Considerations in Testing of Plastics for Biomedical Applications, by Jim Ritchey and Richard Goshgarian
Editor's Introduction.
Analytical testing of specimens is a fundamental process in engineering products. Here two leading lights from US testing equipment supplier Instron provide insight into some of the most important considerations for testing of plastic materials in medical devices.
The Test (Load) Frame
A load frame is the mechanical device used to apply a force to a specimen. These types of systems are used to evaluate a variety of things, including materials, components and packaging. Understanding how they work, performance differences and flexibility is very important to end users, whether the company is making biomedical devices, bumpers for automobiles or confectionery.
Load frames come in many different varieties and sizes and generally fall into two general categories, electro-mechanical (EM) or dynamic system.
EM frames use an electric motor and belt system to turn screw column(s) that cause the crosshead to move. Load cells are used to for measuring load, which can be used to determine stress and position encoders are used for measuring extension, which can be used to determine strain. Extensometers can also be used for measuring strain and this will be discussed further later on in this article.
The load cell is built so that it will deform elastically under the loads applied. The amount of elastic deformation of the cell is sensed and converted to an electrical signal which in measured by the conditioning electronics. Strain gauges are fitted in load cells to measure this elastic deformation. They undergo electrical resistance changes when they are “stretched” or compressed. A typical system uses a load cell connected to a bridge circuit to measure minute resistance changes and thus the applied loads or force. The circuit is excited with a signal generated by the load-cell amplifier, and an applied force causes the strain-gauge bridge circuit change the output producing an electrical output proportional to applied force.
Strain Measurements
As mentioned earlier, strain can be measured using extension derived from position encoders as well as from an extensometer. Strain is a measurement of how much a material or part stretches. It is the change in specimen length per unit of original specimen length. The original specimen length is commonly referred to as the gauge length. Using extension from position encoders does not always give an accurate measurement of how far the specimen is stretching. This is for reasons such as compliance, which by definition is the ability of an object to yield elastically when a force is applied. All systems have some compliance, some more than others and in order to make accurate measurements of strain, you must be able to separate system compliance from specimen deformation. When conducting a tensile, flexure or compression test, materials are stretched, flexed or compressed and in all cases, the crosshead will move and extension will be measured. To separate compliance from these measurements, an extensometer can be used. Additionally, software corrections can be used however, some standards specifically require extensometers so it is important that standards being followed are understood.
Strain calculated with crosshead movement is typically okay for materials such as rubber that exhibit large amounts of elongation. However, for plastics and other rigid materials, an extensometer is most often required.
An extensometer is a device that measures extension or strain directly on the specimen. It will accurately measure across 2 points on the specimen, the distance between these two points being the gauge length. There different types of extensometers, the most common are a clip-on style but video based and fully automatic contacting extensometer are also quite common.
Clip on extensometers nearly always use strain gauges and they work similarly to a load cell. Small extensions are measured by using resistance strain gauges to sense displacement. Output voltage is directly proportional to the amount of knife edge travel.
As mentioned previously, the testing standard will typically identify whether or not an extensometer is required. They also indicate the required performance. Some of the most common plastics standards include ISO 527-2, ASTM D 638, ISO 178 and ASTM D790. These cover the more common tensile and flexure tests.
ISO 527-2 in particular is the most common global testing standard for the determination of tensile properties of plastics. This method covers rigid and semi-rigid thermoplastics molding, extrusion and cast materials, including compounds filled and reinforced. This is a material standard that would apply to material suppliers but also to companies that purchase materials used to make some type of end product.
This is a challenging standard in that it requires an extremely accurate strain measuring device for the determination of modulus. The standard, like others, can change over time and recently there has been a change. When changes occur, it often requires new equipment in order to be compliant. In the case of this standard, a new gauge length was added, 75 mm. 75 mm is the new preferred gauge length which will be required for all lab types with the exception of QC, where 50 mm will still be allowed.
The accuracy required to measure modulus for a 50 mm GL is +/-1um and the accuracy for a 75 mm GL is +/-1.5 um. Having the ability to measure strain with an accuracy of +/-1 um is where the challenge comes. To put this in prospective, the thickness of standard copy paper is about 102 microns, the tip of a ball point pen is roughly 500 microns and height of a grain of salt is roughly 60 microns. The extensometer must perform with this accuracy over the whole region in which modulus is calculated. Also, since some labs prefer to use extensometers through break, it must also have the ability track all specimens (including high elongation materials) to failure and also withstand high energy failures. Devices capable of offering this versatility are usually limited to automatic contacting devices. Many plastics will stretch over 700 mm, which far outside the range of a typical clip-on style.
Other changes include ISO 178 (flex).
ISO 178, a common standard for determining flexure properties of plastics now requires the use of an extensometer or strain measuring device. For many who follow this standard, new fixtures and strain measuring devices will need to be purchased.
Medical Devices and Components
The medical device industry is a large producer of a wide variety of components that are constructed of plastic. Several ASTM and ISO standards have been developed to allow manufacturers to both develop and manufacturing highly reliable, effective and safe products. One of the most critical of these devices is medical catheters. These catheters are typically inserted into canals, vessels, passageways, or body cavities usually to inject or remove fluids from the body. Failure of these devices could result in severe patient injury. As a result, mechanical testing is used for quality control, research and development, and is required by the Food and Drug Administration (FDA) guidance documents to demonstrate device safety.
Breaking strength and breaking strain of the tubing are critical measurements of product quality. ISO 10555 is the standard that is typically used for guidance in the testing of these products and it is broken into 5 parts evaluating performance characteristics such as leakage, corrosion resistance, tensile force, flow rate, burst and power injection. ASTM F623 is another standard, but it focuses exclusively on Foley catheters products. Ideal test configuration typically include pneumatic action gripping since they are highly effective at preventing slippage and reducing the clamping stress applied to the specimen. This results in enhanced testing repeatability. Additionally, incorporating a high elongation direct strain measurement either through a contacting mechanical extensometer or non-contacting video extensometer is recommended. Since elongations can high, often greater that 500% strain, devices must have extended capabilities.
Plastic components also play a critical role in the development of both hip and knee implants used in replacement surgeries. Similar to the catheters, there are standard tests that manufacturers and developers can use as guidelines when it comes to evaluate their designs and products. ASTM F 1714 Standard Guide for Gravimetric Wear Assessment of Prosthetic Hip Designs in Simulator Devices’ was developed to evaluate the wear properties of ceramic and polymeric materials used as bearing surfaces in hip joint replacement prostheses. It represents a more physiological simulation than basic wear-screening tests such as pin or ring-on-disk. The ISO 14879 standard ‘Determination of Endurance Properties of Knee Tibial Trays’ provides a common set of test parameters for determining and validating the fatigue properties of different tibial tray designs. Fixtures should be designed to secure one half of the tibial tray, simulating a fully supported condyle. The other unsupported condyle is then subjected to physiologically representative loading which is critical to relevant evaluations of product performance and quality. All of the tests are performed on Dynamic Systems that provide the specific fatigue loading characteristics spelled out by the standards. Accurate strain measurements are required to determine the differences between different designs and products. These are just a small sampling of plastics being utilized in the medical device industry where direct strain measurements provide important information relating to the devices performance and reliability. There are additional measurements that provide material properties and characteristics as well.
Load Measurements
In addition to strain accuracy, load accuracy and the range in which a load cell should be used can have a significant impact on a lab. For companies that test a variety of materials, it is desired to not have to change equipment each time a new material is tested or when a test is changed from tensile to flexure for example. An example of such a company could be a manufacturer of IV bags where the product consists of flexible plastic film as well as rigid plastic for connectors or luer locks.
The load and strain requirements differ drastically. For the case of the plastic film, the load cell will need to be able to accurately measure smaller forces since plastic film is relatively weak. For the rigid plastic used in luer locks and other components, the system will need a larger load range but also, high accuracy required to measure modulus, which happens early on in a test. For most situations today, labs, depending on their equipment and range of materials being tested, will have to change the load cell and extensometer (if used) each time they change materials. This takes time and money. However, recent developments in load cell technology and system controllers have drastically expanded the range in which load cells can be used. For instance, a typical load cell several years back (and for some companies today) would have a usable range from its full capacity down to 1/100th of the capacity; this means a 1,000 N load cell for example could be used from 10 N to 1,000 N. If measurements had to be made below 10 N, the accuracy was not guaranteed. Today, some systems offer a range from full capacity down to 1/1000, so for that same capacity load cell, the range would be 1 N to 1,000 N, which increases the likelihood of it accommodating all of the load accuracy needs.
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About the authors:
Jim Ritchey is currently the director of Instron’s tissue engineering businesses, having worked for Instron for 28 years in a variety of roles ranging from project engineer to sales manager. In his current role he focuses on expanding Instron’s global reach and market exposure through collaborative research formulations and new product development. He regularly presents on topics ranging from testing technology to global trends within medical device and biomaterial development, as well as actively participating in conferences across the globe.
Jim currently serves as chairman on the Executive Committee of the Boston Chapter of the American Society of Materials (ASM), as well as actively participating in several ASTM F04 subcommittees. He holds a Bachelor’s of Science (BSc) degree in Materials Science & Engineering from Cornell University, as well as a Masters of Business Administration (MBA) from Cleveland State University.
In his spare time, Jim enjoys spending time with his two teenage boys, skiing, playing golf, collecting folk art and travelling the globe.
Richard Goshgarian is currently Instron’s Market Manager for the Global Plastics Testing Market, having started as an Applications Engineer, and then spending 5 years as Product Manager for the Electromechanical range working with plastics focused applications. In his current role, Richard is now additionally responsible for Instron’s CEAST range for plastics testing.
Richard holds a BSc in Plastics Engineering as well as an MBA, both from the University of Massachusetts. Richard is an active member of both the ASTM and ISO organisations and continues to work hard to ensure Instron’s compliancy with key standards across the plastics market.