A new material provides a promising new approach to force sensing that allows transparency and may lend itself to a wide range of touch-screen applications.
by David Lussey
QUANTUM TUNNELLING COMPOSITE (QTC) is a material that is being developed to address some of the ongoing challenges of creating a better touch technology. QTC is made from a polymer that has nanoscale conductive particles, each characterized by a "spikey" surface structure, evenly distributed throughout (Fig. 1). The spikes do not actually touch, but when the material has a force applied, such as pressure, the spikes move closer together and a quantum effect occurs in that the electrons leap or tunnel from one spike tip to the next and a current flows – until the pressure is removed. Thus, QTC provides a change in resistance that is proportional to the pressure applied – from almost infinite resistance when not under force to almost zero when pressed (Fig. 2).
Fig. 1: In this illustration of two QTC particles, the electrons "leap" or tunnel from one particle tip to the next. Source: Peratech, artist's rendering.
Fig. 2: QTC in bulk form has a very large resistance range and current-handling capability.
QTC material was discovered in 1996 while the author was trying out different formulas for conductive adhesives. While dismantling one particular experiment, he noticed that the resistance dropped dramatically when he attempted to pull apart two metal strips bound together with one particular formulation. Putting metal particles into a polymer to make a conductive material was not a new concept – what was different was that the metal particles and the adhesive binder had been mixed in a polythene mortar and pestle that imparted low shear forces to the mix. The result was QTC.
Professor David Bloor at Durham University confirmed that conduction was occurring because of a quantum effect – not from the touching of the metal particles. QTC's inventor was able to take out a patent on the manufacturing process to make QTC and founded the company Peratech in 1997. Peratech has continued to research QTC and currently has over 100 QTC patents worldwide. Durham University has a department devoted to investigating the properties of QTC.
In Search of Applications
The first few years of the company's life were spent investigating QTC – how to make it reliably and how changes to the polymer, the conductive materials used, and the size and shape of the particles resulted in QTC materials with very different performance characteristics. For example, the overall resistance range can be chosen for the task (Fig. 3), and the sensitivity can range from being so sensitive that a thin film of QTC can act as a microphone to being so insensitive that it takes the weight of a tank to activate it. It can also respond with a smoothly variable change in resistance in the same manner that a conventional variable resistor performs or have a threshold pressure in which it changes from a very high resistance to virtually nothing, just like a switch; in which state it can carry large currents.
Fig. 3: This is a typical QTC-based ink for touch screens.
When the pressure is removed, the resistance returns to what it was originally – a cycle that can be repeated time and time again. Also, the anisotropic properties of QTC can be controlled to impart functionality to applications such as whiteboards, touch screens, and other large-area devices. Leading whiteboard maker and Peratech licensee Egan Teamboard have pioneered the commercial use of QTC in large-scale applications with the new T3 VRT Interactive Whiteboard.
A key feature of a QTC pressure sensor is that there is no air gap, so there is no possibility of a contaminant getting between two contact points. Similarly, as there is no "make and break" between contact points, there is no possibility of arcing, making the technology interference-free and safe for potentially explosive environments.
One of the first applications for QTC was in clothing in which controls for iPods and similar devices were integrated (Fig. 4). Effectively, a flexible, textile, solid-state switch, the QTC could be washed or dry cleaned, crumpled, and stretched. A number of design wins were achieved with high-end ski brands and suit makers, but the volumes were small.
Fig. 4: This ski jacket has QTC switches to control an iPod.
In the search for high-volume markets, Peratech also considered mobile phones. One of the main points of failure on a mobile phone is the keypad, as dust or liquids can contami-nate the contacts, or the plastic of a collapsible dome switch can lose its resilience and fail to spring back. Peratech, therefore, approached the mobile-phone manufacturers offering to solve this problem for them. However, this was not viewed as a high-priority problem compared to other development challenges. The growing popularity of smartphones was creating a different problem in that the human–machine interface (HMI) was not keeping up with advances in features. A number of phones used a small joystick-like device to navigate a cursor around the screen. However, it was rather crude to have four switches for up, down, left, and right that moved the cursor in the required direction, but only at one speed. By replacing the on/off switches with QTC sensors, a variable response was now possible. The harder one pushed, the faster the cursor moved, or the faster one scrolled through a list of contacts, or the higher an avatar leapt.
Peratech designed a QTC-based navigation keypad that was licensed by Samsung in 2010 and is currently commercially available in a number of mobile phones. This feature of being able to have a variable response that depended on the amount of pressure applied effectively provided a third dimension of input. The challenge was how to bring this third dimension of input to a touch screen when QTC is a black or gray opaque material.
Peratech's solution was to print a small set of QTC dots 10–20 μm thick around the periphery of the screen (Fig. 5). When pressure is applied to the screen, the dots are compressed against the phone case, enabling this third dimension of input to be achieved. This solution was licensed to Nissha, one of the largest manufacturers of touch screens in the world, to supply mobile-phone manufacturers and mobile-device OEMs and should eventually appear in a shipping product.
Fig. 5: QTC is added around the periphery of a touch screen.
Transparency and Touch
Peratech began looking for ways to incorporate its technology as a powerful solution within the touch market. In order to do that, it was necessary to work out a way to employ QTC over the entire area of the screen to give not only pressure sensitivity but also the coordinates of where the pressure was being applied. Developers did this by implementing a solution that used the same structure of well-known resistive touch screens, but replaced that technology's air gap and spacer bumps with an ultra-thin layer of QTC. The challenge was to create a version of QTC that would work at only 6–8 μm thick and at a very low density of particulates, so as to be, for all intents and purposes, transparent. Peratech finally created the right combination of polymer and metal particle size and the resulting film of QTC Clear typically reduces light transmission by less than 1% when applied to a typical touch-screen application. Thus, the structure for the new QTC touch screen is a layer of QTC sandwiched between two layers of conductive material such as silver nanowires or indium tin oxide (ITO), which is, in turn, sandwiched between two transparent sheets of plastic or thin glass. Perimeter conductors are applied in the same way as any other 4-, 5-, or 8-wire resistive touch screen, and the same electrical interface techniques can still be employed.
The top layer of thin glass or plastic can be more rigid than those materials traditionally used for resistive membranes because the QTC can detect tiny deflections of a couple of microns from a force of as little as 5 grams. A harder front surface, such as a sheet of thin glass, also means that traditional problems with resistive technology, such as scratches and top-sheet incursions, can be greatly reduced and that "shaped" screens with compound curves can be produced. One feature of QTC's material is that virtually no current flows unless a force is applied, which is important for battery-operated devices. Just like traditional resistive, this overcomes the power-management challenges of capacitive-based designs that require more energy while searching for a touch event. Lower power consumption in this case also has the advantage of reducing the radiated emissions signature of the screen.
QTC touch screens are also being designed to offer competitively high levels of x-y resolution. As QTC gives a proportional response to touch, the responses from adjacent intersections on an X and Y matrix can be interpolated to provide a highly accurate position. For example, using a 10-bit sampling of the resistance gives 1024 levels of resistance change for the X coordinate and similarly for the Y coordinate, making it ideal for applications in which accuracy is important.
However, unlike typical resistive solutions, because QTC changes resistance with pressure, this technology provides the opportunity to detect three dimensions of input – x, y, and z. For example, the speed that you scroll through a contact database increases as you press harder. Menu icons can be arranged in three dimensions that you "fly" through to find what you need more quickly. The width of a line being drawn can vary with the pressure applied, which is important for writing characters in some languages.
As the QTC material can be a layer only 6 μm thick, the material costs are minimal and the resulting touch screens should cost the same or less to manufacture than current touch-screen designs, especially as existing resistive or capacitive control electronics can be used. Importantly, QTC touch screens can be made to sizeable dimensions, making possible a wide range of uses from desktop computers to automotive and from in-store displays to interactive control interfaces.
The production of QTC touch screens can be simple, as the QTC material is supplied as an ink that is screen-printed directly onto the electrode layers of one of the substrates. This makes it easy for existing resistive manufacturers to quickly switch from printing spacer dots to printing the QTC layer.
Potential Uses for QTC Touch Screens
QTC touch-screen technology offers the potential to expand touch-screen usage by removing constraints of size, excessive power consumption, system design complexity, and SNR issues. And QTC's ability to respond to very small deformations or pressure changes suggests novel ways of constructing touch screens. For example, the QTC layer could reside behind a thin sheet of metal or similar material – even wood, with small holes to allow light through to create "secret till lit" buttons. This capability would enable a robust interface for applications such as automobile dashboards or consumer-electronic devices such as printers, washing machines,etc., where a glass or plastic screen might be easily broken.
In summation, QTC technology shows considerable promise and has been generating commercial interest. Companies that have been placing patents for the use of QTC in future products include Apple, Nike, Philips, EADS, MasterCard, Samsung, and others. Developers at Peratech believe that new practical uses for this novel material will continue to be discovered. In terms of QTC's use as a platform for touch technology, challenges of scalability and reliability with regard to specific applications are the focus of current work. Peratech has already signed with one major display manufacturer (that it is not at liberty to disclose) and is in search of additional partners.
UK: D. Bloor et al., A metal-polymer composite with unusual properties, J. Phys. D: Applied Physics 38, 2851-2860 (2005). Available at: http://www.iop.org/EJ/abstract/ 0022-3727/38/16/018.
US: D. Bloor et al., Metal-polymer composite with nanostructured filler particles and amplified physical properties, Appl. Phys. Lett.88, 102–103 (2006). Available at: http://link.aip. org/link/?APPLAB/88/102103/1. •