A novel window into 'smart' glass

From fixing bones to making antibacterial surfaces, Michael Allen talks to the researchers making glass that has additional functionality and performance

Glass is ubiquitous in everyday life. Being highly transparent, stable and durable, it's an important material for a myriad of applications, from simple windows to touch screens on our latest gadgets to photonic components for hi-tech sensors.

The most common glasses are made from silica, lime and soda. But for centuries additional ingredients have been added to glass to confer properties such as colour and heat-resistance. And researchers are still working on glass, seeking to give it further functionality and improve its performance for specific tasks, creating increasingly hi-tech glass and what could be referred to as "smart" glass.

Smart materials aren't easy to define, but broadly they are designed to respond in a specific way to external stimuli. In terms of glass, the most obvious "smart" application is for windows – in particular, controlling the amount of light that passes through the glass. That way we can boost the energy efficiency of any building: reducing the heat in the summer, while keeping it warm in colder weather.

The colour or opacity of some smart glass can be changed by applying a voltage to the material, thereby altering certain optical properties – such as absorption and reflectance – in a way that is reversible. Such "electrochromic" smart windows can control the transmittance of certain frequencies of light, such as ultraviolet or infrared, on demand, or even block them altogether. The application of this technology is popular not only in buildings, but also in electronic displays and tinted car windows.

Indeed, electrochromic windows are ahead of other technologies in this field, and have already been commercialized. But despite working well, they have some obvious disadvantages. They are quite complex and expensive, and retrofitting them to older buildings generally requires installing new windows, window frames and electrical connections. They are also not automatic – you need to switch them on and off.

To address some of these issues, researchers have been working on thermochromic windows, which are triggered by changes in temperature instead of voltage. One big attraction is that they are passive – once installed, their properties change with the ambient temperature, with no need for human input. The dominant method for creating such thermochromic windows is applying a coating of vanadium dioxide to glass (Joule 10.1016/j.joule.2018.06.018), but other materials such as perovskites can also be used (J. App. Energy 254 113690). These materials undergo a phase transition, becoming more or less transparent as the temperature changes, an effect that can be tuned for different conditions.

While vanadium dioxide shows a lot of promise for smart windows, there are obstacles to overcome. Due to its strong absorption, vanadium dioxide produces an unpleasant brownish-yellow tint and further work is needed on environmental stability (Adv. Manuf. 6 1). A recent review also suggests that although these technologies could provide significant energy savings, more research is needed on their use and impact in real-world settings. For example, the energy performance of thermochromic windows has been found to vary a lot between different cities using the same film type, but far less so between different film types used in the same city (J. App. Energy 255 113522).

But hi-tech glass doesn't end with smart windows. Researchers have found that if they add more unusual metals to glass, it can help to protect solar panels and make them more efficient (see box: Improving photovoltaic cover glass). Bioactive glass, meanwhile, can help us regrow bone and other tissues (see box: Fixing bones and other tissue), while new etching processes could allow us to add multiple functions to glass without the need for surface coatings (see box: Anti-reflective, self-cleaning and antibacterial). And although not traditional optical glasses, new phase-change materials could help create lighter and more compact optical systems (see box: Non-mechanical control of light). Finally, glass might one day even be able to heal itself (see box: Immortal glass).

It might seem surprising, but not all sunlight is good for solar cells. While photovoltaic units convert infrared and visible light into electrical energy, ultraviolet (UV) light damages them. Just like a case of sunburn, UV light negatively impacts the carbon-based polymers used in organic photovoltaic cells. Researchers have found that the damage from UV light makes the organic semiconductor layer more electrically resistant, reducing current flow and the cell's overall efficiency.

This issue isn't limited to organic cells. UV light also hampers the more common silicon-based photovoltaic, which consists of a stack of different materials. The silicon-based photoactive layer is sandwiched between polymers that protect it from water ingress, and this unit is then topped with a glass cover, which further protects it from the elements while allowing sunlight through. The problem with UV light is that it damages the polymers, allowing water to penetrate and corrode the electrodes.

Paul Bingham, an expert in glass at Sheffield Hallam University, UK, explains that to improve solar-panel efficiency "the overriding direction of travel in the past few decades has been to make the glass clearer and clearer". This means removing chemicals that colour the glass, such as iron, which produces a green tint. Unfortunately, as Bingham explains, this lets more UV light through, damaging the polymer further.

Bingham and his colleagues have therefore been going in the other direction – they have been chemically doping glass such that it absorbs damaging UV light but is transparent to the useful infrared and visible light. Iron is still not an ideal additive, as it absorbs some visible and infrared wavelengths, and the same is true for other first-row transition metals such as chromium and cobalt.

Instead, Bingham's team has been experimenting with second- and third-row transition elements that would not normally be added to glass, such as niobium, tantalum and zirconium, along with other metals like bismuth and tin. These create strong UV absorption without any visible colouration. When used in the cover glass, this extends the lifespan of photovoltaics and helps them maintain a higher efficiency, so they generate more electricity for longer.

The process also has another benefit. "What we’ve found is that many of the dopants absorb UV photons, lose a bit of energy and then they re-emit them as visible photons, so fluorescence basically," Bingham says. They create useful photons that can be converted to electrical energy. In a recent study, the researchers showed that such glasses can improve the efficiency of solar modules by up to about 8%, compared with standard cover glass (Prog. in Photovoltaics 10.1002/pip.3334).

In 1969 biomedical engineer Larry Hench, from the University of Florida, was looking for a material that could bond with bone without being rejected by the human body. While working on a proposal for the US Army Medical Research and Design Command, Hench realized that there was a need for a novel material that could form a living bond with tissues in the body, while not being rejected, as is often the case with metal and plastic implants. He eventually synthesized Bioglass 45S5, a particular composition of bioactive glass that is now trademarked by the University of Florida.

A specific combination of sodium oxide, calcium oxide, silicon dioxide and phosphorus pentoxide, bioactive glass is now used as an orthopaedic treatment to restore damaged bone and repair bone defects. "Bioactive glass is a material that you put into the body and it starts to dissolve, and as it does it actually tells cells and bone to get more active and produce new bone," says Julian Jones, an expert in the material, from Imperial College London, UK.

Jones explains that there are two main reasons the glass works so well. First, as it dissolves it forms a surface layer of hydroxycarbonate apatite, which is similar to the mineral in bone. This means it interacts with bone and the body sees it as a native, rather than foreign, object. Second, as it dissolves, the glass releases ions that signal cells to produce new bone.

Clinically, bioactive glass is mainly used as a powder that is formed into a putty and then pushed into the bone defect, but Jones and his colleagues have been working on 3D-printed scaffold-like materials for larger structural repairs. These are inorganic–organic hybrids of bioactive glass and polymer that they refer to as bouncy Bioglass. The 3D-printed architecture provides good mechanical properties, but also a structure that encourages cells to grow in the right way. In fact, Jones has found that by changing the pore size of the scaffold, bone marrow stem cells can be encouraged to grow either bone or cartilage. "We’ve had a huge amount of success with bouncy Bioglass cartilage," Jones says.

Bioactive glass is also being used to regenerate chronic wounds, such as those caused by diabetic ulcers. Research has shown that cotton wool like glass dressings can heal wounds, such as diabetic foot ulcers, that have not responded to other treatments (Int. Wound J. 19 791).

But Jones says the most common use of bioactive glass is in some sensitive toothpastes, where it prompts the natural mineralization of teeth. "You have sensitive teeth because you have tubules that go into your nerve cavity in the centre of the tooth, so if you mineralize those tubules there is no way into the pulp cavity," he explains.

At University College London, researchers have been etching nanoscale structures into the surface of glass to give it multiple different functions. Similar techniques have been tried in the past, but it has proved challenging and complicated to structure the glass surface with fine enough detail. Nanoengineer Ioannis Papakonstantinou and his colleagues, however, have recently developed a novel lithography process that allows them to detail glass with nanoscale precision (Adv. Mater. 33 2102175).

Inspired by moths that use similar structures for optical and acoustic camouflage, the researchers engraved a glass surface with an array of sub-wavelength, nanoscale cones to reduce its reflectiveness. They found that this structured surface reflected less than 3% of light, while a control glass reflected around 7%. Papakonstantinou explains that the nanocones help bridge changes between the refractive index of the glass surface and that of air, by smoothing out the usually abrupt air-to-glass transition. This reduces scattering and therefore the amount of light that reflects off the surface.

The surface is also superhydrophobic, repelling droplets of water and oils so that they bounce off cushions of air trapped in the nanostructures. As the droplets roll off, they pick up contaminates and dirt, making the glass self-cleaning, as Papakonstantinou explains. And as a final benefit, bacteria struggle to survive on the glass, with the sharp cones piercing their cell membranes. Focusing on Staphylococcus aureus – the bacteria that cause staph infections – scanning electron microscopy has shown that 80% of bacteria that settle on the surface die, compared with around 10% on standard glass. According to the researchers, this is the first demonstration of an antibacterial glass surface.

Light is generally controlled in optical systems by moving parts, such as a lens that can be manipulated to change the light's focal point or steer a beam. But a new class of phase-change materials (PCMs) could change the properties of optical components without any mechanical intervention.

A PCM can switch between having an organized crystalline structure to being amorphous and glass-like when some form of energy, such as an electrical current, is applied. Such materials have long been used to store data on optical discs, with the two phases representing the two binary states. But these materials have not really been used in optics beyond such applications, because one of the phases is normally opaque.

Recently, however, researchers in the US have created a new class of PCMs based on the elements germanium, antimony, selenium and tellurium, known as GSST (Nature Comms 10 4279). They discovered that while both the glassy and crystalline states of these materials are transparent to infrared light, they have widely different refractive indexes. This can be exploited to create reconfigurable optics that can control infrared light.

Juejun Hu, a materials scientist at the Massachusetts Institute of Technology, says that instead of having an optical device with one application, you can programme it to have several different functions. "You could even switch from a lens to a diffraction grating or a prism," he explains.

The properties of PCMs are best utilized, Hu says, by creating optical metamaterials, in which nanoscale, sub-wavelength structures are fashioned on the surface and each is tuned to interact with light in a specific way to create a desired effect, such as focusing a beam of light. When an electrical current is applied to the material, the way the surface nanostructures interact with the light changes as the material's state and refractive index switches.

The team has already demonstrated that it can create elements such as zoom lenses and optical shutters that can quickly switch off a beam of light. Kathleen Richardson, an expert in optical materials and photonics at the University of Central Florida, who worked with Hu on the GSST materials, says that these materials could simplify and reduce the size of sensors and other optical devices. They would enable multiple optical mechanisms to be combined, reducing the number of individual parts, and remove the need for various mechanical elements. "Multiple functions in the same component makes the platform smaller, more compact and lighter weight," Richardson explains.

"You can bend the laws of physics, but you can't break them," says Paul Bingham, who specializes in glasses and ceramics at Sheffield Hallam University, UK. "Fundamentally, glass is a brittle material and if you apply enough force over a small enough part of the glass then it's going to break." Still, there are various ways that their performance can be improved.

Consider mobile phones. Most smartphone screens are made from chemically toughened glass, with the most common being Gorilla Glass. Developed by Corning in the 2000s, this strong, scratch-resistant yet thin glass can now be found in around five billion smartphones, tablets and other electronic devices. But chemically strengthened glass is not completely unbreakable. In fact, Bingham's phone screen is broken. "I dropped it once and then I dropped it again and it landed on exactly the same point and that was game over," he says.

To improve the durability of glass screens further, Bingham has been working on a project entitled "Manufacturing Immortality" with polymer scientists at Northumbria University, led by chemist Justin Perry, who have developed self-healing polymers. If you cut these self-healing polymers in half and then push the pieces together, they will, in time, join back together. The researchers have been experimenting with applying coatings of such materials to glass.

If you apply enough force, these screens are still going to break, but if you dropped one and cracked the polymer layer it could self-heal. This will happen under ambient, room-temperature conditions, although heating them up a bit, by leaving them somewhere warm for example, could speed up the process. "It's about improving lifetimes of products, making them more sustainable and making them more resilient," Bingham says. And it could be useful for many products that use glass as a protective layer, not just smartphones.