HEALTH


molecule (amorphous indium gallium) in the active layer. The resulting closely packed, hexagonal nano-networks can detect subtle glucose changes in tear fluid. ‘This scaleable technique improves sensing by about two- to three- orders of magnitude – so well within the range of detecting glucose in tears,’ says Herman. Their goal is to integrate the sensor in a contact lens where it operates as part of an artificial pancreas. The sensor could transmit real-time glucose information to a wearable pump that delivers the hormones needed to regulate blood sugar: insulin and glucagon. The team is investigating using a


capacitor to store charge and power the sensors. ‘The capacitor could be charged using radiofrequency communications,’ explains Herman. ‘As the sensors would be very small and would not run all the time, ideally they would not use a lot of power.’


In theory, Herman says more than 2500 biosensors – each measuring a different bodily function – could be embedded in one millimetre square patch of an IGZO contact lens. ‘We can integrate an array of sensors into the lens and also test for other things: stress hormones, uric acid, pressure sensing for glaucoma and characteristic protein biomarkers of cancer risk. We can monitor many compounds in tears – and since the sensor is transparent, it doesn’t obstruct vision; more ‘real estate’ [area] is available for sensing on the contact lens.’ Once they are fully developed, the biosensors could transmit vital health information to smartphones and other wi-fi or Bluetooth-enabled devices. However, Herman says it could be a year or more before a prototype biosensing contact lens is ready for animal testing.


Dual purpose lenses Researchers in South Korea are also working on contact lenses to monitor glucose in tears, but their devices are also designed to measure high intraocular pressure. Intraocular pressure is the largest risk factor for glaucoma, a leading cause of blindness. The team – led by Jang- Ung Park at Ulsan National Institute of Science and Technology and Hong Kyun Kim at Kyungpook National


transparent, with a slightly visible spiral antenna. The circuits operate at a radio frequency so power sources are not required, the team adds. To monitor intraocular pressure,


This study can be used to diagnose diabetes and glaucoma by implementing two types of transparent electronic sensors in the production of smart contact lens sensors. We are now a step closer to the implementation of a fictional idea for a smart contact lens like in the films Minority Report and Mission: Impossible


Jang-Ung Park Ulsan National Institute of Science and Technology 2500


Number of biosensors - each measuring a dif- ferent bodily function – that could potentially be embedded in a one millimetre square patch of an IGZO contact lens


Researchers have dem- onstrated real-time glu- cose detection on a live rabbit eye and in vitro wireless monitoring of intraocular pressure of a bovine eyeball. Intraocular pressure is the largest risk factor for glaucoma, a leading cause of blindness.


University – has demonstrated real-time glucose detection on a live rabbit eye and in vitro wireless monitoring of intraocular pressure of a bovine eyeball. Their sensor measures both glucose and intraocular pressure simultaneously based on different electrical responses. ‘This study can be used to diagnose diabetes and glaucoma by implementing two types of transparent electronic sensors in the production of smart contact lens sensors,’ says Park. ‘We are now a step closer to the implementation of a fictional idea for a smart contact lens like in the films Minority Report and Mission: Impossible.’ The sensor’s key components


are graphene and a graphene-silver nanowire (AgNW) hybrid structure, which has enhanced electrical and mechanical properties while still maintaining transparency and ‘stretchability’. The team integrated the sensors with resistance, inductance and capacitance circuits and placed them onto soft contact lenses. All the components are


the team placed a layer of silicone elastomer between two inductive spirals made of graphene-AgNW hybrid electrodes in a sandwich structure.3


High-intraocular pressure


increases the radius of curvature of the cornea. As pressure rises, the cornea stretches and the dielectric layer in the sensor – an electrically non-conductive layer – starts to thin, and this increases the capacitance of the circuit. At the same time, the spiral coils also start to expand and this increases the inductance. The sensor embedded in the contact lens transmits the changes in both to the wireless antenna. Whereas inductance and capacitance vary with structural changes in the device, allowing the detection of intraocular pressure, the circuit’s resistance responds to molecular binding. To detect glucose, the team use glucose oxidase immobilised on channels in the graphene using a pyrene linker. The enzyme catalyses oxidation of glucose to gluconic acid and reduction of water to hydrogen peroxide. Hydrogen peroxide, a reducing agent in the system, is oxidised to produce oxygen, protons and electrons. The concentration of charge carriers in the channel, and thus the drain current, increases at higher concentrations of glucose, and this affects the resistance in the circuit, which can be detected and measured.


Although the team admits that


the precise diagnosis of glucose may require further sensor development, they say that the contact lens sensor should be sufficient for screening for prediabetes and daily glucose monitoring. They also expect that the simple pyrene-chemistry involved would allow for a multiplexed array of graphene sensors, tuned to detect numerous disease-related biomarkers in tear fluid. According to the team, the


sensors still worked when the lens changed shape, and when exposed to various substances in human tears. Furthermore, since the electronic sensor is inserted into a soft contact lens, they claim it should feel comfortable and note that the


20 08 | 2017


DR. HONGRUI JIANG


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