Durometer is arguably the most common polymer attribute discussed by medical catheter engineers. For example, an ‘80A thermoplastic polyurethane (TPU)’ reference includes the durometer: 80, A scale. A measure of polymer hardness, durometer is directly applicable to certain catheter requirements. Soft polymer tips minimize vascular trauma. Harder catheter liners are necessary to resist abrasion from wires and instruments. Durometer also correlates to modulus, a measure of polymer flexibility that is important for catheter shafts.
The durometer hardness scale was developed by Albert Shore in the 1920s. It measures the depth of indentation into a polymer surface by a rigid object, known as a ‘presser foot’. Indentation depth is dependent on hardness of the polymer, shape of the presser foot, pressure applied to the presser foot and duration of pressure.
American Society for Testing Materials (ASTM) method D2240 establishes 12 durometer scales for a wide range of materials. These scales use slightly different measurement systems to accommodate materials tested; yet, each includes measurements between 0 and 100, with high values indicative of harder materials. For medical plastics, the two most common durometer scales are A and D.
Shore A scale uses a steel rod presser foot with a 35o conical taper that ends in a 0.031 inch (0.79mm) diameter flat tip. The tip is applied with 1.8 lbs (822 grams) of force to the test piece. Shore D uses a similar steel rod with a 30oconical taper and a 0.004 inch radius (0.1mm) rounded tip, that is applied with 10 lbs (4.5 kgs) of force.
The final value of the hardness depends on the depth of indentation by the presser foot. If the tip penetrates 0.1 inch (2.54 mm) or more, the durometer is 0 for the scale. If it does not penetrate at all, the durometer is 100 for the scale. Shore values between 0 and 100 are proportional to the indentation between 0 and 0.1 inches.
In cases of zero or maximum indentation, an alternative scale is required to accurately measure hardness. Durometer scales often overlap. For example, a durometer of 90 on the A scale (i.e., 90A) is equal to a durometer of 40 on the D scale (i.e. 40D).
Polymer flexibility is most accurately determined by flexural modulus, which measures deformation under load. However, durometer is sometimes used as surrogate for assessing flexibility of competing polymers, since a higher durometer correlates to a higher modulus.
Polyether block amide copolymer (e.g., Pebax brand by Arkema) is a common catheter shaft polymer. Grades with 35D, 55D and 70D durometers have increasing flexural moduli of 3.0 ksi (21 MPa), 24.7 ksi (170 MPa) and 56.6 ksi (390 MPa), respectively.