Advanced contact lenses, known as smart contact lenses, have the remarkable capability to display visible information on our corneas and facilitate access to augmented reality. These lenses serve multiple purposes, including vision correction, health monitoring, and disease detection for individuals with chronic conditions like diabetes and glaucoma. In the future, they might even capture and transmit wearers' sensory experiences to cloud-based data storage. However, the realization of this potential depends on the development of a suitable and safe battery to power these lenses. Existing rechargeable batteries rely on metal-containing wires or induction coils, making them inappropriate for ocular use due to discomfort and associated risks.

Scientists from NTU Singapore (Singapore) have successfully created a flexible battery, as thin as a human cornea, which stores electricity when immersed in saline solution, thereby offering the potential to power smart contact lenses. The NTU-developed battery is built from biocompatible materials and is devoid of wires or toxic heavy metals found in lithium-ion batteries and wireless charging systems. It features a glucose-based coating that interacts with sodium and chloride ions in the surrounding saline solution. The water within the battery functions as the conduit for generating electricity. The battery can also harness power from human tears, which contain sodium and potassium ions in lower concentrations. Testing with a simulated tear solution indicated that the battery's lifespan could be extended by an hour for every twelve-hour wearing cycle. Moreover, it can be conventionally charged using an external power source.

The innovation was put to the test using a simulated human eye. Merely 0.5 millimeters thick, the battery draws power from basal tears, the continuous tears that form a thin film over our eyeballs. The battery's glucose oxidase coating reacts with the sodium and chloride ions in tears, generating current within the contact lenses for embedded devices to function. It was demonstrated that the battery produces a current of 45 microamperes and a maximum power of 201 microwatts, sufficient for powering smart contact lens. Lab tests revealed that the battery could be charged and discharged up to 200 times, while typical lithium-ion batteries endure around 300 to 500 charging cycles. The research team suggests that the battery should be immersed in a suitable solution rich in glucose, sodium, and potassium ions for at least eight hours during sleep to facilitate charging. The NTU team aims to further enhance the battery's electrical output and collaborate with contact lens manufacturers to implement this technology.

“Although wireless power transmission and supercapacitors supply high power, their integration presents a significant challenge due to the limited amount of space in the lens,” said co-first author Ms. Li Zongkang, a PhD student from NTU’s EEE. “By combining the battery and biofuel cell into a single component, the battery can charge itself without the need for additional space for wired or wireless components. Furthermore, the electrodes placed at the outer side of the contact lens ensures that the vision of the eye cannot be obstructed.” 

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