In current biomedical research, the classification of the mechanical properties of biological materials is becoming significantly important. With recent advancements in biomedical engineering science, innovative materials have been developed for replacing human tissues.
In order to develop such new tissues, factors like lifespan, resistance to inflammations, biocompatibility, and mechanical properties of these tissues need to be evaluated. Hence, a technique is required that allows one to examine the mechanical properties of these novel materials. Such a technique can also prove useful in a number of ways. For instance, infected tissues and cells tend to modify their stiffness with respect to healthy ones.
Nevertheless, tissues are a complex class of materials since they contain complicated structures. But, when compared to commonly tested materials, tissues are usually much softer. A technique that can help in determining the mechanical characteristics of such soft materials can therefore prove advantageous.
Instrumented Indentation Systems
Figure 1. Ultra Nanoindentation Tester on a Compact Platform (a) and Table Top Nano Indentation Tester (b).
Instrumented indentation provides a simple way of determining the mechanical properties of a variety of materials and eliminates the need for complicated sample preparation. Anton Paar offers instrumented indentation systems such as the Table Top NHT system and the Ultra Nanoindentation Tester (UNHT), which are compact and can be easily set up in medical research centers. These instruments provide easy yet comprehensible output. Even inexperienced users can understand the measurements results delivered by these instruments.
For over two decades, the instrumented indentation technique has been widely used for many types of materials. The special active surface referencing principle combined with the use of sophisticated materials and electronics almost removes the issue of frame compliance and thermal drift.
The UNHT system employs two individual force and displacement sensors and easily identifies the adhesion effects, which are usually seen in very soft materials. However, it remains unclear whether today’s nanoindentation systems can be used for indentation of biological tissues and other types of soft gels. During a joint project between ETHZ and Anton Paar, significant problems were encountered when devising the indentation technique for these types of materials.
The concept of applying instrumented indentation on very soft materials was fairly new and common methodology was not available at that time. Hence, it was decided to begin the indentation process on soft elastomers, since substantial experience has been acquired in this area.
Figure 2. Flowchart of the experimental work for development of a dedicated methodology for indentation of extremely soft materials.
Indentation of Soft Elastomers
For the indentation experiments on elastomers, three types of Vishay photoelastic sheets were utilized that ranged from hard to soft. With the help of the UNHT system, measurements were carried out using spherical and Berkovich indenters in load controlled mode.
The indentations were done at various loading rates with a 120 second hold at the maximum load. The rationale behind this experiment was to optimize the indentation parameters, like the radius of the indenter, feedback control parameters, and the applied loads and loading rates.
Figure 3. Typical load-displacement plots for the three types of Vishay photoelastic sheets.
The above figure shows the standard indentation force-displacement curves for the three tested Vishay elastomers. Using the Oliver & Pharr method, the elastic modulus was determined and was found to match with the manufacturer's data, while the elastic modulus was overestimated for the softer elastomers. This reason was attributed to the visco-plastic properties of the elastomers, which resulted in higher values of elastic modulus for the softer samples.
Indentation of Gels
For indentation of gels, the measurements were carried out on three types of decorative gels that had similar characteristics to gels used for replacing biological tissues. The initial experiments on gels helped in optimizing the feedback control parameters and establishing the optimal indentation process.
Although the UNHT system feedback control was optimized for a wide range of materials, it had to be configured for very soft materials. Hence, the optimization of the feedback control parameters for indentation on soft gels was an important step for characterization of gels by instrumented indentation.
Adhesion Effect and Determination of Contact Point
Adhesion effect is a major issue that is normally encountered during the indentation of gels and other soft biological tissue. The fact that the entire indentation information, including the pull-on and pull-off adhesion is recorded, allows users to decide where the contact point will be defined.
The above experiments show that instrumented indentation is a suitable technique for determining very soft gels, which exhibit similar structure and properties of biological tissues.
This information has been sourced, reviewed and adapted from materials provided by Anton Par.
For more information on this source, please visit Anton Paar.