Smart clothes

‘Joining two materials with very different mechanical properties is currently an enormous challenge in science and engineering,’ says Andre Studart of the Swiss Federal Institute of Technology (ETH) in Zurich. When hard, stiff materials are deposited directly on a flexible, soft substrate, very high mechanical stresses develop at the interface and the material can fail, he explains.

ETH researchers think they may have solved this problem by mimicking the way tendons connect to bones. Tendons are attached to bone by collagenous fibres that continue into the matrix of the bone. Studart and colleague Rafael Libanori have developed a stretchy material made from polyurethane reinforced with nanometre sized particles of aluminium oxide and laponite, a synthetic clay. The stiff regions hardly deform and can protect delicate electronics, but the soft part can stretch by 350%.¹

The novelty lies in the way these components are distributed within the material, explains Studart. ‘The concentrations of molecular, nano- and micro-reinforcing elements within the polyurethane are adjusted to control their resulting mechanical properties. Films of individual composites with different compositions are assembled in a way that provides a smooth soft-to-hard transition, reducing significantly stress at the interfaces between the layers.’ In one of the many possible architectures, bulk-graded composites can be designed to be hard on the top surface, while still being as soft as skin on the bottom layer.

‘At the moment,’ Studart says, ‘we have a material that proves the concept that locally reinforced surfaces can protect fragile electronic devices from failure upon extensive stretching of an underlying rubber-like material. The vision is that this technology can be integrated into textiles in the near future.’

To Studart’s knowledge, other examples of similar smart fabrics have been made by using much stiffer and less stretchy substrate materials like polyimide. ‘This limits not only the stretchability but also the flexibility,’ he says. ‘With our materials, it should be possible to protect the devices even upon strong stretching and bending.’

But the ETH material still needs further development and design modifications. Studart comments: ‘In our material, we observe that failure occurs on the rubber-like substrate and not on the stiff islands or electronic components themselves. This is very promising since it suggests that the reliability over time will be determined by the relatively durable polyurethane base material. A lot of development work is now needed to make the material resistant to washing and applicable to any fabric.’

The team is also developing the material to house high-performance transistors. This could result in specialised textiles used to acquire biomedical data. Studart believes it should also be possible to insert energy harvesting or storage devices as well as memories onto these structured substrates.

One of the companies commercialising stretchable electronics is MC10, a Massachusetts-based firm co-founded in 2008 by John Rogers at the University of Illinois, US, to develop products from his work on stretchable circuits. The company uses extremely thin silicon chips sandwiched in a stretchable polymer and connected by tiny conductors in a concertina configuration that can stretch to about 60%, about the same as the body’s soft tissues.

‘The challenge is taking traditional electronics literally “out of the box” and into new forms that can bend, stretch, and twist with the natural world,’ says MC10’s Kevin Dowling. ‘This makes it possible to use electronics in entirely new ways, such as on or inside the human body, allowing better data collection and with all the benefits of traditional electronics, including high quality data and standard manufacturing processes. We’re thinning silicon and transferring interconnected ‘islands’ of this silicon to flexible, polymeric substrates.’

The result is described as ‘an accordion made of silicon’ and MC10 is now manufacturing the electronic circuits that can be woven into fabrics, moulded onto flexible devices, or even adhered to body parts. The company is working on several products, including a skullcap, developed in collaboration with Reebok, which can be worn under sports helmets and will be on sale later this year. The skullcap, with embedded MC10 sensors, measures impacts to the head rather than the helmet, and allows athletes to assess whether they need medical attention.

A different approach to integrating electronic components into textiles has recently been shown by another ETH-Zurich group. Christoph Zysset and his team have demonstrated that textiles with interwoven electronic components – such as sensors on flexible plastic strips and conductive filaments – can be mass-produced on conventional ribbon looms and washed, one of the main obstacles to combining advanced textile application and electronics in a garment.²

The researchers attached thin-film electronics and miniaturised, commercially available chips to plastic fibres. This allowed them to integrate a large number of microchips and other microelectronic elements directly into the architecture of the material. As the microchips are encapsulated by epoxy strips, the fabric can be washed several times in a washing machine using a mild detergent, without damaging the ‘e-fibres’.

‘We were the first group [to] integrate electronics on flexible plastic strips into woven textiles during the fabrication of the textile,’ says Zysset. ‘To enable the integration of the electronics, we had to adapt the fabrication process [used to place] metal layers on flexible plastic substrates to achieve sufficient adhesion of the metal layer, preventing it from breaking when exposed to high strain values as they appear in woven textiles.’

The team used standard textile machines that weave the e-fibres into 5cm-wide bendable ribbons, reports Zysset. So far the team has produced two demonstration objects: a tablecloth with temperature and humidity sensors and woven-in LEDs; and an undershirt that measures body temperature. Zysset explains: ‘The shirt does not measure the body temperature as, for example, an in-ear thermometer would; it rather measures the temperature between skin and clothing layers. This temperature is relevant to prevent heat stress, especially when wearing protective clothing such as fire-fighter jackets.’

However, power supply is a major issue. Zysset explains: ‘Our demonstrators worked so far either with a table-top power supply or they were powered using a a laptop computer – for example, for the temperature sensing shirt.’ One future solution might be an attachable button containing a battery.’

Meanwhile, Ana Claudia Arias’ team at Berkeley University, US, is printing electronics directly onto substrates such as cloth and plastics. She is working with organic materials, such as polythiophenes, that can be processed from solution to make inks. ‘This means we can use an inexpensive technique like screen-printing – just like when a graphic is printed onto to a t-shirt – to print the ink onto substrates like plastics or cloth,’ she explains. ‘The organic ink material can contain conductors, dielectrics, semiconductors, batteries and memory.’

The novelty, she says, lies in being able to deposit all the electronics in an integrated and straightforward way. In contrast, silicon cannot be made into inks and each component has to be fabricated in a different process.

Since the organic electronics are flexible, they can wrap around the body. Arias is investigating medical applications, producing sensors that can be printed onto material that would monitor the body’s vital signs such as temperature and blood pressure. Presently, she is working on a strip of material that could be worn ‘like a Band-Aid’ that is both durable and washable.

The basic structure involves a slice of polyvinylidene difluoride sandwiched between two electrodes. Exerting pressure on the polymer changes the polymer’s electrical characteristics, altering the voltage measured between the two electrodes. Using existing technology for thin-film batteries and a layer of circuitry including common electrical elements, a device as thin as a piece of tape becomes a functional, bendable pressure sensor. ‘This is just the beginning,’ Arias says, ‘wearable sensors that measure environmental and biological signals can open up many applications for people who play sports, are in the hospital, or just want to monitor their daily health.’

In the search for flexible electronics for textiles, a team of scientists from the Universities of Exeter and Bath in the UK claim to have created the most transparent, lightweight and flexible material for conducting electricity.³ GraphExeter is adapted from graphene, the thinnest substance capable of conducting electricity. The team is now developing a spray-on version, which could be applied straight onto fabrics, mirrors and windows.

‘Graphene is an ideal candidate for transparent optoelectronic devices. A sheet of carbon just one atom thick, it has spectacular transparency and electrical conductivity,’ explains lead researcher Monica Craciun at the University of Exeter. However, graphene made currently is not nearly conductive enough to replace indium tin oxide (ITO), the most widespread transparent conductor in optoelectronic applications. ‘ITO is 10 ohms/square, whereas the best pristine graphene is between 100 Ohm/square and 1500 Ohm/square depending on the number of layers and the fabrication method,’ explains Cracium.

So the team developed a graphene-based material in which they sandwiched ferric chloride molecules between the flexible and transparent layers of graphene. This improved version of graphene outperforms the electrical properties of ITO and of any other known carbon nanotube-based transparent and flexible conductor currently used in optoelectronics, Craciun reports. ‘It has an outstandingly high electrical conductivity and optical transparency. By introducing ferric chloride between the graphene layers, the charge density of graphene is heavily increased which results in a record low sheet resistance of 3 Ohm/square.’

The development of smart materials that are able to display information would have a radical impact on our society, Craciun believes. Since textiles are ubiquitous, the ability to embed display-based information and communication devices into wearable textiles ‘would transform clothing into mobile phones, displays with electronic newspapers or GPS-activated maps, and would certainly facilitate exchanges between individuals and communities. Developing these materials is also essential for other societal needs, such as biomedical monitoring, communication tools for sensory-impaired people, and personal security.’