Monthly Spin-Off #11 [Testing of Key Material Properties]

Overview

Biocompatibility is an essential part of medical device evaluation. Whether the interactions between said device and the host (both by the device to the host and by the host to the device) cause toxicity, injury, or rejection, will decide if a device is allowed to come into contact with the human body (1).  One determinant of biocompatibility has to do with the properties, both physical and chemical, of the material(s) that makes up a device. Physical properties are, oftentimes, more straightforward to measure, and this piece will describe how BioSurfaces goes about testing a few of these properties for its proprietary Bio-Spun™ material technology.

Three of these physical properties are often tested in conjunction with each other, these being the tensile strength, elastic (Young’s) modulus, and ductility of the Bio-Spun™ materials. The evaluation of properties, such as the three listed, allows BioSurfaces to better understand how its Bio-Spun™ materials may interact with cells and tissues, make determinations of which material may lend itself to certain applications, and maintain the quality of the materials the company produces.

Sample preparation

In order to do testing for tensile strength, elastic modulus, and ductility, the Bio-Spun™ materials are cut into “dog-bone-like” segments similar to the one shown in figure 1(Dimensions not accurate to BioSurfaces testing).

Figure 1. “Dogbone” shaped specimen example.(2)

This particular shape, outlined in the ASTM standard D638 (2), is the primary specimen shape for uniaxial tension tests due to it reducing “the influence of stress concentrations induced by loading grips'' of the apparatus that the specimens will be placed into (3). BioSurfaces uses similar test specimens which follow ASTM D882 (4), which is similar to D638, however D882 is specifically for plastics of thickness less than 1.0 mm. The cutting of the specimens is done with a laser cutting machine. This method works best for the Bio-Spun™ materials, however die cutting, injection molding, and machining are other popular methods for plastic materials (2).

After cutting, the thickness of each Bio-Spun™ material specimen is measured at the narrow section of the “dogbone” with the use of a digital thickness gauge. These thickness measurements (in micrometers [um]) are documented and will be used later for data analysis of the corresponding specimens.

Data Collection

The testing of the specimens is done on a motorized force testing instrument. This device is capable of exerting both tensile and compressive force on materials and measuring the amount of force in relation to time and or distance. For testing tensile strength, elastic modulus, and ductility, the specimens are placed into two grips (one on each end of the specimen). The cut specimens are then put under tension and pulled, sometimes until the break or failure of the specimen. The test parameters for each of these properties have slight differences in terms of rate at which the specimen is pulled and size of the specimen being used (all in accordance with ASTM D882). The force (Newtons [N]) versus distance (millimeters [mm]) data is collected by the instrument and is exported for data analysis.

Material Property Analysis

Ductility - The calculation for ductility of the Bio-Spun™ materials is the simplest of the three. The aim is to determine the amount that the specimen stretches before it fails. To determine this, the strain must be found by equation 1 below. 

Equation 1. 

L is the distance between the two grips holding the specimen at failure, which can be extracted from the data collected on the force testing instrument. This distance is divided by L0, which is the initial distance between the two grips prior to running of the test. Solving for strain will give the relative elongation of the specimen under the tensile forces (ie. a strain of 0.20 would mean the material stretched 20% longer than the initial length of the stretched segment). This strain value has no units. The ductility of each specimen is documented and an average done for all the specimens taken from a specific material.

Tensile Strength - In order to determine the tensile strength of the Bio-Spun™ materials, the data collected from the force testing instrument must be transformed. First, all the distance values for a specimen must be converted to strain values using equation 1. Second, all the force values for a specimen must be converted to stress values using equation 2 below.

Equation 2. 

To do so, The force data must be divided by the width (converted to meters [m]) of the “dogbone” shape used. Following this, the data is divided by the thickness (converted to meters [m]) of the specific specimen the data came from. The outcome of these calculations will be the stress data (in Pascals [Pa]) which corresponds to the strain data previously obtained. The stress can be plotted versus the strain to create a stress-strain curve as shown in figure 2. 

Figure 2. Example of a Stress-Strain Curve.(5)

In this graph there are two strength values of interest. First is the yield strength. This is the amount of stress the material can withstand before it undergoes permanent deformation or permanent change in its shape. The second strength value is the ultimate tensile strength. This value is the maximum amount of stress the material can withstand before it begins to fail or break. Both of these numbers are recorded for each specimen and will vary between materials.

Elastic (Young’s) Modulus - Another property that can be found on the stress-strain curve found in figure 2, is the elastic or Young’s modulus. This modulus is a quantification of the “material’s resistance to non-permanent deformation” (6), occurring before the yield strength previously described. During non-permanent deformation the material is able to return back to its original shape after tension forces are removed. Young's modulus can be found by determining the slope of the curve in the linear region. This can be achieved by dividing the change in stress by the change in strain over this linear region. This modulus, in pascals, can also be referred to as the stiffness of the material. This value is calculated for each specimen tested and documented along with the rest of the material properties. 

Conclusion

This is just a small subset of material properties that are tested at BioSurfaces. There are many more factors that account for the biocompatibility of medical devices, which is required for FDA approval (1). BioSurfaces’ testing of material properties on its proprietary Bio-Spun™ materials plays an important role in deepening the understanding of their function and potential use cases. In addition, determination of such properties provides a standard to test products against, while outlining places where such products can be improved upon in the future. 

By: Jacob LeDo

References

1. Biocompatibility: Key Considerations for Medical Devices (Monthly Spin-Off #6), Biocompatibility Testing - Benchtop Evaluation of a Material’s Interaction With the Body’s Cells.

https://www.biosurfaces.us/single-post/biocompatibility-key-considerations-for-medical-devices-monthly-spin-off-6

2.  ASTM Standard D638-14, 2014, "Standard Test Method for Tensile Properties of Plastics," ASTM International, West Conshohocken, PA, 2014, DOI: 10.1520/D0638-14, www.astm.org/d0638-14.html

3. Zhang, Weihong, and Yingjie Xu. Mechanical Properties of Polycarbonate: Experiment and Modeling for Aeronautical and Aerospace Applications. ISTE Press Ltd., 2019. 

4. ASTM Standard D882-10, 2010, "Standard Test Method for Tensile Properties of Thin Plastic Sheeting," ASTM International, West Conshohocken, PA, 2010, DOI: 10.1520/D0882-10, www.astm.org/d0882-10.html

5. Wikipedia contributors. (2024, January 25). Stress–strain curve. In Wikipedia, The Free Encyclopedia. https://en.wikipedia.org/w/index.php?title=Stress%E2%80%93strain_curve&oldid=1198945947

6.Modulus of Elasticity. Instron. https://www.instron.com/en/resources/glossary/modulus-of-elasticity#:~:text=What%20is%20Modulus%20of%20Elasticity,permanent%2C%20or%20elastic%2C%20deformation