It's taking time for wearable technology to move from technological curiosity to practical reality. Some of the obstacles are practical; others are more about social acceptability and attitudes. And some of it involves the tendency for effective wearable technologies to become mentally invisible.
Early digital watches were sold as techno-gadgets when LEDs were a novelty. Today, an LED watch is a retro fashion statement. Beyond the wristwatch, wearables have been in use for more than a decade in some industries, although they are largely tablet or PDA-style devices held onto a sleeve or belt by straps so they can be carried around warehouses and manufacturing plants easily. Outside of that environment, aesthetics play a large role. Like everything else people put on their bodies, wearable technology is clearly subject to the vagaries of fashion. Five years ago, everyone seemed to have a Bluetooth earpiece stuck to the side of their head. Today, it seems to be the preserve of salespeople and taxi drivers, with their decisions to wear this technology driven more by legislation about using mobile phones at the wheel of a car than treating electronics as a form of jewellery. While at the Massachusetts Institute of Technology, Thad Starner and colleagues built wearable systems that gave the wearer the look of the Borg – Star Trek's iconic cybernetic aliens. Although the technology was anything but discreet, Starner and others showed what was possible with a head up display and interface devices designed for use on the move. Developed during the 1990s, there was little scope for a voice-driven interface. So, the wearables pioneers adopted what they called the Twiddler – a chorded keyboard that attached to the wrist so that they could type simply by curling their fingers around the bar that held the keys in place. Like a stenographer's keyboard and the Microwriter personal digital assistant launched in the 1980s, the Twiddler relies on combinations of button presses to record a single keystroke. The Microwriter was designed so that the user's fingers rarely had to move from one key to another – the only digit that had a choice of keys was the thumb. The Twiddler, however, supports 16 discrete buttons to provide thousands of possible key chords and includes a small joystick using a design similar to the Trackpoint used by IBM's laptop PCs. Like the Microwriter, those who have learned to use the Twiddler generally love it – but these specialised keyboards take time to learn, which limits its immediate usability. Voice control is easier to master, but potentially introduces other issues. Today's take on the wearable display is Google's Glass, a project for which Starner has provided much of the direction. Since the devices first appeared in the early 2000s, microdisplays have shrunk significantly. Although the screens in early devices were only around 1cm across, they were generally held by a relatively bulky arm attached to a headset. The arrival of organic light-emitting diode (OLED) displays has resulted in a thinner screen overall which requires less energy to power than the older LCD-based designs. As a result, Glass's display is much lighter overall and is meant to clip on to a pair of spectacles. This, in turn, has introduced some issues. Even before it has been launched as a product, Google Glass is causing concern about privacy and the idea that people's actions in public could easily be recorded discreetly by anyone with a pair of the camera-armed spectacles who happens to be facing in their direction. However, the use of voice control, in which commands are delivered with the phrase "OK, Glass", is likely to be a giveaway in public. Similar augmented-reality wearables could improve the mobility of people with severe disabilities. Worn as a pair of spectacles, the idea behind the Smart Specs system developed at the University of Oxford is to provide users who have very limited vision with additional information about their surroundings. The spectacles view the surroundings and interpret them for the wearer, using colours or lighting patterns generated using LEDs built into the lens pieces to indicate what they have found. By attaching to a visor or spectacles, Glass or Smart Specs do not have to conform too closely to the body's profile. But it is one of the few wearable product concepts for which that is not a concern. For example, a number of companies see healthcare devices as important for the development of the wearable-technology market. But, for that market, long-term comfort will be a key factor. Some devices are already familiar to those who run or cycle heavily. Sports watches, such as those made by Garmin, accept low-power radio transmissions from heart-rate straps worn around the chest. Generally, the straps just keep the sensors in place – a small rigid section at the front contains the processing electronics. This makes the strap flexible enough to not be uncomfortable during exercise, although some pressure does need to be applied to take reliable readings. Other sports systems are based on accelerometers so that they can record motion. The Nike Fuel band, for example, uses the swinging motion of a wrist while someone is walking to determine how many steps they have taken during the day. Using this approach, the electronics for the Fuel can fit into a bracelet that is heavier than most jewellery but which can be worn for much of the day. To provide a faster response to people with heart disease, help rehabilitate others after a stroke and even prevent repetitive strain injuries at work, researchers have proposed developing more sophisticated motion-tracking systems that use sensors attached to various parts of the body. For these systems to be comfortable to wear for long periods, they need to stretch and flex in the same way as skin. This need is leading to new directions in semiconductor research. Several years ago, Nokia showed off its Morph concept – the company's idea of a flexible, wearable phone that could wrap around the wrist like a bracelet. When removed and unravelled, it would look more like a mini-tablet computer. Devices like the Morph are likely to need radically different materials to those in use today and this will have a long-term effect on wearables in general. The most obvious candidate for flexible electronics is plastic. Work on OLEDs, such as those used in Glass, is helping to develop simple processors that can be inkjet printed onto a flexible surface. The performance of these thin-film polymers is poor – arguably at the same stage of development that silicon CMOS was in the early 1970s – but they are potentially cheap to make and may be more robust to washing than today's silicon-based devices. However, water is currently the organic semiconductor's biggest enemy. One of the problems that is holding back OLEDs from being used commercially in lighting is the lack of a good sealing material that can keep water out. When water breaks through the surface, it quickly disrupts the organic polymer layers, causing them to break down and fail. Organic polymers are not the only candidates for flexible electronics. Metal oxides can be made thin enough to bend without cracking and some of them have another benefit – they are transparent. This, in principle, makes it possible to embed circuitry in clothing that is practically invisible. Some of these oxides have been in use for a long time. During the Second World War, manufacturers used transparent conductive oxides to keep aircraft free of ice – the resistance of the film to current passing through it heated the film and the glass on which it sat. Later, indium tin oxide (ITO) moved into widespread use as a conductive coating for flat-screen displays and touchscreens. Unfortunately, one of its constituent elements – indium – is a very rare and expensive metal and proving harder to mine over time. Electronics manufacturers have a strong incentive to find other options. Oxides of zinc may provide the kind of performance currently offered by ITO. However, many of the more promising candidates rely on rarer elements, such as hafnium. Although ITO is used mainly as a near-invisible wire, thin-film oxides have the potential to make flexible integrated circuits. Thin-film semiconductors based on metal oxides date back to the 1960s, although these early materials were n-type only. Hiroshi Kawazoe and colleagues at the Tokyo Institute of Technology developed a p-type thin-film oxide material in 1997, providing the core elements for diodes and transistors. Even with these materials, there is an issue. As with the organic polymers used in printed electronics, they have poor conductivity, especially in the form most amenable to low-cost mass production. The best materials have a conductivity more than ten times worse than the materials used today in silicon ICs. However, the performance of oxides tends to be better than the lowest grade of silicon, the amorphous form often used in solar panels. Made at comparatively low temperatures, and with a structure similar to that of amorphous silicon, researchers have demonstrated zinc oxides with mobilities of around 30cm²/Vs – five times lower than that of the polysilicon used for very short interconnects on ICs. The crystalline silicon used in transistor channels in CMOS devices is closer to 300cm²/Vs. Graphene may prove useful as a low-resistance way of interconnecting circuits formed using other polymers or oxides. Researchers at Northwestern University in the US recently developed a graphene-based ink that, when deposited as a film, is less than 20nm thick and several hundred times more conductive than previous attempts to produce a form of the material that can easily be sprayed or printed onto a surface. Although polymeric graphene behaves like a metal, it is difficult to produce in this form. The resistance between individual flakes of the planar material is generally much higher, but some of this resistance is due to the conditions needed to process sheets of graphene into flakes that can be used in an ink. Oxidation at the edges greatly reduces the conductivity. The Northwestern technique uses more benign conditions that help preserve graphene's surface chemistry. The problem for graphene as a universal flexible material is that there is no clear way to turn it into a semiconductor that behaves in a similar way to silicon. It is possible that future graphene devices will work quite differently to CMOS devices, but these designs remain experimental. If they are made to work, however, the material could greatly boost the performance of flexible electronics. In the meantime, one approach may be to use stretchable conductive polymers purely for wiring and put the active electronics into discrete silicon ICs that sit on islands joined by the interconnect. This technique was demonstrated at the 2005 International Electron Device Meeting (IEDM) by researchers from Princeton University using a gold impregnated polymer. For communicating between sensors and body-worn computers at longer distances, wires will generally restrict movement too much. This puts the focus on body-area wireless networks. One option is to use the low-energy version of Bluetooth, which has been promoted as a potential body-area network, or Zigbee, originally developed for home networking. Earlier this year, Microchip Technology unveiled a cheaper alternative called BodyCom that, rather than use radio-frequency transmissions, relies on the body's own electric field. The BodyCom network uses capacitive coupling to let wearable sensors pass data to other systems as soon as the person touches a place on the receiver. Ericsson demonstrated a system based on the same technique at the Consumer Electronics Show in 2012, claiming it could operate at speeds of up to 10Mbit/s. Designed for lower cost and to run on simple 8bit microcontrollers, the BodyCom system operates at much lower data rates – up to 10kbit/s – which may be enough for passing medical data such as heart-rate or limb movements. However, some of the initial applications have been for security and safety. One Microchip customer developed a system that lets a bike helmet communicate with the bike itself – it will not start if the user is not wearing the helmet. Security and privacy are likely to become concerns for health-monitoring applications. Few systems in use today employ any form of encryption and protocols such as Bluetooth and Zigbee tend to have limited support. But concern over patient data being prone to sniffing by third parties will probably lead to the development of more secure variants for body monitoring. Wearable technology could also be used to enhance wireless communications. At last year's Sophia Antipolis Microelectronics conference (SAME), researchers from the University of Nice Sophia-Antipolis and the University of Rennes described a flexible metamaterial antenna that could be worn by firefighters to improve their chances of sending and receiving radio messages while reducing their own exposure to the radio energy. The final issue is powering the devices. Although researchers are making progress on more comfortable, flexible electronics for wearables, battery technology is not making similar strides. In principle, battery chemistries such as lithium-polymer are amenable to being formed into odd shapes and potentially even being made flexible. But they also run the risk of bursting into flames. Energy harvesting may be a viable option, with body heat and movement providing two sources of power for electronics. While neither technique can harvest more than very low levels of energy, these may be enough to drive motion sensors and low-energy capacitive coupling-based communication to a battery-powered handset or bracelet. As wearable technology develops, we are likely to see more products along the lines of the Exmobaby 'onesie', which records a baby's heart-rate and motion for parents concerned about sudden infant death syndrome, as well as devices such as Glass that form computers around the body. When we will stop regarding them as unusual is another matter.