Investigating Enamel Nanostructure with Nanoindentation

Thought LeadersDr. Shahrouz AminiMax Planck Institute of Colloids and InterfaceDepartment of Biomaterials

An interview with Dr. S Amini, conducted by Jake Wilkinson, MSc

Why are you interested in biomaterials?

As a teenager, I was amazed by nature and wondered why people lost their teeth permanently, while sharks regenerated their teeth regularly? These questions were always on my mind.

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My primary interest was solid mechanics. During my master’s program, I got familiar with biomaterials, and started a project with the School of Dentistry on tooth restorative materials. The interest in biological materials further developed during my Ph.D. program in Biological and Biomimetic Materials Laboratory (BBML) at Nanyang Technological University, Singapore.

My project was mainly focused on biological hard tissues. Lately, I moved to the Max Planck Institute of Colloids and Interfaces, Department of Biomaterials, where I have the chance to closely collaborate with our different research groups and explore different biological models. Currently, I am working on structural properties and mechanical responses of damage-resistance biological models, such as shark teeth enameloid and human teeth enamel.

How do synthetic composites compare to the properties of teeth?

Minerals and proteins, the building blocks of the natural enamel, entangled in complex shapes to display the properties that go far greater than their individual properties. These hierarchical structures, which are composed of available and simple building blocks, are far beyond anything that we can produce synthetically. In addition, we are not able to mimic the gradients in synthetic materials that we can see in biological materials, such as the human tooth. These gradients have a crucial role in mechanical response and performance of the biological models.

We can now artificially use ceramic or polymer composites to repair our teeth, but we never regain the original properties since we induce artificial interfaces and replace the graded structure of the tooth with a homogenous material with mismatched properties. Nature has adopted complex design strategies to achieve high-performance biological composites that overcome harsh external stimuli, while hindering stress-mismatch failure occurring during cyclic loading.

How have you been using nanoindentation to further your understanding of biomaterials?

Mechanical characterization of materials dates back a few hundred years. The diverse studies resulted in a comprehensive data base for mechanical response of materials. However, due to technological limitations, engineers were not able to characterize the mechanical response of the microstructures to differentiate the behaviour of the building blocks, or the role of their spatial arrangements.

Thanks to recent advances in mechanical characterization techniques, such as atomic force microscopy and nanoindentation, we are now able to characterize the microstructural features of the materials. These advances allow us to see the role of micro- and nanostructures on the mechanical response of the overall structures. For example, we can understand how presence of minerals and their arrangement can affect the hardness or toughness of the samples.

What imaging techniques do you use alongside nanoindentation as part of your research?

I have used different imaging techniques, such as optical and electron microscopy, micro CT imaging, and Raman spectroscopic imaging, alongside my nanoindentation studies. Combining these techniques, we are able to correlate the structural and mechanical properties of the samples.

How does performing a nanoindentation experiment on biological material compare to conventional nanoindentation experiments?

There are lots of studies surrounding homogenous, inorganic or polymeric materials. Therefore, most of the testing methodologies are developed for these materials.

Conversely, organic phases are sensitive to humidity and temperature, and they can simply denature if not stored and prepared properly. As a result, specific protocols are needed to make sure that the samples keep their native characteristics.

Why is the use of an environmental chamber important for your research?

We frequently use humidity chambers so that the extracted mechanical properties are not affected by dehydration. The effect of dehydration depends on degree of mineralization of the samples. For example, if it's a highly mineralized sample, dehydration does not significantly affect the mechanical properties. However, for soft samples that are highly organic, like skin and muscle, then environmental control (mainly humidity) is necessary. Tissues need to be tested at their functional environment; otherwise, they won’t have the same mechanical response.

Where do you expect our understanding of biomaterials to take us?

Our understanding of biomaterials continues to develop. The systems we can use to measure them are becoming increasingly sophisticated, empowering us to discover more and more about their structure and behaviour. My field is based on the characterisation of these materials not producing them, although the breakthroughs we make feed directly to the material engineers who use this information to create increasingly powerful materials.

New engineering techniques are bringing us closer to creating biomimetic materials, which will be able to match those we see in the natural world. For example, 3D printing will allow us to emulate the complex 3D or interwoven structure seen in biological models that results in its amazing properties. We will be able to accurately replicate the patterns we see in biological materials and create engineering materials that accurately mimic them. However, we are still limited by size and materials.

What value do you see in expert-led conferences such as Nanobrüken?

From my personal experience, even with the access through the internet and journals, scientific groups with different backgrounds all around the world are not connected in the way they should be. Having these interdisciplinary seminars and discussions, scientists can come up with new ideas and innovative solutions for their own research. Furthermore, you get some cases of a team working to design or develop a test method for their research; however, a similar method that has already been applied for years in a different field can be customized for your own research.

Conferences focusing on techniques instead of research area, like Nanobrüken, are helpful, because no matter what your background, you can attend and get new ideas in other fields. It facilitates the sharing of ideas and helps us all perform our research more effectively.

About Dr. Shahrouz Amini

Dr Amini is a post-doctoral researcher at the Max Planck Institute of Colloids and Interface, Department of Biomaterials (Supervisor: Prof. Peter Fratzl). With his research background on the mechanical characterization and properties of biological materials, Dr Amini is conducting studies on damage tolerant biological models such as tooth enamel.

Shahrouz received his PhD degree from Nanyang Technological University, Department of Materials Science and Engineering (Advisor: Prof. Ali Miserez). In 2016, he was awarded a “Research Excellence Award” for his PhD work on mantis shrimp dactyl club and its toughening strategies.

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