Nanogirl Michelle Dickinson: The secret structure of cactus spines

Even deceptive soft and fluffy looking cacti can have aggressive and painful spines.

COMMENT: When it comes to plants, nothing is easier to care for than a cactus. A bit of sand, some sunshine and a spritz of water can result in a pot plant for those who want to do less and get more.

Convenience, however, can come at a cost and anybody that has been pricked by a cactus spine knows how painful and difficult to remove they are. Some spines are easier to remove than others and new research out this week used some leftover pork chops to show us why.

Originating from desert regions, cacti thrive in bright sunlight and dry conditions. Instead of having wide flat leaves like most other plants, cacti have evolved to have modified leaves which form sharp spines that protrude from the main plant body.

Cactus spines make great armour, providing essential protection for what would be a highly desirable thirst quenching plant in the dry conditions of the desert.

The spines can also have other purposes with some acting as a light diffuser helping to distribute light evenly all over the plants surface. Downward pointing curved spines can help to trap and condense water forming droplets which drip onto the roots below and some spines can even look like blades of grass helping the plant with camouflage.

The spines of most cacti are sharp and designed to stick into anything that comes into range. Even the deceptive soft and fluffy looking ones can still have aggressive and painful spines.

To study the structure and efficiency of cactus spines, researchers took spines from six different types of cactus and studied how they behaved when punctured into different materials.

Using chicken meat and pork shoulder samples they measured how much force it took to pierce one single cactus spine through the skin and into the animal tissue. Publishing their results in the journal Proceedings of the Royal Society B the researchers found that smooth, pointy spines easily punctured through animal skin and into the muscle, but they also slid back out just as easily.

The researchers concluded that this design could make the spine useful as a defence mechanism warning animals to stay away from the plant and deter them from coming back through a quick painful stab.

Other spines had microscopic barbs or hooks on the end of them and these required much less force to puncture the meat than the smooth spines. Instead of sliding back out, the barbs acted like a knife and an anchor, preventing the spine from being pulled out by cutting into then catching onto muscle fibres for more grip.

In one species called the jumping cholla cactus, the spine design of overlapping hooks created enough grip that one single spine was able to puncture and lift up a whole sample of pork shoulder without breaking or being pulled out. When the researchers eventually removed the barbed spines from the pork they found that many of the barbs were missing, likely left in the animal tissue which might explain why cactus injuries stay painful even after the visible spine has been removed.

The researchers believe that instead of offering protection, these hooked spines are designed to stick into the animal causing the cactus body to tear apart. The torn piece of cactus can then be transported to a new location where it eventually falls off the host and can start growing as a new plant once it hits the ground.

Who knew that a common houseplant and some left over pork meat could eventually help engineers to design new bio-inspired materials with much better gripping, sticking and stabbing abilities.

• Dr Michelle Dickinson, creator of Nanogirl, is a nanotechnologist who is passionate about getting Kiwis hooked on science and engineering. Tweet her your science questions @medickinson