. Citric acid is very good at removing free iron from stainless steel surfaces. It is getting much wider acceptance due to the improved environmental and safety effects as compared to Nitric acid. Citric acid is not as good of an oxidizer as Nitric, but this disadvantage is easily overcome with the addition of oxidizing rinses to its post treatment steps.

Electropolishing: Electropolishing is the best passivation method in terms of removing free iron from stainless steel surfaces.  It also forms an oxide layer that can be up to 20 angstroms thick to yield a higher chromium oxide/iron oxide ratio. It is a commonly accepted passivation method listed in the infamous ASTM A967 passivation standard.  Electropolishing yields passive properties AND it has the additional potential benefits of removing burrs or scratches from the surface and providing a more glossy appearance.

Cadence currently has any of these three methods available for passivation, including Nitric Passivation (to ASTM A967-05 Nitric 2), Citric acid passivation, and Electropolishing.  

Any of these methods can be an acceptable alternative so we offer all three for maximum process flexibility for our customers. 

For more information on your passivation needs, please contact us today… 

This specification was very primitive in nature, attempting to simply identify gross defects.  Some highlights of the spec were:

• No blade shall contain a nick greater than .004” – A nick was defined as a chipped-out, broken-out, indented, or bent out piece of metal, a semicircular projection or any similar gap, indentation, or projection.  The specification went on to define the allowable limits of numbers and sizes of smaller nicks.
• No blade shall contain a feather greater than .005” in length - Feathers were identified as a thin, curled, or turned edge, not removed by honing or buffing.  The specification also went on to define the allowable limits of smaller feathers, etc. 
• There shall be no burs, jags, or other defects on the cutting edge.  A bur was defined as a piece of metal projection not inherent to a smooth uniform surface.  A jag was defined as several small toothlike projections or similar indentations, individually smaller than a nick, and collectively greater than .015” in length.
• Blade inspection was to be done at 10x magnification on a binocular microscope with two 15watt lamps.

This sharpness specification was cancelled without replacement in 1996.   Best practices today for assuring blade sharpness or needle tip sharpness include:

• clear specification of all geometries on an engineering drawing (angles, surface finish, burr, etc.)
• use of cut force or penetration force testing that has correlated test limits with actual functional performance

For additional guidance on sharpness, see our incisionlab
 

The cornerstone and foundation of a multitude of surgical procedures, medical needles are often improperly viewed as a “commodity” rather than precision-engineered devices. However, a growing number of medical needles are taking a major role in today’s state-of-the-art surgical devices, and their function as a “delivery device” for specialized procedures is very critical.

The first step for any medical needle based project should be “DEFINE”, a true brainstorming session aimed at identifying Functional and Manufacturing aspects of design and performance so that they can be matched with the expertise and capabilities of the contract manufacturer.

The following are 5 tips on how to make better medical needles:

1.) Optimize for the Application

- Fluid vs. Solid Delivery - What is being delivered? Does the diameter of the deliverable require special consideration for the tubing I.D.?

- Surface finish requirements (Ra) could be play a significant role from a multitude of parameters including cleanliness, ease-of-delivery, damage to the implant etc.

- Coring vs. non-coring tip configuration – is this a required differentiator and how critical is the non-coring aspect?

- Material selection to meet application requirements (non-magnetic, MRI, hardness, chemical composition)

2.) Material & Needle Tip Consideration:

- Which part of the body is being penetrated? (tissue vs. bone)

- Number of insertions per procedure? Is the needle being used on a one-time basis or expected to perform in multiple insertions? This aspects impacts both material selection and tip geometry

- Flat vs. Lancet Grind? Standard vs. Short bevel? Non-Coring? Needle “sharpness” needs to be identified from a functional standpoint.

- Material selection to meet tip requirements and edge retention

3.) Performance Criteria

- PF - Penetration Force or FTF (Force To Fire). Although one would expect a needle that requires low penetration force, many of today’s applications demand a performance criterion that is above the acceptable norm. Consideration should be given to:

  -> Why is penetration force critical in this application? (tissue trauma, site location, competitor’s performance)

- Straightness/alignment of cannula to hub or tubing/wire in general.

  -> Why is this critical feature and what is the impact on patient safety, inspection, assembly, and handing?
 

4.) Inspection Criteria

- Magnification – requirement vs. industry standards for in-process and/or final inspection. Why is the additional magnification necessary and is it justified from a cost standpoint.

- Sampling Plans – should be discussed up front along with all supporting documents to assure ultimate process capability.

- Cleanliness – define “clean” along with verification parameters.

5.) Packaging

This critical discussion item should address

   -> tip protection to protect sharps

  -> in-transit protection

  -> receiving requirements and handling

  -> Expectation for cleanroom receiving or processing

 The foundation of success in surgical device manufacturing is based on careful and detailed upfront evaluation of all design, functional and performance criteria. The five tips above will help make a better medical needle.  If you know of other key items then please leave us a comment…

In addition to IncisionTech’s Adrenaline rapid prototyping response service, new machining capabilities include tolerances to 10 microns, modular tooling configurations, thread wherling, hob milling, helical and cylindrical turning, live tooling and more.

Materials include Brass, Stainless Steel, Titanium, Peek, Ceramic and more. The machining center area is supported by a dedicated tooling and engineering team.

IncisionTech, a division of Cadence, Inc. (Staunton, VA) is an OEM medical device and component manufacturer. The Cranston, RI needle and machining technologies center boasts over 110 employees. Cadence, Inc. with manufacturing sites in Staunton, VA and Cranston, RI is a publicly held corporation with over 225 employees.

300 Series Stainless Steels - Austenitic stainless steels offer excellent corrosion resistance for the harshest surgical environments. Non-magnetic properties combined with high toughness at all temperatures make these grades of stainless steels an excellent selection for most surgical applications. These austenitic stainless steels also provide more corrosion and shock resistance than 400 series martensitic steel, but sacrifice some wear resistance, hardness, strength, and durability.

301 302 304 304L 316 316L

400 Series Stainless Steels, Heat Treated

In general, these materials exhibit excellent toughness and hardness properties and offer good corrosion resistance. Suitable for medical and surgical applications, 400 series martensitic steel is much more corrosion-resistant than regular carbon steel and can be sharpened and pointed to equally-keen sharpness. IncisionTech maintains an extensive inventory of Razor Blade Stainless steel in thicknesses from .010”-.062” thick, as well as 420 “Cutlery Grade” Stainless steel.

410 420 420 (cutlery grade) 440A modified 440A (aka Razor Blade Stainless Steel)

17-4 & 17-7 PH Stainless Steels

Precipitation hardened austenitic stainless steels offer excellent corrosion resistance for the harshest surgical environments and superior edge strength to many 300 series stainless steels. This class of materials also provides more corrosion and shock resistance than 400 series martensitic steel, but sacrifices some wear resistance, hardness and durability.

High-Performance Zirconia Ceramic

Recent developments have made it possible to produce extremely sharp blades from transformation-toughened zirconia (ZrO2), commonly referred to as "zirconia ceramic". Although not recommended for high-shock applications, the Rc 75 hardness and low friction coefficient make zirconia ceramic an interesting option. Its component life can be more than 100 times that of conventional steel. Other characteristics of Zirconia that make it an attractive material in some applications include, superb corrosion resistance, non-magnetic, and high electrical insulation properties.

Nitinol

Originally developed 40 years ago in the Naval Ordinance Labs, Nitinol has recently found a multitude of novel surgical applications. Made from nearly equivalent amounts of Titanium and Nickel, this material can be processed in ways that optimize its mechanical properties for “superelasticity” or “shape memory.” As the availability of Nitinol has been growing, IncisionTech has had the opportunity to partner with medical device OEMs in developing some of the most innovative cutting and piercing solutions for Nitinol on the market today.

Titanium

Titanium is a standard material for many implants such as hip and knee joints and bone screws due to its total resistance to body fluids and high fatigue strength. Additionally Titanium has proven to be more compatible with MRI (Magnetic Resonance Imaging) and CT (Computed Tomography) than some other materials. IncisionTech has worked with a number of medical device OEMs to create innovative Titanium-based solutions.