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An Introduction to Pinch Testing Hydrophilic Coatings for Medical Devices

Pinch testing is one of the primary methods used to test the performance of a hydrophilic coating on medical devices. A pinch test is where a coated rod, wire, or tube is literally pinched between two surfaces inside a gripper, a known load is applied, and then a motorized unit pulls and pushes the coated item through the gripper.

Testing Factors

Different testing environments can lead to differing results. Here are some of the factors that can yield different results:


Differences in the applied force that the gripper is meeting the coated object will provide different results.


The sample being tested is pushed/pulled between grippers. The speed at which sample is traveling can also affect the results. Smooth transition between push and pull cycles versus a sudden stop/start motion can have an effect as well.


The pad shape determines the contact area. If the gripper pads are rectangular, a large area will be contacted on the surface of the test object. The pinch force is therefore spread out over a wider area and could be a reason for different results.


The pads that are typically used are silicone or delrin. Either pad type can be used, but there will be variations in the results of the different tests.


The solution choice is potentially the largest variable in achieving matching testing results. For example, hydropgels with an HA-based coating (like Biocoat’s HYDAK Thermal coatings) will demonstrate excellent lubricity and durability in PBS but show higher coefficients of friction in DIW medium. Interestingly the opposite is true for PVP-based coatings. In DIW, PVP coatings will perform well but exhibit degradation in PBS. The temperature of the medium can also play a role in variations

What do we learn?

After the First Few Cycles

The coefficient of friction

After Repeated Testing Cycles

Durability of the coating

After Our Testing is Complete

Biocoat provides our customers with a friction coating report. We also provide the coated sample materials to our clients to run their own testing on their in- house equipment.

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