Biomaterials: Mechanical Testing for Medical Devices
Crucial ideas for medical device development include biocompatibility, biomechanics, and biofunctionality. This article will examine the mechanics of an exemplar biological tissue, the bone, and mechanical testing of biomaterials that are of use in designing medical equipment and devices.
Image Credit: Shutterstock/Roman Zaiets
For medical device design, extensive testing is a complete necessity. Biomechanics, the field that lays the foundations for the mechanical properties of biological tissues such as bones, ligaments, tendons, and muscles, needs investigation and testing of biological tissues under a variety of loading conditions.
Although biomechanics applies principles from engineering to biological systems, it also necessitates considered analysis of the biological tissue. A medical device can only be correctly validated if the features of the tissue it is substituting as well as the tissue it is to be in contact with are understood.
The mechanical properties of tissues differ with age and varying conditions, some of which taken into account are biological and environmental factors. Additionally, a majority of tissues are composite and viscoelastic materials, which means their mechanical properties change from one point to the next.
Bones, the foundation stones of the skeletal system, are composite materials that contain both fluid and solid phases. Water, which can be found in the organic matrix or in canals and cavities, accounts for up to a quarter of the total bone weight. Solid phases grant bones their hard structure as well as flexibility and resilience.
Bones are able to self-repair and change their shape, and their mechanical properties are dependent upon changes that the body experiences. It is also vital to register that the composition, and in consequence, specific characteristics of bone, vary by age, sex, types of bone and bone tissue, and additional factors.
There are a large amount of terms in use to classify bone. This article will briefly examine the two tissues that bones consist of: cortical or compact bone tissue, and cancellous or spongy bone tissue. The dense cortical bone tissue is linearly elastic and forms the outer cortex of bones and the diaphysis region. The fibrous layer that covers every bone (with the exception of joint surfaces) is named periosteum. Cancellous bone tissue is the non-uniform mesh structure enclosed by the cortical bone.
Bones are detected as either cortical or cancellous, which may depend on the level of porosity and organization. Although the level of porosity can also transform with age or with disease and altered loading, when distinguishing between cancellous and cortical bones, cancellous bone will frequently have higher porosity in comparison to the cortical bone.
To analyze the material properties of bone in an appropriate manner, its mineral content needs to be considered. Bones demonstrate a higher ultimate tensile strength (UTS) and Modulus of Elasticity if they have higher mineralization. Contrarily, higher mineralization will frequently reduce toughness. Cortical bone demonstrates higher Modulus of Elasticity in comparison to cancellous bone and has the most desired characteristics for torque resistance. Cancellous bone, to contrast, has a greater capacity to store energy, thus it can withstand much greater strains before failing in addition to being able to resist high compression and shear forces.
Bone shows different mechanical properties in its various regions and in various directions. The following graph illustrates the stress-strain behaviors tested in four different directions.
Stress strain curves at different testing directions
Due to the anisotropic behavior mentioned previously, most frequently mechanical behaviors such as elastic behavior, creep behavior, and strain rate sensitivity are analyzed for each specific bone type with identical microstructure and identical environmental conditions. The following graph is a usual stress-strain curve of generic bone tissue.
A typical stress strain curve
The elastic region of a material’s stress-strain curve is the region in which the material will continue to revert to its base state if the applied load is taken away. The portion from the beginning of the curve up until a material’s yield point is representative of a elastic region in a material. Beyond the yield point, where the characteristics of bone may be irreversibly changed, plastic deformation begins to happen. The final strength is indicative of the greatest amount of force the bone can resist before rupture. The characteristic of bone and skeletal structure as well as the direction of the applied load decide whether fracture will occur.
Biomechanics to keep in mind for medical devices include the ability of an implant to resist compressive, tensile, and shear forces, variations and degrees of freedom, and the device’s mechanical properties such as elastic modulus, yield strength, and elongation to failure. Materials like metals and alloys, ceramics, and polymers are of use in developing medical devices.
Metallic implants have an organized, 3D crystalline structure and are predominantly used for loading bearing (for example hip and shoulder implants, fixation devices). Dependent upon the properties of the metal selected, highly-reactive metallic surfaces which counter surrounding tissues frequently need additional metallic treatments or, if it is permissible, the use of other biomaterials on the outer surface.
Ceramics, alternatively, are non-metallic and non-organic, their compressive strength is amongst the greatest, but they demonstrate poor tensile properties. The most typical use of ceramics is in dental implants. Polymers are organic materials which consist of repeated units. Their benefits include the controlled degradation rates and ease of manufacture.
Polymers can also be granted a specific shape for the specific application. From a mechanical standpoint, polymeric materials have different degradation mechanisms and will frequently demonstrate wear debris and will fatigue under constant loading.
Stress strain curves of different materials
Time-dependent features of the behavior of materials can be analyzed running creep and recovery tests, stress relaxation, and dynamic tests.
Creep and recovery tests can be performed with the application of a certain amount load and maintaining that load, consistently, for a specified amount of time, then suddenly taking it away to observe the response of the material in question. This phenomenon happens in tissues or, medical devices that substitute such tissues, are subjected to a constant force, and over time, the tissues lengthen.
Stress relaxation testing involves the material to be strained up to a point and stress response to be examined while strain is held constant. Contrasting with creep, stress-relaxation happens in tissues that are stretched and held at a fixed length.
Dynamic oscillatory testing applies a harmonic stress to facilitate the measuring of the material strain response.
Note: When a force-deformation test is run on biological tissues, the loading and unloading curves will not be identical because of the energy transferred to friction. This phenomenon, hysteresis, can be analyzed when dynamic mechanical testing is being run.
The standards of testing for medical devices are frequently designed for the specific medical device that will undergo testing, this is due to the fact that desired biomechanical properties per material and per application vary.
In the United States, Food and Drug Administration (FDA) categorizes medical devices into Class I, II, or III on the basis of their risks and the regulatory controls required to give a reasonable assurance of effectiveness and safety. Depending on the product being designed, the manufacturers of medical device equipment will adhere to FDA CFR 21 part 11 obligations.
ADMET’s MTESTQuattro controller software aids in meeting the requirements of FDA CFR 21 Part 11 by generating a documentation trail whenever there are modifications to test procedures. These changes lead to the creation of a record which gives an indication of what was modified, when, and the persons doing so.
MTESTQuattro also enables CFR 21 Part 11 administrators to reduce the access users have within the program on an individual basis. This makes sure that the way in which tests are conducted, i.e. test procedures, can only be changed by an authorized. MTESTQuattro gives the ability to export data in a secure format for inclusion in CFR Part 11 submissions.
Medical devices and equipment that will be tested include:
- Artificial Hip Implant Prosthesis
- Artificial Shoulder Implant Prosthesis
- Metallic Bone Plates and Fixation Devices
- Metallic Bone Screws
- Intramedullary Rods
- Dental Implants
- Luer Lock/Luer Taper Fittings
- Spinal Implant Constructs
- Vascular Stents
Featured Configurations for Biomechanics Testing
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This information has been sourced, reviewed and adapted from materials provided by Admet, Inc. – Materials Testing Equipment.
For more information on this source, please visit Admet, Inc. – Materials Testing Equipment.