Perspective www.acsnano.org

Smart Reinvention of the with Graphene Kyoungjun Choi and Hyung Gyu Park* Nanoscience for Energy Technology and Sustainability, Department of Mechanical and Process Engineering, Eidgenössische Technische Hochschule (ETH) Zürich, Tannenstrasse 3, Zürich CH-8092, Switzerland

ABSTRACT: With potential benefits to the 71 million contact lens users worldwide, contact lenses are being reinvented in the form of smart wearable electronics. In this issue of ACS Nano, Lee et al. report on the fascinating functions of a graphene-based smart contact lens that is able to protect eyes from electromagnetic waves and dehydra- tion. Graphene and two-dimensional materials can be exploited in many opportunities in the development of smart contact lenses. Here, we briefly review and describe prospects for the future of smart contact lenses that incorporate graphene in their platforms.

n 2014, we saw the “smart” reinvention of contact lenses as external devices such as smart phones, tablet computers, or an ideal ocular platform for wearable electronics.1 This other wearable devices. A plan also exists to integrate into smart I smart contact lens (Figure 1) aimed to monitor glucose contact lenses a tiny light-emitting device and a built-in camera levels in tears to diagnose diseases such as diabetes easily and capable of signaling, recording, and projecting images directly noninvasively. into our field of vision. Although these goals may sound like Remarkable progress in micro-nanomanufacturing now science fiction, several market leaders today are steadily enables us to imagine that contact lenses could be incorporated embodying these concepts. with wireless devices and miniaturized sensors. A wireless Early work on contact lenses dates back to 1508: Leonardo antenna in the lens could transmit data that are collected and da Vinci is credited with conceptualizing a contact lens through analyzed by sensors and microprocessors, respectively, to a water-filled glass hemisphere over the eye. However, it was not until 1949 that the first corneal lens via (methyl methacrylate) (PMMA) was introduced. Despite its glass-like clarity, the bare oxygen permeance of PMMA has engendered a search for alternative materials that have sufficient oxygen permeability. In order to meet the oxygen transmission requirement, a wide variety of polymers have been investigated from polyhydroxyethyl methacrylate hydrogel to siloxane- and fluorosiloxane-containing hydrogels.2 The quest for optimal polymeric materials was soon followed by an early concept of the smart contact lens, which involved incorporation of a drug- delivery system and an intraocular pressure sensor for glaucoma diagnosis within the lens. In this issue of ACS Nano, Hong and colleagues demonstrated extended function of the smart contact lens by coating one side of a hydrogel lens with graphene to protect the eyeball from both electromagnetic (EM) wave damage and dehydration discomfort.3 Graphene’s remarkable electrical, mechanical, and chemical properties have called for a reliable and efficient synthesis route, and today, chemical vapor Figure 1. Prototype of the contact lens, developed deposition (CVD) is the most common method of supplying collaboratively with Novartis, to detect glucose levels. Reprinted graphene for various applications.4 In particular, graphene’s with permission from ref 1. Copyright 2014 Macmillan Publishers Ltd.

© XXXX American Chemical Society A DOI: 10.1021/acsnano.7b03180 ACS Nano XXXX, XXX, XXX−XXX ACS Nano Perspective densely packed, hexagonal lattice made of carbon atoms, which incorporate inorganic LEDs. Even though the structural issue prevents water penetration across the graphene film, has led to influenced the resistance upon wearing the graphene-covered its potential use as eye protection from dehydration.5 Although contact lens in a case of a live rabbit eye, no unusual behavior graphene is nearly transparent optically, its optical thickness by the rabbit was observed during the 5 h observation period. often enlarges for EM waves in the MHz and GHz ranges.6 According to Hong and colleagues, an electromagnetic Along with high electrical and thermal conductivities, these interference (EMI) shielding effect is a promising feature of features of graphene promise immediate adaptability to the graphene use in a smart contact lens.3 In order to facilitate data smart contact lens. communication and power charge, a wireless antenna In this Perspective, we provide a brief overview on and interacting with EM waves is likely essential for the smart describe prospects of feasible functions of graphene for smart lens; however, continuous exposure to EM waves may harm contact lenses. Outstanding electrical properties such as low eyeballs by low-temperature burns or dehydration.3,7 Despite sheet resistance and considerable charge carrier mobility enable the use of a metal film as an EMI shielding strategy, the primary graphene’s use in transparent electrodes and fast-response mechanism of the shielding is the wave reflection, not sensors, suitable components for a wearable device. Interactions adsorption, thus bringing about secondary issues to the between external magnetic fields and electrons in graphene help surrounding. On the other hand, by efficiently absorbing the − to shield EM waves in certain frequency ranges. Due to the EM energy up to 90% due to high carrier density (>1012 cm 2 mass impermeability of the pristine graphene lattice, graphene for doped graphene), graphene coating is considered one of the can mitigate ocular dehydration. Finally, in order to adjust near- rising EMI shielding strategies today.6 Figure 2 shows an and far-sightedness, graphene can change the focal length of a polymeric soft contact lens. Metal electrodes are an essential component of electronic devices and provide desired electrical properties; for wearable devices, however, their use could be limited by mechanical failure and opacity if future wearable electronics, including the smart contact lens, demand more flexibility, stretchability, and transparency. Especially for the smart contact lens, the relatively bulky film of the metal electrode can be constrained for issues in biocompatibility, corrosion, and oxygen permeation, thereby hampering daily wear.7 Ever since the CVD on Cu foil graphene synthesis method was developed in 2009, graphene has been intensively considered as a potential alternative to metal electrodes and transparent indium−tin−oxide electrodes. Having lots of grain boundaries, CVD-grown graphene with Young’s modulus of 1 TPa and spring constant of 1−5 N/m is as strong as mechanically exfoliated graphene, the first isolated type of graphene. These values are among the best mechanical properties measured so far, enabling graphene transfer onto fl Figure 2. (a) Before and (b) after irradiation of electromagnetic exible and rough surfaces. Low and uniform sheet resistance of wave onto egg white passivated by graphene-covered and normal 2 250 Ω/sq measured over 400 × 300 mm of graphene is on a contact lenses. Reprinted from ref 3. Copyright 2017 American 4,8 par with those of the transparent electrodes. Interestingly, Chemical Society. graphene sheet resistance is invariant in stretching (<12%) and tensile strength (<6.5%), evidenced by the complete recovery of the electrical property.9 One-atom-thick graphene is rather example of the shielding effect: an egg white sample underneath transparent, adsorbing only 2.3% of incident light in the entire a graphene-coated soft contact lens is barely damaged, whereas ultraviolet−visible−infrared wavelength range. These unique another sample covered by a bare soft lens is denaturalized characteristics enable researchers to delve into electronics during the irradiation of a strong EM wave, similar to the 4G applications ranging from flexible electronics and organic light- and Bluetooth communication frequencies, in a microwave emitting diodes (LEDs) to energy devices and wearable oven.3 electronics. The large specific surface area of 2630 m2/g, which enables As described in this issue of ACS Nano, Hong and colleagues facile functionalization, adds to the advantages of graphene for were interested in utilizing graphene as a multifunctional film the smart contact lens.11 The unique band structure of pristine material on a soft contact lens substrate.3 The concept of the graphene further enables it to respond and to sense quickly. graphene-based smart contact lens can be justified if Based on these benefits, graphene functionalized with glucose considering the feasibility of miniaturized and thinner circuitry, oxidase can selectively respond to glucose. If utilizing the electrical characteristics, mechanical stability, and transparency. charge-neutral-point shift phenomenon, a field-effect transistor For example, CVD-grown graphene is not only semimetallic, out of the functionalized graphene can detect a glucose level in with in-plane sheet resistance as low as 593 Ω/sq, but also able the range of 3.3−10.9 mM that brackets most of the reference to lie on a flexible surface like a lens. Hybrid design of graphene range of medical examination or screen test for the diagnosis of and silver nanowires can further decrease sheet resistance of 30 diabetes.12 Monitoring glucose levels from tears could Ω/sq at the expense of transparency (94%).10 Superb accurately approximate bloodstream glucose levels, given that mechanical stability at a strain of 27% with invariant resistance the tear film is closely associated with blood across a tear− indicates nearly perfect conformal coating of the lens to blood barrier.

B DOI: 10.1021/acsnano.7b03180 ACS Nano XXXX, XXX, XXX−XXX ACS Nano Perspective

Figure 3. (a) Relative water vapor transmission rate (WVTR) of graphene showing the dependence on the substrate and the number of layers. The red bar shows the relative WVTR of monolayer graphene on hydrogel contact lenses (hioxifilcon A). Black bars show how relative WVTR of graphene on a polyethylene terephthalate (PET) substrate changes with layer number. All the bars are normalized by P P WVTR of each polymer substrate. (b) Schematic of oxygen penetration through nanoporous graphene and hydrogel contact lenses. 1, s, and P R R 0 are the external, interlayer, and internal pressures, respectively. NPG and HG are permeation resistances of graphene and hydrogel, respectively. (Inset) Oxygen permeances for various contact lens materials (supplier data) and porous graphene (authors’ characterization). A black dotted line indicates a desirable oxygen permeance level to avoid hypoxia-related complications such as for corneal anoxia with extended (or overnight) wear for closed eye conditions.21

The tear film, which is composed of three layers, plays sheet resistance of the graphene are as low as 0.45 kΩ/sq, important roles in cleaning up and lubricating the eyes as well which is comparable to those of CVD-grown graphene. as in providing clear vision.7 Dehydration of the eyes, called Depending on voltage and frequency, the electro-displacement keratoconjunctivitis sicca, or dry eye syndrome, can occur when of the elastomer can vary from 29 to 946 μm with optical tears either are no longer produced or they evaporate quickly. If transparency of 57%. Even though high voltage is still needed left untreated, dry eye syndrome can bring about severe to bend the elastomer, the focal length change reported in the complications including impaired vision or loss of sight. literature offers the potential to use graphene for the multifocal The gastight lattice of CVD-grown graphene could prevent lens component. dry eye syndrome by inhibiting excessive evaporation of tears. In fact, mechanically exfoliated, pristine graphene is imperme- PROSPECTS able against gases including helium. Water vapor transmission Google’s invention of the smart contact lens for sensing glucose of CVD-grown, large-scale graphene is as negligibly small as − in tears has garnered worldwide interest in wearable electronics. 10 4 g/m2·day for the first few hours with high transparency The idea of a lens equipped with a tiny display and a camera and mechanical stability kept unaltered.5,13 Thus, CVD-grown that can project images into the user’s own field of vision is graphene can keep the hydrogel contact lens hydrated, although being pursued by other market leaders such as Samsung and graphene does not cover the eyes directly. Otherwise, if the lens fi Sony. In the speedy course of this development, there are a few is dehydrated, the tear lm on the eye dwindles away, oxygen research topics that deserve attention. Oxygen permeation can permeation decreases, and the contact lens refractive index be ranked high on the list. Because permeance is inversely increases. Hong and colleagues report water vapor transmission proportional to thickness, the three-layer (lens, circuitry, and rates to be 0.1181 g/cm2 per day for a hydrogel soft lens and 2 protective coating) structure of smart contact lenses that has 0.0791 g/cm per day for a graphene-covered soft lens after a 7 been validated thus far could cause such hypoxia-related day long measurement (Figure 3a). The 30% decrease in the complications as red eyes and corneal neovascularization. If water vapor transmission rate is attributable to a screening ff graphene takes the place of the circuit layer, the mass e ect of the graphene-coated contact lens. impermeability of graphene may impede oxygen accessibility The ability for a single lens to be multifocal is another to the cornea. Nevertheless, this potential risk might be fascinating feature that adds to the promising advantages of addressed by use of nanoporous graphene membranes.16 Figure graphene-incorporated smart contact lenses. As people grow 3b illustrates the possible pathway of oxygen across a older, they inevitably develop a symptom called presbyopia, nanoporous graphene layer that we characterized and a ffi which leads to di culty in focusing on near objects. There are hydrogel contact lens from suppliers. As the oxygen molecules two types of state-of-the-art multifocal lenses. One design, travel to the eyeball, they would encounter two resistances, called an alternative design, is similar to bifocal glasses and is from the nanoporous graphene and from the polymer contact composed of two zones in a contact lens for distant vision at lens. Because the oxygen permeance of the nanoporous the center and near vision at the edge of the lens; the other graphene is greater than that of the contact lens by many design, called a simultaneous design, has both portions for orders of magnitude, the permeation resistance, RNPG, coming distant and near visions in front of the pupil. With the from the nanoporous graphene becomes negligible.17 There- electrically induced deformation behavior of dielectric fore, without the concern of hypoxia, nanoporous graphene can elastomers, research on using deformable membranes as optical help embed ultrathin circuitry within the smart contact lens. lenses is ongoing to enable the active membrane to change its Loss of tears by water evaporation could be controlled by curvature reversibly from flat to a concave or convex shape engineering the pore sizes of the nanoporous graphene. In this under an electrical bias. Recently, a transparent actuator way, the cornea can be exposed to a clinically safe amount of integrated with chemically exfoliated graphene film has been oxygen and be protected from anoxia-level dehydration. demonstrated.14 In order to achieve the electro-deformation However, the successful utilization of nanoporous graphene behavior of the lens, both transparent elastomers and in smart contact lenses depends critically on the establishment conductive materials are needed.15 Reported values of the of synthesis options at large and intermediate scales.

C DOI: 10.1021/acsnano.7b03180 ACS Nano XXXX, XXX, XXX−XXX ACS Nano Perspective

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D DOI: 10.1021/acsnano.7b03180 ACS Nano XXXX, XXX, XXX−XXX