Nature inspires new methods of making porous materials

The phenomenon of porosity gradation — where materials show variations in their underlying structural cavities — is found throughout nature in tree trunks, bones and beehives among other structures.

It allows for maximum strength-to-weight ratio and the efficient transferring of forces in order to minimise stresses and avoid whole structure failure.

For instance, a crack in the branch of a tree will not lead to the felling of the tree in the same way that a broken ankle will not lead to collapse of the whole leg.

Dr Carmen Torres-Sanchez of the Department of Mechanical Engineering at Heriot-Watt University, seeks to study these instances of porosity gradation in nature in order to develop better artificial materials.

‘We mechanical engineers are obsessed with how solid, homogenous materials behave, using concepts such as Young’s modulus and bulk modulus — but when you look at nature, there’s nothing that is really homogenous out there,’ Torres-Sanchez told The Engineer.

Current manufacturing methods for making porous materials, however, cannot easily mass produce graduated, heterogeneous structures.

Torres-Sanchez, along with colleagues at Strathclyde University, is experimenting with low-frequency ultrasonic irradiation to ‘excite’ molten polymers as they begin to foam. Once the material solidifies, this effectively traps different porosity distributions throughout the solid matrix.

The approach has allowed the team to generate polymeric foams with porosity gradients closely resembling natural cellular structures such as bones and wood. The technology opens up new opportunities in the design and manufacture of bio-mimetic materials that can solve challenging technological problems. This might include bridge building and construction in earthquake zones, improved vehicle and aircraft efficiency and even longer-lasting more biocompatible medical prosthetics.

‘There are different ways of achieving it, but at the end of the day what we want to achieve is to tailor mechanical properties for a specific purpose — for example, preferential bending or to mimic bone for better prosthetics,’ Torres-Sanchez said.