Lubricious Coatings: Pinch Test Primer
Lubricious coatings for medical devices come from various companies, chemistries, and calibres. Differentiating the "men from the boys" can be difficult when it comes to coatings, however. I do go into some detail in this in my white paper on hydrophilic coatings, but here I would like to expand on one method for determining lubricity and durability: pinch testing.
Conceptually, pinch testing is exactly what it sounds like. A coated rod, wire, or tube is literally pinched between two surfaces inside a gripper, and then a motorized unit pulls and pushes the coated item through the gripper. The motorized unit could be a type of mechanical tester, such as an Instron, which keeps track of the force and displacement during the test. A few cycles will yield results on coefficient of friction for the lubricious coating, and a few more cycles will determine durability.
Sounds simple, right?
Determining if a coating is durable and lubricious, in favor of, or despite all claims about the coating, requires testing on a machine such as this, yet results from different machines are almost incomparable to one another. Here are the factors that come into play:
Pinch Force - What force is the gripper applying to the coated object? Many competitors in this field will put an almost non-existent pinch force on their coatings and then show that the coatings remain lubricious over many cycles of testing. It makes pretty graphs, but the coating is still not durable, per se.
Pad Material - The gripper pads are made from some material. What material is that? Material matters. A soft material like silicone gives a much easier test, than say, a hard plastic. A common trick in this industry is to run a pinch test with a silicone pad and show durability out to tens of cycles,when in actuality, if that pad was switched for something harder, like polyethylene, the coating would fail within a few cycles.
Pad Shape - Pad shape determines 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. Again, this makes for an easier test, compared with a shape that gives a line contact area.
Granted, not all hydrophilic coatings would need to withstand forces like this. It depends on the application. Moreover, pinch testing (and other methods of characterizing lubricity and durability) has not been correlated to clinical function. Rather, it is a basis for making an engineering decision on a coating. No one really knows how many cycles on a pinch tester equates to how many passes in and out of the vasculature during a surgery, for example. Something still needs to be the basis of a decision during the design phase. Plus, which coating do you want on that neurovascular catheter? The one that fails in 20 cycles or the one that fails in 100?
However, understanding this data is still important. A lot of marketing material from lubricious coating companies purports great things, when in reality, the tests are customized to make the coating look good. The best thing to do is to get a bunch of samples of coatings from a bunch of coating companies and put them ALL through the same test. A head-to-head comparison is the way to go.