Smart contact lens battery charged by human tears
Researchers at the Nanyang Technological University (NTU) in Singapore have devised a micrometer-thin battery that can power smart contact lenses and be charged with tears, a university press release said.
Contact lenses have long been used for vision correction. With technological advancements, companies have been working to make smarter versions, connecting to devices like smartphones and displaying information close to the wearer's eyes. Such applications require the lenses to have an internal battery.
Interesting Engineering has previously reported how companies have managed augmented reality displays on smart contact lenses. These devices use extremely thin batteries with induction coils and wires made up of metals, which carry risks for the user's eyes. A research team led by Lee Seok Woo, an assistant professor at NTU's School of Electrical and Electronic Engineering (EEE), has now devised a battery that does use any metals.
The battery is made up of biocompatible materials and has a glucose-based coating. This coating can react with sodium and chloride ions in the water inside the battery, serving as circuitry for generating electricity. Since sodium and chloride ions are also found in tears, the lenses could practically be charged as the user wore them.
"This research began with a simple question: Could contact lens batteries be recharged with our tears?" said Woo in a press release. "Previous techniques for lens batteries were not perfect as one side of the battery electrode was charged and the other was not. Our approach can charge both electrodes of a battery through a unique combination of enzymatic reaction and self-reduction reaction." Since the approach uses glucose and water, they are safe for the user and the environment after the life is completed.
Since the battery is thinner than a millimeter, the biggest question is how much energy it can pack and how long it can be used. To test out their invention, the researchers worked with a simulated eye model.
Their experiments found that the battery can produce a current of 45 microamperes and a maximum power of 201 microwatts. This amount of power will suffice for running the device for most of the day.
This is where the charging with tears also helps. When kept in a simulated tear solution, the battery life could be extended by an additional hour for every twelve hours of usage. When not in use, the battery can be kept in a concentrated saline solution that contains high amounts of glucose, potassium, and sodium ions for charging.
The team found that the battery can undergo 200 charge-discharge cycles. "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 Li Zongkang, a Ph.D. student at NTU who was involved in this work. "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."
The team is now working on increasing the amount of electric current the battery can discharge and partnering with contact lens companies to bring this to the market.
The research findings were published in the journal Nano Energy.
Smart contact lenses developed for medical and personal applications will require miniaturized power supplies, with integrated batteries providing promising options. However, charging batteries in small wearable devices is challenging because it is difficult to transfer electrical power through a miniaturized wired connection or wireless transmission unit. Herein, we develop safe, tear-based batteries integrated into contact lenses that are charged by biofuel during their storage. Enzymatic reactions of glucose oxidase and self-reduction of conducting polymer are utilized to charge the cathode and anode, respectively. The electrodes are embedded into a contact lens and discharged in an artificial tear solution followed by charging in a glucose solution via a bio-reaction (called bio-charging). The bio-charging battery shows a discharging capacity of 45 μA cm− 2 and a maximum power of 201 μW cm− 2, with its performance verified over 15 cycles. The bio-chargeable battery can also be charged conventionally by an external power supply.Abstract