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Material selection and coatings are important factors to consider when dealing with friction in the design of 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 per cent, adds the company.

‘The design of these seals is developing into a science of its own with friction playing a major role,’ said Ted Ahrenholtz of Minnesota Rubber and Plastics.

‘Friction in these pass-through seals is very 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,’ he added.

Ahrenholtz points out some of the variables that have a profound impact on friction and those that Minnesota Rubber and Plastics (MRandP) deals with in designing its pass-through seals for the medical sector.

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 and is important in designing high-quality trocar seals, according to 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 MRandP 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 a lot by material, application and, above all, the lubrication state and/or coating and is a central element to the ‘feel’ of a trocar.

‘Contrary to most thinking, sliding a rod (cannula) through a seal is never smooth,’ said 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; 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 MRandP’s processes can reduce friction by as much as 90 per cent,’ he added.

Minnesota Rubber and Plastics

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