Saline-powered ultrathin battery developed for smart contact lenses

Scientists from Nanyang Technological University, Singapore (NTU Singapore) have developed a flexible battery as thin as a human cornea, which stores electricity when it is immersed in saline solution and could one day power smart contact lenses. Smart contact lenses are high-tech contact lenses capable of displaying visible information on corneas and can be used to access augmented reality. Current uses include helping to correct vision, monitoring wearers’ health, and flagging and treating diseases for people with chronic health conditions such as diabetes and glaucoma. Smart contact lenses could also be developed to record and transmit everything a wearer sees and hears to cloud-based data storage.

However, in order to achieve this, a safe and suitable battery must be developed to power smart contact lenses. Existing rechargeable batteries rely on wires or induction coils that contain metal and are unsuitable for use in the human eye. The NTU-developed battery is made of biocompatible materials and does not contain wires or toxic heavy metals, such as those in lithium-ion batteries or wireless charging systems. It has a glucose-based coating that reacts with the sodium and chloride ions in the saline solution surrounding it, while the water the battery contains serves as the ‘wire’ or ‘circuitry’ for electricity to be generated.

The battery could also be powered by human tears as they contain a low concentration of sodium and potassium ions. Testing the current battery with a simulated tear solution, the researchers showed that its life would be extended an additional hour for every 12-hour wearing cycle it is used. The battery can also be charged conventionally by an external power supply.

Associate Professor Lee Seok Woo, who led the study, said previous techniques for lens batteries were not perfect as one side of the battery electrode was charged and the other was not. NTU’s approach can charge both electrodes of a battery through a combination of enzymatic reaction and self-reduction reaction. Along with the charging mechanism, it relies on glucose and water to generate electricity, both of which are safe to humans and are less harmful to the environment when disposed of, compared to conventional batteries.

Co-first author Dr Yun Jeonghun said the most common battery charging system for smart contact lenses requires metal electrodes in the lens, which are harmful if they are exposed to the naked human eye, while induction charging requires a coil to be in the lens to transmit power, much like a wireless charging pad for a smartphone. “Our tear-based battery eliminates the two potential concerns that these two methods pose, while also freeing up space for further innovation in the development of smart contact lenses,” Yun said.

Associate Professor Murukeshan Vadakke Matham from the NTU School of Mechanical & Aerospace Engineering said the battery should be suitable for human usage, as it is based on glucose oxidase, which occurs naturally in humans, and is powered by chloride and sodium ions, such as those in our tears. “The smart contact lenses industry has been looking for a thin, biocompatible battery that does not contain heavy metals, and this invention could help further their development to meet some unmet needs of the industry,” Matham said.

The researchers have filed for a patent through NTUitive and are also working towards commercialising their invention. The findings were published in the scientific journal Nano Energy.

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