(Nanowerk Spotlight) As wearable technologies like smart watches and fitness trackers continue to grow in popularity, scientists are searching for ways to make these devices more environmentally friendly. Currently most wearables are made from non-biodegradable components, meaning they pile up in landfills after disposal. But researchers from Beijing Normal University in China have developed a fully biodegradable and biocompatible “ionotronic skin” that could enable transient wearable electronics to reduce electronic waste.
The term “ionotronic” refers to materials that conduct ions (charged particles) rather than electrons like traditional electronics. Ions move through the skin material to transmit signals instead of electrons flowing through wires.
The ultimate aim of this research is to create electronic devices with zero e-waste footprint, a crucial step in mitigating the growing electronic waste crisis.
Schematic illustration of the designed double-network ionotronic skin and its mechanism of degradation in PBS solution. (Reprinted with permission by Wiley-VCH Verlag)
The ionotronic skin is made from a double network of natural polyelectrolytes, substances found in nature that conduct ions. One network contains carboxylated chitosan (CCS), a derivative of chitosan that is biocompatible and provides good ionic conductivity. The other network is made from polymerized sulfobetaine methacrylate (SBMA), a zwitterion material that is also biocompatible. According to the researchers, the SBMA and CCS polymers are connected by hydrogen bonds and electrostatic interactions to form a unified skin material. Glycerol and water in the blending system improve the cross-linking and make the network more cohesive.
The ionotronic skin’s rapid degradation – a key advantage – is attributed to the water-soluble nature of the carboxylated chitosan (CCS) and the easily separable fragments of polymerized sulfobetaine methacrylate (SBMA).
The material is designed to degrade quickly in salt solutions because the electrostatic bonds between SBMA molecules can be easily broken, separating the zwitterion dimers. When soaked in phosphate buffered saline at room temperature, the material completely breaks down in just 3 days. This is much faster than previous attempts using materials like polycaprolactone and polylactic acid (PLA), which have high modulus and poor compliance with human skin.
The transient nature also gives the ionotronic skin an advantage over non-biodegradable devices in reducing electronic waste, which has grown into a pressing issue with e-waste projected to reach 74.7 million tons by 2030.
Importantly, the salt solutions in which the ionotronic skin degrades quickly mimic physiological fluids, such as blood and interstitial fluid. This characteristic further boosts its potential for use in medical implants and other applications that come in direct contact with the human body. When implanted, the skin would naturally break down into harmless components, eliminating concerns about long-term device retrieval or disposal.
In addition to being biodegradable, the ionotronic skin exhibits other properties that make it well-suited for wearable applications. Through careful macromolecular engineering, the researchers optimized the chemical structure to achieve the desired characteristics. It has high ionic conductivity, similar to levels seen in human skin, which allows it to efficiently transfer signals. The skin material is also highly flexible with good adhesion strength, allowing it to stretch and stick well to human skin without losing conductivity.
The researchers demonstrated the ionotronic skin’s capabilities by using it to measure a variety of human electrophysiological signals. When adhered to volunteers’ wrists, the skin successfully recorded electrocardiogram (ECG) patterns like commercial electrodes. It also accurately measured electromyography (EMG) signals from arm muscles and electrooculography (EOG) signals generated by eye movements. The skin even picked up the faint electrical patterns of electroencephalograms (EEG) reflecting brain activity.
Most impressively, the team showed that the ionotronic skin can be implanted to act as a biodegradable electrode. They integrated the skin with the sciatic nerve of a bullfrog to measure neural action potentials and stimulate muscle contractions. Over 3 days, the signals remained strong until the material fully dissolved, avoiding the need for device removal surgery.
The biocompatible and highly conductive ionotronic skin overcomes key challenges in transient wearable electronics. Because it can degrade completely in physiological fluids, it could be used for skin patches, biosensors, and medical implants that don’t generate e-waste after use.
While promising, more work is needed to perfect the ionotronic skin for commercial applications, like ensuring it works well with conventional electronic components. But this advance suggests that environmentally-friendly transient electronics could soon displace traditional non-biodegradable wearable devices. The researchers believe their strategy of using natural polyelectrolyte derivatives could pave the way for green, biodegradable electronics that minimize ecological impacts.
While the ionotronic skin presents a step toward sustainable electronics, there are still some challenges to be addressed. One key concern is its compatibility with existing electronic components, most of which are not designed for biodegradable or transient materials.
Furthermore, while the skin has shown promise in laboratory settings, its durability and performance in real-world conditions have yet to be thoroughly tested. Cost is another factor; biodegradable materials often come with a higher price tag, which could limit widespread adoption initially.
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