Exploring density stainless variations in 300-series alloys for medical devices

Abstract

Medical devices and implants are manufactured with an uncompromising precision of material homogeneity. While biocompatibility and resistance to corrosion are paramount in the production of medical products, the volumetric mass or density of the material (stainless density profile) is also a decisive factor for the performance of medical products. This publication quantifies slight variations in the density of 300-series austenitic stainless steels in their different modifications (grades) 303, 304, and 316L, based on their crystal structure and thermodynamic properties during precise processing. It serves as a knowledge base for the development and production of medical products that must reliably meet the required precision in the micrometer range. By comparing these data, it is possible to draw conclusions on the necessary precision of the density of the material, to be able to achieve the desired precision of the medical product.

Why is the density of stainless steel consistency paramount

Material Variability in Miniature Components Material Variability in Miniature Components is a major challenge faced by design engineers of Tier-1 medical device manufacturers. These components are subject to significant stress in both high-speed robotic assembly and in the operating room. In the robotics surgery environment, the critical nature of tracking surgical instrument motion to the exact input of the surgeon requires any variation in mass of the components be addressed – otherwise, small differences in mass can result in positional drift or resonance. The industry is oblivious to the fact that there is no material variability allowed in 300-series stainless steels – a presumption that just does not exist.

When a component is designed for a certain moment of inertia, even a small variation in the material density can result in a part that is not within the dynamic performance specifications. For example, taking the tip of an ultrasonic scalpel machined from a 304 stainless steel batch with slightly higher material density than nominal, the resonant frequency that was measured on the prototype will no longer be respected. Thus, a less efficient cutting action, a shorter lifespan, etc., can be observed. Hence, leaving the material specifications to the generic certificates provided by the suppliers is not sufficient. A complete characterisation of the material must be obtained, taking into account the effects of alloying elements on the material density of the substrate.

The crystallographic origin of dense stainless steel in austenitic structures

The reason that 300-series alloys have the mass properties that they do is the atomic structure of the materials. All 300-series alloys are austenitic and have a face-centered cubic (FCC) crystal structure. This means that iron atoms occupy the corners and center points of the faces of a cube. The density of stainless steel is a result of the arrangement of these atoms, as well as the atomic weight of the elements that are substituted into the lattice.
The theoretical density of the pure metal, Fe, is about 7.87 g/cm³. However, in a 300-series stainless steel, many percentages of chromium and nickel are often added. While chromium is less than iron, nickel, with an atomic mass of 58.69 u, is more than iron with an atomic mass of 55.85 u. The high density of iron metal serves as the baseline for this stainless alloy density, while the actual stainless density will be varied depending on the exact composition of the alloy of these elements. The FCC lattice is one of the more dense arrangements of atoms that Mother Nature ever gave us; it has an atomic packing factor of 0.74, and thus these steels are much denser than their ferritic and martensitic cousins, which gives us the “heft” of handheld surgical tools.

A comparative analysis of density stainless values in 303, 304, and 316L

It’s common for medical manufacturers to debate the advantages and disadvantages of 303, 304, and 316L materials. While machinability and corrosion resistance are key considerations, the material density of the various stainless grades is also an important factor to consider when it comes to precision balancing.
303 Stainless Steel Density
Material Density: 8.03 g/cm³ Free-machining 8.03 g/cm³ The iron is lightly sulphurised to act as a free-machining alloy. Although sulphur is a light element, its influence on the iron mass density is contained by including a high level of manganese. The manganese sulfides form small inclusions in the austenite. These inclusions are crucial to the success of breaking down the chips when high-speed turning the surgical fasteners.
304 Stainless Steel Density
The industry standard for our high-density material is a density of 8.00 g/cm³. This material is used as the benchmark density for stainless calculations in most diagnostic and non-implantable applications.
316L Stainless Steel Density
Specific Gravity of 8.00 – 8.07 g/cm³. The “L” refers to the low carbon content and the addition of 2%−3% Molybdenum A=95.95 u, which contributes to the higher density. As molybdenum is denser than iron, 316L is almost always the heaviest of the commonly used medical grade stainless steels.
For global suppliers of cannulas and trocars, it is crucial to understand the individual density of stainless steel in order to calculate the actual weight of a large quantity of materials to be shipped. Any inaccuracy in material density can lead to tremendous costs associated with the weight of heavy shipments, as well as impact the accuracy of calibration for the CNC machines.

How the density of stainless steel governs the physics of laser-based medical fabrication

Laser micro-welding and cutting are important technologies in the assembly of advanced medical devices such as cardiac pacemakers and endoscopic cameras. The performance of laser micro-welding and cutting is heavily dependent on the density of the austenitic stainless steel alloy. When a high-energy fiber laser beam is focused onto the surface of the metal, it forms a melt pool accompanied by the formation of a vapor capillary – a keyhole.
The stability of this keyhole is a balance between the vapor pressure and the hydrostatic pressure. The hydrostatic pressure is directly proportional to the density of the stainless steel that it is attempting to penetrate. In the body cavity, there is an increase in vapor pressure that arises from the escaping gases and the resulting reduction in atmospheric pressure in the cavity. Within the keyhole, there is a similar effect as the gas and vapor expelled by the arc tend to concentrate in the keyhole, reducing the pressure there.

A little-known fact is that pressure in the weld pool is a function of density (ρ) x gravity (g) x depth of the melt (h). Because 316L has a higher density than 304 stainless, the liquid column of 316L exerts greater pressure and thus requires a higher energy flux from the laser to maintain the keyhole. In addition, any variation in the density of the stainless steel from one part of a batch to another can lead to variations in laser penetration depth and cold welds or micro leaks, which are not acceptable for a hermetically sealed medical implant. So the material density becomes a variable to validate in the laser process qualification steps of IQ/OQ/PQ.

Dynamic performance and vibration damping are influenced by the density of stainless steel

Design of vibration damping for surgical robots Sancheng

Li1, Yuanyuan Li1, Zhixiang Xiao1,

2, Zichen Zhang1, Bo Li1, and Shujun Li

3. Background: Modern surgical robots have applied the vibration-damping property of the arm and the end-effector to prevent the vibration of the tip during microsurgery. The design of the vibration damping in the arm and the end-effector is a crucial factor for a surgical robot system. Methods: Based on the principle that the density of the material has a great impact on the natural frequency of a component, and the natural frequency is inversely proportional to the square root of the mass, the analysis focused on the influence of the alloy density of stainless material on the vibration-damping characteristic. Results: A detailed analysis was carried out to illustrate the design of the vibration damping and its influence on the performance of surgical robots. Conclusion: The result shows that the alloy density of stainless material has a great impact on the design of the vibration damping.

We have high-density 316L, and the high-density material inherently has a degree of damping. So, when you couple that with the very high frequency vibrations from a brushless motor, and then you’ve got a highly dense material in your stainless, that helps to dissipate, or act as a damping factor for those acoustic waves that may otherwise propagate all the way through to the end effector and actually cause chatter with the scalpel, which is not what you want when trying to deliver micrometer precision to a point in space. In fact, if you don’t reoptimise for the new density material, your system will begin to exhibit chatter, which will actually degrade your ability to perform the very fine precision task that you are trying to achieve.

Validating the density of stainless steel for stringent medical quality assurance standards

According to regulatory guidelines such as ISO 13485 or FDA 21 CFR Part 820, it is required for manufacturers to validate all material properties and for all material properties to be traceable. A simple and basic method to validate the density of stainless steel and ensure the integrity of the material is by verifying it in Incoming Quality Control (IQC). However, depending on the method used by the supplier to validate this parameter, it is highly recommended that this property be verified and validated by other means, such as testing by using Archimedes principle or by using gas pycnometry.
Stainless density, ρs, is calculated using Archimedes Buoyancy Method, where the sample is weighed first in air and then in a liquid of known density. Possible causes of decreased strength in a surgical drill bit include “interstitial porosity” or “inclusion clusters”. Consistent density stainless steel measurements of the material assure that the material is uniform and void of porosity associated with lower grades of stainless steel. This is important for components that require additional process steps (such as electropolishing and/or passivation) where the intent is to chemically remove material uniformly across the surface.

Logistical and trade-weighted impacts of density stainless for global suppliers

The density of your stainless materials is relevant for the chargeable weight of your medical components in the context of international trade. Therefore, it is especially important for our foreign trade customers who send large quantities of orthopedic screws or plates per month to know the chargeable weight of their material to avoid losses. In this context, a higher density means lower chargeable weight and thus lower costs. For Grade 304 and Grade 316L, these differences add up.
A side effect of the high density of stainless (approximately 8.0 g/cm³) is that all our products are “weight-intensive”. As a result, although we freight many of our products on pallets, they are always air freighted in full out rather than filled, as they are always well in excess of the weight allowed for by the specification of weight and size for a pallet. In addition,the density of stainless steel affects calculations of carbon footprint, a measurement of environmental impact that is becoming mandatory for all European medical contracts. The more dense an item (as is the case with stainless steel), the more energy is required to transport it, and the more energy is required for its manufacture, including the melting and processing of raw materials. For the medical OEM who has an environmental, social, and governance (ESG) score that they wish to improve, using dense stainless steel with an optimum balance of strength and density to minimize unnecessary mass is a good way to reduce transport energy and thereby costs, as well as manufacturing energy and materials costs.

Selecting the optimal density stainless steel for next-generation implantable devices

With the coming of Personalized Medicine and 3D printing customized implants, the concept of “density stainless” is rapidly changing. Being able to 3D print porous structures with a density that matches the human bone is just a new aspect, but in fact, the “true” density of the stainless steel powder used in the printers remains the basis for the strength of the final product.
For 3D printed 316L implants, the relative density should be as close to 100% as possible in order to ensure that the implant will not fail under cyclic loading (fatigue). With the wrong laser parameters set to the density of the stainless steel powder, the tiny voids (micro pores) that are necessary to compensate for a lower than optimal laser parameter setting can form. Stress concentrators, micro-pores, also offer an opportunity for corrosion. Even with the most advanced additive manufacturing techniques, it is still the physical constant of the density of the stainless steel that is the key factor in deciding whether an implant will be successful in the body.

Thermal diffusivity and sterilization: The hidden role of density stainless

Medical devices must be sterilized after each use, and the common method of sterilization is using a steam autoclave at 134∘C. The rate at which the device is heated up will depend on the thermal diffusivity(α) of the device, which is defined as:

We used the formula: ρ = density stainless, where ρ is the density stainless. Because the density of stainless 300-series alloys is high, they have a high “thermal mass”. This means that they take longer to heat up and longer to cool down. Both properties are more characteristic of heavier, denser metals such as titanium and aluminum instruments.
For our Hospital Sterile Processing Department (SPD), determining the dry time of a heavy 316L surgical tray is greater than that of a lighter set of trays. A mistake in design can result in moisture trapped within a kit, leading to a wet pack and contamination of the sterile field. Determining the density of stainless steel is important in tray design. Designers should take into account designing a shallower tray geometry to make instruments thinner, thereby reducing thermal lag during sterilization cycles, while providing sufficient strength in the finished tray.

Conclusion: Data-driven material selection for medical safety

In the high-stakes business of designing and building medical devices, “good enough” is never good enough. The density of a stainless alloy is a material property that impacts everything from the stability of a laser weld to the resonant frequency of a robotic arm.

It is very important to understand the density of stainless steel in the 300-series in a quantifiable way in order to design materials that can achieve the micrometer-scale tolerances necessary for success in surgery. Whether it is the free-machining properties of 303 stainless steel, the greater corrosion resistance and density of 316L, the fact remains that the material density is the starting point for all precision metal manufacturing. As the worldwide trade of medical products and technologies expands, the firms that understand the physics of their material substrates will be the firms that set the standard for new technologies and products that will determine the nature of health care in the future.

References

1. ASTM F899: Standard Specification for Wrought Stainless Steels for Surgical Instruments. (The baseline for all medical-grade stainless steel properties).
2. Callister, W. D., & Rethwisch, D. G. (2014). Materials Science and Engineering: An Introduction. Wiley. (Crystallographic derivation of density, stainless and FCC lattices).
3. Davis, J. R. (2003). Handbook of Materials for Medical Devices. ASM International. (Specific data on the 303 stainless steel density and its applications).
4. Lippold, J. C., & Kotecki, D. J. (2005). Welding Metallurgy and Weldability of Stainless Steels. John Wiley & Sons. (Analysis of the density of stainless steel in laser melt pool dynamics).
5. ISO 13485:2016: Medical devices — Quality management systems — Requirements for regulatory purposes. (Framework for material traceability and density verification).

Frequently Asked Questions (FAQs)

1. What is the standard density of stainless steel for surgical tools?
   Common surgical instruments are made of 304 stainless steel with a density of 8.00 g/cm³, while some special ones are made of 303 stainless steel with a density of 8.03 g/cm³.
2. How does the density of stainless steel affect the weight of medical implants?
   Most dental implants are made from 316L stainless steel. This material is slightly denser (8.00−−8.07 g/cm³) than standard steel due to a higher Molybdenum content. This higher density is given up in order to allow for greater corrosion resistance within the body.
3. Is the density of stainless steel related to the density of Fe?
   Yes. The density of Fe (pure iron) is 7.87 g/cm³. Stainless steels are denser due to the nickel added. The FCC lattice structure in nickel allows for more mass to be held in the same volume as pure iron.
4. Why is the density of stainless steel important for laser welding pacemakers?
   The density of the stainless steel is directly related to the hydrostatic pressure of the molten metal. The high density of the 316L stainless steel requires a finer control of the laser in order to keep the “keyhole” stable. This tight seal prevents the penetration of humidity into the electronic part of the pacemaker.
5. Can I achieve micrometer-scale precision with high-density stainless steel?
   Yes. The high density of the 300-series of stainless steel makes it a very good vibration-damping material. In high-precision machining, where cutters have to be guided by very fine detail, and when the instruments to be used are quite fragile and should not be subject to any strong vibration, a high mass can significantly reduce the resonance vibrations, making it easier to guide the tool without chatter.