Pass through seals For Trocars

Written by: Chris Shaw | Published:

Material selection and coatings are two important factors to consider in dealing with friction when designing pass-through seals for endoscopic trocars. According to Minnesota Rubber and Plastics, 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," reports Ted Ahrenholtz of Minnesota Rubber and Plastics. "Friction in these passthrough 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."

Mr. Ahrenholtz points out some of the variables that have a profound impact on friction and those that MR&P deals with in designing its pass through seals for the medical community. They include the lubrication state, the 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 (lbs) and is important in designing the highest quality trocar seal, according to Mr. Ahrenholtz. 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 MR&P uses 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 than 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 Mr. Ahrenholtz. "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, which 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 minimise 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 and coatings on the trocar seal material can substantially reduce the coefficient of friction," reports Mr. Ahrenholtz. "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%."

MR&P molds and assembles medical devices requiring high tolerance components that facilitate device function. Device designs include seals that isolate fluids and gases, internal bosses for aligning actuating mechanisms containing seals and springs, and snap-fit designs that lock the assembly together without the use of adhesives. It produces needle-free injection devices, surgical instruments, hand sets, diagnostic instruments, valves, pumps, connectors, diaphragms, plunger tips, seals, laboratory equipment and disposables, among other devices.

MR&P's corporate-wide quality systems include: ISO 13485, ISO-TS 16949, and ISO 9000 in all facilities. These facilities operate under a corporate-wide Environmental Management System registered to the International Standards Organisation series ISO-14001.


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