Compostable electronics

Imagine being able to dispose of your old smartphones and tablets by just throwing them in the compost. That’s the world that could be ushered in by the latest work on biodegradable electronics. Jon Evans reports

The steady expansion of electronic devices into every corner of our lives has led to a matching expansion in electronic waste. It’s estimated that over 50m t of electronic waste are now produced each year, of which less than 20% is recycled. Meaning the rest either gathers dust in our homes or ends up in landfills.

How much better if these old devices could simply be dumped in the compost to gradually degrade away into harmless chemicals. This is the bright, green future of biodegradable electronics that Ravinder Dahiya, Professor of electronics and nanoengineering at the University of Glasgow in the UK, is currently working towards.

‘The idea is that electronic devices get converted into useful products, and compost is one of them,’ says Dahiya. ‘So, there’s no waste at all.’ To that end, Dahiya and his group are working with various partners, including the British semiconductor company ARM and the UK National Physical Laboratory, on the Green Energy-Optimised Printed Transient Integrated Circuits (GEOPIC) project. This project aims to design electronics that are more easily recycled into new forms or naturally degrade, and recently received a £1.5m grant from the UK Engineering and Physical Sciences Research Council (EPSRC).

In addition to helping reduce electronic waste, biodegradable electronics could also prove valuable for medical applications, specifically for creating sensors and other devices that are implanted into the human body. Rather than needing to be removed with a further surgical procedure, these sensors and devices would be naturally broken down by the body into harmless compounds when no longer needed. Dahiya and his group are exploring these medical applications as well.

To produce a biodegradable electronic device, all the materials it is made from need to be biodegradable, including conductors, semiconductors, insulators and the substrate that houses all these components. This isn’t actually too much of a problem because it turns out there are numerous biodegradable options (see below).

The main challenge then isn’t finding the biodegradable materials required to produce all the components in an electronic device. Rather it’s finding cost-effective ways to join these materials together to produce biodegradable electronic devices that work just as well as conventional, non-biodegradable electronics. These devices must also degrade at the right time and under the right conditions, rather than while they’re still being used.

‘The main challenges are scaling up to a small-scale microprocessor level and working out how to integrate various circuits together, including in three-dimensions,’ said Dahiya. ‘The microprocessor should have a performance on a par with conventional silicon-based electronics and it should degrade at a time of our choosing.’

Dahiya and his group recently achieved the first step in this integration, by developing working transistors from biodegradable materials (Adv. Electron. Mater.; doi: 10.1002/aelm.202200170). They produced these transistors using a novel, inexpensive printing technique they developed called in-tandem contact-transfer printing. This involves printing nanomaterials on a surface and then picking them up by pressing a stamp with a defined pattern onto the surface. This stamp is then pressed on a second surface to deposit the nanomaterials in the defined pattern.

The idea is that electronic devices get converted into useful products, and compost is one of them, so there’s no waste at all.
Ravinder Dahiya Professor of electronics and nanoengineering, University of Glasgow, UK

To produce biodegradable transistors with this technique, Dahiya and his group grew vertical zinc oxide nanowires over a surface and then essentially scraped this surface over a silicon wafer, transferring the nanowires so they lay horizontally on the wafer. Next, they pressed a stamp comprising lots of small, raised squares onto the wafer to pick up the nanowires, and then deposited them as lots of individual squares onto a flexible magnesium foil coated in silicon dioxide. Finally, they printed gold electrodes on two opposite sides of each square, such that the nanowires connected the two electrodes.

Each square is a separate transistor, with the nanowires forming the semiconducting channels between the two electrodes and the magnesium foil coated in silicon dioxide forming the gate that turns the transistor on and off.

‘Our latest paper describes biodegradable transistors on magnesium foil,’ says Dahiya. ‘It still uses some materials that are non-biodegradable, such as the gold electrodes, but they can be recovered. Once the whole device degrades into a solution through standard processes, you can extract the gold.’

There are two basic ways to control the degradation process. One way is to use materials that only degrade under specific conditions that they don’t encounter in normal use, such as the raised temperatures in composts. But this isn’t an option for devices that need to function and degrade under the same conditions, such as implantable devices. In that case, the devices can be embedded in biodegradable polymers that protect them from these conditions for a set period of time, dictated by the thickness of the protective polymers.

Eco power

Electronic devices consist of more than just circuits and microprocessors; however, they also require an energy source to power the device and often a display to show information, and so researchers are working to develop biodegradable versions of these as well. For example, Hong Jin Fan and his colleagues at Nanyang Technological University in Singapore recently developed a biodegradable zinc battery (Adv. Sci., 2022, 9, 2103894).

In a battery, power is generated by ions flowing through a liquid electrolyte between an anode and a cathode, with this flow driving an associated movement of electrons that produces electricity. A separator is also required to prevent the electrodes coming into contact with each other and causing a short circuit.

In Fan’s biodegradable battery, a piece of paper acts as both the electrolyte and the separator. It’s not a standard piece of paper though, but rather cellulose filter paper reinforced with a polyacrylamide hydrogel, then soaked in a solution of potassium hydroxide and lithium hydroxide. Finally, the anode and cathode are simply printed on the opposite sides of the paper, with the anode ink comprising a mixture of zinc and carbon black and the cathode ink comprising a mixture of nickel hydroxide and carbon black.

The idea is that zinc ions flow through the paper from the anode to the cathode, with the soaked hydrogel-impregnated paper acting as both the electrolyte through which the ions flow and the separator that keeps the electrodes apart. Fan and his colleagues showed that this battery could be charged by a solar cell and produce enough electricity to power a small electric fan for 45 minutes but would also degrade completely when buried in soil for four weeks.

Taking a similar paper-based approach, Gustav Nyström and his colleagues at the Swiss Federal Laboratories for Materials Science and Technology (EMPA) in Dübendorf recently produced a biodegradable supercapacitor (Adv. Mater., 2021, 33, 2101328). Like a battery, a supercapacitor generates power from the movement of ions in an electrolyte, but in a supercapacitor this movement is of oppositely charged ions.

Charging a supercapacitor causes these oppositely charged ions to be separated on the surface of an anode and cathode. Electricity is then generated when these ions move back towards each other. Because the ions are held on the surface of the electrodes, rather than being intercalated inside, supercapacitors can’t store as much energy as a battery, but they can be charged and discharged much faster. They are thus used for applications that require rapid delivery of small amounts of power, such as powering camera flashes.

To produce a biodegradable supercapacitor, Nyström and his colleagues printed layers of four different gel-like materials on top of each other. The bottom layer is a nanocellulose gel that acts as a substrate. On top of this, they print a current-collecting layer made of carbon black, and then an electrode layer made of a mixture of carbon black and nanocellulose. Finally, they print an electrolyte layer made of a mixture of nanocellulose, glycerol and sodium chloride.

These four layers are then simply folded over to create a supercapacitor in which a central electrolyte is surrounded by two electrodes and two current collectors. Nyström and his colleagues showed that this supercapacitor could power a small electric clock but would completely degrade away in a couple of months when placed in compost.

Degradable displays

Just as there is more than one way to produce a biodegradable power source, it turns out there is more than one way to produce a biodegradable display. Gerardo Hernandez-Sosa and his colleagues at the Karlsruhe Institute of Technology in Germany recently developed a biodegradable display made from a conducting polymer that works by electrochromism (J. Mater. Chem. C, 2020, 8, 16716). Meanwhile, Wei Huang and his team at Nanjing Tech University in China developed a biodegradable display made from fish scales that works by electroluminescence (ACS Nano, 2020, 14, 3876).

Electrochromism involves a material changing colour or opacity when a voltage is applied. To produce a biodegradable electrochromic display, the Hernandez-Sosa team coated a gelatin-based electrolyte onto a cellulose-based substrate. Onto this, they printed small rectangles of the conducting polymer PEDOT:PSS, as the electrochromic layer, which they connected with gold electrodes. When they applied a voltage to the electrodes, the PEDOT:PSS rectangles changed from transparent to dark blue, with an electrochromic contrast of 32%, within around three seconds. But when placed into compost, most of the display, apart from the gold electrodes, degraded away within a few weeks.

By contrast, electroluminescent materials generate light when a voltage is applied. To produce a biodegradable electroluminescent display, Huang’s team first developed a way to produce transparent films from fish gelatin derived from waste fish scales. They then combined this film with silver nanowires to form an electrode, by pouring a liquid precursor of the film over a surface coated with the nanowires and letting the precursor dry.

Next, the team coated the film with metal-based electroluminescent compounds and then added a layer of silver nanowires mixed with a water-soluble synthetic polymer to form the other electrode. The resulting display was flexible and would light up when a voltage was applied but would also dissolve in water within a few minutes.

At the moment, these displays are very basic, but then the first biodegradable electronic devices will only be very basic. Early applications will probably include implantable sensors for measuring things like temperature and pressure, and simple solar cells. Smartphones that can be thrown into the compost are still a long way off.

Although maybe not that long. ‘As far as the GEODISC proposal is concerned, we need to demonstrate biodegradable electronics within three years,’ Dahiya says. ‘So that means by the end of 2025 we should be able to see something.’