In touch

C&I Issue 5, 2015

Touchscreens are one of the most sophisticated devices that we come into contact with every day, yet they hardly existed a few years ago. So the idea of making your own probably sounds crazy; but that’s exactly what scientists at Saarland University in Germany are suggesting. They say that touchscreens can be made from simple materials – using screen printing or inkjet printing – and provide a host of products with interactivity.

Simon Olberding and colleagues in the department of computer science at Saarland have developed a technique called PrintScreen. Their system allows non-experts to design and make customised interactive displays based on thin-film electroluminescence – that is, made with substances such as zinc sulphide, doped with manganese, which emit light when a current is passed through it.

The screens are typically made by printing four layers of ink onto a wide array of substrates, including PET, paper or even wood. The four layers would typically be a silver conducting layer, which acts as one electrode; a barium titanate-based dielectric layer; a phosphor layer, which emits light when stimulated by an electron; and a translucent conducting layer, which acts as the second electrode. Displays of any shape can be designed using a standard vector graphics program, and made by a simple manufacturing process using commercially available conductive inks. Olberding comments: ‘The technique allows the “personal fabrication” of interactive displays, which could be used in a range of mobile or wearable computing applications.’

There are two versions of PrintScreen, which use either inkjet printing or screen printing. The inkjet printing method offers ‘instant fabrication’, while the screen printing method is slower, but makes screens of higher quality – and can be used on a wider range of substrates. One downside of the inkjet method is that it cannot be used to apply the phosphor layer because the phosphor particles are too large to fit through the nozzles. The answer, says Olberding, will be to rely on a certain amount of ‘pre-fabrication’: suppliers, such as specialist print shops, would prepare sheets of material that are ‘pre-printed’ with phosphor, over which the end user can then print using commercially available conductive inks. ‘People can then print instant circuits, using their home printer,’ says Olberding.

The team has already produced a small screen on a PET substrate, using screen printing, for playing ‘ong – a simple interactive game in which a paddle is moved left or right. The screen was printed in three layers: a conducting bottom layer, which has all the horizontal wires; a middle phosphor layer; and a conducting top layer, which has the vertical wires. At each intersection of vertical and horizontal wires, there is a pixel. Incorporating two capacitative touch buttons allows the user to control the paddle.

Displays made in this way are all about customisation, so are not designed to compete with mass market displays. But Olberding says that the fabrication process can still be improved – by making it faster or more automated. ‘Having a fully inkjet solution would be desirable, but this is for the future,’ he says.

The technology might also find use on short-run commercial products that require an interactive display, or could even be used by businesses like restaurants – which could design and print interactive menus, for example.

Sound technology

Researchers at Nanyang Technological University (NTU) in Singapore have also developed a low-cost touch-sensitive system that can be used on a range of flat surfaces, such as whiteboards, glass or even wooden table tops.

Most touchscreens are either resistive or capacitative. When resistive screens are touched, two internal surfaces are pressed together. These screens are generally robust and low cost and work even if the user is wearing gloves. They are used in applications such as touch-sensitive till displays in restaurants. Capacitative screens are more sophisticated – and expensive – and form the basis of devices like smartphones. A surface conductor, such as indium tin oxide (ITO), detects a distortion in surface capacitance when it is touched.

By contrast, the NTU technology, Statina – speech, touch and acoustic tangible interfaces for next-generation applications – exploits the vibration of sound waves propagating through a solid surface. Using low-cost vibration sensors and a specially developed algorithm, the system can pinpoint the location of a light tap on any surface. When equipped with web-cameras, it can also track the movement of multiple fingers or objects on any surface.

Leading researcher, Andy Khong, assistant professor in the school of electrical and electronic engineering at NTU, says that because sound waves propagate through matter at a certain speed, it is possible to derive the location of the touch based on when each sensor picks up the signal. ‘Our system can transform surfaces such as wooden tables, aluminium, steel, glass and even plastics into low-cost touchscreens,’ he says. ‘In future, you could play computer games or draw sketches on walls or windows, since almost all surfaces can be made touch-sensitive.’

Retrofitting the system onto an existing flat-panel TV, for example, would transform the TV into a touch-sensitive screen at a fraction of the cost of a new touch-sensitive screen. Once hooked up to a computer, the modified TV screens could then be used as interactive billboards, mall directories and even as digital whiteboards that can track what is drawn or written.

Khong and his team are now working to commercialise their invention by developing a more compact system and expanding its capabilities to include tracking of fingers and stylus movements using optical cameras.

Carbon future

Carbon nanotubes (CNTs) are likely to be a key success factor in future touchscreens. According to US-based research firm TechNavio, CNTs have the potential to replace ‘fragile and expensive indium tin-coated films, which are currently used in applications like liquid crystal displays and touchscreens’. But in order to exploit these materials, scientists need to be able to make them defect-free on a large scale and be able to control their structural properties.

Traditionally, CNTs are made at high temperature, and are built up one atom at a time with help from a catalyst. However, controlling the exact nature of the end product – such as tube diameter and structure – is very difficult. Recently, a team at the NanoScience Centre of the University of Jyväskylä, Finland, in collaboration with researchers at Harvard University in the US, have proposed a new way of making CNTs, which involves rolling up sheets of graphene, and therefore a potential solution.

The researchers have carried out computer simulations to show that ‘nanoribbons’ of graphene can be ‘twisted’ into tubular shapes. While the work is still theoretical – and only exists as computer models – Pekka Koskinen, a research fellow at the centre, comments: ‘Experiments have shown that nanoribbons are easier to make with precision. If you can then twist this structure, you’ll get a much better nanotube.’ He likens the process to twisting on a strap – which will eventually roll itself into a tube. The computer simulations show that a similar effect would happen at the molecular scale, he says.

The difficulty will be in rolling up a sheet that is only nanometres in size. Even some of the atomic-scale manipulation techniques would find it difficult to get this right because the ribbons are very flimsy, explains Koskinen. ‘For now, we are waiting to see if any experimental techniques can be developed to put this into practice,’ he says.

Meanwhile, as Faisal Ghaus, vp of TechNavio, points out: ‘Vendors are increasingly adopting new production technologies to help them diversify their product capabilities from traditional markets to new, emerging markets.’

Canatu of Finland, for example, is making carbon nanobuds (CNBs), a hybrid of CNTs and fullerenes. Canatu says that CNTs are chemically inert and the only way to bond them to other molecules is to ‘damage’ them to create reaction sites. But this also generates defects, which reduces the performance of the materials. Canatu’s technology combines reactive fullerenes with CNTs – no defects are created and the CNTs retain their properties.

The company uses CNBs to make flexible, transparent, conductive films for a number of touch-sensitive products. The most recent is a touch panel for car dashboards, which could one day replace mechanical controls. The demonstrator product was developed with Schuster – which makes decorative plastic parts – and Display Solution, a developer of customised LCDs.

In addition to automotive applications, CNBs are also being used in more traditional products: iTouchworks Optoelectronics, a Chinese manufacturer of touch modules, has begun using the technology to make touch sensors, which are likely to be used in portable and wearable consumer devices.

Shrinking metal alternative

But it’s not only CNTs that could replace expensive materials like ITO. In Finland, scientists at research organisation VTT are using a new method of making nanometals, for example, of copper, cobalt and iron as well as more exotic metals and alloys, to do just that.

The team’s main focus is to use the nanometals in conductive inks, which could be used to replace more expensive metals in, for example, solar cells or touchscreens. Larger touchscreens could be a particular beneficiary, as the basis of their capacitive sensors – indium tin oxide (ITO) – is not conductive enough at that size, according to Ari Auvinen, principal scientist at VTT.

‘Silver meshes are already used for large screens,’ he says. ‘But these are very expensive. Copper nanoparticles would be cheaper.’ The downside is that copper is relatively unstable, as it is prone to oxidation. ‘If we could manage this in the inks, it could compete with silver, and be much cheaper,’ he says.

Most nanometals are made using flame synthesis, which is similar to the way that carbon black is produced, says Auvinen. VTT’s production method starts with cheap metal salts – usually chlorides – which are vaporised in nitrogen (or argon). The vapour is then fed into a furnace and reacted with hydrogen gas at 900°C – resulting in pure metal vapour and hydrogen chloride.

‘The metal vapour has a low vapour pressure and nucleates to particles,’ says Auvinen. The particles are typically 50–80nm in size. They are made at a rate of around 200g/day at VTT’s pilot plant, at a cost below €100/kg. Although there are no plans to expand the size of the pilot plant, Auvinen says VTT is looking to license the process to allow commercial companies to produce nanometals on a larger scale.

Dogs have their day

In the main, touchscreen technology has been designed and developed for humans. However, the technology now looks set to be extended to animals, specifically working dogs, such as those used to guide the blind and deaf.

Researchers at Georgia Institute of Technology, US, working for the project Facilitating Interactions for Dogs with Occupations (Fido), have already developed a special jacket for working dogs. By interacting with sensors on the jacket, such as by biting, pulling or touching, the dogs can pass on specific instructions to their owners, such as ‘doorbell’. ‘Currently, dogs can only communicate with people by barking or through body language. Sometimes that isn’t good enough,’ says project leader, Melody Moore Jackson of the school of interactive computing. ‘The sensors can give them a “voice” they’ve never had.’

The team is currently investigating the use of touchscreens for working dogs. The idea would be for dogs to pass on instructions by touching coloured buttons on a screen. The team is assessing the ability of a number of dogs to touch two large coloured dots – blue and yellow – in the correct order. The colours were chosen carefully because dogs can tell them apart, but find it hard to differentiate between red and green, for example.

The screen had to be chosen carefully. A typical capacitative screen could not be used because moisture from the dog’s nose would cause the cursor to freeze. An ultrasonic screen was rejected for the same reason, so the team settled on an infrared touch surface.

The first priority is for the dogs to help their owners – but the idea of them searching the Internet for pictures of cats is now one step closer.

Lou Reade is a freelance science writer based in Kent, UK

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