Material selection and coatings are two important factors to consider in dealing with friction when designing pass-through seals for endoscopic trocars. Engineers at Minnesota Rubber and Plastics (MRP) state that specifying the correct material and coating can reduce friction in these applications by as much as 90%.
The design of these seals is developing into a science of its own with friction playing a major role. Friction in these pass-through seals is extraordinarily complex. The amount of force it takes to slide one surface past another is affected by many variables influencing the surgeon’s skill and quality of the procedure.
Some of the variables that have a profound impact on friction, and those that MR&P deals with in designing pass through seals for the medical community, include the lubrication state, material modulus, surface finish, temperature, geometry of the part, and direction of the relative forces.
Surgical instrument insertion and retraction through a seal must feel smooth and easy. The more the material grabs the shaft of the instrument, the more force it will take to manipulate the instrument.
Quantifying seal friction is measured as frictional force (pounds) and is important in designing the highest quality trocar seal. Frictional force is directly related to the coefficient of friction (COF) of the material. For the most part, COF (measured using ASTM D1894 test) is what the engineers at MR&P use for comparisons as it is a sealing system measurement rather than a measurement of the property of the material.
When moving a surgical instrument or cannula through a trocar seal, the inertia that it takes to start moving is different from the inertia that it takes to stay moving. This is called the static (starting) COF and the dynamic (moving) COF. This difference will vary tremendously by material, application, and most of all, the lubrication state and/or coating, and is a central element of the feel of a trocar.
“Contrary to most thinking, sliding a rod (cannula) through a seal is never smooth,” states Ted Ahrenholtz of Minnesota Rubber and Plastics. “While it may feel smooth, in reality it is not. There is always a stick/slip force that is present where the seal flexes to accommodate the cannula’s movement, and then rebounds back to a stable state. In general, the smaller the difference between the static and dynamic COF, the smoother the inertia feels. The best seal design and the most appropriate material formulation will minimize this stick/slip phenomena. However, it is important to note that some degree of friction will always be present in all seals used for this purpose.”
Surface Finishes, Coatings
However, surface finishes and coatings on the trocar seal material can substantially reduce the coefficient of friction – the smoother the surface, the greater the COF. A matte finish can greatly reduce the amount of friction on a very smooth surface. However, maximum friction reduction is achieved with the correct coatings applied to the material surface. Depending on the substrate and its application, PTFE, parylene, chlorination, and MR&P’s proprietary processes can reduce friction by as much as 90%.