By Siegfried Bauer and Martin Kaltenbrunner
Living organisms are made largely from soft substances that have the ability to stretch, and to heal when damaged. By contrast, everyday electronic appliances are made mostly of hard, brittle materials incapable of self-repair. Scientists desire to narrow this gap between the animate and digital worlds; this will require new approaches in materials science. On page 411, Oh et al. report their use of non-covalent-bonding mechanisms to make high-performance organic semiconductors that are intrinsically stretchable and healable. This allowed the authors to fabricate transistors — elementary electronic components used in amplifiers and logic circuits — entirely from stretchable materials in thin films, potentially opening the way to new generations of wearable electronic devices.
In his 1940 book The Nature of the Chemical Bond and the Structure of Molecules and Crystals, the chemist Linus Pauling beautifully described the non-covalent origin of hydrogen bonds: “It has been recognized in recent years that under certain conditions an atom of hydrogen is attracted by rather strong forces to two atoms, instead of only one, so that it may be considered to be acting as a bond between them.” Being weaker than covalent bonds, hydrogen bonds easily rearrange themselves within networks of macromolecules, a fact that has been exploited as a self-healing mechanism in polymers.
By contrast, hydrogen bonds were thought to be detrimental in organic semiconductors. Research into organic electronics in the 1990s focused mostly on π-conjugated molecules, in which electrons are highly delocalized — an important characteristic for efficient charge transport in a semiconductor. Intermolecular hydrogen bonds were expected to interrupt electron delocalization. But in the past few years, it has emerged that the possible structural basis for building organic semiconductors is much larger than was previously anticipated4, and, particularly surprisingly, that charges can move effectively in materials that contain intermolecular hydrogen bonds5. Oh et al. have taken that surprising finding to a new level, by incorporating hydrogen bonds into polymers not only to make the materials tolerant to stretching, but also to allow impressive charge transport. They achieved this by tailoring the molecular structure of semiconducting polymers to incorporate crystalline parts, which are responsible for charge transport, and amorphous regions crosslinked through hydrogen bonding, which take up mechanical strain without greatly impairing the material's electronic properties (Fig. 1). The authors report that films of these polymers crack in response to severe elongation, but that the cracks can be healed by treating the materials with solvent vapour and heat. The healed polymers almost completely recover their initial electronic behaviour.
So far so good, but the next step is to use the materials to make a stretchable electronic device. Three general strategies have been developed to achieve this goal. The first involves placing rigid semiconductors on small flexible 'islands' embedded in or placed on elastomers (rubbery materials) that are connected by stretchable wires6. This approach also allows high-performance, off-the-shelf microelectronic devices to be integrated onto stretchable materials.
The second strategy is to laminate thin foils containing flexible inorganic7 or organic8 electronic components onto pre-stretched elastomers. When the elastomer is relaxed, the laminated system wrinkles, thus allowing the device to be subsequently re-stretched in the direction that the elastomer was first extended.
The third approach takes advantage of stretchability at the molecular level, either by using percolating networks of nanostructured conductors or semiconductors — typically tubes or wires — in elastomers9, or by incorporating softer materials into a stiff semiconducting polymer without disrupting charge-transport pathways10. Oh and colleagues' strategy falls into this latter subcategory, but the authors' use of materials that can heal themselves adds a previously unavailable feature to intrinsically stretchable electronic devices.
Oh et al. prepared stretchable transistors from their polymers as proof-of-concept devices, and observed that the devices maintained their electrical performance even after 500 stretching cycles at strains typical of most practical applications. Remarkably, the authors found that the transistors also maintained high charge-carrier mobility when mounted on human limbs and subjected to various common movements, such as twisting of the hand, folding of the arm and stretching of the elbow — as needed for wearable electronic devices.
It should be noted, however, that the authors' transistors are not as durable or stretchable as state-of-the-art devices based on islands and wrinkles, leaving plenty of room for future development. Moreover, the voltage required to operate the new transistors is large (of the order of tens of volts), which lowers the energy efficiency of the devices and is thus a concern for autonomously operating wearables. Smaller voltages will be essential for electronic devices that contact the human body. The voltage problem might be alleviated by greatly reducing the transistors' dimensions — most notably, the thickness of the electrically insulating layer between the control electrode and the transistor's semiconductor — although this would be challenging to achieve. It should also be noted that heating for prolonged periods with solvent vapour may be an impractical means of healing devices worn on human skin.
Perhaps the ultimate goal for scientists developing flexible electronics is to produce something that behaves like human skin: the stretchable organ that covers and protects our bodies, enables us to feel touch, pain and temperature, and triggers a healing process when wounded. A perfect technological imitation may not be possible, but Oh and colleagues' work is a milestone in the search for electronic skins that behave much like their archetype. In the shorter term, healable soft electronic devices hold promise for truly bionic and smart electrical appliances, and might revolutionize future generations of wearables.
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