Glue – A sticky subject

Science news alert! Let’s give some attention to slugs, which at the moment are somehow less depressing than the normal news.

The only attention I have given to slugs in recent years is trying to prevent them from getting into the kitchen in our student house. We failed in our prevention techniques, and yes I did step on one at 4am and it was a dark moment for both me and the slug, but at least I survived the encounter. The slug wasn’t so lucky.

Harvard University’s Wyss Institute for Biologically Inspired Engineering have been exploring slug slime as a potentially useful adhesive – and recently created a ‘bio-glue’ that could solve the current problems with medical glues, which include toxicity and an inability to stick well to wet or moving surfaces.

The new slug-inspired bio-glue works differently because it has two layers: the adhesive substance and a ‘dissipative matrix’ which basically acts as a shock absorber, and dissipates energy – this means that the energy from movement that would normally break the adhesive bond does not reach the adhesive bond as quickly, allowing the bio-glue to continue to stick even on a moving surface. This has been tested by the Harvard University team on a tear in a pig heart, which remained sealed by the glue throughout thousands of simulated heartbeats.

The adhesive layer is unusually strong, binding to biological tissues through three mechanisms: electrostatic attraction between the positively-charged polymers of the adhesive and negatively-charged cells on the biological surface, covalent bonds between atoms, and physical interpenetration which is where the polymer chains become entangled and start to diffuse into the tissue surface. If you want more information on adhesive mechanisms (of course you do), I will explain these a bit further down.

The combination of the two layers creates a unique hydrogel (a hydrophilic polymer network) which is extremely useful in medical science. It has been tested on pig tissues, wet and dry, and binds more strongly than other medical glues. It has been tested in living rats and mice and retains its strong bonding properties without damaging or sticking to any surrounding tissues. It could be used as a patch to stick to broken tissue surfaces, or as an injectable glue for internal injuries, as a new way to deliver drugs into the body, or a way to attach drugs to their target organs. The Harvard team are now looking at developing the glue further to make it biodegradable, so that once its purpose has been served it will be decomposed by the body.

The bio-glue needs a lot more testing, but the research so far looks extremely promising for the future of medical and surgical glue.

Current medical glues in use are cyanoacrylates, a family of strong adhesives that include superglue. Medical glues have been engineered based on superglue, but with less toxicity and skin irritation due to a longer organic backbone, which slows down degradation and is therefore less toxic to skin tissues. Cyanoacrylates work by rapidly polymerising when exposed to the hydroxide ions in water, so normal atmospheric humidity is enough to set the glue. The long polymer chains bind surfaces together but don’t deal well with excessive moisture or movement, so are difficult to use when too much blood is involved, or if the wound you are trying to seal is moving, an example being a tear in the heart or lung systems.

Untitled
2-octyl cyanoactrylate (left) and n-butyl cyanoacrylate (centre) are medical glues based on superglue, methyl cyanoacrylate (right)

 

Adhesives are extremely common in everyday life, ranging from sticky tape and glue sticks to construction and aerospace engineering. However, just like foam, magnets, or the brazil nut effect, adhesion does not have one single theory that scientists can agree on. It’s dependent on the types of surfaces and the type of adhesive used to hold them together. Five different mechanisms for adhesion have been proposed and widely accepted:

velcro-200543117-001

Mechanical – think about how Velcro works: tiny hooks and tiny loops interlock to hold the two sides together – this is mechanical adhesion.

 

DNA molecules

Chemical – Also called chemisorption – the separate sides undergo a chemical reaction which forms ionic or covalent bonds, attaching the surfaces together. Hydrogen bonds between surfaces are also chemical adhesion: for example the two strands of DNA are held together and twisted by hydrogen bonds.

water-drops-on-leafDispersive (or adsorptive) – This kind of adhesion is what causes water droplets to stick to surfaces, or those stickers that you can stick onto windows without using any glue. It is caused by van der Waals forces, which is the weak electrostatic attraction between two particles where one particle has a region of negative charge and the other has a region of positive charge, causing a weak bond. This can happen between molecules that are charged (permanent dipoles) or molecules that have no charge but randomly become slightly charged due to movement of their electrons (induced/temporary dipoles).

Quick side note- have you ever wondered how a gecko can stick to the ceiling? It’s not due to a sticky substance like slug slime: it’s because of dispersive adhesion. Gecko’s feet are covered in tiny tiny hairs, each one a billionth of a metre wide, and these tiny hairs have tiny van der Waals interactions with the molecules of the surface that the gecko is standing on. There are so many hairs packed into such a small area of the gecko’s feet, that the resulting adhesive effect is amazingly strong, considering it’s based on such weak attractive forces.

gecko.jpggecko-feet.jpg

balloon static

Electrostatic – when the surfaces are charged, one positive and one negative, it causes a difference in charge between the surfaces which creates an electrostatic force. An everyday example would be a balloon sticking to the ceiling due to static: when you rub the balloon it acquires a negative charge, which will repel electrons away from the surface of the ceiling, creating a slight positive charge and allowing the balloon and ceiling to stick together.

Diffusive – This could be described as similar to mechanical adhesion (Velcro), but on a much smaller scale, at the molecular level. Take the polymer chains in superglue as an example: the polymer chains start to interlock with the surface that they are sticking to, whilst remaining entangled enough that they don’t stop sticking to each other. I can’t find a fun enough picture for this one, so you’ll have to imagine some molecular-level Velcro and be happy with that.

So how did we start off with slugs and end with balloons and polymer chains? I suppose it’s just another example of how everything is connected and science can be ridiculously random sometimes… Or just another example of how easily I can end up on a long and (questionably) interesting tangent. But thanks for reading if you made it this far! And by the way, don’t use salt in the kitchen to get rid of your slugs. It doesn’t deter them, it just kills them in an unnecessary amount of mess… RIP kitchen slug from my student house, I would like to dedicate this blog post to you.

 

 

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