How to Measure the Physical Properties of Medical Devices

Regenerative Medicine research is poised to change the way we know health care. This cutting-edge technology demands new tools and solutions that are able to keep up with the development in personalised medicine. Texture Analysis, which has long been used by the pharmaceutical, food and cosmetic industries, is increasingly being used to evaluate properties of medical devices.

Texture Analysis refers to calculating properties such as firmness, adhesiveness, compressive strength, swelling/absorption, in addition to relaxation behaviour, fatigue and brittleness. Regulatory agencies, including the FDA, have used the TA.XTplus Texture Analyser to evaluate medical devices. In addition, the patent literature increasingly relies on texture analysis to validate claims.

In its most basic configuration, the instrument carries loadcells ranging from 500 g to 750 kg which provide the force measurement for compression or tensile testing of samples. The arm of the instrument moves up or down at speeds from 0.01-40 mm/s collecting data at very precise resolution – at up to 2000 points per second.

The mechanical function of medical devices and engineered tissues is a primary endpoint for the successful regeneration of many biological tissues. It is critical that the mechanical function be characterised and compared to initial benchmark function to determine quality control standards for the next generation of health care.

Typical Texture Analysis Tests for Medical Device Products

How to Measure Actuation Force of Metered Dose Inhalers

The metered-dose inhaler (MDI) delivers a precise, reproducible dose of drug accurately to the deep lung. It is also well accepted by patients who depend on MDI’s in their treatment and therefore it is essential to constantly improve this technology. Patients need to rely on their inhaler to provide them with the required medication which is often at a crucial time when physical strength may be very low.

The metering valve, whose performance is assessed with this test, is a critical component of a finished metered-dose inhaler. The interaction between the elastomeric components and the formulation can significantly affect product performance. A change in formulation, for example implementing a new propellant system, may necessitate a complete redesign of the valve system in order to successfully deliver the drug with the minimum of difficulty.

The Inhaler Support Rig provides a holding device for the inhaler so that it can be tested with a hemispherical probe which best mimics the action of the finger for drug delivery. The force required to push down on the inhaler to deliver the drug is measured. This assesses the performance of the metering valve.

Metered Dose Inhaler test and typical comparative graphs

Metered Dose Inhaler test and typical comparative graphs

How to Measure Syringeability

An assessment of syringeability is required to assess the potential effects of formulations and their subsequent mechanical consequences to reach an optimal performance. The syringeability of each formulation is determined using a Universal Syringe Rig on a Texture Analyser. This innovative attachment effectively measures the aspiration and extraction forces of syringe pistons. The extraction force quantifies how easily the injectable material is expressed during syringe depression and material discharge. This measured force impacts patient comfort during injection and affects how the material is received when deposited under the skin. This rig also allows manufacturers to determine the force required to withdraw the plunger from the syringe. The method enables syringe manufacturers and pharmaceutical companies to precisely quantify the performance of their products so that they are able to optimise usability and patient comfort, while guaranteeing quality control and product safety. The universal design allows a wide range of syringe types and sizes (from 5-50mm diameter) to be supported securely for testing.

Formulations are transferred into identical plastic syringes to a constant height. The content of each syringe is fully expressed using the Texture Analyser in compression mode and the resistance to expression is determined from the area under the resultant force-time plot. Increased work of syringeability is denoted by increased areas under the curves.

Universal Syringe Test Rig and assessment of force to expel syringe contents

Universal Syringe Test Rig and assessment of force to expel syringe contents

How to Measure Hypodermic Needle Sharpness

Needle sharpness, which is an important characteristic for patient comfort, may be determined by a puncture test where the needle is held by a Tensile Grip on the Texture Analyser and is pushed through a standard substrate. The force required for insertion and extraction is measured.

Hypodermic Needle Testing and typical comparative graphs

Hypodermic Needle Testing and typical comparative graphs

How to Measure Stent Mechanical Strength

Due to the challenging service environment that stents face in vivo, the radial stiffness of a stent is critical to its function of keeping the arteries open and eliminating abrupt closure or restenosis. Restenosis has been correlated with geometric properties of stents, such as the number of struts, the strut width and thickness, and the geometry of the cross section of each strut. A large number of stents with different geometric and mechanical features are available on the market. The therapeutic efficacy of stents depends largely on their mechanical properties, thereby influencing the choice of stents for treating specific tissues.

By performing texture analysis tests on the stents, researchers can improve design and performance. A compression test using a cylinder probe is the most common test to measure structural integrity.

The stent is positioned centrally under a cylinder probe and compressed to a chosen distance. During the test the force is shown to increase gradually as compression continues. The distances at 3 force values are recorded as measures of increasing stiffness.

Stent compression test and typical comparative graphs

Stent compression test and typical comparative graphs

For a full summary of typical texture analysis tests that can be performed on medical device products:

This information has been sourced, reviewed and adapted from materials provided by Stable Micro Systems Ltd.

For more information on this source, please visit Stable Micro Systems Ltd.

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