Simulating a Spring with the ElectroPuls

As the number of patients seeking “in-home” medical care grows larger, medical device companies find themselves catering to an increasing demand for safer, more user-friendly biomedical solutions. 

In an attempt to mitigate the injection process, medical device companies have developed the auto-injector, a spring loaded syringe that delivers a prescribed amount of drug subcutaneously into the bloodstream, forgoing the need for physician oversight and allowing patients to self-administer medication from the comfort of their own home. To design the ideal auto-injector, the delivery time of the syringe must be optimized so that drug release is neither too long nor too short, thus reducing the level of discomfort experienced by the patient. The delivery time can be optimized by choosing the appropriate spring and preload for a given syringe/drug system, which is currently achieved by subjecting springs with varying degrees of stiffness to different loads. However, medical device companies need to be able to get through this iterative auto-injector design process more quickly, and testing a large number of springs is time consuming and less than economical. Simulating the spring eradicates the supply chain issues of multiple springs testing, thereby significantly reducing the required design time. Instron solves this control challenge using Modal Control, an advanced feature of the 8800 controller that creates a composite channel to allow the applied load to vary with actuator position. The user simply needs to specify the initial preload and spring stiffness in order for the system to adjust itself based on the actual load seen by the syringe. A force vs. time graph is produced, from which the Calculations Module feature in WaveMatrix, determines the dispensing time and stall force of the syringe. The user can then determine if the chosen parameters for spring stiffness and preload have yielded an ideal auto-injector dispensing time for a given drug. 
 

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Upper Yield Calculations—Discontinuous Yielding / YPE Material

Following a recent lab visit, I thought it was worth writing a quick post to share an example of how labs sometimes incorrectly calculate yield strength. While visiting a testing lab for unrelated issues, my colleagues and I were discussing how the customer runs both continuous and discontinuous material with the same method (Non-YPE and YPE). For historical reasons, they have updated their Series IX methods to Bluehill 3 with little knowledge of why, when or by whom the methods were setup.

It later transpired in the conversation that some of the lab's material was falling below their specification for yield strength. They are required to test every coil of product before shipment; therefore, materials not meeting the yield strength requirement are of major concern. The material has had to be reworked in the past in the hopes that the yield strength could be increased slightly. Otherwise, the material would be scrapped.

The calculation used for both continuous and discontinuous material was an offset yield of 0.2% strain. Based on this calculation the stress values were determined for product verification. For a continuously yielding material, it is recommended to use the offset yield calculation. However, for a discontinuously yielding material, using this calculation would result in an inaccurate measurement of stress after a potentially significant load drop. The main international metals standards such as ISO 6892 and ASTM E8 both recommend using upper yield strength (ReH or UYS) to calculate this value correctly.

By changing the software algorithm to a different Bluehill 3 standard calculation, we were able to increase the calculated yield strength by around 10%. This meant they no longer had to rework the material as the higher stress values were always well within specification. This modification to their test method helped reduce testing turnaround time, increased the reported material strength, reduced shipment delays, and removed the need for costly and time-consuming reworking process.

The graph below shows an example of how the two calculations differ. The results of the offset yield calculation were on average 21% lower than the upper yield calculations. Checking that calculations are correctly evaluating data could result in a significant improvement of results. Often methods were setup and have not been reviewed on a regular basis.

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What Does Load Measurement Accuracy of 0.5% of Reading Down to 1/100th, 1/500th, 1/1000th of a Load Cell's Capacity Imply?

First of all, there are two important pieces of information here:

(1) Load accuracy: ± 0.5% of the reading

(2) Lowest possible load within this accuracy: 1/100, 1/500, 1/1000 of load cell capacity

For example, let’s assume your test system has a load measurement accuracy of ± 0.5% of reading down to 1/100th of the load cell’s capacity. When a load cell of 10 000 N (10 kN) capacity is used on this test system and a measurement of 1000 N is read, the actual load measurement can be any value from 995 N to 1005 N. For this 10 kN capacity load cell, the minimum possible load that is guaranteed to meet the load accuracy of ± 0.5% of reading is 100 N.

Now, imagine you have a newer system with a new 10 kN load cell and the load measurement accuracy of ± 0.5% of reading is now extended down to 1/1000th of the load cell’s capacity. The load measurement accuracy of ± 0.5% of reading remains, but now the minimum possible load that is guaranteed to meet the load accuracy of ± 0.5% of reading is extended down to 10 N.

So what does a larger load accuracy range mean to you? Here are some of the benefits you will see:

- Less initial cost since less load cells are needed

- Lower subsequent service costs as fewer load cells need to be verified annually

- Less operator error and improved throughput (number of specimens tested per hour) by not having to change load cells between different test types such as tensile and flexure tests.

- Higher confidence in test data for low load tests.

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Industry Insights Series: David Fry

Working alongside our friends at AZOM, they developed a series of engaging interviews with a few of our global colleagues. In the recent series, I was able to discuss metals testing requirements with a focus on the latest European metals standard, ISO 6892 and high precision strain measurement with the AutoX750.

We'd appreciate hearing from you some suggestions for additional interview topics that would benefit your testing and applications. Read more

Instron at Tissue Engineering Conference in Istanbul

 

Instron TGT went on the road for the TERMIS (Tissue Engineering and Regenerative Medicine International Society) meeting in Istanbul, Turkey at the end of June. Despite the "social unrest" in Istanbul, the conference was well attended with over 500 attendees from around the world, including Argentina, Chile, Korea, Russia, and New Zealand.

We introduced our combination of products, with a demonstration of the 5944 and Bluehill^® and the LigaGen Bioreactor System. The attendees were excited to learn about the acquisition and the new solutions that Instron could provide to tissue engineering and regenerative medicine. We estimate that 30—40% of the researchers were familiar with Instron, including those already using Instron systems to characterize materials for tissue engineering or to test and characterize tissue engineered products.

Tissue engineering conferences are an exciting combination of disciplines and this one was no exception. An array of topics were covered, ranging from Biomaterials; Scaffold Designs and Fabrication Techniques; Tissues & Cells; Stem Cell Science; Genetic Modifications; and Bioreactors.

We look forward to attending the remaining 2013 regional meetings in Shanghai (October 23—26) and Atlanta, GA (November 10—13).

Want to know what tissue engineering and regenerative medicine is all about? Check out Dr. Atala's TED talk on bioengineered tissues. Read more

Equipment Considerations for a Well-aligned Test

When testing stiff or brittle materials, such as composites, alignment is crucial—even instances of slight misalignment can throw off test results.

Misalignment in test frames takes two forms: concentricity misalignment, where the centerline of the upper grip/fixture is offset from the centerline of the lower grip/fixture, and angularity misalignment, where the two centerlines are at contrasting angles. Both forms cause extra stress on the specimen and can affect mechanical properties such as modulus, maximum elongation, and tensile strength.

The alignment of a system is so critical to test results that organizations, such as Nadcap, strictly regulate misalignment. To meet these requirements and reach the most accurate results, the test operator needs to carefully assess his or her station, including the testing frame itself to the alignment cells used to verify alignment.

In addition to being accurately aligned, the frame should also be stiff so that it deflects very little at the load.

Next, it’s necessary to align the frame with the grips that will actually be used. This means that if an alignment is performed with one set of grips, but then replaced with a second set of grips, both sets of grips must be realigned. (To prevent removing grips, it is helpful to choose grips that can accommodate multiple tests including compression.)

Rigid and symmetrical grips produce the best alignment results. Under load, symmetrical grips will deform symmetrically from front-to-back/from side-to-side; therefore, the grips will not bend the specimen. Plus, the more rigid a grip is the less it will deform under load.

Once the frame and grips are aligned, now the specimen needs to be aligned. The operator oversees this process. To minimize user error, grips with specimen stops indicate where to place the specimen every time. If the specimen is loaded in different positions, even in a perfectly aligned system, large variation will occur.

To ensure that platens are parallel to the degree that Nadcap requires, one rigid platen and one locking, spherically-seated platen are required. These types of platen can be compressed against each other to ensure that they are parallel and can then be locked to prevent them from being misaligned.

Alignment cells are an essential part of any alignment. These cells, when used in conjunction with software, provide valuable information on how to correct a system’s misalignment. The dimensions of the cell should be as close as possible to the specimens being tested. On occasion, standard or stock alignment cells do not fit with the samples being tested. In these instances, the design of a custom strain-gaged alignment cell may be necessary. Since fabricating a strain-gaged alignment transducer can be a time-consuming process, plan this step well in advance.

Any given system has small misalignment errors due to tolerances of the frame and the parts used. To adjust for these small errors, it is helpful to have a fixture which can be used to make small adjustments to the concentricity and angularity of the load string. This is especially handy for future alignments and will save you time and effort down the road.

As always, feel free to contact us with any questions. Read more

Question from a Customer: Bagley Correction

Question:
Can we compare viscosity results between polymers tested at the same temperature and identical shear rates but on two different capillary rheometers?

Answer:
As an example, let’s consider two capillary rheometers with the following specifications and test parameters.

Rheometer A:
Barrel diameter = 9.55 mm
Capillary diameter = 20 mm
Capillary length = 1 mm
Test temperature = 150 C
Shear rates = 100, 200, 300, 400, 500 1/s

Rheometer B:
Barrel diameter = 15 mm
Capillary diameter = 10 mm
Capillary length = 1 mm
Test temperature = 150 C
Shear rates = 100, 200, 300, 400, 500 1/s

Results obtained from a single test performed each on Rheometer A and Rheometer B are generally not comparable.

In order to compare any number of viscosity results from two or more different capillary rheometers, it is very important to normalize the pressure data by introducing a correction called Bagley Correction post testing. This correction applied to the pressure data will generate true viscosity values which will then be comparable between different capillary rheometers. This correction is necessary because of the “entrance effects”, which occur when the molten polymer is pushed through the larger barrel diameter into the smaller capillary diameter. These “entrance effects” generally are different between Rheometers A and B. The Bagley Correction compensates for these differences, thereby making the results between the two comparable. To find out more about the Bagley Correction, and how to apply it, please refer to this blog post. Read more