A Guide to Mechanical Testing of Medical Devices

Your Ultimate Guide to Mechanical Testing of Medical Devices

Welcome to ADMET’s guide on Mechanical Testing of Medical Devices. In this comprehensive resource, we delve into the critical importance of rigorous mechanical tests for ensuring the safety and efficacy of a wide range of medical products, from orthopedic implants and tissue engineering to medical adhesives and catheters. Learn how our state-of-the-art testing systems, compliant with ASTM and ISO standards and FDA 21 CFR part 11, provide unmatched reliability in evaluating these life-essential devices.

What are Medical Devices?

The definition of a medical device as defined by the Global Harmonization Task Force is any instrument, apparatus, implement, machine, appliance, implant, in vitro reagent, software, material or other similar or related article that does not achieve its primary intended action in or on the human body solely by pharmacological, immunological or metabolic means and that is intended for human beings for:

• the diagnosis, prevention, monitoring, treatment or alleviation of disease;
• the diagnosis, prevention, monitoring, treatment or alleviation of, or compensation for an injury;
• the investigation, replacement, modification, or support of the anatomy or of a physiological process;
• supporting or sustaining life;
• controlling conception;
• disinfecting medical devices; and
• providing information for medical or diagnostic purposes by means of in vitro examination of specimens derived from the human body.

What Medical Devices Require Mechanical Testing?

Implantable Medical Devices

An implantable device or a prosthesis is a medical device that is supposed to replace a missing anatomical structure, function as a human organ, support a biological structure, or enhance the functionality of an organ, and which does not solely attain its functions by means of pharmacological, immunological, or metabolic means.  Some implants are also used for diagnostic purposes.  These devices are called implantable due to the fact that they are introduced into the human body partially or fully, surgically or by other means for a short or long period or for a lifetime. 

Orthopedic Implants

Musculoskeletal diseases including trauma create a need for biomedical implants to reconstruct bone and its associated soft tissues. With the increased activity of an aging population, the number of orthopedic devices being implanted worldwide is continuing to climb. These orthopedic implants include devices for fracture fixation, joint replacement, tumor reconstruction, soft tissue repair; and fusion, reconstruction, or stabilization of the spine.

Mechanical testing of these orthopedic implants may involve measuring implant rigidity, testing how many cycles it takes until it breaks, or how the implant influences the rest of the body around it. Regardless of the circumstance, it is important to recognize that the way in which an implant is tested should always attempt to represent the way in which it is mechanically loaded in the body.

Tissue Engineering

The loss or failure of an organ or tissue is one of the most frequent, devastating, and costly problems in human health care.  Tissue engineering applies the principles of biology and engineering to the development of functional substitutes for damaged tissue.  Scaffolds to support the constructive remodeling of injured or missing tissues or organs can be composed of synthetic or naturally occurring materials and can be degradable or nondegradable.  These scaffolds are also engineered to have specific mechanical or material properties that closely resemble those of the tissue replaced.  Ultimately, the scaffold should facilitate the attachment, migration, and proliferation of the cell population required for structural and functional replacement of the target organ or tissue.  When implanted into the body, the scaffold will be subjected to in vivo physiological stresses and strains.  Thus, it is necessary to develop scaffolds that have mechanical properties similar to those of the native tissues.

Mechanobiology

Biomechanics is the science of the movement of a living body, including how muscles, bones, tendons, and ligaments work together to move. The European Society of Biomechanics defines biomechanics as “the study of forces acting on and generated within a body and the effects of these forces on the tissues, fluid or materials used for the diagnosis, treatment or research purposes.”

Mechanobiology is the opposite and studies the mechanisms by which cells sense and respond to mechanical stimuli. It focuses on how physical forces and changes in the mechanical properties of cells and tissues contribute to development, cell differentiation, physiology, and disease. By understanding how mechanical forces induce changes at the molecular, cellular, and tissue levels, mechanobiology provides insights into tissue physiology, disease development, and relevant therapeutic strategies. This field is leading to advancements in constructing engineered tissues and organs, repairing and regenerating damaged tissue and providing therapy for diseases.

Medical Adhesives

In biomedicine, the performance of adhesives for hard and soft tissues in the presence of fluids is crucial. Medical adhesives require strong wet adhesion while subjected to various physiological conditions. Bone and tooth cement are hard tissue adhesives. In joint replacement surgery, the bone cement fills and localizes in the space between the implant and the bone, thereby acting as an implant fixation and transferring the mechanical load from the implant to the bone. In dental applications, restoratives are adhered to tooth enamel and dentin. The reliability of the bond over a long period of time is key in hard tissue adhesives. Tests are performed in tension and shear to measure the mechanical properties of the hard tissue adhesive immediately after application, after 24 hours, and after several months of being stored in a fluid at 37C.

Soft tissue adhesives are used for transitory or short-term purposes, where they can be removed or degraded when wound healing has progressed sufficiently. For the integration of the adhesive with soft tissues that are surrounded by wet tissue fluid or blood, the adhesive needs to be spread easily on the surface and show effective wet adhesion in an adequate working time. Before curing, all adhesives are low-viscosity liquids that can efficiently flow into the pores of, in this case, biological tissue. After the polymerization phase, the cohesiveness of the adhesive increases, and the interface between the adhesive and the tissue is altered mechanically by interlocking of the adhesive with the porous tissue surface. Overall, the strength of the cured adhesive joint is the result of a balance between the cohesiveness of the adhesive and its adhesiveness to the tissue. The mechanical strength of a tissue adhesive is an important parameter in its overall effectiveness. Tissue samples bonded by an adhesive are placed in a fluid bath at 37C and the mechanical properties of the tissue/adhesive bond under tensile, shear, and peel forces are determined.

Patch-type or tape adhesives are used in medicine owing to their advantages, including reduced operation times and enhanced tissue handling in a large area. Patch or tape adhesive removal causes injuries that range in severity from skin irritation to permanent facial scarring or restriction of motion. A patch or tape that provides secure fixation with quick, easy, damage-free removal is a longstanding need. Tapes with backing adhesives that possess high shear strength and low peel force are key to providing proper device fixation and injury-free removal.

Catheters and Tubing

Catheters are used for cardiovascular, urological, gastrointestinal, neurovascular, and ophthalmic applications. They may be left inside the body either temporarily or permanently to allow for drainage, drug administration, or access by surgical instruments. Accordingly, catheters may be inserted into vessels, skin tissue, body cavities or the brain. Thus, depending on their use and intended location in the body, their tubes vary in diameter and stiffness and come with or without balloons that hold them in place.

Breakage or separation of any catheter can be catastrophic. Polyurethane material degradation during in vivo may cause deterioration of the mechanical properties of medical catheters. The best way to ensure that catheters are safe, retain their quality, and perform successfully is to subject them to a series of tests to measure their mechanical properties and durability.

Catheter mechanical tests include:

Ballon Cycle Fatigue – Balloons must withstand multiple inflations during clinical use to avoid inducing device failure or vessel damage.

Ballon Rated Burst Pressure – Measuring the maximum pressure a balloon can withstand before bursting is known as the rated burst pressure (RBP).

Flexibility/Kink Resistance – Measuring the ability of the device to bend in order to accommodate a predetermined clinically relevant radius or angle it will be required to negotiate during access and delivery.

Coating Friction Test – Lubricity/Pinch Test – The purpose of this test is to assess the lubricity and durability of coatings applied to catheters. The most common test used for measuring surface coating friction is a pinch test, where catheters are pinched between two pads with a known normal/pinching force while using a linear actuator to pull and push the catheter through the pads. Cycling the catheter between the pads multiple times will result in increasing friction readings due to coating degradation.

Peak Tensile Force / Tensile Bond Strength – Measuring the maximum force during uniaxial tensile tests to determine the bond strength at locations where adhesives, thermal fusion, or other joining methods are used for bonding components of the delivery system.

Three-Point Bending – The rigidity of a catheter is measured using a three-point bend test. The sample rests on two lower supports and a force is applied at a constant displacement rate midway between the supports. The test stops when the displacement reaches 0.2 x span length.

Torque Strength – Rotate the proximal end of the catheter while the distal end is fixed and routed through a tortuous path simulating anatomical conditions. Record the number of turns to failure.

Torsional Bond Strength – Determine the torque required to cause failure of the joints and/or fixed connections in the catheter system.

Medical Fluid Connectors or Luer Fittings

The luer fitting or luer-lock fitting is a commonly used connector in the medical industry. They are used to connect two medical devices in a liquid-leak-proof and mechanically secure manner. Applications for these male and female tapered, interlocking fittings include but are not limited to, syringes, needles, stopcocks, IV sets, and diagnostic and therapeutic catheters. The rigid connectors are available in a range of metals and thermoplastics, the selection of which is determined by use.

Mechanical tests are used to assess the mechanical properties of these devices when forces in the form of axial loads, unscrewing, and overriding are applied. Leakage tests are used to assess whether or not air/water is allowed to pass between the connection of a reference conical fitting and the small bore connector under test when subjected to either positive, negative, or vacuum pressures. Stress cracking, a test where samples are connected, conditioned, and then examined for cracks and leakage is also performed.