The selection of seal material is central to a properly functioning, quality trocar. The seal and its materials must meet strict and demanding performance requirements that differ by trocar device and design. The material must be highly durable to accommodate multiple instrument insertions and removals. It must have a long shelf life with minimum compression set, and it must be tough and resistant to tearing. The seal material must be pliable with good memory so that it stretches properly while maintaining a seal during use. At the same time, it must have the ability to rebound to its original shape when instruments are removed. Surface friction has to be low so that surgical instrument insertion is smooth and the seal can hold its shape around the instrument as it moves during the procedure.

Friction is an extraordinarily complex subject and critical in trocars. The amount of force it takes to slide one surface past another is affected by many variables. These variables include the lubrication state, the material modulus, the surface finish, the temperature, Duckbill seals are molded from a specially formulated rubber material. They stretch to accommodate instrument insertion and manipulation.the geometry of the part, and the amount and direction of the relative forces. Such variables are particularly relevant to trocar seals because the surface friction and design of the seal can profoundly effect the surgeons' feel for their instruments. 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 takes to manipulate the instrument. The more force it takes to manipulate the instrument, the greater the potential for an adverse effect to the patient or the surgeon. Designers and engineers, therefore, must focus on friction reduction.

When there is a force pressing two surfaces together and they are moving past each other as in a seal, it is impossible to calculate or predict such frictional force with accuracy. It can only be measured through experimentation. The results of such experiments are expressed as a coefficient of friction (COF). COF is what is used for comparisons, as it is a sealing system measurement rather than a measurement of the property of the material. ASTM D1894 is a standardized test that can be used to accurately measure COF.

When a surgeon moves a surgical instrument or cannula through a trocar seal, the energy that it takes to start moving is different from the energy it takes to stay moving. The energy to start moving is called static COF, whereas the energy to keep moving is called dynamic COF. The difference between static and dynamic COF can vary tremendously by material and application. The difference between the two can greatly affect the feel of a surgical instrument as it passes through a trocar. In general, the lower the COF and the smaller the difference between the static and the dynamic COF, the smoother the feel.

It is important to note that surface textures, surface coatings, and the presence of fluid can substantially reduce the COF and the relative difference between static and dynamic COF (See Figure 1). In most dry seals, there is usually a stick-slip action whereby the seal flexes to accommodate a mating surface's movement; it subsequently pops back to a stable state. When fluid is present, the cannula can hydroplane, resulting in a significantly reduced COF