Sticking with it

Paul Boughton

Clever design and clever chemistry have helped to develop ways of fastening that could revolutionise the way that components are put together. Lou Reade reports.

Adhesives can’t be made much stronger or stickier than they already are, while mechanical fasteners would seem to have come as far as they can. But a series of new ideas could breathe new life into products that need to be fastened or stuck together. UK-based Rotite, for example, has developed a novel connector technology.

Two flat discs – each with a raised ‘pattern’ on its surface – are twisted together to make a secure connection.

The company’s founder and technical director, Stuart Burns, says that the concept could be used across a range of industries – from aerospace and defence through to cosmetics, medical and even DIY. The two discs can be identical, or ‘male and female’. They are pressed together and rotated, by anything between 10 and 360˚ to form a firm connection.

Burns describes the concept as a ‘low profile helicoidal dovetail’. Examples of potential applications include: attaching a walkie-talkie or radio to a soldier’s tunic; securing under-bonnet automotive components, such as hoses, quickly and effectively; and as a replacement for buckles or clasps in a variety of clothing and textile applications. Burns works to a simple design mantra: “Get it off the screen and into your hand as soon as possible.”

He says that 3D printing has has played a crucial role in the development – acting as both a prototyping and a manufacturing technology. “It’s the cornerstone of our technology,” says Burns. “Without 3D printing we would not exist, and the idea would still be on the shelf. There’s really no way to ‘draw’ the concept – it’s impossible.”

As well as being a visualisation tool, he says it also acts as a ‘thinking tool’ – helping to accelerate understanding of the concept. “This is a novel geometric concept,” he says. “To get an object and play with it helped us to understand the geometry and to know where the development may go.”

The company has looked at a variety of geometries. Some, for example, require a small twist, while others require more rotation in order to ‘lock’; some have identical faces, while others connect two different faces; others can incorporate added functionality, and do not always have to be ‘flat’.

In addition to the expected ‘male’ and ‘female’ geometries, Rotite has also developed ‘hermaphroditic geometries’. “These are previously unimaginable geometries,” says Burns. “As we’ve played with the 3D shapes, this helps to stimulate the design process. As we went on we looked at extra functionality. To start with it was just a mechanical interface. We then realised there were other areas to look into, like locking mechanisms and switches.”

Rotite has filed a UK patent, and is looking to license the concept. Burns says that the company has taken the last three or four years to develop as many variants as possible.

“If we’d rushed straight to market we would have had one or two examples. Instead, we have hundreds – all of them quantified by 3D printing. There are so many variables that we can’t possibly understand where they will all go,“ he says. “There are so many opportunities: I’m very excited about it.”

Best foot forward

US scientists have created micro-scaled polyurethane (PU) structures that mimic the way that a gecko’s foot sticks to a surface. Conventional adhesives have a lot going for them, as they can support enormous forces.

When a glued structure fails, it is often the material itself that shears – while the adhesive join itself remains intact. But adhesives are not effective in all environments.

The vacuum of space is a particular weak point, as are aqueous environments – such as under the sea or even inside the human body. Because of this, scientists are looking to develop adhesives that work in new ways.

Copying the gecko’s foot is a good starting point. The pads on a gecko’s feet are composed of millions of tiny ‘hairs’, which make such intimate contact with the floor – or ceiling – that forces of molecular attraction (called Van der Waals forces) come into effect.

“You can’s usually get close enough contact with a surface for this to happen,” says Noshir Pesika, assistant professor in the chemical and biomolecular engineering department at Tulane University in the US, who led the research.

The other advantages of the gecko’s foot include: it leaves no sticky residue behind; and, while it sticks strongly, it peels off very easily. Mimicking this latter ability could allow structures stuck in this way to be easily ‘repositioned’ if applied incorrectly.

To create the adhesive material, Pesika has used a technique normally used in the production of microchips – called photolithography – to create special adhesive surfaces that mimic the gecko’s foot. These tiny polyurethane structures have microscopic features on the surface that stick through ‘attraction’ only.

The bulk of the structure is a cylinder; a series of tiny ‘fibres’ protrude from its surface. These measure around 20 x 20 x 7 microns. It is these tiny fibres, or pads, that make intimate contact with a surface and stick to it via molecular attraction.

Despite the microscopic nature of Pesika’s synthetic structures, they are still around 1,000 times larger than the equivalent one on the gecko’s foot. However, the tiny fibres on the gecko’s foot are quite stiff; Pesika’s synthetic structures have been made more pliable, to improve surface contact.

He estimates that his structures are around 100 times weaker than a real gecko’s foot. But he says that a 1ft x 1ft pad of his material could support the weight of a human.

So far, his structures – which were made at the Naval Research Laboratory in Washington DC, US – are about 1.5 inches square.

A key attribute of the gecko’s foot is its anisotropy – meaning that it sticks firmly when sheared in one direction, but weakly in the other. It is this quality that allows the gecko to run quickly across the ceiling.

Pesika has reproduced this effect by mimicking the structure of the gecko’s foot, tilting the small ‘columns’ at an angle to create the anisotropy.

He points to a number of potential markets: space travel, where the harsh vacuum of space can destroy conventional products; and in aqueous environments – which could include the inside of the human body.

“It could be used to make special sutures, which surgeons could ‘stick and replace’ in order to put them in the correct position,” he says.

An initial product would probably take the form of a tape, in which the ‘sticky’ surface is protected until needed. But launching a commercial product is some way off as many challenges remain.

“It would have to be made in a continuous process, rather than the batch process we’re currently using,” he says.

The next stage is to test the structures on substrates with varying degrees of roughness, to see how large a ‘pad’ is needed to stick a certain weight to each surface. And, with the ‘molecular’ design all but finalised, he will also look for new materials that might improve the performance: he initially chose to use PU because it is commercially available and flexible enough to have a range of qualities.

“We’re looking to fine-tune which polymer we actually use,” he says.

German stick

Separate to this, other Fraunhofer scientists are working to produce adhesives from renewable materials. More than 820,000 tonnes of adhesive were produced in Germany in 2010, according to the German Adhesives Association.

Most were derived from petroleum. A few renewably sourced adhesives are available, for products such as wallpaper pastes and glue sticks.

In collaboration with Westfälische Hochschule, University of Applied Sciences and the companies Jowat, Logo tape, and Novamelt, scientists at Fraunhofer Institute for Environmental, Safety and Energy Technology (Umsicht) are working on new formulas of renewably sourced adhesives to develop a pressure-sensitive adhesive for industrial applications.

Products that use pressure-sensitive adhesives include adhesive bandages, self-adhesive labels and adhesive tapes. They have demanding requirements: they must remain permanently adhesive at room temperature; gentle pressure should be enough to stick them to most substrates, yet they must be removed without leaving a residue.

To do this, the adhesive force must match the application. Pressure-sensitive adhesives are based on backbone polymers, which give the adhesives their inner strength (or cohesion).

The researchers are using polylactic acid as a starting material for new backbone polymers. What makes the material attractive is its low production cost, because lactic acid is already produced on an industrial scale – so costs are in the region of prices for conventional backbone polymers. However, the researchers will have to develop a completely new formula.

“The properties of polylactic acid are completely different from those of the polymers used to date, such as polyacrylates and styrene-based block copolymers,” said Stephan Kabasci, who heads the Umsicht renewable resources business unit.

Fraunhofer Umsicht is also working with Achilles Papierveredelung Bielefeld, Jowat, and Deckert Management Consultants to develop adhesive systems that meet the quality requirements of laminated packaging, while being compostable.

The researchers are focusing mainly on water-based dispersion adhesives, in which the adhesive components are dispersed very finely in water. These are applied to one side of the product and joined while wet.

Underwater curing

One of Noshir Pesika’s aims, with his ‘gecko’ research’, is to develop an adhesive that will work underwater – and this is something that the natural world is very good at.

Another example is the buoy barnacle (Dosima fascicularis), which produces a special adhesive that it uses to attach itself to flotsam.

The adhesive is so strong that it is almost impossible to break down into its constituent parts using ordinary solvents. The adhesive can also cure underwater.

Now, researchers at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (Ifam) in Bremen are trying to discover which particular amino acids make up the relevant proteins in this adhesive.

“Once we’ve done that, the next step will be to recreate the adhesive proteins in the laboratory,” said Ingo Grunwald, an expert in biological adhesives at Ifam.

These types of ‘bioadhesive’ are mainly of interest in medical applications, such as for closing incisions or replacing and supporting the pins and screws used to treat bone fractures.

A 2011 book called ‘Biological Adhesive Systems’, which Grunwald co-edited, lists a host of potential applications of ‘biological’ glues, ranging from cardboard packaging and labels for glass bottles, through to ways to heal parts of the body such as skin, teeth and internal organs. 

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