Hydrogels, shown in Figure 1, are materials that are extremely soft and have a high liquid content. These materials have been recently used in numerous areas of clinical and biological research, for example from tissue regeneration and osteoporosis through to hemorrhage control. There are many hydrogels that are believed to be promising candidates for regeneration or replacement of many different types of tissues or even as growth substrates for other types of soft tissues present in the human body. An in-depth knowledge of the mechanical properties of hydrogels is therefore required for appropriate and effective application of these materials for specific growth, cellular regeneration and tissue replacement1,2. For instance, the mechanical and structural properties of the growth substrate can serve as a biomechanical modulator of cellular behavior and thus, determine the quality and function of the growing cell. Further, it has been found that homeostasis of tissues can be largely influenced by the elasticity of the hydrogel substrate3. This process is critical for efficient regeneration of tissues. Measurement of elastic – and in general term mechanical – properties of hydrogels used in biomedicine are therefore extremely important.
Figure 1. Typical hydrogel samples in a Petri dish.
This article presents the results of measurements of creep and mechanical properties of different types of soft polyacrylamide hydrogels using a new nanoindentation device for bioindentation known as Bioindenter (BHT). The Bioindenter has been specifically developed for use in the biomechanical domain which typically needs different sample handling and testing conditions when compared to ‘traditional’ hard materials.
Based on the successful Ultra Nanoindentation Tester (UNHT), the nanoindentation device leverages its exceptional high resolution and thermal stability in both displacement and force measurements. There is a need for normal loads in micronewtons and displacements in tens of micrometers because the biological materials are very soft, and at the same time good thermal stability is critical to determine the materials’ creep properties. The indentation process has been adapted and established for automated testing in liquids as well as testing of samples with uneven surfaces. These indentation procedures enable the measurement of both elastic and time-dependent properties of biomedical and biological samples.
Polyacrylamide (PAAm) hydrogels have been considered for the purpose of this analysis, as they are a common growth substrate employed in many biological laboratories for tissue replacement or cell cultivation. The mechanical properties of PAAm hydrogels can be easily customized to achieve elastic moduli ranging between ~10 kPa and up to ~200 kPa, and thus they are considered to be suitable candidates for demonstrating the bioindentation procedures over a wide range of elastic properties.
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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.
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