Metal part manufacturers who supply the medical device industry have recently seen significant improvements in the calibre of their work. Since the introduction of statistical process control, ISO 9000 certifications, and other initiatives, the product quality of these alloys has never been higher.
Today, a large selection of usable metals is available to medical equipment manufacturers and implanted devices. Manufacturers have a wide range of options when deciding which grade or alloy best suits an application, thanks to an understanding of the variations between these materials and the different options each one offers. Surfaces that are sterile-friendly, bright, and clean can be paired with high strength and formability.
Device manufacturers want guarantees that the components and materials they purchase are the best available for the task at hand, given the extreme difficulty of many modern surgical procedures and the constant worries about patient comfort and safety. More emphasis than ever before is being placed on technology that operates the first time flawlessly, every time, as a result of the advent of minimally invasive and noninvasive procedures, which allow surgeons to operate without making direct eye contact with the operative field.
Given that consumers are continuously looking for less expensive, more effective options, metal components purchased from stamping houses and fabricators, as well as the raw materials from which they are created, are receiving further scrutiny. Metals are still necessary for many essential medical applications, even though plastics continue to make significant advancements in the medical industry, including substituting metals in some equipment. To help the medical equipment industry achieve its aims for higher quality levels and improved device performance, designers now have access to various metal materials and forms constantly expanding.
How does metal selection play a role?
Especially in small cross sections, metals are selected for pieces that need to be exceptionally strong and stiff. They are also the ideal choices for components that must be formed or machined into complex shapes, such as blades, points, and probes; for mechanical components that must work with other metal components, such as gears, triggers, slides, and levers; for components that must be sterilized in high-heat conditions; and for any other parts requiring mechanical or physical properties that are superior to those of polymer-based materials.
Metals often offer a firm, brilliant surface that is ideal for easy cleaning and sterilizing. Materials like stainless steel, nickel alloys, titanium, and titanium alloys are especially prevalent in medical equipment because they meet the stringent cleaning criteria of most healthcare applications. These applications exclude those metals that oxidize on the surface in an uncontrolled and destructive manner, such as steel, aluminum, or copper. These high-performance metals have unique properties, some restrictions, and incredible versatility. When dealing with these materials, which frequently also call for a novel approach to design, product engineers who are more accustomed to working with standard metals or plastics will find a plethora of possibilities.
- 300-series stainless steel is the most popular option when a metal component is required to construct a medical device. These alloys have excellent mechanical and physical qualities and a variety of surface finishes, including reflective and matte, and are reasonably simple to cold form or fabricate into specialized pieces. They are also essentially non-corrosive.
- 17 to 25% chromium and 8 to 25% nickel are present in stainless steel alloys. By producing a strong, sticky, invisible chromium oxide coating on the alloy surface, chromium helps stainless steel resist corrosion. This film is capable of self-healing if it sustains mechanical or chemical damage. Molybdenum additions of up to 7% are occasionally used to improve corrosion resistance further.
- 301, 304, 304L, and 305 are the most often used stainless steel grades for medical items. The alloy composition of these versions varies slightly, and the choice of which grade to employ depends on factors like formability, corrosion requirements, or the desired thickness and temper availability. The other crucial design factors are surface strength and condition.
- While maintaining good ductility and toughness, stainless steel alloys can be cold-treated to high tensile and yield strengths. Depending on the composition and the quantity of cold work, they have yield strengths between 30 and 200 ksi at room temperature. The metals’ metallurgical structure is described by the designation “austenitic” or “martensitic” for stainless steel grades. This occurs after the metals are heated above their critical temperature and quickly cooled. The alloy composition, heat treatment time and temperature, and the final structure are determined by the temperature and heat treatment duration. Compared to martensitic grades, austenitic stainless steels are stronger and more formable.
Austenitic Stainless Steel
- Most medical device parts are manufactured from austenitic stainless steel, containing between 16 and 20 percent chromium and 6 to 14 percent nickel. Chromium provides the requisite corrosion resistance by creating a durable, undetectable, and adhering chromium oxide coating on the alloy surface. This film is capable of self-healing if it sustains mechanical or chemical damage. To further improve corrosion resistance, some grades receive molybdenum additives of up to 7%.
- The frames, springs, anvils, cartridge slides, and jaws of surgical staplers constitute a virtual showcase of 300-series stainless-steel components used in modern medicine. Various grades of stainless steel wire are used to make the staples themselves. Catheters, diagnostic equipment, and other items are used for 300-series stainless steel. Stainless grade 316 is an alloy with a higher nickel content that is sometimes specified for braces due to its high creep strength at high temperatures. When welding is anticipated, the low-carbon versions of the corresponding alloys, grades 304L and 316L, are advised.
- The 400 series of stainless steels, frequently used for surgical tools, are less corrosion resistant than the 300 series but can be heat treated to higher degrees of strength and hardness. In contrast to the two most popular grades, 410 and 420, these materials have no nickel and just a small quantity of chromium. 410 is the general-purpose grade, but 420 is more hardenable and has a higher carbon content.
- Surgical instruments are also made of type 410 martensitic stainless steel. Although it has a lower corrosion resistance level than any 300 series grades, it can still be heat treated to increase its strength and hardness. Type 410 has no nickel and just 11.5 to 13.5 percent chromium.
- The designer may use one of the precipitation-hardening stainless steels, such as 17-7 PH or 17-4 PH, when a part requires extra strength and stiffness, like in equipment housing, for instance.
- The sole difference between these metallurgical hybrids and stainless type 301 is the addition of trace amounts of copper, aluminum, phosphorous, or titanium.
- After being moulded into its final shape, a part is given an age-hardening treatment, which causes the phase transformation caused by the extra components.
- Intermetallic compounds are created. As a result, they are dramatically raising the part’s hardness and strength—up to 40% more often.
- Ultra-high dependability pieces, or those that remain inside a patient after surgery, are made of pure titanium, the most expensive and inert of the regularly used metal materials. Replacement joints, pacemaker cans, and other metal implants are some examples. Titanium alloys are also employed in the medical industry, primarily for components when stainless steel cannot fulfil the required standards for strength, hardness, corrosion resistance, or other factors.
- Titanium is an excellent biomaterial because it provides the strength of steel at less than two-thirds the weight. Due to its remarkable capacity to develop a protective oxide film that adheres closely to the surface when exposed to air or other oxidizing agents, it exhibits exceptional corrosion resistance. This passive layer will regenerate after surface damage to the metal and resists all sorts of corrosion.
- In terms of fabrication and the usage of available alloys, one of the most demanding applications in the medical industry is also one of the most unusual. Since the popular hip-joint replacement is meant to last a lifetime, it needs materials with high strengths, great wear resistance, and excellent corrosion resistance. Designs from different manufacturers vary slightly, but one common kind includes a cobalt-chromium stem that fits into the femur and a titanium-alloy cup that fits into the hip socket. The cobalt-chromium head is attached to the cup.
- Manufacturers of titanium hip stem joints typically favour Ti-6AL-4V titanium alloys, especially the low oxygen ELI grade, for implantable applications in both bar and plate form. When a product is forged, a bar, available in diameters up to seven inches, is the most typical beginning material.
Why are certain products preferred?
Foil, strip, sheet, wire, rod, bar, and plate are just a few of the several medical industry-required forms in which stainless steels, hardenable alloys, and titanium alloys can be made. Since medical device components are frequently small and complicated, automatic stamping presses are typically utilized to create the shapes. The optimum beginning materials for this kind of processing are strips and wire, the most often used materials. Various sizes are available for these mill forms. Strip, for instance, ranges in thickness from ultra-thin foil of 0.001 in. to 0.125 in., whilst flat wire is available in thicknesses of 0.010 in. to 0.100 in. and widths of 0.150 in. to 0.750 in.
Which metal is suitable for the medical device?
Although from the user’s perspective, stainless steel, titanium, and nickel-based alloys are considerably more sophisticated than more typical materials, they also offer a significantly greater range of capabilities. These “metallurgical animals” can alter their mechanical characteristics by heating, cooling, and quenching. They can be further modified during processing if needed. For instance, rolling out metals with narrower gauges can harden them, and annealing can restore their characteristics to a precise temper for economic shaping.
These unique metals offer several exceptional benefits in many medical items, including unparalleled corrosion resistance, high mechanical capabilities, a large variety of surface treatments, and excellent production versatility once designers are comfortable with the complexity of the materials.
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