Miniscule diatoms boast massive strength

 

Researchers from Caltech, US, have identified diatom shells as the 'strongest' biological material reported ever, following mechanical testing and electron microscopy studies on these so-called frustles.

 

Analysis reveals the structures to be light-weight and strong with exceptional structural integrity that could provide insight into evolutionary design.

 

Diatoms are unicellular algae housed within a lightweight protective silica shell perforated with pores.

 

As Professor Julia Greer from Materials Science and Mechanics at Caltech points out: "Silica is a strong but brittle material; when you drop [it], it shatters."

 

"But architecting this material into the complex design of these diatom shells creates a structure that is resilient to damage," she adds.

 

 

To test the specific strength of the structures, Greer and colleagues first carried out three-point bending experiments on 3.5 micron square cross-sectional beams taken from the frustles. 

 

Tests were performed within a custom-built in situ FEI Quanta SEM equipped with a InSEM nano-indentor; load and displacement were continuously measured as researchers captured video footage of deformation.

 

Analysis revealed that failure by brittle fracture took place at an average stress of 1.1 GPa, and the researchers showed that with a relative density of 30%, the frustle has the highest strength-to-density ratio of 1702 kNm/kg.

 

"[This specific strength] is a value well above that of other natural cellular, composite and silk materials including bamboo, the mollusc shell and spider silk," highlights Greer.

 

Microstructural analysis using a FEI Technai F30 TEM showed that the frustle is made almost entirely of amorphous silica with some local regions within its basal plate - the inner layer of the shell - displaying a nanocrystalline microstructure.

 

Further fracture analysis and finite element simulation suggested that these different regions of the shell resisted fracture differently.

 

For example, the basal plate is punctuated by reinforced pores, called foramen, which were found to deflect crack propagation. Meanwhile failure would take place at the honeycomb pattern of pores within the outer layer, or cribum layer, of the shell.

 

"The presence of pores delocalises the concentrations of stress on the structure," says Greer. "These results demonstrate the natural development of architecture in live organisms to simultaneously achieve light weight, strength and structural integrity."

 

The researchers now intend to use the design principles from diatoms to create resilient, bioinspired artificial structures.

 

Research is published in PNAS.