Scott Dailey
Historically, the MedTech devices industry has relied heavily on stainless steel mechanical cable in the manufacture of motion control systems commonly found in endoscopic surgical instruments. The storied popularity of stainless steel cable among such device makers has been due to the material’s strength, durability, corrosion resistance and cost-effectiveness, all of which are time-tested benefits of this cable type. Specifically with surgical robotics however, the growing preference for tungsten cable, over stainless steel cable, has been both pronounced and immutable. But why this shift in cable material type? Certainly, stainless steel cable has proven itself worthy of the loads, cycle counts and flexibilities required of surgical robotic end effectors.
So then why? Why the departure among surgical robotics makers from the time-tested authority stainless steel cable has been known to be, in favour of a lesser proven tungsten substitute?
Material Properties: A Comparison of Stainless Steel and Tungsten Cable
Perhaps the most critical factors powering the adoption of tungsten cable in surgical robots is the difference in material properties compared to stainless steel.
Tensile Strength
Tungsten cable outperforms stainless steel in tensile strength, enabling smaller cable diameters to be used in the construction of the mechanical cable, without compromising durability and loadbearing capacity. Stainless steel, for example achieves a tensile strength of 621 MPa, while tungsten enjoys a tensile strength of 980 MPa. This makes tungsten about 30% stronger than steel. Consequently, surgical robots using tungsten cable to achieve motion will also use less space, while achieving the critical factor of safety the robot’s designers require.
Fatigue Resistance
Although brittle when forces come downward on tungsten wire, when under a load tungsten cable possesses superior flexibility and fatigue resistance, which makes tungsten cable ideal for applications where devices experience repeated bending, tension, pitch, yaw, and various other fluid movements meant to mimic a surgeon’s arms, hands, and fingers. This increased fatigue resistance prolongs the life of the surgical robot, vital for medical devices requiring high precision and lasting performance when undertaking emergent or differently critical medical procedures.
Flexibility
Although stainless steel mechanical cable is known for its flexibility, depending upon the cable’s construction, tungsten additionally possesses superior flexibly. No matter the wire count (i.e., cable construction), stainless steel is a characteristically springy cable material. Whether a 1×7 cable construction, or a 7×49, stainless steel cable maintains a level of rigidity inherent to the material itself. So even at larger wire counts, which increases mechanical cable flexibility, stainless steel cable will always want to return to its manufactured, linear geometry. Conversely, tungsten cable, like a strand of cooked spaghetti hanging from the rungs of a fork, will lazily dangle when suspended similarly. Tungsten cable’s flaccidity allows the cable assembly an easier time going around sharp radii, some of which are as tight as the tip of a children’s Crayola Crayon. This added flexibility also means slower fatigue rates of mating parts, like a pulley, which would experience less fatigue per cycle due to tungsten’s sinuous state of motion over pulley systems.
Corrosion Resistance
While stainless steel cable is known for its exemplary corrosion resistance, tungsten cable outperforms stainless steel cable in this area as well. Tungsten cable’s resistance to corrosive environments, including body fluids and saline solutions, for example, make it a preferred choice for surgical robots due to their likelihood of making contact with human fluids and tissue.
Biocompatibility and Sterilization Resistance
Both stainless steel cable and tungsten cable are radiopaque and biocompatible materials, meaning that each can be plainly seen using radiographic equipment, and are both safe when contacting human tissue. However, tungsten cable demonstrates higher resistance to sterilization techniques, such as autoclaving, radiation, or chemical sterilant, which ensures devices maintain their integrity and performance after multiple sterilization cycles. And while many tungsten cables being used in the motion control systems of surgical robots are meant for single usage, this is not always so.
Tungsten’s superior strength, potentially smaller footprint, and its biocompatibility advantages, quickly begin to make clear why so many surgical robotics makers are abandoning their stainless steel cable assembly designs for tungsten replacements.
Tungsten Density
Adding to the intrinsic benefits of tungsten cable is its higher density than stainless steel. Previously mentioned was tungsten’s radiopaque characteristics. But it is in the material’s radiopacity that tungsten increases its advantageousness. For instance, the sheer density of tungsten offers the material better visibility under radiographical imaging, making it easier for surgical staff to manage robotic interplay between the robot’s end effectors and a patient’s fragile anatomy.
Innovations in Tungsten Cable Technology
Also driving the growing appeal of tungsten cable in surgical robotics is the innovation being achieved among the small quantity of manufacturers producing these sophisticated tungsten cable assemblies. For instance, it is not especially uncommon for a miniature mechanical cable assembly to have some flexibility surrounding tolerances. But this is almost entirely untrue of mechanical cable assemblies being produced for a surgical robot.
So tight are tolerance requirements for surgical robotics cable assemblies, that even the slightest deviation in a ball, sleeve, or hypotube diameter or thickness can mean the assembly fails in the field. And given the “field” is an operating room, where the stakes can’t be higher, a single failure due to out-of-tolerance fittings could mean catastrophe during surgery.
Additionally, for all its elemental strengths, wolfram is actually quite brittle. Therefore, when struck like a hammer, swaging fittings to tungsten cable is a more delicate process than using stainless steel cable to execute the same manufacturing operation. Striking tungsten cable too hard with swaging dies or an industrial press may result is cracking or shattering tungsten wire filaments. For this reason, applying forces to tungsten cable is a delicate operation that, once perfected, must then be replicable thousands or perhaps millions of times over the component’s entire production life.
A Natural Transition
Numerous surgical robotics makers have successfully transitioned from stainless steel cable assemblies to tungsten cable assemblies. And while tungsten wire can cost as much as 7x the cost of comparable stainless steel wire, tungsten’s superior physical characteristics makes the spend worth the investment.
Tungsten’s strength, corrosion-resistance, flexibility, biocompatibility and radiopacity all coalesce to lend to improvements in patient outcomes too, because the robot itself is only as reliable as the sum of its components. Because tungsten plays nicer with mating parts and surfaces, meets, and exceeds cycle count demands, and introduces no harm to the patient, stainless steel has experienced a kind of Darwinian exit from many the motion control systems of tomorrow’s marquee surgical robots.
Surgical Robotics Technology
