Good impression

Technique adapted from biotechnology research cuts testing of mechanical properties in materials from weeks to hours. Stuart Nathan reports.

Miniaturisation is a continuing trend in many areas of technology, but researchers at the Massachusetts Institute of Technology have found that making things smaller can also make them faster. Testing the mechanical properties of materials is a time-consuming task, but according to the MIT team, reducing the size of the samples cuts the time needed to screen a large number of materials from weeks to days.

The technique grew out of biotechnology research. Looking for a material to support the growth of human stem cells, chemical engineers Daniel Anderson and Robert Langer made what’s known as a combinatorial array of compounds —very small quantities of related materials, deposited using automated equipment on to a glass slide.

Their research caught the eye of materials scientist Krystyn Van Vliet, whose laboratory specialises in how the surface features of a material affect the way it supports cell growth.

One of the most important parameters for this is the surface hardness, which Van Vliet tests using a nanoindenter. Analoguous to the standard indentation test for materials, nanoindentation provides the same information about properties such as elasticity and hardness, but requires much smaller amounts of material.

The spots Anderson and Langer had made for their research were the right size for nanoindentation testing, said Van Vliet, and this gave her the idea that a similar system might be able to screen a large number of minute material samples in a very short time.

Van Vliet’s team created a new combinatorial array, consisting of spots of some 1,700 different polymers. ‘Each dot was a combination of two different monomers, or building blocks, so we could map out the effects of the percentage of each monomer on the properties of the material,’ she said.

Testing using the nanoindenter gave a measurement of the stiffness of all of these monomers within 24 hours.

Using traditional techniques, which require the synthesis of a large bulk mass of the polymers, sample preparation, and individual sample testing, it would have taken many weeks to test every combination, Van Vliet said. Because of this, development of new materials has tended to be a long drawn-out process, depending on calculation of the exact properties needed for a particular application and careful synthesis of a few samples. But using the combinatorial approach and nano-scale testing, this could change considerably.

‘Instead of trying to engineer perfect materials, let’s make thousands at the smallest scale we can, and see if we can find some materials with unexpected or interesting properties,’ Van Vliet said.