Globally, the use of medical consumables continues to rise due to an aging population and the increasing prevalence of day surgeries. As the use of medical consumables continues to rise, the testing required to ensure that these products meet specifications also increases. Syringes and other medical products with Luer connectors are the most common medical consumables found in the industry, as they are used to connect a wide variety of medical products. Luer connections come in two forms: either a slip or tapered connection, or a threaded locking connection. Most commonly, Luer connectors are used to connect a needle to a syringe body or to connect various types of medical tubing (such as a catheter) to an IV bag, or a needle to catheter.

When considering a biaxial test machine, users should carefully assess the method development procedure. (Credit: Instron)

In order to measure both the safety and usability properties of these products, they must be tested with a biaxial system that measures both force and torque simultaneously. Traditionally, biaxial systems are difficult to set up, require specialized operator training, and require complex software programming. In addition, a variety of mechanical testing equipment suppliers offer different types of biaxial test systems that can further confuse users. This article discusses the most common biaxial tests performed in the medical device and pharmaceutical industries, as well as discuss the differences between biaxial test systems and what users should consider before designing a testing protocol.

When and Why to Test

Biaxial testing of Luer connectors that are used to connect a needle to a syringe body is one of the most common uses of this type of mechanical test. (Credit: Instron)

In practice, assembling the Luer slip or Luer lock connector requires a healthcare practitioner to apply an axial downward force onto the connector while simultaneously rotating the connector to lock it into place. Due to the simultaneous pushing and twisting motion required to assemble a Luer connection, Luer slip and lock testing is one of the most common biaxial tests in the medical device industry. ISO 80369-20 outlines the mechanical testing procedures for testing a variety of connection properties of Luer connectors including assembly, as well as various properties associated with overriding and disconnecting the connector.

In addition to testing Luer locks and slips, another common need for biaxial testing in the medical device industry is for testing bone screws. Bone screws are most commonly made from titanium due to the fact that titanium's inertness, biocompatibility, and mechanical strength closely match those of bone. In cases of severe fracture, bone screws are most commonly used to hold a fracture fixation plate in place, which allows the bone to heal in the correct orientation. In many cases, a fracture fixation plate and bone screws are temporary, and the patient must undergo a second surgery to remove them once the bone is fully healed. In order to ensure easy insertion and removal, bone screws are most commonly tested to the ASTM F543 standard. ASTM F543 is composed of four testing annexes that are designed to quantify a bone screw's insertion properties, override properties, axial pull-out properties, and, when applicable, the self-tapping properties of self-tapping bone screws. Unsurprisingly, the testing of bone screws requires measurement of force, axial displacement, torque, and rotational displacement, thus requiring a biaxial testing system.

Biaxial testing is extremely common in the orthopedics industry, and despite bone screw testing being more of a static biaxial test, most biaxial testing performed on orthopedic devices is designed to measure fatigue. For example, ASTM F1717 describes test methods for quantifying the axial and torsional properties of spinal fixation devices in fatigue. Orthopedic applications require linear-torsion fatigue testing because orthopedic devices remain permanently implanted in a patient's body, and most withstand at least a decade of wear.

While biaxial testing is very common in the medical device industry, packaging of consumer products such as childproof medicine bottles, mouthwash containers, and makeup containers also requires the same push and twist mechanism. Despite the simplicity of these applications, it is critical for global consumer brands to test their product's packaging to ensure both ease of use as well as safety. In many cases the packaging is just as important as the product inside, and if the package is difficult to open or easily opened by a child, the repercussions could be catastrophic.

Focusing further on product usability, many manufacturers of high-end products require the usability of their products to be superior to that of their competition. With a biaxial test system, packaging labs can simulate a variety of product-use cases to quantify product usability attributes and ensure that their products are performing up to the desired standard. For example, a high-end makeup brand can test a variety of usability properties on the packaging of mascara, lipstick, and other cosmetics by determining the torque needed to unscrew a mascara wand while maintaining a zero axial force on the container, followed by an axial tensile test to determine the force require to pull the mascara wand out of the mascara container. While this is one simple example of biaxial testing on a consumer product, the biaxial testing on consumer packaging is nearly limitless.

Choosing the Right Test System

Biaxial testing systems are used to measure axial and torsional forces required to insert and remove bone screws. (Credit: Instron)

From Luer connectors and spinal implants to mascara containers, with such a variety of biaxial testing applications, it can be difficult for users to determine what type of biaxial test system is right for them. In general, biaxial test systems can be broken down into three different types of systems: torsional test systems that allow users to use dead-weights to hold a static force, static biaxial test systems that allow for full axial test control and simple rotational control, and biaxial fatigue test systems that allow for full axial and torsional test control.

Torsion systems that allow users to use dead-weights to hold a static axial force are often the simplest systems. Many users prefer these test systems when rotational or torque testing is the primary mode of test control. More specifically, applications that require a product or material to be held in tension or compression and rotated or twisted to failure are best suited for torsion-only systems. In addition, labs that need to quantify shear stress, shear strain, or shear modulus are best suited for a torsion system, as this identifies that rotation or torque is the primary mode of control.

Static biaxial test systems that allow full axial test control and simple rotation are most commonly used for Luer connectors, bones screws, and consumer products given the versatility of these testing systems. Universal testing systems, also known as static test systems, are most commonly used for tension, compression, and simple cyclic testing. Often a rotational unit can be added to this test system (this is often referred to as a torsion add-on). Static test systems equipped with a torsion addon are designed for simultaneous axial and rotational testing. One common requirement of users testing medical devices, specifically Luer connectors, is the ability to hold a torque value during a test, while simultaneously performing some sort of axial control. When users look to identify a biaxial test system for Luer lock testing to ISO 80369, it is critical that users choose a system that is capable of holding a torque value while also having the full ability to control the axial testing parameters of linear displacement and force.

As mentioned, linear-torsion fatigue test systems are most commonly used in the orthopedic industry. However, biaxial fatigue testing also extends to other industries that require long-term fatigue characterization such as dental and aerospace applications. While bone screw testing is most commonly conducted on static test systems with a torsion add-on, some laboratories perform bone screw testing using a linear-torsion fatigue system. It is important to note that nearly all testing performed on a torsion-only system and a static biaxial test system can also be performed on a linear-torsion fatigue system. One of the primary reasons why a lab would choose bone screw testing on a linear-torsion fatigue system is if the lab has other applications that require fatigue testing. This is very common in orthopedics labs, as the majority of orthopedic devices have to be tested to simulate years of use in a patient's body.

While there are a variety of mechanical testing systems on the market that can be used to measure force, axial displacement, torque, and rotational displacement, one of the most important differences impacting their users is the type of software used to program these systems. Biaxial testing requires dual-axis programming, which can be intimidating. Considering this, it is critical that the software be intuitive and easy to use. Research has shown that a touch-based user interface is most intuitive. Software programs that have been optimized for a touch interface tend to be easier to use, because the icons are larger and the interface is flat, which eliminates the need for the right-click mechanism and enables all parameters to be clearly displayed on one interface along with descriptive icons. A flat user interface is especially helpful for programming a test method for axial and rotational control.

When considering a biaxial test machine, users should carefully assess the method development procedure to ensure that the axial and rotational axis can easily be programmed for testing speed, and that starting and stopping axial and rotational control are easily set for a variety of scenarios. In additional to the software interface, the software program should have an extensive library of calculations that ensures the user has flexibility to automatically calculate test results. In many cases, as in Luer lock testing, ISO 80369 specifies multiple required steps in order to assemble and disconnect a Luer fitting. This requires multiple axial and rotational ramps or steps to be programmed, and a variety of duplicate calculations need to be reported. For instance, a maximum torque during assembly and maximum torque during disassembly both need to be reported in a given test.

Overall, when determining which biaxial test system is best for an application, users should consider their test requirements, the future requirements of their lab, and the flexibility and intuitiveness of the software program. While biaxial testing is often considered complex and difficult to program and set up, with an intuitive user interface, users can easily write testing methods without compromising on the flexibility or performance of the system.

This article was written by Elayne Gordonov, Biomedical Market Manager and Software Product Manager, Instron, Norwood, MA. For more information, visit here.