Surgical Robotics Technology

Stainless Steel Cable’s Role in MedTech: Understanding its Common Constructions

Stainless Steel Cable's Role in MedTech

In 1926, the first stainless steel cable was surgically implanted into a human. Ninety years later, in 2016, I also received a stainless steel cable implant to repair my severely damaged left shoulder. And though nowadays, tungsten is the preferred cable material for surgical robotics, before tungsten, stainless steel cable was frequently used in the motion control of a surgical robot. Broadly speaking, the versatility and reliability of stainless steel wire rope have made it a mainstay among countless MedTech device applications.

The truth is that stainless steel mechanical cable is the single most popular wire rope material used among MedTech motion control and surgical device makers. Yet interestingly, it was only as recently as 1967 that the Olympus Corporation first used a cable-driven mechanical motion system in one of their endosurgery innovations.

Since then, stainless steel cable has been the go-to cable option for makers of the world’s most advanced endoscopic instruments and surgical robotic motion systems. While endoscopic instruments have a fascinating history dating back to ancient Greek and Roman times, the occupational and ergonomic motion dynamics wishlists physicians have for these devices have gone unchanged throughout the centuries.

So, whether you’re a pioneering German physician named Philipp Bozzini, who, in 1806, was the first to illuminate the urinary tract with his endoscope, the Lichtleiter or “light conductor,” or are counted among the world’s foremost authorities on minimally invasive bladder surgery today, the control you have over the device’s motion has remained among the instrument’s biggest sources of continuous improvement. And that control is driven, in part, by stainless steel cable more than any other material type.

Why Stainless Steel Cable?

Stainless Steel wire rope (or cable, if you prefer) is produced from iron, chromium, nickel, and additional elements, depending upon the grade of stainless steel being produced.

In MedTech, stainless steel cable is ideal for a host of reasons, but chief among them is its chemical composition. For instance, 304 stainless steel, arguably the most common grade of stainless steel cable used in MedTech motion control systems, contains both chromium and nickel, which yields the cable a corrosion resistance and durability ideally suited to share space with the human body. 316 stainless steel cable, however, uses about 2%-3% molybdenum in its manufacturing process, which further reduces the potential for stainless steel to pit or form crevice corrosion, because the addition of molybdenum adds an extra stratum of protection from chloride ions, which are found in saline-present environments.

Stainless steel’s corrosion resistance, combined with its strength, durability, resistance to wear, and of course its biocompatibility rounds out the reasons this material type is coveted by MedTech.

Common Stainless Steel Cable Constructions in MedTech

While there are many cable constructions used to produce stainless steel wire rope, this article will focus on the most common constructions in MedTech devices like endoscopes and robotic-assisted surgical systems.

1×7 Stainless Steel Cable

1x7 Stainless Steel Cable

Among the most common stainless steel cable constructions used within the tight spaces allocated to endoscopes is a 1×7 construction. This relatively rudimentary construction possesses a single, core wire, wrapped in six additional wires – hence 1×7. Oftentimes such cables are disposable and thus must be manufactured with a sensitivity to production costs. So, though not the highest of priorities, an endoscopic device maker must strike a balance between achieving the requisite factor of safety, at a price point that allows the device maker to grow the product line ambitiously. This last point is especially critical, given the fierce nature of competition among endoscopic device ecosystems.

What’s more, a 1×7 stainless steel cable is not known for its flexibility. This rigidity is desirable because this variety of cable is often fed into small mating components. Therefore, the cable must possess a stringency that prevents it from kinking, kicking back, becoming stuck, or producing a “bird caging” event, which is a fault that occurs when a cable’s wires are separated by axial forces. Not ideal where tight radii need to be achieved, because again, a 1×7 stainless steel cable is springy, in its nature, and consequently does not want to bend tightly by its design.

Used frequently in motion control subsystems, 1×7 cable is often contained inside conduit of one type or another and is used in rigid routing methodologies, rather than in facilitating motion actuation.

Another common MedTech stainless steel wire rope construction is 7×19. This cable comprises 133 wires, which is produced by combining seven, 1×19 constructions, which is a single core wire, wrapped in 18 wires.

7×19 Stainless Steel Cable

7x19 Stainless Steel Cable

7×19 stainless steel cable, conversely, is known precisely for its flexibility, making it an ideal MedTech device cable where the material needs to traverse narrow, small radii. Unlike a 1×7 cable, a 7×19 will not abruptly bounce back into a linear geometry when a bend is removed. Although stainless steel cable possesses a natural elasticity, no matter its construction configuration, 7×19 possesses enough wires that the cable’s natural flexibility becomes malleable. In MedTech, 7×19 is commonly used in handheld surgical instruments, that often contain levers and switches that the physician uses to control the device’s end effector(s).

Although tungsten mechanical cable has proliferated among robotic-assisted surgery, the cable constructions used to produce this highly sophisticated wire rope are characteristically different than their stainless steel counterpart. Common cable construction found among tungsten cable applications do not relate to those used in endoscopic instruments. Although less cost-effective, comparatively speaking, ultrafine tungsten cables, which are produced in diameters found in surgical robotics, possess a “deadness” that is highly desirable to surgical robotics makers. As mentioned, it will lay in your hand like a shoelace, falling from each end of your palm without a trace of mechanical resistance.

However, because this article discusses the popularity of and justification for the use of stainless steel cable in MedTech, it is recommended that you contact your tungsten cable supplier to discuss its potential to support your unique MedTech device application.

1×19 Stainless Steel Cable

Possessing a seemingly comparable stiffness to that of a 1×7 cable construction, a 1×19 stainless steel possesses a slightly lower modulus of elasticity due to the distribution of load across more wires than those contained in a 1×7. This makes 1×19 modestly more flexible, but still stiff enough to characterize it as “comparably stiff” to a 1×7, while also like the 1×7 cable, a preferred stainless steel cable construction among handheld, endosurgical instruments. Like a 1×7 construction is a single core wire, wrapped in additional wires, the 1×19 also contains a core wire at its center encircled by an additional 18 wires.

A 1×19 stainless steel cable offers the user a smoother surface, as well as more strength — both of which are characteristics produced by the presence of a higher wire count. Yet, a 1×19 stainless steel cable still yields an inelasticity that remains highly prized within endosurgical applications. The rigid behavior, combined with its increased strength, makes it ideally suited in applications where guiding a cable through twisting, narrow spaces cannot be at risk of hysteresis, or for that matter, another unwanted operational fault that, in cable making circles, is known as backlash, which is a phenomenon that can be found in poorly performing push-pull cables, like those used to manipulate a vertical stabilizer in aircraft.

Diameter and Break Strength

Although MedTech buyers sourcing stainless steel cable are known to purchase cable diameters as small as .018”, for example, an interesting fallacy exists in cable making that deals directly in size and strength. Take, for instance, a 7×19 stainless steel cable, versus a 1×7 of the same diameter. Is a 7×19 cable, which again contains 133 wires, stronger than a 1×7 cable, which only contains seven wires? The answer is a certifiable no. Because the wires comprising the 7×19 are significantly smaller, in this use case than those used in a comparable 1×7 cable construction, the 1×7 yields the user a higher strength-to-diameter ratio. So, where flexibility is a design priority, the device maker must select a cable diameter that is not compromised by the minuscule sizes of the cable’s individual wires. Likewise, where rigidity is desirable, the wires should be larger, to ensure both the stiffness of the cable isn’t achieved in exchange for the cable’s required strength.

Stainless Steel Cable: A Fundamental Component of the MedTech Industry

Although a perfunctory overview, it’s easy to see why this cable material continues to proliferate among the world’s endosurgery device makers. Its corrosion resistance, biocompatibility, paired with its miniature size, and herculean strength, make it a perfect material to produce and support motion in incredibly small, tight, claustrophobic spaces within which a surgeon’s hands and fingers simply would not. And even where the surgeon can fit her digits, modern surgical medicine is today obsessively in pursuit of methods that shorten hospitalizations and more swiftly restore quality of life.

To ensure you understand the stainless steel cable construction your endosurgical applications require, ask the experts at Carl Stahl Sava Industries, Sava has been producing the world’s medical cables for over 50 years, and is always thrilled to discuss the next generation of endosurgical instrumentation with you.

Scott-Dailey

Scott Dailey

This article was written by Scott Dailey, Vice President of Sales & Marketing for Carl Stahl Sava Industries. Sava is a New Jersey, USA manufacturer of small, miniature, and ultrafine medical mechanical cables and cable assemblies.

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  • 4 North Corporate Drive Riverdale, New Jersey 07457-0030
  • +1 973-750-9578
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