The polyurethane rods in the sponge connect and bend triggering internal friction. Credit: Harsh Jain
Kitchen sponges — a network of soft polymeric rods — can act as memory devices, an experiment has shown. The memory can also be erased and rewritten, potentially helping mimic the architecture and functioning of the human brain, allowing memory storage and computation in the same location.
When Harsh Jain, at the National Centre for Biological Sciences in Bengaluru, squeezed a kitchen sponge cube between two metal plates, he could imprint external deformities in the shape of alphabets using a motorised rod. The sponges remembered the shapes. When he removed the pressure, the impressions vanished. If he etched the shapes without external pressure, the sponge would not remember them.
This memory storage is different from a memory foam’s elasticity, which cannot be reprogrammed, say Jain, and co-author, Shankar Ghosh, at the Tata Institute of Fundamental Research in Mumbai, where the experiments were performed.
Using a model based on friction, they explain that when the material transitions from an elastic regime to a ‘pseudo-plastic’ one, it retains the deformations due to frictional locking of the rods that make up the polymeric sponges. The polyurethane rods connect and bend, giving rise to internal friction, following basic physics principles.
“The phenomenon we observe in soft systems are applicable to a variety of soft biological systems,” says Jain, suggesting that biological memory should be considered in terms of both mechanical processes and chemical pathways. Bones also contain a network of rods, which are also present in wood and in leaves.
The concept of mechanical memory in biological systems is an emerging area of research, focused on how cells and cell nuclei can store ‘memories’ of past mechanical environments and forces. Recent research shows that information can be encoded within the protein polymer network or actin cytoskeleton, which forms the mechanical scaffolding within cells and regulates cell shape.
Potential applications of the study findings also include the fabrication of auxetics, unusual materials that widen when stretched and narrow when compressed, and reprogrammable Braille displays that require systematically creating deformation patterns.
Arathi Ramachandran at the Indian Institute of Science (IISc) in Bengaluru, says that it’s easier to test such simple models through experiments than complex ones. However, IISc’s Gondi Kondaiah Ananthasuresh recommends further exploration beyond the basic friction model outlined in this paper because mechanical properties, although common in everyday objects, such as light switches and latches, exhibit complex behaviour at the microscopic or atomic scales.
