Cats’ paws allow safe landing from jumps. The hierarchical network structure of the paw pads absorbs and dissipates the energy of impacts. The novel composite protects a raw egg from breaking from dropped from 50 cm.
(a) Cats possess thick paw pads to absorb the impact of jumping from height. (b, c) Subcutaneous histological images of a cat’s paw. (d) Schematic flow chart of the fabrication of the hierarchical composite by dip-coating, freeze-drying and flow-filling processes. (e) Image of a 100 x 100 x 8 mm composite pad. The remarkable flexibility of the material (f) twisted in arbitrary shapes, (g) bent into a cylinder and (h) worn as a wrist guard.Inspired by a cat’s ability to land silently and safely on its paws from great heights, researchers from Zhejiang University in China and Bristol Composites Institute in the UK have designed a soft, flexible and energy-dissipating material [Lu et al., Applied Materials Today 25 (2021) 101222, https://doi.org/10.1016/j.apmt.2021.101222].
“The proverbial ‘nine lives’ of cats are… due in large part to their impact-resistant paw pads,” points out Faxiang Qin, one of the authors of the study. “By mimicking the real biological multiscale architecture of cat paws, we have designed and fabricated a highly impact-resistant metacomposite with an ingenious hierarchical network microstructure.”
The pad of a cat’s paw is a three-layered structure of an outer skin or epidermis, a thicker layer of tissue or dermis, and a soft subcutaneous layer that acts as the main energy absorber. This crucial layer consists of bundles of stretchy collagen fibers arranged in porous, crisscrossing networks. The structure is made up of large and small chambers filled with fatty tissue, which act like bouncy cushions to absorb and dampen the energy of impacts with the fibrous network maintaining the overall integrity and mechanical strength.
“Each isolated soft adipose compartment wrapped by hard elastic collagen acts as a small hydrostatic system responsible for the energy dissipation,” explains Qin.
To mimic this complex multiscale architecture, Qin and colleagues led by Hua-Xin Peng, designed a metacomposite based on a global cellular network of open-cell polyurethane (PU) foam with large porous chambers. A metacomposite is a material that takes its properties from both the composite fillers as functional units and their prescribed ordered structure at single or multiple length scales. The PU skeleton is dip-coated with graphene oxide (GO) and multiwalled carbon nanotube (MWCNT) nano-inks to give the structure strength and robustness, while a freeze-drying technique is used to introduce GO/MCWNT membranes and chambers. Finally, the foam is filled with polyborondimethylsiloxne (PBDMS) – or ‘silly putty’ – to give the necessary ‘bounce’. The combination of a ‘hard’ polymer wrapped around a ‘soft’ one makes full use of the strengthening properties of the one while retaining the energy absorbing properties of the other.
The metacomposite demonstrates remarkable damping and impact energy absorption properties, demonstrating the ability to dissipate almost 100% of impact energy in low-kinetic energy drop tests. In fact, an egg wrapped in a 5-mm-thick sheet of the novel metacomposite can withstand a drop from 50 cm without breaking, the researchers show.
The new material has great potential for protective wearable items such as wrist guards for sport or aerospace applications, the researchers believe. As well as being soft, flexible and impact resistant, the metacomposite is creep-resistant, strong, of tailorable stiffness, and possesses shape recovery capabilities. The researchers are now working on optimizing the fabrication process to improve damping performance and target practical applications, says Qin.