There are a number of interaction forces which have a significant role in the interesting mechanical properties that arise at the nanoscale. Nanoscale forces (mechanical not chemical) have been attributed to allowing Geckos to hang on glass surfaces by a single toe! On a geckos footpad, there are hair-like setae which have nanoscale β-keratin spatulae (~200nm in width) at the ends.
The action of a gecko adhering to a surface is not chemical, it’s completely mechanical and due to the unique design of the setae. If a gecko is not in contact with a climbing surface, the setae are curved toward the body and the spatulae are misaligned. But, when the gecko is planted on a surface, the setae bend outwards – flattening out the setae onto the surface. With this action, the spatulae tend to align with the material. If every setae (6.5million) and spatulae aligns – a single gecko could lift 113kg!
Of course, this is amazing. But the icing on the cake is that geckos can detach their feet in 0.015 seconds. So, it is an easily reversible adhesion. Additionally, geckos adhesion/detachment is not hugely dependant on the surface. Experiments have been carried out where scientists analyse how geckos walk on both hydrophobic and hydrophilic surfaces – only a 2% deviation in adhesion/detachment was seen. We know that gecko Setae are highly hydrophobic (repel water), since van der Waals force is the only mechanism which allows two hydrophobic materials to adhere in air, we can deduce the reason for adhesion to be van der Waals. Van der Waals forces are a weak interaction distance dependent force which encompasses three contributions – the permanent dipole-dipole attraction Keesom force, the dipole-induced dipole interaction Debye force, and the induced dipole-induced dipole London force. Another amazing feature of Gecko adhesion is the self-cleaning abilities. Geckos actually get cleaner with repeated adhesion and detachment because the dirt/pollen/dust is energetically inclined to stick to the climbing surface rather than the geckos toe!
After scientists saw the incredible adhesion properties Geckos have, they got to thinking – how can we replicate this synthetically? We could have adhesives that adhere strongly, detach easily and even self-clean! We all know about issues with adhesives, they are typically adherent only (one way) or saw with something like velcro tape, it gets unusable after repeated use due to dirt build-up. How can we use this Gecko design knowledge to make special glues or to carefully pick up and put down small/delicate items or even climb walls like Spiderman? Scientists have made Gecko tape and have actually incorporated this technology into robotics, by making an artificial mechanical gecko. See the fantastic video below! Maybe we could translate this technology to help humans climb walls!
For more details on Gecko adhesion, please see this fantastic paper: You have access Gecko adhesion: evolutionary nanotechnology Autumn et. al.
Nanomaterials bridge the gap between the state of matter transition of molecules to bulk solid materials. There are distinctive size-dependent properties of nanomaterials which arise mostly as a result of the large surface area to volume ratio, which I discussed in my Nanoscale Phenomena post. At this small size, often times the material length is less than the de Broglie wavelength of the materials charge carrier (electrons and holes) or less than the wavelength of light. In this instance, the periodic boundary conditions (which are theoretical boundaries around a single unit cell – which could be extrapolated to form a full lattice) break down or there may be a change in the density of atoms at the non-crystalline surface of the nanoparticle. This leads to interesting nanoscale forces, like van der Waals forces and others.
Nanoparticles dispersed in a high dielectric constant solution typically develop an induced surface charge. As like charges repel – nanoparticles of the same surface charge also repel thus preventing nanoparticles from clustering together. These surface charges can arise in a few different ways – ionization or a dissociation of surface charge groups, for two dissimilarly charged surfaces in close proximity – charges can “hop” between the surfaces, and an uncharged surface could adsorb or bind to ions in solution. An electrostatic double layer may form to neutralise a charged surface which results in a zeta potential, which is the electric potential between the particle surface and the edge of the double layer – this potential is in the order of milliVolts. A higher zeta potential is a consequence of a larger number of surface charges and results in a higher stability.
Capillary forces are as a result of the formation of a liquid meniscus. This force must be considered for powder and granular samples, multi-particle systems interacting with each other or with a surface, nanoparticle assembly/self-assembly and static friction or stiction forces in nano/microelectromechanical systems. Concave shaped capillary bridges are typically attractive whereas convex bridges are repulsive.
At the nanoscales other than van der Waal and electrostatic forces, other forces like hydration, solvation, and structural forces are relevant. These forces can, of course, be positive or negative, but can also oscillatory or much stronger than van der Waal or electrostatic forces at short distances. Each of these nanoscale mechanisms and interactions are vital for progressing all sorts of technologies from defense to medical.
Now, because the main focus of this website is on nanomedicine – let’s talk medical applications. The first medical adhesive I think of are plasters. There are so many issues with plasters that I can think of – they either don’t stick very well or they are a nightmare to take off (ouch) and they seem a little unhygienic. In theory, the properties of Gecko adhesion could really help here. Research has been carried out to make gecko-inspired tissue adhesive by mimicking the nanotopography seen in gecko feet. These researchers created a biocompatible and biodegradable elastomar which with further research could be invaluable for the medical field. This is of course not the only bioinspired material – in fact there is a whole area of research dedicated to it and this link is a great way to stay up-to-date with the most exciting research!