Challenges in Torsion Testing for Biomedical Applications
Biomaterials, implantable devices, medical consumables, orthopedics, and pharmaceutical products are all subjected to multi-axis mechanical loading at some point during their product lifecycle. The most common type of multi-axis test in the biomedical industry is bi-axial testing of applying both axial load and torque to a specimen. In recent years, standards bodies such as ASTM and ISO have added torque and axial requirements to many standards. For example, Annex 4 was added to ASTM F543 in 2013 to require the axial tapping force to be reported for self-tapping bone screws while simultaneously undergoing torsion loading. Bi-axial testing in the biomedical industry is not only driven by ASTM and ISO standards, but also by functional testing. For example, a person with diabetes may need to dial an insulin pen to the correct dose then push down on the pen to inject the medicine. Regardless, if biomedical companies are complying with standards or performing functional testing, the prevalence of bi-axial testing is rising. With this rise in tension and torsion testing, we are finding most companies interpret bi-axial testing to be over complicated.
However, not all bi-axial systems require complex method or test set up and companies should not shy away from performing the correct bi-axial test due to complexity. Bi-axial testing is not scary, but there are testing parameters that need to be defined by the user before testing that are viewed as common challenges. These challenges are knowing how to test and designing the correct fixturing.
Knowing how to perform bi-axial testing is generally defined by reading standards, gathering test requirements, and thinking about the future to determine what equipment is needed. To simplify things, we are able to break bi-axial systems up into three main categories: torsion system with constant axial load that is often a pulley with a weight attached, simple bi-axial systems capable of monotonic axial loading and torsional loading, and fatigue systems capable of monotonic and cyclical axial and torsional loading and control. For example, a bone screw test to determine maximum torque may only require a constant compressive load to hold the screw into place and torsion to drive the screw into bone material. While a luer-lock test may require a specimen to be monotonically loaded and then torqued to a certain value and unloaded after.
Lastly, more complex applications that require a bi-axial system are often in the orthopedic industry. Long-term torque and axial loading on a specimen or component for durability testing requires the highest level of loading and control, and because of that will need a more advanced system. Overall, defining test requirements based on standards and future product development and knowing how to test is the first step and challenge in performing bi-axial testing.
Fixtures: Customized or Standard
After determining how to test, the next step is determining how to grip the specimen or what fixturing is needed. When it comes to fixturing for biomedical components and materials, the fixture almost always ends up being something specific to the customer’s component or consumer product. Pharmaceutical and plastics companies who manufacturer child-proof medicine bottles that require push and twist testing often have different diameter, shape, and size bottles, as well as different cap sizes. How to grip these different geometries and finding a fixture that can handle all specimen types is the challenge. We have worked with numerous companies to design fixturing as well as supplied standard grips or 3-jaw chuck that can do 80% of a customer’s applications.
We have found that supplying a standard grip or fixture is most economical for customers and provides them with a baseline to machine additional components or fixturing for the remaining 20% of their testing. For low-force and low-torque applications, many customers use 3D printers to print custom fixtures, which enable rapid fixture prototyping. It is important to remember that the fundamentals of the machine, that is, the axial force and torque transducers, software control, and torsion actuator are all integrated and fixturing should not be considering an overwhelming final piece to the solution.
Overall, knowing how to test and defining the equipment requirements and fixturing are small challenges when compared to the benefits of performing bi-axial testing to simulate functional product use or to meet ASTM and ISO standards. Many medical products are subjected to multi-axis mechanical loading, so it is crucial for companies to start considering if bi-axial testing is appropriate for them.
As a Finnish start-up company, Synoste is making a name for themselves in the industry through the development of next-generation, patient-friendly medical solutions for the treatment and correction of skeletal deformities. Their first product: a fully implantable device for lower limb lengthening that utilizes smart materials and novel technologies.
The current treatment for limb lengthening relies on devices with an external frame connected directly to the bone to allow for the lengthening mechanism to work. This is extremely uncomfortable for patients and is prone to complications like infection, misalignment and re-fractures of the bone. Patient’s mobility is also affected and they have to make regular trips to the hospital for the lengthening procedure to be conducted.
Synoste’s philosophy of patient friendliness, usability, and device reliability has driven them to develop a less painful and precisely controllable treatment device that lowers the complication rate and enables faster recovery to normal life.
As they went through their product development process, material testing became a larger and larger part of the program. But without direct responsibility for the testing instrument they shared with other departments, their research was exposed to significant risk.
Read more on how Synoste responded to their immediate needs and fulfilled their requirements for long-term plans.
Webinar: Overcoming Challenges & Perceptions of Bi-Axial (Torsion + Axial) Testing for Biomedical Applications
In this webinar, Instron Application Specialists Elayne Schneebacher and Elena Mangano will discuss the common challenges associated with dual-axis testing and, through application-specific examples, will help uncover that torsion testing is not that scary after all.
We recently participated in Georgia Institute of Technology’s Bioengineering Techniques Symposium. At the symposium Jim Gleason and Kent Wallace discussed staying current with the newest technology for tension, compression, flexural, and fatigue materials testing. During the presentation, they demonstrated current technology for measuring strain to an accuracy of 1 micron, discussed contacting and non-contacting strain measuring techniques, and demonstrated full field of view strain measurements using Digital Image Correlation (DIC). With a solid focus of cutting-edge strain measurement, our local team enjoyed speaking with professors and graduate students who came to the symposium to gather general information on materials testing, gain advice for their current testing, and those that came to inquire about materials testing equipment for their future research and testing needs.
The applications of ElectroPuls™ Test Instruments vary from investigating the behavior of biomaterials and biomedical fixtures to testing materials and components for aerospace and automotive industries, as well as for metals, plastics, or athletic footwear. Additionally, the ElectroPuls instruments are also preferred for biomedical applications due to its environmentally-friendly design and its suitability for use in a clean environment.
Since the end of 2014, around 40 articles have been published that discuss materials testing using an ElectroPuls instrument. Most of the articles include research that has been done in the biomedical field. A few examples of certain articles are presented below.
PresentationsChallenges in QC Medical Device Testing
About Strain Measurement
- Biomedical Testing Solutions
- Medical Devices & Biomaterials Videos
- ElectroPuls Videos
- Quality Magazine: The Challenge of Developing Testing Standards for Non-Standard Technology
- Quality Magazine: Why Medical Device Innovation Needs Quality Programs