Ivy and the Nanoparticles: The sticking power of Ivy

I find English Ivy to be a fascinating plant for many reasons. One being the changes that it makes from the juvenile stage to the adult stage. Another reason, and the topic of this post, is the way that ivy climbs structures such as trees and walls by the aid of little rootlets.

This is something that has long been observed and was described by Charles Darwin in his book The Movements and Habits of Climbing Plants where he noted two things about the secretions of the rootlets.
Firstly that:

'As the discs soon adhere firmly to such smooth surfaces as planed or painted wood, or to the polished leaf of the ivy, this alone renders it probable that some cement is secreted' 

He later described the colour of the cement:

'the rootlets of the Ivy, placed against glass, barely adhered to it, yet secreted a little yellowish matter.'

It would seem that after these discoveries, the secretions that provide such successful adhesive properties, were not investigated again until 2007 by Mingjun Zhang and his team at the University of Tennessee in the US.

The aerial rootlets are grown only by the ivy during its' juvenile stage. They can be seen as discs that consist of between four to seven tendrils. To study the tendrils and the secreted materials they encouraged the ivy to climb onto silicon wafer and piece of mica for a week. They then removed these branches so that they could see the traces left on the silicon and mica surfaces. They used Atomic Force Microscopy to get a very close view of the secretion.

They found that the particles were around 70 nm in diameter and very uniform. Each image they took showed a large number of nanoparticles. This provided support for the hypothesis that the nanoparticles play an important role for ivy climbing on surfaces. Also that they are directly related to the capability for the ivy to affix itself.

Not only did they find the immensely small size of the nanoparticle globules that are secrected, but they found 19 different compounds within the secretion. The compounds contain oxygen, nitrogen, and sulphur. Compounds that are well known for their ability to create hydrogen bonds. This suggests that the nanoparticles rely on hydrogen bonding to attach to different surfaces. While hydrogen bonds are known to be weak bonds, over many rootlets, this provides enough force for the ivy to climb surfaces.

The secreted material was shown to by yellow, as documented by Darwin. It is gradually secreted as a gel, with the research team observing that water is evaporated as the gel dries. Once the drying is complete, the stem is firmly attached to the surface. Due to the method of surface climbing, that is, using the rootlets for attachments - the secreted nanoparticles can adhere to various surfaces due to their very small size.

Therefore it seems that it is the many rootlets secreting this weak adhesion along with the hydrogen bonds that are the forces enabling the surface climbing of ivy.

Further Research
As this research was completed some time ago, I wanted to look at what research had been done since 2007. I searched Mingjun Zhang and found that he has been part of some very interesting ivy-related projects. These include understanding the adhesion mechanics of ivy nanoparticles, which could potentially inspire the design and fabrication of nano-bio-materials and the UV protective capability of the nanoparticles of juvenile ivy rootlets, which could be an alternative to metal oxide nanoparticles in sunscreen applications.

It just goes to show that English Ivy, considered to be mundane and a pest to some, is actually a plant at the forefront of research. And most importantly, it doesn't live in some tropical barely explored island - it lives here, with us. Let's take the time to look at it more fondly in future, considering the benefits it give to us and for the secrets it will provide in time.

Resources
Zhang M., Liu M., Prest H. & Fischer S. (2008). Nanoparticles secreted from ivy rootlets for surface climbing., Nano letters, PMID: 18355053 Darwin, Charles (2011-03-24). The Movements and Habits of Climbing Plants (Kindle Locations 1438-1439 and 1824-1825). Kindle Edition.