A team have studied the phenomenon that a material in a single piece is not always best equipped to to resist external strain and may use their findings to help create biomaterial-inspired artificial materials.
A single layer of epithelial cells stretched (left) and then released (right). On the right, cracks clearly visible along the cell junction lines. The cracks originated from the coupling of the epithelial tissue with the hydrogel substrate – Credits: L. Casares and X. Trepat, IBEC, Barcellona
Biological tissues are more likely to crack gradually in several places rather than severely in a single location, making them particularly resistant. A theoretical study conducted by a team of researchers from SISSA provides details on the basic mechanism behind this phenomenon.
The team observed this phenomenon experimentally in epithelial cell cultures, and the results of the study may take them a step closer to the creation of artificial materials that exhibit bio-inspired features. Such materials could be useful in a number of fields, including medicine. The journal Physical Review Letters has published the research findings.
Remarkably high resistance is shown by biological tissues like skin, bones, blood vessels, etc.; even when subjected to continuous deformation, stretching and bashing, they don’t break or tear. This behavior is due to their contradicting property by which they crack at multiple spots simultaneously, instead of breaking at a single or a few spots.
This new study provides a detailed explanation of this behavior. The research work is a collaborative effort between scientists at the International School for Advanced Studies (SISSA), Trieste, and the team at Polytechnic University of Catalonia.
“Strange as it sounds, a system capable of fracturing at several points is far more resistant than one that fractures in a localized fashion”, explains Alessandro Lucantonio, SISSA researcher and first author of the study together with Giovanni Noselli, also from SISSA.
Antonio DeSimone, a SISSA professor, headed an Italian team that conducted theoretical analysis of the phenomenon based on the experimental analysis data provided by a Spanish group, whose findings were reported earlier.
A single layer of epithelial cells integrated to a hydrogel substrate was the basis for the computer simulation produced by DeSimone’s team. The cell layer was first subjected to stretching, and then released.
“Surprisingly, the cracks never appeared under stretch but only after release”, explains Noselli. “We also observed – and this came quite as a surprise as well – that cracks appeared in many places, along the cell junction lines where one cell is in contact with another.”
The authors have emphasized the importance of the hydrogel substrate representing the extracellular matrix into which the biological tissues are embedded. The hydrogel can be considered as a sponge that has water trapped inside.
“It’s the presence of this substrate that facilitates multiple cracking: when the system is compressed the fluid trapped in the hydrogel pores is forced inside the small cracks at the cell junctions in the epithelial cell layer, causing them to open,” explains Lucantonio.
Computer simulations enabled the researchers to determine the particular features of the hydrogel that brought about distributed cracking. The scientists explained the contradictory effect of multiple cracking thus: “having to force several fracture points, the overall energy required for system failure increases” says Noselli. “Systems that undergo distributed cracking are therefore more resistant than others where the fracture occurs in a localized manner”.
Biomimetics
The research work conducted by DeSimone and his team established that materials exhibiting resistance similar to biological tissues could be highly useful in a number of applications in various fields.
“The possibility of regulating the permeability of a film by mechanical strain or the possibility of releasing drugs in a controlled manner through a membrane”, explains DeSimone, ”are of great interest for biomedical applications”.
DeSimone and his colleagues are currently working on the project SAMBAbiomat. This project is based on “biomimetics”, which is a study of natural materials and processes with the aim of using them for new technological applications.
“Materials and mechanisms like those investigated in our latest paper may, for example, be applied in small ‘devices’ (microrobots) to be used in ‘tasks’ that are useful to humans.”
Further studies into the field of biomimetics were conducted by De Simone and his team for investigating the motion of microorganisms and algae, which could be used for designing microscopic mobile devices in the future.