Pharmaceutical tablets show viscoelastic stress-strain behaviour when the powder compact is being loaded and unloaded, and this behaviour continues once the load has been fully removed. This can cause problems for tablet manufacturers such as capping, when either the upper or lower part of the tablet separates from the main body when ejected from the press or during the handling process.
Viscoelastic properties are functions of the compression conditions as well as the formulation. It is important to investigate every instance of capping to ensure the problem is not repeated (which is very costly) and to measure the viscoelastic properties of new formulations before they reach the mass manufacturing stage.
Measuring Viscoelasticity
The High Tolerance Powder Compaction Rig allows powders to be compacted to high forces, simulating the tablet forming process. Relaxation behaviour at every stage is easily investigated using this rig. As the tablet can relax in both the axial and radial directions, this can cause difficulties when ejecting the tablet from the die. After compression in the compaction rig, this exact process is replicated and the ejection force recorded.The apparent failure viscosity and toughness of compacted tablets can also be calculated from deformation and failure measurements under diametral loading after tableting. This requires a simple flat probe compression, but some thought must be given to the loading conditions and hold time of this type of test. If there is a split present in the tablet to enable it to be snapped in two, it must be aligned in the same orientation for each sample.
Almost all biological materials are viscoelastic because their constituents also have viscoelastic behaviour. Their viscoelastic mechanical properties are generally important in their characteristic functions. These softer materials are often measured using indentation or compression, although many biological materials are in sheet form. In these cases, puncture is an ideal test technique. The Film Support Rig holds a sample around a circular aperture allowing a spherical probe to put it into biaxial tension, allowing relaxation properties to be investigated.
 |
| Tests for Viscoelastic behaviour – left to right: High Tolerance Powder Compaction Rig; Film Support Rig; Testing a Gel sample; Tensile Grips for testing Film Tensile Strength |
Skin is one such example of a film sample. The viscoelastic properties of human skin are critical to its protective function, surgery, dermatology, impact biomechanics and forensic science. Samples of cadaver skin are often the subject of mechanical testing in medical research, but synthetic analogues are also widely used in the cosmetics and personal care industries. Both sample types are suitable for testing under biaxial tension.
Hydrogels are almost always viscoelastic and this property is important to their function as uses in the human body. They are highly absorbent polymer networks and due to this high water content they have a flexibility very similar to bodily tissues. Anything that is designed to be incorporated into the human body (tissue scaffolds, implants, drug delivery systems) must have its mechanical properties fully researched. Apart from rheology measurements and dynamic measurements (which can be carried out using a Texture Analyser, but are more difficult and time-consuming), simple static measurements can easily be carried out on freestanding Gel samples, loading to a set force or distance and observing the force or distance relaxation over time.
Compression bandages are often used to help with recovery after surgery, muscular injuries and ulcers. They are made up of viscoelastic fibrous materials. Understanding the stress relaxation behaviour of these bandages is useful in determining the pressure exerted on limbs while worn by a patient when the bandage is applied to different strains, and how that pressure changes with time. The viscoelastic behaviour of a textile material depends on the material type, the applied tension and the tightness of the structure, and these factors could play a prominent role in interface pressure variation over time.
Bandages can easily be cut to form straight sided or dog-bone shaped samples suitable for uniaxial tensile testing. The range of Tensile Grips by Stable Micro Systems allows for many different sample types to be securely gripped under tension.
There is a Texture Analysis test for virtually any physical property. Contact Stable Micro Systems today to learn more about our full range of solutions.
For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.
The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is
available to measure and analyse the textural properties of a huge range of
food products. Our technical experts
can also custom design instrument fixtures according to individual
specifications.No-one understands texture analysis like we do!
To discuss your specific test requirements, click here...
 |  |  |
No comments:
Post a Comment