Curved needles are used within the biomedical industry for wound closure procedures. Their curved shape is specifically designed to work with surgical sutures and improve a physician's ability to maneuver the needle. The curved geometry is also optimal for ensuring that an even and consistent tension is applied to the suture during the procedure.

Key Considerations for Testing

During product research and development, penetration testing is key to identifying a needle coating with the ideal balance of lubricity and durability. This ensures that the needle can be used reliably for the minimum number of throws to close a wound. Bend testing can also be used during the material selection process or as an in-process quality control check for manufacturing consistency. A material defect causing the tip of the needle to break could cause undue pain for a patient.

During the manufacturing process, penetration testing primarily serves to ensure that the needle retains its sharpness and that the applied coating is performing as expected, as the lubricity of the coating is a main driver of the penetration force. Since a curved needle will be threaded multiple times in practice, the penetration test must be performed a minimum of 10 times per needle to ensure that coating will not wear out over repeated use, causing an increase in the penetration force. The result will be a maximum penetration force for each run, as well as qualitative evaluation of the coating's durability from penetration force range.

Testing Standards

There are two main standards dedicated to assessing the functionality of curved needles: ASTM F3014 for penetration testing & ASTM F1874 for bend testing. It should be noted that ASTM F1874 was formally withdrawn in 2020 with no replacement. However, manufacturers may still perform internal qualifications based on the procedures outlined in the standard.

Materials Testing System

Bend testing of curved needles most closely resembles a cantilever bend test with a fixed platform or edge and a rotating needle. The test is performed until there is plastic deformation of the needle, with the objective of identifying the yield bend moment, the yield bend angle, and the maximum bend moment.

Unlike straight needles, which are used with a linear motion, angular motion is necessary to best simulate the real-world use of a curved needle. Instron's 68SC-1 Curved Needle Tester converts the motor rotation into angular movement with an encoder registering the angle during the test. A 5 or 10 N load cell is typically mounted to the base, with the appropriate penetration or bend fixture attached. For penetration testing, a manual or automated turret is required to allow for the 10 tests on a single needle. The turret is ideal for repeatability and prevents the need to adjust the needle in between test runs. The needle holder is adjustable to accommodate a wide range of curved needle lengths and radii. 

Testing Tips and Tricks 

In this “first phase” of ASTM F3014, the medium to be used for the penetration test is not provided, leaving the decision up to the manufacturer. Membranes utilized for transdermal patches or products like Parafilm©, which have some similar mechanical properties to skin, can be appropriate for this testing.